WO1999003665A1

WO1999003665A1 - Improvements in or relating to gas assisted injection moulding

Info

Publication number: WO1999003665A1
Application number: PCT/GB1998/001879
Authority: WO
Inventors: Terence Colwyn Pearson
Original assignee: Gas Injection Limited
Priority date: 1997-07-15
Filing date: 1998-07-14
Publication date: 1999-01-28
Also published as: AU8447298A; EP1005409A1; GB9714807D0

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

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┬¼

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.