Flocculation after injection molding in ceramic suspensions
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The flocculation of an unstabilized suspension of fine ceramic particles is advanced as the explanation for the formation of cracks in a "liquid" suspension. The development of cracks was observed several minutes after reheating wax-based ceramic moldings above the melting point of the wax and was accompanied by phase separation of the wax from the molding. Calculations of the acceleration of particles under London dispersion forces in a viscous fluid show the "time to impact" as a function of initial separation distance, fluid viscosity, and particle size. This is compared with the intercollision time calculated from classical flocculation theory, and it is shown that for crowded suspensions initial interparticle distances are such that the London force field cannot be neglected. Methods of preventing the flocculation are described.
I. INTRODUCTION Injection molding, slip casting, and compaction represent the three main classes of fabrication processes for manufacturing engineering components from powders. The former, along with the related plasticforming processes,1 differs from the other two in that particles are not in contact with their neighbors after filling the die cavity. This arises from the free volume fraction of organic binder which is necessary to allow fluid flow.2 It is only when this excess is removed during pyrolysis, solvent extraction, or capillary flow that particles actually make contact in preparation for sintering. It follows that changes that take place in the organic phase both during and after molding can influence interparticle distances. These include the strains associated with relaxation of residual stress3'4 or with orientation5'6 or the nonuniform removal of binder.7'8 These changes affect the positions of particles before sintering. Popular interest in sintering processes has shifted, in recent years, from the classical three-sphere model,9 with its tacit assumption of continuity, to the importance of local differences in packing as the source of strength-limiting defects.10"12 Although injection molding does not have the inherent problem of interagglomerate defects which plagues compaction operations, any process that rearranges particles such that the pattern of contacts is irregular is a potential cause of defects. An important source of movement has been identified in this work, and attempts are made to account for it and to show how it can be avoided. In low viscosity organic vehicles such as waxes, fine particles may become mobile when the molding is reheated above the wax-melting point. The low viscosity of the continuous phase may, if the suspension is not correctly stabilized, allow particles to approach each other under London 2386
J. Mater. Res., Vol. 9, No. 9, Sep 1994
http://journals.cambridge.org
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dispersion forces. This is found to cause floes which develop into domains that are separated by large fissures. This leads to the surprising proposal that cracks can form in a ceramic body at a stage when no binder has been expelled and where rhe
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