Mould basics: parting line, pins, core and vents

Short answer: a sleeve mould is a two-part hex block that splits along one line, closes one way because of pins, is filled through two gaps beside a hanging core, and lets air out through vents at the high points. The core is what holds the canal shape while the outer body cures round it. Get those 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 general mould-making descriptions come from Smooth-On's mould-making tutorials. Those tutorials make soft rubber moulds around a solid model. This tool makes a hard printed mould around a soft cast, and hangs a printed core inside it to leave the canal hollow, so most of the core and vent detail below is our own reasoning, and says so. For adults (18+).

• Parting line
• Pins and sockets
• The core
• Pour opening
• Vents
• Wall thickness
• Undercuts
• Pry slots
• Sources

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 sleeve's outer body, a hexagonal prism, 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, six pairs round the hex. The sockets are cut a little loose so a print that comes out fat still closes; see pins, bands and fit.

The core

Our own design, not from the Smooth-On tutorials: a two-part mould around a solid model has nothing to leave a hollow with. A sleeve needs a hole down its middle, so a third printed piece, the core, holds that space open while the silicone cures round it, then comes back out.

• Entry face up. The mould stands with the sleeve's entry, the open end a body slides into, at the top, under the pour opening. The closed tip is at the bottom of the cavity. That is the reverse of a solid mould, where the model usually stands base-down.
• The bar. The core doesn't stand on the floor of the cavity; it hangs from a bar that crosses the pour opening, so nothing needs to support it from below except the tip.
• Two feet, two sockets. A foot at each end of the bar drops into a socket cut at the rim corners. The feet stop the core turning and carry its weight on the mould rim, so it stays centred without touching the sides of the cavity.
• Tip spigot, tip socket. A spigot on the core's tip sits in a socket at the bottom of the cavity. It centres the tip end the same way the feet centre the top, and it leaves a small hole through the finished sleeve's tip, the tip vent, which the demould step uses to break the suction when the core is pulled back out. Turn it off in the tool for a sealed tip, and accept a little more drift at that end.
• Prints bar down. The core is exported as its own file and printed standing on the bar, tip up, so the tip's finish and the canal's finish come off the printer the right way up. No hole runs down its middle: a bore that size would leave under 1 mm of wall in a 16 mm core, so any relief is cut into the surface instead. Print settings.

Pour opening

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] Here the pour opening is the sleeve's own entry face, with the core's bar crossing it. That leaves two gaps, one on each side of the bar, and you pour through both. 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 outer body has such a pocket; a textured core adds its own risk of trapped air facing down into the pour, which the guide covers separately: brush a thin coat of silicone into a textured core before you close the mould. Whether the tool found every vent for your shape is a thing to test with a real pour, so watch the first fill.

Wall thickness

The silicone wall is whatever is left between the core and the outer body at each point along the sleeve: outer body radius minus canal radius, all the way from the entry to the tip. Too thin, and the wall tears when the core is pulled back out or when the sleeve is used. The tool's check blocks a design under 6 mm of wall anywhere and warns under 8 mm. That is a working floor, not a tested figure, chosen because the wall is also the part that has to survive the core being pulled out through the entry after the cure. Demoulding.

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 an outer body that leaves along the split, and the printed core is hard too, so a texture in the canal has to let go of the core as it's pulled back out through the entry, not just leave the mould. The cast is the soft part in both directions. Silicone at 30 A or below bends out of detail. [FAQ 138] That forgives a gentle ridge, and does not forgive a deep hook. If the designer's checks flag a shape, believe them.

Pry slots

The printed halves meet flush, and a little silicone that creeps into the joint glues them shut. With nothing to grip, opening the mould is hard work. The designer cuts a slot across the joint at each corner, between the band grooves: two per corner, or one on a mould shorter than 80 mm, 3 mm wide and up to 6 mm deep, never closer to the cavity than the wall. Pins come first: a mould too small for both gets no slots. Push a flat screwdriver or a coin into one and twist; the halves pop. This comes from the author's own casting, not from a listed source.

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 the core and how much silicone.