Two-Piece Mold Design — Parting Plane, Seal Ridge, Locator Pins, Undercuts

The elements that make a two-piece mold seal, align and release the cast, explained one by one so you can pick the right parameters.

A two-piece mold splits the model at one height into a top and a bottom shell. It looks simple, but meeting three conditions at once — it must not leak, it must align exactly, and the cast must come out — takes a handful of features. The mold generator adds all of them automatically; knowing why makes the parameters much easier to choose.

The parting plane

The height at which the mold splits. The best spot is usually the model's widest outline, so the top half pulls straight up and the bottom straight down with nothing hooking on either side.

The split height slider (30–70 %) sets this. When you load a model the generator pre-scans many heights and suggests the one with the fewest undercuts.

Undercuts — where the cast gets caught

Any part of the cast that hooks on the shell when you pull the halves apart is an undercut: mold material sitting on top of a downward-facing surface. The underside of a mushroom cap, under a character's chin, behind an ear.

The generator paints undercut areas red on the model and shows depth and share in a banner. Fix in this order: ① apply the suggested split height → ② re-upload the model in another orientation → ③ if still deep, the shape needs a three-piece mold or a core.

Seal ridge and groove

Where the halves meet there is a tapered ridge on one half and a matching groove on the other, running around the cavity. The trapezoidal ridge finds the groove by itself and stops silicone from creeping into the gap right at the cavity edge.

The ridge can follow the model's silhouette (follow silhouette) or be a rectangular ring (rect ring). The silhouette version uses less material and produces shorter flash.

Tapered locator pins

The ridge seals; the four corner cone pins align. Being cones, they self-centre even when inserted at an angle and leave no side play once seated. In the stats panel, a play value of ±0.25 mm or less means a good fit.

Printers shrink holes by different amounts, so there is a locator fit setting. If the pins will not seat fully on your first print, choose loose; if the halves rattle, choose tight and reprint.

Cavity gap and open casting

A cavity gap of 0 gives a solid cast of the model. A positive gap makes the cavity that much larger than the model; press in the core plate (an extra part exported in open-casting mode) and a silicone skin of exactly that thickness is formed. This is how squishies, doll skins and filled soft toys are made.

Wall thickness and lightweighting

6 mm walls are the default. Large shells can go down to 4 mm; the seal ridge and pins need room, so it will not go below 3 mm. Turning on waffle hollows the inside of the shell into a grid of pockets, saving 30–50 % filament. A 2.5 mm skin around the cavity always stays solid, so nothing leaks.

Holding the model

With a cavity gap above zero the model (or core) floats inside the shell and needs something to hold its position. The generator offers end tabs on the model's flanks, grips on the parting line, or pins underneath. Each leaves a small mark on the cast, so pick the one that lands somewhere inconspicuous. With a gap of 0 the cavity hugs the model and no holder is needed.

Next: sprues and air vents — how silicone actually fills the shell.