3D Printing the Mold Shell — Material, Layers, Orientation, Post-Processing
PLA vs PETG, printing without supports, layer settings for a smooth cavity, dimensional tuning for the locator pins, and release agents.
The inside of the mold shell becomes the surface of your cast. That makes shell printing different from an ordinary "it just needs to hold together" print: everything is about cavity surface quality.
Material
| Material | Pros | Watch out |
|---|---|---|
| PLA | Easiest, dimensionally accurate, silicone barely sticks to it | Deforms above ~60 °C (car interiors, heat guns). Do not put it in an oven to speed curing |
| PETG | Tougher, a little more heat tolerant, can be clear | Silicone sticks slightly more than to PLA — release agent is mandatory. Watch for stringing |
| ABS/ASA | Heat resistant | Shrinkage and warping can stop the halves mating |
| SLA resin | Smoothest surface | Uncured resin inhibits platinum silicone — wash and post-cure thoroughly |
Start with PLA. Turning on the generator's waffle structure cuts material by 30–50 %.
Slicer settings
- Layer height 0.12–0.16 mm — reduces stair-stepping on curved cavity surfaces. 0.2 mm works but the layers show on the cast.
- 3–4 walls, 5 top/bottom layers — the seal ridge and pins are made entirely of walls, so wall count matters.
- Infill 15–20 % — lower is fine with waffle on.
- Supports off — shells from the generator are designed to print support-free with the cavity facing up. Supports leave scars inside the cavity.
- Brim — rarely needed thanks to the wide base; use 5 mm if PETG corners lift.
- Ironing — a smoother parting face seals better. If your slicer has ironing, enable it for top surfaces.
Orientation
Place both halves exactly as exported: parting face (the one with the ridge and groove) up. Do not flip or stand them. Cavity-down needs supports; standing up makes the pins grow sideways and distort.
Dimensional tuning
FDM printers tend to make holes slightly small and bosses slightly large. The generator's locator fit setting adjusts the clearance between pins and holes:
- Pins will not seat fully on the first print → choose loose (0.25 mm)
- The halves rattle (play of 0.5 mm or more in the stats) → choose tight (0.10 mm)
- Setting your slicer's XY compensation to −0.05 to −0.1 mm improves accuracy across the board.
Post-processing
- Parting faces — flatten any blobs or strings on the top surfaces with the back of a blade or 400 grit. Anything trapped here makes the mold leak.
- Cavity surface — if layer lines on the cast bother you, sand lightly with 600 then 1000 grit, or spray a thin coat of filler primer and sand. Do not over-sand or you lose detail.
- Pin test — mate the halves and wiggle them. The generator's show gaps toggle previews the designed clearance.
- Release agent — a thin coat on the cavity and parting faces. A dedicated spray release is best; otherwise a very thin smear of petroleum jelly or diluted dish soap. Petroleum jelly is usually fine with platinum silicone, but test first.
Shell life and storage
A PLA shell typically survives dozens of pours. When the seal ridge wears and starts leaking or the pins loosen, reprint from the same STL. Store the halves mated, away from sunlight and heat.
Related: two-piece mold design · troubleshooting
Upload an STL, get a mold
Generate a 2-piece mold STL with seal ridge, locator pins, sprue and air vents — free, in your browser. Your file never leaves your device.
Open the mold generator →