Silicon rubber mold making is the foundation skill for anyone building prop replicas. Once you can make a reliable mold from a master pattern, you can cast unlimited copies in resin, plaster, or urethane foam — which is exactly how professional prop houses produce multiples of the same piece.
This guide consolidates the mold-making process developed and refined over years of building replicas for the B9 robot, Mystic Seer, and other classic sci-fi props.
Why Silicon Rubber
Platinum-cure silicone rubber (also called addition-cure silicone) is the material of choice for detailed prop molds because it captures surface texture down to fingerprints, releases cleanly without a mold release agent in most cases, and can be demolded without damaging the master pattern.
Tin-cure silicone (condensation-cure) is cheaper but has drawbacks: it shrinks slightly during cure, can be inhibited by sulfur in some clays, and has a shorter shelf life. For prop work, platinum-cure is worth the extra cost.
Choosing Your Silicone
Shore A hardness is the key number. It measures how firm the cured rubber is on a scale of 0–100.
- Shore A 10–20 — Very soft, stretchy. Good for undercuts and flexible molds but tears easily. Use for simple shapes.
- Shore A 20–30 — The sweet spot for most prop molds. Firm enough to hold detail, flexible enough to demold without tearing.
- Shore A 40–60 — Stiffer. Better for flat molds and pieces without undercuts. Longer mold life.
Common products in the Shore A 20–30 range: Smooth-On MoldMax 20, MoldMax 30, Dragon Skin 20. The MoldMax series is particularly popular for prop work because of its long pot life and reliable cure.
What You Need
Before starting, gather:
- Silicone rubber (two-part, platinum-cure)
- Digital gram scale
- Mixing cups (disposable)
- Mixing sticks
- Mold release (Ease Release 200 or Vaseline — even though platinum-cure usually doesn’t need it, releasing the master after the first pour is easier with a light coat)
- Mold box materials
- Sulfur-free clay (if building a two-part mold)
Building a Mold Box
The mold box contains the liquid rubber while it cures. It can be permanent (wood, foamcore) or temporary. LEGO bricks are the most popular solution among prop builders — they snap together in any configuration, seal reasonably well without glue, and can be disassembled and reused indefinitely.
For small pieces under about 6 inches, build a LEGO wall around your master with 1–2 inches of clearance on all sides and at least 1 inch below the deepest point of the master.
For larger pieces, foamcore glued with hot glue works well. Seal the corners and any gaps with clay or hot glue before pouring.
Preparing the Master
The master is the original piece you want to copy. It can be:
- Carved from foam or wood
- Sculpted from epoxy putty or polymer clay
- A found object (toy part, hardware piece, commercial item)
- A 3D-printed part
Whatever the material, the master must be sealed before molding. Unsealed porous materials (raw foam, unfired clay, unsealed wood) will absorb the silicone and bond to it. Seal with:
- Spray lacquer (2–3 light coats, let dry fully)
- Shellac
- Smooth-On’s Smooth-Prime urethane primer
After sealing, apply a thin coat of petroleum jelly (Vaseline) or Ease Release 200. This ensures clean release even with platinum-cure silicone.
One-Part vs. Two-Part Molds
One-part (block mold or glove mold) — You pour rubber over the entire master. Works when the master has a flat back and minimal undercuts. Simple, fast, good for relief carvings and flat-backed pieces.
Two-part mold — You mold each side separately with a clay bed dividing them. Required for any piece with significant depth on both sides (a full round head, a box, a figure). More complex but necessary for three-dimensional pieces.
Most B9 robot parts and prop replicas require two-part molds.
Mixing and Pouring
Platinum-cure silicone comes in two parts (typically labeled A and B or Part 1 and Part 2). Mix ratio is specified by the manufacturer — weigh both parts on a gram scale, not by volume. Eyeballing ratios is the most common cause of sticky, uncured rubber.
- Weigh Part A into a clean cup
- Weigh Part B into the same cup
- Stir slowly and thoroughly for 3–5 minutes, scraping the sides and bottom
- Pour in a thin stream from a height of 12–18 inches — this helps air bubbles pop before the rubber reaches the master
- Fill to cover the master by at least ½ inch on all sides
Pot life (working time) varies by product — typically 30–60 minutes for most prop-grade silicones. Work at room temperature; heat accelerates cure, cold slows it.
Cure Time and Demolding
Most platinum silicones cure in 16–24 hours at room temperature. Resist the urge to demold early — undercured rubber tears.
To demold a one-part mold: flex the rubber walls away from the master and work it free. If it sticks, flex more — don’t pull straight.
For a two-part mold: remove the mold box, separate the two halves, and remove the master. Clean the parting line of any flash.
Common Problems
Sticky spots on the cured rubber — Usually caused by sulfur inhibition (sulfur in modeling clay can prevent platinum-cure silicone from curing). Switch to sulfur-free clay, or test your clay by pressing a small amount against the silicone before committing to a full pour.
Bubbles in the mold face — Air trapped during mixing. Stir more slowly, pour thinner, or apply a thin brush coat of silicone to the master first and let it gel before the main pour.
Mold tears on demolding — Master has sharp undercuts, or the silicone Shore A is too hard. Softer silicone or silicone with a flexible mold jacket helps.
Mold won’t release master — The master wasn’t sealed or mold release wasn’t applied. If this happens, gently work vegetable oil around the edges to lubricate before forcing.
Next Steps
Once your mold is complete, the next step is casting resin parts in it. For multi-part assemblies like the B9 robot, you’ll cast individual sections and bond them — see the B9 robot construction overview for how mold-made parts fit into a full build.
For mold-making practice pieces before tackling a major prop, the devil head mold project is a good starting point — it’s a single compact shape with manageable undercuts.