A more representative mechanical testing of green state investment casting shell

The influence of de-waxing method and post de-wax treatment on the flexural strength of ceramic shells has been evaluated. De-waxing by autoclave generates the highest strength when dry samples are tested, significantly stronger than when de-waxed by refrigerating. The autoclave-dewaxed shells were found not to be fully de-waxed but to have a coating of wax and some wax penetration into the face coat. This and the curing of the sol by the autoclave process were shown to be responsible for the higher strength.
Treating the shells after initial de-waxing to simulate the conditions in the autoclave by either boiling in water, steaming above water or autoclaving at 180 °C and 0.8 MPa caused changes in the reported flexural strength. For samples initially de-waxed by autoclave the strength was reduced, boiling caused the greatest strength reduction and re-autoclaving gave the least change. Testing wet always gave lower strengths compared to dry.However, initial dry strength was never regained on further drying. The samples prepared by refrigeration lost strength when boiled or steamed but gained strength on autoclave treatment. The strength built to be 57% of the samples de-waxed by autoclave in the dry state. This occurred because the sol cured; however, the shells being essentially free from wax never gained the strength of the samples de-waxed initially by autoclave.