Reaction Injection Molding of Silica-Alumina Mixtures Using Heterocoagulation
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REACTION INJECTION MOLDING OF SILICA-ALUMINA MIXTURES USING HETEROCOAGULATION 0 VIJAY V. PUJAR*' , JAMES D. CAWLEY* , PING HU**, AND L. JAMES LEE** *Dept. of Materials Science and Engineering, and **Dept. of Chemical Engineering, The Ohio State University, Columbus, OH 43210.
ABSTRACT A colloidal processing technique based on the heterocoagulation of selected ceramic oxide systems, and analogous to reaction injection molding of polymers, is described. Commercial colloidal silica and boehmite sols, which have oppositely charged particles in the pH range 2.09.0, and mixtures of these suspensions with micron sized oxide powders, have been used to determine whether a liquid to solid transformation could be induced when the two materials are simultaneously injected at high velocities into a mold cavity. Rheological changes have been measured as a function of relative flow rates of the two suspensions, solids loading and oscillatory frequency. Viscosity is observed to increase by as much as six orders of magnitude under optimized conditions. INTRODUCTION Injection molding of ceramics has the potential to form net-shape ceramic components, particularly of complex geometries. It is an attractive alternative to processes such as slip casting, since it is a high throughput technique and also has the potential of good dimensional control. Most injection molding mixtures consist of a ceramic powder dispersed in an organic binder/plasticizer mixture, which undergoes a liquid to solid transformation in the mold cavity forming a green compact with the correct geometrical information. The main drawback of the technique is removal of the large amounts (usually 30-40 vol.%) of binder phase during subsequent thermal processing, which often results in tile introduction of structural flaws and distortions in the components. Although several innovative binder removal methods have been developed to overcome these problemst, it has been recognized by several independent investigators 2-7 that an aqueous based binder system offers several fundamental advantages for molding powders that require subsequent thermal processing. The processing technique described here, explores heterocoagulation (which occurs when two suspensions with oppositely charged particles are brought together) as a possible
mechanism for reaction injection molding (RIM)9 of selected oxide ceramics. RIM is a well
developed method for processing polymersg. . The technique essentially involves the simultaneous injection of two highly reactive monomers into a mold cavity through a runner, as shown schematically in Figure 1, where they react rapidly to produce a solid part by chemical
cross-linking or physical changes such as domain formation and crystallization. The work reported here represents exploratory experiments using the well-known alumina-silica system as a model. The feasibility of our approach is demonstrated using colloidal boehmite, AIO(OH), and silica sols (which have 11.3 and 15.5 vol. % solids respectively in the as-supplied condition). Experiments were
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