Draft angles & undercuts: molds that release
Most demolding disasters are not a release-agent problem - they are a geometry problem baked in before the mold was ever printed. A cast can only leave a rigid mold by sliding in one direction, and two things decide whether it slides freely: the draft angle on the walls, and whether there are any undercuts catching the part on the way out. Get both right and even a stiff PLA mold releases cleanly. Get them wrong and no amount of wax or oil will save you - the cast is mechanically locked in. This guide covers how much draft to use, how to spot undercuts before you print, and what to do when a shape simply cannot pull straight out.
What a draft angle is and why molds need it
A draft angle is a slight taper applied to the vertical walls of a mold cavity so that the opening is a touch wider than the bottom. Instead of walls that run perfectly straight up and down, they lean outward by a degree or two in the direction the cast is pulled. That small taper is what lets the part break contact with the wall the instant it starts to move, so it slides the rest of the way out freely.
Without draft, a vertical wall stays in full contact with the cast along its entire height as you pull. That creates two problems: friction along the whole surface, and suction as the cast tries to separate from a tight-fitting cavity. On an FDM-printed mold the layer lines add mechanical grip on top of that. The result is a part that feels glued in - you end up forcing it, which marks the cast, cracks brittle materials like plaster, or tears a softer mold. A degree or two of draft removes nearly all of that resistance for free.
How much draft do you actually need
There is no single magic number - it scales with how grippy the surface is and how deep the cavity runs. The deeper and more textured the wall, the more taper it needs to break free cleanly.
- Smooth, shallow rigid molds: 1 to 2° is plenty. This is the everyday default for a polished or epoxy-coated cavity.
- Textured surfaces or deep cavities: 3 to 5°. Texture and raw layer lines grip hard, and depth multiplies the contact area, so give them more taper.
- Tall, narrow features: 5° or more. A deep, slender pocket is the hardest thing to release and benefits from generous draft.
- Flexible silicone or TPU molds: little to none. A flexible mold peels and stretches off the cast, so it does not rely on draft the way a rigid mold does.
When you are unsure, round up. Extra draft costs almost nothing visually on most parts and turns a fight into an effortless pop-out. It is far easier to add a degree than to rescue a stuck cast.
| Surface / finish | Recommended draft | Why |
|---|---|---|
| Smooth rigid mold (polished / coated) | 1 - 2° | Low friction, so a slight taper is enough to break contact. |
| Textured surface (raw layer lines, matte detail) | 3 - 5° | Texture and ridges grip the cast; more taper clears them faster. |
| Deep, narrow cavity | 5°+ | Long contact length and suction make depth the hardest case to release. |
| Flexible mold (silicone / TPU) | ~0° | The mold flexes off the cast, so draft is largely unnecessary. |
What counts as an undercut
An undercut is any feature that mechanically locks the cast into the mold so it cannot move straight out along the release direction. Think of a lip, an overhanging rim, a recessed groove, a bulge that gets wider below the opening, or any re-entrant shape that hooks back on itself. Where draft is a matter of degree, an undercut is a hard catch: the part physically cannot pass it without the mold or the cast deforming.
The simplest way to find one is the "can't pull straight up" test. Picture grabbing the finished cast and lifting it straight out of the mold along the pull direction. If every surface either moves away from the wall or slides parallel to it, you are fine. If any feature would snag - a lip the cast has to climb over, a wall that bellies outward below the rim - that feature is an undercut, and a rigid mold will trap the part there.
Spotting undercuts before you print
Catching undercuts on screen is far cheaper than discovering them mid-demold. A few quick checks:
- The pull-direction test. Pick the direction the cast leaves the mold, then mentally sweep the whole part along that axis. Any surface that faces back toward the pull direction is an undercut.
- Slicer preview. Load the mold into your slicer and look at how the cavity walls are oriented. Walls that overhang the layers below them often signal geometry that will also trap a cast.
- Look for negative draft. A wall that tapers the wrong way - narrower at the top, wider toward the bottom - is negative draft, which is effectively a built-in undercut along the whole wall. Rotate the model and check that every wall opens up toward the parting surface, not away from it.
Fixing undercuts
Once you have found an undercut, there are three ways out, in rough order of preference:
- Redesign to remove it. The cleanest fix. Soften a sharp lip into a draftable slope, fill a recess, or reorient the part so the offending feature points along the pull direction instead of across it. If the undercut serves no functional purpose, design it away.
- Split into a two-part mold. When the feature has to stay, divide the mold along a parting line so each half pulls off in a direction that clears the undercut. This is how most real-world molds handle shapes that cannot release in a single pull.
- Switch to a flexible mold. A silicone or TPU mold stretches and peels off the cast, releasing mild undercuts that would lock a rigid mold solid. If the geometry is fiddly, flexibility often beats fighting the split. See silicone molds vs direct-print molds for which way to go.
Choosing the parting line
The parting line is where the two halves of a mold meet, and where you place it decides how well the mold works. The guiding rule is to split at the widest cross-section - the silhouette or "equator" of the part. From that line, every surface tapers inward toward each half, which means both halves draft away from the seam and lift off cleanly.
Two more things to get right:
- Keep it on a non-critical surface. A parting line always leaves a faint witness seam on the cast. Run it along an edge, a back face, or somewhere it will be sanded or hidden, not across a show surface.
- Align the draft to it. Both halves should taper away from the parting line so neither half has a wall running back toward it. If you find a wall drafting the wrong way relative to the seam, the parting line is in the wrong place.
Two-part molds done right
A two-part mold only works if the halves go back together in exactly the same position every time and stay there under pour pressure. A few essentials:
- Registration keys or alignment pins. Add matching pins-and-sockets (or cones) along the parting surface so the halves seat in one precise position. Without them the seam shifts and the cast gets a step at the parting line.
- Clamping. Liquid casting material pushes the halves apart. Plan for clamps, bands, bolts through flanges, or an outer shell to hold the mold shut while the cast sets.
- Sealing the seam. Even well-mated halves leak thin material. A light wipe of soft wax or modelling clay along the parting line stops flash and keeps the seam crisp.
From there, treat the seam like any other mold surface: apply release before each pour and keep the print walls clean. See mold release agents for what to use, and best print settings for molds for getting the cavity walls smooth enough to release in the first place.
FAQ
How much draft angle do I need?
For a smooth rigid mold, 1 to 2 degrees of taper on every vertical wall is usually enough. For textured surfaces or deep cavities use 3 to 5 degrees, and go higher still for tall, narrow features. Flexible silicone or TPU molds flex off the cast, so they need little to no draft. When in doubt, more draft is safer - it costs almost nothing and dramatically eases release.
What is an undercut in mold making?
An undercut is any feature that mechanically locks the cast into the mold so it cannot be pulled straight out along the release direction - a lip, an overhang, a re-entrant shape, or a wall that gets wider below the opening. The classic test is to imagine pulling the cast straight up out of the mold: if any feature would catch, that feature is an undercut.
How do I make a two-part mold?
Split the model along a parting line placed at its widest cross-section, so each half drafts away from the seam and pulls off cleanly. Add registration keys or alignment pins so the halves seat in exactly the same position every time, plan a way to clamp them together against pour pressure, and keep the parting line on a non-critical surface to hide the seam witness line.
Do silicone molds need draft angles?
Not really. A flexible silicone or TPU mold stretches and peels off the cast, so it can release shapes with vertical walls and even mild undercuts that would lock solid in a rigid mold. You still benefit from a little draft on deep or detailed features, but flexibility is what lets silicone molds handle geometry rigid molds cannot.