Draft angles & undercuts for 3D printed molds

A draft angle is a slight taper on the walls of a part so it can slide out of a mold; an undercut is any feature that hooks back and locks the part in. In a rigid mold, vertical walls and undercuts are the reason casts stick or break. The fixes are simple: give the shape a gentle taper, put the mold's parting line at the widest point so neither half has an undercut — or use flexible silicone, which bends around undercuts. Here's how each works with printed molds.

What draft does

Picture pulling a straight-sided cup out of a tight sleeve: friction holds it along the whole wall, and any suction makes it worse. Taper the sides by a degree or two and the moment the part lifts, it breaks contact everywhere at once. That's draft. It matters most in rigid molds — a printed two-part mold, a rigid tray, a plaster mold — because the mold can't flex to let go.

• A common rule of thumb is about 1–3° of taper on walls in the direction the part comes out. More is easier to release.
• Textured or matte surfaces need more draft than smooth ones — the texture grips. 3D-printed layer lines count as texture.
• Silicone molds are forgiving — because they flex, they often release parts with little or no draft.

Where draft comes from: your model

Draft is a property of the shape you cast. If you design the master in CAD, add a small taper to vertical faces (most CAD tools have a draft feature). If you download a model, look for ones that already widen toward the opening side. Printing the mold doesn't add draft to a part that has none — so plan it into the master, or choose a mold type that doesn't need it.

What an undercut is

An undercut is any part of the shape that would have to pass back through a narrower opening to come out: a lip, a recess under an overhang, a handle loop, a bulge below a narrower neck. In a rigid one-piece mold, an undercut traps the cast completely. You have three ways around it.

1. Split the mold at the widest section

The classic answer is a two-part mold with the parting line running around the object's widest cross-section. Each half then only has to pull away from surfaces that open outward — no undercuts on either side. In the two-part box mold generator, set the Seam position to that widest point and use Rotate X/Y/Z to orient the model so the seam can sit where it needs to. See two-part molds: keys & pour spouts for seam and alignment details.

2. Use a flexible silicone mold

When a shape has many undercuts or fine detail, a silicone mold is the easy route: silicone stretches over undercuts and peels away. Print a silicone mold box, pour silicone around your master, and cast into the flexible result — draft becomes a nice-to-have rather than a must.

3. Redesign the feature

Sometimes the cleanest fix is to change the master: fill a small recess, split a handle into a separate piece, or flatten an overhang. If a rigid cast keeps sticking in the same place, look there for an undercut you didn't notice.

Draft in a casting tray

The casting tray generator builds a grid of cavities from your model. In Rigid tray mode, the part of the model below the Seam position becomes the cavity — so place the seam where the shape stops widening, or the cast will lock in. In Silicone-pour mode it prints a master inside a trough, and the Trough wall draft setting (5° by default) tapers the trough walls so the cured silicone block lifts out cleanly.

Quick checklist

• Rigid mold? Give walls about 1–3° of taper, more on textured surfaces.
• Put the parting line at the widest cross-section so neither half has an undercut.
• Lots of undercuts or detail? Use silicone.
• Still sticking? Add a release agent and check the demolding technique.

FAQ

How much draft angle do I need?

For rigid molds, about 1–3° is a common starting point; textured surfaces need more. Flexible silicone molds usually need little or none.

Can I add draft in the mold generator?

Draft on the part comes from your model, so add it when you design the master. What the generators control is the seam position (to avoid undercuts) and, in the casting tray's silicone mode, the draft of the trough walls.

My cast is stuck — is it an undercut?

If it's stuck in one spot, likely yes. Move the seam to the widest section, redesign the feature, or switch to a silicone mold. For other causes, see casting troubleshooting.