Once you’re comfortable with basic resin pours, several advanced techniques can dramatically improve results — eliminating bubbles, reducing weight, matching specific colors, and producing consistent copies across a production run. This guide covers the techniques that make the difference between a decent casting and a professional-quality one.

For the fundamentals, see plastic resin casting introduction.

Pressure Casting: Eliminating Bubbles

Air bubbles in a casting come from two sources: air whipped into the mix during stirring, and outgassing from the resin chemistry itself. Basic technique reduces bubbles by slow mixing and thin pours, but pressure casting eliminates them entirely.

How It Works

A pressure pot (a modified paint pressure pot — Smooth-On sells purpose-built casting pressure pots) is pressurized to 40–60 psi while the resin cures. At elevated pressure, any remaining air bubbles are compressed to a fraction of their original size and become invisible in the cured piece. The mold goes into the pot immediately after pouring, before the resin gels.

Equipment

A 2-gallon or 5-gallon paint pressure pot, modified with a pressure inlet valve and gauge, and sealed with a silicone gasket. Purpose-built casting pressure pots are sold by Smooth-On and similar suppliers, but DIY modifications of harbor freight paint pots are common in the prop community and cost a fraction of the commercial version.

Process

  1. Mix resin and pour into mold as normal
  2. Seal the mold (for two-part molds) or cover the pour opening
  3. Place in the pressure pot immediately
  4. Pressurize to 40–60 psi
  5. Allow to cure fully under pressure (30 minutes for Smooth-Cast 300, longer for others)
  6. Release pressure slowly before opening the pot

Result: Castings with zero visible bubbles, even from mixes that would normally produce moderate porosity. Essential for clear or semi-translucent castings where bubbles are most visible.

Rotational Casting: Hollow Parts

Solid resin castings become very heavy at scale. A B9 robot torso cast solid would be impractically heavy. rotational casting produces a hollow shell — strong enough for prop use, a fraction of the weight.

Technique

  1. Mix a fresh batch of resin
  2. Pour into the closed mold
  3. Seal all openings and begin rotating the mold in all planes before the resin gels
  4. Continue rotating while the resin coats the mold walls and gels (approximately 3–4 minutes for Smooth-Cast 300)
  5. Set aside in normal orientation to finish curing

The walls built up this way are typically 3–6mm thick depending on how much resin was used and how vigorously the mold was rotated. Multiple layers can be built up for a thicker wall.

A second technique — slush casting — pours a small amount of resin into the open mold, tips and rotates to coat the walls, pours out the excess, and allows the thin layer to cure. This works for one-part open molds.

Urethane Foam Fills

Casting rigid urethane foam into a hollow piece fills it with lightweight structural material that adds rigidity without significant weight. This is the technique used for the B9 robot’s structural components — outer skins cast in urethane resin, interior filled with two-part expanding urethane foam.

Smooth-On FlexFoam-iT and Foam-iT series are the standard products. They’re two-part foams that expand 2×–20× their liquid volume depending on the product.

Pour into a sealed hollow shell and allow the foam to expand and cure. The pressure can distort flexible molds, so rigid outer shells (fiberglass or rigid resin) are preferred for foam fills.

Colorants and Tints

Casting resin in color avoids the need to paint through the entire thickness — if the piece chips or scratches, the color shows through rather than white resin underneath.

SO-Strong (Smooth-On) — Universal colorants that mix into urethane resin. A small amount goes a long way — 1–2% by weight is typical. Available in a full color range including transparent colors for semi-translucent castings.

Alumilite Dyes — Similar concentrated tints, compatible with most urethane systems.

Metal powders — Bronze, aluminum, iron, copper powders mixed into resin create cold-cast metal pieces. Mix at 80–90% powder by weight, cast normally. After demolding, sand the surface to expose the metal particles — the piece looks and polishes like metal. Used extensively for B9 robot components that need a metallic finish.

Cold Cast Bronze Example

For the B9 robot’s aluminum-look components:

  1. Mix fine aluminum powder into Smooth-Cast 300 at high ratio (80% powder by weight)
  2. Cast normally
  3. After demolding, sand with 220-grit to expose the aluminum surface
  4. Polish with Brasso or steel wool
  5. The result looks like machined aluminum

This technique replaces spray chrome paint (which looks fake at close inspection) with actual metal particles in the resin surface.

Silicone Mold Life and Maintenance

A good silicone mold lasts 50–200 pulls before degrading, depending on the resin type, mold geometry, and how much force is used in demolding. To extend mold life:

  • Apply mold release before every pour (or every 3–4 pours minimum)
  • Demold gently — flex rather than peel
  • Store molds flat or in a position that doesn’t permanently deform them
  • Keep molds away from heat

When a mold starts to tear at the pour gate or vent holes, those areas can be repaired with fresh silicone (brush-on platinum silicone, cured overnight). This extends mold life significantly.

Production Casting: Running a Series

When casting 10 or more copies of the same piece, efficiency matters:

  • Pre-measure cups of Part A and Part B before the session starts
  • Have everything staged: molds cleaned and released, cups ready, timer set
  • Work in 3–5 pour batches — pour multiple molds from one mix batch where pot life allows
  • Build a casting rotation: while the first pour cures, prep the next mold

Consistent room temperature (68–75°F) matters for consistent results. Cold rooms slow cure and can cause problems with thin sections. Hot weather shortens pot life — work early in the day in summer.

For a large-scale project like the B9 robot construction, a full casting session produces all the components needed for a single robot section. Having a systematic approach prevents wasted resin and ensures all pieces come from the same production run with consistent color and finish.

Frequently Asked Questions

How does pressure casting eliminate bubbles in resin castings? A pressure pot is pressurized to 40–60 PSI while the resin cures. At elevated pressure, any remaining air bubbles are compressed to a fraction of their original size and become invisible in the cured piece. The mold goes into the pot immediately after pouring, before the resin gels, and pressure is released slowly before opening.

What is rotational casting and when is it used for B9 robot components? Rotational casting produces a hollow shell by pouring resin into a closed mold, sealing all openings, and rotating the mold in all planes before the resin gels. The resin coats the mold walls and gels into a shell typically 3–6mm thick. It is used specifically for large B9 components like the torso that would be impractically heavy if cast solid.

How are urethane foam fills used in B9 robot structural components? Rigid urethane foam is cast into hollow shells to fill them with lightweight structural material that adds rigidity without significant weight. The technique uses two-part expanding urethane foam (Smooth-On FlexFoam-iT or Foam-iT series) that expands 2x–20x liquid volume. The outer skins are cast in urethane resin and the interior is then filled with expanding foam.

What is the cold-cast aluminum technique for B9 robot components? Fine aluminum powder is mixed into Smooth-Cast 300 at 80% powder by weight and cast normally. After demolding, the surface is sanded with 220-grit to expose the aluminum surface, then polished with Brasso or steel wool to achieve a machined aluminum appearance. This replaces spray chrome paint with actual metal particles in the resin surface.

What colorant systems are compatible with urethane casting resins? SO-Strong universal colorants from Smooth-On mix into urethane resin at 1–2% by weight and are available in a full color range including transparent colors for semi-translucent castings. Alumilite dyes are a comparable alternative. Metal powders including bronze, aluminum, iron, and copper create cold-cast metal finishes at high concentration ratios.

How long does a silicone mold typically last and how can mold life be extended? A good silicone mold lasts 50–200 pulls depending on resin type, mold geometry, and demolding force used. Mold life is extended by applying mold release before every pour, demolding gently by flexing rather than peeling, storing molds flat, and keeping them away from heat. Small tears at pour gates can be repaired with brush-on platinum silicone cured overnight.