6.3. Loose Sintering

This method is quite widely used for manufacture of highly porous parts like filters. Basically, metal powder is poured or vibrated into a mould which is then heated to the sintering temperature in an appropriate atmosphere. The form and complexity of shape which can be made by this method depend to a large extent on the flow characteristics of the powder. Since shrinkage usually takes place during sintering, only shapes where this can occur without mould constraint causing cracking can be used. The characteristics required of the mould material for loose sintering are:

( i) It should be easily machined or formed into the required shape, ( ii) it should withstand the sintering temperature without appreciable deformation and ( iii) it should not weld to the powder during sintering. For most applications machined or welded metal moulds are used, although graphite may also be employed where no reaction with the powder is likely, and even here a refractory mould wash may suffice to prevent reaction. Since no pressure is applied to the powder any unreducible oxide skin on the particle will prevent metal-to-metal contact and inhibit sintering. For this reason it is virtually impossible to loose sinter aluminium powder.