S Krug

Residual stresses and cracking in large ceramic injection mouldings subjected to different solidification schedules

In ceramic injection moulding, the moulding dimensions and the residual stresses are related to the hold pressure history during solidification in the cavity. In conventional moulding, the residual stresses are generally compressive at the surface. In this work, an insulated sprue device was made. It allows prolonged pressure transmission to the moulding and this can result in residual tensile stresses at the surface of thick section mouldings. At very low holding pressures (<9 MPa) or under conditions of premature gate solidification, a reversal of the sign of surface stresses occurs with compressive stresses near the surface. The appearance of cracks during binder removal was related to these residual stresses. For 25 mm thick mouldings which were subjected to a low but persistent hold pressure of 5 MPa no defects developed during the binder removal stage.

Differential sintering in ceramic injection moulding: particle orientation effects

Abstract The sintering shrinkage in three orthogonal directions was measured in sections cut from large rectangular injection moulded ceramic blocks. Two powders were used: a comminuted alumina and a chemically-derived zirconia. Pronounced anisotropy of shrinkage was observed in the former whereas the equiaxed zirconia showed uniform shrinkage throughout the moulding. The pattern of shrinkage anisotropy was consistent with that observed by other investigators and indicates that slight particle anisotropy can introduce differential sintering. The sintering shrinkage in the alumina powder was found to be most pronounced perpendicular to the main flow direction.

Transient effects during catalytic binder removal in ceramic injection moulding

Abstract This work is part of a comprehensive study of the incidence and cause of defects in large section ceramic injection mouldings. In previous related work, the defects associated with mould-filling and with solidification and packing in the cavity were identified. Such defects sometimes manifest themselves only after binder removal or after sintering and so may be causally connected with those stages by mistake. In the present work, defects which do indeed arise from the binder removal stage itself as a result of minor process interruptions are isolated. These are identifiable by a pattern of defects which are distinct from those due to solidification and are independent of solidification conditions. This work reports on the systematic study of perturbations to conditions in the binder removal oven in order to track the origin of these defects and hence distinguish them from shrinkage-related defects.

Packing and solidification in ceramic injection moulding

The occupation of volume in large ceramic injection mouldings solidified under different pressures using two moulding techniques is considered. Large (60 × 45 × 25 mm) alumina injection mouldings were prepared in cavities fitted with conventional (metallic) and insulated [poly(ether ether ketone)] sprues. These provide quite distinct solidification histories which, in turn, influence the physical properties of the moulding and the extent and type of defects it contains. In the former case, the gate solidified after 26 s whereas, in the latter, the hold pressure could be applied for over 240 s. In insulated sprue moulding, the advance of the solid liquid boundary during packing and solidification was traced by fractography. Pressure was varied from 5 to 120 MPa. The interdependencies of moulding mass, apparent density, local density, polymer crystallinity, and microstructure were accounted for as a function of pressure and pressure transmission method. Changes in polymer crystallinity due to different cooling rates at different positions in the mouldings were insufficient to account for observed density differences. Systematic changes in the mass as a function of hold pressure were related to macroporosity in conventional mouldings and to microporosity in insulated sprue mouldings.

Solidification of large section ceramic injection mouldings under low hold pressures

Many of the problems of making large ceramic injection mouldings have been identified with solidification in the cavity, particularly with the level of hold pressure applied during cooling. In this work, a wide range of hold pressures (0.1–120 MPa) was used to compensate for thermal contraction of large mouldings. A new pneumatic technique for applying pressure was designed. An insulated sprue was used to prolong solidification of the gate. These techniques provide a combination of high injection pressure during mould filling (95 MPa) with hold pressure control in a range from 0.1–1 MPa during solidification. Large (25 × 45 × 60 mm) mouldings made at these low hold pressures did not have shrinkage-related cracking and the organic vehicle could be safely removed.