By Printara · Published 11 October 2026 · 21 min read

How to Make a Silicone Mold from an STL: Printed Mold Box vs Printed Shell

You have an STL and you want copies of it in resin, wax, plaster or soap. The route that works for almost any shape is not to print the mold itself. You print the master and a rigid container, pour liquid silicone around the master, and the cured silicone becomes the mold. The container is either a mold box (a simple box the silicone fills) or a shell (a jacket that follows the master at a constant distance).

This guide covers when to use which, how to prepare the STL, where the parting line and the keys go, how thick the silicone and the printed walls should be, where to put the pour spout and the vents, how to work out how much silicone to buy, print settings, the pour itself, demolding, and the most common failures.

Disclosure: Printara makes the Mold Maker described in the tools section of this guide. Everything before that section works with any tool, including a box you build by hand.

Why silicone, and what the printer is for

A common view among people who cast regularly is that molds printed in rigid plastic are fussy, and that silicone molds are better and reusable. For most shapes that is right. A rigid printed mold only lets go of a cast if nothing on the part leans back over the parting line. Its layer lines print into every copy, and it needs release agent every time. Silicone stretches over undercuts, copies the surface of the master and peels off.

So the 3D printer has two jobs: making the master, if you do not already have one, and making the rigid parts that hold liquid silicone while it cures (and that later hold the cured silicone in shape while you cast). Keep four objects apart in your head, because guides and tools mix them up:

1. The master: the object you want copies of. A print of your STL, or any original.
2. The printed box or shell: the container. It is reusable.
3. The silicone mold: the flexible negative that cures inside the container. This is what you cast into.
4. The cast: the copy, in resin, wax, plaster, soap or something similar.

When is a rigid printed mold enough? When the shape has no undercuts, a flat back or an obvious parting plane, and some draft (1 to 2 degrees or more on walls that run in the direction the mold opens). Simple soap bars, tiles, coasters and plaster blocks work. Figures, faces, hands and anything with texture want silicone.

Mold box or shell: how to choose

Both are printed containers for silicone. The difference is how much silicone they take and how much printing and sealing they need.

A worked example shows why the choice matters. To keep the numbers checkable, take an egg-shaped master 80 x 70 x 150 mm (an ellipsoid; a real figure will be close but not identical) and give it 10 mm of silicone everywhere:

• Master volume: 4/3 x pi x 40 x 35 x 75 mm = about 440 cm3.
• Box: the inside is 100 x 90 x 170 mm = 1,530 cm3. Silicone = 1,530 minus 440 = about 1,090 cm3, or about 1.22 kg of a silicone with a specific gravity of 1.12.
• Shell: the master grown by 10 mm in every direction is about 811 cm3. Silicone = 811 minus 440 = about 371 cm3, or about 0.42 kg.

Same master, same 10 mm of silicone, and the box takes about three times as much. On a figure with arms, a weapon or a cloak the gap is bigger, because the box also fills all the air between the limbs and the body. On a squat, blocky master there is little difference, and the box is simpler. That is the whole decision: the box saves effort, the shell saves silicone.

Step 1: Prepare the master STL

Make it watertight. A mold is the space around your model, so the model has to be a closed surface: no holes, no flipped faces, no loose internal shells. Most slicers report and repair simple problems on import, and Blender's 3D-Print Toolbox add-on finds non-manifold edges. Scans and sculpts are the usual offenders.

Check the size and the units. Read the dimensions in millimetres after import. A model exported in inches arrives 25.4 times too small. Decide the final size now, because the silicone copies whatever you give it. Platinum silicone barely shrinks (Smooth-On lists less than 0.001 in/in for Mold Star 30). Tin-cure silicone shrinks a little more over time. If your casting material shrinks, use the value from its datasheet to scale the master, not a guess.

Choose the opening. Decide which face of the master becomes the opening of the finished mold, the place where resin or wax will go in later. Usually that is the base or the back: flat, hidden, and where a small pour mark does not matter. The master stands on that face, and the silicone covers everything else.

Look at thin parts. Swords, fingers, antennas and thin capes are where silicone tears and where air hides. Thicken them, remove them and cast them separately, or split the master. A master that is too big for your printer or your silicone budget can be cut into parts first, for example with the Printara STL Splitter.

Finish the surface. Silicone copies everything, including layer lines. Sand and prime FDM masters, or print the master on a resin printer. FDM prints are porous, so seal them.

Resin masters need a full cure. Platinum silicone can stay sticky against resin that is not fully cured. Smooth-On's procedure: wash the print in isopropyl alcohol, then soap and water, let it dry, and UV post-cure it for at least 6 hours, turning it so light reaches every surface, including deep recesses. Then seal it with a clear acrylic spray or an epoxy coating and let that dry completely. If you print the master specially for molding, use a clear or translucent resin: UV light gets through it and cures it all the way, and Smooth-On advises against opaque resin for this job.

Test when in doubt. Put a small blob of mixed silicone on a spot that does not matter and leave it for the full cure time. If it cures firm, the pour will too. Keep sulfur-based modelling clay, latex gloves and tin-cure silicone residue away from platinum silicone: each of them can stop it curing. Wear nitrile gloves, and let any sealer dry completely before you pour.

Step 2: Parting line and keys

Unless the mold is an open-faced tray, the silicone has to open somewhere to let the master out. You have three common ways to do it.

One-piece block, cut open. Pour one block, let it cure, then cut it open with a fresh blade along one side. Cut straight where the blade is close to the model and in small zigzags away from it, so the two flaps key back into each other. Stop part of the way; the uncut part keeps the block in one piece. This is the hobby standard for figures and miniatures.

Two-part silicone mold. Pour the first half against a parting bed (clay, or a printed bed with key bumps), apply release agent, then pour the second half. Silicone sticks to silicone, so the release is not optional. Smooth-On lists three options: a release made for silicone (such as their Ease Release 200), a mix of 2 parts isopropyl alcohol and 1 part dish soap, or petroleum jelly thinned with mineral spirits. Test it on a small scale first.

Shell skin. In a shell, the silicone is one skin around the master. It comes off through the opening at the base, or you cut it along a line as above if the master locks in.

Where the parting line goes. Along the widest silhouette, so nothing is wider than its own way out. Along edges and on hidden sides, away from faces and lettering. If no plane works for the whole master, that is the sign to cut the silicone instead of splitting it on a plane.

Keys. Keys make sure the mold closes the same way every time.

• In a two-part silicone mold: half-spheres or cones on the parting face, at least three, placed unevenly so the halves only fit one way.
• In a shell: keys on the outside of the silicone sit in pockets in the shell, so the skin cannot shift or sag when you cast. The shell parts themselves locate on each other with flanges, pins or tabs, and close with bolts, clips or bands.
• Between printed parts that fit together, start with about 0.2 mm of clearance on an FDM printer with a 0.4 mm nozzle, and print one test pair before the whole set.

Step 3: Silicone thickness, walls and clearances

These are starting points, not datasheet values. Check your silicone's datasheet for hardness and tear strength. A platinum silicone around Shore 20A to 30A is a common choice for resin, wax and plaster casts: soft enough to peel over undercuts, firm enough to keep thin walls straight.

Step 4: Pour spout and vents

In a box, the top is open. Pour into one corner, or the lowest point, never onto the master, in a thin stream from a height. Smooth-On suggests 2 to 3 feet (about 60 to 90 cm); the falling stream stretches and pops bubbles. Let the silicone find its own level and rise up over the master. Fill until it covers the highest point by your planned thickness.

In a shell, the silicone goes in through a spout at the top, runs down and fills from the bottom. The air it pushes ahead of it has to leave somewhere, and it collects at every local high point of the cavity:

• under a chin, a nose or a hat brim,
• under outstretched arms or a cloak,
• at the top of the shell ceiling, if the spout is not the highest point.

Give every one of those spots a vent: a small hole (3 to 5 mm works with most pourable silicones) from the high point to the outside, ending above the level of the spout. Pour until silicone appears in every vent, then plug them or let them fill, and top up the spout as the level settles. Make the spout a funnel and give it some height, so it holds a little extra silicone while air works its way out.

For the casting later, the opening of the mold is usually at the base. For hollow casts (slush or roto casting), a fill and drain port in the base lets you pour resin in, turn the mold so it coats the walls, and pour the rest out.

Step 5: Work out how much silicone you need

Silicone is the expensive part of the job, so measure before you buy.

Box

silicone (cm3) = inside length x width x height of the box (cm) - master volume (cm3)

Most slicers and mesh tools show the volume of a model: PrusaSlicer shows it in the object info, and Blender's 3D-Print Toolbox has a Volume check. This is the method Smooth-On describes: box volume minus model volume.

Shell

silicone (cm3) = master surface area (cm2) x silicone thickness (cm) x 1.25

Area times thickness on its own comes out low, because the outer side of the layer is bigger than the master. In the egg example above, area x thickness gives about 302 cm3, while the real layer is about 371 cm3: about 23 percent more. On thin parts such as arms, swords or fingers the difference is bigger still, so treat the result as a minimum. If the tool that builds your shell shows the volume, use its number.

Then convert to weight

grams = cm3 x specific gravity (g/cm3)

The specific gravity is on the silicone's technical datasheet. Mold Star 30, for example, lists 1.12 g/cc. Add about 10 percent for what stays in the cup and the spout. For the egg example: box 1,090 cm3 x 1.12 = 1,221 g, plus 10 percent = about 1.34 kg. Shell 371 cm3 x 1.12 = 416 g, plus 10 percent = about 0.46 kg.

No numbers? Measure. Smooth-On's easy method: stand the master in the box, pour water over it to the planned level, then pour the water into a measuring jug. That is the volume of silicone you need. Dry the box and the master completely before you pour silicone.

Mixing amounts. Most silicones are mixed by weight in a fixed ratio from the datasheet (Mold Star 30: 100 parts A to 96 parts B by weight, or 1:1 by volume). Weigh, do not guess.

Step 6: Print the box or shell

Before the full print, print one key or clip pair to check the fit on your printer.

Step 7: Assemble, mix and pour

1. Fix the master to the base. Glue it down and run a bead of hot glue or clay round the joint, so silicone cannot creep underneath and push up a large bubble. A printed base with a peg that the master sits on does the same job.
2. Close and seal the container. Assemble the walls or wedges, tighten the bolts or clips, and run hot glue, clay or tape along every seam from the outside. Stand the whole thing in a tray, so a leak costs you silicone and not your bench.
3. Release, if the datasheet asks for it. Many platinum silicones release from a sealed, smooth master without help; some combinations need a light mist of release agent. Follow the silicone maker's table.
4. Work warm. Room-temperature silicones are made to cure at about 23 C. A cold room, cold material or a cold master slows or stops the cure.
5. Weigh and mix. Weigh part A and part B to the ratio. Mix slowly for the time the datasheet says, scraping the sides and the bottom of the cup. Under-mixing is the most common reason a mold never cures. Slow stirring also folds in less air.
6. Get the air out. If you have a vacuum chamber, degas the mixed silicone: it rises, foams and collapses as air leaves. Without one, mix a small batch first and brush or stipple a thin detail coat onto the master, then mix the rest and pour.
7. Pour. Thin stream, into the lowest point, away from the master (box), or through the spout until every vent shows silicone (shell). Watch the pot life: Mold Star 30 gives you 45 minutes; fast silicones much less.

Step 8: Cure, demold and cast

Cure fully. Leave it undisturbed for the full cure time on the datasheet (Mold Star 30: 6 hours at room temperature), longer in a cold room.

Demold.

• Box: take off the clamps or bands, pull the walls away, lift the block off the base. Cut it open along your planned line, or separate the two halves.
• Shell: undo the bolts or clips, lift the wedges away, take the silicone skin off the base and pull the master out through the opening, cutting along a line if it locks.

Keep the master and the printed container. If the mold tears or wears out, you can pour a new one in the same container.

Cast. Put the silicone back into its box or shell when you cast, so it keeps its shape. Use a release agent suited to your casting material before each pour: Smooth-On reports that this alone can as much as double the number of casts a mold survives. For resin casts, a pressure pot shrinks bubbles in the cast (it does nothing for the mold itself). How long a mold lasts depends mostly on what you pour into it: candle makers casting beeswax report hundreds of casts from one mold, while a chemically harsh resin may give only a handful.

Food. A printed box or shell is never food-safe as printed. For chocolate, ice or anything else that touches food, cast the mold in a silicone that is explicitly sold as food-grade, and follow its maker's instructions.

Common failures and fixes

Let a tool build the box or shell

Everything above can be drawn by hand in CAD: a box is a few rectangles, a shell is an offset of the master cut into pieces. The fiddly parts are the keys, the vents, the parting flange and getting every seam to line up. Several browser tools now build these parts from an STL. We make one of them, the Printara Mold Maker, so here is what it does and what it does not do, followed by a short comparison with the others.

What the Printara Mold Maker does

• You load a master as an STL, check its dimensions and scale it to the size you want to cast. The master can come from anywhere: your own sculpt, a scan or a model made in Printara. A closed master goes in untouched; a master with holes in its surface is rebuilt on a fine grid so it can be molded, and the report says so.
• You choose one of eight mold types: an adaptive shell (a printed jacket that follows the model at silicone thickness: a base plate plus two to four wedges, one pour through the spout and an optional fill port in the base), a one-piece cut mold (the open box poured as one silicone block, with molded guide grooves that show where to cut it open), an open box (a base plate and two wall halves that key together, held by a rubber band), a two-part silicone box (a printed parting bed, frame and sprue for two pours), a tray (one open-top cup, with up to 36 cavities), a printed two-part mold without silicone (only for masters with almost no undercuts, 2 degrees of draft by default), a press mold or stamp, and a vessel with an inner core for vases, planters, cups and candle holders cast hollow. It suggests a tray for a flat master and an adaptive shell for anything else.
• You set the silicone thickness and the shell wall, and choose how the shell closes: bolts through tabs, spring clamps on a parting flange, a flange with bolts, printed clips, or hooks and rubber bands. Registration keys are sized and placed automatically, or you place them yourself. Shrink compensation has presets for platinum and tin silicone, resin and wax.
• The mold is built on your own computer, in a current Chrome, Edge, Firefox or Safari, whenever it can run the engine. For a slow device or a model that may not fit in its memory, Printara servers can build it instead, for credits. Both use the same engine. A simple master takes seconds, a detailed one up to a few minutes.
• Before you print, you can make the master or the shell transparent, check highlighted air traps, move through a fill preview that shows a level and a volume estimate, and pull the parts apart.
• The download is a ZIP: one STL per printed part in print orientation, the same parts in the assembled pose, your master, a README with print settings, warnings and numbered casting steps, and a report with volumes, air pockets, vents and the hardware to buy (for example the number and size of bolts). There is no 3MF or slicer project in it. You can save the mold set to your Library and open it again later.

The air-trap highlight and the fill preview are checks, not a guarantee: degassing, pour speed and vents still decide whether a cast comes out clean.

Watch the workflow

This Printara Academy lesson builds an adaptive shell around a bust in about five minutes, from a scaled master to a saved mold set. The same lesson is on the Academy lesson page, with narration in seven languages and no account needed to watch. It is a design walkthrough, not a physical casting test.

Browser mold makers at a glance

As of October 2026, read on each vendor's own pages on 11 October 2026. The full comparison has more rows, prices and the source of every line.

Where the others are ahead today: Meshcast has a generator for each craft, a free library of ready-made molds and a large guide library; MoldStudio3D has eleven product designers, ceramic slip-casting molds and presentation tools; and Mold Studio needs no account at all for its box and form-fitting molds. Where the Printara Mold Maker fits: eight mold types in one tool, all free when built on your own computer, a README and a hardware list with every build, and air-trap and fill previews before you print. It suits people who want the whole set in one place or prepare their masters in Printara.

No printer, or no time? Printara Atelier in Prague makes silicone molds and resin or plaster casts by hand. The marketplace also has a Molds category with mold sets made in the Mold Maker, such as this torso bust adaptive shell.

FAQ

Can I pour silicone straight into a 3D printed mold?

You can pour silicone into a printed box or shell: the print is the container and the cured silicone is the mold. Printing a rigid mold and casting resin or wax straight into it only works for simple shapes with no undercuts and some draft.

Is a printed mold box or a printed shell better?

A box is simpler to print and seal and suits small, squat masters. A shell uses far less silicone on tall, large or thin masters: in our worked example the same master needed about three times more silicone in a box than in a shell with the same 10 mm thickness. A shell needs vents at every high point.

How thick should the silicone be around the master?

As a starting point, 10 to 15 mm in a box and 6 to 10 mm in a shell, and not below about 5 mm anywhere. Thin silicone tears after a few casts. Go thicker on deep or very detailed areas.

How much silicone do I need for my STL?

For a box: inside volume of the box minus the master volume. For a shell: master surface area times silicone thickness, plus about 25 percent (more on thin parts). Multiply cm3 by the specific gravity from the datasheet to get grams, and add about 10 percent for the cup and the spout.

Do I need a vacuum chamber?

It helps, especially for fine detail, but it is not required. Without one, brush a thin detail coat onto the master first, then pour the rest slowly in a thin stream into the lowest point of the box, away from the master.

Will platinum silicone cure on a resin 3D print?

Only if the print is fully cured. Wash it, UV post-cure every surface (Smooth-On recommends at least 6 hours, turning the print), seal it with clear acrylic or an epoxy coating, and test a small blob of silicone first.

What release agent do I need for a two-part silicone mold?

Silicone sticks to silicone, so the parting face needs a release made for silicone, a 2:1 mix of isopropyl alcohol and dish soap, or thinned petroleum jelly. Test it on a small scale first.

Can I make a food-safe mold from an STL?

Printed plastic is not food-safe as printed. Use the printed box or shell to cast a mold in a silicone sold explicitly as food-grade, and follow its maker's instructions.

How many casts will a silicone mold last?

It depends mostly on what you cast. Wax is gentle and can give hundreds of casts; harsh resins wear a mold out much faster. A release agent before each cast and demolding promptly both extend its life.

Can a tool build the mold box or shell from my STL?

Yes. Several browser tools build the printed parts from an STL, among them Meshcast, MoldStudio3D, Mold Studio and the Printara Mold Maker. They differ in mold types, free allowances and where the geometry is computed. Our comparison lists each one's features and prices from its own pages, as of October 2026.

Does my master have to be made with AI or in Printara to use the Printara Mold Maker?

No. The Mold Maker works from any STL master: your own sculpt, a scan or a model made in Printara. It is open to every Printara account. A mold built on your own computer is free; a build on Printara's servers costs 10 to 30 credits, depending on the mold type.

Material figures in this guide come from Smooth-On's public technical data and FAQ, checked on 11 October 2026: the Mold Star 30 datasheet and their FAQs on estimating rubber, bubbles, cure inhibition, pouring without a vacuum, rubber that will not cure, SLA prints, silicone to silicone release, mold life and platinum vs tin. Mold Maker details are from the Mold Maker page and the AI Mold Maker guide on the same date. Always follow the datasheet of the silicone you actually use.

About Printara. Printara is an AI-powered creative manufacturing platform for creating 3D models, dioramas, laser-cut designs, molds and printable products. Questions or corrections: hello@printara.co.