Injection moulding is a forming technique used extensively in the plastics industry. In the basic form of this technique, molten plastic is forced through an orifice into an otherwise sealed mould cavity. It is common for the cavity to be made of metal, typically tool steel. Heat flows from the plastic into the cavity until the plastic cools and sets. The piece, which is now rigid can be ejected from the cavity.

Injection moulding is an attractive forming technique for other materials because of its speed, low labour content, ability to make complex shapes in one process, and ability to be automated. Variants of the technique, which are generically known as Powder Injection Moulding (PIM), have been developed for metals, ceramics and other particulate materials.

In general the PIM techniques use thermoplastic binders, filled with a particulate, which can be formed in a similar way to plastic materials. However many of these systems have the disadvantage that after forming, the pieces must have the thermoplastic binder removed to enable the particulate to be sintered by the action of heat. The binder removal process can be slow and complex and lead to high production losses.

To overcome some of the problems associated with the removal of thermoplastic polymer binders, several systems have been developed which use solvent, typically water-soluble binders. These binder systems have the advantage that once the pieces have been dried, to remove the solvent, the remaining binder components can be removed with less disruption to the structure of the shaped particulate component. In addition some of these binder systems are attractive because of their lower relative cost per item.

One less favourable feature of these solvent based systems is that it is difficult to achieve the same degree of resistance to deformation that is achieved with thermoplastic polymer binders. When trying to eject the piece from the mould, distortion may occur because of the forces acting on the piece.

Specifically, distortion can result from the suction forces applied by the tool surfaces.

According to the present invention there is provided injection moulding apparatus, the apparatus comprising a mould having a mould cavity, with at least part of the material forming the moulding surface of the mould cavity being porous.

The porous material is preferably air permeable.

Preferably only part of the mould surface is formed by porous material.

The remainder of the mould surface is preferably partially or wholly formed of metal, and desirably steel.

The porous material preferably comprises a plastics material, a porous metal, or a porous composite material. The porous material may comprise a porous epoxy resin aluminium composite, or a sintered metal such as stainless steel or bronze.

The pore size of the porous material is preferably such that material being moulded does not substantially enter the pores, and the pore size may be less than 25 micrometers.

Means may be provided to permit a fluid to pass through the porous mould surface to aid release of a moulded item.

In an alternative arrangement the porous material may be in the form of a coating on at least part of the inner face of the moulding surface. The coating may comprise epoxy porous aluminium or a porous metal. The coating may be provided on a steel substrate and could be machined to a correct profile.

Alternatively, the coating could be applied by spraying and desirably plasma spraying.

The invention also provides powder injection moulding apparatus for producing items of ceramic hollow ware, the apparatus being according to any of the preceding seven paragraphs.

The apparatus preferably comprises a mould formed of a number of components including a component defining the base of the mould cavity which component is not made of porous material, and a component or components defining the side of the mould cavity at least part of which component or components is made of a porous material.

The mould preferably also comprises a component defining the inner core of the mould cavity, which component may be made of porous material.

The mould may also comprise a component defining the lower end of the inner core of the mould cavity, which component may be made of non-porous material. The mould may also comprise an ejector ring, which is desirably not made of porous material.

The invention also provides a method of injection moulding using apparatus according to any of the preceding nine paragraphs.

The invention yet further provides a method of powder injection moulding using apparatus according to any of said preceding nine paragraphs.

The material being moulded preferably comprises a binder which is solvent based, and preferably water based. The binder may comprise any of PVA ; cellulose derivatives such as methyl cellulose; polysaccharides such as agaroids; starches; or combinations of any of these materials, perhaps with other materials.

The material being moulded may be any of : ceramics and particularly any bone china, structural ceramics or functional ceramics; steel; ferrous or non- ferrous alloys ; cermets; powder and fibre reinforced composites; powder phenolic resins; or foodstuffs.

A fluid may be passed through the porous mould surface following moulding to aid release of a moulded item, and the fluid may comprise a mould release agent. Alternatively air may be blown through the porous mould surface.

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawing which is a diagrammatic cross sectional view through moulding apparatus according to the invention.

The drawing shows powder injection moulding apparatus 10 suitable for producing ceramic cups made for example from bone china. The apparatus 10 defines an appropriate shaped mould cavity 12. The mould cavity 12 is defined by the following six parts. A base 14 made of tool steel. Two side cheeks 16 are provided made of a porous epoxy resin/aluminium composite such as Metapor HD1OOAL available from Portec AG of Switzerland. This has a permeability index of 56 as measured by the manufacturer.

The mould cavity 12 is also defined by an ejector ring 18 made of tool steel. A core 20 of the above porous/permeable material is provided with a non-porous core end 22 also made of tool steel.

The apparatus 10 is usable as follows. The powdered bone china is produced by a conventional technique and mixed with a PVA/cellulose/water binder system. This is injected into the apparatus 10 at a temperature of 85°C to 90°C with the apparatus at a temperature of 8°C to 10°C, and moulded for forty seconds. The resultant shaped piece is handleable but not as rigid as could be produced for example using a thermoplastics binder route. The components of the mould cavity made of the above porous/permeable material enable release therefrom without damage to the delicate parts such as the side walls 24 and handle 26.

There is thus described apparatus which permits the above relatively low cost and environmentally friendly binder system to be used which can be driven off using conventional techniques, without the resultant moulded piece being damaged during removal from the mould. The apparatus is of relatively conventional construction and can thus be inexpensively manufactured and formed to meet particular circumstances or requirements.

Various modifications may be made without departing from the scope of the invention. Whilst the above described example relates to powder injection moulding of ceramic materials, the invention could be used with a wide range of other materials such as other ceramics and particularly structural ceramics or functional ceramics; steel; ferrous and non-ferrous alloys; cermets; powder and fibre reinforced composites; powdered phenolic resins; soap; or foodstuffs, and particularly items such as beef burgers and the like which require shaping. The invention can also be usable in other types of injection moulding.

The invention is usable with other aqueous binders such as cellulose derivatives and especially methyl cellulose; polysaccharides and especially agaroids; and starches. The invention could also be used with non-aqueous binders. Other porous/permeable materials could be used such as other similar composites; sintered metals such as stainless steel or bonze; or other porous materials such as porous plastics materials. The selection of material is dependent on the ability to form, for instance by machining or casting, or the pore size. In general this has found to be preferable below 25 micrometers for bone china, but the pore size may differ dependent on the particle size of material being moulded and/or the moulding pressure. The choice of material also depends on permeability, abrasion resistance, heat transfer and mechanical stability.

In an alternative arrangement a coating of porous/permeable material could be provided on an otherwise non-porous material. For instance a solid block of tool steel could be rough machined to give an oversized cavity. A coating of for example epoxy porous aluminium could be applied, cured and subsequently machined to a correct profile. An alternative would be to provide a thin porous coating of for instance porous metal, and this could be applied to a steel substrate by spraying and particularly plasma spraying.

A fluid such as a mould release agent may be passed through the porous part of the mould to aid release of the moulded item. Alternatively, air may be blown through the mould to aid release. The operating temperatures outlined above may be varied. For instance, the material could be injected at a temperature within the range 50°C to 110°C, and the apparatus may be at a temperature of between 5°C and 30°C.

Various other modifications may be made without departing from the scope of the invention, and combinations of the above features could be provided.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.