GB2343651A

GB2343651A - Plastics moulding

Info

Publication number: GB2343651A
Application number: GB9824678A
Authority: GB
Inventors: Victor Philip Slemmings
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1998-11-10
Filing date: 1998-11-10
Publication date: 2000-05-17
Also published as: GB9824678D0

Abstract

In producing large plastics mouldings 10 using a cold runner system, there is a problem with sink marks in the areas where there is a large thickness of plastics (eg the runners 18). To avoid this, gas under pressure is introduced into the mould, on the B surface (i.e. the back face that is at least less visible or out of sight) of the moulding, in those areas where sink marks would be anticipated. The gas forces the A surface (i.e. the front or visible face) of the moulding against the corresponding mould wall so that it solidifies without sink marks. The mould surface can be roughened in certain areas 32 so that the gas only flows in those areas between the solidifying moulding and the mould wall, and does not penetrate other areas. The mouldings are vehicle instrument panels.

Description

Plastics Moulding This invention relates to plastics moulding techniques, for injection moulding plastics articles, especially large plastics articles such as motor vehicle instrument panels.

For the purposes of this specification, large plastics articles are those which require a mould equipped with multiple feeds, normally by way of a hot runner or a very complex cold runner system. Small plastics articles are those where the mould can be fed from a single runner.

Examples of large plastics articles in the automotive industry are instrument panels and bumpers. An example of a small plastics article is a glovebox.

Injection moulding is conventionally carried out by forcing molten plastics material into a closed mould, and then allowing the plastics to solidify in the mould. When solidification is complete, the mould is opened and a product of the desired moulded shape is produced.

It is conventional within the industry to refer to such moulded products as having an'A'surface and a'B' surface. The A surface is the front face which will be visible when the product is in use, the B surface is the back face which will be out of sight, or at least less visible than the A surface when the product is in use.

The molten plastics enters the mould through a sprue or gate and flows into a runner or runners. The runners distribute the molten plastic to the various parts of the mould cavity. Both hot and cold runner systems are known.

In a hot runner system, the runner is maintained at an elevated temperature so that the plastics remains molten, even when the mould is full, and pressure is maintained, forcing molten plastics into the mould cavity. Then, as the plastics shrinks while cooling, the shrinkage can be made up by a continuing feed of molten plastics into the mould cavity so that there are, if the mould and runner systems are properly designed, no'sink'marks on the product. By maintaining the runner temperature high enough to keep the plastics in the runner molten, a subsequent moulding cycle can follow directly after a first product has been removed from the mould.

In a cold runner system producing large size plastics mouldings, relatively large runner passages are provided within the mould to allow the molten resin to flow quickly around the mould cavity. These runners (which form part of the final moulded product) are called integrated cold runners. Other runners also connect with the sprue to provide a manifold from which molten plastics can flow into the mould cavity. The plastics which solidifies in these runners does not form part of the finished product, and is broken off from the product after moulding. These runners are called non-integrated cold runners.

As the plastics in the filled mould cools and solidifies, the associated shrinkage results in the surface of the product, where the cold runner is located, shrinking away from the mould wall to cause a sink mark.

Hot runner systems have the disadvantage that the equipment to heat the runners is expensive and a possible cause of breakdown.

US Patent 3 345 687 describes a moulding process in which gas under pressure is forced into the mould during the cooling phase, between one mould surface and the semimolten plastics material to force the cooling plastics material, which at that time is plastically deformable, against the mould wall, so that the product when released from the mould has no sink marks.

The invention seeks to obtain the product quality achievable with a hot runner system, but without hot runners.

According to the invention there is provided a method of forming a plastics product by injection moulding, wherein the product is designed to have integral cold runners on a B surface, and wherein gas under pressure is applied to faces of the cold runners on the B surface after injection and during cooling, to force the semi-molten plastics material into contact with the A surface of the mould.

The gas (which may be air, nitrogen or any other gas inert relative to the plastics material being moulded) may be pumped from a gas reservoir into the mould cavity, and may be returned to the reservoir after the plastics has solidified, before being pumped back into the mould in the next and subsequent mould cycles.

The gas can be pumped in at a pressure up to 300 bar.

The invention also provides a mould for forming a plastics product by injection moulding, wherein the mould has grooves in which integral cold runners will be formed on a B surface, and means for introducing gas under pressure is introduced into the grooves after injection and during cooling, to force the semi-molten plastics material into contact with the A surface of the mould.

The surfaces of the grooves preferably have a roughened finish, whilst the surrounding mould surfaces have a smooth finish. The roughness may be produced by grit blasting the appropriate areas of the mould surface so that small channels are formed, between the surface and the molten plastics injected into the mould, along which gas under pressure can flow.

The roughened finish should be continuous along the whole length of the grooves but can extend laterally over part only of the width of each groove. Preferably the roughened finish extends over at least the deepest part of the groove.

The means for introducing gas under pressure includes openings leading into the grooves, the openings preferably being fitted with valves. The valves will be opened at the point in the mould cycle when gas is to be introduced, either by the pressure of the gas itself, or by an external actuator. Typically gas will be introduced when the mould is practically completely full. A gas reservoir may be provided with means for pumping the gas from the reservoir, through the openings and into the mould, and with means for recovering the gas to the reservoir after the plastics has solidified.

This principle can be applied not only to the backs of cold runners in the mould, but also to any mould part where the volume of plastics is such that sink marks might occur on cooling. Thus, according to a second aspect of the invention, there is provided a method of forming a plastics product by injection moulding, wherein gas under pressure is applied to those areas of the plastics product face on the B surface where the volume of plastics is such that sink marks might be expected after injection and during cooling, but not to other areas, to force the semimolten plastics material into contact with the A surface of the mould.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an instrument panel moulding for motor vehicle; Figure 2 is a section on the line W-W; Figure 3 is a section on the line V-V; Figure 4 is a view taken in the direction of the arrow Z from Figure 2; Figure 5 is a section through an non-integral cold runner separated from the moulding; and Figure 6 is a scrap view of the'B'surface of a mould in accordance with the invention.

An instrument panel moulding is generally indicated with reference numeral 10. This moulding has a generally convex form and has a number of openings 12 which are formed as part of the moulding and which are designed to accommodate instrument modules, ventilation outlets and the like.

The panel will be moulded in a correspondingly shaped twopart mould, with molten plastics being injection through a sprue 14 into a (non-integrated) cold runner 16. From this runner, molten plastics flows at both ends in to the mould cavity. Within the mould cavity, on the side which will be the reverse side of the instrument panel (the B face), a number of integral cold runner passages 18 are arranged. These passages are formed in the mould as grooves in the mould wall, and allow a relatively large volume of molten plastics to travel towards the corners of the mould. These cold runners are indicated in Figure 1 by pairs of dotted lines. They extend from both ends of the non-integral cold runner 16. Figure 3 shows a typical section through the instrument panel where the cold runner is shown at 18 and the regular thickness of the instrument panel moulding is shown at 20. The front face of the instrument panel is shown at 22.

It is well known that the presence in a moulding of thick section plastics will result in sink marks on the moulded product's surface, once the product has cooled. To avoid this however the mould in which the panel is formed is provided, at the bases of the parts of the mould where the cold runners will be formed, with air inlets which will allow air or another gas to be forced into the mould, during solidification, to apply a pressure against the rear face of each of the runners to press the front face of the panel (the A surface) into intimate contact with the opposite surface of the mould, thus ensuring that the inevitable shrinkage of the plastics results in deformation of the back of the moulded product, rather than the front.

Furthermore, in order to ensure that the air pressure acts on the correct parts of the runners 18, the surface of the mould with which the cold runners will come into contact is provided with a slight roughening. It has been shown (see European Patent 0 592 525) that this measure is effective to direct the air flow along only those parts where the slight roughening exists and to avoid the air forcing its way between the smooth surfaces of the mould and the moulded product.

Figure 6 shows the surface 28 of a mould, with a groove 30 in which a cold runner 18 will be moulded. The base of the groove has a roughened area denoted by stippling at 32, and gas openings at 34 which open into the roughened area.

In operation, molten plastics is injected into the whole of the mould and flows throughout the mould cavity, filling the runners 18 and the cavity volume between.

Initially the plastics will fill the whole of the cavity and will be in contact with all mould faces. However as the plastics cools, it shrinks and the effect of this shrinkage is noticeable where there is a large volume of plastics, ie where the runners 18 are located. This shrinkage is most apparent on the A surface, above a runner 18. To avoid the shrinkage from adversely affecting the A surface, gas is pumped in to the grooves 30 through the openings 34. Because of the roughening at 32, the gas will be able to flow between the solidifying plastics surface and the mould wall, but will not ba able to flow between the plastics surface and the adjacent smooth walled part of the mould. The pressure of the gas will force the A surface of the solidifying plastics (which at this stage is still plastically deformable) against the front face of the mould so that when the plastics finally solidifies, no sink mark is visible on the A surface. Instead, the shrinkage is apparent on the B surface, in the form of a slightly shallower rib or cold runner 18. This is not visually significant.

Figure 2 shows how the sprue 14 communicates with the nonintegral cold runner 16 which then has gates 24 through which the molten plastics close into the main part of the mould cavity. After moulding, ejector pins 26 eject the non-integral cold runner 16 from the mould, snapping off the plastic remaining at the gates 24.

Figure 4 shows an end view of an integral cold runner 18 with a scar at 28 where the cold runner 16 has been snapped off, and a roughened area 29 which was initially in contact with the roughened area 32 of the mould, but which has been forced away from the mould wall to take up the shrinkage of the plastics which has occurred during solidification.

By using air or gas pressure to act against the backs of the cold runners to perfect the front surface of the mould, the need for hot runners is avoided, thus resulting in considerable cost and complexity savings.

Claims

Claims 1. A method of forming a plastics product by injection moulding, wherein the product is designed to have integral cold runners on a B surface, and wherein gas under pressure is applied to faces of the cold runners on the B surface after injection and during cooling, to force the semi-molten plastics material into contact with the A surface of the mould.
2. A method as claimed in Claim 1, wherein the gas is nitrogen.
3. A method as claimed in Claim 1 or Claim 2, wherein the gas is pumped from a gas reservoir into the mould cavity, and is returned to the reservoir after the plastics has solidified, before being pumped back into the mould in the next and subsequent mould cycles.
4. A method as claimed in any preceding claim, wherein the gas is pumped in at a pressure up to 300 bar.
5. A mould for forming a plastics product by injection moulding, wherein the mould has grooves in which integral cold runners will be formed on a B surface, and means for introducing gas under pressure is introduced into the grooves after injection and during cooling, to force the semi-molten plastics material into contact with the A surface of the mould.
6. A mould as claimed in Claim 5, wherein the surfaces of the grooves have a roughened finish, whilst the surrounding mould surfaces have a smooth finish.
7. A mould as claimed in Claim 6, wherein the roughness has been produced by grit blasting the appropriate areas of the mould surface so that, in use, small channels will be formed, between the surface and the molten plastics injected into the mould, along which gas under pressure can flow.
8. A mould as claimed in Claim 6 or Claim 7, wherein the roughened finish is continuous along the whole length of the grooves.
9. A mould as claimed in any one of Claims 6 to 8, wherein the roughened finish extends laterally over part only of the width of each groove.
10. A mould as claimed in Claim 9, wherein the roughened finish extends over at least the deepest part of the groove.
11. A mould as claimed in any one of Claims 6 to 10, wherein the means for introducing gas under pressure includes openings leading into the grooves, the openings being fitted with valves which are normally closed but which can be opened to admit gas.
12. A mould as claimed in any one of Claims 6 to 11, wherein a gas reservoir is provided with means for pumping the gas from the reservoir, through the openings and into the mould, and with means for recovering the gas to the reservoir after the plastics has solidified.
13. A method of forming a plastics product by injection moulding substantially as herein described with reference to the accompanying drawings.
14. A mould substantially as herein described with reference to the accompanying drawings.
15. A method of forming a plastics product by injection moulding, wherein gas under pressure is applied to those areas of the plastics product face on the B surface where the volume of plastics is such that sink marks might be expected after injection and during cooling, but not to other areas, to force the semi-molten plastics material into contact with the A surface of the mould.