GB2143767A

GB2143767A - Blow moulding an article whose diameter varies along its length

Info

Publication number: GB2143767A
Application number: GB08415680A
Authority: GB
Inventors: Tatsuya Nakagawa; Yasuo Ezaki
Original assignee: Ex-Cell-O Corp
Current assignee: Ex-Cell-O Corp
Priority date: 1983-06-27
Filing date: 1984-06-20
Publication date: 1985-02-20
Also published as: SE8403166D0; SE8403166L; AU570691B2; SE465363B; JPH0359813B2; GB8415680D0; GB2143767B; JPS606429A; AU2978484A

Abstract

A molten plastic material is extruded to form a parison, and the amount of material extruded is increased to increase the extruded wall thickness of the parison and the gas pressure inside the parison is increased, corresponding to a larger diameter (D') portion of the article to be blow moulded in a blow mould (12), and the extruded wall thickness of the parison is decreased and the gas pressure inside the parison is decreased, corresponding to a smaller diameter portion (D) of the article to be blow moulded. Thus, there is obtained a parison which is uniform in thickness and whose cross-sectional area or diameter varies generally in compliance with the shape of the article to be blow moulded. The apparatus is provided with means for automatically varying both the extruded wall thickness and gas pressure, in the same required sense. <IMAGE>

Description

SPECIFICATION Method and apparatus for manufacturing a hollow plastic product having different diameters BACKGROUND OF THE INVENTION 1. Field of the invention This invention generally relates to blow molding technology and particularly to a method and apparatus for manufacturing a plastic product whose cross-sectional area varies along its longitudinal axis.

2. Description of the Prior Art When manufacturing a plastic pipe whose crosssectional area or diameter varies at least locally along its longitudinal axis in accordance with the prior art blow molding technology, the thickness of a parison is first determined according to the size of that portion of a plastic pipe to be blow-molded which has the largest cross-sectional area or diameter. In such a case, however, there is formed a relatively large flash along those portions of a molded plastic pipe which have smaller cross-sectional areas. Such a prior art method is disadvantageous because it is then necessary to remove the flash which could constitute a loss of material. In particular, in the case of a plastic pipe which is used as an air intake duct of an automobile engine, it is often required to provide a bellow structure at the small diameter portion of a pipe.Thus, the formation of a flash at the small diameter portion of a pipe where a bellow structure is provided causes to deteriorate the function of the bellow structure.

Under the circumstances, it has been proposed to form a parison 1 whose thickness varies depending on the size of cross-sectional area of a resulting molded pipe, as schematically shown in Figure 1. The parison 1 formed according to this method is generally constant in outer diameter, but its thickness is increased at those portions which are located at larger cross-sectional sections A, A of a mold cavity. In this case, however, after blow molding, a smaller cross-sectional portion B may be made uniform in thickness as indicated in Figure 2a, but the larger cross-sectional portion A tends to be non-uniform in thickness as indicated in Figure 2b. In other words, if the larger cross-sectional portion A is approximately oval in shape, the top and bottom rounded parts tend to be larger in thickness as compared with the straight side parts.

SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to obviate the above-described disadvantages of the prior art and to provide an improved method and apparatus for manufacturing a plastic pipe.

Another object of the present invention is to provide an improved blow molding method and apparatus for manufacturing a plastic pipe whose crosssectional area varies at least partly along its longitudinal axis.

A further object of the present invention is to provide an improved method and apparatus for fabricating a plastic pipe having a complicated shape without forming a substantial flash.

A still further object of the present invention is to provide an improved method and apparatus for fabricating a plastic pipe having a uniform thickness and thus strength even if its shape is rather complicated.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration showing the prior art method for manufacturing a plastic pipe whose cross- sectional area changes along its longitudinal axis; Figure 2a and 2h are cross-sectional views of the parison 1 after blow molding taken at lines IIA - IIA and Is - IIB, respectively, in Figure 1; Figure 3 is a schematic illustration showing a system constructed 110 accordance with one embodiment of the present invention; Figure 4 is a graph showing the time-dependent variation of pre-blow pressure p and amount of extruded material m, which is useful for explaining the operation of the system shown in Figure 3; and Figure 5 is a schematic illustration showing that a parison 1 formed by the system of Figure 3 is placed inside of a mold cavity as a preparation for blow molding.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figure 3, there is schematically shown a parison forming system which is constructed in accordance with one embodiment of the present invention and which constitutes part of a blow molding apparatus of the present invention.

As shown, the present blow molding apparatus includes a parison extruder 2 inside of which is provided an auger screw 3. Although not shown specifically, it is to be noted that the parison extruder 2 is connected at its base end to a hopper (not shown) containing therein a quantity of synthetic resin material, such as polyethylene and polypropylene, as is well known for one skilled in the art.

Typically, a heater strip is provided as wrapped around the extruder 2 so that the synthetic resin becomes molten as fed along the extruder 2 when the screw 3 is driven to rotate in a predetermined direction. A nozzle 4 is provided at the forward end of the extruder 2 and it is comprised of a vertically movable mandrel 4a and an outer cylinder 4b which is fixedly attached to the forward end of the extruder 2. There is defined an annular passage 5 between the mandrel 4a and the outer cylinder 4b, which is in fluidic communication with passage 2a inside of the extruder 2.

The bottom end of the mandrel 4a is formed to be convergent in shape, and the inner peripheral surface of the outer cylinder 4b is formed to project inwardly at its bottom. Thus, a parison extrusion opening E is defined between the bottom end of the mandrel 4a and the inner peripheral surface of the outer cylinder 4b. The size or passage width of the opening E may be varied by moving the mandrel 4a vertically with respect to the outer cylinder 4b, so that the amount of parison extruded through the opening E per unit time may be suitably adjusted by controlling the relative position between the mandrel 4a and the outer cylinder 4b.

That is, when the mandrel 4a is so located with its bottom end surface flush with the bottom end surface of the outer cylinder 4b at height Ho in the illustrated embodiment, the amount of extruded parison material gradually increases as the mandrel 4a is moved upward toward the position indicated by the dotted line with respect to the outer cylinder 4b.

The mandrel 4a is provided with a center hole 4c as extending therethrough and this hole 4c is used to supply a pressurized gas into a parison formed as extruded through the opening E. The mandrel 4a has its top portion extending out of the nozzle 4 and forming part of a hydraulic cylinder 5, which is operatively connected to an oil pump 7 through a servo-valve 6, which, in turn, is connected to a computer 8. Thus, the servo-valve 6 is operated in accordance with commands from the computer 8 to control the supply of operating oil from the pump 7 to the hydraulic cylinder 5 thereby adjusting the position of the mandrel 4a with respect to the outer cylinder 4b or the cross-sectional area of the opening E.

As also shown in Figure 3, the center hole 4c is fluiddynamically connected to a pressurized gas source 10 through a gas pressure control device 9.

In the illustrated embodiment, the gas pressure control device 9 includes four branched flow passages B1 through B4 which are connected at each end to a common flow passage leading either to the center hole 4c or to the pressurized gas source 10. Each of the four branched flow passages is provided with a pressure regulator 9a and a pair of valves 9b, 9b located upstream and downstream of the pressure regulator 9a. It is to be noted that one of the valves 9b, 9b may be discarded; however, with the provision of a pair in each of the branched passages as in the illustrated embodiment, the gas pressure can be adjusted more accurately and with excellent responsiveness. Each of the valves 9b is connected to the computer 8 so that the opening and closing condition of each of the valves 9b can be controlled automatically in accordance with commands from the computer 8.

A relief flow passage P is provided as branching our of the common flow passage extending between the gas pressure control device 9 and the center hole 4c, and a relief valve 11 is provided in the relief flow passage P. The relief valve ills connected to the computer 8 so that its open and closed condition is also controlled automatically in accordance with commands from the computer 8.

When the relief valve 11 is open, the relief flow passage P and thus the center hole 4c is in communication with the atmosphere.

Now, the operation of the system shown in Figure 3 will be described below. Prior to the initiation of extrusion of a parison, each of the pressure regulators 9a in the gas pressure control device 9 is appropriately set at a desired pressure level. For example, the pressure level of the pressure regulator 9a provided in the first branched flow passage B1 is set such that the pressurized gas is 1.2 Kg/ cm2 may be supplied into a parison when this flow passage B1 is selected for operation by opening the corresponding valves 9b, 9b. In addition, the pressure regulators 9a of the remaining branched flow passages B2, B3 and B4 are set at 1.3, 1.4 and 1.5 Kg/cm2, respectively.With these four branched flow passages B1 through B4 having differently set pressure levels, the pressure of a gas to be supplied into a parison may be changed at four different levels by selecting one of the four passages for operation. Besides, if it is so structured that two or more of the branched flow passages may also be selected for operation at the same time, an increased variety of gas pressure may be supplied into a parison. It is assumed that the pressurized gas source 10 supplies a pressurized gas approximately at 5 Kgism2 to the gas pressure control device 9.

With all of the valves 9b closed and only the relief valve 11 open, the extruder 2 is set in operation to initiate the extrusion of a parison through the opening E. While the parison material is extruded through the opening E, the relative position of the mandrel 4a with respect to the outer cylinder 4b and the pressure of a gas to be supplied into a parison are suitably adjusted under the control of the computer 8. That is, in accordance with a program and data relating to a product to be manufactured, which are stored in the computer 8, the amount of parison material to be extruded and the pre-blow pressure or the pressure of the gas to be supplied into an extruding parison are appropriately controlled.As a result, there is obtained a parison whose thickness is uniform and whose cross- sectional area or diameter varies along its longitudinal axis generally in compliance with the shape of a mold cavity. This aspect will be described further in detail with reference to Figures 4 and 5 below.

Figure 5 schematically shows a mold half 12 provided with mold groove 12a having a particular shape. As is obvious for one skilled in the art, after placing the parison 1 in the mold groove 12a, another mold half provided with a corresponding mold groove is brought into contact with the mold half 12 and the two mold halves thus brought into contact are clamped to be ready for introducing a pressurized gas into the parison 1 for blow molding. Thus, the mold grooves define a mold cavity whose shape correspond to the outer peripheral shape of a plastic product to be blow-molded. It is to be noted that a product to be blow-molded using the mold half 12 is complicated in structure with its diameter or cross-sectional area changing continuously along its longitudinal axis.

In forming the parison 1 to be placed into the mold groove 12a of the mold half 12 shown in Figure 5, the extrusion of parison through the opening E is initiated with the gas pressure p inside the parison 1 at atmospheric pressure and the amount m of extruded parison material at minimum. Thus, at first, a small diameter parison portion 9A is formed corresponding to a small diameter section GA of the mold groove 12a and placed therein. Under the condition, the bottom end surface of the mandrel 4a is located at height Ho in flush with the bottom end surface of the outer cylinder 4b.Then, corresponding to a first larger diameter section G5 of the mold groove 12a, the pair of valves 9b, 9b in the first branched flow passage B1 are selectively opened under the control of the computer 8, and, at the same time, the servo-valve 6 is operated to have the hydraulic cylinder 5 actuated thereby causing the mandrel 4a to rise with respect to the outer cylinder 4b to increase the amount of extruded parison material in accordance with the increase in diameter in the mold groove 12a. Of importance, it is so controlled by the computer 8 that the pre-blow pressure is supplied after a time delay T, as from the initiation of increase in the amount m of extruded parison material.As shown in Figure 4, a rise in the amount m of extruded parison material is not so sharp and it has a particular slope relating to the properties of the parison material, so that it is necessary to control the operation for the pre-blow pressure p inside of the parison 1 to rise similarly, which can be done easily by appropriately operating the pair of valves 9b, 9b in each of the branched flow passages in the present system shown in Figure 3.

By operating the valves 9b and the servo-valve 6 as described above, the amount m of extruded parison material increases by Am and the pre-blow pressure of 1.2 Kg/cm2 is introduced into the extruding parison 1. The condition of extruding an increased amount of parison material is continued for a time period T, and the condition supplying the pressurized gas at the pre-blow pressure of 1.2 Kg/cm2 is continued for a time period T2 for the first larger diameter section GB. Thereafter, the conditions are turned to the initial conditions corresponding to the next smaller diameter section GA. That is, under the control of the computer 8, the relief valve 11 is opened and the valves 9b, 9b in the first branched flow passage B1 are closed so that the servo-valve 6 is operated to switch the direction of supply of operating oil to the hydraulic cylinder 5. As a result, the mandrel 4a returns to the initial position with its bottom end surface flush with the bottom end surface of the outer cylinder 4b at height Ho Similarly with the case of increasing the extruded amount m of parison material and the preblow pressure p as described above, it is also necessary for the extruded amount m and the preblow pressure p to decrease to the initial values approximately with the same slope.However, as far as the time to initiate the decrease is concerned, the associated valves should be so operated that the extruded amount m of parison material starts to decrease with a time delay of TB as from the time when the pre-blow pressure p starts to decrease. The reason why such a time delay is provided between the operations for controlling the extruded amount m of parison material and the pre-blow pressure p is that, as shown in Figure 5, the material tends to be scarce in an expanding section Q where the parison 1 suddenly expands from the smaller diameter portion 9A to the larger diameter portion g, so that an increased amount of material should be supplied for this section Q1 thereby allowing to prevent this portion of the parison 1 from becoming too thin. The similar arguments hold true for a contracting section Q2.

Thereafter, corresponding to a second larger diameter section G5, of the mold groove 12a, there is formed a second larger diameter portion g0, of the parison 1. In this case, since the diameter D' of the second larger diameter section G5, is larger than the diameter D of the first larger diameter section GS, the pre-blow pressure p and the extruded amount m of parison material are set to be correspondingly higher.That is, under the control of the computer 8, the mandrel 4a is moved upward beyond the previous point and the second branched flow passage B2 is selected for operation thereby setting the pre-blow pressure at 1.3 Kgicm2. Similarly with the previous pre- blow step, a time delay T is also provided and its length in time is selected to be longer in light of expansion rate at the second larger diameter section G5,. Holding time periods T,', T2, and time delay Tis3' are also determined corresponding to the length L' and the diameter D' of the second larger diameter section G5, and the valves 9b, 9b are operated by the computer 8 according to these previously determined values.

As described above, in accordance with the present invention, the operating conditions for the valves 9b, 6 and 11 are previously determined in compliance with the shape of a plastic product to be manufactured and these conditions are stored as data in the computer 8. Thus, these valves are appropriately operated under the control of the computer 8 so that there is obtained the parison 1 having locally expanded portions g, and 9B/ uniform in thickness. As the parison 1 is formed as extruded from the nozzle 1 through the openeing E, it is continuously placed into the mold groove 12a along its longitudinal axis from one end to the other.

With the parison 1 placed in the mold groove 12a of the mold half 12, the other mold half (not shown) is brought into mating contact with the mold half 12 and the two mold halves in contact are then clamped. Then, a pressurized gas at a pressure higher than the level of pre-blow pressure p is introduced into the parison 1 inside the clamped mold halves so that the parison 1 is pressed against the surface of the mold cavity thereby carrying out the blow molding. The result ing product will have a shape whose cross-sectional area varies substantially along its longitudinal axis, but its thickness will be the same at the smaller and larger diameter portions GA, G5 and G5,.

The gas pressure control device 9 for controlling the level of pre-blow pressure p in the above-described embodiment includes four branches, paral lel flow passages B1 through B4. However, the number of branched passages should not be limited only to four, but it may be determined appropriately in accordance with a desired shape of a product to be fabricated. Alternatively, the gas pressure control device 9 may include a single passage provided with a single variable flow control valve whose pressure setting condition is variably controlled by the computer 8. In place of the computer 8, use may be made of a sequence control circuit comprised, for example, of timers for controlling the operation of the valves to adjust the pre-blow pressure p and the extruded amount m of parison material.

While the above provides a full and complete disclosure of the preferred embodiments of the present invention, various modifications, alternate constructions and equivalents may be employed without departing from the true spirit and scope of the invention. Therefore, the above description and illustration should not be construed as limiting the scope of the invention, which is defined by the ap

Claims

pended claims. CLAIMS

1. A method for manufacturing a hollow plastic product whose cross-sectional area varies at least locally along its longitudinal axis, said method comprising the steps of: extruding a molten plastic material from a nozzle to form a parison while varying its amount of extrusion per unit time and a pressure inside said parison in accordance with a particular shape of said product to be manufactured; placing said parison in a mold cavity; and introducing a gas at an increased pressure into said parison inside the mold cavity to carry out blow molding.

2. The method of Claim 1 wherein, during said extrusion step, the pressure inside the parison is increased if the amount of extrusion is increased and decreased if the amount of extrusion is decreased.

3. The method of Claim 2 wherein, during said extrusion step, the pressure inside the parison and the amount of extrusion are increased at substantially the same first rate and decreased at substantially the same second rate.

4. The method of Claim 3 wherein, during said extrusion step, the pressure inside the parison is initiated to increase after a first time delay from the initiation in increase of said amount of extrusion and the pressure inside the parison is initiated to decrease after a second time delay from the initiation in decrease of said amount of extrusion.

5. The method of Claim 4 wherein, during said extrusion step, the length in time of each of said first and second time delays is varied depending on the size of the cross-sectional area of the corresponding portion of said product to be manufactured.

6. Apparatus for manufacturing a hollow plastic product whose cross-sectional area varies along its longitudinal axis, said apparatus comprising: extruding means for extruding a molten plastic material to form a parison, said extruding means including extrusion amount adjusting means for adjusting the extruded amount of said plastic material; gas pressure adjusting means provided as connected between a pressurized gas source and the interior of said parison; and control means connected to said extrusion amount adjusting means and said gas pressure adjusting means, said control means controlling said extrusion amount adjusting means and said gas pressure adjusting means such that the extruded amount is increased when the pressure inside the parison is to be increased and the extruded amount is decreased when the pressure inside the parison is to be decreased according to a particular shape of said product to be manufactured.

7. The apparatus of Claim 6 wherein said extrusion amount adjusting means includes an outer cylinder, a mandrel extending through said outer cylinder as separated therefrom thereby defining an opening therebetween and movable with respect to said outer cylinder and cylinder means connected to said mandrel for moving said mandrel with respect to said outer cylinder thereby changing the size of said opening.

8. The apparatus of Claim 7 wherein said mandrel is provided with a center hole extending therethrough and said gas pressure adjusting means includes at least one pressure regulator and at least one valve provided in a flow passage extending between said pressurized gas source and the center hole of said mandrel.

9. The apparatus of Claim 8 wherein said gas pressure adjusting means further includes a relief valve capable of connecting the interior of said flow passage to the atmosphere.

10. The apparatus of Claim 9 wherein said cylinder means includes a hydraulic cylinder connected to said mandrel, an oil pump and a servovalve interposed between said hydraulic cylinder and said oil pump for controlling the flow of oil therebetween, and said control means includes a computer connected to said at least one valve, said relief valve and said servo-valve.

11. Method for manufacturing a hollow plastic product substantially as herein described with reference to Figures 3 to 5 of the accompanying drawings.

12. Apparatus for manufacturing a hollow plastic product substantially as herein described with reference to Figure 3 to 5 of the accompanying drawings.