Atlas of Casting Defects
Negative
Faceted metal indentation sometimes accompanied by a positive metal scab
N/A
Flat featureless surfaces
N/A
Scabbing, Reverse Buckle
Visual, appears as scab with indentation under scab
Failure of bond between 1st and 2nd layer of shell. The first layer is pushed or pulled into the mold cavity usually during dewax. Sometimes the shell cracks and metal fills the gap between the layers producing a scab.
| Area | Possible Cause | Potential Correction |
|---|---|---|
| Shell | Incomplete wetting between 1st and 2nd layer |
Blow off loose primary coat stucco. Ensure immersion time in 2nd layer slurry is adequate to wet-out the primary layer |
| Shell | Etch too strong – too tacky |
Reduce etch strength or time |
| Shell | Drying rate of 2nd layer too high |
Increase drying room humidity |
| Shell | Poor bond between prime and backup layer |
Ensure adequate keying between the primary and first backup coat Use a coarser or more angular primary coat stucco blow off loose or excess stucco |
| Shell | Moisture trapped behind the primary coat |
Ensure adequate drying of the mold prior to dewaxing |
| Shell | Differential expansion stresses between the primary and secondary coats |
Ensure the thermal expansion of the primary coat is compatible with that of the shell coats |
| Shell | Rapid pressure release during autoclave dewaxing |
Autoclave blowdown should be gradual and take 2 minutes or more |