Vacuum Casting for Small Batch Parts: Mold Design Tips

Vacuum casting is ideal for production runs of 20 to 100 parts, giving you good material options without the cost of injection molding tooling. The process starts with a master pattern, which you suspend inside a casting frame. You then pour silicone around it to create a flexible mold. Once cured, the mold is cut open, the pattern removed, and the mold is ready to produce accurate polyurethane parts.

The concrete thing to get right is the mold design, specifically the location of the runner and the parting line. Always place the parting line on a single plane, and make the runner at the lowest point of the cavity. When you pour resin in, it fills from the bottom up, which pushes air ahead of it. If you also create a small vent at the highest point of the cavity, air will escape cleanly and you will avoid trapped gas bubbles.

You also need to use vacuum wisely. After you cut and close the mold, place it inside the vacuum chamber with the runner sitting under a cup of mixed resin. Pull a vacuum before you open the valve from the cup. The pressure differential draws the resin into the mold, and any remaining micro bubbles stay in the resin rather than on your part's surface.

Remember to keep wall thicknesses uniform, ideally around 2 to 3 millimeters, to prevent sink marks and warping. The silicone mold naturally flexes, so undercuts are fine, just avoid sharp internal corners. With these design rules, you'll produce bubble-free parts in under an hour, making vacuum casting the most practical short-run process for many components.

#InjectionMolding# #Welding#