Cast copies instead of printing ten

Turn a model into a printed mold, then pour silicone, resin, wax, plaster or soap as many times as you like.

Printing ten of something takes ten prints. Casting ten takes one print and an afternoon, and the copies can be in materials a printer cannot produce at all: soft silicone, clear resin, scented wax, plaster, soap, chocolate. The mold is just the negative of your model, split so the casting can get back out — and the splitting is the part worth understanding.

A ball releases at its equator and nowhere else. That sentence is the whole subject. Move the seam of a spherical mold a few millimetres up or down and the widest part of the casting is now *inside* one half, gripped by material it has to squeeze past — an undercut. The mold generator measures this rather than assuming it, which is why the Release row changes as you drag the split, and why a box shape never shows the problem at any height. Try it on a sphere once; it teaches the idea faster than any diagram.

Best split is a measurement, not a suggestion. When the part is trapped, the generator has already worked out the height at which it would come free. Typing that number is quicker and more reliable than nudging the seam until the warning stops, and it puts the seam where the part wants it rather than where you happened to stop.

Undercuts you cannot split away are a different problem. A model with a genuine overhang — a handle, a hook, an undercut lip — cannot be freed by moving a flat parting plane anywhere, because no flat plane exists that frees it. Reporting that is done; *solving* it needs a non-planar parting surface or a flexible mold, and the usual maker answer is the second one: print the mold, cast a flexible silicone intermediate, and cast the real thing out of that.

The mold is a copy, and that cuts both ways. It takes its own duplicate of your model when you make it, so deleting or moving the original afterwards leaves the mold untouched — convenient. It also means a change to the model does not reach the mold, so a mold made halfway through a design is a mold of a part that no longer exists. Make it last.

Print the mold in something that does not mind the pour. PLA is fine for silicone, soap and plaster. Resin cures warm and wax is poured warm, so PETG is the safer choice there. A mold is also one of the few prints where a slightly rough inner surface is a feature rather than a defect — it gives silicone something to key against — though for a glossy casting you want fine layers and a smooth-walled model going in.

Clamp harder than feels necessary. Two printed halves under a pour will spread at the seam and leave you a flash line to trim off every copy, and printed clamps are cheap. A clamp generated at the mold's own width takes ten minutes to print and pays for itself on the second casting.

1. Finish the model first. The mold holds its own private copy, so editing the original afterwards does not update it — make the mold last, or make it again.
2. Select the body, then open the Generate panel and pick Casting mold (under Molds & casting). It is one of the few tiles that needs a selection before you click it.
3. Choose Two-part box to pour into, or Open face to press into. A press mold is easier to print and fill; a two-part box is what anything with a back needs.
4. Read the Release row. It measures whether the casting can actually come out of the split it has chosen, and it flips to Undercut the moment it cannot.
5. If it is trapped, look at Best split — it names the height that frees the part — and set Split height to it rather than hunting by eye.
6. Set the Pour hole wide enough for whatever you are pouring. Silicone and resin flow; wax and plaster are less forgiving, and a narrow one traps air behind the pour.
7. Run the print check and export. Print both halves, clamp them — a printed clamp will do — and pour.