Scabbing or Delamination
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
Causes of this defect are evident in the wax and shell portions of the process; to cure, R&R recommends taking the following actions:
Wax is too tacky – over etched.
Reduce etch time, rinse better.
Hollow patterns – implode during autoclave.
Fill hollow pattern sections with wax.
Poor bond between the first coats and the backup shell.
Use coarser stucco or intermediate stucco (which will leave a rougher surface to which the first backup coat can bond) and blow off excess stucco, check on drying etc.
Slurries need to be applied in an even, uniform coat. When the slurry viscosity is too high or the tree is not drained properly, heavy layers of slurry are left behind. These heavy areas do not dry properly and leave a soft, poorly cured moisture pocket behind. When the part is dewaxed, this moisture tries to escape and causes the primary coats to pop off. This typically occurs in the second prime.
Fines in the stucco.
If the primary stucco contains a high level of fines (dust), it will dust over the slurry coat, keeping the actual stucco from coating the shell properly. The next slurry layer will not adhere well to this fine particle surface.
Poor slurry control.
Institute a slurry control program.
Prewet is used to stop the previous shell layers from absorbing excess binder from the slurry. The shell will shine after the prewet drains off. Excess prewet can also be washed into an area by the next slurry layer causing a liquid pocket. This pocket becomes a void when the binder evaporates leaving a weak area.
Excess stucco build up.
Sometimes the primary stuccos will build up too heavily, that is, a given stucco coat has too many layers of grain for that individual coat. This happens because they are fine and light and require very little slurry contact to stick. If this excess stucco is not removed, the next slurry coat cannot penetrate the stucco properly and causes a weak bond between layers. As a result, one way to reduce this spalling is to blow off excess stucco between primary coats.
Slurry is gelling.
Replace the slurry.