US2226447A - Magnetic heater - Google Patents

Description

Dec. 24, 1940. 5M|TH ET AL 2,226,447

MAGNETIC HEATER Filed Feb. 25, 1959 Patented Dec. 24, 1940 UNITED STATES PATENT OFFICE MAGNETIC nsaraa Graydon Smith, Cambridge, and Albert Allen,

Sharon, Mass., assignors to heed Prentice Corporation, Worcester, Mass, a corporation of Massachusetts Application February 25, 1939, Serial No, 258,494 9 Claims. (CL 219-41) This invention relates to magnetic heaters and more particularly to magnetic heaters for heating thermoplastic or heat plasticizable materials in preparation for molding. i

This application is a continuation-in-part of our copending application, Serial No. 181,250, filed December 23, 1937.

Electric heaters for heating thermoplastic material have been proposed before but when electro-magnetic forces have been used, the heating has been primarily inductionheating produced by generating eddy currents in the heating body or through utilizing the heating body as the secondary element of a step down transformer.

We employ a varying magnetic field to produce the desired heat in the heater body, but we produce this heat through magnetic hysteresis effect as far as possible and minimize induction heating such as is generated by eddy currents in the heater body.

Induction heaters are well known to the art, and are of two types. One type, operating on the transformer principle, produces heat by causing a heavy circulating current to fiow through the material to be heated and is generally used for melting brass and other non-magnetic materials. The second type, known as the high-frequency induction furnace, employing frequencies from about 360 to 100,000 cycles per second, producesheatbyeddy currents. 'It is a characteristic of the high frequency induction furnace to produce the greatest heat at. the surface'of the object to be heated, thus it has been used to advantage for heat treating machine parts such as cams where the surface must be hardened without heating the inside of the part to the critical temperature.

The induction heating principle has been usedin numerous applications where intense or rapid heating is desired, and has been used in molding processes, as for heating rubber molds.

However, heating by eddy currents is inherently unsuited to the heating of thermoplastic material for injection molding, since it produces surface heating. What is desired in a thermoplastic heater is a uniform heating throughout the heater body so that a maximum transfer of heat can be obtained without danger of burning the material. To accomplish this we employ a principle which we shallterm magnetic heating,

which produces heat by hysteresis when a varying electro-magnetic field isimpressed on a heater body made of a material having high coercive force. Moreover we take special steps to minimize eddy current heating. Some eddy current heating is unavoidable in such a structure, but the essence of our invention lies in making the hysteresis heating effect large compared to the heating by eddy currents, in contradistinction to induction heating where eddy currents are the principal source of heat.

Thus by relying chiefly upon hysteresis losses for the production of heat, we accomplish uniform heating throughout the heater body with the result that every particle of the material subiected to heat, is uniformly heated. Where eddy current heating is relied upon, the heating is not uniform for the heating effect is a skin effect, the heat produced being greatest near the outer surface and decreasing to zero at the center.

The intensity of the heating can be readily adiusted as desired by varying the intensity of the magnetic field, as for example, by varying the applied voltage, or changing the number of turns in the coils.

It is thus evident that the effectiveness of our invention lies, in large part, in making the hysteresis effect large compared to the induction or eddy current effect, so that the desired heating will be essentially uniform throughout the body of the heater. We have foundit desirable, in order to make hysteresis large and eddy currents small, to employ a material having high coercive force and high resistance, to'employ a low frequency magnetic field, and to so proportion the magnetic heater that the flow of eddy currents is impeded; as is more fully set forth hereinafter.

Hysteresis "may be thought of as due to some kind of friction between the molecules of the magnetic material. The varying magnetic field to which we subject the magnetic heater body causes each'molecular magnet to reorient itself with the flux each cycle, and the resulting friction produces the desired heat. Certain materials possess high magnetic friction or are said to have a high coercive force and such materials are highly desirable for our purposes since they producethe desired heat with magnetic fields of moderate intensity. Since the undesired eddy current heating is proportional to the square of will be evident that it is very helpful to be able to employ weak fields. Magnetic steel alloys, such as are commonly used in permanent magnets, as a class exhibit the high hysteresis, high coercive force and high retentivity which we desire. These may be magnet steel alloys as described and examples given onpages 294 to 298 inclusive, of the fifth edition of Standard HandbQQK f Electrical Engineers published by Mcthe flux density (other things being equal), it

Grew-Hill Book Company or may be other alloys as described and examples given on pages 1679 and 1680 of volume III of Hutchinson's Technical and Scientific Encyclopedia, published by MacMillan Company. By way of illustration, the following alloys may be used: Chromium steel: tungsten steel. and cobalt steel.

Eddy currents are circulatory currents in the magnetic body induced by the varying field, in much the same way that currents are induced in the secondary of a conventional transformer. These currents are inversely proportional to the specific resistance of the heater body, so that a material having high resistance is very desirable fcr the heater body.

Thus a material having high hysteresis and high resistance is very effective in producing uniform heating, and any metal or alloy exhibiting these characteristics would be suitable for our heater core provided that it has the necessary mechanical strength.

We also prefer to utilize alternating currents of relatively low frequency for producing the varying magnetic field. In a given magnetic heater of the type described, a reduction in frequency may lower the effect on the eddy current heating, but it will increase the hysteresis effect. Therefore a reduction in frequency makes the heating more uniform. Thus in contrast to high frequency induction heaters where frequencies of 5,000 to 100,000 cycles per second are usual, we prefer to use commercial frequencies such as 60; 50; or 40 cycles per second. We do not wish to restrict ourselves to these frequencies however, as other uncommonly low frequencies, such as 10 cycles, are within the scope of this invention.

It was mentioned in a preceding paragraph the eddy currents are inversely proportional to the resistance of the path through which they flow. and the advantage to be gained from using a material of high specific resistance for the heater body was pointed out. It is also possible to increase the resistance of the eddy current path by suitable shaping of the heater body, as by introducing slots to cause the currents to flow through a circuitous route, as is more fully described in our copending application Ser. No. 181,250 of which this is a continuation in part.

Other features of our invention reside in the provision of a perforated conduit heater and in the provision of an electro-magnetic field struc ture particularly suitable for magnetic heating.

An object of the invention is to heat thermoplastic material uniformly throughout.

Another object of the invention is to heat thermoplastic material magnetically.

Another object of the invention is to heat thermoplastic material magnetically while eliminating so far as is possible induction heating by eddy currents.

Another object of the invention is to utilize a magnet steel alloy having high coercive force as a magnetic heater.

Another object of the invention is to provide in a magnetic heater, a closed magnetic circuit including as one leg a magnetic heater body having high coercive force, and as other legs laminated sheet cores.

Other objects of the invention will be apparent from the following description and from the drawing.

A preferred embodiment of the invention will now be described with reference to the d mnw of which:

Fig. 1 is a view in longitudinal section, of one embodiment of an assembled magnetic heater incorporating features of this invention;

Fig. 2 is a sectional view along the lines 2-4 of P18. 1, and

Fig. 3 is a sectional view along the lines 3-3 of Fig. 1.

The magnetic heater preferably is divided into two portions, the cylindrical portion 5 in which preliminary heating is accomplished, and the tapering perforated or chambered portion 8 herein shown as having the converging bores or channels I. and in the outer end of which the outlet nozzle I is screwed. Eight bores or channels I are shown in the tapering portion 8 of the heater but it will be understood that the number of these channels may be varied and their shape modified within the scope of the invention. Both heater portions I and 8 are formed from magnet steel alloys having high coercive force in order that suitable heat may be produced therein under the influence of a varying magnetic field as is hereinafter described.

The heater portion 8 is fitted within the cylindrical heater portion 6 and is attached thereto in longitudinal alignment by cap screws 9 which extend through circular openings in a shoulder of the heater portion 8 and are screwed into tapped corresponding openings in the cylindrical portion 5. The entire heater comprising the two portions I and I is supported from cooperating feeding mechanism by the cap screws i0, fragments of which are shown.

The cylindrical heater portion 5 contains a centrally located solid torpedo or pineapple Ii with walls equi-distantly spaced from the inner walls of the cylindrical heater portion 5 to form an annular passage I! through which the thermoplastic material to be acted upon flows. The torpedo II is supported by a threaded end II which is screwed into a tapped opening in the central portion H of the converging heater 0, and performs the function of splitting smoothly and in stream-line flow, the body of thermoplastic material entering the channel II, for passage into the channels I which are in longitudinal alignment with the channel II. The torpedo H is also preferably formed of magnet metal having high coercive force, or may be a permanent magnet and it provides relatively great surface contact with the thermoplastic material passing through the channel l2, and heats the material through hysteresis losses as do the heaters I and I.

The torpedo ll may be replaced with the annular plunger disclosed in the copending application of Charles 8. Bird, Leon 1". Marsh and Graydon Smith, entitled "Heating apparatus for in- Jection molding machines."

The channels 1 in the heater 6 converge throughout. their length and meet in the common channel I. which is located in alignment with the channel II in the nozzle l.

The two shoulders II and II of the heater portion 8 are each cut away to provide, in the embodiment illustrated. ten equi-distant plane surface; for receiving the radial legs II and i I respectively, of bridge-shaped laminated sheet cores II, which are fitted tightly against the heater 8. The cores II with their legs II and I! and the cylindrical heater I form a closed magnetic circuit, of which one portion (20) is an elcctro-magnetic core and of which the other portion (I and II) is a coercive magnet core. For the P se of this description and for the claims, electro-magnetic and coercive magnet cores are, defined thus: "An electro-magnetic core is a laminated core comprising metal sheets, such as silicon steel, in which all losses are kept as low as practical. A coercive mag net core isa magnetic core in which hysteresis losses are very high and eddy current losses are relatively low.

The electro-magnetic cores 20, with their legs ii and I! also form bridges around the cylindrical heater 5, and within thebridgedspaces 21 between the legs I! and 10 are supported the spaced annular coils 2|, ,ea'ch coil containing a plurality of turns of wire, the coils being adapted to be connected electrically in series with respect to each other for energization from, for example, a 60 cycle alternating current source, though obviously other frequencies may be employed or the current may be pulsating or interrupted direct current.

The converging portion i of .the heater likewise has in the embodiment illustrated, ten plane faces provided on each of its shoulders 22 and 23 for receiving the legs 24 and II of the electromagnetic cores 2! which like the cores 20 are formed from laminated metal sheet. The cores 2. likewise bridge the spaces around the converging heater 8, and within the bridged spaces 2| between the legs 22 and 23 are supported the annular coils 2! which like the coils II are composed of a plurality of turns and which are adapted to be connected in series and to an alternating or pulsating direct current supply source.

The electro-magnetic cores 2! and the coils II are supported on the cylindrical heater portion 5 and the cores 2! and the coils II are supported on the heater portion i by suitable structure not herein illustrated, but which may be of the nature disclosed in the copending application of Graydon Smith, Serial No. 259,201, filed March 1, 1939, and entitled Ma etic field structure.

The torpedo H is tapered to form with the outer heater portion 8 the diverging annular channel II, as shown in Fig. 3, and the central portion ll of the heater portion I is oppositely tapered to form the converging channels I. The pointed end and tapering body of the torpedo serves to divide the incoming material into an annular stream and, together with the heat received from the walls, prepares the material for passage into the channels I as it moves through the passage l2. With this arrangement, minimum resistance .to flow with maximum heating surface contact for the thermoplastic material, are provided.

In operation, low frequency alternating or pulsating direct current supplied to the coils 2i and 20 causes through the electro- magnetic cores 20 and 2 respectively, varying magnetic fields around and in the magnetic heaters land 6 respectively and in the inner core or pineapple II. By including the magnetic heaters 8 and C as elements in closed magnetic circuits, the varying magnetic field is produced much more readily and eflclently, thus reducing the current which the coils must carry for a given heating effect. The use of a closed magnetic circuit furthermore confines and directs the field, thus minimizing stray magnetic fields and consequent stray heating of other parts of the machine to which the heater is attached.

The cylindrical magnetic heater I serves as a preliminary heater and acts upon the solid thermoplastic material entering the channel I! to soften it suiilciently for it to how easily from the channel I! into the converging channels 1. The

incoming material while in the preliminary heating stage is divided into an annular stream by the torpedo, as above described, preparatory to its introduction into the channels 1. The con- 5 verging magnetic heater 6 then provides increased heating surface on the material and additional heat to reduce the material to the desired plastic condition for extrusion through the nozzle Ito suitable molds or dies.

It will be apparent from the foregoing that we prefer. that our heaters have as high a ratio as possible of hysteresis loss to eddy current loss. This may be accomplished by lowering the eddy current for a given hysteresis effect or by in-lii creasing the hysteresis effect for a given eddy current loss. Thus we produce uniform heating throughout the material being handled and so are able to provide the necessary heat without burning any portion of the material through the 20 formation of hot spots. I

While one embodiment of the invention has been described for the purpose of illustration, it should .be understood that other material and arrangements of material maybe suggested by 25 those skilled in the art for providing magnetic heating through a high ratio of hysteresis loss to eddy current loss without departure from the essence of the invention. Moreover, while'in describing the apparatus we have referred to its use in connection with the molding of thermoplastic materials, it is, of-course, not limited to any particular type of material capable of being rendered plastic by heat, and the term "thermoplastic" is herein used in that broad sense.

Having thus disclosed our invention and described an illustrative embodiment thereof, we claim as new and desire to secure by Letters Patent:

1. Amagnetlc heater comprising a perforated 40 magnetic element adapted to receive and to heat thermo-plastic material and possessing the characteristic of having relatively high hysteresis losses and relatively low eddy current losses when subjected to a varying magnetic field.

2. A magnetic heater comprising a perforated magnetic element adapted to receive and to heat thermo-plastic material and possessing the characteristlcs of having a high ratio of hysteresis to eddy current losses when subjected to a varying magnetic field.

3. A magnetic heater comprising a perforated magnetic element adapted to receive and to heat thermo-piastic material and possessing the characteristics of having relatively high hysteresis and relatively low eddy current losses and having high electrical resistivity to the flow of eddy currents when subjected to a varying magnetic field.

4. A magnetic heater comprising an integral 60 cyindrical core and an integral hollow tapering core bolted together and providing a longitudinal passage, both of said cores being steel of high coercive properties, separate series of electromagnetic bridges engaging the cylindrical and tapering cores respectively. and separate series of electro-conductive coils disposed about the two cores within said bridges.

5. A magnetic heater for thermo-plastic material, comprising an integral core of steel having high coercive properties and having a passage longitudinally therethrough for said material, means providing a plurality of electro-magnetic bridges disposed about and engagi g the core and having relatively low hysteresis losses when subjected to a varying magnetic held, and electro-conductive coils disposed about the core within the bridges, whereby to heat said material uniformly, as it passes through the core, by relatively high hysteresis losses in the core.

6. A device for heating thermo-plastic material, comprising a body oi steel having high coercive properties and having a solid inner centrally disposed portion so spaced from the outer portion as to provide passage longitudinally through the body between the inner and outer portions !or receiving said material, and means for producing relatively high hysteresis losses in said body for heating said material as it passes through the body between said portions.

7. A device for heating thermo-plastic material, comprising a body of steel having high coercive properties and including an inner core providing an annular passage thereabout and extending longitudinally within the body and diverging to a predetermined position within the body, the body having a plurality of relatively small passages in communication with the annular passage at .said position and converging therefrom away from the annular passage to the exit end oi the body, and means for producing hysteresis losses in the steel body for heating said material as it passes through the passages to said exit end of the body.

8. A device ior heating thermo-plastic material. comprising a body or steel having high coercive properties and having passage longitudinally therethrough for receiving said material, the body possessing the characteristic of having relatively high hysteresis losses when subjected to a varying magnetic ileld, cooperating magnetic means possessing the characteristic'oi' having relatively low hysteresis losses when subjected to said varying magnetic iieid, said means bridging a portion oi the length of said body and forming with the same a closed magnetic circuit. and an electric winding in the bridged space between said means and the body adapted to be energized by a pulsating electric current for producing a varying magnetic iield.

9. Heating apparatus for thermoplastic material or the like, comprising an elongated body oi term-magnetic metal which exhibits high hysteresis loss when subjected to an alternating magnetic held, the body having a plurality of passages longitudinally therethrough for receiving said material and having non-conductive gaps between the passages to reduce eddy current therein, and means arranged to heat the body by subjecting it to eiectro-magnetic action.

GRAYDON SMITH. ALBERT ALLEN.

Images