GB2430643A

GB2430643A - Injection impact compression moulding

Info

Publication number: GB2430643A
Application number: GB0519939A
Authority: GB
Inventors: Peter Reginald Clarke
Original assignee: Im Pak Technologies Ltd
Current assignee: Im Pak Technologies Ltd
Priority date: 2005-10-03
Filing date: 2005-10-03
Publication date: 2007-04-04
Also published as: GB0519939D0

Abstract

A method for injection impact compression moulding an article. A mould is placed between the relatively movable platens of a press, the mould comprising a cavity plate 18 with a depression 40, a core plate (12 Fig 2) having a projecting core (46) at least part of the outer surface of which is cylindrical, and a closure plate 14 movable relative to the core plate (12, Fig 2) and the cavity plate and having a surface in sealing contact with the cylindrical outer surface of the core 46. When the mould is closed, the core 46, the cavity plate depression (40) and the closure plate 14 together define a mould cavity having the shape of the article to be moulded. The method comprises the steps of moving the platens of the press to close the mould, injecting a predetermined dose of plastics material into the mould cavity to fill the mould cavity only partially, allowing the core 46 to rebound away from the cavity plate 18 during the injection of the plastics material into the cavity to expand the volume of the cavity, and advancing the core plate (12, Fig 2) by means of the press to reduce the volume of the cavity after the completion of the injection to compress the injected plastics material to fill the mould cavity. In the invention, the press is controlled, during the movement of the core 46 to compress the injected plastics material, to vary the velocity of the core as a function of the position of the core relative to the cavity plate depression. A further embodiment relates to a moulding machine for carrying out said method.

Description

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INJECTION IMPACT COMPRESSION MOULDING

Field of the invention

The present invention relates to a method and apparatus for injection impact compression moulding (herein referred to as I2CM)

Background of the invention

The I2C moulding process is already known from the Applicants' earlier International Patent Applications W002/058909 and W005/068157 and is useful for forming articles having a large length to thickness ratio.

The mould used in the I2CM process comprises a cavity plate, a core plate and a closure plate movable relative to one another by the platens of an injection moulding press.

The cavity plate has a depression of the same shape as the outer surface of the article to be moulded. The plate has a projecting core which may be received within the depression of the cavity plate, the outer shape of the core corresponding to the shape of the inner surface of the article to be moulded. The closure plate is provided to seal between the core and the depression of the cavity plate so that the mould cavity can remain sealed in different positions of the core plate.

In operation, the mould is closed by applying a relatively small force to the core plate. Plastics material is injected under pressure into the mould cavity to fill the cavity only partially and during this injection the core plate moves back from the cavity plate to allow the plastics material to be admitted. Once a predetermined dose of the plastics material has been injected into the cavity, the core plate is once again moved in a direction to reduce the volume of the mould cavity but this time with sufficient force to cause the plastics material to flow into the thin walled sections of the cavity and to fill the entire cavity.

The rebounding of the core plate followed by its advancing a second time towards the cavity plate is referred to as a shuffle and characteristic of the I2CM process. A closure plate separate from the cavity plate and the core plate is required because it is essential during this shuffle for the cavity defined between the core, the depression and the closure plate to remain sealed so that no plastics material can escape.

The I2CM process is not to be confused with injection compression moulding (1CM) which is used to create articles, such as compact discs and lenses, where compression is required after injection primarily to compensate for the shrinkage of the plastics material as it cools. In the 1CM process, at the end of the injection phase, the mould cavity is completely full and as the plastic material cools down the volume of the cavity is reduced to achieve a moulded article having good optical properties. By contrast, in the I2CM process, the mould cavity is at first only filled partially with plastics material and the closure of the mould by the press platens is relied upon to force the plastics material rapidly into the thin-walled regions into which the injection pressure alone would not allow it to penetrate.

Hitherto, the I2CM process has been used for moulding articles such as the drinking cups and margarine tubs having very thin walls (less than 1 mm) . The process is not however restricted to articles having very thin walls and can equally be used for articles such as test tubes and bottle preforms which have relatively thick walls, but still a large length to thickness ratio.

Moulding a long thin cylindrical article with a significant wall thickness by the I2CM process should in principle be no different from moulding a drinking cup or a margarine tub. However, in practice the difference is that a shuffle of only a few millimetres is required when a cup is being formed but the shuffle movement may need to be a few centimetres when forming a test tube or a bottle preform.

The present invention is predicated on the realisation that more accurate control is required over the movement of the core during this extended shuffle movement if mouldings of consistently high quality are to be achieved.

Summary of the invention

In accordance with one aspect of the present invention there is provided a moulding machine comprising a press having relatively movable platens and a mould mounted between the platens and defining a cavity for injection impact compression moulding of an article, the mould comprising a cavity plate formed with a depression, a core plate having a projecting core at least part of the outer surface of which is cylindrical and a closure plate movable relative to the core plate and the cavity plate and having a surface in sealing contact with the cylindrical outer surface of the core, characterised in that means are provided in the press or in the mould to enable the relative velocity of the core and cavity plates to be varied as a function of the distance between the core and cavity plates as the plates of the mould approach one another to close the cavity.

In accordance with a second aspect of the invention, there is provided a method of injection impact compression moulding an article, which comprises placing a mould between the relatively movable platens of a press, the mould comprising a cavity plate formed with a depression, a core plate having a projecting core at least part of the outer surface of which is cylindrical, and a closure plate movable -4-.

relative to the core plate and the cavity plate and having a surface in sealing contact with the cylindrical outer surface of the core, the core, the cavity plate depression and the closure plate together defining when the mould is closed a mould cavity having the shape of the article to be moulded, the method comprising the steps of moving the platens of the press to close the mould, injecting a predetermined dose of plastics material into the mould cavity to fill the mould cavity only partially, allowing the core to rebound away from the cavity plate during the injection of the plastics material into the cavity to expand the volume of the cavity, and advancing the core plate by means of the press to reduce the volume of the cavity after the completion of the injection to compress the injected plastics material to fill the mould cavity, characterised in that the relative movement of the core and cavity plates is controlled at least during the compression of the injected plastics material, such that the relative velocity of the core and cavity plates varies as a predetermined function of the distance between them.

Preferably, the core is at first accelerated to reduce the mould cavity volume rapidly and is subsequently decelerated as the core approaches its final position.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figures 1 to 11 all show a section through a mould of the present invention at different stages in the moulding of a preform by the I2CM process.

Detailed description of the preferred embodiment

The mould shown in the drawings is made up of two sets of plates which are shown in Figs. 1 and 2 separated from one another. The set of plates shown to be left is mounted on the moving platen of an injection moulding press while the set of plates shown to the right is mounted on the fixed platen, which is connected to an injection screw (not shown) The set of plates mounted on the movable platen of the injection moulding press comprises a support plate 10, a core plate 12, a closure plate 14 and a splits plate 16.

The stationary set of plates comprises a cavity plate 18, a hot runner plate 20 and a dosing cylinder plate 22. Unlike the plates mounted on the moving platen, the stationary set of plates never move relative to one another and may be regarded as a single subassembly. The purpose of the hot runner plate 20 and the dosing cylinder plate 22 is to inject a predetermine dose of plastics material into the mould cavity at the appropriate time in the operating cycle to be described below.

Within the stationary subassembly, a molten plastics material is injected by the injection screw into the dosing cylinder plate 22. The latter incorporates valving 24 to direct the incoming plastics material into a dosing cylinder 26 of predetermined capacity. Once the dosing cylinder 26 is full, the valving 24 is operated to disconnect the cylinder 26 from the injection screw and connect it instead to a depression 40 in the cavity plate 18 which constitutes the outer surface of the mould cavity. A piston 28 connected to an actuator 30 displaces the molten plastics from the cylinder 26 and the plastics material then flows by way of heated conduits in the hot runner plate 20 to the mould cavity.

The support plate 10 fixed to the moving platen of the injection moulding press has mounted within it an air cylinder 42. A horizontal platform 44 is also bolted to the underside of the support plate 10, its purpose being to carry and guide the core plate 12.

The core plate 12 has a core 46 projecting forwards from it, the core defining the inner surface of the mould cavity. The core 46 comprises a hollow cylinder with a closed hemispherical front end and its rear end has a flange sandwiched between separate plates 12a and 12b that are permanently secured to one another to form the core plate 12. The two-part construction of the core plate 12 is to permit a cooling medium to flow through the core 46. The core plate 12 is mounted on a carriage 48 that follows tracks 49 of the support platform 44 thereby enabling the core and the core plate to move relative to the support plate 10 from left to right in the plane of the drawing.

The closure plate 14 is guided for movement relative to the core plate 12 on pillars 50. The closure plate 14 carries a sealing element 52 that mates with the cylindrical outer surface of the core 46 and seals against it at all times. The closure plate 14 is suspended from two arms 60 (only one been visible in the drawings) which rest with their rear end on guide surface 61 on the support plate 10 and which ride at their front end on guide surfaces 62 secured to the cavity plate 18. The purpose of the support arms 60 is to ensure correct alignment of the core 46 with the cavity plate 18. Also guided on the support pillars 50 is the splits plate 16 which carries a pair of splits 64.

Pins 70 are mounted on the closure plate 14 and pass through the splits plate 16 to engage in bores 72 formed in the cavity plate 18. Hydraulically operated collets (not shown) near the entrance to the bores 72 grip and preferably apply a preload to the ends of the pins 72, to clamp the closure plate 14 and the splits plate 16 to the cavity plate 18 at certain times during the operating cycle.

At the cornniencement of an injection moulding cycle, the various components of the mould adopt the positions shown in Figure 1. Here, the core plate 12 rests against the support plate 10 and the splits plate 16 rests against the closure plate 14, while a gap remains between the core plate 12 and the closure plate 14. At this point, the mould is fully open and the article moulded in the preceding cycle has just been ejected off the core 46. It will be noted that the splits 64 have a conically tapering extensions received in the sealing member 52 of the closure plate so that the splits 64 are held together radially.

The first step in the cycle is shown in Fig. 2 in which the air cylinder of 42 is activated to push the core plate 12 away from the support plate 10.

In the next step, shown in Fig. 3, the moving platen of the injection moulding press advances the core plate 12, the closure plate 14 and the splits plate 16 towards the cavity plate 18. The first contact occurs between the arm 60 and the guides 62 and this ensures that all the plates are aligned with one another and parallel to one another.

When the pins 70 of the cavity plate 14 engage in the bores 72 of the cavity plate 18, the collets mentioned previously are operated to lock the closure plate 14 and the splits plate 16 to the cavity plate 18.

It will be noted that the splits 64 also have formations projecting from their front end which are received in a conically tapering recess in the opposing face of the cavity plate 18. Once again, this interlocking prevents the splits from separating while the mould is closed. The locking of the closure plate 14 and the splits plate 16 to the cavity plate 18, not only maintains the cavity sealed but prevents the splits 64 from separating during the subsequent movements of the core plate 12.

Continued advancement of the support plate 10 by the moving platen of the injection moulding press displaces the core plate 12 to the position shown in Fig. 4 in which the core fully penetrates into the depression 40 of the cavity plate 18 and reduces the mould cavity to its minimum volume, this volume corresponding to the shape of the desired preform. The preform has a test tube like end portion defined between the depression 40 and the core 46 and a screw threaded portion defined between the splits 64 and the outer surface of the core 46. The axial end of the cavity is defined by the closure plate 14. The pressure being applied at this point to maintain the cavity closed is only the pressure of the air cylinder 42, not the pressure of the press of the injection moulding press.

In Fig. 5, the actuator 30 is operated to displace the piston 28 and inject plastics material in the manner described previously from the cylinder 26 into the mould cavity. As the plastics material is injected, it cannot flow into the thin walled portions of the cavity and instead the pressure build-up causes the core 46 and the core plate 12 to rebound away from the cavity plate 18, thereby widening the gap between the core plate 12 and the closure plate 14 while closing the gap between the core plate 12 and the support plate 10. During this time, the closure plate cannot move because it is locked to the cavity plate 18 by the collets acting on the pins 70.

With plastics material now filling the bottom of the cavity and filling the sides of cavity either only partially or not at all, the core 46 is driven into the cavity by the force of the press of the injection moulding press acting on the support plate 10. This movement forces the plastics material present in the cavity up the side walls and into the screw thread defined by the splits 64. Because the cavity was reduced to its minimum volume before plastics material was injected into it, little air remains in the cavity prior to the cavity being filled and such air as is present can readily be vented during the compression of the plastics material.

As earlier mentioned, the length of the compression stroke of the core when manufacturing a preform for a two litre bottle is of the order of 2. 5 cms, which is considerably greater than when making thin-walled cups and margarine containers. With thin-walled articles, it was only found necessary to regulate the pressure applied to the core plate to ensure that it was sufficient to overcome the resistance to flow of the plastics material. However, with the longer stroke required to manufacture a bottle preform, it has been found that, to achieve mouldings of consistently high quality, it is not sufficient to regulate the pressure of the press but it is important to regulate the velocity profile of the core. Hitherto, because of the constant pressure and the geometry of the press, the velocity merely decreased monotonically as the core approached its final position. By contrast, it has been found that at the commencement of the compression stroke, it is desirable for the core velocity to be increased and for the core to be decelerated more sharply as it approaches its final position. In this way, the bulk of the cavity is filled as rapidly as possible, allowing little time for the skin to cool down in the cylindrical part of the preform and the final stage of the compression at high pressure ensures that material continues to flow to form the screw thread at the end of the cylindrical portion. One may employ a press having control over the speed of movement of the platen, as the velocity profile may need to be modified to suit the particular configuration of the article being moulded, but if the press does not offer such functionality then the same - 10 - result may be achieved by incorporating additional hydraulic cylinders within the mould to move parts of the mould relative to the machine platen with controllable velocity.

The closed position of the mould is shown in Figs. 7 and 8. In both these figures, the plastics material which has been injected into the cavity has been compressed by the movement of the core and now occupies the entire mould cavity. Fig. 8 shows that while the mould is in this position the valving 24 is reversed so that once again plastics material from the injection screw is readmitted into the dosing cylinder 26, in readiness for the next cycle.

In the next step, shown in figure 9, the mould is fully opened by first releasing the collets acting on the pins 70 and then withdrawing all four of the movable plates 10 to 16 away from the cavity plate 18. At this stage, the splits 64 have still not been separated and therefore the moulded article is pulled out of the depression 40 in the cavity plate 18 and remains around the core 46.

As shown in Figure 10, the closure plate 14 is next moved away from the core plate 12 to commence the ejection of the moulded article off the core 46 and this ejection is continued in the next step, shown in Fig. 11, by the movement of the splits plate 16 away from the closure plate 14. Once the interlock between the splits 64 and the sealing member 52 has been disengaged, the splits 64 are separated by an actuator (not shown) to release the formed article.

Once the splits plate 16 returns to its position against the closure plate 14, after the moulded article has been ejected, the components again adopt their position shown in Fig. 1, ready for the commencement of the next operating cycle.

Claims

- 11 - CLAIMS

1. A moulding machine comprising a press having relatively movable platens and a mould mounted between the platens and defining a cavity for injection impact compression moulding of an article, the mould comprising a cavity plate formed with a depression, a core plate having a projecting core at least part of the outer surface of which is cylindrical and a closure plate movable relative to the core plate and the cavity plate and having a surface in sealing contact with the cylindrical outer surface of the core, characterised in that means are provided in the press or in the mould to enable the relative velocity of the core and cavity plates to be varied as a function of the distance between the core and cavity plates as the plates of the mould approach one another to close the cavity.

2. A method of injection impact compression moulding an article, which comprises placing a mould between the relatively movable platens of a press, the mould comprising a cavity plate formed with a depression, a core plate having a projecting core at least part of the outer surface of which is cylindrical, and a closure plate movable relative to the core plate and the cavity plate and having a surface in sealing contact with the cylindrical outer surface of the core, the core, the cavity plate depression and the closure plate together defining when the mould is closed a mould cavity having the shape of the article to be moulded, the method comprising the steps of moving the platens of the press to close the mould, injecting a predetermined dose of plastics material into the mould cavity to fill the mould cavity only partially, allowing the core to rebound away from the cavity plate during the injection of the plastics material into the cavity to expand the volume of the cavity, and advancing the core plate by means of the press to reduce the volume of the cavity after the completion of the injection to compress the injected plastics material to fill - 12 - the mould cavity, characterised in that the relative movement of the core and cavity plates is controlled at least during the compression of the injected plastics material, such that the relative velocity of the core and cavity plates varies as a predetermined function of the distance between them.

3. A method as claimed in claim 2, wherein the core is at first accelerated to reduce the mould cavity volume rapidly and is subsequently decelerated as the core approaches its final position.