3D Printed Molds for Resin Casting

Rigid printed mold or silicone mold?

Print the mold and cast in it directly when the part has good draft, no undercuts, and you want dimensional accuracy. Print a master and pour silicone around it when the part has fine surface detail, shallow undercuts, or you plan to run dozens of casts.

A printed mold is fine for a handful of parts. A silicone mold made from a printed master is fine for many, and it forgives geometry that a rigid mold cannot.

Print settings for FDM molds

FDM prints are porous along layer boundaries. Seal the cavity with a thin coat of epoxy, sanding sealer, or clear spray, then wet-sand. Sealing also stops resin wicking into the print and locking the part in.

Print settings for resin (SLA/MSLA) molds

Resin printers give you a cavity surface that needs almost no finishing, which is their main advantage here. The catch is cure inhibition: incompletely cured photopolymer poisons platinum-cure silicone and bonds aggressively to casting resin.

• Wash thoroughly, then post-cure longer than you would for a display print.
• Let printed molds sit for a day before first use. Residual monomer keeps migrating to the surface.
• Test cure a scrap against your silicone before pouring a whole mold's worth.
• Resin molds are brittle. Keep walls thick and avoid over-clamping.

Release agents and mold prep

Do not skip release agent, even on a mold with good draft. Options in rough order of aggressiveness:

• Paste wax: durable, builds up in fine detail, best on smooth geometry.
• PVA release: forms a film barrier, water-washable, good for stubborn combinations.
• Silicone spray: fast and easy, can leave a slick surface finish and interferes with painting later.
• Mold release specifically sold for your resin: always the safest choice.

Reapply between casts. Most first-cast successes followed by a stuck third cast are release-agent depletion, not a mold problem.

Pouring and curing

1. Clamp the halves evenly across the center of the block, not just at the edges.
2. Mix resin slowly to limit entrained air, and let it sit briefly so large bubbles rise.
3. Pour into the sprue in one continuous stream, tilting the mold slightly so resin climbs one wall rather than splashing into the cavity.
4. Stop when material reaches the vent. That is the fill indicator.
5. Let it cure fully. Demolding early is the most common cause of torn and deformed parts.
6. Open the mold slowly and evenly. Prying one corner cracks printed molds.

For bubble-free casts, a pressure pot at 40 to 60 psi during cure collapses the bubbles you could not avoid. Vacuum degassing before the pour removes the ones in the mix.

How long a printed mold lasts

Expect five to twenty casts from a sealed FDM mold, more with gentle handling and good release discipline. Exothermic resins accelerate wear because each cure cycle heats the cavity. If you need dozens of parts, pour a silicone mold from the printed master and cast in that instead.