Part 1 of the plastic casting reference. Content is being restored from archive sources.
Topics covered originally:
- Two-part urethane resin selection
- Mixing and pour technique
- Pressure-pot vs vacuum-degassing
- Demolding cured castings
Related: silicon rubber mold-making reference (/molding1.htm), plastic casting part 2.
Frequently Asked Questions
What two-part urethane resin is recommended for first-time plastic casting? Smooth-On Smooth-Cast 300 is the most-recommended starting resin — it has a 7-minute pot life (enough time to mix carefully), a 30-minute demold time (allowing fast iteration), and is the most documented urethane for prop casting. Beginners should avoid polyester resin (worse smell and shrinkage), epoxy casting resin (long cure times), and resins with 3-minute pot life (too fast).
What is the correct mixing procedure for two-part urethane resin? Part A and Part B are measured into separate cups (or by weight on a gram scale), then Part B is poured into Part A (not the reverse). The mix is stirred slowly for 30–45 seconds, scraping sides and bottom, until uniform. Fast stirring or shaking whips air into the mix and causes bubbles. The resin must be poured within the pot life window — 7 minutes for Smooth-Cast 300.
What happens if the resin mix ratio is wrong? Incorrect ratios produce sticky or rubbery results that never fully cure — too much Part A (not enough catalyst) leaves the piece soft. Contamination of mixing cups with partially cured resin from a previous pour can also cause accelerated or incomplete cure. Fresh mixing cups and sticks for every pour are essential to consistent results.
How should liquid resin be poured into a mold? For one-part molds, resin is poured in a thin stream from 8–12 inches height to let it self-level and allow surface bubbles to escape. For two-part molds, the halves are closed and secured with rubber bands or clamps before pouring through the pour gate until resin appears at the vent holes. Cold molds below 70°F should be warmed to room temperature before casting.
Further Reading from Authoritative Sources
- resin casting — Wikipedia’s resin casting article provides authoritative background on casting materials, processes, and the types of defects that beginners encounter.
- polyurethane resin — Wikipedia’s polyurethane article covers the chemistry of isocyanate-polyol two-component systems that underpin urethane resin casting materials.