Abstract:
An injection compression moulding apparatus comprises two mould parts which define a mould cavity and are mounted on two relatively movable platens of an injection moulding press. An actuator is arranged in series with the platen action on at least one of the mould parts and is controlled in synchronism with the movement of the platens to enable the relative speed of the two mould parts during a mould closing phase of each article moulding cycle to be modified, the speed of relative movement of the mould parts being the vector sum of the speeds of movement and the platens and the actuator.

Description:
FIELD OF THE INVENTION 
   The present invention relates to apparatus for injection compression moulding of plastics material. 
   BACKGROUND OF THE INVENTION 
   It is known that large, thin articles are difficult to form by injection moulding. The reason is that the gap between the two parts of the mould is small and the distance that the material has to travel is too long for the pressure applicable by the moulding machine to be available at the far end of the gap from the injection point for driving the plastics to fill the mould. In short, the “flow path thickness ratio” is too long. 
   Conventionally, thin articles are formed by vacuum or pressure forming where a sheet of plastics material is stretched to conform to the shape of a mould. Such techniques are limited in their application as they cannot produce articles of even wall thickness or articles that have regions of increased or reduced wall thickness. This is because only one surface of the article is being moulded and the thickness at any point is determined exclusively by the thickness of the original sheet and the extent of its deformation. 
   This problem has been overcome by EP 1360057, which discloses injecting molten plastics material into an open mould forcing the plastics material to fill the mould by closing it at high pressure after injection. This method enables the use of cheaper materials which do not flow as well yet ensures that thin wall sections can be easily made. 
   It was known prior to EP 1360057 to move part of a mould in order to apply additional compression after having injected a plastics melt into a mould cavity in the conventional manner. This process, which was known as injection compression moulding (ICM) offered advantages of longer flow lengths, thinner walls and a lower level of material stresses. This made and still makes the process suitable for moulding such articles as CDs and DVDs (because of improved internal stresses) and vehicle body and instrument panels (because of improved impact resistance). 
   The known ICM processes differed from EP 1360057 in the extent of the compression of the plastics melt by the closing of the mould cavity. In the newer technology, the relative displacement of the mould parts is in excess of ten times the final mould thickness and may be as great as two hundred times the final moulding thickness. This was in contrast with known ICM processes, where a corresponding movement of typically twice the final wall thickness was used. 
   The present invention is concerned with an improvement of the apparatus described in EP 1360057. 
   According to the present invention, there is provided an injection compression moulding apparatus comprising two mould parts which define a mould cavity and are mounted on two relatively movable platens of an injection moulding press, wherein an actuator is arranged in series with the platen acting on at least one of the mould parts and is controlled in synchronism with the movement of the platens to enable the relative speed of the two mould parts during a mould closing phase of each article moulding cycle to be modified, the speed of relative movement of the mould parts being the vector sum of the speeds of movement and the platens and the actuator. 
   The present invention is based on the realisation that, during the final stage of closing the mould speed is a more important parameter than pressure in achieving moulded articles of uniform thickness and good surface finish. Though injection moulding presses are capable of applying and maintaining the desired high pressure once the mould has been closed, their speed of movement just before the mould is fully closed, is not necessarily optimum. For thinner sections, the speed may be too low and, for thicker sections, the speed may be too high and uncontrollable. 
   In the case of a press employing a toggle mechanism, the mechanical advantage of the lever system acting between the hydraulic cylinder and the platen augments the closing pressure but at the same time it slows down the rate of movement of the platen. Rather than attempt to modify the press, the present invention overcomes the problem by placing an actuator in series with at least one of the platens to supplement the relative movement of the platens of the press, the actuator being preferably built into the mould. 
   The actuator may be a fluid operated piston preferably a hydraulic piston. 
   In the impact moulding apparatus of EP 1360057 it is necessary to allow one of the mould parts to move towards the platen while injection of plastics material into the mould cavity is taking place. To accommodate both these features in a compact manner, the actuator of a preferred embodiment of the invention is constructed as an annular first piston receiving second piston that acts on said one mould part. 
   Conveniently, the two pistons are forced apart by means of a force sufficiently weak to allow said one mould part to move towards the associated platen when plastics material is injected into the mould cavity before the latter is fully closed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic perspective view of an apparatus of the invention, 
       FIG. 2  is a section through a punch and die used  FIG. 1  to produce an oxygen impermeable blank, and 
       FIGS. 3 to 7  are sections through the injection mould of  FIG. 1  at different stages of a cycle in which a container is formed. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows schematically an apparatus  10  for forming a square container of the type commonly used, for example, in packaging margarine. Currently, such containers are often formed by thermoforming. In this process, a flat sheet is stretched by means of an applied vacuum and optional mechanical assistance to conform to a female mould. The process has disadvantages in that the base of the formed container is thicker than it needs to be, while the sides and the corner are weakened unnecessarily. In other words, the best use is not made of the plastics material. Furthermore, the plastics material does not contain an oxygen barrier and this reduces the shelf life of the packaged comestible product. 
   Conventional injection moulding can alternatively be used to form such an article but the length to thickness ratios are such that expensive material have to be used. Even when using low viscosity plastics materials, one is obliged to make the sides of the container thicker than necessary. If injection moulding is used, one may have to use more than one injection gate, especially if the container is rectangular. One has also to modify the thickness of different regions of the base so that the flow resistance favours the molten plastics material flowing towards the corners, thereby giving a bow tie effect. As with the thermoforming, the plastics material is not oxygen impermeable. 
   The containers produced by the above processes are oxygen permeable because the composition of the containers is homogeneous throughout their wall thickness. Because no oxygen impermeable plastics material is readily available that is safe to come into contact with food, one can only produce a food safe oxygen impermeable container if the walls are inhomogeneous, allowing the oxygen barrier to be formed by a layer that does not come into contact with the food. 
   The apparatus in  FIG. 1  can be used to produce a container having three (or more) layers in its walls. As the thickness of the walls of the container is small, the thickness of the oxygen barrier must of necessity be even smaller. For this reason, aluminium is chosen as the oxygen barrier because even a sheet having a thickness measured in microns can act as an efficient oxygen barrier. A sheet of aluminium of this thickness cannot however be stretched and once a piece has been separated from a web, it is very flimsy and difficult to handle. The use of aluminium as a barrier thus poses several problems and these all are addressed in the design of the illustrated apparatus. 
   The apparatus  10  in  FIG. 1  comprises a stamping mechanism  12 , a transfer mechanism  14  and an injection-compression mould  16 . The stamping mechanism  12  produces an aluminium foil blank of the same shape as the finished container. The transfer  14  mechanism transfers each blank after it has been formed from the stamping mechanism  12  to the cavity of the injection-compression mould  16  and there the blank is coated on both sides with a plastics material to form the finished container. 
   The stamping mechanism as shown in  FIGS. 1 and 2 , comprises a punch  20  and a die  22 . The aluminium foil  24  is in the form of a continuous web extending between a supply roll  26  and a take-up roll  28 . The movement of the punch  20  towards the die  22  will at first cut out piece of foil from the web and that piece is then deformed into a blank of the desired shape as it is crushed between the punch  20  and the die  22 . Because the foil cannot be stretched, it creases in the corners in the same was as take-away aluminium containers, which are of course of much thicker material. 
   The foil  24  may be of aluminium or an aluminium alloy with a thickness of around 25 microns. As an alternative, the foil may  24  comprise aluminium which has already been coated with plastics films on both sides. In either case, it is important that the surface of the blank should be capable of bonding to the plastics material later to be injected around it and this may be assisted by passing the foil through a corona discharge or spraying the surface of the foil with a preparation that improves adhesion. It will be appreciated that if the foil comprises coated aluminium then the walls of the finished container will have five layers rather than just three. 
   The transfer mechanism  14  is not shown and will not be described in detail as its construction is not important to the present invention. It may for example comprise a robot arm designed to pick a blank out of the die  22  or off the punch  20  and transfer it to the mould cavity where it will be encapsulated in plastics material. Vacuum and compressed air can be used to pick up each blank from the stamping mechanism and to set it down in the mould cavity. 
   The injection-compression mould  16  operates on the same principle as that described in WO 02058909. In this case, however, plastics material is injected from both the core side and cavity side of the mould to cover both sides of the inserted blank. The injection-compression process can best be understood from reference to  FIGS. 3 to 7 . 
     FIG. 3  shows a mould cavity  30  formed in a stationary female mould part  32 . A core  34  is carried by a core plate  36  and is surrounded around its rim by a spring biased conically tapering sealing ring  38 , which operates in the manner taught in PCT/GB2004/005422. The core plate is moved towards and away from the female mould part  32  by a platen  40  of an injection moulding press. The platen  40  can form part of a conventional injection moulding machine, and it can for example be operated hydraulically, either directly or through a toggle mechanism. Specially shaped guide fingers  35 , as taught by PCT/GB2004/005414, cooperate with correspondingly shaped slots  33  to align the mould parts with one another accurately even before the mould cavity is fully closed. 
   The platen  40  acts on the core plate  36  through an arrangement comprising a back plate  41  housing two series connected concentric pistons  42  and  44 . The piston  42  is movable relative to the piston  44  and the latter is itself movable relative to the back plate  41  which is fixed to the platen  40 . The purpose and method of operation of the pistons  42  and  44  will be described in more detail below. 
   Each of the core plate  36  and the female mould part  32  carries a respective dosing cylinder  50  and  52 . The use of dosing cylinders is important in all cases where an article is formed by injecting a plastics material into a partially open mould, as injection back pressure cannot be relied upon to determine the full dose. In the present embodiment, two such dosing cylinders are needed because it is also important to set accurately the relative proportions of the plastics material lying on the opposite sides of the foil in the finished article. Valves  80  and  82  connected the dosing cylinders  50  and  52  either to the mould cavity or to an injection screw. The valve  82  of the dosing cylinder  52  of the female mould part  32  is always connected to an injection screw  54  (see  FIG. 1 ) whereas the valve  80  of the dosing cylinder  50  of the core plate  36  is connected to an injection screw only when the mould is closed through communicating passages  54  and  56  which are opened and closed by valves  58  and  60 . The dosing cylinder  50  of the core plate  36  may either be connected to the same injection screw  54  as the dosing cylinder  52  or to a second injection screw, depending on whether the same plastics material is to be used for both the inside and the outside of the container. Passages  62  and  64  controlled by gate valves  66  and  68  connect the valves  80  and  82  of the dosing cylinders  50  and  52  to the mould cavity. 
   A cycle commences with the parts of the mould in the position shown in  FIG. 3 . In this position the piston  44  is fully retracted into the back plate  41  and the piston  42  is fully extended to create a small gap between the core plate  36  and the back plate  41 . The mould is fully open and the two dosing cylinders are charged with a dose of a plastics material. The two doses need not be equal but the combined dose corresponds to the volume to be injected into the mould cavity to form the finished container. 
   After the transfer mechanism  14  has positioned a blank  70  in the mould cavity  30 , the back plate  41  is advanced by the platen  40  to the position shown in  FIG. 4 . While a gap still remains between the core plate  36  and the back plate  41 , the core  34  is pushed into the mould cavity displacing trapped air from the cavity. The force acting to push the core  34  into the mould cavity is a weak air pressure acting between the pistons  42  and  44 . 
   In the next stage of the cycle, shown in  FIG. 5 , the pistons of the two dosing cylinders  50  and  52  are operated by external actuators  72 ,  74  to pump the two stored doses of plastics material into the non-circular base of the mould cavity. It is important for the operation of the two actuators  72 ,  74  to be accurately synchronised and a simple manner in which this can be achieved is to use a single hydraulic cylinder to act on both of them as the mould approaches its fully close position thereby ensuring that the injection pressures on the opposite sides of the blank  70  are always matched to one another. At the same time, the valves  80  and  82  and the gate valves  66  and  68  are positioned to admit the molten plastics material into the mould cavity. 
   The injection pressure is sufficient to force the core plate  36  away from the female mould part, opposing the weak air pressure which acts on the piston  42  and closing the gap between the back plate  41  and the core plate  36 . The ability of the core to move limits the pressure that can be reached by the plastics material in the base of the cavity so that it cannot travel up the sides of the cavity. Because of this, the injected plastics material first spreads to cover the entire base of the mould cavity and separates the mould parts only sufficiently for the predetermined dose of the plastics material to be accommodated within the perimeter of the base. Throughout this time, the sealing ring  38  remains in contact with the female mould part  32 . 
   After completion of the injection step, the compression step is effected in the manner shown in  FIGS. 6 and 7 . Because the plastics material cools down as it travels up the sides of the mould cavity, compression needs to take place very rapidly and for this respect the speed of movement of the platen  40  may not be sufficient. To augment the speed with which the plastics material is compressed, the piston  44  is hydraulically operated to supplement the movement of the core  34  caused by advancing the platen  40 . 
   As the plastics material flows to fill the cavity, the cavity is vented through a narrow gap between the sealing ring  38  and the core  34  but this gap does not allow the plastics material to enter into. Because of the way that the ring  38  seals against the core  34  when the mould is fully closed, it leaves only a minimal witness mark on the finished article. 
   In the final step shown in  FIG. 7 , the mould is fully closed by advancing the platen to its end position and the plastics material is maintained under compression as it cools in the cavity. The piston  44  is allowed to retract into the back plate  41  in readiness of the next operating cycle. While the mould is fully closed and the passages  58  and  60  are in register with each other, the gate valves  54  and  56  and valve  80  are opened to admit plastics material into the dosing cylinder  50  of the core plate  36 . At the same time, a fresh does of plastics material is also directly supplied to the dosing cylinder  52  of the female mould part  32  past its valve  82 . The stroke of the piston of each of the dosing cylinders  50  and  52  is adjustable to allow precise setting of the quantities of plastics material injected on each side of the aluminium blank. 
   This terminates the operating cycle and on returning to the position in  FIG. 3  the formed article is ejected and a new blank is placed is in the mould cavity, as previously described. 
   The fact that the aluminium blank is creased need not mar the appearance of the finished article as the plastics materials may be opaque. On the contrary, the ability to use plastics materials of different colour on the inside and outside of the container may enhance its visual appeal. 
   It can thus be seen that the invention allows the moulding of an article that will improve the shelf life of comestible products by incorporating an effective hidden oxygen impermeable barrier layer.