Holes or leaks can be created from improper support removal, rough handling, over sanding, or from the pattern geometry or CAD file design defects.
Successful casting yields depend on sealed patterns. Because the pattern is dipped in slurry to produce a “shell”, any small hole can allow slurry to run into the interior of the pattern creating a “non-metallic inclusion” in the casting.
Using Accura Patch, fill all but one vent or drain-hole in your part. For more information on patching a hole use the Accura Bond and Patch User Guide.
Several methods may be incorporated to check for holes:
A visual inspection for holes will find the obvious holes, but is limited by human error.
Use of a vacuum leak tester (see bottom middle photo) will verify whether a model is fully sealed. Using CAD, create a tapered tube (leak check nipple) that can be merged on to the part or build the nipple independently on the SLA system and attach it to the part after building. Use the center of the tube pattern and attach the vacuum hose to the nipple. Begin pumping air from the pattern, stop when the gauge reaches 10-15 in/Hg (250-380 mm/Hg). If the gauge holds pressure the part is sealed. If the gauge reading drops, the pattern has at least one hole. The nipple may be left on for the foundry to verify vacuum after shipment of the pattern to the foundry (they will plug it with wax before casting). It is imperative that the nipple be placed on an area that will make it easy for the foundry to remove after casting.
Using a pressure regulator (see bottom right photo), attach a low-pressure airline with 1-10 psi (7 - 70 kPa) to the nipple (described above) and pump air in. The smaller the part, the less pressure is required. Feel the surface or listen with a stethoscope to find the leak(s). Seal the hole(s) using Accura Bond and Patch User Guide and repeat the vacuum check.
A simple vacuum leak tester or a pressure regulator will help you quickly and easily identify leaks in a QuickCast build style pattern.