Mould basics: parting line, pins, vents and undercuts
Short answer: a two-part mould splits along one line, closes one way because of pins, is filled through a funnel, and lets air out through vents at the high points. Get those four right and the cast comes out whole. This page names each part, says what the maker of the silicone says about it, and shows where the designer puts it.
What is sourced and what is not. The descriptions come from Smooth-On's mould-making tutorials. Those tutorials make soft rubber moulds around a model. This tool makes hard printed moulds around a soft cast, so the last section is our own reasoning, and says so. For adults (18+).
Parting line
The parting line is where the two halves meet. Smooth-On has the maker draw it on the model first, at the place where the halves will separate with least resistance. [Smooth-On] In their two-part tutorial a slab of non-sulfur clay, about 1 cm thick, makes the dividing plane while the first half is poured. [Smooth-On] Here the halves are printed, so no clay is needed: the tool splits the shape along its centre line. The cast carries a faint seam there, and the flat faces are why the printing page cares about ironing.
Pins and sockets
Without a guide the halves can shift and the cast comes out with a step in it. Smooth-On presses small acorn nuts into the clay so the finished halves carry matching bumps and pockets, called registration keys, which make them register with each other. [Smooth-On] The designer prints tapered pins on one half and sockets in the other. The sockets are cut a little loose so a print that comes out fat still closes; see pins, bands and fit.
Funnel
A funnel gives the silicone a wide place to pour into. Smooth-On adds one to the model in clay for exactly that reason. [Smooth-On] In the designer's mould it is cut into the top face, with the shape standing tip down beneath it. See the section drawing.
Vents
Air in the cavity has to leave as the silicone rises, or it stays as a bubble on the surface of the cast. Smooth-On puts vents over the highest points of the top half, and makes them by punching a sharpened brass tube through the cured mould. [Smooth-On] The idea carries over: any pocket where air could sit needs a thin channel to the outside. The tool cuts vents where the shape has such a pocket. Whether it found every one for your shape is a thing to test with a real pour, so watch the first fill.
Undercuts
An undercut is a part of the shape that folds back on itself so the cured rubber cannot slide off. Smooth-On calls the result mechanical lock, and treats it as the thing to plan against: it fills the cavity with clay, or splits the mould into more pieces. [Smooth-On]
Our reasoning, not a source: the printed halves are hard, so a two-piece split only works for a shape that leaves along the split. The cast is the soft part. Silicone at 30 A or below bends out of detail. [FAQ 138] That forgives a gentle ridge or a flared base, and does not forgive a deep hook. If the designer's checks flag a shape, believe them.
Sources
Read on 2026-09-20. Manufacturer pages: Smooth-On tutorials and FAQ. The full list is in docs/GUIDE-SOURCES.md, section 8. Next: printing the halves and how much silicone.