Two-part molds: alignment keys & pour spouts explained

In a two-part mold, alignment keys are the pins or ridges that make the two halves seat in exactly the same position every time, so the seam lines up and your cast doesn't come out with a step around its middle. The pour spout is the opening you pour the casting material through. Get these two features right — plus a sensible fit clearance and seam placement — and a two-part mold goes from frustrating to reliable. Here's what each one does and how to set it.

Why a two-part mold needs keys at all

A two-part mold splits around your object so you can pull it out after casting. But two loose halves can shift sideways against each other, and even a small shift shows up on the cast as a ledge or offset along the seam line (the "parting line"). Alignment keys — also called registration keys — solve this: a bump on one half drops into a matching pocket on the other, locking the two into one repeatable position. Line them up, and the seam becomes a thin line instead of a step.

Types of keys: pins vs tongue & groove

The two-part box mold generator gives you a Half-to-half seal choice with three options:

• Pins — round pegs on one half seat into holes on the other. Simple, strong, and quick to align. They're a great default for rigid casts where a hairline seam is acceptable.
• Tongue & groove — a continuous ridge runs around the whole seam and drops into a matching channel. Because the join is continuous rather than a few points, it seals liquids best — the better pick for thin, runny pours like resin that would otherwise creep out of the seam.
• None — no keys; the halves rely on external retention alone. Only sensible for simple shapes with bands or clamps holding everything tight.

Rule of thumb: runny pour or a fine seam matters → tongue & groove; simple rigid cast → pins.

Fit clearance: the gap that makes keys work

Keys have to actually fit together after printing, and 3D printing isn't perfectly dimensionally exact. That's what Fit clearance is for — a small designed gap between the pin and its hole (and between tongue and groove) so the halves close fully without forcing. A value around 0.2 mm is a good starting point on a well-tuned printer.

• Too little clearance and the halves won't seat — they rock on over-tight keys or need to be hammered together.
• Too much clearance and the keys are sloppy, letting the halves shift — the exact problem keys are meant to prevent.

If your first mold is tight, increase the clearance a touch; if it's loose, decrease it. It's the single setting most worth dialing in for your machine.

The pour spout: getting material in (and air out)

Once the mold is closed, you need a way to fill it. The Pour hole is the funnel-like opening into the cavity. Two settings shape it:

• Pour hole Ø — the diameter of the opening at the model surface. Bigger pours faster and suits thick materials; smaller leaves a neater mark to trim off the finished cast.
• Pour hole position — slide the spout along the seam to sit above a high point of the cavity. Pouring into a high spot lets the material fill downward and pushes air ahead of it.

Air is the other half of the problem: as casting material flows in, trapped air has to escape or you get voids. Pouring slowly into the highest point helps air rise out through the same spout; for complex shapes, a small vent channel at another high point lets the last of the air out. Pour in a thin, steady stream to keep bubbles from being whipped in.

Seam placement and demolding

Where the two halves meet — the Seam position — decides how easily the cast comes out and where the parting line lands on the finished piece. Put the seam across the object's widest cross-section so neither half wraps around an undercut and traps the cast. A well-placed seam means the halves lift straight off; a bad one locks your casting in. Place it where a faint parting line won't matter cosmetically, since you'll usually clean it up afterward.

To hold the closed mold together while it cures, the generator adds external retention (like bands) and optional finger pockets — thumb notches that make the halves easy to pull apart once the cast has set.

Putting it together

1. Choose a seam across the widest section so the mold opens cleanly.
2. Pick tongue & groove for liquid pours or pins for rigid casts.
3. Set fit clearance near 0.2 mm and adjust after a test print.
4. Position the pour spout over a high point and size it for your material.
5. Band the halves shut, pour slowly, let it cure, then use the finger pockets to open up.

FAQ

My cast has a step along the seam — what went wrong?

The halves shifted. Either your keys are too loose (reduce fit clearance) or you had keys set to "none". Switch to pins or tongue & groove and tighten the clearance.

Resin leaks out of the seam. How do I stop it?

Use tongue & groove, which seals a continuous line rather than a few points, band the halves tightly, and don't overfill. A tiny bit of clay or tape along the outside seam helps as backup.

Do I need a separate vent hole?

For simple shapes, pouring into the highest point usually lets air escape through the spout. For complex cavities with multiple high spots, add a small vent at the other high point so trapped air can get out.