FORMING ARTICLES FROM COMPOSITE MATERIALS

Abstract: Composite mouldable material is supplied via a hopper (1a) into a double-screw type compounder (2). Further components can be added via a further hopper (1b). The hot melt mixture from the compounder (2) passes through a non-return valve (5) into a holding chamber where it accumulates, driving back a plunger (3) until a complet charge is held in the chamber. The plunger (3) is then driven into the chamber to expel the material through an extrusion die or into a mould (13). An oscillatory packing pressure can be applied to the mold (13) by a plunger (20).

Description: FORMING ARTICLES FROM COMPOSITE MATERIALS The present invention relates to the formation of moulded or extruded articles from composite mouldable materials. There is an increasing demand for high integrity articles produced, for example- from composite materials. Examples of such composite materials are thermoplastic or thermosettlng polymers with organic or inorganic particulate and fibrous fillers and ceramics with wax or polymer binders. Basic requirements are conformity of compounding, with good dispersion of composite elements, breaking down agglomeration, careful temperature and pressure control of the melt and freedom of shrinkage defects. Attempts to satisfy these requirements at present necessitate a separate compounding, extruding, and pelletising operation to produce uniform granules. This sequence of operations requires temperatures to produce a hot melt with subsequent cooling to produce the granules ready for the following injection moulding or extrusion operation which in turn requires careful temperature and pressure control of the reheated hot melt and cooling in the mould. According to the invention, there is provided a method of forming articles from composite materials in which the constituents of the composite material are mixed and heated in a mixer, the heated mixture is accumulated in a holding space from which it is then expelled under pressure into a mould or through a die. Advantages of the invention include 1. one-heat history which may be specially important with heat sensitive materials and additives. 2. reduced fibre breakage and molecular degradation. 3. closer control of processing conditions and uniformity of composition which can improve performance. 4. elimination of extra handling, with its problems of ingress of dirt, exposure to humidity, etc. 5. reduced labour cost. Also according to the invention, there is provided apparatus for forming articles from ouldable composite material comprising a mould or die for forming the articles and mixing means for mixing the constituents of the material, wherein the mixing means are arranged to deliver the heated mixed material into a holding chamber having means for subsequently driving the heated material therefrom into a mould or through a die to form an article. The holding chamber is advantageously provided with heating means to maintain material at the required temperature. For the mixing stage a twinscrew or multiscrew compounder is \ preferred, presenting the advantages of a) lowering the thermal input into the material (i.e. the product of heating time and temperature) with greater control of residence time. This is important with heat sensitive polymers and polymers containing heat sensitive additives. b) reduced shear history of the material. This is important with shear-sensitive polymers and additives, especially fibrous additives. c) Improves dispersion of finely divided particulate additives and improves breaking down of agglomerates. d) improves conveying difficult-to-feed materials such as polymers containing high loadings of mineral fillers. Advantageously, the means for driving the composite material from the holding chamber comprises a plunger which during delivery of the material from the mixing means is allowed to be pushed back by the material as it is delivered, optionally against a resistance. Preferably, the material is delivered through a passage in the plunger, the passage including a non-return valve. The mixing means may then be mounted for. movement with the plunger. The invention will now be further described by way of example with reference to the drawings in which: Figure 1 shows diagrammatically a mixing and injection moulding apparatus in axial section, Figure 2 shows diagrammatically a modified form of the apparatus of Figure 1, Figure 3 is an axial sectional view of a twin screw compounds which may be used in the apparatus of Figure 1 or Figure 2, showing one of the screws and its enclosing barrel and Figure 4 is a view corresponding to Figure 3 of a modified compounder. \ In the apparatus shown in Figure 1, the material and additives are delivered to a hopper la from a simple non-heated mixer or by a number of commercially available measuring devices (not shown in the drawing). In certain cases further additives, or chopped fibres, can be added by means of a weight feeder from a second hopper lb. Unit 2 is a twin or multiple screw compounder which applies controlled shear and conductive heating, effective dispersion and * positive pumping action to the ingredients and, if necessary, effects devolatisation of the material, as described below with reference to Figures 3 and 4. The resulting dispersed material is passed as a homogeneous melt through a sealed connection 23 from the unit 2 to a plunger 3 along a passage 4 therein, through a check valve 5 in the plunger 3 and into a holding or injection chamber 6 in a pressure vessel 7. The melt causes a build-up of pressure in chamber 6 and this causes the plunger 3 to be pushed back within pressure vessel 7, thus enabling the gradual increase in volume of chamber 6 to be measured and controlled so that, when the correct and necessary amount of material has been accumulated, the action of compounder 2 may be stopped until the next cycle when the next measured quantity is required from it. The movement .during the. filling of-chamber 6 is transmitted to a crosshead 9 and piston and cylinder assemblies 8a and 8b, the consequent oil displacement from which may be resisted to impose a back pressure to cause some compacting of the material in the injection chamber 6. The relative movement between crosshead 9 and crosshead 19 constitutes the necessary means of injecting the material into the mould from the chamber 6 via a passage 10, and a nozzle 12 which is pressed against the mould 13 by the action of a piston and cylinder assembly 14. Under the action of the power stroke of the two pistons and cylinders 8a and 8b, the material in the chamber 6 is injected into the mould. The discharge from the compounder into the injection chamber 6 ensures the retention of the principle of the highly desirable 'first in first out' discharge of\ the melt into the mould. The 'in line' configuration of this construction maintains a unidirectional flowpath. A plunger 11 operated by a piston and cylinder assembly 20 will then move forward, close off the passageway 10 at gate 21 and if desired apply controlled pressure upon the material Injected in the closed mould. Also if desired, the plunger 11 can be reciprocated at predetermined frequency and pressure, operating between predetermined limits, which can be set, to.enable additional material from the injection chamber to be transferred under a pre-set pressure into the mould. This facility will assist compacting the material in the mould with the objective of eliminating internal, microporosity cracks and voids in the moulding. Thus, the main objective of using an oscillating packing pressure in injection moulding is to ensure that thick sectioned mouldings, or parts of mouldings have sufficient time to freeze off in the cavity before the gate, sprue, runner or any other narrower section separating the thick section of the mould cavity from the injection moulder nozzle frozen off. This is achieved by the periodic compression and expansion of the melt in the thick sections which causes enhanced shearing of the melt in the narrower sections. The result of this action is that 'the internal heat generated by the melt shearing in the narrow sections delays the freezing-off time for these sections. In addition, a significant amount of control over the freeze-off time for the thin sections can also be obtained by changing the oscillating pressure profile. One of the main effects produced by the oscillating packing pressure is the elimination of voiding and surface shrinkage in thick- sectioned components. A significant degree of control over post moulding residual stresses may also be achieved and utilised to counteract stresses due to differential thermal shrinkage. The complete assembly of mixer 2, crosshead 9 and pressure vessel 7 may be slidably mounted on two rails 15, for example circular bars so that the action of cylinder 14 may be used to separate nozzle 12 from mould 13 for processing or maintenance purposes if so required. The compounder unit 2 is driven by a flange mounted hydraulic motor 22. The whole of the plastics processing parts of this mechanism will be temperature controlled, preferably but not exclusively by electrical resistance heaters 16, 17, 18 divided into controllable areas as appropriate to their requirements. In case the volume of material required to fill the mould is greater than the volume of the chamber 6 at maximum stroke of the plunger 3, the control mechanism can be altered to keep the compounder 2 pumping material by its own inherent pumping pressure generated by the rotating action of the screws into the mould until it is filled, when the pulsating plunger 11 will take over applying compaction and the final pressure required. The hydraulic power pack of the machine can provide, with suitable controls and interface with sequence controls, the operational requirements of the described compound-injection unit coupled with " the mould closing, mould opening action of an injection moulding machine. In the modified arrangement shown in Figure 2 (where parts corresponding to parts in Figure 1 have reference numbers increased by 30) the twin screw compounder 32 is located out of the line of the rest of the machine. This arrangement increases the lateral dimensions of the injection unit, but may have advantages for machines which require a large Injection volume to be supplied by the injection cylinder. The compounder 32 supplies plasticised material to the rear of the injection chamber 36, via a crosshead 39. The material passes down the injection chamber 36 and through a non-return valve 35 situated at the end of the plunger 33. The pressure created at the end of the plunger forces it to retract and a melt pool is generated at the front of the injection chamber 36 (i.e. its left hand portion as seen in Figure 2). When sufficient melt has been obtained, the compounder (32) stops the supply of material and the shot is injected into the mould cavity 43 by the \ application of hydraulic pressure to the injection cylinder 38. Once the cavity is full the packing of the mould is the same as that described for the other machine configuration. The oscillating packing pressure device 50 can also be employed if required. The main functions of the twin-screw compounds 2, 32 are to convey, melt, uniformly homogenise and deliver material and additives to the accumulation point on the moulding machine for subsequent injection into the mould cavity 13,43. Although in principle any design of twin-screw extruder capable of meeting these requirements could be utilised, including co-rotating, counter-rotating, fully, partially or non intermeshing variants, the preferred design is a co-rotating closely intermeshing twin-screw extruder (as indicated diagrammatically in Figure 2) with proven conveying and mixing capabilities. These qualities have been obtained using a co- rotating twin-screw extruder with trapezoidal-section screw flights and screw channels and screw diameter for example of 40 mm. Such a design permits a high degree of intermesh between the screws resulting in a high positive displacement efficiency as material is conveyed from one screw to the other, thereby facilitating the transport of material through the machine. Effective mixing is achieved by a variety of means, including judicious design of clearances between the screws resulting in interchannel leakage flows and a complex shearing and motion of the fluid passing through the extruder. Inclusion of special screw forms, such as reverse helix screws and/or mixing elements, for example, slotted discs tri- or bi-lobal cam shaped kneading elements, can also intensify distributive and dispersive mixing processes. Figures 3 and 4 show two forms of suitable screw geometry. Only one screw is shown in Figures 3 and 4. The two or more screws of the compounder are of the same construction, and driven with the same direction of rotation, with their threads intermeshing. Similarly the barrel 61 of the compounder is also segmented into lengths of four times the screw diameter, which may vary in design are chosen and assembled according to the processing needs, being held together by tapered clamp rings. Each screw ' 60 is assembled from lengths 60a, 60b, 60c, 60d, 60e, 60f which conveniently are of a standard length (for example four times the diameter of the screw) or combine to form standard lengths. In the embodiment shown in Figure 3, the barrel or casing of the compounder is assembled from a feed section 61a having a feed port 62 (to communicate with the hopper la in Figure 1), two compression sections 61b and 61c a devolatilsation section (or additional feed port section) 61d formed with twin ports 63 and a final metering portion 61e. As can be seen in Figure 3, the feed section 61a is shorter than, e.g. half the length of the other sections. The screw section 60d is of zero (or even negative) pitch thereby presenting a high resistance to the passage of material past it. The screw section 60a, 60b and 60c are of progressively finer pitch and thereby- force the material past the section 60d with substantial heating of the material into a melted state. The coarse pitch screw section 60e conveys the material under low pressure to the final metering screw section 60f. Where volatile constituents are to be removed from the mix, suction may be applied to the ports 63. The embodiment shown in Figure 4 differs from that shown in Figure 3 in that the screw section 60e is shortened by a length equal to that of a further melting element 60g" which is inserted between a half-length compression element 60h (of similar thread to the element 60c) and a half-length metering element 60j of similar pitch to the metering element 60f of Figure 3. The flexibility in the choice of components for both the screws and the barrel as discussed above in relation to Figures 3 and 4 also enables the use of specially hardened barrel portions or screw portions could be restricted to only those points along the compounder where from abrasive feedstock or additives is most prominent. This avoids the cost of using barrels and screws which are hardened throughout the length of the compounder which would be\ the case with one piece screw and barrels. Also, the overall length of the screws and barrel may be increased or decreased as desired by appropriate selection of the numbers and lengths of the screw and barrel sections.

Claims: CLAIMS 1. A method of forming articles from composite materials in which the constituents of the composite material are mixed and heated in a mixer, the heated mixture is accumulated in a holding space from which it is then expelled under pressure into a mould or through a die. 2. Apparatus for forming articles from mouldable composite material comprising a mould or die for forming the articles and mixing means for. mixing the constituents of the material, wherein the mixing means are arranged to deliver the heated mixed material into a holding chamber having means for subsequent driving the heated material therefrom into a mould or through a die to form an article. 3. Apparatus according to claim 2, wherein at least the holding chamber is provided with heating means to maintain material at a required temperature. 4. Apparatus according to claim 2, wherein the mixing means comprises a twinscrew or multiscrew compounder. 5. Apparatus according to claim 4, wherein the compounder comprises a barrel formed in separate interchangeable lengths « fastened together, and the screws of the compounder are formed in separate lengths which are selected from a range of lengths having different characteristics and are fastened together. 6. Apparatus according to claim 2 wherein the means for driving the material into a mould or through the die comprises a plunger βlidably drivable in the holding chamber and having a non-return valve to pass material from the compounder Into the holding chamber. 7. Apparatus according to claim 6 wherein the plunger has an internal longitudinal passage leading from the compounder, through the non-return valve into the holding chamber. 8. Apparatus according to claim 7 wherein the compounder, plunger, holding chamber and outlet therefrom are arranged in line. 9. Apparatus according to claim 2 and including means for exerting an oscillation packing pressure on. material in the mould. 10. Apparatus according to claim 9, wherein the means for exerting the packing pressure comprises a reclprocatory plunger active on material in the passage between the holding chamber and the mould.

Inventors: ALLAN PETER STEWART, BEVIS MICHAEL JOHN, GASPAR EMERY, HORNSBY PETER RIDSDALE, ALLAN, PETER, STEWART, BEVIS, MICHAEL, JOHN, GASPAR, EMERY (DECEASED), HORNSBY, PETER, RIDSDALE
Applicants: POOLES RICHARD INGRAM EXECUTOR, ROBINSON RICHARD MACLEOD EXECU, TURNER JOHN CHRISTOPHER EXECUT, UNIV BRUNEL, EMERY GASPAR
Application year: 1986
Main classification: B29C45
IPC codes: B29B7, B29C45, B29C47