By Printara · Published 11 October 2026 · 19 min read

How to Prepare a 3D Model for a Split Mold: Seams, Keys and Clearance

Make the master a closed solid at its final size, choose the direction each part of the mold comes off, and put the parting line on the model's widest outline as seen from that direction, so nothing on either side hooks back over the seam. Then add draft if the mold is rigid, registration keys on the flat face around the cavity, clearance on every printed part that slides into another, and a pour channel and vents that let the air out. Check every seam in a section view, and print one seam as a test before you print the whole set.

A split mold is any mold that opens along seams: a rigid two-part mold printed in plastic and cast into directly, a two-part silicone mold poured in two halves, or a printed shell in wedges that holds a silicone skin. The preparation is the same for all three. What changes is what the seams must release: in a rigid mold every surface of the cast, in silicone everything except the small undercuts the rubber stretches over, in a shell only the smooth outside of the silicone.

This guide covers the model and the printed parts. Silicone thickness, how much silicone to buy and the pour are in How to make a silicone mold from an STL.

Disclosure: Printara makes the Mold Maker described in the last section, and the screens in this guide come from its Academy lesson. Everything before that section works with any CAD program or mold tool.

Step 1: Make the master a closed solid at its final size

Close the mesh. The cavity is made by subtracting the master from a block, and that only works on a closed surface: no holes, no flipped faces, no loose shells inside, no faces cutting through each other. Your slicer reports most of these when it imports the STL, and Blender's 3D-Print Toolbox finds non-manifold edges and intersections. Scans and sculpts are the usual problem.

Set the final size. Scale the master in millimetres to the size you will cast and check the dimensions after import: a file exported in inches arrives 25.4 times too small. If your casting material shrinks, scale by the figure on its datasheet.

Decide on thin parts now. A sword, a staff or an arm held away from the body is the hardest thing to mold in two halves: the seam runs along its whole length, and a thin part is the first to snap as the cast comes out or to tear the silicone around it. Cut such parts off and cast them separately, with a pin hole to glue them back. The STL Splitter or your slicer's cut tool does the cut.

Choose the hidden face. Decide which face of the cast nobody sees: the underside of a base, the back of a plaque. The pour channel usually enters there, because its stub has to be cut off and sanded.

Finish the surface. Every mold copies layer lines: sand and seal an FDM master, or print it in resin and cure it fully (steps in the silicone mold guide).

Step 2: Choose the opening direction and find the undercuts

Each part of a split mold comes off in a straight line. Choose that line first: the parting line follows from it.

The sight-line test. Switch your viewer to an orthographic view and look straight down the opening direction, then straight up it. Every surface you can see from above can be formed by the upper half, and every surface you can see from below by the lower half. A surface you cannot see from either side is an undercut: it hooks behind something and locks a rigid mold. In CAD, a draft analysis does the same job by colouring faces by their angle to the opening direction (Fusion has one under Inspect).

What to do with an undercut:

• Turn the model. A different opening direction often turns an undercut into an ordinary surface.
• Step the parting line around the feature, so the half that can see it forms it.
• Let silicone take it. Rubber stretches over small undercuts such as the curl of a lip or the gap under a chin. A rigid printed mold cannot.
• Add a part: a third mold piece, or a loose insert that comes out with the cast.
• Change the master: cast a hooked weapon separately, or fill a gap that does not need to be open, such as the space between an arm and the body.

Step 3: Put the parting line on the widest outline

Follow the outline. The parting line belongs on the outline you see when you look down the opening direction: the widest part of the model, where the surface stops facing one half and starts facing the other. Anywhere else, one side hooks back over the seam. On a bust or a figure that opens front to back, that outline runs over the top of the head, down the sides of the head and neck and along the shoulders, so the face sits wholly in the front half.

Hide it. Every seam leaves a line, and some flash (thin fins of cast material) wherever the halves do not close perfectly. Run the seam along edges, folds and the back or underside, never across a face or lettering. A seam on a sharp edge cleans up with a blade; one across a smooth curve needs sanding and shows.

Keep the parting face flat where you can. A flat face can be printed face down on the bed, seals well and takes keys easily. Step or curve it only where the outline forces you to, and give the side walls of each step at least 3 degrees of slope, the minimum Protolabs sets for mold faces that slide past each other, so the halves slide together instead of rubbing.

Carry it outward. Past the cavity, the parting face continues as a flat flange, wide enough to hold the keys with a few millimetres of face on each side of them. That flange carries the keys, the bolts or clips, and the seal.

In a printed shell the wedge seams only have to release the outside of the silicone, which is a smoothed copy of the master. Fewer wedges mean fewer seams to seal; more wedges let each one come off a deeply curved silicone surface. The Printara adaptive shell uses two to four.

Step 4: Add draft where the mold is rigid

Draft is a slight taper on the walls that run along the opening direction, so they move away from the mold as soon as it opens.

Rigid mold: required. A straight wall in a rigid mold drags along its whole length, and a wall that leans the wrong way locks. Give every wall that slides against the mold at least 2 degrees, up to 5 where the shape allows: Formlabs recommends 2 to 5 degrees for printed injection molds, and the Printara printed two-part mold uses 2 degrees by default. Printed layer lines grip like a texture, and molders add draft for texture (Protolabs asks for 3 degrees on a light one and 5 on a heavy one), so FDM molds and deep walls want the top of the range.

Silicone: not needed. Silicone flexes off the cast. Draft still makes deep, narrow pockets easier to demold.

Adding it. On a CAD model, use the draft tool on the faces that run along the opening direction. A sculpted mesh rarely takes clean draft, which is one more reason to mold sculpts in silicone.

Step 5: Add registration keys

Keys put the halves back in the same place every time. Without them the halves shift sideways and every cast gets a step along the seam.

• Shape: cones or half spheres. They centre themselves as the halves close and let go easily. Straight pins need more clearance and bind if the print is slightly off.
• Number and spacing: at least three, spread as far apart as the flange allows, and placed unevenly (or one of them larger) so the halves close only one way.
• Size and depth: 5 to 10 mm across on a palm-sized mold. On printed parts, make each socket about 0.5 mm deeper than its key is tall. A key that bottoms out holds the faces apart, and the gap fills with flash.
• Position: on the flat flange, clear of the cavity edge and of the outer edge. A key that breaks into the cavity marks every cast; one too close to the outside snaps off.
• In a shell mold there are two sets: bumps or pockets molded into the silicone that match the shell or the base plate, so the skin goes back in the same place for every cast, and the flanges, tabs or seat that locate the printed parts on each other.
• Mark the parts. Emboss a letter or an arrow on the outside of each piece, so you can tell them apart and see which way up they go.

Step 6: Set the clearances between printed parts

Where two printed parts touch, the contact is one of two kinds. Sealing faces, such as the parting faces or a wedge sitting on the base plate, must meet flat with no gap. Locating parts, such as keys in sockets, pins in holes, a plug in its port and bolts in their holes, need clearance or they will not go together.

How much. On an FDM printer with a 0.4 mm nozzle, start at 0.2 mm per side: a 6 mm key gets a 6.4 mm socket. The right value depends on the printer: the fabrication lab guide of the Creative Computing Institute (University of the Arts London) suggests 0.1 to 0.15 mm per side for alignment pegs on its Bambu printers, 0.2 mm on its Ultimakers, and 0.3 mm on the Ultimakers for parts you take apart by hand. On a resin printer, test 0.1, 0.15 and 0.2 mm per side on a small pair; exposure and post-cure change the fit.

Bolt holes. Model them at the bolt's normal clearance size, 4.5 mm for M4 (metric clearance hole chart). Small printed holes come out undersize, so print one first, or drill the holes out to size after printing.

The first layer. The first layer squashes wider than the model. Where a mating face meets the bed, as on every flange of a wedge printed standing, that lip props the seam open at the bottom. Check that your slicer's elephant foot compensation is on (Prusa suggests about 0.2 mm for a 0.4 mm nozzle) or model a small chamfer along that edge.

The slicer seam. Each layer starts and ends at a seam that leaves a small bump. On a flange printed standing up, keep that seam off the mating face with your slicer's seam position setting or seam painting.

Warping. A long, thin flange can curl at the ends as it cools, and the seam then gapes. Use a brim on long parts, and lay a straight edge across each flange after printing.

Step 7: Place the pour channel and the vents

Liquid poured in at the top fills the cavity from the bottom up, and the rising level pushes the air ahead of it. Every place where air can collect needs a way out, higher than the way in.

Pour channel. Bring it in at a thick, hidden part of the cast, usually the base, and widen its top into a funnel so you pour into a small reservoir, not a pinhole. The stub it leaves is cut off and sanded, so keep it away from detail.

Vents. Stand the mold the way you will pour it and find every local high point of the cavity, where the rising level closes over a pocket of air. On a figure poured upside down through its base, those are the parts that point down when it stands: the chin, the fingertips of a hanging hand, the point of a lowered sword. Give each one a vent running upward and out. End every vent above the top of the pour spout, or the material runs out of the vent before the mold is full; the University of Edinburgh's mould-making guide gives the same rule.

Put the channels on the seam. In a two-part mold, model the pour channel and the vents as rods attached to the master, centred on the parting plane and running out to the edge of the mold. Each half then gets a half-round groove of each channel, and the cast comes out with its pour and vent stubs attached. Vents can also be cut into cured silicone afterwards: Smooth-On's two-part mold tutorial punches them with a sharpened brass tube where the cast surface does not matter, and props the mold at 45 degrees while the resin goes in, so the air can get out.

For the silicone pour in a shell the same rules apply one step earlier, but the air now collects under every overhang of the master as the silicone rises: under the chin, a hat brim or an outstretched arm. Pour through a spout at the top, give each of those spots a vent (3 to 5 mm works with most pourable silicones), and keep pouring until silicone shows in every vent. Where a vent would cut through detail, lay a wire from the pocket to the seam and pull it out after the cure. The build report of the example mold on the AI Mold Maker page suggests exactly that, or tilting the mold, for a small air pocket under the master. Details in the silicone mold guide.

How to review seams, clearance and assembly

Review a printable mold in three passes before you print it: look at every seam in a section view, list every place where printed parts touch and give each the right gap, and walk through assembly and demolding in the order you will do them at the bench. Then confirm the result on a printed test section.

Seams, in section. Cut through the closed mold at several heights. In CAD, use a section view. In Blender, import the parts in their assembled positions, press Alt+B and drag a box: the view is clipped to it, which cuts the parts open. At each seam, check that:

• the parting faces meet with no gap and no step;
• the cavity outline continues smoothly across the seam;
• vents and the pour channel cross the seam cleanly or sit wholly in one part;
• there is flat face around the cavity for the seal, and the seam crosses no detail you care about.

Clearance, interface by interface. List every place where two parts meet:

Also look for the opposite fault: two printed parts that overlap in the assembled model. An overlap means something will not close.

Assembly and demolding, in order. Walk through it as you will at the bench: the master on its base, then each part in turn. Does each part go on along a straight line without hitting the one before? Can your hand and a tool still reach every bolt or clip once its neighbours are on? Then reverse it: which part comes off first, and can the master or the cast come out without prying against detail? Count the hardware while you are at it.

What to check before printing a mold shell

Before you print a mold shell, check the size of the master inside it, the silicone gap and the wall thickness, that every seam closes flat on keys that cannot bottom out, that the spout and vents reach the cavity, and that each part can print with its mating faces clean. Then print one seam as a test section before the whole set.

Print a test section

A test section finds fit problems for a fraction of the filament and time of the full set. With your slicer's cut tool (see how in PrusaSlicer or Bambu Studio), cut a band 15 to 20 mm tall out of two neighbouring parts, at a height that includes a key and a bolt hole or clip. Print the bands with the settings you will use for the full set, then check that:

• the keys go in by hand and the faces close fully;
• no light shows through the closed seam when you hold it up to a lamp;
• the bolts pass through both holes and tighten without bending the flange;
• a straight edge lies flat across each mating face.

If something fails, change the clearance or the slicer settings and print the band again, not the whole set.

What are the limits of a digital mold preview?

A digital mold preview checks geometry: whether the parts fit together in the file, where air could be trapped as the cavity fills, and roughly how much volume it holds. It cannot see your material, your printer or your bench: how the silicone flows and cures, where bubbles come from, how accurately the parts print, how much the walls flex, or whether the cast comes out. Use it to catch design mistakes before printing, then let a test section and a first cast decide.

Printara's own pages say the same: the Mold Maker calls its air-trap highlight and fill preview a check, not a guarantee, and the AI Mold Maker page says to cast once and check the result before you print or sell a whole set. The Academy lesson the screens in this guide come from is a design walkthrough, not a casting test.

Doing it in the Printara Mold Maker

Everything above can be drawn by hand in CAD. The Printara Mold Maker builds the printed parts from an STL master instead, flanges, keys and vents included. For a split mold:

• Mold types that open along seams: an adaptive shell (a base plate and 2 to 4 wedges around a silicone skin), a two-part silicone box (a printed parting bed, frame and sprue for two pours with release agent between them), a printed two-part mold (rigid, no silicone, only for masters with almost no undercuts, 2 degrees of draft by default) and a one-piece cut mold (one silicone block with molded guide grooves that show where to cut it open).
• Your master: a closed mesh goes in untouched. One with holes in its surface is rebuilt on a fine grid so it can be molded, and the report says so; the original stays in the download.
• Keys and vents: registration keys are sized and placed automatically, or you place them yourself; vent holes can be added by tapping the model.
• Closures: bolts through tabs, spring clamps on a parting flange, a flange with bolts, printed clips, or hooks and rubber bands, with a list of the hardware to buy.
• Shrink compensation with presets for platinum and tin silicone, resin and wax.
• Before you print: a transparent master or shell, highlighted air traps, a fill preview with a level and a volume estimate, and an exploded view.
• The download is a ZIP: one STL per part in print orientation, the same parts in the assembled pose to check the fit, your master, a README with print settings, warnings and numbered casting steps, and a report with volumes, air pockets, vents and hardware. There is no 3MF or slicer project in it.

Limits. It takes STL files only, it needs an account before the editor opens, and its previews have the limits listed above. Other browser tools are ahead in places, such as a free library of ready-made molds or downloads without an account: see the browser mold makers comparison. The full workflow is in the Academy lesson Make a printable mold shell for silicone casting.

FAQ

Do silicone molds need draft angles?

Usually not: silicone flexes off the cast and stretches over small undercuts. Draft still makes deep, narrow pockets easier to demold. Rigid printed molds do need it: at least 2 degrees on every wall that slides against the mold, up to 5 where the shape allows, and the top of that range on FDM prints.

How much clearance do 3D printed mold parts need?

Start at 0.2 mm per side on keys, pins and plugs printed on an FDM printer with a 0.4 mm nozzle, and adjust between about 0.1 and 0.3 mm after a test print. Sealing faces get no clearance: they must meet flat. Make each socket about 0.5 mm deeper than its key so the faces close first.

How many registration keys does a two-part mold need?

At least three, spread as far apart as the flange allows and placed unevenly, or with one larger, so the halves close only one way. Cones or half spheres centre themselves. Keep them clear of the cavity edge.

Where should the parting line go on a figure?

On the widest outline seen from the opening direction. For a figure that opens front to back, that runs over the top of the head, down the sides and along the shoulders, so the face sits in one half. Keep it on edges and hidden sides, away from the face and lettering.

Can a digital mold preview guarantee a bubble-free cast?

No. An air-trap preview shows where air could be trapped by the shape of the cavity. Bubbles from mixing, from the pour or from a porous master are invisible to it. Degassing, pour speed, vents and a first test cast decide the result.

Can I cast resin straight into a 3D printed split mold?

Only for simple shapes with no undercuts and at least 2 degrees of draft. Every cast copies the cavity surface, layer lines included, so sand and seal an FDM cavity or print the mold in resin, and use a release agent every time. Keep heat in mind: PLA deforms above about 60 °C, and casting resin warms up as it cures, more in thick sections. For figures and textured masters, print a shell or a box and cast a silicone mold instead.

Sources checked on 11 October 2026: Formlabs, injection molding with 3D printed molds (2 to 5 degrees of draft); Protolabs, draft guidelines (more draft for texture and for faces that slide past each other); Prusa, elephant foot compensation and PLA (deforms above 60 °C); the Creative Computing Institute, clearance and tolerance; the metric clearance hole chart; Smooth-On, preparing a two-part mold for casting; University of Edinburgh uCreate, making silicone moulds. Mold Maker details are from the Mold Maker page, the AI Mold Maker guide and the Academy lesson on the same date. Always follow the datasheets of the materials 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.