Blender add-on · user guide

Sformo

Work out how many parts a mold needs, where it splits, and build the blocks to print.

Point Sformo at a mesh and it answers three questions: can this be cast at all, how many mold pieces will it take, and where does the mold split. Then it builds the actual mold blocks — hollowed, split, keyed, vented — as watertight solids you can export and print.

A fluted drum in blue surrounded by four exploded mold staves and two end plates. — **A stave mold built by Sformo.** The blue drum is the casting; around it are the four staves and the two end plates, slid apart for inspection. Each stave carries the fluted negative on its inner face.

Section oneInstalling

In Blender, open Edit ▸ Preferences ▸ Add-ons, choose Install from Disk, and pick sformo.zip. Tick the box next to Sformo to enable it.

The panel lives in the 3D viewport sidebar. Press N in the viewport and choose the Sformo tab. Requires Blender 4.0 or newer; it has no other dependencies.

Section twoQuick start

Select your model. One mesh object, ideally watertight. Select the model itself — not a mold piece you made earlier.
Press Analyze. The model is coated in a colour heatmap and a results box appears telling you the mold type and the recommended number of parts.
Read the heatmap. Red means trouble — see below. Green means the surface pulls free.
Press Build Mold Blocks. The finished pieces land in a new collection called <model>_mold.

Important Everything below the Analyze button stays greyed out until you have analysed the selected object. If the Mold blocks section looks disabled, that is why — press Analyze first. Analyses are held in memory, so they are lost when Blender restarts or the add-on reloads.

Section threeReading the heatmap

After Analyze, every face is tinted by how it behaves when the mold is pulled apart. The same key is shown in the panel, under Colour Key.

Green — releases toward +pull. This face lifts out with the first half. Nothing to do.

Blue — releases toward −pull. Same, but it belongs to the opposite half.

White — parting band. The silhouette, where the two halves meet. This is where the seam will run.

Yellow — low draft. Releases, but drags on the way out. Tolerable; a steeper wall would be kinder.

Red — undercut. Trapped. The mold physically cannot open here without a loose piece.

Grey — end cap. Radial molds only: a surface facing along the axis, needing an end plate.

An organic head form split into a green half and a blue half with a white band and a pink loop along the division. — **A two-part split, viewed across the pull direction.** Green on one side, blue on the other, the white parting band between them, and the traced parting curve in pink riding exactly along the division. Red patches are genuine undercuts in the deep folds.

Reading tip If the model looks almost entirely one colour, you are probably looking straight down the pull axis and seeing only one half. Orbit ninety degrees — a two-part split only reads properly when viewed across the pull.

What to do about red

Red is not automatically a failure. The panel tells you how much of it matters: Undercut regions: 4 (+55 negligible) means four undercuts are big enough to need a loose mold piece, and fifty-five are specks — nostril creases, hair crevices, tool marks — that no mold-maker would treat as separate parts. Only the significant ones are counted into the part total. Press Select Undercuts to jump into Edit Mode with those faces selected so you can see exactly where they are.

A carved head on a pillar, mostly blue, with a broad red shelf where the head overhangs the narrower shaft. — **A real undercut.** The broad red shelf is where the head overhangs the narrower pillar — the mold cannot open past it. Smaller red slivers along the hair are the sort of surface detail that gets filtered out as negligible.

Section fourThe three mold types

Sformo works down a ladder and reports the simplest mold that actually works. With Strategy set to Auto it picks for you; you can also force any rung.

Type	What it is	Suits
One-part press	An open-faced mold, pressed and lifted straight off	Reliefs, tiles, anything essentially flat-backed
Two-part	Two blocks meeting on a flat parting plane	Most free-standing forms
Radial / staved	A ring of staves around an axis, plus end plates	Columns, drums, vases — anything turned

A fluted drum is the clearest case for the difference: pulled sideways as a two-part mold, the flutes lock it. Split into staves around its axis instead and it releases with no undercuts at all — which is why Auto chooses the stave mold for it.

A fluted drum coated green with grey bands, with pink lines running up each flute crest. — **A radial analysis.** Green means every side face releases outward; grey bands are the end caps that need plates. The pink lines are the traced parting curves, one running up each arris ridge — the crest between two flutes.

Section fiveAnalysis settings

Setting	Default	What it does
Strategy	Auto	Which mold type to test. Auto walks the ladder and reports the simplest that works.
Axis	Z	The pull direction, or the axis of revolution for radial molds. The camera button beside it sets the axis from your current view.
Min Draft	2.00	Degrees. Faces shallower than this are flagged yellow as dragging.
Staves	0	Radial only. 0 lets Sformo choose a count that divides the flutes evenly.
Find Best Axis	off	Searches 27 directions for the one with least undercut. Slow on heavy meshes — see below.
GPU Overlay	on	Draws the heatmap as a live overlay that writes nothing into your mesh. Turn it off to bake a colour attribute instead.

Performance Analysis is fast — a 7,000-face model takes a fraction of a second. Find Best Axis multiplies that by twenty-seven. On a 79,000-face mesh it goes from about four seconds to thirteen. Leave it off unless you genuinely do not know which way the mold should pull.

Section sixParting lines

Two buttons, for two different purposes.

Suggest Parting Lines

Marks the parting line on the model itself, as UV seams plus a vertex group called SFORMO_parting. Use this when you want to see the line on the mesh, or to drive other modelling work from it.

Trace Parting Curves

Builds a separate, editable curve object named <model>_parting. On a two-part split you get closed loops following the silhouette; on a radial form you get one curve running up each arris ridge. Use these as guides, or as geometry to build from.

Worth knowing The mold builder does not use these curves. A traced curve can only follow existing mesh edges, so a flat n-gon face — a plain cylinder's lid, for instance — forces it to detour around the rim rather than cut across. The mold split is computed independently from the true silhouette, so it is correct even when the drawn curve wanders.

Section sevenBuilding the mold

Build Mold Blocks produces watertight solids in a new collection named <model>_mold. What you get depends on the mold type:

Two-part — _moldA and _moldB: two blocks, hollowed to the model, split on the silhouette plane, with registration pegs on one face and matching sockets on the other, plus a pour spout and vents.
Radial — _stave00… and _plate_top / _plate_bot: the staves around the form and the two end plates, with keys on the stave seams.

Two mold blocks opened apart with a blue cylinder between them, registration pegs at the corners. — **A two-part mold.** The cavity is hollowed into both halves, with registration pegs at the corners of the parting face and the pour channel grooved down the middle. The blue cylinder is the casting it produces.

Setting	Default	What it does
Wall %	20	Thickness of mold material around the model, as a percentage of its size.
Spout %	7	Radius of the pour channel.
Reg. Keys	4	How many registration pegs to place on the parting face.
Key %	5	Peg radius.
Clearance %	0.4	Fit gap cut into the sockets so printed halves actually go together. Raise it if they bind.
Draft°	1.5	Taper on the mold's outer walls and on the pour and vent channels, so cast sprues pull free.
Vents	2	Air channels from the high points of the cavity. Without them, trapped air stops the pour filling.
Vent %	2	Vent channel radius.
Pour Spout	on	Whether to cut the pour channel at all.
Seam Keys	on	Radial only. Keys on the stave seams so staves align to each other.
Explode %	0	Slides the finished pieces apart so you can see inside. Live — drag it any time. 0 is the assembled, export-ready position.

All the size settings are percentages of the model, not absolute distances, so the same values work whether you are casting a thumbnail or a column drum.

Section eightExporting to print

Set Explode % back to 0. The pieces return to their exact assembled positions.
Select the mold collection in the Outliner — <model>_mold.
File ▸ Export ▸ STL (or 3MF), with Selection Only ticked. Export each piece separately if your slicer prefers one solid per file.
Check the scale in your slicer. Sformo works in Blender units and does not assume millimetres.

Every piece is built watertight, so slicers should accept them without repair. Print the parting face down where you can — it is the flattest surface and the one whose accuracy matters most.

Section nineTroubleshooting

The panel stops after Analyze — no results, no Mold blocks section: The selected object has no analysis. Select the source model (not a mold piece or a curve) and press Analyze. Analyses live in memory and are cleared when Blender restarts.
“Mesh changed since Analyze — run Analyze again”: You edited the mesh, so the stored per-face results no longer line up with it. Press Analyze again. This guard is deliberate; without it the tools would fail on stale data.
“Silhouette is not planar — flat split will leave undercuts”: This model needs a curved or stepped parting surface, which Sformo cannot build yet. It will still produce blocks, but they will not open cleanly. Organic forms — faces, figures — usually land here.
Analysis is slow: Turn off Find Best Axis and set the Axis by hand.
A radial analysis picks a strange axis: The camera button beside Axis sets it from your view and leaves Axis on Custom. For a column or drum, set it back to Z — or whichever axis the form is actually turned about.
The numbers look wrong on an imported mesh: Sformo assumes a clean, watertight surface. Open edges, duplicate interior faces, or inverted normals all produce misleading results. Check with Mesh ▸ Clean Up first. Joined objects are a common culprit — joining leaves internal faces that read as undercuts.
The mold has modifiers baked in — or doesn't: Analysis runs on the base mesh. If the object has live modifiers you will get a warning; apply them first if the shape they produce is the shape you mean to cast.

Section tenKnown limits

Stated plainly, so nothing surprises you at the printer:

Parting surfaces are flat only. Forms whose silhouette does not lie in a plane get a warning rather than a working mold.
Loose pieces are reported, not built. Sformo counts the side-actions an undercut form needs; it does not model them.
Print tolerance is untested. Clearance % is a starting guess — cast one test piece and adjust before committing to a large mold.
No clamp-band grooves on stave molds yet.