Plastic Resin Casting — Part 2

Part 2 of the plastic resin casting series. See the updated advanced casting guide.

Frequently Asked Questions

What advanced resin casting techniques are covered in Part 2? Part 2 of the plastic resin casting series covers pressure casting to eliminate bubbles, rotational casting to produce hollow shells, urethane foam fills for lightweight structural interiors, and colorant systems for pre-tinting resin before casting. These techniques build on the basic one-part mold casting covered in Part 1.

What pressure pot is needed for pressure casting? A 2-gallon or 5-gallon paint pressure pot modified with a pressure inlet valve and gauge, sealed with a silicone gasket, is used for pressure casting at 40–60 PSI. Purpose-built casting pressure pots are sold by Smooth-On and similar suppliers, but DIY modifications of hardware store paint pots cost a fraction of commercial versions and are common in the prop community.

How does rotational casting produce a hollow B9 robot shell? Approximately 20–30% of the mold’s cavity volume of resin is poured into the closed mold. The mold is rotated slowly in all axes for the first 5–7 minutes, coating all interior surfaces as the resin gels. The result is a hollow shell with wall thickness determined by resin volume and rotation duration. This technique makes large B9 components like the torso practical by keeping them lightweight.

What is the difference between foam fills and rotational casting for hollow parts? Rotational casting produces an accessible hollow space inside the casting — appropriate for pieces needing interior electronics access or interior assembly space. Urethane foam fills (using two-part expanding foam poured into a finished hollow shell) permanently fill the interior with lightweight structural foam, adding rigidity without significant weight, but the interior becomes inaccessible.

Further Reading from Authoritative Sources

• rotational casting — Wikipedia’s rotational molding article provides authoritative background on the industrial process that the hobbyist rotational casting technique is derived from.
• urethane foam — Wikipedia’s polyurethane foam article covers the chemistry and properties of expanding urethane foam systems used in the structural fill technique.