IMPROVEMENTS IN OR RELATING TO GAS ASSISTED INJECTION MOULDING

Abstract: A mould for a gas assisted method of injection moulding has first and second mould parts defining a mould cavity. A series of upstanding ribs extend from the surface of the mould defining the mould cavity. These ribs produce complementary channels (30) in a moulding produced in the mould. These channels (30) split the mould cavity into a plurality of communicating sub-sections (31). Channels are provided splitting the ribs into sections and forming, during the moulding process, connecting ribs (35) of plastic between the plastic adjacent the channels (30). These ribs contribute to the lateral rigidity of the moulded product. The sub-sections enable an otherwise uncontrollable void to be divided into controllable smaller voids.

Description: IMPROVEMENTS IN OR RELATING TO GAS ASSISTED INJECTION MOULDING The present invention relates to apparatus for and method of gas assisted injection moulding. Injection moulding of thermoplastic materials is a well-known process. There are however practical limitations on the moulding of plate-like mouldings of comparatively thick sections over larger areas. Typically it is normally not economically or technically viable to injection mould thicknesses of 6mm or more. The times required for cooling and the transformation of the plastic from a liquid or molten state into a solid state are likely to be excessive. Also it is difficult to continue injecting additional plastic into the mould cavity to compensate for the volumetric shrinkage of the material as it cools and solidifies. One solution is known to be the moulding of so-called 'structural foam', in which the material has a closed cellular structure formed by mixing gas with the molten material or by producing gas from a chemical blowing agent which is pre-mixed with the plastic, and then activated at the plastic moulding temperature. Structural foam however has inherent disadvantages of a poor surface finish and comparatively long moulding time cycles due to the cooling times required. Gas Assisted injection Moulding is becoming well known and established as a version of injection moulding in which a fluid, normally gas, is injected into the plastic when still molten or semi-molten, to form hollow continuous channels within the plastic within the mould cavity. The gas may be injected into the flow of plastic at the moulding machine nozzle, or at carefully selected positions in the mould cavity, or a multiple of cavities in one mould, or into plastic feed runners within the mould. The process is based on the known principle that the plastic forms a skin when locally cooled adjacent to the mould cavity surface, which is cooled to a temperature below the softening temperature of the plastic, as the plastic flows into the mould cavity. In each case the gas will flow into the path of least resistance, which is normally at the centre of the thicker sections of the mould cavity where the plastic is least viscous, i.e. it is most fluid or runny. It is also well known by those familiar with the process that gas will flow from high to low pressure positions within the molten plastic, and will normally tend to flow in the same direction of the streams of flow of the plastic when filling the mould cavity from positions at or near to the plastic feed gate position, towards the last-to-fill positions in the mould cavities. The gas thereby displaces molten plastic from the middle of the thicker sections, forcing it to continue flowing towards the as yet unfilled extremities of the mould cavity. This results in the formation by the gas of hollow continuous channels within the said thicker sections. Successful implementation of the process is dependent on precise control of moulding machine and gas injection conditions. As far as the moulding machine is concerned, it is necessary to inject a controlled volume of plastic at a controlled speed of injection into each mould cavity, in the case of multiple cavities in a mould, it is necessary to balance the flow of plastic into each cavity so that a prescribed weight or volume of plastic is injected into each cavity consistently during successive moulding operations. As far as the gas is concerned, it is necessary to precisely control the injection of gas at a prescribed pre-set and variable cycle of pressures and times over a series of phases, which are the subject of a previous patent application by the inventor of this invention. When the gas is injected directly into the mould cavity, gas injection assemblies previously disclosed in patent applications by the same inventor, and using the principle of Sealed Injection of Gas, are preferably used. They may be positioned at one or more locations within each mould cavity or feed runner, or external to the mould cavity at a satellite position. It is known that if gas is injected into a comparatively thick section of a plate-like moulding, the gas will form a randomly variable void which will be different in shape and contour moulding after moulding. As an alternative it is normally advisable to combine a thinner plate-section with one or more ribs to provide the required rigidity. A more suitable design combines a thinner plate section and one or two thicker ribs into which gas is injected and controlled. In Gas Assisted Moulding the gas pressure is used as a medium for controlling the pressure within the plastic in order to force the plastic towards the surfaces bordering the mould cavity in order to precisely reproduce the profile and the surface textures of the mould cavity. This is in contrast to the conventional injection moulding process when the moulding machine creates the pressure within the plastic whilst it cools and solidifies, and at the same time forces more plastic into the cavity to compensate for natural volumetric shrinkage of the plastic when cooling. A further requirement of the gas assisted process is to release the gas pressure before the mould opens by exhausting the gas to avoid bursting of the moulding component after ejection from the mould cavity. Another version of Gas Assisted Moulding of comparatively thick sections referred to as 'the structural web process", comprises injecting gas into the plastic which forms skins adjacent to the mould surfaces, which are connected by randomly formed webs which then support the surfaces. This process also has the disadvantages of being difficult to control consistently. and it has comparatively long time cycles. Conventional injection moulding is a well known method of manufacturing components for footwear and even for moulding complete one-piece shoes, sandals or slippers. The process is particularly widely practised for making shoe soles and heels. PVC, polyurethenes and thermoplastic rubbers in a wide range of formulations and a wide diversity of physical properties are used, as well as other plastic materials which are capable of being moulded by injecting when in a molten state into a forming mould tool cavity under pressure. However injection moulding has the inherent limitations previously referred to. The basic objective of embodiments of this invention is to use the principles of Gas Assisted Moulding to mould comparatively thick sections over larger plate-like or circular shapes, and at the same time to control and direct the gas, forming channels into pre-determined positions and configurations which avoid the random formation of uncontrolled and variable voids in the moulded section. According to one aspect of the present invention, there is provided apparatus for gas assisted injection moulding comprising a mould having first and second mould parts defining a moulding cavity, means extending from the surface of the mould bordering the cavity for splitting the cavity at the surface into communicating sub-sections, means for introducing material to be moulded into the mould cavity and means for introducing a gas into the mould to pressurise the material in the mould. According to another aspect of the invention there is provided a method of gas assisted injection moulding comprising the steps of supplying material to be moulded to a mould, splitting the material supplied in the mould into two or more linked sub-sections and supplying gas to the mould to pressurise material to be moulded in the mould whereby to control the entry of gas into the sub-sections and thereby into the mould. The invention also comprises moulded products made by the above apparatus or method. In preferred embodiments comparatively thicker sections are moulded with a combination of externally formed dividing channels and internal continuous hollow channels or voids wholly within the moulded plastic material, formed by the injection of gas into the molten plastic material. External dividing channels are positioned so that the spaces between them are filled with plastic material, in which hollow internal channels are formed with comparatively controlled and predictable wall thicknesses and sectional configurations. The external dividing channels may be formed by male, web-like features in the mould tool. They may be designed and positioned to provide an advantageous configuration relative to the application requirement of the product. They may be positioned from one or both sides of the mould cavity. The in plate-like mouldings combination of external channels and the spaces or distances between them are selected in order to provide spaces which are relatively square or rectangular in cross-section, and of dimensions which will allow the penetration of the plastic with gas during the moulding process, and for the formation of moulded skins to be as uniform as possible and avoiding the formation of thick solid sections of plastic as far as is practicable. In other thick section mouldings such as shoe heels, the external dividing channels are used to prevent the formation of large, uncontrolled voids of gas by the separation of the gas flows into smaller controlled flows of gas. Lateral strength or rigidity may be provided across the external dividing channels by including ribs formed by plastic connecting the thicker sections in which internal continuous hollow channels have been formed by the gas. The connecting ribs will be sufficiently small in thickness to prevent the ingress of gas from one thick section to another, but at the same time to provide sufficient tensile and compressive strengths to prevent the structure flexing laterally. The positioning and frequency or spacing of connecting ribs will be dependent on the practical and physical requirements of the application. In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example with reference to the accompanying diagrammatic drawings, in which :- Figure 1A Illustrates a variable and uncontrolled void which would be created by gas injected into a flat thick section shoe sole and heel shape. Figure 1 B Illustrates a section through the centre line X-X of Figure 1A illustrating the variable and uncontrolled gas void section. Figures 1C-1 E respectively illustrates sections A-A, B-B and C-C, taken from Figure 1A and confirming variable position and thickness of gas void in the shoe sole. Figures 2A-2C, respectively illustrate the likely effect of injecting gas into a shoe heel section, and the variable and uncontrolled gas void which would result. Figures 3A-3D Illustrate the application of the invention to a typical shoe sole and heel. Figures 4A-4D correspond to figures 3A to 3D and illustrate the application of two gas injection positions in the formation of a typical shoe sole and heel, Figures 5A-5D correspond to figures 3A to 3D and illustrate the injection of gas at three or a multiple of gas injection positions in the formation of a typical shoe sole and heel, Figures 6A-6D correspond to figures 3A to 3D and illustrate the combination of the shoe sole and heel of figure 3 with a fitted but separate inner shoe sole. Figure 7A-7D correspond to figures 3A to 3D and illustrate the combination of the shoe sole and heel of figure 3 with an in situ moulded inner shoe sole - manufactured by a two-component injection moulding. Figure 8A-8D correspond to figures 3A to 3D and illustrate the combination of the shoe sole and heel of figure 3 with an in situ moulded outer shoe sole which is manufactured by a two-component injection moulding. Figures 9A-9C illustrate a shoe heel with typical external dividing channels and internal gas channels. Figures 10A-10C illustrate a modification of the arrangement of figures 9A to 9C. Figure 1 1 A Illustrates a cross-section of an insulated container or drinking mug taken along the line B-B of figure 1 1 B, Figure 1 1 B is a cross-section taken along the line A-A of Figure 1 1 A, Figure 12A illustrates the container or drinking mug of figures 1 1A and 1 1 B with a mating lid. Figure 12B is a cross-section taken along the line A-A of figure 12A Figure 13A illustrates the effect of injecting gas into a circular thick section plate-like moulding, resulting in an uncontrolled and variable gas channel formation, Figure 13B is a cross-section along the line A-A of figure 13 A, Figure 14A illustrates the application of the invention so that the circular thick section plate-like moulding of figure 13 includes radial dividing external channels and the resultant, more uniform flow of gas between the external dividing channels. Figure 14B is a cross-section along the line B-B of figure 14A. Referring to the drawings. Figures 1A to 1 E show a possible conventional mould 1 for producing a flat thick section shoe sole and heel shape 2. The mould 1 comprises first and second mould parts 3 and 4 which come together along a mould split line/face 5 and which define a mould cavity 6 in which the shape 2 is moulded. A moulding material inlet 7 and a gas inlet 8 lead into the mould cavity 6. Plastic material injected through inlet 7 into the mould contacts the surfaces of the mould bordering the cavity and begins to solidify on cooling. Gas injected into the plastic material through inlet 8 tends to produce a void 9 in the more molten material in the centre of the mould cavity 6. The shape and size of this void is uncontrolled and unpredictable. The thickness of the wall of plastics material surrounding this void 9 will also vary leading to uneconomic use of moulding material and to an unsatisfactory moulded product. Figures 1 C, 1 D and I E show how the section can vary at different points. Figures 2A to 2C show the problem described above in a moulded high shoe heel 21. Here a mass of plastics 22 forming the heel surrounds a void 23. The split line/face of the mould (not shown) is referenced 24 and the gas and plastic inlets 25 and 26 respectively. Figures 2B is the view in the direction of arrow X in figure 2A and figures 2C is the cross-section along line A-A in figure 2C. Figures 3A to 3D show the invention applied to a sole and heel shape of the sort shown in figures 1A to I E. Figure 3A is a plan view of the sole and heel, figure 3B is a cross-sectional view taken along the line C-C in figure 3A and figures 3C and 3D are cross-sectional views taken along the lines A-A and B-B respectively in figure 3A. In comparison with the arrangement shown in figures 1A to I E, the mould of the invention comprising a series of upstanding ribs extending from the surface of the mould defining the mould cavity. In the resultant moulded product shown in figures 3A to 3D, these ribs produce complementary shaped channels 30 extending into the product from the external surface thereof. The ribs which produce these channels 30 extend from the surface of the mould bordering the mould cavity in which the sole and heel is formed. The ribs in the mould and the corresponding complementary channels 30 formed in the moulding have the effect of splitting the mould cavity defined by the mould parts in which the moulded product is formed into a plurality of communicating sub-sections 31. T e gas injector for the mould is indicated at 32 and the plastic entry at 33. When molten plastic is injected it flows into the sub-sections and as explained earlier begins to cool and solidify firstly on the mould surfaces and the ribs. When gas is injected, it finds its way preferentially through the most molten plastic forming voids 34 in each of the sub-sections. The large uncontrolled void of the arrangement of figures 1A to 1E is replaced by a plurality of smaller controlled voids 34. Channels between the ribs split the ribs into sections and form subsidiary connecting ribs 35 between plastic adjacent the ribs in the mould. Such connecting ribs will provide lateral tensile and compressive strength to prevent flexing of the component laterally. The connecting ribs are normally positioned at right angles to the axis of the dividing channel. Figures 4A to 4D illustrate a modified form of the moulding shown in figures 3A to 3D to which figures 4A to 4D respectively correspond. In this modification the single gas injector 32 is replaced by two gas injectors 42 resulting in a modified gas void and channel arrangement. Parts or features in the modified embodiment bear corresponding reference numerals, but with the first digit changed from 3 to 4. Figures 5A to 5D illustrate another modified form of the moulding shown in figures 3A to 3D to which figures 5 A to 5D respectively correspond. In this modification the single gas injector 32 is replaced by three gas injectors 52 resulting in a modified gas void and channel arrangement. Parts or features in the modified embodiment bear corresponding reference numerals but with the first digit changed from 3 to 5. Figures 6A to 6D illustrate a modified form of the moulding shown in figures 5A to 5D to which figures 6A to 6D respectively correspond, in this modification a separate inner shoe sole 66 is fitted. In other respects the embodiment corresponds to that of figures 5A to 5D and corresponding parts or features bear corresponding reference numerals but with the first digit changed from 5 to 6. Figures 7A to 7D illustrate a modified form of the embodiment shown in figures 6A to 6D to which figures 7A to 7D respectively correspond. In this modification the inner shoe sole 76 is moulded in situ on another material and bonded thereby by means of a conventional two component injection moulding process. In other respects the embodiment corresponds to that of figures 6A to 6D and corresponding parts or features bear corresponding reference numerals but with the first digit changed from 6 to 7. Figures 8A to 8D illustrate a modified form of the embodiment shown in figures 7A to 7D to which figures 8A to 8D respectively correspond. In this modification an in situ moulded outer shoe sole 87 of another material is provided instead of an inner shoe sole by means of a conventional two component injection, in other respects the embodiment corresponds to that of figures 7A to 7D and corresponding parts or features bear corresponding reference numerals but with the first digit changed from 7 to 8. Figures 9A to 9C respectively correspond to figures 2A to 2C. Gas is fed to the mould by means of a gas injector 95 and plastic by means of a plastic feed 96. Figures 10A to IOC respectively correspond to figures 9 A to 9C but the gas and plastic feeds are from the bottom of the heel rather than the top. Corresponding parts/features have the same reference numerals preceded by the number 10 rather than 9. In the case of these shoe heels, as often included in ladies shoes, externally formed dividing channels 90, 100 of varying depths are provided to control and guide the formation of gas channels 94, 104 of restricted thickness and to avoid uncontrolled void or varying positions and configurations of the gas. For shoe heels, the inclusion of such controlled channels will contribute to the effective weight reduction in the moulded component. A normal but not essential requirement of the invention is of a design configuration incorporating external moulded dividing channels so that the shoe sole or heel is preferably covered with an inner sole in order to hide or cover the dividing channels. Such inner soles may be fixed or fitted free and separate from the moulded component, as is normal practice in shoe construction as illustrated in Figures 6A to 6D. Inner soles are often used for added comfort of the wearer of the shoes. The above described manufacturing process and equipment enables shoe soles, heels and other footwear components to be made with advantageous designs to give controllable and beneficial compressibility with difference in compressibility at selected and designed positions, with controlled stiffness, and with desired flexibility. Shoe soles, heels and other footwear components in which hollow channels or voids are formed with comparatively consistent cross-sections, profiles and material skin thicknesses, and at locations and positions where required may be made. Th e hollow channels achieve reductions in production costs principally from reductions in weight, and therefore cost of material. Reductions in moulding time cycles also result from the reductions in general wall thickness of the material. There will be further reductions in shoe construction costs by reducing the number of individual components, and in reducing complexity and manual labour costs involved in assembly of the shoe components. Other design features can be included in the moulded components, which may otherwise be impracticable when using conventional high pressure injection moulding techniques. This results from the substantial reduction of in-mould pressures when using Gas Assisted Moulding, compared with conventional moulding. This facilitates the use of more delicate materials, sometimes fabric materials, to be incorporated in the moulding process when previously they would have been damaged due to the higher pressures. Also there may be economies in mould tool construction, in machine investment costs and operating costs, resulting from the lower in-mould pressures and therefore lock force requirements. Alternative mould construction materials can be used, resulting in further cost savings. it is also possible to manufacture designs of shoe soles and heels which are more fluid and flexible, thereby conforming to individual foot shape, by injecting a liquid instead of a gas and by leaving the liquid in situ in the moulded component and preventing " its exhaust by sealing off the point of entry or exit of the liquid. Alternatively it is possible to pressurise the moulded component by injecting and sealing off gas at a pre-determined prescribed pressure, which would then produce a pneumatic effect, with the shoe sole or heel upper surface conforming to the shape of an individual's foot contours with added comfort. The design of shoe sole or heel or another component is formed by a combination of external formed dividing channels and internal continuous hollow channels or voids wholly within the moulded plastic material. The external dividing channels may be positioned so that the spaces between them are filled with plastic material, in which hollow internal channels are formed with comparatively controlled and predictable wall thicknesses and sectional configurations. The external dividing channels are formed by the male, rib-like features in the mould tool. They may be designed and positioned to provide an advantageous configuration relative to the human foot profile and shape in order to provide a variable compressibility and profile to optimise comfort of the shoe. The combination of external channels and the spaces or distances between them are selected in order to provide spaces which are relatively square or rectangular in cross-section, and of dimensions which will allow the penetration of the plastic with gas during the moulding process, and for the formation of moulded skins to be as uniform as possible and avoiding the formation of thick solid sections of plastic as far as is practicable. The invention has a wide variety and range of applications. One basic benefit will be the provision of heat insulation by the moulded product. The combination of plastic, which is a relatively poor conductor of heat, the hollow internal gas channels, and external dividing channels provide an advantageous means of restricting the conduction of heat, and therefore in providing an effective means of insulation. One such application is the moulding of one-piece food or liquid or other material containers in which it is desired to retain either cold or hot temperatures. One example is a drinking cup or mug or tumbler a shown in figures 1 1 A and 1 1B. In this application, external formed dividing channels 1 10 and continuous hollow internal channels 1 14 partially cover the base 1 15 and side walls 1 16 of the container. Such product may also be combined with a lid 127 which may be moulded in plastic, with or without the features of this invention as shown in figure 12A and 12B. Other applications for the invention will be for the design of components for refrigeration equipment which require a combination of insulation, rigidity and functional and aesthetic designs. Such applications could include doors, lids, shelves, side walls and compartmental dividers. Further economy in weight of material and improved flexibility and/or reduce resistance to compression, may be provided by including a blowing agent in the moulding material which will create a fine cellular structure within the moulded material. This reduces the density of the material, and therefore the weight and cost per unit. It can also add sponginess and flexibility if required. The principle of applying the inventive process to the production of the products, with one or more of the advantages cited, includes : feeding the plastic material into the mould cavity at an advantageous position, preferably at one end and at the edge of the component, alternatively the material may be fed directly into the component using recognised moulding methods of incorporating a hot runner in a mould (preferably fitted with valve gates which can be shut at the end of the plastic injection time), or other known methods of injecting plastic directly into a mould cavity. the gas may be injected either directly into the plastic in the mould cavity or into a feed runner, so that the gas enters the mould cavity at the same position as the plastic feed gate. Further the design and configuration of the dividing channels may require the injection of gas at two or more selected positions, the depth of the external dividing channels which are formed by the male ribs or webs within the mould cavity construction are selected with the objective of providing a reasonable depth of solid material at the base of the dividing channel, so that it will provide a sufficient period of time before wearing through as a result of loss of material to expose the external dividing channel, also with the objective of preventing gas from flowing from one thicker section gas channel to an adjacent gas channel during the moulding process. The cross-section of the external dividing channels varies with different applications and different materials. However it is preferable that the channels should be at a depth of between 90% and 60% of the total thickness of the component. The width of the external moulded channels varies from material to material, and is influenced by the depth of the mould rib and the requirement to conduct heat from the rib when plastic is being cooled. The thicknesses therefore vary, preferably between 1 and 4mm. The thickness of the rib also requires a generous draw taper to facilitate ejection of the moulding from the mould cavity. The thickness of the connecting ribs is dependent on the plastic material characteristics, and is a compromise between the requirement to provide lateral rigidity and strength between the thicker gas channel sections without allowing ingress of gas from one channel to another during the moulding process. The thickness of the connecting ribs will therefore preferably be between 1 mm and 3mm. It is also necessary to include generous draw tapers to facilitate ejection from the mould. The number of connecting ribs is dependent on the lateral rigidity or strength required and the material being used. The distance between the connecting ribs is therefore preferably between 10 and 30mm. The principle of using external dividing channels combined with thicker sections, in which hollow continuous gas channels are formed, may also be applied to shoe heels as well as ladies high heels. However it is a practical requirement of the mould construction that the external dividing channels are in the direction of the draw of the opening of the mould, and therefore the depth of the dividing channels varies according to the contour of the shoe heel. It is also necessary to include connecting ribs joining the thicker sections in which gas forms the hollow continuous channels. Similar principles as described above apply to the external dividing ribs in shoe heels. The basic principle of combining externally formed dividing channels with internal hollow gas channel sections can also be applied to the moulding of furniture components, and in particular table tops or chair seat bases. A typical example is illustrated in Figures 14A and 14B. Figures 13A and 13B show an uncontrolled and variable gas void 139 which would be formed if a gas was injected into a flat thick section plate-like circular component. Figures 14A and 14B illustrate the addition of externally formed dividing channels 140, which have the effect of dividing the gas flows 144 into relatively controlled flows of gas between each dividing channel. In this embodiment gas and plastic are fed through the same inlet 142. It will be appreciated that the above embodiments have been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims: CLAIMS 1. Apparatus for gas assisted injection moulding comprising a mould having first and second mould parts (1 ,2) defining a moulding cavity(6), means extending from the surface of the mould bordering the cavity for splitting the cavity at the surface into communicating sub-sections (31 ), means for introducing material (33) to be moulded into the mould cavity and means for introducing a gas (32) into the mould to pressurise the material in the mould. 2. Apparatus as claimed in claim 1 , in which the means extending from the surface of the mould comprise one or more male projections extending from at least one of the mould parts (1 ,2) into the mould cavity (6). 3. Apparatus as claimed in claim 2, in which the means extending from the surface of the mould comprise one or more male projections extending from both mould parts into the mould cavity. 4. Apparatus as claimed in claim 2 or 3, in which one or more channels are provided through the or each male projection. 5. Apparatus as claimed in claim 2, 3 or 4, in which the or each male projection is elongate. 6. Apparatus as claimed in claim 5, in which the or each male projection follows the external contour of the mould along at least a part of its length. 7. Apparatus as claimed in any of claims 2 to 6, in which the means for introduci g gas (32) are disposed in relation to the or each male projection to supply gas into those parts (34) of the mould cavity at least partially defined by the or each male projection. 8. Apparatus as claimed in any of claims 1 to 7, in which the moulding cavity is for a combined shoe sole and heel. 9. Apparatus as claimed in any of claims 1 to 7, in which the moulding cavity is for a shoe sole or heel. 10. Apparatus as claimed in claim 9, in which means are provided for moulding an inner shoes sole and bounding it to the combined shoe sole and heel in a two component injection moulding process. 1 1 . A method of gas assisted injection moulding comprising the steps of supplying material to be moulded to a mould (1 ,2), splitting the material supplied in the mould into two or more linked sub-sections (31 ) and supplying gas to the mould to pressurise material to be moulded in the mould whereby to control the entry of gas into the sub-sections and thereby into the mould. 12. A method as claimed in claim 1 1 , in which the material is split by means of one or more male projections extending from at least one of the mould parts (1 ,2). 13. A method as claimed in claim 12, in which material (35) to be moulded is supplied to one or more channels extending through the or each male projection. 14. A method as claimed in claim 1 1 , 12 or 13, in which gas is supplied to the mould to form voids (34) within the molten plastic in the mould. 15. A method as claimed in claim 14, in which the voids (34) are continuous, hollow channels in the material. 16. A method as claimed in any of claims 1 1 to 15, in which the sub¬ sections (31 ) are disposed and the gas is controlled to produce in the material in those sub-sections hollow channels (34) having controlled and predictable wall thicknesses. 17. A method as claimed in claim 14, 15 or 16, in which the voids (34) have substantially square or rectangular cross-sections. 18. A method as claimed in claim 13, or in any of claims 14 to 17 when appendant directly or indirectly to claim 10, in which the thickness of the or each channel is chosen so that, in operation, gas does not penetrate the molten material (35) therein. 19. A method as claimed in claim 18, in which the dimensions of the or each channel are chosen to provide lateral rigidity to a moulding. 20. A method as claimed in any of claims 11 to 19, in which a combined shoe sole and heel is moulded. 21. A method as claimed in claim 20, in which a shoe sole or heel is moulded. 22. A method as claimed in claim 20, in which an inner shoe sole is moulded and bonded to the combined shoe sole and heel in a two component injection moulding process.

Inventors: PEARSON TERENCE COLWYN, PEARSON, TERENCE, COLWYN
Applicants: GAS INJECTION LTD, PEARSON TERENCE COLWYN, GAS INJECTION LIMITED, PEARSON, TERENCE, COLWYN
Application year: 1998
Main classification: B29C45
IPC codes: B29C45, B29D31