GB2251201A

GB2251201A - Compensating for article contraction during injection moulding cooling

Info

Publication number: GB2251201A
Application number: GB9028184A
Authority: GB
Inventors: Henry Tooze
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-12-29
Filing date: 1990-12-29
Publication date: 1992-07-01
Also published as: GB9028184D0

Abstract

A follow up pressure device (14) (for so-called second stage injection) for a mould tool (22), comprising an accumulator device incorporating a variable volume chamber (15) adapted, in use, to be charged with a quantity of material being moulded during the injection process, and to be gradually discharged during cooling of the mould tool (22), by the delivery of material into the mould cavity under predetermined conditions of pressure and volume. The invention also includes a mould tool (22) provided with a device (14) and an injection moulding machine provided with such a mould tool. The device may utilise a diaphragm, as an alternative to the spring loaded plunger (16). <IMAGE>

Description

INJECTION MOULDING This invention relates to the injection moulding of plastics or rubber and in particular to a follow up pressure device for a mould tool; to a mould tool provided with such a device; and to an injection moulding machine (such as a rotary, vertical or horizontal moulding machine or a multiclamp machine) provided with such a mould tool.

In the moulding of plastics, viscous but flowable plastics material is injected via a sprue into the or each mould cavity of a mould tool, and after the injection step, the tool requires cooling, usually by pumping cooling water through coolant passageways of the tool, to ensure the article being moulded has sufficient handling strength before the mould can be parted for the article removal and a repeat of this cycle. As the plastics undergoes various temperature changes the structural integrity of the article may suffer and imperfections, such as porosity, may form. Loss of integrity is accentuated with thick walled articles, whilst in some applications where safety is involved e.g. natural gas pipe fittings, integrity requirements are paramount.

Various means have been proposed in the past to feed additional plastics material, under varying conditions of pressure and volume to the mould cavity during cooling and hence shrinkage to fill and pack the mould tool cavity with a view the mitigating if not eliminating the formation of imperfections such as porosity, but such means have all drawn on the basic energy of the moulding machine, have been relatively complex and hence expensive, and have not in practice solved the problem in a sufficiently reliable manner.

According to a first aspect of the present invention, there is provided a follow up pressure device for a mould tool, comprising an accumulator device incorporating a variable volume chamber adapted, in use, to be charged with a quantity of material being moulded during the injection process, and to be gradually discharged during cooling of the mould tool, by the delivery of material into the mould cavity under predetermined conditions of pressure and volume.

According to a second aspect of the present invention, there is provided a mould tool incorporating a follow-up pressure device in accordance with the first aspect.

According to a third aspect of the invention, there is provided an injection moulding machine incorporating a mould tool in accordance with the second aspect.

The follow up pressure device may constitute a removable, or an integral, part of the mould tool, there being no difficulty in modifying existing mould tools to incorporate, or carry, the follow-up pressure device.

Preferably, the variable volume chamber takes the form of a bore, with which is associated a plunger that is reciprocable in a first, retracting direction to increase the bore volume during the chamber charging stage, and in a second, delivery direction, to force material from the chamber into the article being moulded. As an alternative to a bore and plunger arrangement, a diaphragm arrangement could be used.

In order to ensure that the material in the chamber is at a suitable working temperature, at least one heater is preferably provided in the vicinity of the bore, as is a thermocouple to control activation of the heater(s). In detail, a plurality of in-built cartridge and band heaters may be provided to achieve the desired melt-flow viscosities best suited for the particular plastic or rubber material being moulded.

Conveniently, with the bore and plunger arrangement, the plunger is loaded by a spring arrangement, such as a coil compresion spring, or a pack of Belleville washers, during the plunger retraction stroke, whilst the chamber is being charged.

The spring arrangement may be operable on an enlarged diameter collar attached to a rear end of the plunger, the spring arrangement being located in a housing cylinder, and bearing at its other end on a closure plate of the housing cylinder.

As the loading of the spring arrangement on the plunger would normally exceed the working pressure of the material during injection moulding, and hence inhibit if not prevent the charging of the chamber, preferably mechanical means is provided for compressing the spring and hence negating its loading effect during the injection moulding, which means is conveniently operable upon, or by, movement of the clamp or carriage. Such means may comprise a rod attached to the opposite side of the collar from that to which the plunger is attached and extending to a lever arrangement to cause axial rod displacement and hence spring compression.

The follow-up presure device in accordance with the first aspect of the invention, may be located on, or built into, a body member e.g. a steel block, securable e.g. by bolts, to the mould tool, with the block provided with a supply sprue aligned with the tool sprue and closed at an inlet end by a conventional spring-loaded non-return ball valve, an inlet end of which valve is, in use, engaged by the material delivery nose of the injection moulding machine.

Thus, in detail, upon either the closing of the moulding machine clamp, or the movement of the moulding machine carriage, the coil compression spring is compressed by the use of levers or cams etc., depending on convenience of application, such compression retracting the plunger ready for its bore to be charged more or less simultaneously with plasticized material from the injection moulding machine.On initiating the injection cycle of the injection moulding machine, both the mould tool cavity, or cavities, and the bore of the follow-up pressure device are charged with plasticized material, and when the control system of the injection cycle determines that the requisite quantity of plasticized material has been injected to fill completely the mould tool cavity and the chamber of the follow-up pressure device, the injection moulding machine is retracted, ceasing to apply pressure and the ball valve closes so that the pressure is contained within the mould tool and the bore of the follow-up pressure device, and simultaneously, or thereabouts, the compression spring is released e.g. by the opening movement of the clamp of the injection moulding machine on reverse movement of the carriage of the injection moulding machine, thereby increasing the pressure on the thermoplasticized material within the bore of the follow-up pressure device by the resulting spring loading and hence causing progressive discharge of thermoplasticized material, rendered flowable by the action of the heaters, from the bore of the follow up presure device and delivery into the article being moulded, via the sprues, during cooling of the article takes place, to achieve the stability of form required, this second stage feeding of plasticized material into the tool after termination of the injection moulding step ensuring true shape and integrity of structure in the moulded article, thus overcoming the tendency of prior art systems to misform products, resulting in relatively high scrap rates, due to contraction of the thermoplasticized material upon cooling and inadequate, or non-existant, second phase material supply.

In detail, the variable volume chamber, e.g. the bore, is drilled into the body member so as to intersect the supply sprue, while the spring housing is formed by drilling a counterbore coaxially with the bore. An outer end of the spring housing may be closed by a bearing plate, with an adjusting nut to adjust the spring pressure housed within a recess in a spring location nut.

Thus, in principle, the invention feeds a metered amount of plasticized material under pressure into the cooling mass of a moulded article(s) within a given mould tool cavity, or cavities, with a view to ensuring that the moulded article is true to form, and sound in shape and structure, hence overcoming the problem of mis-shapen forms due to material contraction as cooling takes place within the cavity, or cavities, of the mould tool. The invention also provides for considerable power saving compared with prior art proposals, as a result of the ability to off-load the injection cylinder after the initial injection stage.

The invention will now be described in greater detail with reference to the accompanying drawing which is a sectional view through a follow-up pressure device in accordance with the first aspect of the invention fitted to a mould tool in accordance with the second aspect of the invention.

In the drawing, a follow up pressure device 14 comprises a variable volume chamber in the form of a bore 15 into which is fitted a plunger 16 attached at one end to an enlarged collar 3 reciprocable in a counter bore 17 serving as a housing for a coil compression spring 5. To the collar 3 is attached a rod 18 in turn attached to a lever 9 pivotable about axis 19 under the action of a vertical plunger 13 operable by the associated injection moulding machine during various stages of the machine cycle. The compression of the spring 5 is adjusted by a screw 6 housed in a spring location nut 7 through which the rod 18 passes, while the bore 15 is in communication with a sprue 20 formed in â steel housing block 4, an inlet end of the sprue 20 being provided with a nonreturn ball valve 11.The block 4 is also provided with a plurality of apertures 1 each to receive a heater and with another aperture to receive a thermocouple 2, the block 4 being secured by bolts 21 to a mould tool 22 with the sprue 20 of the block 4 aligned with the sprue 23 of the tool 22 for the flow of thermoplasticized material to a mould cavity 24.

It will be appreciated that the heaters 1 maintain the material to be moulded at the optimum plasticized temperature; the thermocuple 2 measures the temperature of the block 4 in the vicinity of the bore 15 to control the switching on and off the heaters 1; the plunger 16 and bore 15 serve to meter, under pressure, the volume of flowable thermoplasticized material to be fed into the mould cavity during cooling of the moulded article, within a predetermined bore volume; the block 4 serves to contain all component parts of the follow up pressure device 14 and to withstand the effects of pressures and operational stresses within the device 14; the compression spring 5 serves to apply a predetermined force to the plunger 16, which in turn creates pressure on the plasticized material contained within the bore 15 (and thus the sprues 20, 23 and the mould cavity 24), thus causing flow of the thermoplasticized material progressively from the bore 15 into the cavity 24; the screw. 6 serves to adjust compression of the spring 5; the spring location nut 7 serves to lcoate the spring 5 and maintain spring pressure; a sealing flange face is indicated at 8; the lever mechanism 9 serves to compress the spring 5 during the charging stage and to load the spring to the required tension; cam activators 10, being vertical or horizontal wedge cams, provide an alternative to the lever mechanism 9 to compress and load the spring 5 to the required tension by using movement of the clamp or the injection unit; and the non-return valve 11 serves to seal off the mould cavity 24 etc after completion of the injection stage, to maintain pressure within the mould cavity 24 etc. during cooling and during discharge of material from the bore 15 of the follow up pressure device 14.

Claims

1. A follow up pressure device for a mould tool, comprising an accumulator device incorporating a variable volume chamber adapted, in use, to be charged with a quantity of material being moulded during the injection process, and to be gradually discharged during cooling of the moulded product, by the delivery of material into the mould cavity under predetermined conditions of pressure and volume, to ensure conformity of form, integrity and size, by which is meant second stage injection.

2. A device as claimed in Claim 1, wherein the variable volume chamber takes the form of a bore, with which is associated a plunger that is reciprocable in a first, retracting direction to increase the bore volume during the chamber charging stage, and in a second, delivery direction, to force material from the chamber into the article being moulded.

3. A device as claimed in Claim 1, wherein a diaphragm arrangement is provided as the variable volume chamber.

4. A device as claimed in any proceding claim, wherein at least one heater is associated with the variable volume chamber.

5. A device as claimed in Claim 4, wherein a thermocouple is provided to control activation of the heater(s).

6. A device as claimed in Claim 4 or Claim 5, wherein a plurity of in-built cartridge and band heaters are provided.

7. A device as claimed in Claim 2 and any claim appendant thereto, whereby the plunger is loaded by a spring arrangement.

8. A device as claimed in Claim 7, wherein the spring arrangement is a coil compression spring, or a pack of Bellville washers, during the plunger retraction stroke, whilst the chamber is being charged.

9. A device as claimed in Claim 7 or Claim 8 wherein the spring arrangement is operable on an enlarged diameter collar attached to a rear end of the plunger.

10. A device as claimed in Claim 7, and any claim appendant thereto wherin the spring arrangement is located in a housing cylinder.

11. A device as claimed in Claim 10, wherein at an end remote from the plunger, the spring arrangement bears on a closure plate of the housing cylinder.

12. A device as claimed in Claim 7, and any claim appendant thereto, wherein means is provided for compressing the spring arrangement and hence negating its loading effect during the injection moulding.

13. A device as claimed in Claim 12 wherein the compression means is operable upon, or by, movement of the clamp or carriage.

14. A device as claimed in Claim 12 or Claim 13, wherein the compression means comprises a rod attached to the opposite side of the collar from that to which the plunger is attached and extending to a lever arrangement to cause axial rod displacement and hence spring compression.

15. A device as claimed in any proceeding claim, located on, or built into, a body member e.g., a steel block, securable e.g., by bolts to the mould tool, with the block provided with a supply sprue aligned with the tool sprue and closed at an inlet end by a conventional spring-loaded nonreturn ball valve, an inlet end of which valve is, in use, engaged by the material delivery nose of the injection moulding machine.

16. A device as claimed in Claim 15 wherein the variable volume chamber is drilled into the body member so as to intersect the supply sprue, while the spring housing is formed by drilling a counterbore coaxially with the bore.

17. A device as claimed in Claim 16, wherein an outer end of the spring housing is closed by a bearing plate, with an adjusting nut to adjust the spring pressure housed within a recess in a spring location nut.

18. A follow-up pressure device by which is meant second stage injection substantially as hereinbefore described with reference to the accompanying drawings.

19. A mould tool incorporating a follow-up pressure device in accordance with any proceding claim.

20. A mould tool substantially as hereinbefore described with reference to the accompanying drawings.

21. An -injection moulding machine incorporating a mould tool in accordance with Claim 19 or Claim 20.

22. An injection moulding machine substantially as hereinbefore described with reference to the accompanying drawings.