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Scabbing or Delamination
This defect presents as faceted metal indentations or crusts with depression underneath and ceramic material in between; sometimes accompanied by a scab over the top. The delamination, or rough surface, is copy of a section of shell layers that separated from the shell; sometimes completely separating and others still attached. The shell layers that separated from the shell will also cause negative shell defects if they are present in the shell during casting. The delamination can also remain attached to the shell and just curl inward creating a negative valley or crevice in the casting. If metal penetrates behind this delamination that is still attached to the shell, a scab is formed.
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The causes of this defect are evident in the wax and shell portions of the process. To cure these causes, R&R recommends taking the following actions.