Shell Coats

Shell Building Summary

  • Inspect the assembly prior to dipping for cracks, leaks and solid sections

  • There is no need to avoid the use of etching or pre-wet solutions

  • Take care in dipping to avoid damage to the assembly from buoyancy forces.

 

Currently, foundries often used more coats on shells for QuickCast patterns than they did for wax patterns to provide greater strength to resist expansion of the shell during autoclave and burnout phases. Recent testing has shown, however, that cracking of the shell in the autoclave can be avoided relatively easily without the use of additional shell coats and is discussed in detail in the Pattern Removal section in this guide. Consequently, it is generally not necessary to use more coats for a QuickCast pattern than would be normally used for a wax pattern.

There are exceptions to this rule; if the geometry is particularly challenging, or contains thin walls that are solid rather than hollow, foundries may still elect to add additional coats. Given the cost of QuickCast patterns, adding extra coats is a relatively inexpensive way to increase the probability of a successful casting. Additional methods to add strength include; using wire mesh, adding chopped ceramic fibers or chopped stainless steel wool between coats.

Foundries often use a pre-wet solution to assist in shell building for certain shell situations. The assembly is dipped in the pre-wet solution to assist slurry to flow into tight areas and around complex features. A number of foundries avoid using pre-wet solutions with QuickCast patterns because they believe that contact with the pre-wet solution may damage the pattern. While we have not done extensive testing, suppliers of pre-wet solutions have assured us that pre-wet solutions are much less aggressive than etching solutions and if etching solutions have no effect, pre-wet solutions certainly will not.

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Adding extra backing layers to the shell mold will help overcome the higher stresses placed on the mold by specific QuickCast style pattern  geometries.