The solidification of large sections in ceramic injection molding: Part II. Modulated pressure molding
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Cylindrical moldings of 20 and 40 mm diameter were injection molded with the application of modulated hold pressure using a well-characterized alumina-polypropylene suspension. The effect of frequency on sprue solidification was explored. For the smaller moldings, very little extension to sprue solidification time was obtained with pressures up to 140 MPa, and this is attributed to the low reciprocating volume flow. For the larger moldings, pressures of 98 MPa were sufficient to produce moldings with neither voids nor cracks, and the sprue solidification time corresponded to the time needed for solidification of the molding. The use of higher pressures resulted in internal residual stresses which were qualitatively detected by the defomation on annealing of polished diametral sections.
I. INTRODUCTION
II. EXPERIMENTAL DETAILS
The weaknesses of conventional ceramic injection molding for large sections have been described and interpreted in terms of shrinkage during solidification in the cavity.1 The use of a suspension for which thermal properties and equation of state are known2'3 has enabled a quantitative interpretation of shrinkage voids to be made.1 The development of residual stresses during solidification can be described qualitatively in terms of differential thermal contraction and the strains in previously solidified layers caused by a change in pressure in the molten core during the progress of solidification. The quantitative prediction of stresses is possible only for simple geometries at present.4 It was observed that in conventional molding, the hold pressure "window" within which moldings were free from voids at low hold pressures, as well as from cracks at high hold pressures, was closed. Previous work has shown that when modulated pressure is used, the window is opened for thick sections.5'6 In that work, attempts to quantify the solidification process relied on estimates of thermal properties,6 and approximate values of heat dissipated in oscillating flow were obtained. The modulated pressure technique is of particular value where the sprue does not enter the largest section of the molding or where more than one large section is separated by narrow channels. If the sprue does enter the largest section, then a heated sprue can achieve the same purpose with less machine modification.7 In the present work, the solidification of sprue and molding are quantified by a finite difference method in order that the effect of modulated pressure can be compared directly with the conventional molding situation reported previously.
The alumina suspension was identical to that previously described1 having 56 vol. % ceramic based on an atactic-isotactic polypropylene blend with stearic acid lubricant in the weight ratios 4 : 4 : 1 , respectively. It was mixed in an intermeshing corotating twin screw extruder (model TS40, Betol Machinery, Luton, U.K.) with barrel temperatures 190-200-220-220-210 °C feed to die with a rotation speed of 60 rpm. The injection molding machine was a Negri Bossi NB90 (John Brown Mach
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