Plastic Casting â€” Part 2

Part 2 of the plastic casting reference. Content is being restored from archive sources.

Topics covered originally:

Bubble suppression techniques
Coloring resin pre-pour vs post-cast painting
Multi-part molds and registration
Common casting defects

Related: silicon rubber mold-making (/molding1.htm), plastic casting part 1.

Frequently Asked Questions

How are bubbles suppressed in urethane resin castings? Bubble suppression requires both technique and equipment. Pouring resin in a thin stream from 8–12 inches height helps surface bubbles escape. For elimination of interior bubbles, pressure casting in a pot pressurized to 40–60 PSI while the resin cures compresses any remaining air to invisibility — essential for clear or translucent castings where bubbles are most visible.

Should resin be colored before pouring or painted after casting? Both approaches have merits. Pre-pour coloring using SO-Strong universal colorants (1–2% by weight) or Alumilite dyes produces colored-through castings where chips or scratches show color rather than white resin underneath — ideal for pieces that will be handled. Post-cast painting allows more precise color control and is standard for pieces requiring multiple colors or detailed painting.

What are the most common defects in plastic castings and how are they fixed? Surface bubbles from air trapped against the mold face are fixed by applying a thin brush coat of resin to the mold before the main pour. Interior bubbles from fast mixing are prevented by slow, deliberate stirring. Sticky or uncured sections result from wrong mix ratio or contaminated cups. Warped parts indicate early demolding — at 30 minutes the part is still softer than fully cured.

How are registration features used in multi-part mold casting? Registration keys — hemispherical features pressed into the clay bed during mold making — lock the two mold halves together in exact alignment for every cast. The keys ensure that parting line seams align precisely and that the cast piece doesn’t show a stepped or misaligned seam at the join. Keys are placed at the corners of the mold face at least 1/2 inch from the master perimeter.