GB2458876A

GB2458876A - Metering plastics material in method of injection impact compression moulding

Info

Publication number: GB2458876A
Application number: GB0803771A
Authority: GB
Inventors: Philip Hopping; Neil Hodgson
Original assignee: Im Pak Technologies Ltd
Current assignee: Im Pak Technologies Ltd
Priority date: 2008-02-29
Filing date: 2008-02-29
Publication date: 2009-10-07
Also published as: GB0803771D0

Abstract

An improvement is disclosed to the method of injection impact compression moulding articles in which plastics material is injected to partially fill each mould cavity of a multi-cavity mould and the cavity volume is then minimised rapidly to force the plastics material to flow into thin walled sections of the cavity, defined between a mould core 62 and wall of a stationary cavity assembly 52. Instead of metering the plastics material separately to each cavity, a predetermined quantity of plastics material is injected into the cavities by an injection screw or into a single shooting pot (runner) connected to all the mould cavities, the quantity of plastics material injected being set to be equal to or in excess of the quantity required to fill the mould cavities when reduced to their minimum volumes. To equalise the doses received by the individual cavities, the feed gates 55 of the cavities are maintained open during a first part of the impact compression stroke. This allows plastics material to flow between the individual cavities through the hot runner system and thereby equalise the back pressure in all the cavities.

Description

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METHOD OF INJECTION IMPACT COMPRESSION MOULDING

Field of the invention

The present invention relates to a method of injection impact compression moulding.

Background of the invention

In conventional injection moulding, a molten plastics material is injected into a fully close mould until the mould is filled to its maximum capacity. If a multi-cavity mould is used, the plastics material from a single injection screw can be supplied in parallel to all the mould cavities.

As soon as a cavity is filled to its maximum capacity, the flow from the injection screw is automatically diverted by the back pressure to another cavity until all the cavities are full. Consequently, no special steps need to be taken to ensure that the correct dose is metered to each individual cavity.

In injection compression moulding, a plastics material is injected tc fill fully a cavity while it is closed, but bounding a volume greater than that of the article to be moulded. As the plastics material cools, it tends to shrink and, to compensate for this, a force is applied to reduce the volume of the mould so as to compress the plastics material as it is cooling. In this way, it is possible to form articles, such as optical lenses, where the shape and surface finish are of critical importance. Because, once again, the injected plastics material fully occupies the mould cavity, no special steps are needed to meter the plastics material to individual cavities when a multi-cavity mould is used.

Injection impact compression moulding is used to form article having wall sections with a large length to thickness ratio. Because of these thin walled sections, the pressure available from an injection screw cannot be used to fill the mould cavity because the molten plastics material would solidify before it has reached the ends of the thin walled sections. Instead, as is the case in injection compression moulding, the plastics material is injected into the mould cavity while its volume is greater than that of the article to be formed. However, in contrast to injection compression moulding, the plastics material only partially fills the mould cavity and does not enter the thin walled sections. The mould is subsequently closed rapidly to force the injected plastics material to flow into the remaining thin walled sections of the mould cavity. For example, if forming a cup, the initially plastics material lies only in is the base of the cup and the closing of the mould forces the plastics material to flow along the conical wall of the cup until it reaches the upper rim. The impact compression is analogous to the forging of objects made of metal.

The important difference to notice in the case of injection impact compression moulding is that the plastics material does not fill the cavity at the end of the injection phase. For this reason, when using a multi-cavity mould it has hitherto proved essential to meter the doses of plastics material individually to the different cavities.

Such metering has been achieved by the use of piston-cylinder units, termed shooting pots, in the hot runner system. Several shooting pots, each associated with a respective one of the cavities, are filled in parallel to their maximum capacity by the injection screw. The shooting pots are then isolated by valves from the injection screw and each then supplies the metered dose of the plastics material that it contains to its associated cavity.

Object of the invention The present invention seeks to provide a method of injection impact compression moulding that dispenses with the need to meter doses of plastics material to the different cavities of a multi-cavity mould.

Summary of the invention

According to the present invention, there is provided a method of injection impact compression moulding articles, which comprises providing a multi-cavity mould formed of at least two relatively movable parts, providing an feed gate in the mould leading to each cavity, providing a hot runner is system connecting the feed gates of the cavities to a common injection screw, injecting a plastics material by means of the injection screw a predetermined quantity of plastics material to fill all the cavities partially before the parts of the mould have been brought together to the extent necessary to minimise the volume of the cavities, and applying a force to the mould to reduce the volume of all the cavities rapidly so as to compress the plastics material injected into each mould cavity, causing the plastics material to flow and to occupy the entire volume of the cavity, characterised by setting the predetermined quantity of plastics material to be equal to or in excess of the quantity required to fill the mould cavities when reduced to their mrnimum volumes, and maintaining the feed gates of the cavities open during a first part of the application of the force to the mould to compress the plastics material, so as to allow plastics material to flow between the individual cavities through the hot runner system, so as to equalise the back pressure in all the cavities.

The total quantity of plastics material injected into all the cavities is preferably calculated to exceed slightly the dose of plastics material required to fill all the mould cavities after they have been reduced to their minimum volume. On completion of injection, some cavities may contain more than the required dose and others less. During the initial phase of the compression step a back pressure will be build up in each cavity, dependent upon how far the plastics material has managed to travel into the gaps in the cavity within which the thin walled sections of the finished article are formed. Because all the cavities are connected to one another by the hot runner system, the flow through the feed gates will equalise the back pressures and ensure that the plastics material partially fills all the cavities to the same extent. The teed gates are then closed simultaneously to ensure that all the cavities contain the same dose of plastics material and, in the final part of the compression step, this dose is compressed further to fill the mould cavity fully to form the finished article.

If the metered quantity of plastics material is in excess of that required to fill the mould cavities when they are at their minimum volume, it is possible that the excess could be accommodated by the elasticity of the hot runner system, It is however preferred to provide the hot runner system with an expansion chamber comprising a spring biased piston.

Though one may be able to rely on the injection screw to meter the desired quantity of plastics material in each operating cycle, it is preferred to provide the hot runner system with a shooting pot having a piston and an actuator for metering the totai quantity of molten plastics material injected into the mould cavities.

In the case of the latter embodiment, it is possible by placing a spring between the piston of the shooting pot and the actuator to permit the shooting pot to double as an expansion chamber.

Brief description of the drawings

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 a conventional moulding machine, Figure 2 is a section through a mould when fully Op Cfl, Figure 3 is a section through the mould of Figure 1 immediately prior to injection of the plastics melt, Figure 4 is a section through the mould of Figure 1 during the injection of the plastics melt, Figure 5 is a section through the mould of the preceding Figures with the mould cavity fully closed, and Figure 6 shows a runner system which has been modified to permit plastics material to be expelled from the mould cavities.

Detailed description of the preferred embodiment(s) The moulding machine shown in Figure 1 is generally conventional and will therefore only be described in the detail necessary to understand the Injection Impact Compression (110) method of the invention. The moulding machine 10 comprises two stationary bulkheads 12 and 14 connected to one another by four tie bars 16. A conventional mould 8 is shown which is formed of two parts, namely a C stationary part l8a mounted on the bulkhead 12 and a movable parts l8b mounted on a platen 20 that cart 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 s 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 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 ic 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.

This known method of operation has its limitation and cannot be used to form articles that have a very thin wail section. This is because during injection of the plastics material it cools very rapidly on contact with the mould surface and creates a large back pressure that prevents the plastics material from filling the entire cavity.

In the present invention, the injection screw is not relied upon tc produce enough pressure to fill a closed mould cavity. Instead, the screw is used to inject a dose of the melt into the mould while the mould cavity is not closed under pressure and subsequently the mould parts are brought together rapidly using the ram 22 to "forge" the plastics material and force it rapidly to fill every part of the mould cavity.

An operating cycle will now be considered into greater detail with reference to Figures 2 to 5.

In place of a conventional two part mould 18a, 18b as described by reference to Figure 1, an injection impact compression mould assembly 50 comprising four components that can move relative to one another. The first of the four components is a statLionary cavity assembly 52 that defines the cavity 54 and is formed with a feed gate 55 through 0 which the plastics melt is injected into the cavity. The cavity assembly is fixed to the bulkhead 12. The feed gate and the control pin that opens and closes the gate will be described in more detail below in the context of the manner of introducing an accurate dose of the plastics melt into the mould cavity 54.

The other three components, which together constitute the core assembly, are mounted on the moving platen 20 of the moulding machine. A first of the three components, herein termed the pressure plate, is designated 56 and is fixed to the moving platen 20. The second of the components is termed a rim closure plate and is designated 58 in the drawings. The rim closure plate 58 is biased away from the pressure plate 56 by relatively strong springs 60 and is accurately aligned ano guided so that a projecting boss 59 engages in a recess 53 in the mould part 52 of the cavity assembly. The last of the components of the mould is a core 62 which partly defines the mould cavity and has a cylindrical portion that slides freely through and is accurately guided within a through bore formed in the rim closure plate 58.

The core 62 at its end remote from the mould cavity is formed with an enlarged head 64 that is received in the 3s manner of a piston in a chamber 66 formed in the pressure plate 56. A weak spring 68 located in the chamber 66 urges the core 64 away from the pressure plate 55 towards an annular stop plate 70 that is fixed to the pressure plate 56 and surrounds the cylindrical region of the core 62. The enlarged head 64 trapped between the pressure plate 56 and the stop plate 70 forms a lost motion coupling arranged in the line of action between the core 62 and the machine platen 20.

In the embodiment of Figures 2 to 5, the head 64 does not form a seal with the chamber 66 and the stop plate 70 o does not seal against the core 62. Instead, small clearances allow air to escape while damping the movement of the core 62.

Figure 2 shows the mould in the position at the end of is one cycle and the commencement of the next. The cavity is open and the formed article, in this case a drinking cup, is ejected from the cavity in a conventional manner (not shown) . The toggle mechanism now moves the core assembly towards the cavity assembly until the position shown in Figure 3 is reached or at least nearly reached. In this position the boss 59 of the rim closure plate 58 is fully engaged in the recess 53 of the cavity assembly 52 and the strong springs 60 ensure the cavity is fully contained against the egress of plastics melt from the mould cavity even though the core 62 can still move to allow the cavity volume to vary.

In the next step, plastics melts is introduced at relatively low pressure into the mould cavity through the feed gate 55. At this point, the pressure of injection of the plastics melt can push the core 62 back against the action of the weak spring 68. The injection can be timed to occur just before or just after the core reaches the bottom of the cavity 54 so that as the melt enters the cavity it spreads into the corners of the cavity without trapping any gas between the melt and the corners of the mould. The injection pressure is not however sufficient to force the plastics melt into the narrow parts of the mould cavity, in this case the conical wall of the drinking cup.

Lastly, the pressure plate 56 is moved by the platen 20 s to apply a force directly to the core 62 after full compression of the weak spring 68. The pressure resulting from the movement of the core under the force of the hydraulic ram 22 as magnified by the mechanical advantage of the toggle levers is sufficiently great to forge the melt ano. thereby fIll all parts of the mould.

The term "forge" is used in order to stress the speed of closing the mould and the rate of pressure increase within the mould cavity during the closing process.

Typicaily, the mould is closed and maximum pressure is reached within the cavity within a period of less than 0.5 seconds and preferably less than 0.3 seconds. By contrast, in injection compression moulding, after the plastics material has been injected under pressure to fill a major part of the mould and cavity, the pressure is only ramped up progressively to flow the plastics material to fill the remainder of the mould.

When plastics material is normally injected into a mould having multiple cavities, the melt follows the path of least resistance. Thus the melt will first flow to the cavity nearest the feed screw and as that cavity fills its resistance increases so that the melt flows to the other cavities, this being repeated until all the cavities are full. In injection impact compression moulding, by contract, the melt meets little resistance even after a cavity has received its full dose of plastics material and one cannot rely on back pressure to distribute an equal dose to all the cavities.

The process of injection impact compression moulding as described so far is the same as described in EP 1360057. The -10 -solution offered in the latter patent to the problem of balancing the dose of plastics material delivered to the individual cavities of a multi-cavity mould is to use a separate shooting pot for each ot the mould cavities. A metered dose of the plastics material is stored in a working chamber of the shooting pot and this dose is subsequently injected into the cavity. This solution requires the use of multiple shooting pots and increases the cost considerably because each shooting pot requires its own actuator.

The present invention overcomes this problem by omitting the individual shooting pots and instead metering a predetermined quantity of molten plastics material to all the cavities. The total dose should exceed slightly the m quantity required to fill all the mould cavities when they have reached their minimum volume. The dosing is not critical and one can either rely on the injection screw to meter the desired quantity to all the cavities or provide a single shooting pot connected to all the cavities. It is also unimportant if any one cavity receives more or less than the required dose.

During the initial impact compression stroke, that is to say after the core 62 has come into contact with the pressure plate 56 but before the pressure plate has reached the closure plate 58, the feed gate 55 is left open. As a result, as the cores move towards the fully closed position shown in Figure 5, a back pressure will build up in each cavity dependent upon how far the plastics material has been forced into the parts of the cavities that form the thin-walled sections of the finrshed article. Overfilled cavity will develop a higher back pressure than cavity that have received less than the required dose. Because all the cavities are still connected to one another through the hot runner system, molten plastics material may flow out of some cavities and into others in order to balance the back pressures. This flow will ensure that the plastics material -11 -will have penetrated the thin walled sections to the same extent in all the cavities at the time that the teed gate is closed.

Because slightly more plastics material is injected into the mould cavities than is needed to form the finished articles, the excess plastics material has to be contained within the hot runner system. It is possible that the hot runner system may, without the riced to take additional steps, be capable of expanding sufficiently to accommodate this small excess. Alternatively, it is possible to attach an expansion chamber to the hot runner system, as shown in Figure 6.

Figure 6 shows a modified hot runner system 70. The hot is runner system 70 comprises a block 71 having several feed gates 55 at one end, each of which is opened and clcsed by a respective control pin or valve 72 connected to a piston 76.

A runner 74 within the block 71 leads from a port 78, which communicates with the injection screw, to all the feed gates 55. n expansion chamber 80 is connected to a branch of the runner 74. Within the expansion chamber 80, there is mounted a piston 82 acted upon by a strong spring 84.

When the pressure in the runner 74 rises, as a result of build up in the back pressure from all the open feed gates 55, the piston 82 is retracted into its chamber against the action of the spring 84, thereby limiting the pressure in the runner 74 and the cavities to the level needed for all the cavities to contain the desired doses.

The feed gates 55 are closed to complete the impact compression stroke and remain stroke for the remainder of the operating cycle. In the next cycle, as soon as the feed gates open, the piston 82 returns to its rest position and returns the excess plastics material to the runner 74 for _5 injection into the cavities.

-1) -If desired, the expansion chamber can be realised by a shooting pot that meters the desired dose to all the cylinders but whose piston is connected to its actuator by a spring that acts in the same way as the spring 84 to allow the piston at the shooting pot to retract slightly to accommodate any excess plastics material that has been injected into the mould cavities. Such an embodiment would offer the advantage the excess of plastics material in the runner would not increase with each operating cycle as the o quantity of plastics material remaining in the shooting pot would simply reduce the quantity needed to refill the shooting pot in the next operating cycle.

Claims

-13 -CLAIMS1. A method of injection impact compression moulding articles, which comprises providing a multi-cavity mould formed of at least two relatively movable parts, providing an feed gate in the mould leading to each cavity, providing a hot runner system connecting the feed gates of the cavities to a common injection screw, injecting a plastics material by means of the injection screw a predetermined quantity of plastics material to fill all the cavities partially before the parts of the mould have been brought together to the extent necessary to minimise the volume of the cavities, and applying a force to the mould to reduce the volume of all the cavities rapidly so as to compress the plastics material injected into each mould cavity, causing the plastics material to flow and to occupy the entire volume of the cavity, characterised by setting the predetermined quantity of plastics material to be equal to or in excess of the quantity required to fill the mould cavities when reduced to their minimum volumes, and maintaining the teed gates of the cavities open during a first part of the application of the force to the mould to compress the plastics material, so as to allow plastics material to flow between the individual cavities through the hot runner system, so as to equalise the back pressure in all the cavities.
2. A method as claimed in claim 1, further comprising the step of providing the hot runner system with an :s expansion chamber comprising a spring biased piston.

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3. A method as claimed in claim 1, which further comprises providing the hot runner system with a shooting pot having a piston and an actuator for metering the total quantity of molten plastics material injected into the mould cavities.
4. A method as claimed in claim 4, which further comprises placing a spring between the piston of the shooting pot and the actuator to permit the shootrig pot to act as an expansion chamber.