Optimization of heating schedules in pyrolytic binder removal from ceramic moldings
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Optimization of heating schedules in pyrolytic binder removal from ceramic moldings J.H. Song, J.R.G. Evans,a) and M.J. Edirisinghea) Department of Materials Engineering, Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
E.H. Twizell Department of Mathematics and Statistics, Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom (Received 20 July 1998; accepted 15 November 1999)
A model that finds the maximum permissible heating rate for pyrolysis of ceramic moldings is extended to produce multi-segment temperature–time profiles to minimize the binder removal time. The degradation of the polymer and the diffusion of degradation products in solution to the free surface in a cylinder containing 50 vol% alumina and polyalphamethylstyrene is considered. The theory has previously been validated experimentally for fixed heating rates for cylindrical and flat plate geometries and for overpressure debinding. The extended model, presented here, calculates the vapor pressure of monomer over solution and modifies the heating rate to keep this just below ambient pressure. In this way, the temperature follows the maximum allowable rate at each stage to prevent boiling and hence the incidence of defects.
I. INTRODUCTION 1,2
In powder injection molding and the related plasticforming operations3–5 a high volume fraction of powder (0.5–0.7) is usually incorporated in a polymeric organic vehicle and the shear or elongational flow of the suspension is used to confer shape. The organic vehicle is then removed by pyrolysis and the residual assembly of particles is sintered to near full density just at it would have been if assembled by powder compaction. The early stage of pyrolysis is problematic because connected porosity is absent and very low heating rates are necessary. Gravimetric control has been used to adjust the temperature during heating in order to achieve a controlled linear rate of weight loss as the polymer is degraded and displaced.6 This tends to produce a three-stage temperature schedule comprising an initial fast rate, an intermediate low rate during which the bulk of the organic matter is displaced, followed by a final increased rate. Similar procedures have been implemented for the manufacture of ZnO varisters.7 Methods of adjusting the composition of the organic vehicle have been devised to give a slow rate of weight loss as a function of temperature.8,9 These approaches neglect the resistance to mass transport of the degradation product early in the cycle. At this stage, transport of degradation products is by diffusion in solution in the continuous phase, namely the polymer melt. a)
Present address: Department of Materials, Queen Mary and Westfield College, University of London, London E1 4NS, United Kingdom. J. Mater. Res., Vol. 15, No. 2, Feb 2000
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In recent years, this problem has been solved for simple polymers for steady-state10 and unsteady-state diffusion11 and some of these model
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