Round or spherical metal defects or foam-like surface roughness
Indicates that the metal managed to penetrate the primary coat; that air is trapped against the wax pattern and the shell formed around it; or there was a void from an air bubble at the surface of the wax
Causes of this defect are evident in the shell portion of the process; to cure, R&R recommends taking the following actions:
Inadequate defoam characteristics.
Conduct an antifoam test on the primary slurry and adjust, if necessary.
The primary slurry was not stabilized for a sufficient period of time and air bubbles were not allowed to escape from the mix before dipping.
Check that the slurry is being allowed to stabilize before using in process.
There is air in the slurry and the slurry was allowed to stabilize before use.
Check the mixing equipment to verify that it is not creating too much action and pulling air into the slurry. Conduct an antifoam test on the slurry to determine if antifoam should be added.
Poor dipping technique.
Reduce dipping speed and/or adjust dipping angles to eliminate the introduction of air bubbles into the slurry layer.
Dipping technique traps air in detail.
Change dipping technique to avoid trapping air, use air wand to blow slurry into detail, brush or prewet.
Aggressive mixing – pulling in air.
Control mixing speed, use a timer.
Dried slurry formations on the sides of tanks and on propeller shafts introduces air into the slurry.
Clean tank sides and shaft.