Spalling on Edges

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Technical Support

Struggling with a specific casting defect or just need general technical support?

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  • Rough filling in the corners

  • Indicates that there was a poor bond between the first coats and the backup shell in these corners; causing primary coats to pop off (areas of primary coats popping off of edges)

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Causes of this defect are evident in the shell portion of the process; to cure, R&R recommends taking the following actions:

Shell

Improper draining.

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. Check slurry prewet draining times and positions. Drain the slurry away from areas where it may pool. If necessary, use a prewet (see below for caution) or, if possible, use a second primary slurry with a lower viscosity.



Poor slurry control.

High binder solids in the primary slurry will inhibit drying, causing the slurry to dry on the surface and entrap moisture underneath. This moisture will try to migrate out too fast during dewax, causing spalling. Institute a slurry control program. R&R can provide information regarding the necessary testing procedures, if needed. If you currently have a program, increase the frequency or have a sample checked by the R&R technical department to test the accuracy of your results.



Excess prewet.

Prewetting 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.
Note: It is also possible that fines in the stucco or excessive stucco coating is causing adhesion issues between the primary and secondary slurry layers; however, spalling in corners is typically related to residual moisture entrapped in the shell layer and bursting during dewax. Stucco-related adhesion issues are described on scabbing/delamination defect. Drain the prewet away from pockets and out of detail. When the prewet is properly drained, the shell will take on a dull or matte finish.