3D-printed formwork for silicone moulds
The frame is the cheapest part of a kit and the one that decides whether the pour leaks, whether the core drifts and whether the cured block comes out in one piece. How a Mouldery formwork is designed, what the clearances mean, how to print each part and what to do when something does not fit.
Updated 6 November 2026 · 9 min read
Three jobs for one printed frame
A formwork for a poured silicone mould has to hold half a kilo of liquid silicone for a day without leaking or bulging, keep the master and the core exactly where the geometry puts them, and let go of the cured block without prying or cutting. Later it has a fourth job: during casting it goes back around the silicone as a counter-mould, so the soft block cannot bulge under the weight of the concrete. Mouldery answers with four printed parts that work together: a one-piece ribbed frame with a locked split, a base plate with a centring boss, a hollow master screwed to that plate, and a rigid core held down by a bridge. No supports, no bolts along a seam, no bead of clay.
One-piece ribbed frame with a locked split
The frame is a single upright tube, printed as exported. Its wall is a multiple of the line width, 2.4 mm with a 0.6 mm line, which the slicer fills with two perimeters per face meeting in the middle: no infill region, nothing to leak through, nothing to tune. Stiffness comes from outside, with horizontal rib bands 5 mm tall standing 2 mm proud and vertical ribs 3 mm wide, so the flat faces stay flat under the pressure of the pour.
One corner is split from top to bottom. The two edges carry flanges 14 mm long, and the cut is a tongue-and-groove with 0.3 mm of play: the tongue stops the edges sliding past each other and gives the silicone a long path instead of a straight slit. Two 25 mm binder clips on the flanges, one low and one high, keep it shut, and the flanges drop into a notch in the base plate so the frame cannot rotate. After the cure you take off the clips, the corner springs open by a few millimetres and the block slides out. A frame without a split has to be cut, or the block levered out, and that is how moulds get torn.
A master that cannot float
A hollow printed master weighs less than half of the silicone it displaces at a density of 1.10. Left free, it floats, tilts and ends up with rubber under its foot; tape lets go under release spray and weights get in the way of the pour. A Mouldery master is screwed down instead. An M5 nut sits in a hexagonal pocket under it, 8.2 mm across flats and 4.5 mm deep, pressed in cold or with a touch of a soldering iron at 80 °C if the print is tight, and an M5 × 12 countersunk screw comes up through the base plate into that nut. The master sits in a 1 mm recess exactly the shape of its plinth: no silicone gets under the foot, and the opening of the finished mould is the plinth outline.
The base plate is 3 mm thick with a 3 mm centring boss the frame drops over with 0.15 mm of play, four ⌀16 mm push holes to eject the block with the handle of a screwdriver, and a ⌀40 × 1 mm relief disc for later: pressed onto fresh concrete, it gives the vessel a slightly hollowed foot that does not rock.
Rigid core, bridge and the 70 g problem
The core is a hollow cylinder, ⌀48 mm on the 80 mm candle presets and sized to the cavity otherwise, with a 1.2 mm wall, a closed floor, internal ribs and a hub with a 3.4 mm pilot hole for a 4 mm self-tapping screw. It carries 1° of draft, narrower at the bottom, so it slides upward out of its silicone sleeve, and it hangs with 6 mm of silicone under it: the floor of the sleeve, where the wick socket is formed.
A core of that size displaces around 100 ml of silicone and weighs 20 to 30 g, so the rubber pushes it up with about 70 g of net lift. Unrestrained, it rises, tilts and leaves a sleeve of uneven thickness. The bridge, a 20 × 6 mm bar screwed to the core hub, has arrow-shaped ends that drop 3 mm into two pockets in the frame rim with 0.2 mm of play; the pockets centre the core and the two remaining clips, one on each end, stop it lifting. The top of the core stays 4 mm above the engraved pour line, and nothing should flow inside it.
Clearances and why they are tight
Every fit is modelled with a specific clearance for a calibrated printer. They are small on purpose: mould silicone is thick, 12 000 to 25 000 cps, and a 0.15 mm gap behind a 3 mm boss lets through a film, not a flow. The fits are also the seals; the glue bead outside takes care of the film.
| Fit | Clearance | What it does |
|---|---|---|
| Frame on the base-plate boss | 0.15 mm | centres the frame and closes the bottom joint |
| Bridge ends in the rim pockets | 0.2 mm | centres the core, drops in without forcing |
| Split tongue-and-groove | 0.3 mm | closes under the clips, springs open after the cure |
| M5 bore in the base plate | 5.5 mm | free passage for the countersunk screw |
| Nut pocket under the master | 8.2 mm across flats × 4.5 mm | M5 nut pressed in, cannot turn |
| Pilot hole in the core hub | 3.4 mm | a 4 mm self-tapping screw bites without splitting the hub |
Print settings per part
PLA is right for every part: silicone and concrete are cold processes. Use PETG if the workshop gets warm. No part needs supports. Only the master needs finishing, 0.12 mm layers then sanding and sealing as in the silicone guide, because the rubber copies every line.
| Part | Orientation | Layer | Line width and walls | Infill |
|---|---|---|---|---|
| Formwork | upright, as exported | 0.2 mm | 0.6 mm, 2 perimeters | none needed, the wall is solid by design |
| Base plate | flat, boss up | 0.2 mm | 0.6 mm, 4 perimeters | 30 % |
| Master | upright, cavity up | 0.12 mm | 3 perimeters | 15 % gyroid |
| Core | upright, floor on the bed | 0.2 mm | 2 perimeters | none |
| Bridge | flat | 0.2 mm | 4 perimeters | 40 % |
| Wick centring tool | flat | 0.2 mm | default | default |
When the parts do not fit
If the frame will not drop over the boss, the bridge binds in its pockets or the tongue welds into the groove, your printer is laying down too much plastic. Lower the flow by 2 to 4 %, check the first-layer squish, enable elephant-foot compensation and reprint the small part. Do not file the mating faces: a filed fit is an uneven fit, and an uneven fit is a leak. A frame that rattles on its boss is the rare opposite; raise the flow by the same amount. A 20 mm calibration cube measured with calipers tells you which way you are off.
Sealing and leaks
- Seat the plate on a flat bench, drop the frame over the boss, close the split and clip it, both flanges in the plate notch.
- Run a bead of hot glue or a strip of tape around the outside bottom edge. The boss closes most of the path; the bead takes care of a layer line or a speck of plastic.
- Spray non-silicone release on everything the rubber will touch: master, inside of the frame, plate, core. On the first pour this is what lets the frame come off instead of being broken off.
- Pour slowly into one corner. A slow seep at the bottom edge stops with a dab of hot glue; a running leak means the plate is not seated, so stop, decant and start again.
The frame during casting
After the master is out, the cured block goes back into the clipped frame on the base plate, opening up, for every cast. The frame carries the weight of the concrete and keeps the outer faces of the vessel flat; without it a Shore A 15–25 block would bulge under 400 g of wet mix. The core goes back into its sleeve for the same reason, and the bridge holds it down while you vibrate. Store the mould assembled in the frame, flat and out of the sun, with the spare clips.