Process and apparatus for injection moulding

Abstract: A process of producing an injection moulding of plastics material comprises introducing the plastics material in the form of a molten stream into a mould space, and at one or more selected positions injecting a predetermined, finite quantity of a non-plastics fluid under pressure into the stream of plastics material at a controlled rate, the fluid being injected through means (19;90) around which flows the stream of plactics material whereby the fluid is injected directly into the plastics material, the fluid thereafter flowing simultaneously with the plastics material between the mould surfaces to fill the mould space (12) and exerting a pneumatic pressure on the surrounding plastics material (8) to pressurise the plastics material outwardly towards the mould surfaces. The invention further provides an apparatus for produc­ ing an injection moulding in accordance with the process, and an injection moulding produced thereby.

Description: This invention relates to a process and apparatus for injection moulding. More particularly, the invention relates to the introduction of a stream of plastics material into a mould space and the introduction of a fluid under pressure into the path of the stream of plastics material filling the mould. This fluid creates a pressurized mass of fluid or a gas containing core within the plastics material for the purpose of applying an outward pressure on the plastics material urging it towards the mould surfaces until such time that the surrounding wall of plastics material is self-supporting. This outward pressure assists the mould space to be filled and to give the moulding an improved surface finish. A method and an apparatus of the kind stated in the first part of claims 1 and 18 respectively are known from prior art document FR―A―2 256 021. The method of manufacturing articles of synthetic resin disclosed in this citation comprises injecting a molten resin into a mould cavity in an amount less than the volume of the cavity and feeding a mass of fluid under pressure into the cavity, in such a way that the mass of fluid is enclosed by the resin. In one embodiment there is a fluid introducing device having an outlet connected to a pressurized chamber of a cylinder. A piston is mounted to slide in the cylinder and can pressurize a fluid supplied to the chamber from any source through a duct provided with a valve. In another embodiment an inlet is connected to a source of high pressure fluid controlled by a valve. Neither embodiment provides means for introducing a measured quantity of fluid and the controlled utilisation of the measured quantity, which has made it possible to produce moulded articles of a simple or complex design and which are of high quality. Also, U.S. Patent Specification No. 4101617 discloses a method for forming a hollow shaped body from a thermoplastic resin by injection molding. An injection nozzle is positioned within an aperture formed in a mould, and the thermoplastic resin is injected through the nozzle and into the mould cavity defined by the mould. After the mould cavity is partially filled with resin, a valve is adjusted to permit compressed gas to flow into the mould cavity through the injection aperture. In consequence, a resin bubble is formed within the mould cavity, and the compressed gas causes the bubble to expand until the outer walls of the bubble are pressed against the inner wall of the mould cavity. When the hollow shaped body has cooled below the softening temperature of the resin, the pressurised gas within the body is allowed to escape before the mould is opened. Again, there is no disclosure of introducing a measured quantity of the gas or the controlled utilisation of the measured quantity of gas. According to the invention there is provided a process of producing an injection moulded plastics article comprising holding a quantity of non-plastics fluid in a variable volume chamber, and reducing the volume of the variable volume chamber to pressurize the non-plastics fluid to a pressure sufficient to form at least one cavity or core of the non-plastics fluid in the plastics material injected into the mould space, characterised by the process including the steps of determining prior to introducing the plastics material into the mould space a quantity of the non-plastics fluid sufficient to achieve an intended weight reduction in the article being produced as well as a required internal pressure in the article being produced, increasing the volume of the variable volume chamber to hold the determined quantity of the non-plastics fluid, measuring said quantity of the non-plastics fluid by filling said variable volume chamber with the non-plastics fluid at a pressure determined in relation to the increased chamber volume, reducing during the introduction of the non-plastics fluid the volume of the variable volume chamber at a controlled rate to form the cavity or core, continuing to control the rate of reduction of the variable volume chamber to pressurize the non-plastics fluid after the cavity or core is initiated, and maintaining a non-interrupted flow of the non-plastics fluid at a pressure higher than that of the stream of plastics material yet not so high as to cause a rupture in the plastics material defining the cavity or core formed therein, until the plastics material is self-supporting. It is preferred that the introduction of the fluid is controlled so that the pressure of the fluid introduced remains higher than the pressure of the plastics material, to ensure a uniform introduction of the fluid throughout the period of introduction. Some or all of the fluid may be introduced into the stream of plastics material at one or more selected positions within the mould space, or alternatively upstream of the mould space. In one embodiment of the invention it is preferred that the fluid is a gas, which may be air but is preferably nitrogen or other inert gas. Alternatively, the fluid may be a liquid, but again it is preferable that the liquid is inert relative to the plastics material being moulded and not e.g. water which would require additional apparatus to control the effect of the steam produced on contact of the water with the hot plastics material. If desired the fluid may be a mixture of a liquid and a gas. In an embodiment in which the fluid is a gas, venting of the cavity preferably occurs after the solidification and cooling of the plastics material and before the mould is opened. Also, in a case in which at any one selected position the fluid introduced is a gas, the volume of the respective gas filled cavity may be formed by a portion only of the measured quantity of the gas, the remainder of the gas being introduced into the cavity to increase the pressure therein, whereby correspondingly increased outward pressure is applied to the plastics material after the flow of the plastics material through the mould space has stopped. In either case, the pressure in the or each cavity may be relieved by causing the gas to leave the cavity after the article has become self-sustaining. In another embodiment the fluid is a liquid blowing agent or a mixture of a gas and a liquid blowing agent which is injected into the stream of plastics material at a temperature which is below its decomposition temperature, the fluid being heated to its decomposition temperature by the plastics material whereby the fluid creates a closed cell cellular foam core with the gas exerting said pressure on the surrounding plastics material. It is preferred but not necessarily essential that the blowing agent is preheated before it is introduced into the plastics material. In a preferred embodiment the introduction of pressurised fluid commences prior to the termination of the flow of plastics material, whereby fluid and plastics material are introduced into the mould space simultaneously. The invention also provides an apparatus for producing an injection moulded plastics article, comprising a mould, a variable volume chamber, means connecting said variable volume chamber to said mould, means for increasing the volume of said variable volume chamber to receive a quantity of non-plastics fluid, filling means for filling said variable volume chamber with the quantity of non-plastics fluid, and reducing means to reduce the volume of the variable volume chamber to pressurize the non-plastics fluid to a pressure sufficient to form at least one cavity or core of non-plastics fluid in the plastics material injected into the mould space, characterised in that the variable volume chamber is a measuring chamber or is connected to a measuring variable chamber to measure a predetermined quantity of the non-plastics fluid, and in that controlling means are provided which act on said reducing means to control the rate of reduction of the volume of the variable volume chamber to introduce the non-plastics fluid at a controlled rate to form the cavity or core and to pressurise the non-plastics fluid after the cavity or core is initiated to maintain a non-interrupted flow of the non-plastics fluid at a pressure higher than that of the stream of plastics material yet not so high as to cause a rupture in the plastics material defining said cavity or core formed therein until the plastics material is self-supporting. Preferably the non-plastics fluid introducing means is a piston and cylinder arrangement, the cylinder being connected by a passageway projecting into the stream of the plastics material. The cylinder of the non-plastics fluid introducing means may be the measuring chamber for the fluid. Alternatively, the non-plastics fluid introducing means may include a second piston and cylinder arrangement connected in series and upstream of the first-mentioned piston and cylinder arrangement, the cylinder of the second piston and cylinder arrangement comprising a measuring chamber for the fluid for delivering a measured quantity to the variable volume chamber. Preferably the effective volume of the measuring chamber is adjustable to allow the measured quantity of the fluid to be varied. By way of example, specific embodiments in accordance with the invention will be described with reference to the accompanying diagrammatic drawings in which:― Figure 1 is a layout of a mould and a screw ram of an injection moulding press; Figure 2 is a layout of a mould and a screw ram of another injection moulding press; Figures 3 and 4 show alternative modifications of the injection moulding press of Figure 2; Figure 5 shows a different arrangement in which the fluid is injected at a position upstream of the mould space; and Figure 6 is an end view of part of the nozzle and the fluid injection passageway of the arrangement of Figure 3. Regarding Figure 1, a mould 4 for use in an injection moulding press has upper and lower mould parts 10,11 defining a mould space 12. The mould parts 10, 11 are mounted between a fixed platen 9 and a platen 7 movable by a hydraulic ram 5. Molten thermoplastics material 8 is introduced into the mould space 12 by a screw ram 13 having a nozzle 14, feed chamber 15, check ring 16, and heater 40 in known manner. The plastics material enters the mould space 12 from the nozzle 14 through a sprue 18, a runner 17 and a gate 6. During the moulding process it is desired to create a pressure within the plastics material 8 to urge the surrounding plastics material outwardly into contact with the mould surfaces and thereby assist the external surface of the plastics material to take up the precise nature of the mould surfaces. The pressure also assist the mould space to be filled, especially in the case of narrow or extended positions which are normally difficult to fill. This pressure is applied by a fluid injected at at least one selected position, under pressure, into the plastics material as it flows between the moulding surfaces. For this purpose a fluid injection passageway 19 extends, in this embodiment, through the lower mould part 11 into the mould space and protrudes upwardly within the mould space to a point between, preferably substantially mid-way between, the upper and lower surfaces. In this embodiment the fluid is a gas but it could be a liquid. The gas will be described as nitrogen which is the preferred gas since it is inert and reasonably cheap. To transmit the nitrogen to the passageway 19 there is provided a hydraulic ram 39 having a piston 20 and cylinder 21 connected to the passageway 19 via a solenoid operated valve 22, and to a nitrogen supply 23 via a non-return valve 24 (or a solenoid operated valve) and a pressure regulator 25. Downstream of the valve 22, the passageway 19 has a return connection 26 via another solenoid operated valve 27 either to waste or back to the nitrogen supply 23 so that at least some of the nitrogen is recirculated. The cylinder 21 constitutes a chamber for measuring the predetermined, finite quantity of nitrogen it is desired to inject into the plastics material through the passageway 19 in any particular moulding cycle, both to create the required internal pressure and reduction in weight of the resultant product. The stroke of the piston 20 is adjustable so that the effective volume of the cylinder 21 and thereby the quantity of nitrogen to be injected may be varied. The control means of the ram 39 includes a hydraulic pump 28 operated by an electric motor 29 and protected by a pressure relief valve 37, a two-directional solenoid operated valve 30, and a flow control valve 31 having a bypass 32 including a non-return valve 33. In operation the injection chamber 15 of the screw ram 13 is filled with sufficient molten plastics material to form the product to be moulded allowing for the cavity formed by the nitrogen as described below. The mould parts 10, 11 are closed and the moulding pressure applied by the platens 7 and 9 and the ram 5. The stroke of the piston 20 of the hydraulic ram 39 has been pre-set and is in its rear resting position, so that the cylinder 21 is filled with a predetermined quantity' of nitrogen from the supply 23. This quantity is either the minimum amount which is required to reduce the weight of the moulding to the desired level or a predetermined greater amount for the reason explained below. The pressure of the nitrogen in the cylinder 23 is at the pressure set by the pressure regulator 25. A pressure switch (not shown) is tripped indicating that the full clamping tonnage required is applied to the mould parts 10, 11. The plastics material in the injection chamber 15 is then introduced into the mould space 12 through the sprue 18, runner 17 and gate 6 by the screw ram 13. As soon as the plastics material 8 within the mould has passed and immersed the outlet end of the passageway 19, solenoid valve 22 is opened (valve 27 being closed), and the directional valve 30 operated to allow the ram 39 to inject the nitrogen in the cylinder 21 through the passageway 19 into the plastics material at a rate controlled by the flow control valve 31. The nitrogen is thereby injected approximately into the middle of the thickness of the plastics material and forms a channel or channels or other mass 38, as desired, therein. When the screw ram 13 has travelled its full stroke, the piston 20 may have some forward movement remaining, which is completed after the end of the introduction of plastics material. The result is that the nitrogen continues to be injected into the mass 38 formed within the plastics material after the mould space is full, whereby the pressure of the nitrogen in the mass 38 is increased and thereby the outward pressure applied on the plastics material urging it towards the mould surfaces to improve the plastics surface definition. Both rams 13, 39 remain in their full operative positions until the moulded part and the sprue have cooled and their surface skin is self-supporting. Ram 13 may then be withdrawn. Also the directional valve 30 is reversed to allow the ram 39 to return to its rest position, the oil from the ram 39 returning to tank through the non-return valve 33 and the piston 20 being moved by the pressure of the nitrogen in the passageway 19. On completion of the return movement of the piston 20, valve 22 is closed. The pressure of the nitrogen trapped in the cylinder 21, and also the pressure of the gas still downstream of the valve 22, both in the passageway 19 and the mass 38, has thereby halved compared with the previous pressure, i.e. if the pressure was 400 p.s.i. it is now 200 p.s.i. Valve 27 is then opened which relieves the remaining pressure of the nitrogen in the moulded part to the desired level which may be atmospheric pressure or above. The nitrogen emitted passes to atmosphere or at least some of it is returned to the supply 23 for recirculation during a subsequent moulding cycle. The feed chamber 15 of the screw ram 13 and the cylinder 21 of the ram 39 may also be refilled with plastics material and nitrogen respectively in readiness for the next operating cycle. Finally the clamping pressure on the mould is relieved and the mould opened to remove the moulding. The recirculation of some or all of the nitrogen extracted from the moulding during each operating cycle may be an advantage since the nitrogen returned will retain some of the heat from the previous cycle and thus result in the nitrogen injected during the next cycle having a temperature nearer that of the incoming plastics material than would otherwise be the case. It will be appreciated that the embodiment described above allows an exact amount of nitrogen or other gas to be metered and injected into the plastics material. Also the pressure of the nitrogen injected is dictated by the injection pressure of the plastics material. In each moulding cycle the plastics injection pressure may vary. However, the pressure on the nitrogen injection ram 39 will change, i.e. hunt, in accordance with the drift in the plastics pressure (either up or down) automatically subject to the relief valve 37 being set to a pressure value which is greater than the maximum plastics pressure expected. This means that the nitrogen pressure is maintained high enough that it will enter the plastics material and yet not be unnecessarily high, or so high that it would burst the wall or skin of the surrounding plastics material. It is a further advantage that the pressure of the nitrogen in the mass 38 can be increased after the mass is full. In the above embodiment it is envisaged that the flow of nitrogen through the passageway 19 would be continuous so that a single mass 38 of nitrogen is formed within the plastics material of the required size and at a desired position in the moulding both to exert the intended outward pressure on the plastics material and where a resultant cavity does not adversely affect the finished moulding. However, in another embodiment it may be desired to create two or more separate masses 38 of nitrogen in the moulding. This could be achieved by arranging for the flow of nitrogen through more than one position in which the passageway 19 enters the moulding space. It may be sufficient to allow the pressure of the nitrogen to dissipate itself either by return movement towards its entry point or by effecting movement of the adjacent internal mass of plastics material which is still flowable. It will also be appreciated that the chosen position for the or each passageway 19 through which nitrogen or other gas is injected into the mould space 12 may be different from the position shown in Figure 1. For example, the selected position may be adjacent the entry point of the plastics material. It may also be desirable that the angle at which `the nitrogen is injected is substantially with the flow of plastics material between the mould surfaces rather than transverse to said flow. This could be achieved by the exit port of the passageway 19 being in the side wall of the end of the passageway which protrudes upwardly into the mould space. Figure 2 is concerned with a similar embodiment to the embodiment of Figure 1, but employs a liquid blowing agent for injection into the plastics material 8 instead of nitrogen. The liquid blowing agent may be any such blowing agent which is commercially available for use in cellular injection moulding, but is preferably one which liberates nitrogen gas at its decomposition temperature. Thereby, as before, the fluid injected effects a weight reduction and an outward pressure on the plastics material urging it towards the mould surfaces. However, there is a different physical result in that the injected fluid does not form at least one mass within the plastics material which results in a cavity or cavities in the finished moulding, but rather decomposes to mix with the central portion of the plastics material to form a gas containing core 70 in similar fashion to conventional structural foam mouldings. But the present embodiment can provide an important advantage over conventional structural foam mouldings, since if the basic plastics material does not contain a blowing agent, the absence of blowing agent in the outer wall or skin of plastics material during the moulding cycle combined with the effect achieved by pressurising the plastics material against the mould surfaces, means that post-moulding treatment of the external surface of the moulding need not be required. The mould and screw ram of the moulding press remains substantially unchanged except that in this embodiment the chosen position for introducing the plastics material into the mould space 12 is above the centre of the mould space 12. The position of the injection of the blowing agent, i.e. the passageway 19, is directly beneath the entry point for the plastics material. In this embodiment the method of injection involves a dual hydraulic ram arrangement which rams are operated in series, the first ram 50 having a cylinder 51, which is the measuring chamber for determining the finite quantity of blowing agent to be injected during a particular moulding cycle, and the second ram 52 effecting the injection. The reason for this dual arrangement is that the blowing agent is preheated both whilst it is in the measuring chamber 51 by heater 63 and also by the heater 53 of a further pneumatic ram 54 which has a large capacity cylinder 55 acting as a storage cylinder for the blowing agent fed into the cylinder through conduit 82. The cylinder 55 of the pneumatic ram is connected by a non-return valve 56 to the cylinder 51 of the first hydraulic ram, which is in turn connected to the cylinder 57 of the second hydraulic ram 52 via a solenoid operated valve 58 and a non-return valve 59. The cylinder 57 also has a connection 60 to waste including a non-return valve 61 and a solenoid operated valve 62. Both hydraulic rams 50, 52 are operated by a common hydraulic pump 71 and electric motor 72, the pump being protected by a relief valve 81, but the rams have their respective two-directional solenoid operated valves 73, 74 and flow control valves 75, 76 each with its bypass 77, 78 including a non-return valve 79, 80. The operating cycle of this embodiment will now be described. To start with the storage cylinder 55 is filled with liquid blowing agent which is heated by the heater 53. The pneumatic ram is then operated to fill the measuring chamber which is the cylinder 51 of the first hydraulic ram 50. As before the volume of the cylinder may be varied by adjusting the stroke of the ram 50. The metered amount of blowing agent is heated again by heater 63. The mould is closed and the clamping pressure applied. The feed chamber 15 of the screw ram 13 is full of plastics material. The directional valve 73 of the first hydraulic ram 50 is then energised and solenoid valve 58 opened to allow the ram 50 to transfer the blowing agent from the measuring chamber to the injection chamber which is the cylinder 57 of the second hydraulic ram. The plastics material is introduced into the mould space 12. When the end of the passageway 19 upstanding in the mould space is immersed in the plastics material the directional valve 74 is energised to cause the second hydraulic ram 52 to inject the blowing agent upwardly into the centre of the plastics material. The contact between the blowing agent and the plastics material further heats the blowing agent to its decomposition temperature. The decomposition of the blowing agent effects a chemical reaction with the plastics material and the generation of nitrogen gas, the combined effect of which is to create a gas containing core 70 which flows within the surrounding plastics material until the mould space is filled. The effect of the blowing agent is limited by the flow control 76 on the operation of the ram 52 and thereby on the rate of injection of the blowing agent. This is important to ensure that the core does not extend to the surface of the plastics material whereby the outer wall or skin is punctured. On the other hand the internal pressure created by the nitrogen causes an outward pressure on the surrounding solid plastics material which pressurises it against the mould surfaces. When the introduction of plastics material and the injection of the predetermined, finite quantity of blowing agent is complete, the directional valve 74 is energised to cause the hydraulic ram 52 to withdraw its piston, and the solenoid valve 62 is opened to allow any excess pressure or residue of the blowing agent in the moulding, the passageway 19 and the cylinder 57 to be vented to atmosphere. Normally, sufficient dissipation of this pressure is achieved because the cells of the core are generally interconnected back to the point of entry of the blowing agent. However, it is not necessarily essential that the pressure should reduce itself to atmospheric pressure since some degree of internal pressure can be supported by the walls of the moulding. Indeed, in some cases, some of the cells may not be directly interconnected with the entry point but as in the first described embodiment the pressure within those cells can reduce itself to a safe level by effecting movement of the adjacent plastics material which is still flowable. The clamping pressure may then be relieved so that the mould can be opened and the moulding removed. It will be appreciated that during the above described injection step of the blowing agent, the solenoid valve 58 is closed to allow the measuring chamber to be recharged with the blowing agent from the storage cylinder 55 ready for the next moulding cycle. If desired the fluid introduced into the plastics material may comprise a mixture of a gas and a liquid blowing agent instead of a gas or a liquid blowing agent only. In this case, Figure 3 shows the storage cylinder 55 of Figure 2 having a gas entry conduit 83 beneath the liquid blowing agent entry conduit 82 and a mixer 84 operated by an electric motor 85. The gas is preferably argon, nitrogen, or freon. The cylinder 55 is filled with both liquid blowing agent and gas which are formed into a homogeneous mixture by the mixer 84 and simultaneously heated by the heater 53. Alternatively, as shown in Figure 4, the storage cylinder 55 may remain unchanged from the embodiment of Figure 2, and the gas introduced into the blowing agent from a supply 86 through a passageway 87 at a position between the non-return valve 56 and the cylinder 51. The passageway 87 has a non-return valve 88 and in this embodiment the gas and liquid blowing agent are mixed by an alternating direction mixer 89 which homogenises the two constituents to produce a mixture which is passed into the cylinder 51. In each case the operating cycle from thereon is the same as that described with reference to Figure 2. The gas/liquid mixture may be 1:1 or varied as required to give the desired internal cell structure of the resultant moulding. For example, to produce a more cellular structure, the amount of liquid is increased relative to the amount of gas and vice versa if more open cavities or channels are required. It will thus be appreciated that this gas/liquid variation may be employed to achive the combination of (a) being able to increase the internal gas pressure after the introduction of plastics material is complete which is an advantage of the gas only embodiment of Figure 1, and (b) a cellular structure (although not as fine a structure as with liquid blowing agent only) which is desirable to reinforce the outer wall or skin of the moulding. Figures 5 and 6 relate to a modified embodiment which as shown is applied to the production of a moulding having an internal cavity 38 or cavities, as described with reference to Figure 1, but which is also applicable to the embodiment of Figures 2 to 4 in which the moulding has a core or cores resulting from the injection of a blowing agent. The modification concerns the fact that instead of fluid being injected into the stream of plastics material at one or more selected positions within the mould space 12, the fluid is injected upstream of the mould space. The fluid may be injected at any convenient position or positions between the feed chamber 15 of the screw ram 13 and the gate 6 forming the entrance to the mould space 12. In this modified embodiment, the fluid is injected by the ram 39 (or in another embodiment the ram 52) via a passageway 90 having its outlet end at a position which is substantially at the upper end of the space 18 formed during the moulding cycle. For this purpose, the screw 13 has a different nozzle 14 having an end fitting incorporating a spider insert 91 comprising an outer wall 93 joined to a hub 94 by three webs 95, thereby forming three segment-shaped apertures 92 through which the plastics material may flow. The fluid injection passageway 90 passes radially through the outer wall 93 and one of the webs 95 into the hub 94 of the insert. The passageway is then directed axially in the direction of flow of the plastics material to the desired position at which the fluid is to be injected into the plastics material. In operation, as before, when the flow of plastics material has immersed the outlet end of the passageway 90, the metered quantity of fluid is allowed to pass into the plastics material and flow therewith into the mould space 12 thereby forming the pressurised mass 38 (or core 70) which urges the plastics material 8 towards the mould surfaces. The invention is not restricted to the specific details of the embodiments described above. For example, the basic plastics material may contain a blowing agent mixed therewith within the screw ram. However, in the case of each embodiment described above it will be appreciated that an injection moulding may be produced which employs less plastics material and is lighter than if the moulding is solid. The creation of the internal pressure also results in a moulding having an external surface which corresponds closely to the internal surface of the mould and which, unlike conventional structural foam mouldings, may be acceptable without post-moulding treatment such as painting. On the other hand, such treatment may still be carried out, if desired. For example, the moulding may be painted to obtain a particular decorative colour instead of just to hide blemishes in the external surface. Furthermore, mouldings may be produced in a shorter time than is usual in the case of structural foam mouldings.

Claims: A process of producing an injection moulded plastics article comprising holding a quantity of non-plastics fluid in a variable volume chamber (21), and reducing the volume of the variable volume chamber to pressurize the non-plastics fluid to a pressure sufficient to form at least one cavity (38) or core (70) of the non-plastics fluid in the plastics material injected into the mould space, characterised by the process including the steps of determining prior to introducing the plastics material into the mould space a quantity of the non-plastics fluid sufficient to achieve an intended weight reduction in the article being produced as well as a required internal pressure in the article being produced, increasing the volume of the variable volume chamber to hold the determined quantity of the non-plastics fluid, measuring said quantity of the non-plastics fluid by filling said variable volume chamber with the non-plastics fluid at a pressure determined in relation to the increased chamber volume, reducing during the introduction of the non-plastics fluid the volume of the variable volume chamber at a controlled rate to form the cavity or core, continuing to control the rate of reduction of the variable volume chamber to pressurize the non-plastics fluid after the cavity or core is initiated, and maintaining a non-interrupted flow of the non-plastics fluid at a pressure higher than that of the stream of plastics material yet not so high as to cause a rupture in the plastics material defining the cavity or core formed therein, until the plastics material is self-supporting. A process as claimed in Claim 1, characterised in that the fluid is a gas. A process as claimed in Claim 2, characterised in that the gas is nitrogen or other inert gas. A process as claimed in Claim 1, characterised in that the fluid is a liquid fluid. A process as claimed in Claim 4, characterised in that the liquid fluid is a liquid blowing agent. A process as claimed in Claim 5, characterised in that said agent liberates nitrogen gas upon being heated to the decomposition temperature of said agent. A process as claimed in any one of the preceding claims, characterised in that the introduction of the fluid is controlled so that the pressure of the fluid introduced remains higher than the pressure of the plastics material, to ensure a uniform introduction of the fluid throughout the period of introduction. A process as claimed in any one of the preceding claims, characterised in that some or all of the fluid is introduced into the stream of plastics material at one or more selected positions within the mould space (12). A process as claimed in any one of Claims 1 to 7, characterised in that some or all of the fluid is introduced into the stream of plastics material at one or more selected positions upstream of the mould space (12). A process as claimed in any one of Claims 1 to 9, characterised in that the fluid introduced is a gas and wherein the volume of the respective gas filled cavity (38) is formed by a portion only of the measured quantity of the gas, the remainder of the gas being introduced into the cavity to increase the pressure therein. A process as claimed in any one of the preceding claims, characterised in that the fluid introduced is a gas and venting of said cavity occurs after said solidification and cooling of said plastics material and before the mould (4) is opened. A process as claimed in Claim 11, characterised in that the pressure in the cavity (38) is relieved by causing the gas to leave the respective cavity after the article has become self-sustaining. A process as claimed in Claim 11 or Claim 12, characterised in that at least some of the gas released during the step or relieving the pressure in the or each gas filled cavity is retained for recirculation during a subsequent moulding cycle. A process as claimed in any one of the preceding claims, characterised in that the fluid is a liquid blowing agent or a mixture of a gas and a liquid blowing agent which is introduced into the stream of plastics material at a temperature which is below its decomposition temperature, the fluid being heated to its decomposition temperature by the plastics material whereby the fluid generates a gas which creates a closed cell cellular foam core (70) with said gas exerting said pressure on the surrounding plastics material (8). A process as claimed in Claim 14, characterised in that the fluid is preheated before it is introduced into the plastics material. A process as claimed in any one of the preceding claims, characterised in that the introduction of pressurized fluid commences prior to the termination of the flow of plastics material, whereby fluid and plastics material are introduced into the mould space simultaneously. An apparatus for producing an injection moulded plastics article, comprising a mould (4), a variable volume chamber (21), means (19) connecting said variable volume chamber to said mould, means (20) for increasing the volume of said variable volume chamber to receive a quantity of non-plastics fluid, filling means (23) for filling said variable volume chamber with the quantity of non-plastics fluid, and reducing means (20) to reduce the volume of the variable volume chamber to pressurize the non-plastics fluid to a pressure sufficient to form at least one cavity (38) or core (70) of the non-plastics fluid in the plastics material injected into the mould space, characterised in that the variable volume chamber (21) is a measuring chamber or is connected to a measuring variable chamber to measure a predetermined quantity of the non-plastics fluid, and in that controlling means (28―33, 37) are provided which act on said reducing means to control the rate of reduction of the volume of the variable volume chamber to introduce the non-plastics fluid at a controlled rate to form the cavity or core and to pressurise the non-plastics fluid after the cavity or core is initiated to maintain a non-interrupted flow of the non-plastics fluid at a pressure higher than that of the stream of plastics material yet not so high as to cause a rupture in the plastics material defining said cavity or core formed therein until the plastics material is self-supporting. Apparatus as claimed in Claim 17, characterised in that the non-plastics fluid introducing means is a piston and cylinder arrangement (39; 52), the cylinder (21; 57) being connected by a passageway (19) projecting into the stream of the plastics material. Apparatus as claimed in Claim 18, characterised in that the cylinder (21) of the non-plastics fluid introducing means comprises the measuring chamber for the fluid. Apparatus as claimed in Claim 18, characterised in that the non-plastics fluid introducing means includes a second piston and cylinder arrangement (50) connected in series and upstream of the first-mentioned piston and cylinder arrangement (52), the cylinder (51) of the second piston and cylinder arrangement (50) comprising a measuring chamber for the fluid for delivering a measured quantity to said variable volume chamber (21). Apparatus as claimed in Claim 19 or Claim 21, characterised in that the effective volume of the measuring chamber (21; 51) is adjustable to allow the measured quantity of the fluid to be varied. Apparatus as claimed in any one of Claims 19 to 21, characterised in that the measuring chamber (21; 51) has means (63) associated therewith for heating the fluid therein. Apparatus as claimed in any one of Claims 17 to 22, characterised in that the variable volume chamber (21) is connected to said mould (4) at at least one position upstream of the mould space (12). Apparatus as claimed in any one of Claims 17 to 23, characterised in that said controlling means includes flow control means (31) for establishing a rate of introduction of the non-plastics fluid into said cavity (38) or core (70) which rate distributes the plastics material throughout said mould space (12) and prevents the cavity or core, once formed, from becoming too highly pressurized so as to rupture the plastics material. Apparatus as claimed in any one of Claims 17 to 24, characterised in that said reducing means (20) are adapted to continue to apply pressurization to the non-plastics fluid subsequent to the plastics material being forced throughout the mould space (12), such that the plastics material is further forced against the interior surface of the mould space during cooling of the plastics material so as to improve the definition of the resultant article exterior surface and resist any tendency of the solidifying plastics material to shrink away from the mould surface.

Inventors: GAHAN GEOFFREY DAVID, HENDRY JAMES WATSON, GAHAN, GEOFFREY DAVID, HENDRY, JAMES WATSON
Applicants: CINPRES LTD, CINPRES LIMITED
Application year: 1984
Main classification: B29C45
IPC codes: B29C45, B29C44, B29K105, B29L22