GB2025297A

GB2025297A - A method and apparatus for making a metal cap, and an improved metal cap

Info

Publication number: GB2025297A
Application number: GB7918092A
Authority: GB
Original assignee: Toyo Seikan Kaisha Ltd
Current assignee: Toyo Seikan Group Holdings Ltd
Priority date: 1978-05-26
Filing date: 1979-05-24
Publication date: 1980-01-23
Also published as: IL57383A; LU81310A1; GB2025297B; NO791733L; IT1120759B; NZ190543A; NL182127C; NL182127B; BE876490A; FI791674A; CH630863A5; NO151732C; FR2426507B1; DE2920982A1; IL57383A0; NO151732B; IT7923007A0; FR2426507A1; DE2920982C2; NL7904128A

Abstract

A liner layer is thermally adhered to the end of an aluminium crown c- cap 5 by heating the cap with a high- frequency induced current flow in the end of the cap, in which a primer is applied, by conveying the cap along a high-frequency conductor 7,7a, which repels the cap into contact with a guide wall 8a. The cap is supplied 14 with a thermoplastic resin to be the liner layer after the end of the cap is heated to a thermal adhesion temperature. There is a high- frequency current generating circuit and a high-frequency conductor 7,7a which is to be used with the current generating circuit. The cap may have only a portion of its liner layer thermally adhered thereto. <IMAGE>

Description

SPECIFICATION A method and apparatus for making a metal cap and an improved metal cap The present invention relates to method and apparatus for making a metal cap and an improvement in a metal cap. More particularly, it relates to high-frequency continuous heating method and apparatus suitable for making the metal cap by thermally adhering a liner layer to a metal shell.

The metal caps according to the present invention are meant to include cups, caps and crown caps, which are produced by subjecting a metal sheet to a shallow or deep drawing process and further to a thread cutting process, if necessary, and in which a liner is thermally adhered to the bottom of their metal shells, and to further include pilfer proof caps. The metal shells are meant to include those unlacquered, those printed and painted, those coated with an adhesive or primer in its bottom, and those lined with a thermoplastic resin.

The material to be used suitably for the metal shells according to the present invention is meant to include a sheet of non-magnetic metal such as aluminum, its alloy (which will be shortly referred to as "aluminum"), copper or brass. However, a sheet of ferromagnetic metal such as low-carbon steel can also be used in accordance with the later-described conditions.

Among the metal caps, especially those used to seal or plug the mouth of a container such as a bottle or a wide mouth bottle are lined as a packing material with cork, or thermoplastic resins such as vinyl chloride, polyethylene or their blend. Recently,'the use of the latter materials is especially increasing. In this instance, in order to thermally adhere the liner to the inner side of the shell bottom or end, generally speaking, the metal shell having its bottom inner side coated with an adhesive primer is changed with a liner material, and the shell bottom is heated to about 100 to 2000C before or after the charging step.

As a continuous heating apparatus for metal caps, especially for crown caps (which are usually made of a ferromagnetic material such as tinplate or tinfree steel sheet), there has been well known in the art a heating apparatus of the type using heated air as is disclosed in Japanese Patent Publication No. 41-5588. According to the heating apparatus disclosed, the crown caps are heated to about 1 6000 while being conveyed having their flares seated in the counter-sunk projections which are formed above a number of notches formed at a spacing in the circumference of a turn table.

The heating system of this type requires a long time for the heating process, and accordingly it cannot be free from the drawback that its facilities cannot be compact because an elongated heating station is required for accomplishing the high speed heating operations, e.g. 1000 pieces per minute.

This construction is accompanied by a problem that the skirt portions of the metal shells which do not always require heat have to be heated.

In order to eliminate the above drawback, there has been developed another heating apparatus of high-frequency type, which can be referred to the proposal of Japanese Patent Publication No.

47-41398. According to this proposal, the crown caps being conveyed on a conveyor are promptly heated by means of a high-frequency heating coil in the form of a hair pin, which is arranged below the conveyor. However, it has been found by the Experiments conducted by the Applicant that the highfrequency apparatus proposed can ensure the effective heating process for the crown caps made of a ferromagnetic material which can concentrate the magnetic flux but cannot ensure the heating process at least for the metal shells made of a non-magnetic material such as aluminum. More specifically, since the conveyor having a considerable thickness is interposed between the heating coil and the metal shell, the mutual induction coefficient in between takes such a low value that the temperature of the metal shell can be raised little.When, with a view to increase the mutual induction coefficient, the thickness of the conveyor is reduced to an impractical extent thereby to make the heating coil and the metal shell approach, this shell will float and jump up the conveyor so that the heating process is impossible. The jumping actions are caused by the repulsion which is generated by the interaction of the induced current, which is induced in and adjacent to the bottom of the aluminum shell, and the high-frequency coil magnetic field. In the case of the cap of a ferromagnetic material such as a crown cap, the cap itself is magnetized by,the magnetic field resulting from the high-frequency current to establish the attractive force acting in the direction of the high-frequency conductor.Since, in this instance, the attractive force is stronger than the repulsive force resulting from the induced current, no jumping action will normally result.

It is, therefore, a major object of the present invention to provide a high-frequency heating apparatus suitable for continuously and massively heating the bottoms of non-magnetic metal shells at a high rate.

Next, a normal metal cap has its metal shell, in which a liner is thermally adhered to the inner side of the bottom thereof. In this instance, it is desired that the shell bottom be heated as uniformly as possible to a suitable thermal adhesion temperature. This is because the creation of a locally heated portion will frequently invite decoloration, deterioration or burning of the liner at that particular portion.

Therefore, another object of the present invention is to provide a high-frequency continuous heating apparatus which can heat the bottoms of metal shells as uniformly as possible.

On the other hand, recently, prize-offered caps, especially, crown caps are frequently used, and various proposals have been made as to metal caps which can facilitate separation between the liners and metal shells thereof. Here, the prize-offered caps are meant to include those which have their metal shells printed in its inside bottoms with the "Winning" marks or other induction marks such as letters, symbols or figures with a view to promoting the selling of bottles or the like. One of the known constructions of the prize-offered caps of that type for facilitating the separation of the liner is made such that only the center portion of the liner is lightly adhered while leaving the circumferential portion thereof partially or wholly under the non-adhered condition.For example, according to the known constructions, an adhesive primer is applied only to the center portion of the liner side of the bottom of a metal shell, or such ink as will inhibit the adhesion between the liner layer and the adhesive primer is applied to the whole or partial area of the circumference of the bottom inside after the adhesive primer is applied to the whole area of the bottom inside. However, there arises a problem that the both proposals require a special primer applying device. Special problem to the latter is that one step has to be added. Another common problem is that delicate application is required in the applying step in order to prevent the offset of the coating film.

Therefore, a further object of the present invention is to provide a metal cap, in which a thermal adhesion portion is formed only at a portion of the liner layer without requiring the complex step of applying the adhesive primer, as well as method and apparatus for producing the same.

The foregoing and other objects are partially self-explanatory and will be partially described with reference to the accompanying drawings showing the embodiments of the invention and to the graphical presentations illustrating the test results of the examples of the same, in which: Fig. 1 is a top plan view diagrammatically showing an induction heating apparatus for continuously heating aluminum shells according to a first embodiment of the present invention together with the equipments for the pre- and post-stations thereof; Fig. 2 is a vertical section taken along the line Il-Il of Fig. 1; Fig. 3 is a perspective view showing the bottom of a metal shell as well as an induction current circuit arranged in the vicinity thereof;; Fig. 4 is a graphical presentation showing the relationship between the spacing between the shell bottom and the heating coil and the temperature rise (in a relative value) in the shell bottom; Fig. 5 is a top plan view diagrammatically showing a continuous induction heating apparatus according to a second embodiment of the present invention:: Fig. 6 is a partially enlarged front elevation showing the heating apparatus of Fig. 5; Fig. 7 is a vertical section taken along the line VIl-VIl of Fig. 6; Fig. 8 is a diagrammatical top plan view showing one modification of the continuous induction heating apparatus according to the first embodiment of the present invention; Fig. 9 is a diagrammatical top plan view showing a heating apparatus of the type, in which a horizontal guide wall itself is moved to convey metal shells; Fig. 10 is a vertical section taken along the line X-X of Fig. 9; Fig. 1 1 is a top plan view showing a continuous heating apparatus according to a third embodiment of the present invention;; Fig. 12 diagrammatically shows the wiring arrangement corresponding to the case, in which the heating coil of Fig. 1 1 in an arcuate shape is arranged in a linear shape; Figs. 13 to 16 diagrammatically show the wiring arrangements of the heating coil other than that shown in Fig. 12; Fig. 17 diagrammatically shows the wiring arrangement illustrating the heating coil according'to a fourth embodiment of the present invention; Fig. 18 is a vertical section taken along the line XVlll-XVlll of Fig. 17; Fig. 19 is a vertical section of an essential portion showing the arrangement between the heating coil and a ferrite core according to another modification of the fourth embodiment; Fig. 20 is a vertical sectional view showing the arrangement and construction of a heating coil and a metal shell according to a fifth embodiment of the present invention;; Fig. 21 is a sectional view showing an essential portion of Fig. 20 in an enlarged scale; Fig. 22 is a circuit diagram showing the potential differences of the electric conductors of Fig. 20; Fig. 23 is an explanatory view showing the arrangement and construction of another modification of the fifth embodiment; Figs. 24 and 25 are block diagrams illustrating the priciples of a sixth embodiment of the present invention, respectively, in the cases of using a non-magnetic and ferromagnetic metal shells; Fig. 26 is a connection diagram showing a suitable high-frequency induction heating circuit according to the sixth embodiment; Fig. 27 is a chart showing the wave-forms generated at the respective portions of Fig. 26; Fig. 28 is an enlarged vertical section showing the positional relationship between the heating coils and the metal shell; ; Fig. 29 is a sectional side elevation showing a metal cap according to the present invention, in which a liner layer is partially adhered thereto in a thermal manner; Fig. 30 is a partially enlarged section showing the bottom of the metal cap of Fig. 29; Fig. 31 (a) to (d) are diagrammatical views showing several modes of the portions of the liner layer to be adhered and to be left unadhered.

Fig. 32 is a graphical presentation showing the relationship between the heating time and the temperature in the bottom of the metal shell; and Fig. 33 is a graphical presentation showing the temperature distribution in the bottom of the metal shell.

The present invention will now be described in detail in connection with the embodiments thereof with reference to the accompanying drawings.

Referring first to Fig. 1 there is shown a first embodiment of the present invention, comprising a rotary table 2 acting as a conveyor of a series of aluminum shells 5 formed at its circumference with a number of semicircular notches 3 so that the aluminum shells 5 discharged from a chute 4 are loaded one by one into the semicircular notches 3 upside down, i.e. with their ends being positioned below. The aluminum shells 5 thus loaded are conveyed in the direction of rotations of the rotary table 2, as shown in the direction of the arrow in Fig. 1, until they are further loaded on an induction heating apparatus 1.

The aluminum shells 5 are guided to slide on guides (not shown) before and after they enter and leave the heating apparatus 1.

The induction heating apparatus 1 is composed of a high-frequency coil 7 to be supplied with an electric power from a high-frequency oscillator 6, a horizontal guide wall or corridor 8 and a vertical guide wall 1 5. Turning to Fig. 2, the heating coil 7 is composed of two (paired) conductors (which will be shortly referred to as "conductors"), in which the currents flow in the opposite directions.This is because in case the directions of the currents are opposite, as shown in Fig. 3, the currents induced in the bottom and lower skirt portions of the shells 5 made of aluminum or a non-magnetic material can flow in such a large quantity, while establishing a closed circuit 12, as to efficiently effect the temperature rise due to Joule heat, but in the case of the same direction of currents the closed circuit 12 of Fig. 3 is not established so that the current inducing efficiency is too low to substantially effect the temperature rise. For the same reason, no substantial temperature rise is effected when a single conductor is used as the coil.Although the embodiment shown employs a pair of conductors, when the metal shells have a larger outer diameter, it is preferred in view of the temperature rising rate and the uniformity of the temperature distribution in the bottom of the metal shells that more than one pair of conductors are used as the coil.

On the other hand, the spacing between the conductors (measured between their center axes) is made smaller than the diameter (or the smallest length in case the metal shells are not cylindrical) of the bottom of the shells. This is because if one of the conductors is spaced apart from the bottom of the metal shells the resultant heating efficiency is so abruptly reduced to fail to heat the metal shells. With a view to increasing the heating efficiency and making the temperature distribution uniform, moreover, it is preferred that the aforementioned spacing of the conductors is substantially equal to or slightly larger than the radius of the shell bottom and that the center of the shell bottom is positioned substantially on the center line between the conductors.

On the other hand, the inside of the conductor 7 is cooled with water. The diameter of the conductor 7 is increased with that of the metal shells to be heated. For example, when the diameters of the metal shells are 1 5 to 25 mm, 26 to 40 mm and 41 to 60 mm, the preferred diameters of the conductor are about 3 to 4 mm, 5 to 6 mm and 7 to 8 mm, respectively. This is partly because as the diameter of the metal shells becomes the larger it becomes necessary to supply the conductor with the higher current and partly because if the diameter of the conductor is excessively small the losses therein due to Joule heat are accordingly increased whereas if the diameter of the conductor is excessively large the electromagnetic coupling between the conductor and the shells is so accordingly reduced as to deteriorate the heating efficiency.

It is also preferred to prevent the mutual induction coefficient between the conductor and the shell bottom from being delicately influenced by the slight change in the spacing in between, which will often result in the change in the flow rate of the current to be induced in the shell bottom and accordingly in the temperature rise obtainable. In order to prevent the shell bottom and the conductor from being short-circuited, moreover, it is preferred to stabilize the conductor in position during the heating treatment by fixing the conductor by means of an adhesive 1 1 such as an epoxy resin, as seen from Fig.

2, to a bed 10, which is made of either a synthetic resin such as a phenol resin known under the trade name of "Bakelite" (when a vacuum-tube type oscillator is used) or a highly permeable insulating material such as ferrite (when a semi-conductor type oscillator is used).

Although any type can be used as the high-frequency oscillator 6, a vacuum-tube oscillator may suitably be employed for the frequency range of 100 KHz to 10 MHz, and a semi-conductor oscillator such as a transistor or thyristor may also be employed for the frequency range of 5 to 80 KHz.

In the latter case of using the semi-conductor oscillator, the bed 10 is made of a highly permeable insulating material to increase the electromagnetic coupling between the conductor (or the heating coil) and the shell bottom so that both the heating efficiency and the uniformity in the temperature rise in shell bottom can be enhanced. In the former case of the vacuum-tube oscillator, on the contrary, the use of the highly permeable insulating material would raise the impedance of the portion of the conductor corresponding to the spacing among the metal shells because of the high operating frequency so that the flow of current is inhibited. Therefore, less effects could be attained even if the bed 10 is made of such insulating material.

Moreover, the semi-conductor oscillator has another advantage that it requires no output current transformer for adjustment of output impedance so that it can have a high efficiency while reducing its size. This is because the oscillator of this type can be operated at a low voltage so that it can fee its high-frequency power as it is to the conductor (or the heating coil).

When the metal shell made of a non-magnetic material, as has been described before, is conveyed to above the high-frequency conductor, it is made to float and jump thereabove by the repulsive force generated in between. The aforementioned guide wall 8 functions to prevent the metal shell from being prevented from being heated due to the jumping phenomenon and to provide at its lower or inner side a guide sliding surface, on which the edge 5a of the metal shell 5 is conveyed to slide by the conveyor mechanism. At the same time, the guide wall 8 functions to adjust the spacing between the end or bottom 5b of the shell 5 and the conductor, thereby to ensure effective heating of the shell bottom and to maintain the temperature distribution in the shell bottom as uniform as possible.

There is no special limitation to the guide wall 8 so long as it satisfies the following conditions: that it is free from deformation or damage even with the impact coming from the jumping metal shell; that its lower face is not formed with such roughnesses as to inhibit smooth sliding movement of the edge of the metal shell; and that it is free from wear or damage against that sliding movement. In these conditions, the guide wall 8 may preferably be made of a ceramic plate, especially a reinforced refactory glass plate having a thickness of about 5 to 10 mm. Among all, a glass plate of that type having sufficient transparency is preferred because it can provide easy observation of the heating process.

As will be understood from curve a in Fig. 4, in order to increase the mutual induction coefficient between the conductor and the shell bottom so that this bottom can be heated to a predetermined temperature, the spacing between the upper side (top) 7a of the conductor 7 and the lower side 5b of the metal shell 5 has to be less than 1 mm, preferably, 0.5 mm when a highly permeable material is not used. In this regard, the reduction in that spacing to a level smaller than about 0.1 mm is not desirable because of the possibility in short-circuiting.When, on the other hand, the bed 10 is made of a highly permeable insulating material, it is necessary to reduce the spacing between the upper side 1 Oa of the bed 10 and the lower side Sb of the shell 5 be smaller than 2 mm, preferably, 1 mm, as will be understood from curve b in Fig. 4. In this instance, generally speaking, since the ferrite to be used as the highly permeable insulating material is brittled, it is also necessary to protect the bed upper side 1 Oa with an insulating sheet such as a Bakelite sheet having a thickness of about 0.4 mm. It is therefore difficult to reduce the spacing in between to a level smaller than about 0.4 mm.

When the heights of the metal shells of each lot are managed within a preset range, the spacing between the conductor upper side and the shell lower side can be regulated simply by adjusting the height of the guide wall 8.

Incidentally, taking into consideration the case, in which slightly higher metal shells of a subsequent lot are loaded in following the metal shells being worked, it is preferred that the guide wall 8 having its height once adjusted can be shifted above. In this case, a pressure may be applied down to the guide wall 8, if necessary, by means of a pneumatically controlled ram so that the spacing between the shell lower side and the conductor upper side may be within a preset range.

In order to make the temperature distribution in the shell bottom as uniform as possible, moreover, the imaginary plane defined by the upper lines of the two conductors may preferably be extended in parallel with the lower side of the guide wall 8.

The present invention is not limited to the above mentioned arrangement but can be extended to any modifications, such as (1) the shells are conveyed with their ends uppermost under a heating coil in a horizontal plane, (2) the heating coil is located in any plane, e.g. a vertical plane, (3) rotating shells are conveyed in an opposite position to the conductor at their lower skirts, wherein the rotating table acts as a guide wall.

The first embodiment thus far described is directed to the arrangement, in which the spacing between the guide wall and the high-frequency conductor is fixed at a predetermined value according to the heights of the metal shells, i.e. in which the heights of the metal shells of each lot are regulated within a preset range.

Generally speaking, however, the metal shells are usually subjected to an edge trimming process, after a drawing process, so as to have a preset height at their skirts, and further to a bead treating process and a knurling process. Even if, in the actual working processes, the metal shells are of the same kind and have an identical height at their skirts after the edge trimming process, the heights of the metal shells after the bead treating and knurling processes become different by +0.2 mm among the working lots. Therefore, when the height of the guide wall 8 is fixed, the maximum difference of about 0.4 mm is established among the lots in the spacing or gap between the shell lower side Sb and the conductor upper (top) side 7a. The difference of 0.4 mm will result in change of several ten degrees in the temperature rise in the shell bottom, as will also be understood from Fig. 4.

Turning now to Figs. 5 to 7, there is shown another high-frequency continuous induction heating apparatus according to a second embodiment of the present invention, in which the bottoms of the metal shells can be heated as uniformly as possible even with a slight dispersion in the heights of the metal shells. The major difference of the second embodiment from the first one resides in the construction of the guide wall. Therefore, the portions indicated at the same reference numerals as those in Fig. 1 depict the portions having the same functions.

The guide wall 8 is formed into elongated plates having its width (or shorter side) made slightly larger than the outer diameter of the metal shells such that its lower end sides are so tapered as to ensure smooth movements of the edge of the metal shells. The guide wall 8 thus formed is arranged to face the conductor 7. In Fig. 7, incidentally, the width of the guide wall 8 is made slightly larger than the outer diameter of the metal shells but will develop no substantial trouble even if it is nearly equal to or slightly smaller than that outer diameter. On the other hand, the longer sides of the guide wall may either have a curvature in the longitudinal direction or be straight. In either event, the guide wall is arranged substantially along the same arc about the center of the rotary table even if it is composed of plural components.As to the length of the guide wall 8, there is no special restriction, but with a view to facilitating the height adjustment during the working operations that length may preferably be to such an extent as to allow a plurality of the metal shells to simultaneously pass thereunder. With such length, it is possible to minimize production of nonconforming articles during the heating process when the metal shells being heated are replaced by those of another lot having different heights. When, therefore, the overall length of the heating apparatus is as long as about 80 cm as in a later-described Example, it is preferred that the guide wall has a plurality of components.If in this instance, the spacing between the adjoining duie wall components is excessively large, only that portion of the metal shell being conveyed below that spacing is allowed to float until the metal shell as a whole is so tilted as to be obstructed by the inlet of the subsequent guide wall component, thus interrupting the overall working operations. In the worst case, that particular metal shell may be popped up from the heating apparatus.

It is therefore necessary that the adjacent guide wall components be positioned so close to each other as to ensure smooth transfer of the metal shell from one to the other component.

The guide wall or corridor 8 thus constructed is supported on its supporting mechanism 9 which is equipped with holding spring means. The construction of the guide wall supporting mechanism 9 is as follows: A stationary horizontal lever 9b has its center portion fixed to the upper horizontal portion of a bracket 9a which is generally shaped into letter "Z" and which has its horizontal base fixed to the vertical guide wall 1 5. A pair of vertical bolts 9c are inserted vertically movably into the vertical bores which are formed in the vicinity of the both ends of the horizontal lever 96. A pair of generally C-shaped supporting frames 9d have their horizontal center portions fixed to the lower ends of the vertical bolts 9c, respectively, such that their vertical positions can be adjusted by turning respective adjusting double nuts 9e which are screwed onto the upper threaded portions of the vertical bolts 9c and which are positioned upon the stationary horizontal lever 9b. There are interposed between the upper sides of the supporting frames 9d and the lower sides of the stationary horizontal levers 9b a pair of pressure springs'9f, through which the vertical bolts 9c are extended so that resistances by the spring pressures are applied when the supporting frames 9d are to be raised.

The aforementioned guide wall 8 is fixed in a horizontal position to the inner sides of the supporting frames 9d in the vicinity of their both longitudinal end portions by means of a pair of tipped stopper screws 9g.

It is noted here that those parts constituting the guide wall supporting mechanism 9 except for the pressure springs may desirably be made of a non-magnetic material such as brass.

As will be apparent from the following description, by the use of the guide wall supporting mechanism according to the present invention, the differences in the aforementioned gap among the lots can be substantially obviated so that the temperatures in the shell bottoms can be maintained substantially at a preset constant level for all lots.

First of all, before the working operations, the spacings between the lower sides of the respective horizontal guide wall components and the upper side of the bed are adjusted to a level equal to the minimum height of the metal shells by means of the adjusting double nuts 9e. During the working operations, the metal shells are conveyed while being subjected to the floating forces resulting from the electromagnetic couplings with the conductor. When these floating forces are overcome by the sum of the weight of the horizontal guide wall and the biasing forces of the pressure springs, the metal shells can be conveyed, without floating above the bed 10 even if their skirts have different heights, while having their edges and ends sliding on the lower side of the guide wall and on the upper side of the bed, respectively.As a result, the aforementioned gap can be held at a preset level so that the temperature rises in the metal shells can also be held at a preset level. If, in this instance, the metal shells of another lot higher than the minimum level are loaded, the guide wall 8 and accordingly the vertical bolts 9c are raised by their differences from the minimum height This condition will be understood with close reference to Fig. 6.

On the contrary, when the floating forces are stronger than the sum of the weight of the horizontal guide wall and the initial preset pressures of the pressure springs, the metal shells are made to float above the bed. However, since the floating forces are independent of the heights of the skirt of the metal shell, and since if the pressure springs have sufficient lengths the pressures of the springs remain substantially unvaried even with the considerable variation of about +0.2 mm in the vertical positions of the guide wall, the rises of the metal shells due to the floating forces are left invariable irrespective of the heights of the shell skirts. As a result, the aforementioned gap is also kept at a preset level so that the temperature rises in the shell bottoms are unchanged for lots.In this floating case, the metal shells have their edges sliding upon the lower side of the guide wall but their ends kept out of contact with the upper side of the bed.

The following description is made to a concrete example directed to the case, in which the induction heating apparatus according to the present invention is employed to adhere a liner of polyethylene to the inner side of the bottom of an aluminum shell.

An adhesive primer (which is prepared, for instance, by dispersing oxidized polyethylene or maleic anhydride modified polyethylene in an epoxyphenolenamel) is applied to the inner side of the bottom of the aluminum shell having an outer diameter of 28 mm and a height of 1 6 mm. The aluminum shells 5 (having a thickness of 0.2 mm) are then baked and supplied from the chute 4 into the semicircular notches 3 of the rotary table 2. The aluminum shells are then conveyed to the induction heating apparatus having a length of 80 cm so that they may be subjected to the high-frequency induction heating treatment mainly at their bottoms.Under the working conditions that the transistor oscillator has a frequency of 25 KHz and an output of 10 KW, that the spacing between the lower side of the aluminum shell and the upper side of the bed is 0.7 mm, that the number of the aluminum shells to be simultaneously loaded in the heating apparatus is 20, and that the time period for each aluminum shell to pass through the heating apparatus is 1 second, the shell bottoms are heated to a temperature ranging from 140 to 1 600C (which is measured by means of a thermopaint) and discharged from the heating apparatus. The temperature level measured is sufficient to ensure the complete adhesion between the adhesive primer and the polyethylene at such a mould station as will be described later.

The aluminum shells thus heated are further conveyed through the guide into a molten polyethylene pellet supplier (or extruder) 14, where they are supplied at their bottom centers with molten polyethylene pellets. Then, the aluminum shells are further conveyed to the mould station (not shown), where the molten polyethylene pellets are pressed into a sh-eet by means of a cooled punch and solidified into the liners on the inner sides of the shell bottoms, thus producing the desired aluminum caps. Since the shell skirts are only slightly heated there is no fear of the solidification being delayed by the heat supplied from the skirt.

On the other hand, when the spacing between the extruder 14 of the molten polyethylene pellets and the mould station 16 touched in the above is relatively large as in the metal cap producing apparatus shown in Fig. 8 as a modification of the first and second embodiments, the molten polyethylene pellets supplied to the shell bottoms are so cooled by the time they reach the mould station 1 6 that the adhesion strength is deteriorated or that stress cracking is generated in the liner layer during use. in this particular case, it is preferred that the high-frequency conductor 7 is extended through the extruder 14 to such an extent as to reach the mould station 16, as shown in Fig. 8. The molten polyethylene pellet supplier 1 4 to be used is usually of extruder type, in which the molten polyethylene extruded in the form of cord is cut into the desired pellets by means of a steel cutter.If, in this instance, the cutter is heated by this high-frequency induction heating process, the molten polyethylene sticks to the cutter so that this cutter cannot perform its cutting function. In order to prevent this difficulty, the paired high-frequency conductors in the vicinity of the extruder 14 may preferably be arranged such that: (1) they are very close to each other at a position located away from the shell ends (Fig. 8); (2) they are buried relatively apart from the upper side of the bed 10; (3) they are disposed near the upper side of the bed, but relatively apart from each other outside the shell passage; (4) two independent coils are arranged on the both sides of the extruder, being spaced apart from each other in its vicinity.

Although the foregoing description is directed mainly to shells made of a non-magnetic material such as aluminum, even shells made of a ferromagnetic material such as a tin plate may sometimes jump or float in accordance with the conditions determined by the weight, size and shape of the shells, the intensity and frequency of the current to flow through the conductor, and the spacing between the lower sides of the shell bottoms and the conductor. It should be noted that the present invention can be applied even to such case for the foregoing reasoning.

According to the present invention, the bottoms of the metal shells, especially, of the shells of a non-magnetic material can be heated in a higher rate than 1000 pieces per minute by means of such compact heating apparatus. Even with a slight fluctuation in the heights of the metal shells, moreover, their floating strokes are adjusted to a present level by the actions of the pressure springs so that the temperature of the shell bottom heated can be held within a desired preset range. Still moreover, the shell skirts requiring no temperature rise are heated slightly so that the quality of the products can be prevented from being deteriorated and that the losses in the power can be minimized.

Turning now to Figs. 9 and 10 showing an embodiment of an induction heating apparatus 21, in which the horizontal guide wall itself is moved to convey the metal shells, the guide wall 22 is constructed of an endless belt which is driven in the direction of arrow by means of drive pulleys 23. In this embodiment, the metal shells 26 which are loaded on the high-frequency conductor 25 of the heating apparatus 21 from a chute 24 are conveyed on the guide wall 22 while being made to float by the actions of the high-frequency current and carried by the lower side of the guide wall 22. When the metal shells 26 arrive at the outlet of the heating apparatus 21, they are pushed out transversely of the direction of their procession by a plunger 27 until they are loaded into a subsequent station 28.

Similar functions can be performed even if the endless belt acting as the guide wall is replaced by a rotary disc.

In the first and second embodiments thus far described, the high-frequency conductors of the heating coil are arranged in parallel and at an equal spacing from each other in the passage for the metal shells so that the metal shells are always heated at their same portions, while being conveyed, thus being liable to cause irregularity of their bottom portions in temperature rise.

This irregularity in the temperature rise can be reduced in accordance with a third embodiment of the present invention, as shown in Figs. 11 to 16, in which the forward and backward high-frequency conductors constituting the heating coils are arranged in a taper shape along the conveyor passage.

As will be apparent from Fig. 1 the third embodiment is characterized in that the forward and backward conductors 34 and 36 of the heating coils 30 are arranged in a taper shape along the conveyor passage. In Fig.12 illustrating the case, in which the arcuate heating coil 30 of Fig. 11 1 is arranged in a straight line along a passage 1 00, the heating coils 30 are composed of two pairs of forward and backward conductors, i.e. the coil is composed of the forward and backward conductors 34 and 36 which have their spacing reduced from a high-frequency power source 32 in the conveying direction A of the metal shells 5, and the coil is composed of the forward and backward conductors 34a and 36a which have their spacing reduced from the common power source 32 in the opposite direction to the conveying direction.As a result, the high-frequency current from its power source 32 is branched into the forward conductors 34 and 34a, and the branched currents are returned to the power source 32 through the backward conductors 36 and 36å. It is quite natural that the current passages are inverted in case the phases of the high-frequency power source 32 are inverted.

Although, in the embodiment shown, two pairs of the forward and backward conductors are provided for effecting the impedance matching with the high-frequency power source 32, they can be replaced by one pair or several pairs of conductors.

In the embodiment shown, moreover, since the conductors 34, 36, 34a, and 36a are arranged in the taper shape, the metal shells 5 are consecutively heated, while being conveyed on the, passage 100, at such adjacent portions to the respective conductors as are hatched so that their relatively large areas to the diameter of the conductors can be uniformly heated during the conveying operations, as will be easily understood. With close reference to Fig.12, more specifically, one metal shell 5 is heated first at its center portion, and its heated zone is then shifted progressively to its circumferential portion.After that, the metal shell 5 has its outermost circumference heated substantially at the center of a heating station, i.e. the power supply portion from the high-frequency power source 32 to the heating coil 30 and then has its heated zone shifted gradually to its center in the course of its conveyance until its whole area is uniformly heated. Subsequently, the metal shell 5 is transferred to a subsequent adhering step.

In an embodiment corresponding to the arrangement shown in Fig. 12, in which the operating conditions are preset such that the overall length L of the heating coils 30 measured along the passage 100 is 1 m, that the coil spacing at the power supply portion from the high-frequency power source 32, i.e. the maximum coil width W, is 30 mm, that the coil width W2 at the both ends of the heating coils 30 is 10 mm, that the outer diameter of the metal shell 5 is 38 mm, and the conveying speed of the metal shell 5 is 0.8 m per second, the metal shell 5 passed through the heating section for 1.25 seconds, and the temperature discrepancy after the passage of the metal shell 5 (i.e. the ratio in percentage of the difference between the maximum and minimum temperatures to the maximum temperature) was restricted within 10%.As compared to the conventional arrangement having as high as 40% of temperature discrepancy measured immediately after the passage through the heating station with respect to the maximum temperature, it was found that the uniformity in temperature distribution could be remarkably improved according to the present embodiment.

Turning now to Fig.13, a modification of the present embodiment is shown, in which the heating coil 30 is arranged in such a taper shape that it is composed of a pair of the forward and backward conductors 34 and 36 having its spacing increased in the same direction to the conveying direction A of the metal shells. The modification of Fig. 13 finds suitable application in heating work pieces, which are liable to have their outer circumferential portions cooled more than their inner portions, because the center portions of the shell bottoms are heated previously.

Further reference is made to Fig. 14, in which another modification of the present embodiment is shown. The arrangement according to this modification is similar to the unidirectionally taper shape of Fig.13, but has its taper inclination changed at the portions along the passage 100. A transient portion indicated at reference numeral 200 in Fig. 14 has a larger taper inclination than the other portions. As a result, the metal shell being conveyed at a preset speed in the direction of arrow A along the passage 100 is supplied with less heating energy at the transient portion 200 than at the remaining portions.

According to the Experiments conducted by the Applicant, it has been revealed that the taper shape shown in Fig. 14 was remarkably suitable for ensuring the desired uniform heating operations of the work pieces such as aluminum shells of the type, in which the temperature rise in the intermediate portion of the shell bottoms is steeper than the center and outer circumferential portions thereof.

Still another modification similar to that of Fig. 14 is shown in Fig. 1 5, in which a portion B having a steeper taper inclination is formed in the leading end of the passage 100. As a result, the modification of Fig.15 is suitable for supplying higher heating energy to the outer circumferential portion of the work pieces than to the center portion thereof.

Turning to Fig. 16 showing a further modification of the present embodiment, the steeper taper portion B is formed in the vicinity of the trailing end of the passage 100. As a result, this modification is properly used in the application, in which the higher heating energy is to be supplied to the center portion of the metal shells.

Although the modifications thus far described have their heating coils tapered over their overall lengths, the coils may be formed, if desired, with a parallel portion to the conveying passage.

On the other hand, the high-frequency power source 32 to be used in the present invention may be either a vacuum-tube oscillator or a semiconductor oscillator.

As has been described hereinbefore, it is possible to establish the heating temperature suitable for the characteristics of the metal shells to be worked, to uniformly heat the metal shells and to heat a desired portion of the metal shells to a temperature different from that for the other portions in accordance with the kinds of the metal shells.

The bottoms of the metal shells can be heated to have any desired temperature distribution by providing a ferrite core below the heating coils at a portion of the passage of the foregoing embodiments. More specifically, when the ferrite core is buried below the heating coils, the electromagnetic coupling between the heating coil and the metal shells is so increased that the heating efficiency at that particular portion can be improved. As a result, by providing the ferrite core in any position of the conveying passage, the uniform or local heating operation can be accomplished.

This construction will be described in more detail with reference to Figs. 1 7 to 19 showing a fourth embodiment of the present invention. The heating coils according to the fourth embodiment is composed of the forward and backward conductors 34 and 36 which are arranged in the taper shape along the passage similarly to Fig. 12.In the fourth embodiment shown in Fig. 17, more specifically, the heating coil 30 is composed of two pairs of the forward and backward conductors, i.e. the paired ones composed of the forward and backward conductors 34 and 36 which have their spacing reduced from the high-frequency power source 32 in the conveying direction A of the metal shells 5, and the paired ones composed of the forward and backward conductors 34a and 36a which have their spacing reduced from the common power source 32 in the opposite direction to the conveying direction. As a result, the high-frequency current from its power source 32 is branched into the forward conductors 34 and 34a, and the branched currents are returned to the power source 32 through the backward conductors 36 and 36a.It is quite natural that the current passages are inverted when the phase of the high-frequency power source 32 are inverted.

Although, in this embodiment, two pairs of the forward and backward conductors are provided for effecting the impedance matching with the high-frequency power source 32, they can be replaced by one pair or several pairs of conductors.

The fourth embodiment is characterized in that ferrite cores 37a, 37b and 37c can be arranged in any positions below the heating coils 30, as seen from Figs, 1 7 and 18. As shown, a coil bed 38 is made of Bakelite and is formed with grooves, in which the ferrite cores 37a to 37c are fixedly mounted. As a result, at the heating station equipped with the ferrite cores 37a to 37c, the electromagnetic coupling between the heating coil 30 and the metal shells 5 is increased so that the resultant heating efficiency can be accordingly improved.

In this embodiment, moreover, since the conductors 34, 36, 34a and 36a are arranged in the taper shape, the metal shells 5 are consecutively heated, while being conveyed on the passage 100, at such adjacent positions to the respective conductor as are hatched so that their relatively large areas to the diameter of the conductor can be uniformly heated during the conveying operations; as will be easily understood. With close reference to Fig.17, more specifically, one metal shell 5 is heated first at its center portion, and its heated zone is then shifted progressively to its circumferential portion.After that, the metal shell 5 has its outermost circumference heated substantially at the center of the heating station, i.e. the power supply portion from the high-frequency power source 32 to the heating coils 30 and then has its heated zone shifted gradually to its center in the course of its conveyance until its whole area is uniformly heated. The metal shell 5 is then transferred to the subsequent adhering step.

When the work pieces to be heated are the shell bottoms having a disc shape, according to the heating coils having a linear taper shape of Fig. 1 2, the intermediate portion of the disc will be heated more than the center and outer circumferential portions of the same, thus establishing the non-uniform temperature distribution. According to the fourth embodiment, however, the ferrite cores 37a, 37b and 37c are arranged to improve the heating efficiency at the both ends of the heating station, i.e. the zones for heating the center portions of the metal shells 5 and at the center portion of the heating station, i.e.

the zone for heating the outer circumferential portion of the metal shells 5. As a result, these metal shells 5 can be heated to attain a remarkably uniform temperature distribution.

In the fourth embodiment shown, although the ferrite cores are arranged at the both end portions and the center portion of the heating station, their positions can be desirably selected in accordance with the temperature characteristics of the metal shells and with the taper shape of the heating coils.

Moreover, the metal shells can be heated to attain either uniform temperature distribution or locally different temperature distribution.

According to the present embodiment, the spacings between the ferrite cores and the heating coils can be desirably selected, and the eleciromagnetic coupling change between the heating coils and the metal shells due to the provision of the ferrite cores can also be adjusted. It should be noted that the present embodiment can be applied to the metal shells made of a ferromagnetic material such as iron.

Reference is made to Fig. 19 showing another modification of the present embodiment, which has constructional characteristics in that the ferrite core 37 encloses the conductor elements 34 and 36 with a view to increasing the electromagnetic coupling between the conductors and the metal shells 5.

The ferrite core 37 is formed into a shape of letter "E" having two grooves 37x and 37y, in which the conductors 34 and 36 are arranged. As a result, the magnetic concentration by the conductors 34 and 36 is so increased that the electromagnetic coupling between the conductors and the metal shells 5 can be accordingly increased.

As has been described in the above, the present embodiment is advantageous in that the temperature distribution in the bottoms of the metal shells can be desirab!y preset by selecting the positions of the ferrite cores.

The following description is concerned with a fifth embodiment of plural turns of heating coils which are connected with a high-voltage and high-frequency power source of vacuum-tube oscillation type so that is can efficiently heat metal shells made of a non-magnetic material such as aluminum.

The characteristics of the fifth embodiment over the first embodiment shown in Fig. 1 reside in that there are arranged in the heating station three tums of heating coils which constitute the tank coil of the vacuum-tube oscillation circuit so that the electric power from source 6 may be converted into high-frequency oscillations. The construction of the heating station is shown in detail in its cross-section in Fig. 20.

With reference to Fig.20, conductors 40 to 46 constituting the high-frequency induction heating coil are fixedly buried in an arcuate coil bed 47 which is arranged in a stationary position below the outer circumference of a rotary table 46 and which is made of a silicone resin, Teflon or Bakelite (Registered Trade Marks). As a result, the metal shells 5 which are conveyed by the rotary table 46 are subjected to induction heating process at the heating station thus constructed. During this heating process, the metal shells 5 are guided, while being conveyed, by both a vertical wall or a guide side-wall 48 fixedly positioned in the vicinity of the outer circumference of the rotary table 46 and'a guide ceiling or a horizontal guide wall 49 fixedly positioned above and in the vicinity of the outer circumference of the rotary table 46.As has been described before, when the shells 5 of a non-magnetic material are conveyed onto the high-frequency conductor, they are made to float and jump by the repulsive forces established between the conductor and the metal shells. As will be apparent from Fig. 20, since the horizontal guide wall 49 is arranged above the metal shells 5, these shells 5 are conveyed by the rotary table 46 while having their edges sliding on the opposite lower side 49a of the guide wall 49. As a result, the metal shells 5 are moved in a manner to have their lower sides 5a spaced at preset distances from the respective conductors 40 to 45 at all times so that the lower sides 5a of the metal shells 5 are inductively heated in a relativelyuniform fashion.As a result, an adhesive primer 50 which has been applied to the inner side of the shell bottoms Sa can be heated to a temperature enough to effect its adhesion.

In order to efficiently heat the metal shells or the work pieces made of a non-magnetic material, as has been explained before, better results can be attained as the spacings between the conductor and the metal shells are the smaller. As will be apparent from the characteristic curve of the temperature rise in the shell bottoms 5a against the spacing between the lower sides 5a of the shell bottoms and the upper side of the conductors 44 and 45, as shown in Fig.4, it is necessary that the above-specified spacing be smaller than 2 mm, preferably 0.5 to 1.5 mm. If, however, this spacing is excessively reduced, there arises a problem that the danger of dielectric breakdown between the heating conductors and the metal shells is inversely proportionately increased.This problem is solved by the arrangements of the conductors in accordance with the fifth embodiment.

Turning now to Fig.21, there is shown in detail the construction arrangement of the conductors and the metal shells according to the present embodiment. In Fig. 21, the three high-frequency conductors 40. 42 and 44 constituting the forward current passages are arranged in one half of the induction heating station, i.e. in the left-hand half shown at arrow B. On the other hand, the remaining three high-frequency conductors 41. 43 and 45 constituting the backward current passages are arranged in the other half of the induction heating station, i.e. in the right-hand half shown at arrow C. In the embodiment shown, therefore, three turns of the high-frequency heating coil can be constituted.As has been described in the above, since the forward and backward groups of the conductors are arranged in the respective halves of the heating station, the induction currents generated flow in the metal shells, while constituting respective closed circuits, so that the temperature rise due to the Joule heat can be efficiently accomplished. As is well known in the art, therefore, the conductors in each half establish a flux of currents in an identical direction so that efficient electromagnetic inductive operations can be ensured.

The high-frequency power source 6 including the vacuum-tube oscillator has a frequency as high as 100 KHz to 10 MHz. The effective voltage to be generated in the tank coil acting as the heating coil of the power source 6 is as high as about 10 KV. The high-frequency power source 6 has its one terminal connected in series with the respective conductors, i.e. in the order of 40, 41, 42, 43, 44 and 45 and its other terminal grounded to the earth. It is quite important in the fifth embodiment under discussion that the conductors upstream with respect to the earth, i.e. 44(5) and 45(6) in the embodiment shown are arranged in the vicinity of the metal shells or the work pieces 5 and that conductors connected downstream or at a higher voltage side are arranged consecutively apart from the metal shells 5.In other words, as' compared to 44(5) and 45(6), 42(3) and 43(4) are located apart from the metal shells 5, and 40(1) and 41(2) connected at a higher voltage side are located further apart from the metal shells 5. More specifically, the metal shells 5 are normally at the earth potential, and the potentials of the respective conductors with respective to zero potential are as shown in Fig. 22, V6 for 45, V5 for 44, V4 for 43, V3 for 42, V2for41, and V1 of the potential of the higher voltage side for the conductor 40. Thus, these potentials Vs to V1 are established between the respective conductors 45 to 40 and each metal shell 5.In accordance with the fifth embodiment, the conductors at a higher voltage side having higher potential differences, e.g. 40(1) and 41(2) are arranged more apart from the metal shells 5 so that potential gradients can be restrained to a level substantially equal to those between the remaining conductors and the shells. lt is quite natural the conductors arranged more apart from the metal shells have less contribution to generation of induced currents in the metal shells.According to the embodiment under consideration, however, plural turns of coils are employed so that the conductors can be arranged remarleably closer to the metal shells than the prior art, and the conductors at a higher voltage side are partially used to generate the induced currents so that the overall efficiency can be remarkably improved as compared to the prior art. As different from the prior art, moreover, the potential gradients to be established between the conductor at the grounded side and the metal shells cari be restrained to.a remarkably low value so th#:the. tlie- hig~h-frequeney voltage -to-beZgerlerat'ed-can be increased.As a result of the increases in the voltage and in the number of turns of coils and the reduction in the spacings between the conductors and the metal shells, the overall heating efficiency by the induced currents can be remarkably improved.

In the present embodiment, still moreover, the spacings between any two conductors can be suitably selected in accordance with the potential differences in between, and the potential gradients in between are restrained lower than a preset value so that the dielectric breakdowns in between can be prevented. More specifically, although the spacings between the conductors 40 and 42 and between the conductors 42 and 44 in the left-hand half B are preset the same, the spacing between 40 and 44 is preset larger so that the potential gradients between any two of those conductors can be restrained lower than a preset value. In the right-hand half C, the spacing between the conductors 41 and 45 is preset larger than those between any two of the remaining ones so that the potential gradients between any two of the conductors can be restrained lower than the preset value.

In the embodiment shown in 21, still moreover, the conductors 40 and 41 at the power source side are not aligned with the remaining conductors but are arranged in another step so that an advantage can be achieved, in which the respective conductors can be buried in the bed 47 in a relatively compact manner. As shown in a modification of Fig. 2, however, all the conductors can be arranged in a row or in any shape in accordance with the shape of the metal shells. The constituents or parts of the modification of Fig. 23 are indicated at the same reference letters as those of Fig. 21, and their repeated explanations are omitted here.

According to the two-stage arrangement of the heating coils of Fig. 21, however, the contribution of the conductors 40 and 41 at a higher voltage side to the induction heating process is higher than the single row arrangement of Fig. 23 so that the overall efficiency can be accordingly increased to a higher level, which has been revealed twice that obtainable from the modification of Fig. 23 by the Experiments conducted by the Applicant.

In all of the embodiments thus far described, the conductors are made of copper pipes, in which cooling water is made to circulate so as to prevent heat liberation due to the resistance losses of the conductors themselves.

As has been described hereinbefore, according to the present embodiment, the conductors and the metal shells can be arranged in proximity without causing their dielectric breakdown so that the induction heating efficiency can be remarkably improved.

According to the present embodiment, moreover, the relative arrangements of the conductors are preset in accordance with the potential differences in between so that the dielectric breakdowns in between can be prevented.

According to the present embodiment, still moreover, it is possible to provide a high-frequency induction heating apparatus of small size but of high efficiency, which can ensure the heating operations of the metal shells made of a non-magnetic material such as aluminum.

Now, the existing high-frequency induction heating apparatus cannot be free from a problem that the heating energies to be imparted to the respective metal shells become different with the number of the metal shells to be loaded on the heating station. More specifically, the heating coil circuit composed of conductors has its impedance and frequency remarkably varied with the mutual inductances with the metal shells. For example, the load impedance and the frequency of the heating coils remarkably differ between the case, in which 18 shells are simultaneously loaded as the work pieces upon the heating station, and the case, in which only one metal shell is loaded. As a result that the energies to be imparted to the metal shells remarkably fluctuate for such cases, there arises a problem that accordingly high fluctuations take place in the heating temperature.Especially in the step of producing the metal shells, when the number of the metal shells to be continuously loaded on the heating apparatus fluctuates to such an extreme extent that the supply of the metal shells to the heating station becomes zero, this results in the aforementioned problem that the heating temerature remarkably disperses due to the fluctuations in the load impedance and frequency of the heating coil circuit.

The changes in the characteristics of the aforementioned heating coil circuit are different among the kinds of the metal shells. When the metal shells are made of a non-magnetic material such as aluminum, the inductance component of the heating coil circuit is decreased but the resistance component of the same coil is increased by the supply of the metal shells so that the frequency has a tendency to be increased as the number of the metal shells to be supplied is increased. When, generally speaking, the heat coil circuit is controlled to a preset current, the increase in the frequency invites the increase in the energy to be supplied to the metal shells. As a result, when the number of the metal pieces to be loaded on the heating station is decreased, shortage of heat energy takes place.

On the contrary, when the metal shells are made of a ferromagnetic material such as a tin or steel plate, there is exhibited an adverse tendency, in which the frequency and energy are decreased by the supply the metal shells to the heating coils. As a result, it will be understood that overheating takes place when the number of the metal shells of a ferromagnetic material to be fed to the hearing section is decreased.

The above problem can be solved by a high-frequency induction heating circuit according to a sixth embodiment of the present invention, which will be described in the following. First of all, the principles underlying this sixth embodiment will be explained.

Fig. 24 is an explanatory view illustrating the principles of the heating circuit according to the present embodiment, which is operative to control the flow of the current in a heating coil 60 so as to impart an equal calorie to the representative metal shells. Fig. 24 also shows the case, in which the metal shells acting as the work pieces are made of a non-magnetic material such as aluminum.

In Fig. 24: Ei stands for a reference voltage which is usually of variable DC type; Ef stands for a feedback voltage which is usually of DC type when a DC voltage is employed as Ei; Eo stands for an output voltage which is of such high-frequency that is proportional to the difference between Ei and Ef, and is expressed by the following equation; Eo=G (Ei-Ef) (1) Z stands for the impedance of a high-frequency heating coil circuit (i.e. the series circuit of the heating coil 60 and a capacitor 61 having an impedance of 1/we); and H is used to rectify the terminal voltage Ec of the capacitor 61 so as to obtain the feedback voltage Ef, and has the following relationship thereamong:: Ef=HFc (2) When the feedback control as shown in Fig. 24 is carried out, the output voltage Eo is expressed by the Equation:

Therefore, the current to flow in the heating coil 60 is expressed by the Equation:

The frequency of the output voltage Eo is controlled to be substantially the resonance frequency of the heating coil circuit, i.e. the series circuit of the heating coil 60 and the capacitor 61. When a nonmagnetic element is heated with a heating coil, generally speaking, the inductance component of the heating coil is decreased but the resistance component of the same is increased.As a result, the frequency c9 of the output voltage Eo is increased with the increase in the number n of the metal shells to be heated by the heating coil and can be approximated by the Equation: zone (1 +k,n) (5) Likewise, the impedance Z of the heating coil circuit can be approximated by the Equation: Z=R0(1+k2n) (6) Here, w, and Ro stand for the angular frequency of the output voltage Eo and the resistance of the heating coil, respectively, when the metal shells are not heated, and k1 and k2 stand for constants.

If the Equations (5) and (6) are substituted into the Equation is obtained:

Here, since the following relationship holds; k1n 1 (8) the denominator of the Equation (7) can be approximated, as follows:

When the frequency of the output voltage Eo remains constant irrespective of the number n of the metal shells, an equal calorie can be imparted ot the metal shells if the current I flowing in the heating coil is controlled to be constant. When, however, that frequency is varied, the equal calorie cannot be imparted to the metal shells unless the current I in the heating coil is accordingly controlled.

Generally speaking, when the current in the heating coil is constant, the more calorie is fed to the metal shells as the frequency becomes the higher. With this in mind, when the metal shells are made of a non-magnetic material such as aluminum, the frequency is increased with the increase in the number of metal shells. Therefore, in order to restrain the temperature rise due to the increment of the frequency, it is necessary to reduce the current to flow in the heating coil.If the reduction ratio is denoted at k3 and is defined by the following Equation: l=lo( 1 -k3n) (10) then in order to make the temperature rise in each metal shell constant the following Equation has to hold:

From this Equation (11), as will be understood, it is sufficient that the value HG be determined in a manner to satisfy the following Equation:

As has been apparent from the analysis thus far made, if the characteristics are determined in a manner to satisfy the Equation (12), it is possible to effect the heating operation with a uniform energy at all times irrespective of the number of the metal shells being loaded.

Fig. 25 is similar to Fig. 24 but explains the principles of a high-frequency induction heating apparatus for controlling the current in the heating coil 60 so as to impart an equal calorie to each metal shell in case the metal shells used as the work pieces are made of ferromagnetic material such as tin plate or tinfree steel.

In this case, the feedback voltage Ef is proportional to the terminal voltage of the inductance L so that the current I in the heating coil is expressed by the following Equation:

When a ferromagnetic work piece is heated, generally speaking, the inductance component and resistance component of the heating coil are increased.Therefore, the frequency of the output voltage Fo can be approximated by the Equation: a'=co0(1-k1,n) (14) Likewise, the impedance of the heating coil circuit can also be approximated by the Equation: Z-P0(1+k2,n) (15) On the other hand, if the Equations (12) and (13) are substituted into the Equation (11), the following approximation can be obtained:

When the metal shells are made of a ferromagnetic material, the frequency is decreased with the increase in the number of the metal shells. Therefore, in order to restrain the temperature drop due to the decrement of the frequency, it is necessary to increase the current to flow in the heating coil.If the increase ratio is denoted at k31 and is defined by the following Equation: I=10(1+k3,n) (17) then in order to make the temperature rise in each metal shell constant the following Equation has to hold:

From this Equation i18), as will be understood, it is sufficient that the value HG be determined in a manner to satisfy the following Equation:

From the analysis thus far made, it has been apparent what principle underlies the sixth embodiment of the present invention. With this principle in mind, the high-frequency induction heating apparatus will be described in detail.

Turning now to Fig. 26 showing a suitable modification of the high-frequency induction heating circuit including the heating coil of the above-specified heating apparatus in accordance with the present embodiment, a power source 62 has its output supply current to the heating coil 60 controlled through a current control circuit 63, the output of which is fed through a smoothing circuit 64 to a transistor type oscillation circuit 65. The current fed from the transistor oscillation circuit 65 to the heating coil 60 is detected by means of a detecting circuit 66 so that the signals detected are fed to the current control circuit 63.

The power source 62 includes a three-phase AC power source 72, the three-phase output of which is fed to the collector of a control transistor 76 of the current control circuit 63 after it is rectified by a rectifier 73 and has its ripples removed by a smoothing coil 74 and a capacitor 75. The control transistor 76 controls the pulse width of the smooth DC output of the power source 62 so that its output is fed from its emitter to the smoothing circuit 64.

The power source 62 is constructed to include a constant voltage circuit, the constant voltage output of which is fed to the smoothing circuit 64. In the embodiment under consideration, however, such constant voltage circuit may be provided in the previous stage of the power source 62.

The smoothing circuit 64 is constructed to include a smoothing coil 77, a capacitor 78 and a flywheel diode 79 and is made operative to rectify the pulse wave forms having their width controlled so that they may be fed to the respective collectors of transistors 80 and 81 of the oscillation circuit 65.

These transistors 80 and 81 constitute such oscillation switching transistor group together with other transistors 82 and 83 that the respective pairs of the transistors 80 and 83 and the transistors 81 and 82 are simultaneously rendered conductive and inconductive in response to the control signals from a switching control circuit 84. There is connected between the emitter of the transistor 80 or the collector of the transistor 82 and the emitter of the transistor 81 or the collector of the transistor 83 the primary coil 85 of an output transformer, the secondary coil 86 of which is connected with the series resonance circuit of the heating coil 60 and a capacitor 61.The terminal voltage of this capacitor 61 is fed to the switching control circuit 84 so that the resonance frequency of the series resonance circuit is fed as the oscillation frequency of the transistor oscillation circuit 65 to the switching control circuit 84 in response to the variation in the terminal voltage of the capacitor 61 thereby to switch the respective transistors 80 and 83 in accordance with the resonance frequency.

On the other hand, the terminal voltage of the capacitor 61 is also fed to the detecting circuit 66 so that the output current of the heating coil 60 is detected by the detecting circuit 66. This detecting cir;cuit 66 is constructed to include a rectifier which is operative to convert the output voltage of the capacitor 61 in the form of AC signals into a DC voltage.

The control signals of the detecting circuit 66 are fed to the current control circuit 63 so as to control the supply current which is to be supplied from the power source 62 to the oscillation circuit 65 through the smoothing circuit 64. This current control operation will be described in detail with reference to Fig. 27 as well as Fig. 26.

Indicated at reference numeral 200 in Fig. 27 is the terminal voltage of the capacitor 61 of the series resonance circuit, which voltage is shown in the form of high-frequency signals wave forms. The output of the detecting circuit 66, which is rectified from the terminal voltage of the capacitor 61, is depicted in the form of signals 202. These output signals 202 of the detecting circuit 66 are fed to one input terminal of a differential amplifier 90 of the current control circuit 63 through a resistor 91. The other input terminal of the amplifier 90 is supplied with the reference voltage from a reference voltage generator, which is composed of a reference power source 92 and a variable resistor 93, so that the reference voltage supplied may be compared with the aforementioned detected signals 202.On the other hand, there is connected between the input and output terminals of the differential amplifier 90 a control gain adjusting device including a variable resistor 94 so that the control gain of the current to be supplied to the heating coil 60 can be suitable determined by the current control circuit. The amplifier 90 has its output 204 fed to a voltage-pulse width converter 95. This converter 95 is supplied with the output of the power source 62 so that triangular waves having a gradient corresponding to the output voltage of the power source 62 are generated in the converter 95.As is apparent from the wave-form illustrations of Fig. 27, this converter 95 compares the triangular waves 206 and the output 204 of the amplifier 90 so that it feeds "ON" signals to the base of the transistor 76 only during the term while the voltage of the former 206 exceeds that of the latter 204. As a result, the output signals, as illustrated by pulse wave forms 208, are fed from the emitter of the transistor 76 to the smoothing circuit 64, the smoothed output 21 0 of which is further fed to the transistor oscillation circuit 65.

The voltage-pulse width converter 95 shown in Fig. 26 also functions to compensate the fluctuations in the output voltage of the power source 62. More specifically, the triangular waves 206 of the converter 95 have their gradient corresponding to the main voltage of the power source 62 so that the gradient becomes steeper, as shown in a broken line 206a in Fig. 27, in case the main voltage is lowered. As a result, the output pulses of the current control circuit 63 have their width increased as shown in a broken line 208a. As a result, the supply current to be supplied to the transistor oscillation circuit 65 is accordingly increased. In these ways, according to the high-frequency induction heating circuit exemplified in Fig. 26, even the fluctuations in the main voltage of the power source 62 can be compensated without fail.

As has been described hereinbefore, according to the induction heating circuit of the present embodiment, the fluctuations in the high-frequency current of the heating coil 60 are detected to feedback control the supply current from the power source 62 to the oscillation circuit 65 in accordance with the detected voltage of the heating coil 60 so that the heating energy to be imparted from the heating coil 60 to the metal shells can be controlled to a preset level in accordance with the load impedance of the heating coil 60 and the frequency, which are varied with the changes in the number and electric characteristics of the metal shells.In this respect, it should be noted here that the present embodiment is characterized not in that the current level of the heating coil 60 is controlled to a preset value but in that the heating energy ot be imparted from the heating coil 60 to the metal shells is controlled to a preset level in accordance with the number of the metal shells to be loaded and worked by the heating coil 60. Therefore, the characteristics of the present embodiment reside in that the metal shells can always be heated to a preset temperature.

Generally speaking, if the heating coil 6U were supplied with a preset current, the heated temperature of or the heating energy to the metal shells or work pieces should have been held at a preset level. However, even with the load impedance of the heating coil 60 being kept constant, the resonance frequency of the heating coil circuit is resultantly varied with the fluctuations in the load impedance of the heating coil 60 so that all of the metals cannot actually be heated to a uniform temperature. For example, when the heating apparatus shown in Fig. 1 was loaded with different numbers of the metal shells of a non-magnetic material under the condition in which its heating coil was supplied with a preset current, intense fluctuations were found in the heating temperature levels of the respective metal shells.In the following Table 1, there are tabulated the output voltage VB of and the supply current 16 to the smoothing circuit 64 and the heating temperature O of the metal shells against the number of the metal shells loaded when the control gain adjusting device 94 of the current control circuit 63 is operated to effect a high feedback ratio with a view to controlling the supply current to the heating coil 60 and accordingly the output terminal voltage Vc of the capacitor 88 of Fig. 26:: TABLE 1 for constant Vc

Number of Metal VB 1B e Shells (pieces) (volts) (amperes) ('C) 1 95 30 80 9 160 32 fizz 100 18 200 33 130 From the above Table 1, it will be understood that the energy to be imparted to the metal shells, i.e. the temperature rise is lowered as the number of the metal shells of a non-magnetic material to be loaded is decreased when the capacitor terminal voltage Vc is controlled to a constant level.

The present invention is characterized in that the aforementioned heating temperature fluctuations are eliminated by feeding the more current to the heating coil, when the number of the nOn- magnetic metal shells to be loaded on the induction heating apparatus is the less, in accordance with the characteristics of the heating apparatus.

The transistor oscillation circuit in the present embodiment can generate the high-frequency signals with the use of the series resonance circuit of the heating coil and the capacitor, which circuit has such characteristics as to have its current increased with the reduction in the number of the metal shells to be loaded. As a result, without the feedback system including the current control circuit 63 of Fig. 26, a remarkably high current will flow in the heating coil when the heating apparatus is loaded with no metal shell. It will therefore be understood that the current control circuit 63 effects the feedback action to restrain the supply current as the number of the metal shells to be supplied to the heating coil 60 is decreased.As a result, according to the present embodiment, by using the current control circuit 63 to effect not the constant current control but a low feedback ratio, such feedback control characteristics can be accomplished as allow much current to flow in the heating coil 60 when the number of the metal shells to be supplied tofthe heating apparatus is decreased. The above feedback characteristics can be practically performed by operating the control gain adjusting device 94 of the current control circuit 63 with a view to reducing the amplification of the differential amplifier 90.

Turning now to the following Table 2, there are tabulated the supply voltage VB and current It to the oscillation circuit 65 and the heating temperature 0 against the number of the metal shells to be loaded on the induction heating circuit which is subjected to the weak feedback control action according to the present embodiment:: TABLE 2

Number of Metal VB 1B 0 Number of Metal VB I B o Shells (pieces) (volts) (amperes) ("c) 1 132 35 130 9 175 34 130 18 200 33 130 From the above Table 2, it will be apparently understood that a substantially constant heating temperature can be ensured by accomplishing such feedback control operation as to increase the supply current to the heating coil in accordance with the decrease in the number of the metal shells.

Although, in the foregoing embodiments, the description is directed to the case, in which the spacing between the heating coil 60 and the lower side 5a of the metal shell 5 is held at a preset value, the aforementioned control characteristics have to be adjusted in accordance with the variation in the spacing is varied. The cross-sectional shape of the heating coil is formed, as shown in Fig. 28, such that conductors 97 and 98 made of a copper pipe having a diameter of 6 mm are buried in a coil bed 96 made of a ferrite core and such that their sizes a, b, c, d and e are made to have values of 12, 28, 40, 20 and 27 mm respectively.Then, the conditions for making the temperature rises of the metal shells having a length of 60 cm and made of aluminum and steel constant irrespective of the number of the metal shells are deduced by heating the metal shells with the use of the heating coil having the above form.

In case the aluminum shells having a diameter of 28 mm, a height of 15 mm and a thickness of 0.24 mm were heated, the following results were obtained:

g = lmm 6 = 2mm K, 0.0069 0.0049 Kl 0.071 0.052 K3 0.0025 0.0018 (K2 -K33/(K, +K3) 7.29 6.69 When the aluminum shells having a diameter of 38 mm, a height of 17 mm and a thickness of 0.24 mm were heated, the following results were obtained::

g - lmm g = 2mm K1 0.0111 0.0083 K2 0.102 0.082 K3 0.0032 0.0024 (K2 K3#K1 + K3) 6.91 6.88 When the steel shells having a diameter of 28 mm, a height of 9 mm and a thickness of 0.24 mm were heated, the following results were abtained: :

g = 4mm g - 6mm K', 0.0021 0.0014 K ' 0.042 0.023 K ' 0.00076 0.00051 (K'2 - K'3)7(K', + ('3) 31.9 26.4 When the steel shells having a diameter of 40 mm, a height of 12 mm and a thickness of 0.24 mm were heated, the following results were obtained:

9 = 4mm g = 6mm . 0.0039 0.0022 K' 0.149 0.078 K' 0.0013 0.00089 (K'3 K'3),/%K'1 +K'3) 57.8 52.1 The values k1, k2 and k3 appearing in the above Tables were varied with the spacing between the shell bottom side and the ferrite surface, the cross-sectional shape of the heating coil, the diameter of the metal shells and the frequency of the high-frequency power source, and accordingly the value of HG is varied within a range of +100%.

As had been understood from the above results, by controlling the characteristics of the current control circuit in a varying current manner, it is possible to establish the desired uniform heating energy or temperature irrespective of the changes in the nUmber of the metal shells to be supplied to the heating apparatus and in the mutual inductance between the metal shells and the heating coil.

It should be noted here that the control gain of the current control circuit can be obtained by any adjusting device other than the variable resistor of the shown embodiment and that the current in the heating coil can also be detected by a variety of other detecting circuits.

As has been described hereinbefore, according to the present embodiment, in the high-frequency induction heating circuit including the transistor oscillation circuit which is equipped with the heating coil constituting the series resonance circuit, it is possible to provide such a heating circuit remarkably suitable for the high-frequency induction heating apparatus of the type, which is continuously supplied with any number of the metal shells, as can stably supply remarkably uniform heating energy and temperature by accomplishing the varying current control with the use of the current control circuit even if the number of the metal shells to be loaded is changed.

According to the present embodiment, moreover, it is possible to provide such a heating circuit as can efficiently heat the metal shells made of a non-magnetic material such as aluminum.

According to the present embodiment, still moreover, even for the metal shells made of a ferromagnetic material, it is possible to increase the current upon reduction in the supply number of the metal shells so as to ensure a constant heating operation at all times by effecting a more intense feedback control operation thantthe constant current control with the use of the current control circuit.

Although the foregoing description is concerned with the case, in which a transistor is used in the high-frequency oscillation device, similar effects can be obtained even with the use of the highfrequency oscillation device employing a semiconductor switching element such as a thyristor.

The following description is turned to the metal caps such as prize-offered crowns, which have their liner partially heat adhered to the primer layer, as well as the method of and apparatus for producing them.

Fig. 29 is a cross-sectional side elevatiogn showing one example of partially thermally adhered metal caps according to the present invention. With close reference to Fig. 29, an adhesive primer layer 103 is formed directly or through an anti-corrosion coating film upon the inner wall of a metal shell 102 forming the body of a metal cap 1 01. There is formed at the position corresponding to the bottom of the metal shell a liner layer 104 which is in contact with the primer layer 103 at the portion 105 to be left unadhered and at the portion 106 to be adhered. Turning to Fig. 30 showing in an enlarged scale the cross-section of the bottom of the metal cap, portions indicated at the same reference letters as those of Fig. 29 show identical portions.Numerals 102' and 102" indicate the outer and internal anticorrosion coating films of the metal shell 102, respectively. Fig. (a) to (d) are diagrammatical top plan views illustrating the distributions of those portions 105 and 106 between the primer layer 103 and the liner layer 104. Fig. (a) shows an example, in which the portion 106 to be adhered is composed of two bands extending in parallel while dividing the diameter of the liner layer 104 into substantially equal three. In this example, the width of the bands 106 is made substantially equal to or slightly larger than the outer diameter of a high-frequency conductor 11 7, which will be described later.

The portion 105 to be left unadhered occupies the outside and inside of the bands 106 and accordingly the major portions of the outer circumferential areas and the inner area of the liner layer 104. Here, the outer circumferential areas are defined to mean the areas which extend along the circumference thereof.

Turning to Fig. (b), there is shown an example, in which the portion 106 to be adhered is formed into a substantially rectangular band extending through the center portion of the liner layer 104.

In this second example, the band 106 has its width made slightly larger twice of the diameter of the high-frequency conductors 11 7, and the non-adhesion portion 1 OS occupies most of the outer circumference areas of the liner layer 104.

Fig. (c) illustrates an example, in which the adhesion portion 106 is formed into a generally round shape located at the center of the liner layer 104. As will be described later, the circle 106 is actually formed by effecting the heating prn6e's"s' chile turning the metal shell about the two conductors 11 7 which are arranged in parallel with and in the vicinity to each other and which have their current flows in the opposite directions. In this third example, the non-adhesion portion 105 occupies most of the outer circumferential areas of the liner layer 104.

Fig. (d) illustrates an example, in which the adhesion portion 106 is formed in an annular shape in the circumferential area of the liner layer 104. This shape is formed by juxtaposing the two conductors 11 7 below the circumference of the shell bottom and by effecting the heating process while turning the shell.

As a material for the liner layer, it is possible to employ a suitable thermoplastic resin (or rubber) having an elastic property suitable for a packing material, for instance, soft vinyl chloride, styrenebutadiene-styrene copolymer, a linear polyamide resin, a fluorine-contained resin or a polyolefin resin. A polyolefin resin such as polyethylene is especially useful for the liner material of the present invention because it has excellent sanitary, moistureproof and mechanical properties.As such polyolefin resin, it is possible to use polyolefin such as low, medium or high density polyethylene, polypropylene, polybutene1, ethyiene-butene-1 copolymer or ethylene-propylene copolymer, olefin copolymer containing olefin as a major component and a slight quantity of ethylene unsaturated monomer other than olefin such as ethylene-acetic vinyl copolymer, unsaturated carboxylic acid modified polyethylene or polypropylene, or modified polyolefin. The above-recited olefin resins can be used along or in combination of two or more kinds thereof, or can be blended with an elastomer such as ethylene-propylene rubber or butyl rubber so as to improve the elastic properties necessary for the packing material. Moreover, those polyolefins can be blended with antioxidant, thermal stabilizer, lubricant, filler or coloring agent.By using cross linking agent or foaming agent singly or in combination, it is possible to prepare a polyolefin resin which is subjected to cross linking and/or foaming and which has excellent mechanical properties such as elasticity.

On the other hand, the liner layer 1-04 thus prepared may be either in a fiat sheet shape or in such a shape as has its circumferential portion thickened at the portions contacting with the upper open end of a container, as shown in Fig. 29, and as is formed with a ring-shaped projection holding the both sides of the upper open ends, if necessary.

It is important here that the primer layer is a coating film having a thermal adhesiveness with both the liner material made of the above thermoplastic resin and the metal shell itself or the anticorrosion coating 102" which is formed in the inner wall thereof. This is because it is necessary to prevent the liner layer from being separated frdm the metal shell during their transportation or during their feeding operation to the cap loading chute of a charging and sealing apparatus.

As a suitable primer material, it is possible to recite, merely for example, ethylene-acetic vinyl copolymer, ethylene-acrylic ester copolymer, epoxy, polyurethane, polybutadiene, epoxyurethane, modified polyolefin, ionomer, a resin containing one of them as a major component, or an adhesive resin of their mixture. The primer to be used especially suitably in the present invention is a resin which is prepared by dispersing oxidized or modified polyolefin into a base resin for forming the primer coating film. As this base resin, it can recite a thermosetting resin, which is composed of one or a combination of resins such as epoxy resin, phenol resin, amino resin, polyester resin, alykyd resin or thermoset type acrylic resin or other thermoplastic resins.

The oxidized polyolefin is prepared by oxidizing polyolefin such as polyethylene or polypropylene or the copolymer thereof under a molten or dissolved condition. The oxygens in the oxidized polyolefin is believed to exist partially in the form of carboxyl group or carboxylic ester group at the end of the polymerized chain and partially in the form of ether group and ketone group at a middle of the polymerized chain. On the other hand, the modified polyolefin is defined to be that which has been subjected to the grafting modification (in a desirable grafting ratio of 0.001 to 10 wt.%) mainly with unsaturated carboxylic acid or its derivative.

The aforementioned primer is prepared by mixing the solution, which is prepared by dissolving the aforementioned base resin into an:o?g'a'nic sbfve't,S, with the solutiontisthich is p'Pepared by dissolving oxidized or modified polyolefin into heated xylene or decalin. 3 to 30 wt. parts of the oxidized or modified polyolefin are normally contained in 1 00 wt. parts of the base resin.

The quantity of application of the primer layer (in its non-volatile component) is preferred to be about 30 to 1 50 mg/cm2. Moreover, the primer layer may cover the whole area of the inner wall of the metal shell or only the bottom of the same.

In this instance, the temperature in the vicinity of the melting or softening point of the oxidized or modified polyolefin in the primer layer normally becomes its adhesive temperature with the liner layer.

The anticorrosion coating film 102" is not always required in case the whole area of the metal shell inside is covered with the primer layer. In case, however, the coating is to be applied, a known surfacer consisting of phenol epoxy resin, epoxy-amino resin or phenol-epoxy-vinyl resin etc. is selected.

In the case of the prize-offered cap, the "Winning" mark is printed on the anticorrosion coating film 102" or the primer layer.

The metal caps of partially thermally adhered type according to the present invention are produced in the processes, as follows.

The anticorrnsion coating film 102' and/or prints are applied to one side of a metal sheet for a material of the medial shells, preferably, a sheet of aluminium or its alloy, and the other anticorrosive coating film 102" is applied to the other side of the metal sheet (wherein one or both of the coating films 102' and 102" may be omitted). After that, the primer is applied to the coating film 120" and then is baked. Then, the metal sheet thus painted is pressed into a desired cap shape with its primer layer being positioned inside. The metal shell thus prepared is then conveyed through the high-frequency continuous heating apparatus, as shown in Fig. 1, so that is bottom may be heated.

Generally speaking, when the metal shell is made of a magnetic material such as a tin plate, the magnetic flux is concentrated upon the shell bottom so that the shell bottom can be efficiently inductively heated. When, howeveçv the, ìnetalishell is made of a nor,-magnetic1material such as aluminum, the magnetic flux leaks so much that the heating efficiency is accordingly deteriorated. In either case, the induced current flows in the metal shell only at its portions immediately above and in the vicinity of the heating conductor.As a result, the remaining portions of the metal shell are heated only by the heat conduction from those portions immediately above and in the vicinity of the conductor so that the temperature at the former portions is lower than that at the latter portions at the initial stage of the heating process. However, after a preset time elapses, the temperature ,rise at the portions above the conductor becomes gentler and gentle, and the metal itself is an excellent thermal conductor, so that the temperature difference in between is reduced. As a result, it becomes difficult to effect the thermal adhesion of only a portion of the liner layer.That preset time is different For the size, thickness and material of the metal shell, the spacing between the heating conductor and the shell bottom, the diameter and spacing of the conductors, and the intensity and frequency of the high-frequency current.

As shown in Fig. 32, generally speaking, the heating time of the metal shell, i.e. the passing time of the same through the heating apparatus 1 is preferred to be within about one, two and four seconds, respectively, when the diameter of the shell bottom has a value of 15 to 25 mm, 26 to 40 mm and 41 to 60 mm.

Fig. 32 is a graphical presentation of the temperatures (depicted by curves 1 a and 2a) at the center of the metal shell when this shell is subjected to the high-frequency induction heating process so as to obtain the Thermally heated portion of the type shown in Fig. (c) and the temperatures (depicted by curves 1 b and 2b) at the outside of two-third radius from the center, both of the temperatures being plotted against the heating time of the metal shell. The temperature measurements were carried out by means of thermo-paint.

In Fig. 32, the curves 1 a and 1 b illustrate the temperature changes at the shell center and at the outside of two-third radius therefrom in case the tinned shell having a diameter 27 mm and a thickness of 0.27 mm is heated by means of the heating apparatus 1 of Fig. 1, which is constructed to include a high-frequency heating coil having ter-ntizUr spGcing of 7 mmtetween that centers ofathe conductors of a diameter of 4 mm, a length (in one direction of current flow) of 800 mm, and the spacing of 0.3 mm between the conductor upper side and the shell bottom, and a high-frequency power source of 400 KHz.Incidentally, the output is so adjusted that the temperature at the center may always be at 1600 C. From Fig. 32, it will be understood that the temperature difference in between is decreased with the lapse of the heating time.

On the other hand, the curves > and 2b illustrate the temperature changes at the shell center and at the outside of two-third radius therefrom in case the aluminum shell having a diameter of 38 mm and a thickness of 0.2 mm is heated by means of the heating apparatus, in which the inter-center spacing between the conductors is 10 mm, the diameter thereof is 0.6 mm, the length is 1000 mm, and the spacing between the conductor upper side and the shell bottom is 0.3 mm.

The spacing between the conductors can be increased, e.g. as shown in Fig. 31 (a), or decreased, e.g. as shown in Fig.31 (b), in accordance with the desired positions and sizes of the non-adhesion portions. Generally speaking, the diameter of the conductors is increased with the increase in the shell diameter. For instance, in case the metal shells have diameters of 15 to 25 mm, 26 to 40 mm and 41 tp 60 mm, the desir'ed conductor diameters are about 3 mum, 6 mm and 7 mm, respectively.This is partly because the more current has to flow in the conductor as the shell diameter becomes the larger, partly because if the conductor diameter is excessively small the losses in the conductor due to the Joule heat are increased whereas if the conductor diameter is excessively large the electromagnetic coupling between the conductor and the metal cap is so decreased as to deteriorate the heating efficiency, and partly because the conductor diameter is preferably as small as possible in order to effect a non-uniform temperature distribution.

When it is intended to form the non-adhesion portions, as shown in Fig. 31 (cm, while turning the metal shell during the heating process, this can be realized by applying a liner layer of a low friction material (such as polyfluoroethylene resin known under the trade name of "Teflon") to the portions of the semicircular notches 3, with which the metal shells are brought into contact, by applying a liner layer of an elastic material such as silicone rubber having a relatively high friction coefficient to the inner side of the vertical guide wall 1 5 of Fig. 2, with which the metal shells are brought into contact, and by conveying the metal shells such that they have their skirt walls forced into contact with the inner side of the vertical guide wall 15.

The metal shells heated in the above ways are then transferred through the guide (not shown) to the molten thermoplastic resin pellet supplying machine or extruder 14, where they are supplied at their substantial bottom center with the molten thermoplastic resin pellets.

The metal shells are further transferred to the mould station (not shown), where the molten thermoplastic resin pellets supplied are compressed and extended by means of a cooled punch into the# form of a sheet, which is solidified into a liner layer partially heat adhered to a portion of the shell which has been at a higher temperature than the thermally adhesive temperature.

The present invention should not be limited to the above process but can be exemplified by pouring the pellets of the molten thermoplastic resin into the metal shells before heated or by charging these metal shells with such film of the thermoplastic resin as is cut into a preset shape, then by heating the metal shells and by subjecting them to press adhesion.

Since the partially thermally adhered metal caps according to the present invention can have their liner layers easily separated from their primer layers, they can be suitably used as the prize-offered caps.

The separability of the liner layers can be easily adjusted by selecting the combination of the resins constituting the primer layers and line layers or their heating temperature. For example, in case a polyethylene liner is to be adhered to the primer layer made of a base resin containing oxidized or modified polyethylene, the polyethylene of this type having a low modification may be used or its content may be reduced to decrease the adhesion strength so that the separability can be improved.

Moreover, the weak adhesion is initiated from the time when the heating temperature of the shell bottoms is 30 to 400C lower than the softening point of the oxidized or modified polyolefin, and the thermal adhesion strength is increased with the rise in temperature up to a preset level. On the other hand, the temperature of the shell bottom is the highest at its portion just above of the upper side 7a of the conductor, as shown in Fig.2, and is lowered the more apart from the just above portion so that a gradient in the adhesion strength (or separation strength) is generated even in the thermal adhesion portion, as will be apparent from the Embodiment A of the present invention appearing in Table 3.The existence of such gradient is one of the major differences from the conventional metal caps, in which the adhesive primer is locally applied or in which the liner layer is locally applied by printing in adhesion inhibiting ink. The partially thermally adhered metal caps according to the present invention, especially, the metal caps having their bottoms thermally adhered in the vicinity of their centers, as shown in Fig.

31 (c), are characterized, as has been described hereinbefore, in that the adhesion strength is gradually increased toward the center of the sheli bottoms. As a result, there is no fear of the ageing separation of the liner due to the radial residual stress, which might otherwise be produced in the liner when the pellets of a molten thermoplastic resin such as polyethylene are compressed by means of a press to form the liner.

The experimental Examples' conducted by the Applicant will be described in the following.

EXAMPLE 1 90 wt. parts of an epoxy resin, 10 wt. parts of a phenol resin and 7 wt. parts'of oxidized polyethylene (having a mean molecular weight of 6500, a density of 0.98, an acid value of 13.0 and a softening point of 1220 C) were dissolved or dispersed to have a solid content of 30 wt % into a mixed organic solvent (which is prepared by mixing an equal quantity of methyl isobutyl and methyl ethyl ketone) to prepare a paint (or primer).

The paint thus prepared was applied to a bright tinplated sheet having a thickness df 0.27 mm which had been subjected in advance to the deoiling treatment and which had its surface tin melted.

The resultant sheet,was heated in an electric oven at 2000C for ten minutes to prepare a painted tinplated sheet (A) having a paint thickness fo 100 mg/dm2.

After that, the painted tin sheet (A) was partially printed in an annular shape having outer and inner diameters of 26.6 mm and 1 6.6 mm with alkyd ink inhibiting polyethylene adhesion thereby to prepare a partially printed tin sheet (B).

These two painted tin sheets (A) and (B) were pressed in a normal manner with the panted side being positioned inside thereby to prepare a crown cap shell having a diameter of 27 mm. However, the pressing operation of the sheet (B) was accomplished such that the annular printed portion was positioned just at the outer circumference of the bottom of the crown cap shell.

Two kinds of crown caps, i.e. a partially thermally adhered crown cap according to the present invention and a crown cap according to the prior art for comparison were produced of the foregoing two kinds of crown cap shells.

(1) Crown Cap A according to the Present Invention The crown cap shell made of the painted tin sheet (A) was subjected to high-frequency heating process in the manner, as shown in Fig. (c), by means of the heating apparatus of Fig. 1. In this instance, the high-frequency induction heating coil in which the inter-center spacing of the conductors was 7 mm, the conductor has a diameter of 4 mm, and the length thereof was 800 mm, was arranged such that the spacing between its upper side and the shell bottom was 0.3 mm. The temperature distribution at the shell bottom measured by means of thermo-paint was illustrated by curve 1 in Fig. 33 in case the input to the high-frequency heating apparatus of 400 KHz was 15 KW, and the time required for the metal shell to pass through the heating coil was 0.7 seconds.Moreover the revolutions of the metal shell for this time was about 9. 900 mg of molten polyethylene pellets (which has an Ml of 3 and a density of 0.91 8), which were discharged from an extruder (which had a diameter of 40 mm and an L/D ratio of 1 6) equipped with a die having a discharge port of a diameter of 8 mm were poured into the crown cap shells thus heated and were then pressed by means of a cooled punch to produce a crown cap having a low density liner.

(2) Crown Cap B for Comparison The crown cap shell prepared of the painted tin sheet (B) was subjected to high-frequency heating process (for 5 seconds) so that its shell end might be uniformly heated to 1 500C. The crown cap shell thus heated was charged with molten polyethylene of low density, as has been described in the above, to produce a crown cap.

The following measurements and evaluations were conducted for the two kinds of the crown caps.

As to the crown caps immediately after the moulding process and after left at a rodm temperature for one week, samples having a width of 5 mm and a length of 27 mm were cut away from the bottoms of the crown caps, and the separation strength between the tin plate and the polyethylene was measured. These measurements were carried out with the use of Tensilone at a temperature of 200C at a pulling speed of 20 mm/min in the manner of 1 800C separation. The separation strength or friction pull strength was measured by reading the values at four positions from the circumference to the center of the crown caps from a chart. The results were listed in the following Table 3.

The crown cap A according to the present invention has sufficient performances as a cap for a container, whereas the crown cap B according to the prior art for comparison has the following drawbacks. As compared to the crown cap A of the present invention, more specifically, the conventional crown cap B is inferior in seal ability and has its liner adhesion strength remarkably deteriorated due to ageing. Moreover, the crown cap A has its liner adhesion strength increased gradually from the outer circumference to the center, whereas the crown cap B has its liner adhesion strength completely different between the adhesion and non-adhesion portions.As a result, a stress is left in the liner extended from the center to the outer circumference during formation so that the overall adhesion of the liner of the crown cap B cannot endure the action of the residual stress and it is completely lost. Therefore, the crown cap B has its sealability deteriorated for practical use although it is the same type partially adhered crown cap as the crown cap A.

TABLE 3

Items Crown Cap A Crown Cap B 13 mm O 0 Separation / 11 mm 250 0 Strength just after Spacing 8 mm 980 1130 Process from (Kg/cm) Center 4 mm 1140 1130 O mm 1130 1140 13 mm O 0 / 13 mm O O Separation S#ength One Week/ Spacing 8 mm 540 Later / from (Kg/cm) Center 4 mm 890 O Omm 900 900 EXAMPLE 2 80 wt. parts of a bisphenol A epoxy resin having a molecular weight of about 3000 20 wt parts of an amino resin (known under the trade name of "Super-Beccamine P 138") and 4 wt. parts of maleic anhydride modified polyethylene were dissolved or dispersed to have a solid content of 28 wt. % into an organic solvent to prepare paint. The paint thus prepared was applied to an aluminum sheet having a thickness of 0.2 mm which was then heated and dried to prepare a painted aluminum sheet having a paint coating having a thickness of 100 mg/dm2. This coating was printed with letters or picture indicating the "Winning" or "Lost" for offering a prize. The sheet thus painted and printed was pressed in a known manner with its printed side being inside and with the printed letters and picture being positioned at the center, thus preparing a cap shell having a diameter of 38 mm and a height of 17 mm.

The cap shell thus prepared was then subjected to high-frequency heating process in the manner shown in Fig. (c) by means of the heating apparatus shown in Fig. 1. In this instance, the highfrequency induction heating coil, in which the inter-center spacing of the conductor was 10 mm, the diameter of the conductor was 6 mm and the length thereof was 1 000 mm was arranged to have its upper side spaced from the shell bottom by 0.3 mm. When the oscillator had an output of 400 KHz the input to the high-frequency heating apparatus was 1 5 Kw and the time required for the metal shell to pass through the heating apparatus was 1 second, the temperatures at the shell bottom measured by means of thermo-paint were plotted by curve 2 in Fig. 33. Incidentally, the revolutions of the metal shell for that time was about 8.

After that, 1 g of the molten polyethylene pellets were poured in the same manner as that of the Exa mple7 into the heated cap shell and then were pressed by means of a cooled punch to produce a crown cap having the polyethylene liner. A glass bottle having a capacity of 1 litre for carbonate beverage e was sealed up with the crown cap thus produced and was left at a room temperature for three months.

It was found that there was no problem in sealability.

On the other hand, from the evaluations as to openability of the liner of the crown cap made by 20 housewives and children, it was found that the liner could be easily separated to recognize the "Winning" or "Lost" marks. Incidentally, the adhesion strengths measured from the outer circumference to the center in a similar manner to that of the Example 1 were 0 (19 mm), 120 (10 mm), S20 (5 mm) and 815 (0 mm) Kg/cm respectively, wherein the numerals appearing in parentheses indicate the spacings from the center.

Claims

1. A method of making a metal cap having a metal shell and a liner layer of thermoplastic resin which is thermally adhered to the inner wall of the end of said metal shell, comprising the steps of; causing induction current to flow through the end of said metal shell by means of a high-frequency current coil, whereby said end is heated and simultaneously subjected to an electro-magnetic repulsive force, bringing the edge of said shell into contact with the surface of a guide wall disposed in opposition to said coil under said repulsive force, whereby a spacing between said end and said coil is kept in a predetermined range, and conveying said shell in a passage along said coil while sliding said edge on said surface, whereby said end is heated to a thermal adhesion temperature.

2. A method for making a metal cap as claimed in Claim 1, further comprising the step of applying a primer; which is adhesive to said liner layer, to the inner wall of the end of said metal shell before it is heated.

3. A method for making a metal cap as claimed in Claim 2, further comprising the step of extruding a thermoplastic resin to form said liner layer to the inner wall of the end of said metal shell after the end of said metal shell is heated to the thermal adhesion temperature.

4. A method for making a metal cap as claimed in Claim 3, wherein said thermoplastic resin is composed of a polyolefin resin, or a resin essentially containing a polyolefin resin.

5. A method for making a metal cap as claimed in Claim 1, wherein the end of said metal shell is wholly heated to the thermal adhesion temperature so that all said liner layer may be thermally adhered to the inner wall of the end of said metal shell.

6. A method for making a metal cap as claimed in Claim 1, wherein the end of said metal shell is partially heated to the thermal adhesion temperature so that only a portion of said liner layer may be thermally adhered to the inner wall of the end of said metal shell.

7. A method for making a metal cap as claimed in any preceding Claim, wherein said metal shell is made of aluminum or its alloy.

8. A method for making a metal cap as claimed in any preceding Claim wherein the highfrequency current flowing in the high-frequency current coils generated by means of a semiconductor oscillation circuit and has a frequency range of 5-80 KHz, said coil being provided with a highly permeable insulating material below its heating portion.

9. Apparatus for making a metal cap having a metal shell and a liner layer of thermoplastic resin which is thermally adhered to the inner wall of the end of said shell comprising; a high-frequency current generating circuit, a heating coil consisting of at least one pair of high-frequency current conductors juxtaposed at a spacing smaller than the, diameter of the end of said shell and having their current flow directions opposite to each other, a guide wall disposed to face said heating coil, the surface of said wall facing said heating coil being brought into contact with the edge of said shell which is subjected to a electromagnetic repulsive force, whereby a spacing between said end and said heating coil is kept in a predetermined range, means for conveying said shell in a passage along said coil while sliding said edge on said surface, and means for feeding a thermoplastic resin for said liner layer to the inner wall of said shell end.

1 0. Apparatus for making a metal cap as claimed in Claim 9, wherein the surface of said guide wall facing said heating coil is arranged substantially in parallel with said heating coil.

11. Apparatus for making a metal cap as claimed in Claim 9 or 10, wherein said guide wall is a stationary plate whose surface facing the heating coil has a spacing of a predetermined range in respect to said heating coil.

12. Apparatus for making a metal cap as claimed in Claim 9 or 10, wherein said guide wall is supported through a spring so that the metal shell'may be pushed against the repulsive force.

13. Apparatus for making a metal cap as claimed in Claim 12, wherein said guide wall includes a plurality of components arranged along the passage close to each other.

14. Apparatus for making a metal cap as claimed in Claim 9, 10, 11, 12 or 13 wherein said guide wall is made of a glass plate.

15. Apparatus for making a metal cap as claimed in Claim 14, wherein said glass plate is made of reinforced refractory glass.

1 6. Apparatus for making a metal cap as claimed in any one of Claims 9 to 15 inclusive wherein said high-frequency conductors are arranged in a taper shape along the passage.

1 7. Apparatus for making a metal cap as claimed in Claim 1 6, wherein the taper shape of the high-frequency conductors has a uniforrrl gradient along the passage.

18. Apparatus for making a metal cap as claimed in Claim 1 6, wherein the tape shape of the highfrequency conductors has a different gradient at a portion of the passage from the remainder.

19. Apparatus for making a metal cap as claimed in any one of Claims 9 to 18 inclusive wherein a highly permeable insulating material is arranged below a portion of the high-frequency conductors.

20. Apparatus for making a metal cap as claimed in any one of Claims 9 to 19 inclusive wherein the high-frequency conductors are arranged along the passage at the both sides of the thermoplastic resin feeding means such that they are positioned outside of said passage and in proximity to each other in the vicinity of the thermoplastic resin feeding means.

21. Apparatus for making a metal cap as claimed in any one of Claims 9 to 19 inclusive wherein the heating coils are disposed along the passage in the both sides of the thermoplastic resin feeding means, and located apart from each other in the vicinity of said feeding means.

22. Apparatus for making a metal cap as claimed in any one of Claims 9 to 21 inclusive wherein the high-frequency current generating circuit includes a vacuum-tube oscillator; wherein the highfrequency conductors are composed of plural pairs, one member of each pair being arranged at one half of the heating section whereas the other member being arranged at the other half; and wherein the conductors of the pair, which are connected upstream with respect to the ground potential, are arranged in the vicinity of the metal shell whereas the conductors of the pair, which are connected upstream with respect to a higher potential, are arranged apart from said metal shell so that the potential gradients between the respective conductors and said metal shell may be lower than a predetermined value so as to prevent dielectric breakdown in between.

23. Apparatus for making a metal cap as claimed in Claim 22, wherein the spacings between said conductors are preset to correspond to the potential difference in between so that the potential gradients between said conductors may be lower than a predetermined value so as to prevent the dielectric breakdown in between.

24. Apparatus for making a metal cap as claimed in any one of Claims 9 to 21 inclusive wherein said high-frequency current generating circuit includes: a#semiconductor oscillation circuit constituting a series resonance circuit composed of the high-frequency conductors and a capacitor; a detecting circuit for detecting the impedance and frequency variation of said high-frequency conductors: and a current control circuit having a control gain adjusting device and made responsive to the detected signals of said detecting circuit for controlling the supply current to said high-frequency conductors, whereby the control gain of said current control circuit is so selected by said control gain adjusting device as to control the supply current to said high-frequency conductors in a non-constant current manner so that the supply current to said high-frequency conductors can be varied with the reduction in the number of the metal shells to be supplied to the heating section.

25. Apparatus for making a metal cap as claimed in any one of Claims 9 to 21 inclusive wherein the high-frequency current generating circuit includes: a semi-conductor oscillation circuit constituting a series resonance circuit composed of said high-frequency conductors and a capacitor; a detecting circuit for detecting the capacitor terminal voltage of said high-frequency conductors; and a current control circuit having a control gain adjusting device and made responsive to the detected signals of said detecting circuit for controlling the supply current to said high-frequency conductors, whereby the control gain of said current control circuit is so selected by said control gain adjusting device as to control the supply current to said high-frequency conductors in a weaker feedback control action than the constant current control so that the supply current to said high-frequency conductors can be increased with the reduction in the number of the metal shells to be supplied to the heating section.

26. Apparatus for making a metal cap as claimed in any one of Claims 9 to 21 inclusive wherein the high-frequency current generating circuit includes: a semiconductor oscillation circuit constituting a series resonance circuit composed of said high-frequency conductors and a capacitor: a detecting circuit for detecting the terminal voltage of the inductance of said high-frequency conductors; and a current control circuit having a control gain adjusting device and made responsive to the detected signals of said detecting circuit for controlling the supply current to said high-frequency conductors, whereby the control gain of said current control circuit is so selected by said control gain adjusting device as to control the supply current to said high-frequency conductors in a stronger feedback control action than the constant current control so that the supply current to said high-frequency conductors can be decreased with the reduction in the number of the metal shells to be supplied to the heating section.

27. A method of making a metal cap substantially as hereinbefore described with reference to any one of the examples.

28. An apparatus for making a metal cap substantially as hereinbefore described in any one of the embodiments with reference to the accompanying drawings.

29. A metal cap made by a method or apparatus as claimed in any preceding claim.

30. A metal cap wherein a primer layer having a thermal adhesion to a liner layer made of a thermoplastic resin is formed in the inner wall of the end of a metal shell, a liner layer is formed directly upon said primer layer, and a thermally adhered portion is formed only partially between said liner layer and said primer layer.