Causes of cracking in silica sol precision casting mold shells

Improper selection of shell material: Insufficient strength or mismatched coefficients

of thermal expansion of the shell material may lead to a decrease in shell strength

at high temperatures, resulting in cracking. High-strength shell materials with low

coefficients of thermal expansion, such as silica sol or water glass shells, should be selected.

Uneven coating thickness: Uneven coating thickness will cause inconsistent shrinkage

after shell drying, resulting in stress concentration and cracking. Even coating

application should be ensured, avoiding localized accumulation or excessive thinness.

Insufficient shell drying: Moisture in the shell will generate steam at high temperatures

during dewaxing or casting, increasing internal pressure and causing cracking

Strict control of drying time and temperature is necessary to ensure complete drying.

Excessively high dewaxing temperature: Excessively high dewaxing temperatures

will cause rapid expansion of the wax material inside the shell, exerting excessive

pressure and leading to cracking. Dewaxing temperature should be controlled to

avoid excessive heating rates, while ensuring good air permeability of the shell.

Uneven firing temperature: Uneven firing temperature during shell firing may cause

localized overheating or undercooling, creating thermal stress within the shell.

Uniform heating and cooling processes are required to ensure a consistent overall shell

temperature.

Excessively high or low pouring temperature: Excessively high pouring temperature

causes the shell to heat up too quickly, increasing internal stress; excessively low pouring

temperature reduces molten metal flow, increasing pressure on the shell. Strict control

of the pouring temperature is necessary to ensure it remains within a suitable range.

Inadequate casting structure design: Uneven casting wall thickness leads to inconsistent

cooling rates, easily causing thermal cracks at the thickness interface.

Product structure-related: Large differences in thickness and annular structures can easily

lead to surface cracking.

Inappropriate or deteriorated slurry ratio: An inappropriate or deteriorated slurry ratio

results in insufficient surface strength, leading to cracking. The slurry ratio needs

adjustment to ensure slurry quality.

Excessively rapid degassing after dewaxing: Excessively rapid degassing after dewaxing

causes severe shell shrinkage, leading to cracking. The degassing rate needs to be

controlled.

Poor adhesion between the topcoat and the wax model: Poor adhesion between the

topcoat and the wax model can lead to cracking of the topcoat. It is necessary to ensure

the wax model is thoroughly cleaned and that the amount of wetting agent added to

the topcoat coating is appropriate.

Over-drying of the outer surface and under-drying of the inner surface of the topcoat:

Over-drying of the outer surface and under-drying of the inner surface of the topcoat can

lead to cracking of the topcoat. It is necessary to control the relative humidity,

drying time, and airflow rate to ensure proper drying of both the outer and inner surfaces

of the topcoat.

Insufficient wet strength of the topcoat: Insufficient wet strength of the topcoat can lead

to cracking of the topcoat. It is necessary to ensure the wet strength of the topcoat.