Perhaps the number one problem faced by investment casters is shell cracking. This paper focuses on what are believed to be several contributing factors in shell cracking: shell permeability to wax and wax viscosity.

A permeable shell is only effective if the wax viscosity is low enough at dewax temperatures. Similarly, a wax with sufficient fluidity will not prevent shell cracking if the prime coat lacks sufficient permeability to allow it to pass thru during the dewax stage. Coarser, narrower flour distributions are well recognized for better permeability while finer, wider distributions are known for their lower permeability. However, how is shell cracking affected by these when wax viscosity is changed? Further, if the prime coat permeability is improved sufficiently, what range of acceptable wax viscosities may be used while still avoiding shell cracks?

This study will present data on how shell permeability, (adjusted through the number of prime dips, primary slurry flour particle size distribution (PSD)) and wax viscosity influences shell cracking. Several waxes of varying viscosities were injected into a standardized wax pattern geometry. The wax patterns were then assembled and invested in different slurry combinations. Primary slurries with different PSD’s were investigated with various numbers of prime dips. A large number of shells were invested using the previously proven hollow pipe geometry for autoclave testing. Separate shells were invested using PVC pipes to allow permeability and burst test data which is correlated to autoclave crack occurrence.