Face Coat Slurries

Typically at least two slurries are used. The first of these is called the face coat or prime coat slurry. It is made up of very fine ceramic flour usually 200 mesh or finer. This slurry is the first slurry the wax patterns are dipped into and the very fine texture is required to faithfully replicate the fine details of the pattern. It is also the surface that the molten metal comes in contact with, when it is poured into the mold. Because of this, it is very important that the ceramic selected for this first or prime layer be inert chemically to the molten metal being poured. If not, the molten metal will react with the ceramic and lose its desirable metallic character. 

Another characteristic the first dip must have is that it has to wet the wax surface and form a uniform layer. Since waxes are seldom hydrophilic, wetting agents are added to the face coat slurry to enable it to wet the surface. In addition, the wax patterns are typically cleaned in order to remove any traces of the mold release (usually a silicone) which was used during the pattern 

FIGURE 12.40 The pattern ttee is drained of excess slurry and stucco sand is added, the shell is dried and additional coats are added for strength. 

It would be even more desirable if the face coat actually bonded to the wax pattern, but this does not happen. The wetting agents form a layer on the surface of the wax that inhibits actual contact with the colloidal silica particles. DuPont developed and patented a product called Ludox IWS [5]. IWS is a mixture of colloidal silica and zirconia, which actually appears to bond the organic surfaces at least strongly enough to resist it has being washed away by repetitious rinsing. 

If a hydrophobic wax pattern is dipped in a surfactant-water mixture uniform wetting of the surface can be observed. However, if the pattern is rinsed in water, the surfactant is rinsed away and the surface again becomes 

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There are three basic slot-die orientations die facing upward, horizontal, and downward. With the die face pointing upward, coating can be problematic, especially with thin or low-viscosity slurries, since slurries can flow backwards down the web toward the die. The tendency for slurries to back flow on the web can be reduced by changing the orientation of the die to either horizontal or facing downward. The horizontal position also provides the best combination of air purging at start-up, clean operation, and the finest degree of operator adjustment. 

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