Solid Freebody Forming of Ceramics from Polymerizable Slurry
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ABSTRACT A system is described for forming three-dimensional parts from a polymerizable slurry. This liquid is delivered from a fine nozzle to build up the component as a series of layers described from a CAD drawing of the part. This is one of a family of solid freebody forming methods. The layerwise process lends itself particularly to chemically syntheized materials where shrinkage and mass transport are major problems. The technique also has considerable potential for combining various materials into a single component.
INTRODUCTION In solid free body forming a component is described by a CAD system as a series of slices. The slices are deposited as a sequence of layers which are cured or solidified by a moving head such that a three dimensional part is built up. Systems have been developed based on photocurable polymers, solidifying polymer melts, laser fused powder layers, polymer-bound powders and others [11. Difficulties with such systems to date include the low strength of many of the polymer systems and poor resolution. The polymeric components can be used as forms to make molds for later casting of metals or ceramics. The powder components can be sintered to higher density. In the system described here, the part is built up by deposition of a slurry of ceramic powder in liquid acrylic monomer. As each layer is deposited the monomer is heat-cured. When the whole component is formed, the polymer is burnt-out and the ceramic is sintered.
DELIVERY SYSTEM The slurry is contained in a 50 ml polypropylene syringe which was connected to an argon pressure line to drive the liquid through a needle of 0.3-1.3 mm diameter. The delivery was started and stopped by a shut-off valve with pressure venting in the air line, figure 1. CERAMIC MOLDING Slurry Slurries of Alumina (A120 3-0.5 wt% MgO, Ceralox Corp.) in liquid acrylic monomers were prepared. The formulations, based on the work of Janney [2], are shown in Table 1. 1027 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
Air Pressure
VOLN
Syringe
Needle
Part
Figure 1:
Hot Ptaten
Schematic Diagram of Equipment
Similar monomer systems have been used for tape casting of ceramic sheets by various workers 131. Alumina solids loadings up to 53 vol% were used. The acrylate monomers have the advantage of curing very rapidly. Since they are polyfunctional, high degrees of polymerization are not necessary to achieve high molecular weight or gelation. The polymerization rate and brittleness of the resin was controlled by addition of dibasic esters, a mixture of primarily dimethyl glutarate and succinate. Viscosity increases with solids loading as shown in Figure 2. Figure 3 shows slurry flow rates as a function of applied pressure through a 0.33 mm needle,
Deposition and curing Thin layers of slurry were deposited onto a heated platen at about 120°C. The temperature was adjusted to give rapid curing and good shape definition. Oxygen inhibition of the polymerization leaves the surface tacky and improves the degree of bonding between
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