Chris Collins

Tools · fabrication

Mold maker

Turn an STL, 3MF, OBJ or STEP part into what you need to cast it — pick the process first: a printed rigid mould (automatic parting, keys, sprue, risers, vents, bolts, inserts), a silicone mould (a fitted pour box, a printed master, or brush-on silicone in a printed mother mould), a plaster slip-casting mould with printable case moulds, or a metal-casting pattern (lost-PLA investment pattern with sprue, feeder and flask, or a split sand pattern with match plate and core box) — with step-by-step instructions and quantities, exported as STL, 3MF, OBJ, STEP and DXF, laid flat on your printer's bed.

Added · updated 31 October 2026

Works with

How to use it

Pick how you will cast, load a part, and the tool designs what that process needs: a rigid mould split so every piece pulls off, a silicone mould box or mother mould, a plaster mould for slip casting, or a metal-casting pattern with its gating. It cuts the pieces with a real solid-modelling engine, works out the quantities (silicone, plaster, investment, metal), and gives step-by-step instructions for the process. Everything runs in your browser — your files are never uploaded.

How to use it

  1. Pick the process — the four cards at the top:

    • Rigid printed mould: printed pieces you cast resin, plaster, wax or concrete in.

    • Silicone mould, with a method: Poured box round your part (a printed box, rectangular or fitted to the part’s outline), Printed master (the positive is printed too, so you need no original), or Brush-on + mother mould (a silicone skin in a rigid printed jacket).

    • Slip casting: a plaster mould for ceramics, and printed case moulds to cast the plaster in.

    • Metal casting, with a method: Lost PLA / wax (a printable investment pattern with sprue, cup, feeder and vents) or Sand-casting pattern (split pattern, match plate, gating patterns, core box).

      The panel then shows only what that process uses: its tabs (Parting, Silicone, Plaster, Pattern, Metal, Inserts), its view buttons, the casting materials that suit it, and its main exports. Every setting of the old layout is still there under its process.

  2. Load a part. Drop a file on the view or use Open file: STL (binary or ASCII), 3MF (zip package: units, several objects, component and build transforms), OBJ, or STEP / IGES (meshed by OpenCascade at the STEP tessellation chord error). STL and OBJ carry no units — pick millimetres, inches or centimetres. Or pick one of the samples. The mesh is welded, checked (open edges, non-manifold edges, flipped triangles, shells) and repaired where that is safe: duplicate vertices merged, an inside-out mesh turned the right way. A file that is not a closed solid — holes, open or non-manifold edges, flipped faces, debris, a slit, overlapping bodies, or a file the strict reader rejects (a wrong STL triangle count, a truncated ASCII STL, a 3MF pointing at missing vertices) — is repaired automatically with the mesh repair pipeline, with a progress bar, and a notice says what was fixed and the largest deviation from the original (see Automatic repair below).

  3. Orient it (Part tab): rotate 90° about X, Y or Z, Lay flat (largest flat face down), scale. For a silicone mould the parting is horizontal, so turn the part first; for a one-part block mould, stand it on its flat face; a lost-PLA pattern is poured from the top.

  4. Look at the draft (Draft view). Each face is green if it releases when the mould pulls along the chosen direction, blue-green if it releases the other way (the other half of a two-part mould), yellow if its draft is below the threshold (it slides out but may drag), and red if it is an undercut: it faces against the pull, or a ray from it along the pull hits the part again.

  5. Auto-split finds the parting (rigid mould, slip casting, sand pattern; see below). The Release view colours every face against the pull of the piece it ended up in, and draws each piece’s pull arrow. The Parting tab lists the pieces, their pull directions and any undercut left.

  6. Override anything: drag a parting plane in the view (or use its slider), add an X, Y or Z plane, delete one, change the number of pieces, or set a piece’s pull direction. Back to automatic (or ⟳ Auto-split) discards your edits.

  7. Set up the process in its tabs: keys, sprue, risers, vents, bolts and the casting material (Features); the silicone, box shape, master or mother mould (Silicone); the plaster and case moulds (Plaster); the metal, gating, flask, match plate and cores (Metal); magnets, nuts and other inserts (Inserts, see Inserts).

  8. Read "How to make it" at the top of the panel: numbered steps for the process and method you picked — printing, release agent, mixing and pouring, curing, demoulding and, where it matters, safety — with the quantities from the last build (silicone, plaster, investment, metal). The same steps are in the README of the ZIP.

  9. Mould (or Pattern) view: the pieces, an exploded-view slider, a section plane, X-ray, the casting (the part plus sprue, risers and vents; the silicone; the flask) and a measuring tool (click two points).

  10. Export: the formats that matter for the process are up front — usually everything in a ZIP with a README, all pieces in one 3MF, STEP — and the rest under More formats. Tick Lay pieces flat and arrange on the bed to get print-ready plates for your printer, and Combine all pieces into one file for a single STL (see Exports); the Plates view and the top-down preview show the layout. Save project writes a JSON file with the part, its orientation, the settings and your planes, to load again later.

Press ⛶ Full screen (or F) for the whole window; [ hides the side panel; Esc leaves.

How the automatic parting works

  • Analysis mesh. Long triangles are bisected (no edge longer than 1/30 of the part’s diagonal) so that every triangle’s centre says where it is; up to 6,000 area-weighted faces are sampled.

  • Pull directions. 64 directions spread evenly over a sphere (a Fibonacci lattice), the six world axes and the part’s principal axes. For each direction and face: is the face an undercut (normal pointing against the pull by more than about 0.6°, or blocked — a ray from the face along the pull hits the part again, tested with a bounding-volume hierarchy)? Is its draft below the threshold?

  • Set cover. A greedy set cover picks the fewest directions that between them release (almost) every face — reported under the piece table.

  • Pieces. The mould block is cut by planes into a binary space partition. Each piece pulls in one direction and may only move away from, or slide along, the planes that bound it, so pieces can never jam against each other. A piece’s cost is the area of its faces that its pull does not release, plus a small charge for low-draft faces and for pulls off the world axes. For every candidate plane normal (world axes, principal axes, set-cover directions) every split position is scored exactly with prefix sums. The first cut is forced; after that the piece with the biggest improvement is split again, looking up to two cuts ahead when a plain greedy split leaves undercuts, until nothing is trapped, the piece limit (6 on Auto) is reached, or another piece would not pay for itself. Cells that pull the same way across a plane are merged into one piece.

  • Disassembly. Pieces from later cuts come out first; a piece that carries key pins into a sliding neighbour comes out before it.

The cylinder, sphere, spool and mushroom checks in the tests: a sphere parts at its equator, a spool splits lengthwise through its axis, a mushroom parts at the underside of its cap, and the cross-drilled sample block (blind holes in three faces) needs four pieces.

Features in detail

  • Keys sit on parting planes where at least one neighbour pulls straight off the plane, clear of the cavity, the block edge, channels and bolts, spread as far apart as possible. Pins are on one piece, sockets (pin + clearance) in the other. Typical clearances for printed keys are 0.2–0.4 mm per side; check with print tolerances and fits.

  • Sprue, risers, vents. Pouring is along a chosen axis (default +Z, so the top of the mould as printed). High points of the part — vertices with nothing higher within a few millimetres — are where air collects. The highest gets the sprue (a tapered channel and a pour cup cut into the block), the next ones risers, the rest vents. A high point under an overhang cannot be vented with a straight channel and is reported as an air trap to drill. Bottom fill runs the sprue down beside the part and a runner under it to its lowest point, so the cavity fills from the bottom up.

  • Bolts (M3–M6, 1/4") go through both pieces of a plane where both pull straight off it, with a hex nut trap at one end; the README gives the bolt length. Pry slots are notches across the parting line on the two longest outside edges.

  • Shrinkage allowance scales the cavity up. Presets are typical starting values — always use the figure on your material’s datasheet: urethane and epoxy casting resins 0.1–1 %, plaster ≈ 0 (it expands slightly as it sets), casting wax 1–2 %, concrete ≈ 0.05 %, pewter ≈ 0.6 %, zinc alloys ≈ 0.7 %, aluminium 1.3 % (the pattern-maker’s 5/32 inch per foot).

  • Volumes. Fill volume = the cavity inside the block (part + sprue, risers, vents, runner), in ml, and its mass at the material’s density. Print estimate: shell = surface × 1.2 mm, the rest at the infill percentage, as in the STL viewer and filament cost tools.

  • Materials. The list shows the materials a printed mould suits — resins, plaster, wax, concrete, and pewter only with a heat-proof mould. Molten zinc and aluminium belong to the metal-casting process (an old link that names them still works).

Making a rigid mould

  1. Print the pieces in PLA or PETG — 3–4 walls, 15–25 % infill, 0.12–0.2 mm layers — each on its back face (Lay pieces flat does this). Sand or fill the cavity faces if the casting should be smooth.

  2. Seal the cavity faces (filler primer or a thin brush-on epoxy) and coat them with a release agent for your material: mould-release wax or spray for resin and wax, soap or a thin oil film for plaster and concrete.

  3. Put any inserts on their pins or in their pockets, close the mould on its keys and bolt or band it; seal the seams outside with clay or hot glue.

  4. Mix and pour the fill volume shown under Estimates (+10 %), slowly into the cup, until the vents and risers show material.

  5. Let it cure as the datasheet says. Fast resins get hot: a large pour can soften PLA, so pour in layers or print in PETG.

  6. Open the pieces in the disassembly order (pry slots at the parting line), and cut off the sprue, risers and vents.

Resin and epoxy: nitrile gloves, eye protection, ventilation. Never pour molten metal into a PLA or PETG mould.

Silicone mould box

Silicone mould → Poured box round your part makes a printable box around the part for pouring a silicone mould (its settings are on the Silicone tab). The part is surrounded by the chosen silicone thickness. Two-part: the box is a ring of walls and a bed — the printed equivalent of the clay bed: a block that fills the box up to the parting height, with the lower half of the part sunk into it, hemispherical dimples for keys and a half-round groove for the pour spout. Pour the first half, flip, push the bed out, apply release agent, pour the second half. One piece, cut open: walls, a base with a pour cone that holds the part, and optional cut-line guides — small ridges at the parting height that mould a groove to cut along. One-part block (open face): walls and a base the part is glued to on its flat face; the flat face becomes the open face you pour into. The parting height defaults to the horizontal plane with the least undercut. Silicone volume = box interior − part, given for each half of a two-part mould, with parts A and B by weight for the rubber you pick (typical mix ratios and densities; pot life and the rest are in Mould making).

Box walls: rectangle, fitted, or fitted convex. A rectangular box wastes silicone in the corners round a part that is not box-shaped. Fitted to the outline takes the part’s footprint (its outline seen from above), grows it by the silicone thickness, then closes it — grows it a little further and shrinks it back by the same amount — so narrow notches between arms or legs fill in and the walls stay smooth enough to print and to pour into. Fitted, convex wraps the grown footprint in its convex hull: simpler walls, a little more silicone. The walls, the bed (two-part), the base and the cut-line ridge all follow the outline; the keys are spread round it, inset from the walls and clear of the part. The Silicone tab and the README say how much silicone the fitted walls save against the rectangle (for the figurine sample, about a third). Every point of the part’s footprint keeps at least the silicone thickness to the walls.

Making a silicone mould in a box

  1. Print the walls and the bed (or the base); seal the bed or base into the walls with clay or hot glue.

  2. Two-part: press the part into its pocket in the bed. One-part block: glue it to the base on its flat face. Cut mould: glue it onto the pour cone. Brush release agent on the part and the bed.

  3. Weigh parts A and B (the Silicone tab gives both for each pour), mix, scrape the sides, degas or pour high and thin into a low corner, and let it rise over the part.

  4. Two-part: when cured, turn it over, push the bed out, brush release (petroleum jelly, or a silicone-to-silicone release — without it the halves bond into one block) on the silicone face, and pour the second half.

  5. Demould, open the halves (or cut the one-piece mould open along the guide groove, zig-zag), and take the part out.

Platinum silicone can stay tacky against resin prints, some primers and sulphur clays: test a patch first.

Undercuts in silicone. Silicone stretches, so the silicone modes use a relaxed test (the Release view and the Silicone tab): a face that the part shadows along its half’s pull, or that leans back against the pull by up to 30°, stretches (yellow); only a face leaning back more than 30° is trapped (red) — the rubber would have to turn more than 120° round it, so expect tearing, or cut the silicone there. The pull is up above the parting height and down below it (up everywhere for a one-part block). The rigid undercut area is given beside it for comparison.

Printed silicone master (a mould for a silicone mould)

Pick Silicone mould → Printed master and the tool prints the positive as well — the part itself, fused to the box — so the silicone mould can be made without a physical original (old links with master=1 open it). Everything else is as for the box: the silicone thickness, box wall, parting height, keys and sprue settings, and the silicone mould you end up with. Only what you print changes.

  • One-part block: one master — box walls and a floor, with the positive standing on the floor on its flat face (fused to it) and a fill line: a groove round the inside of the walls the silicone thickness above the part’s top, with 4 mm of wall above it. Turn the cured block over and the part’s flat face is the open face you pour into. If the part has no flat face (under 15 % of its footprint), it stands on a pour cone instead; the mould is then cut or stretched open to take the master and each cast out.

  • Two-part, split masters: two masters, one per silicone half. Each is a box with a flat parting plate at the parting height, that half of the positive standing on it, and the keys: master A (the lower half, turned over to print) has hemispherical bumps, which leave dimples in silicone A; master B has sockets at the same places, which give silicone B the matching bumps. In the print frames the positions are mirrored (A is turned over about X). The sprue is a cone and the vents thin pins lying on the parting plates, from the part’s outline to the same edge of both boxes (the outline’s highest point in Y gets the sprue, other local high points with a clear straight path out get vents), so both halves' channels line up when the mould is closed and stood on that edge to pour. The plate is at least the key radius + 1.5 mm thick so the sockets do not break through.

  • Two-part, classic: print a full positive and the bed of the two-part box; pour half one round the positive set in the bed, flip, release, pour half two. Trade-off: split masters can be poured at the same time, need no release coat between halves, and can be reused to make more moulds, but the halves are cast separately and fit only as well as the two flat plates and the keys do; the classic way gives halves cast against each other — an exact fit — at the cost of two pours and a release coat.

  • One piece, cut open: the positive is fused onto the pour cone of the base.

  • Walls: the cured silicone has to come out of the box. Drafted, one print flares the inside of the walls outwards (3° by default) so the block pulls straight out; Separate, bolted prints a floor and two U-shaped wall halves with full-height lugs and M3 holes, bolted together and sealed to the floor, that come apart like the case moulds.

  • No clay shrinkage: unlike slip casting, nothing is scaled for clay. An optional Casting shrinkage % scales the positive for the resin, plaster or wax you will cast (default 0).

  • Surface: silicone copies every print line. Print the positive at 0.08–0.12 mm layers (FDM) or 0.03–0.05 mm (MSLA), with its detailed side up or its long faces vertical; sand 240 → 400 → 600 and seal with a filler primer or a brush-on epoxy coating. Smallest useful feature: about 0.4 mm detail and 0.8 mm walls or pins on FDM, 0.1 / 0.3 mm on resin; vent pins under 1.6 mm (FDM) snap off.

  • Cure inhibition: platinum-cure silicone can stay tacky against some resin (SLA / MSLA) prints, some primers and paints and sulphur-based clays. Post-cure resin prints beyond normal, seal them, and cure a test patch first; tin-cure silicone is not affected. Check your materials with the inhibition checker in Mould making.

  • Silicone: the volume each master casts (box inside up to the fill line, less the master — keys and channels included) and its weight, with parts A and B by weight for the chosen rubber.

Brush-on silicone with a printed mother mould

For a large or awkward part, a thin skin of silicone backed by a rigid shell uses far less rubber than a poured block. Pick Silicone mould → Brush-on + mother mould. The tool makes:

  • The skin: the part grown by the skin thickness (6 mm by default) all round — a true offset of the part (a Minkowski sum with a sphere, on a simplified copy of it, so it is smooth rather than exact in the finest detail). Its volume, split at the parting height, gives the silicone to mix for each half.

  • The mother mould (jacket): a shell 4 mm thick round the skin, split at the parting height into an upper and a lower half. Each half has a flange along the split (14 mm wide, 6 mm thick each side by default) with keys between the halves (hemispherical bumps on the lower flange, sockets in the upper) and bolt holes (M3–M6, 1/4") spaced evenly round it, between the keys.

  • Locating keys for the skin: hemispherical dimples in the inside of the shell, spread over the upward- facing top and the downward-facing bottom of the skin. The silicone fills them (poured) or you build up bumps there (brushed), so the skin cannot shift in the shell.

  • The skin spacer (tick Skin spacer): the lower half of the skin, printed — the stand-in for the clay of the classic clay-up — so you can pour the silicone instead of brushing it. Pour and vent holes are then cut in the shell: at the top of the upper half, and at the bottom of the lower half (which is poured turned over).

Brush it on. Seal a porous part and coat it with release agent. Brush a thin first coat of unthickened silicone into every detail and blow out bubbles; when it is tacky, build up with thickened coats (the maker’s thixotropic additive) to the skin thickness — usually 4–6 coats — brushing extra where the shell has its dimples. Check the thickness with a pin, trim high spots so the shell halves seat fully, bolt the flanges. To demould, unbolt, lift the shell off, cut the skin open along the parting line (a zig-zag registers) and peel it off. To cast, put the skin back in the shell and bolt it up.

Or pour it. Lay the spacer in the lower shell and set the part in it (the spacer takes the part’s lower half exactly where the shell expects it). Put the upper shell on, bolt it, and pour the upper half of the skin through the pour hole until it shows in the vents. When it has cured, turn it over, take off the lower shell and the spacer, brush release on the silicone face, refit the shell and pour the lower half.

Which process?

Process You print Original needed? Undercuts Good for

Rigid printed mould

The mould pieces

No

None allowed (planar pieces)

Resin, wax, plaster, concrete; a few casts

Silicone box round an original

A box (and bed), rectangular or fitted

Yes

Silicone stretches (up to 30° back-lean)

Many casts of a part you have

Printed silicone master

The box with the positive

No

As the silicone box

Many casts of a part you only have as a file

Brush-on silicone + mother mould

The shell halves (and a skin spacer)

Yes

Silicone stretches; the skin is cut open

Large or awkward parts, little silicone

Slip casting

Case moulds for the plaster

No

None allowed (rigid plaster)

Ceramics (scaled 11–15 % for the clay)

Lost-PLA investment pattern

The pattern with its sprue, cup and vents

No

Anything (the pattern burns out)

Aluminium, bronze, brass, silver, gold — one cast per print

Sand-casting pattern

Split pattern or match plate, gating, core box

No

None (draft 1–2° helps)

Aluminium and bronze, larger parts, many casts

Mold for a mold (slip casting)

Pick the Slip casting process to design a plaster mould for slip casting ceramics and the printable moulds that make it (old links with mom=1 open it too). A stage switcher appears: Part → Plaster mould → Case moulds; the plaster and case-mould settings are on the Plaster tab.

Stage 1, the plaster mould. The part is first scaled up for the clay’s total shrinkage (drying
firing, as a share of the wet size: wet = fired ÷ (1 − s)). Presets are typical values — earthenware about 11 %, stoneware 12.5 %, porcelain 14.5 % — and every body differs: fire a shrinkage bar of your own slip and enter its figure (see the Digitalfire Reference Library, Shrinkage). The mould gets thick plaster walls (30 mm by default; under 20 mm is flagged, as thin plaster cracks and saturates quickly). Instead of a sprue, risers and vents there is a spare: a funnel from the middle of the part’s top up through the top of the block, holding about half the volume of the cast wall (surface × your target wall thickness ÷ 2 — a rule of thumb) so the mould stays full as the plaster draws water out of the slip; turned over, it is also the drain. A part with an open top (a cup, a hollow crown) is moulded from the outside: the opening is filled to its rim and the spare covers it. The spare is part of the cavity the parting is computed for, so it always comes out of the mould — usually split down its middle by the main parting, or in a piece that lifts off upwards — and the planner is biased towards few pieces. Registration is by natches: large hemispherical or conical plaster bumps and dimples with clearance, nothing thin to chip. There are no bolt holes; optional grooves round the block locate straps or rubber bands (left off each piece’s back face, which is the face its case leaves open).

Plaster quantities. For a mould volume V and consistency c (parts of water per 100 parts plaster by weight; USG No. 1 Pottery Plaster’s normal consistency is 70), plaster = V ÷ (c/100 + 1/2.65) — the slurry is the water plus the plaster’s own volume, specific gravity ≈ 2.65 — and water = plaster × c/100, with 10 % added for waste. One cubic foot at consistency 70 works out at about 58 lb of plaster and 40.6 lb of water. The dry mould weighs plaster × 1.186, the gypsum’s water of crystallisation (CaSO₄·2H₂O ÷ CaSO₄·½H₂O = 172.17 ÷ 145.15).

Stage 2, case (mother) moulds. For each plaster piece the tool makes a printable box to cast it in, open on the piece’s back face (the face opposite the part, chosen automatically; you can change it) so you pour the plaster in from there and screed it flat. The box is cut into a base — the side that carries the part’s impression and the natches, pulled straight off the plaster — and four walls pulled straight out sideways (or two L-shaped walls, or a single tray). Recesses in the plaster’s sides (strap grooves, dimples) are formed by the wall on that side. Where the spare runs out through the open face it gets its own spare core with a handle, pulled out upwards. Clamp lugs with 3.4 mm holes (M3 bolts or zip ties) hold the parts together, optional ribs stiffen the walls, and the piece number and an up arrow can be raised on a wall so they are cast into the plaster. The same release test as for moulds checks that every face of the plaster piece comes away from the case part against it; the Release view shows it, and anything caught is reported.

The README in the ZIP gives the workflow: print the cases, bolt and seal them, release agent (mould soap, or a wax film on plastic), mix and pour the plaster, dry the pieces thoroughly, strap the mould together, pour slip, top up the spare, wait for the wall, drain, demould at leather hard, cut off the spare, fettle, dry and fire — with the caveats.

Metal casting

Pick Metal casting and a method. The part is first scaled up by the metal’s shrinkage. Presets (typical values — your alloy supplier’s figures rule): aluminium A356 / LM25 1.3 % (the pattern-maker’s 5/32 in per foot), tin or silicon bronze 1.6 %, yellow brass 1.5 %, lead-free pewter 0.6 %, zinc alloy (Zamak 3) 1.0 %, sterling silver 1.2 %, 14 ct gold 1.3 %, with their densities and pouring temperatures. In investment casting the mould’s own expansion makes up part of the shrinkage, so jewellers often scale less, or not at all.

Feeders by the modulus method. A section stays liquid longer the larger its modulus, volume ÷ cooling surface (Chvorinov’s rule). A feeder (riser) must freeze after the casting it feeds; the rule of thumb is a feeder modulus of at least 1.2 × the casting’s. For a cylinder as tall as 1.5 × its diameter, cooling through its top and side, the modulus is D·1.5 ÷ 7, so D = 5.6 × the casting modulus. The tool uses the whole part’s modulus — a rule of thumb, not a solidification simulation: a part with one heavy section and thin walls elsewhere wants the feeder on the heavy section, which is where it puts it.

Lost PLA / lost wax (investment)

One printable pattern: the part, a short gate neck at its heaviest up-facing section (chosen from the points with a clear path straight up, by the thickness under them), a feeder on the sprue just above it (optional), the sprue (auto: from the casting modulus) up to a pour cup whose rim is the top of the flask, up to two extra gates branching from the sprue, and vents — thin rods from the other high points straight up to the top, so air can leave. All of it is joined (booleans) into one closed print.

The flask is a cylinder round the sprue with the investment clearance (12 mm by default) all round and underneath; the tool gives its minimum size and the smallest common steel flask that fits (2½ × 2½ in up to 8 × 10 in). Investment = flask − pattern; for a gypsum-bonded investment mixed at 38–40 parts water to 100 parts powder by weight (use your investment’s datasheet), powder = volume ÷ (water ratio + 1 ÷ 2.4), plus 10 % — the powder’s specific gravity of about 2.4 is an assumption; the maker’s chart rules. Metal = the pattern’s volume (part, gating and cup) × the density, + 10 %.

  1. Print the pattern in PLA (castable PLA such as PolyCast burns out cleaner) with 2–3 perimeters and 0–10 % infill: less plastic to burn, and less expansion to crack the investment. Smooth it; heavy primer leaves ash.

  2. Fix the cup to a flask base so its rim is level with the top of the flask; brush on a debubbliser.

  3. Mix the investment (weigh both), 2–3 minutes; vacuum, pour down the side of the flask, vacuum again; leave it to set for 2 hours.

  4. Burn out: about 150 °C for 2 h (the water steams off), up to 370 °C for 2 h, then 730 °C for 4–6 h, until no smoke or soot comes out of the sprue. PLA does not run out like wax: it softens, swells and has to burn, so ramp slowly and hold long. Wax can be steamed or melted out first and needs a shorter top hold. Gypsum-bonded investment must not go above about 760 °C (it breaks down, giving porous castings).

  5. Cool the flask to its casting temperature (aluminium 200–400 °C, bronze, brass, silver and gold 480–650 °C), melt the metal and pour in one steady stream until the cup stays full.

  6. When the button has set, quench the flask in water to break the investment out, and cut off the sprue, gates and vents.

Sand-casting pattern

The part is split in two along the first parting plane of the automatic parting (forced to two pieces; move the plane to change the split). The pieces are shown in the flask’s frame: parting horizontal, the cope half above it, the drag half below. A part that lies wholly on one side is a one-piece flat-back pattern in the drag.

  • Dowel pins: two pins (Ø6 mm, or Ø3 mm when the parting face is small) in the drag half and matching holes in the cope half, placed inside the part’s parting face where both halves are thick enough, as far apart as they can be.

  • Draft check: every face against its half’s pull (up for the cope, down for the drag). The Pattern tab reports the area with less draft than the threshold (1.5° in the presets; 1–2° is usual for sand) and any undercut; the Draft and Release views show where.

  • Gating by the 1 : 2 : 2 rule of thumb for an unpressurised system (as for aluminium): sprue choke area : runner area : total gate area. The sprue (auto Ø from the casting modulus, at least 12 mm) tapers wider at the top so it draws up out of the cope; the runner and gate(s) lie in the drag under the parting, entering the part at one end; a feeder sized by the modulus method stands on the heaviest up-facing section (open riser in the cope). They are separate printable patterns.

  • Match plate (option): one print with a plate (8 mm) in the parting plane, the cope half on top and the drag half and the runner underneath, in register, with two Ø10 mm holes for the flask pins — move them to suit your flask.

  • Cores: a straight through-hole along the parting normal or lying in the parting plane (found where the part’s outline seen along that axis has a hole) gets a core: the hole is filled in the pattern and core prints stick out of it at both ends, so the sand mould has seats for the core. The tool prints the core (as a shape reference) and a core box split along the core’s axis, with keys, open at one end to ram the core sand in.

  • Flask: at least 30 mm of sand round the pattern and gating (your setting), the drag at least 40 mm deeper than the pattern below the parting, the cope 50 mm over the pattern or feeder for the sprue head; sand at about 1.5 kg per litre.

    1. Print and smooth the patterns, seal them (sand sticks to print lines) and dust them with parting powder. Ram the cores in the core box with core sand and harden them.

    2. Drag: the drag half pins up, face down on the moulding board with the runner and gate; the drag flask round it; riddle sand over, ram in layers, strike off, vent with a wire, turn it over.

    3. Cope: the cope half on the pins, parting powder, the cope flask, the sprue on the end of the runner and the feeder on the heavy section; ram, draw the sprue and feeder, cut a pour basin.

    4. Lift the cope, draw the patterns, repair loose sand, set the cores, close the flask.

    5. Pour fast and steadily, keeping the sprue full; shake out when cool and cut off the gating.

Safety, whatever the method. Full-face shield over safety glasses, long leather gloves, a leather or wool apron and spats, cotton or wool clothes (synthetics melt onto skin), leather boots. Pour over dry sand, never concrete. Water and molten metal make a steam explosion: every flask, mould, tool, ingot and piece of scrap that touches the metal must be dry and pre-heated — never pour into a damp or cold mould, and never onto damp plaster or investment. Investment powder contains crystalline silica: wear a P2/P3 (N95 or better) mask when mixing it, and vent the kiln outdoors while PLA burns out.

Inserts

The Inserts tab (rigid moulds and silicone moulds) holds parts that are cast in: a magnet (Ø × height), a hex nut (M3–M8), a heat-set or threaded insert (M2–M6, typical sizes), a wire or rod that sticks out of the casting, or a custom cylinder (with or without a bore). Pick the type and size, press Click on the part to place and click the part in the view — or type the coordinates and the tool puts it on the nearest point of the surface. Its axis is the surface normal there, or the pull of the mould piece it sits against.

  • Pin: for anything with a bore (a nut, a threaded insert, a bored cylinder) a pin stands from 3 mm inside the mould wall, across the cavity, through the insert to its far end — a little under the thread’s minor diameter, so the casting cannot fill the thread. The pin belongs to the piece the insert sits against; it has to run along that piece’s pull or it locks the piece, and the tool warns if it does not.

  • Pocket: a magnet, a rod or a plain cylinder sits in a socket cut into the mould wall (0.15 mm clearance) — a magnet 1 mm deep, so it stands that much proud of the casting; a rod as far as it sticks out.

  • Silicone, printed master: the positive gets a hole where each pin goes (the silicone fills it: a rubber post through the insert) and a boss where each pocket goes (the silicone gets the socket).

  • Silicone round your original (box or brush-on): the tool cannot change your part, so the README says what hole to drill for each post, and it prints a stand-in disc for each pocket to glue on.

The inserts are listed in the README with the step to set them before closing the mould. Turning or scaling the part removes them.

Exports

The Export tab puts the formats that matter for the process first (for a lost-PLA pattern: the ZIP, one STL and 3MF; for a sand pattern also STEP and the DXF of the parting) and the rest under More formats. Two check boxes apply in every process and stage (rigid mould, silicone box, masters and mother mould, plaster mould and case moulds, metal patterns):

  • Combine all pieces into one file — STL: one mesh holding every piece, side by side so nothing overlaps (the STL (one file) button); a two-part mould becomes one printable file. OBJ (an object per piece), 3MF (one build plate, each piece a separate object) and STEP (one solid per piece) hold every piece anyway; this moves them apart, on z = 0, in the orientation they were designed in.

  • Lay pieces flat and arrange on the bed — each piece is turned onto a flat face: its back face (the face its pull points to, opposite the cavity) or a master’s floor when that is a flat outside face, otherwise the largest flat outside face that is not on the cavity side; turned about Z to its smallest footprint, dropped to z = 0 and packed on the bed (shelves, largest first, each piece may turn 90°) with the spacing between pieces, each plate centred. Beds: Bambu Lab A1 mini (180 × 180), A1, P1S / P1P and X1C / X1E (256 × 256), H2D (325 × 320, one nozzle), Prusa MK4 (250 × 210), Prusa MINI+ (180 × 180), Ender-3 (220 × 220), Voron 2.4 350 (350 × 350) or custom. If the pieces do not fit they go on more plates, written as numbered files (…-plate-1.3mf, …-plate-2.3mf in a ZIP) and the tool says so; a piece bigger than the bed has a plate of its own. Single-piece STL buttons write that piece laid flat.

The Plates view shows the layout in 3-D, and the Export tab a top-down preview of each plate (the footprints numbered against the piece list). Both settings, the bed and the spacing are remembered and go in share links.

  • STL: binary, one file per piece. 3MF: one package, each piece a named object with a display colour. OBJ: one file with an object per piece, and an MTL with the colours.

  • STEP (AP214): each piece as a solid body (MANIFOLD_SOLID_BREP) — Fusion 360, Onshape, FreeCAD and SolidWorks open them as solids you can add to, fillet or cut. They are faceted: flat faces bounded by straight edges, not NURBS, so a curved cavity stays a set of facets. Coplanar triangles are merged into single flat faces (holes included) to keep the file small; turn that off to get one face per triangle. The tests read the files back with OpenCascade.

  • DXF (AutoCAD R12 ASCII, millimetres): for each parting plane, the block outline, the parting line (where the plane cuts the part), key circles (pin and socket), bolt holes and channels, laid side by side in plane coordinates; plus part sections at the Z heights you list. Layers: OUTLINE, PARTING_LINE, KEYS, BOLTS, CHANNELS, SECTION, TEXT. Insert into a Fusion 360 sketch, or use for laser-cut or CNC mould boxes.

  • ZIP: all of the above, the scaled part, project.json and a README with the steps for the process. A metal pattern’s ZIP holds stl/, pattern.3mf, pattern.obj, pattern.step, the parting DXF (sand), part-scaled-for-shrinkage.stl and a README with the metal, gating, flask, investment or sand, the steps and the safety notes. A brush-on mould has no DXF (no flat parting to draw). With either plate option ticked it also has plates/: a 3MF and a combined STL per plate and, when laid flat, each piece laid flat (plates/pieces/); in slip-casting mode the case moulds' plates are in 2-case-moulds/plates/. In slip-casting mode the ZIP has both stages — 1-plaster-mould/ (the plaster pieces as reference: STL, 3MF, STEP, DXF) and 2-case-moulds/ (a folder per plaster piece with its case parts as STL, a 3MF and a STEP, plus one 3MF of all of them) — and the 3MF / STEP / OBJ / STL buttons export whichever stage is shown.

Automatic repair

When a loaded file is not a closed solid, the tool runs the staged repair from mesh repair — weld, remove degenerate and duplicate faces, remove tiny shells, orient consistently and outward, split non-manifold edges, stitch slits and fill holes, union overlapping shells — and stops as soon as the mesh is a valid solid. A dismissible notice above the view sums it up, for example Closed 3 holes, welded 412 vertices, removed 2 tiny shells; max deviation 0.02 mm:

  • View details opens the report: what was found, what each stage did, triangles, volume and size before and after, and the deviation.

  • Open in Mesh repair ↗ takes the original file to the repair tool (new tab), where the problems are drawn on the model and every setting can be changed; from there Open in mold maker brings the repaired model back.

  • Use original anyway appears when the file could also be loaded the old way (open holes capped with simple fans from their centre): that keeps the file as close to the original as possible.

  • If cleaning and filling are not enough, the only way left is a voxel remesh, which rebuilds the surface from a grid and loses detail smaller than a voxel; the tool asks before doing that.

Large models. A file of more than 400,000 triangles (AI image-to-3-D exports and 3-D scans often have millions) gets an offer before any analysis runs: Simplify to ~300k collapses edges within a small tolerance (manifold-3d) and reports the largest deviation (for a 2.1-million-triangle, 63 mm model: 225,000 triangles, 0.06 mm), or Keep all triangles. A mould needs nowhere near that many, and the cutting is far faster and lighter on memory with fewer; Use all triangles goes back.

The repaired mesh is what the mould is made from (re-orienting or scaling does not repair it again). Turn it off with Repair broken meshes automatically on the Part tab: a broken file is then refused with the reason, as before.

Limits

  • Planar partings only. Pieces are bounded by flat planes. A part whose silhouette is not flat (a twisted blade, a curved handle) leaves either some undercut or an extra piece where a hand-modelled stepped or curved parting surface would do better. No side-action cores or loose pieces inside the part; through-holes need a parting plane through them.

  • Approximate analysis. Undercuts are judged per face on a sample of up to 6,000 faces against 70 or so directions, with one ray per face; a thin lip smaller than the sampling can be missed, and flexible moulds (silicone) release far more than the rigid test assumes. The plan is a good starting point, not a guarantee — look at the Release view and the section plane.

  • Air traps and flow are geometric only: no fluid simulation or solidification modelling. In a rigid mould risers are placed, not sized; metal patterns size the feeder and gating by rules of thumb (below).

  • STEP output is faceted, not NURBS, and STEP input is meshed before anything else happens.

  • Very large meshes (several hundred thousand triangles) work but are slow; the STEP of a 100,000-triangle mould is about 60 MB.

  • Shrinkage presets are typical values, not your material’s.

  • Silicone: the relaxed undercut rule is a rule of thumb — soft silicone (Shore 10–20A) releases more, firm (40A) less. Mix ratios and densities are typical; weigh by your datasheet. Split masters assume a horizontal parting plane; their channels run to the +Y edge only.

  • Plates: shelf packing of bounding rectangles, not true nesting, so it can use more plates than a slicer’s arrange would. Multi-plate 3MF is written as separate files, not as one project with several plates (that format differs between slicers).

  • Fitted silicone boxes apply to boxes poured round an original and to the classic and cut printed masters; split and one-part printed masters keep their rectangular (drafted) boxes.

  • Brush-on mother mould: the skin is an offset of a simplified copy of the part (a Minkowski sum), so very fine detail is smoothed in the shell — the silicone itself still forms against the real part. The parting is one horizontal plane; the flanges follow the shell’s outline at that height, and a bolt can be blocked by a shell that bulges out above it. Large parts take a few seconds.

  • Inserts: placed on the part as oriented, by position, not tied to a feature; a pin is a straight cylinder (no thread), and a silicone post is soft — it locates a nut, it does not clamp it. Not for slip casting or metal.

  • Metal casting is geometry and rules of thumb, not a casting simulation: no fluid flow, solidification or porosity prediction. The feeder is sized from the whole part’s modulus; gates and vents are straight rods (check that an extra gate does not pass through the part). Lost-PLA burnout and flask temperatures are typical schedules — follow your investment’s datasheet. Shrinkage presets are typical; investment expansion offsets some of it.

  • Sand patterns: one parting plane (the first of the plan), so a part that needs a stepped or loose-piece pattern is reported as undercut. Cores only for straight through-holes along the parting normal or lying in the parting plane that show as a hole in the part’s outline along that axis; blind holes, angled or curved holes are not cored (drill them, or make the core by hand). The core is printed only as a shape reference: make the real core from core sand in the core box.

  • Slip casting: casting time, slip behaviour and the spare’s size are rules of thumb, not a model of water uptake. Two L-shaped case walls pull out diagonally, so they suit plaster pieces with plain sides; with strap grooves or natches on the sides use four walls. Plaster pieces cut by more than one parting plane are cased in a box round the piece, which can need the open face set by hand.

Related: STL, OBJ and 3MF viewer, OpenSCAD in the browser, print tolerances and fits (key clearance), epoxy and resin pour calculator (mixing part A and B for the fill volume), filament and print cost, batch, tolerance and shelf life (when a silicone or printed mould beats printing every part), box maker, model splitter (same mesh engine), mesh repair (fix broken STL and 3MF files).

Engines: manifold-3d 3.5.4 (Apache-2.0) for the booleans, occt-import-js 0.0.23 (LGPL-2.1, Open CASCADE Technology) for STEP and IGES, and three.js 0.147 for the view — loaded from jsDelivr the first time they are needed.

Linking to an example

Share link (under the title) copies a URL with your settings — never your part. A link can also be written by hand: /tools/mold-maker/?example=bracket&pieces=auto&key=cone.

Parameter Meaning

example

Sample part: knob, rook, figurine, bracket, crossblock, spool, sphere, pulley; or a preset: knob-silicone-master, knob-split-master, figurine-split-master, figurine-plates, figurine-fitted-box, figurine-brush-on, bracket-inserts, figurine-lost-pla, rook-sand-cast, pulley-sand-core (a preset’s settings never override parameters in the same link)

process

rigid, silicone, slip or metal

sil

Silicone method: box (poured round your part), master (printed master) or brush (brush-on + mother mould)

cast

Metal method: lost (lost PLA / wax) or sand (sand-casting pattern)

mode, mom, master

The old switches, still read: a link with them and no process opens the process they meant (mode=silicone → silicone box, master=1 → printed master, mom=1 → slip casting)

pieces

auto or 2–6

draft

Draft threshold in degrees

wall

Wall / margin around the part, mm

mat, shrink

Casting material (urethane-resin, epoxy, plaster, wax, concrete, pewter, zinc, aluminium, custom) and shrinkage %

key, keyd, clear, nkeys

Key shape (hemi, cone, square, dovetail, none), size mm, clearance mm, keys per plane

sprue, sprued, gate, pour

Sprue on/off, diameter, top or bottom gate, pour axis (+z, -z, +x …)

risers, riserd, vents, ventd

Number and diameter of risers; vents on/off and diameter

bolts, bolt, nbolts, pry

Bolt holes on/off, size (M3…M6, 1/4"), per plane; pry slots on/off

silicone, gap, boxwall, guides

Silicone mould two-part, cut or block (one-part, open face), silicone thickness, box wall, cut-line guides

mstyle, mwalls, bdraft, mshrink, rubber

Two-part masters split or classic; walls draft or split; wall draft °; casting shrinkage %; rubber pt20, pt30, pt40, tin25, tin20, none

box

Silicone box walls: rect, fitted or convex

skin, shell, flange, flanget, skinkeys, spacer

Brush-on: skin thickness, mother-mould wall, flange width and thickness (mm), locating keys per half, skin spacer (1)

ins

Inserts, one string: typesize-or-Øheightdepthboreaxisxyznxny~nz, joined by _ (type magnet, nut, heatset, rod, custom; axis normal or pull)

alloy, ashrink

Metal (aluminium, bronze, brass, pewter, zinc, silver, gold) and its shrinkage %

msprue, feeder, gates, flaskclear, invest

Metal sprue Ø (0 = auto), feeder on/off, gates (1–3), investment round the pattern mm, water per 100 powder

matchplate, platet, pind, cores, margin

Sand: match plate on/off and thickness, dowel pin Ø, cores on/off, sand round the pattern mm

flat, combine, bed, bedw, bedd, spacing

Lay pieces flat and arrange on the bed; combine into one file; bed bambu-a1mini, bambu-a1, bambu-p1, bambu-x1, bambu-h2d, prusa-mk4, prusa-mini, ender3, voron350, custom; custom bed X and Y mm; spacing mm

view

part, draft, release, mold or plate

stage

Slip casting stage: part, plaster or case

pwall, clay, cshrink, slipwall, spared

Plaster wall mm; clay body (earthenware, stoneware, porcelain, custom) and total shrinkage %; cast wall target mm; spare diameter (0 = auto)

natch, natchd, natchclear, nnatch, straps

Natch shape (hemi, cone, none), size, clearance, per parting; strap grooves

plaster, consist

Plaster preset and consistency (water per 100 plaster by weight)

casewall, case, ribs, lugs, emboss

Case wall mm; case parts walls4, walls2 or tray; ribs, lugs, embossed number

Presets: figurine, two-part mould · cross-drilled block, four pieces · bracket, release check · sphere, draft view · knob in a silicone box · knob, printed silicone master (one-part) · knob, two split silicone masters · figurine, split masters with bolted walls and vents · figurine masters laid out on an A1 mini · figurine mould, laid flat on a bed · rook, cone keys, bottom fill · figurine, plaster slip-casting mould · knob, case moulds for the plaster · rook in porcelain, case release check · figurine, fitted silicone box · figurine, brush-on silicone with a mother mould and skin spacer · bracket with a magnet and a nut cast in · figurine, lost-PLA aluminium pattern · rook, sand-cast split pattern · bronze pulley on a match plate, with a core box · knob in sterling silver, lost PLA.

The tool keeps its usual page flags: ?bare=1 (just the tool), ?embed=1 (for an iframe: settings live only in that page load), and #part, #parting, #features, #inserts or #export opens a tab (a tab the process does not use falls back to Part).

Runs entirely in your browser; nothing you type leaves this page.