How to cast plaster, jesmonite & concrete
Plaster, jesmonite, and fine concrete all pour straight into a rigid 3D-printed mold - no silicone insert in between. That keeps the workflow short (one print, one pour), but it also means the mold geometry has to do all the work of holding the cast and releasing it cleanly. This is the field guide: which mold type to pick, what draft angles to design for, the right release agent for each material, mix ratios, and the troubleshooting tree for the failure modes I keep seeing on Discord. We recommend PETG over PLA for these molds - the moisture and mild alkalinity will pit PLA after a few pours.
What you'll need
PETG filament ~$15
PETG over PLA for direct-pour rigid molds. Plaster is mildly alkaline once mixed and stays damp for hours; PLA pits and yellows over a few pours, PETG doesn't.
Buy →Plaster of Paris or Jesmonite AC100 ~$12
Plaster of Paris is the cheapest - chalky finish, brittle, sets in 30 min. Best for prototype casts and decorative figurines that don't get handled.
Jesmonite AC100 is the upgrade pick - water-based acrylic + mineral filler, holds detail like plaster but cures hard and impact-resistant. Studio favourite for trays / coasters / jewellery dishes you'll actually sell.
Fine concrete / GFRC for outdoor / structural pieces. Avoid bagged sidewalk concrete - the aggregate is too coarse to copy fine detail.
Buy →Mold release ~$8
For plaster: cheapest option is plain vegetable oil wiped on with a brush. For Jesmonite and concrete: use a dedicated PVA release or soft-soap solution - oil-based releases can repel water-based casts and cause fish-eyes.
Buy →Mixing bowl + scale + stirring stick ~$10
Flexible silicone bowl beats a rigid one - cured leftovers peel out clean. A kitchen scale that reads to 1 g is plenty accurate for plaster.
N95 dust mask + nitrile gloves ~$12
Always wear a mask when handling DRY plaster, jesmonite, or concrete powder. Silica and gypsum dust both cause cumulative lung damage with repeat exposure. Once the powder is wet you can take the mask off; gloves stay on (plaster is hard on skin).
Buy →No silicone insert needed for plaster - that's the whole point of the Plaster mold type. Lower cost-per-cast than any other casting workflow Meshcast supports.
Design the mold for direct pour
Pick the Plaster mold type, not the 2-part
In Meshcast, the Plaster mold type and the 2-part mold type look similar in the canvas - both clamshell a master between two printed halves. The plaster variant has three differences that matter:
- Wider step seal on the parting plane. Plaster mixes are gritty (mineral filler in jesmonite, gypsum crystals in plaster) and a tongue-and-groove leaks where the grit jams in the gap. A visible exterior step + clearance seats reliably.
- Printed snap clamps instead of relying on rubber bands. Rubber bands creep open under the 1-2 hour set time. The snap clamps stay locked.
- Sharp-edged outer envelope (no inside chamfer at the lid joint). Plaster pulls out flatter against a hard square edge than a beveled one - gives a cleaner foot on the cast.
Bake draft into the master
This is the #1 reason rigid-mold casts fail to demold. Rigid molds don't flex like silicone - every vertical wall must be drafted (sloped outward toward the parting plane) or the cast will lock in.
| Wall height | Draft angle | Top width gain per side |
|---|---|---|
| 10 mm | 1° | 0.17 mm |
| 30 mm | 2° | 1.05 mm |
| 60 mm | 3° | 3.14 mm |
| 100 mm | 3° | 5.24 mm |
Add draft in your CAD (Blender: Solidify modifier "Even Thickness" off + an adjusted normal taper, or Fusion 360: Modify → Draft) before exporting the STL. Meshcast can't recover detail the master doesn't have.
Print the halves and clamps in PETG
Plaster of Paris hits ~60°C while setting (exothermic crystallisation). Jesmonite + concrete stay near room temp. PETG handles all three without warping; PLA goes soft above 55°C and will sag visibly on the first plaster pour.
Slice settings: 0.2 mm layers, 3 walls minimum (the cavity surface IS the cast surface - fewer walls leaks vertical-wall lines into the finish), 15% gyroid infill. Print the snap clamps lying flat for max layer-line stiffness.
Quantities: 1× of each mold half + at least 2 snap clamps for a small mold, 4 clamps for anything over 80 mm. Print them all at once.
Pour day
Release the cavity
Pick the release agent by cast material:
| Material | Release agent | How |
|---|---|---|
| Plaster of Paris | Vegetable oil, soft soap, or petroleum jelly | Wipe a thin film on with a brush. Don't pool - pooled oil makes pits. |
| Jesmonite AC100 | Soft-soap solution OR Mann PVA mold release | Brush on, let dry 5 min. Oil-based releases repel the water-based Jesmonite and cause fish-eyes. |
| Concrete / GFRC | Form-release oil (Crisco works) or commercial concrete release | Brush a thin film. Concrete also damages PETG over time - re-release before every pour. |
Clamp the halves shut
Mate the two halves - the upper half slides down onto the lower half's perimeter rim. The exterior step should be visible all the way around. Slide the printed snap clamps over the seam from the outside, evenly spaced around the perimeter. For tall molds, add a clamp every 50 mm of seam.
Mix the cast material
Always weight-based, never volume - humidity changes how much powder sits in a measuring cup.
| Material | Ratio (by weight) | Mix time | Pot life | Demold |
|---|---|---|---|---|
| Plaster of Paris | 2 parts plaster : 1 part water | 2 min, stir slow | 5-8 min | 30-60 min (brittle until dry next day) |
| Jesmonite AC100 | 2.5 parts base : 1 part liquid | 1-2 min, no lumps | 10-15 min | 4-6 h (full strength 24 h) |
| Fine concrete / GFRC | 4 parts mix : 1 part water (or per bag) | 3 min, paddle on drill | 20-40 min | 24 h (full strength 28 days) |
For plaster, sprinkle the powder into the water rather than the other way round - gives a smoother slurry with fewer lumps. Wait 60 s for the powder to "slake" (absorb water), then stir.
Pour and vibrate
Pour through the funnel gate in a thin stream from one corner. This pushes air ahead of the pour rather than trapping it under each new layer. Once the cavity is full, vibrate the mold for 30-60 s by tapping the sides firmly with a wooden spoon or running an electric toothbrush against the outer wall - both shake out the bubbles that ruin a smooth finish.
Watch the top vent: when it stops bubbling, the cavity is fully evacuated. Stop vibrating.
Wait for full set, then demold
Resist opening early. Plaster looks set in 30 min but is fragile until the next day - a corner snaps off in the demold. Jesmonite is rubber-firm at 2 h, peel-out at 4-6 h, full strength at 24 h.
When ready: pop the snap clamps off, slide the top half straight up, then tap the outside of the lower half with a rubber mallet a few times to break the surface seal. Lift the cast out by a flat surface (never a thin protruding feature - they snap).
Troubleshooting
The cast locked in and snapped trying to demold
Almost always missing draft. Vertical walls in a rigid mold need at least 1-2° outward slope. Re-design the master with draft, or switch to the Silicone mold type which can flex over straight walls.
The cast surface has tiny pits / pinholes
Trapped air. Vibrate longer, pour in a thinner stream from higher up, and tilt the mold to a 10-15° angle while pouring so air can roll out of corners. For plaster, you can also paint a thin "wash" coat into the cavity first, then pour the bulk.
The seam left a visible flash line on the cast
The two halves weren't clamped tight enough. Add another snap clamp, or wrap with a rubber band on top of the clamps. A 0.2-0.3 mm flash line is normal and can be sanded off easily on plaster / jesmonite; for concrete, file it down with a diamond file.
Plaster sets too fast - I can't pour fully before it stiffens
Add 1-2 g of cream of tartar per kg of plaster to slow the set. Or pour at colder water temperature (4-10°C). Reduce mix volume to what you can pour in 3 min - plaster doesn't keep.
Jesmonite came out chalky / weak
Under-mixed or under-ratioed liquid. Always 2.5:1 base:liquid, mix to a fully smooth cream - no streaks visible. If you can see white powder clumps in the mix, keep stirring.
The mold halves don't line up cleanly after a few pours
PETG can warp slightly from heat + moisture cycling. Reprint the affected half. To extend mold life: don't pour boiling-hot plaster, dry the mold fully between pours, and store flat.
Concrete is sticking to PETG even with release
Concrete grinds at the PETG over time. Re-coat with release before every pour, never reuse a release coating, and consider epoxy-coating the mold cavity for high-volume production runs.
Made by @meshminds3d. Some product links are affiliate links - they cost you nothing extra and help keep this tool free. Got stuck? Email me a photo and I'll help you debug.