Hot Bulging or Creep
Thickening, oversized or inflated casting
Similar to buckling, but typically without the sharp peak
Indicates that there was hot deformation of the ceramic mold under pressure of metal, forcing the shell to move or creep at a given temperature and pressure
Causes of this defect are evident in the shell and metal portions of the process; to cure, R&R recommends taking the following actions:
Weak hot strength of shell.
Increase SiO2 level, burnout time or temperature. Use a stronger shell system.
Hot deformation due to shell materials and presence of alkali materials.
Use a more refractory material in backup slurry or as stucco.
Shell firing temperature is too high/hot.
Reduce/control shell temperature.
Shell is too thin.
Add more coats.
Low backup refractory loading, low silica solids in backup binder, inadequate refractory choice or part geometry (large, flat surfaces).
Stiffen surfaces or check backup slurry and refractory.
Metal is too hot.
Reduce/control metal temperature.