Casting troubleshooting: symptom → cause → fix

Cause Air whipped in while mixing, or air that never escaped the cavity as the pour rose past detail.

Fix Stir slowly with the stick below the surface. Pour a thin stream from a height so bubbles break on the way down, then tap or vibrate the filled mold for 30 seconds.

Best fix Brush a thin first coat into the cavity, let it grab, then pour the bulk. The visible surface is then formed by brush rather than by a falling stream.

Cause Too little or the wrong release, an undercut locking the part into a rigid mold, or rough printed walls the cast has gripped.

Fix Re-coat with the correct release for that material, add draft to the master, and smooth the cavity. Those are three different fixes for three different causes, so work out which one you have before buying anything.

Rescue Temperature. A rigid mold expands faster than the cast in warm water and contracts faster in the freezer. Try one, then the other, and never lever with metal.

Cause Demolding before a full cure is the number-one reason by a distance. A brittle mix (too much water, or too much powder) and locked geometry come next.

Fix Wait for the full cure time on the datasheet, not the demold time, and weigh the mix rather than measuring by volume.

Note Cure time and demold time are different numbers on purpose. Opening at the demold time is fine; handling it hard before the cure time is not.

Deeper Plaster and concrete have their own failure modes here, ratio by weight, thin-section cracking, and two rules that flip between the two materials: plaster cracking, crumbling or not setting.

Cause The material set before it reached the deepest features: trapped air in the detail, a mix that is too thick, or a formula that kicks too fast.

Fix Add small vent channels at the deepest points, thin the mix slightly, and work cooler to buy pot life. Brushing a first coat into the detail also solves this.

Cause Not a casting problem. The cast is a faithful negative of the cavity, so you are reading the print’s layer lines straight off the wall.

Fix In order of effort: print the mold at 0.12 mm, sand the cavity, coat it with epoxy and polish, or print it in resin. Each one is permanent and improves every cast after it.

Cause Heat. Plaster and many resins release real heat as they cure, and PLA starts softening from around 60 °C.

Fix Reprint in PETG or better. Meanwhile, pour thinner sections, work in a cool room, and stand the mold in a shallow tray of cool water while it cures.

Cause The halves are not closing tightly, or something is holding them apart: a stray blob of filament, cured material on the mating face, or an elephant’s foot on the first layer.

Fix Clean both faces, then clamp properly rather than relying on bands alone. Tape, clay or hot glue around the outside of the seam seals the last fraction of a millimetre.

Accept A 0.2–0.3 mm flash line is normal on any two-part mold and comes off in seconds.

Cause Working as designed, and the most common misunderstanding we get. A mold reproduces the outside of your model and nothing else. An internal void has nothing holding it in place while you pour, so it cannot be part of a two-part mold. The same goes for a through-hole: you get the outer silhouette and the hole fills with cast material.

Fix, option 1 Slip-casting. Fill a plaster mold, wait while the absorbent wall draws water out and a skin forms, then tip the still-liquid centre back out. Waiting time sets the wall thickness. See the plaster guide.

Fix, option 2 A separate core: print the inner shape as its own piece with its own registration so it is held in the cavity while you pour around it. Nothing in the generator does this for you today.

Cause Almost always the repair pass, not a detail setting. When an upload is not watertight we rebuild it into a solid by voxel remeshing, which resamples your surface onto a grid and is blocky by nature. A clean mesh skips that path entirely and keeps its detail.

Fix Repair the model first, in Optimize, and re-upload the result. Or re-export from your CAD or sculpting tool as one closed body. Only after that is Model detail worth raising, since that setting controls how much a very heavy mesh is decimated before upload, not how the rebuild looks.

How to tell If you saw a "we rebuilt your model" notice, or the preview already looked faceted before you generated, the mesh was the cause.

Cause The size check compares your model against a build volume, and the default is not your printer.

Fix Set your actual machine in the viewer bar's Bed menu first. A lot of "too big" reports are simply a 220 mm printer being measured against a bigger default, or the reverse.

If it really is too big Scale it down, or keep the size and cut the printed mold up afterwards with the print splitter, which is designed for exactly that and adds alignment pins at the cuts.