GB2402906A

GB2402906A - Plastics injection compression moulding apparatus

Info

Publication number: GB2402906A
Application number: GB0314551A
Authority: GB
Inventors: Peter Reginald Clarke
Original assignee: Coraltech Ltd
Current assignee: Coraltech Ltd
Priority date: 2003-06-21
Filing date: 2003-06-21
Publication date: 2004-12-22
Also published as: GB0314551D0

Abstract

An apparatus is disclosed for injection compression moulding of an article which comprises a press having a pair of relatively movable platens 12, 20 and a mould formed of two separable halves 50, 60 mounted on the respective platens and defining between them a mould cavity. One mould half 50 is formed with a core 54 and a pre-close plate 56 movable axially relative to the core 54 such that, during closure of the mould, the pre-close plate 56 contacts the other mould half and closes the mould cavity before the core reaches its end position. The movement of the platens 12, 20 of the press is controlled such that the two mould halves 50, 60 are first advanced by the press towards one another to reach an end position in which the volume of the mould cavity is minimised. Then, the core 54 and one of the platens 20 together retract away from the other platen 12 to increase the volume of the mould cavity at the same time as plastics material is admitted into the mould cavity. Lastly, the two mould halves are advanced towards one another a second time to compress the injected plastics material and force it to fill the mould cavity.

Description

PLASTICS MOULDING APPARATUS

The present invention relates to apparatus for injection compression moulding and in particular is an improvement over the apparatus described in the [.pplicant's copending International Pat. Apple. No. PCT/GB02/00306.

The latter PCT application is directed to a mould made up of two halves for mounting between the platens of an lo injection moulding machine for injection compression moulding of a thin walled article. The movable mould half comprises a pressure plate and is movable continuously from a mould open position to a mould closed position. The movable mould half also includes a core that is movable relative to the pressure plate in the direction of opening and closing of the mould, and a rim closure (or pre-close) plate surrounding and in sealing engagement with the core.

The fixed mould half, the core and the pre-close plate together define the mould cavity and each is movable relative to the other two in the direction of opening and closing of the mould. A spring applies a force to urge the core away from the pressure plate towards the position of minimum mould cavity volume. The force applied to the core by the spring is such as to permit the cavity volume to be increased by the injection of a molten plastics material into the mould cavity. When the pressure plate later moves towards the mould closed position, it makes direct contact with the core and reduces the mould cavity to its minimum volume, and this applies a compression force to cause the injected plastics material to flow, so as to fill and pack the entire mould cavity.

To summarise, the core first moves forwards into the fixed mould half to expel air from the cavity, next it moves backwards as plastics material is admitted into the cavity and finally it moves forwards again in order to compress the molten plastics material in the mould cavity.

The latter proposal suffers from the disadvantage that the mould needs to have a large number of parts that can move relative to one ancCher and the invention seeks to achieve the same injection compression mouldir.g technique using a less comple.< mould.

According to the present invention, there is provided apparatus for injection compression moulding of an article which comprises a press having a pair of relatively movable JO platens and a mould formed of two separable halves mounted on the respective platens and defining between them a mould cavity, wherein one mould half is formed with a core and a pre-close plate movable axially relative to the core such that, during closure of the mould, the pre-close plate contacts the other mould half and closes the mould cavity before the core reaches its end position, characterized by means for controlling the relative movement of the platens of the press such that the two mould halves are first advanced by the press towards one another to reach an end position in which the volume of the mould cavity is minimised, the core and one of the platens together retract away from the other platen to increase the volume of the mould cavity at the same time as plastics material is admitted into the mould cavity, and the two mould halves are subsequently advanced towards one another a second time to compress the injected plastics material and force it to fill the mould cavity.

The invention differs from that in PCT/GB02/00306 in that the retraction of the core during the injection of the plastics material into the cavity is enabled not by allowing the core to move relative to the platen but by the core and the platen moving together away from the other platen. In this way, the requirement for a pressure plate and a means for urging the core away from the pressure plate is obviated and the number of components is thus reduced.

It is possible to control the moving platen in one of two ways to achieve injection compression moulding. In the first way, as in PCT/GB02/00306, the pressure used to inject plastics material into the cavity is used to force the retraction of the core. The platen that moves with the core is simply allow to coast and the drive to it from the press is disengaged.

It is preferred however for drive to the platen of the JO press to be reversed as this allows the plastics material to be injected at lower pressure into the mould cavity, there being no need for the injection pressure to overcome any spring force of inertia force acting on the core.

The invention is made possible by the fact that injection moulding machines have advanced to the point where they allow the movement of the platen to be controlled with great speed and precision. The manner in which the moving platen is driven is not fundamental to the present invention and it may be either hydraulic or electrical. In the matter case, the moving platen may for example be driven by a screw rotated by an electric motor.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an injection moulding machine, and Figures 2 to 5 are sections through the two halves of the mould at different phases of the operating cycle.

The moulding machine shown in Figure 1 is generally conventional and will therefore only be described in the detail necessary to understand the injection compression method of the invention. The moulding machine 10 comprises two stationary bulkheads 12 and 14 connected to one another by four tie bars 16. The mould 18 shown in Figure 1 (which - 4 is not ir accordance with the present invention) is formed of two halves, namely a stationary half 18a mounted On the bulkhead 12 and a movable half 18b mounted on a platen 20 that can slide along the tie bars 16. The platen 20 is moved towards and away from the bulkhead 12 by a hydraulic ram 22 that is mounted on the bulkhead 14 and is connected to the platen 20 by a toggle mechanism which comprises levers 24a pivoted on the bulkhead 14, levers 24b pivoted on the platen 20 and levers 24c pivoted on the ram 22, the lo other ends of all three levers 24a, 24b and 24c being pivoted to another. The levers of the toggle mechanism are shown in their position when the mould cavity is open and to close the mould cavity, the ram moves to the right as viewed so that the levers 24c move into a more vertical position and act on the levers 24a and 24b to move them into alignment with one another, thus moving the platen 20 and the mould part 18 towards the closed position.

A heated screw feed mechanism 30 heats and compresses granules drawn from a hopper by rotation of the screw to form a plastics melt and the screw can also be moved axially to inject the melt into the mould cavity through a set of runners.

As earlier indicated, the machine of Figure 1 is already known for injection moulding. Conventionally, the mould cavity is closed and the injection screw is advanced to provide all the necessary pressure to inject sufficient melt to fill the cavity. After the plastics material has set in the mould, it is opened, the formed article is ejected and a new cycle is commenced.

The present invention is implemented by substituting for the mould halves 18a and 18b, the mould halves shown in Figures 2 to 5 and by suitably controlling the ram 22 so that the moving platen moves first to the right, and then slightly to the left before again moving to the right. 5 -

The first movement to the right closes the mculd cavity and expels air from it. The movement to the left expands the mould cavity and coincides with the introduction of the molten plastics material into the cavity. The last movement to the right compress the plastics material and forces it to fill and pack the mould. The advantage of compressing the plastics material by moving the mould halves together is that it is possible to mould article with much greater length to thickness ratio.

Referring now to Figures 2 to 5, a mould for forming a drinking cup comprises a first mould half 50 mounted on the moving platen 20 and a second mould half 60 mounted on the stationary bulkhead 12. The first half 50 is itself formed of two parts, namely a plate 52 formed integrally with a core 54 and a pre-close plate 56 that can move axially relative over the core and seals against it in all positions. Springs 58 (which could be replaced by hydraulic or pneumatic actuators) urge the core plate 52 and the pre close plate 56 apart.

The second mould half 60 is formed with a cup-shaped recess 62 of complementary shape to the core 54. In the closed mould position shown in Figure 5, the recess 62 defines in conjunction with core 54 and the preclose plate 56 the mould cavity having the desired shape of the finished article. A valve or gate 64 controls the introduction of molten plastics material into the mould cavity.

The purpose of the pre-close plate 56 is to close the mould cavity while still allow the core 54 to move to vary its volume. As the two mould halves 50 and 60 are brought together by movement of the platen 20, the pre-close plate 56 seals against the second mould part 60 and thereafter the core plate is advanced until the springs 58 are fully compressed and air is displaced from the mould cavity as shown in Figure 3.

When the core 54 is retracted, the cavity is expanded as shown in Figure 4. It should be understood that this view is only diagrammatic and that the degree of retraction has been exaggerated in the interest or clarity. Figure 4 shows the valve 64 open and a quantity of plastics material 66 within mould cavity. It should also be understood that at this stage the plastics material fills the base Of the would and has travelled as far as its viscosity will allow it up the side walls of the cup.

As earlier mentioned, the movement of the core plate 52 is preferably caused by the retraction of the platen 20 which is timed to coincide with the injection of plastics material but it could alternative be caused by the injection of the plastics material under pressure.

When the core 54 is again move towards the second mould half, as shown in Figure 5, the injected plastics material is compressed to pack the mould cavity.

The pre-close plate 56 can include formations that define the shape of the finished article. For example, in the case of the illustrated mould for making a drinking cup, it is possible to form the rim of the cup by an annular recess that is formed either the second mould half 60 or in the pre-close plate 56.

Claims

1. Apparatus for injection compression moulding of an article which comprises a press having a pair of relatively movable platens and a mould formed of two separable halves mounted on the respective platens and defining between them a mould cavity, wherein one mould he' f is Armed with a core and a pre-close plate movable axially relative to the core such that, during closure of the mould, the pre-close plate lo contacts the other mould half and closes the mould cavity before the core reaches its end position, characterized by means for controlling the relative movement of the platens of the press such that the two mould halves are first advanced by the press towards one another to reach an end position in which the volume of the mould cavity is minimised, the core and one of the platens together retract away from the other platen to increase the volume of the mould cavity at the same time as plastics material is admitted into the mould cavity, and the two mould halves are subsequently advanced towards one another a second time to compress the injected plastics material and force it to fill the mould cavity.

2. Apparatus as claimed in claim 1, wherein the pressure used to inject plastics material into the cavity serve to force the retraction of the core.

3. Apparatus as claimed in claim 1, wherein the drive to the platen is to be reversed in order for the core and the platen to retract away from the other platen.

4. Apparatus for injection compression moulding of an article, constructed and adapted to operate substantially as herein described with reference to and as illustrated in Figures 2 to 5 of the accompanying drawings.