How to Cut, Simplify and Rescale an STL Model

An STL file is rarely ready as it comes. The model may be larger than the print bed, the mesh denser than the printer can see, the scale someone else's, and the way it sits on the bed decides whether the print snaps the first time it is bent. Four operations, four decisions, and one rule shared by all of them: each is done to the model, not to the price.

Cutting and splitting the model

A model larger than the build volume is not an obstacle, it is a decision about where to cut. Our FDM machine has a 300 × 320 × 325 mm build volume and the resin printer far less, so a half-metre figure comes out in two or three parts glued after printing.

A plane cut is available in every common program: PrusaSlicer and Bambu Studio have a knife tool inside the slicer, Blender cuts with a Boolean modifier, Meshmixer with Plane Cut. The difference is not the tool, it is where you put the plane.

• Cut where the seam disappears anyway: along an edge of the shape, at the junction of two elements, along the line of a figure's clothing. A flat cut halfway up a smooth cylinder will always show.
• Add pins and sockets before you cut. Two 4 mm pins with 0.2 mm clearance align the parts on their own, with no fiddling over the angle while the glue sets.
• Each part must remain a closed solid after the cut. A slicer normally caps the section itself, but an export from Blender after a plain Boolean can leave an open hole.

Simplifying the mesh

A 3D scan, or a CAD model exported at maximum accuracy, can carry several million triangles and weigh hundreds of megabytes. The printer will not see most of them anyway, because it slices the model into layers 0.06 to 0.28 mm high, and a triangle smaller than a layer has no way of showing up in it.

Simplification, or decimation, cuts the triangle count while keeping the shape. Blender does it with the Decimate modifier in Collapse mode, Meshmixer with Reduce, MeshLab with the Quadric Edge Collapse Decimation filter. A sensible target for printing is one to three hundred thousand triangles; below ten thousand the polygons start to show on arcs.

Rescaling

Rescaling looks like the simplest of the four operations and is the most treacherous, because it changes everything at once. Shrinking a model to 60 percent shrinks the wall thickness, the pin diameter and the width of the lettering by exactly as much.

The factor for a scale change is the inverse ratio of the denominators: from 1:6 to 1:10 you multiply by 6/10, that is 0.6. Volume follows the cube, so a figure reduced to 60 percent of its height uses about 22 percent of the material. That is also why the price drops more steeply than the eye expects.

After every reduction, check the thinnest feature separately. A 0.8 mm wall taken down to 60 percent measures 0.48 mm and stops being printable on FDM, because the nozzle is 0.4 mm and such a wall is a single path with no margin. In jewelry the same applies to prongs: a thin prong will not hold the stone once shrunk.

Print angle on the bed

Orientation decides three things at once, and none of them can be fixed after printing. First, supports: any surface leaning more than roughly 45 degrees from vertical needs support, and support leaves a mark. Second, stair-stepping on curves, which grows more visible the closer a surface comes to horizontal. Third, strength.

Strength is the one most often forgotten. An FDM print is built from layers bonded to each other, so it comes apart along a layer more easily than it breaks across the material. A hook, a handle or a lever placed so that the force tries to separate the layers will fail at a load the same part, turned ninety degrees, survives several times over.

Three typical cases

• A loaded mechanical part: layers across the working force, even at the cost of supports and a longer print.
• A figurine: tilted a dozen or so degrees off vertical, so that no large plane lies parallel to the layers and collects stair-stepping across the face.
• A castable resin pattern: at an angle giving the smallest cross-section in each layer, because that is what sets the peel force off the vat.

Check before sending

Before you send a file for a quote, open it once more and look at four things: whether every part fits the build volume, whether the mesh is closed after the cut, whether the thinnest feature still makes sense at the new scale, and whether the saved file is really the edited one. The last one happens more often than you would think.

Mesh repair, file formats and export from common programs are covered separately in the guide How to prepare an STL file for 3D printing.