Designing a part so it prints well

Every rule below comes out of a number this shop's machines actually run: the 0.42 mm extrusion width, the 2-wall default, the 0.12 mm to 0.28 mm layer range, and the build volumes on the floor. None of them is a figure copied from a filament vendor's data sheet, and none of them is a tolerance, because this shop has not measured one on its own parts.

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Filament parts

Draw walls in multiples of 0.42 mm.

One pass of the nozzle lays a bead 0.42 mm wide. A wall drawn at 0.84 mm is exactly two passes and comes out solid. A wall drawn at 0.63 mm is one and a half, so the slicer either overlaps the beads and thickens the wall or leaves a gap down the middle of it.

From the 0.42 mm extrusion width this fleet runs.

The default shell is 0.8 mm, which is 2 passes.

That is right for most parts. A part that carries load wants four or five passes instead, because the load-bearing shell is the walls and not the infill, and a fourth wall adds a third to it. The wall setting is on the quote form, so this is a choice rather than something to design around.

From the 2-wall default in the quote form.

Overhangs past about 45 degrees from vertical need support, and support leaves a mark.

Molten plastic laid on air sags. Below 45 degrees each layer still rests mostly on the one under it, so it holds. Past that it needs something under it, and the surface where support was removed is rougher than the rest of the part. Chamfering an overhang to 45 degrees is usually cheaper than supporting it.

From how a fused deposition layer bonds to the one below it.

Orientation decides strength, and the weak axis is across the layers.

A printed part is strongest along a layer and weakest between two of them. A hook printed flat and a hook printed upright are the same file and different parts. Saying which direction the load comes from, in the notes on the quote, is worth more than any material change.

From layer adhesion being weaker than the bulk polymer.

Use heat-set inserts rather than printed threads on anything that will be opened twice.

A printed thread in a soft filament strips. A brass insert pressed in with a soldering iron takes seconds and gives a machine thread. Design the hole for the insert, not for the screw.

From how printed threads behave in filament, on the shop floor.

Anything over 500 x 500 x 500 mm has to be split and joined.

That is the largest machine on the floor. A part past it is not refused, it is printed in pieces and bonded, but a split designed into the model at a joint that was going to exist anyway is always better than a split chosen by whoever prepares the file.

From the large-format build volume in the printer list.

Resin parts

Every enclosed cavity needs a drain hole.

The part comes out of a vat of liquid. A sealed hollow fills with uncured resin that cannot be washed out, so the part stays heavy, keeps curing, and eventually cracks. Two holes are better than one, because one hole traps air.

From the part being lifted out of a liquid vat.

Cured photopolymer is stiff and brittle, so design it as a detail part, not a hinge.

Resin resolves detail no nozzle can lay down, and that is what it is for. A snap fit or a living hinge in cured resin snaps. If the part has to flex or take repeated load, it belongs on the filament side.

From the mechanical behaviour of cured photopolymer.

Anything over 300 x 250 x 250 mm is an FDM part.

That is the resin build volume. There is no large-format resin machine here, so a part past this size is either split or printed in filament, and for most parts filament is the right answer anyway.

From the resin build volume in the printer list.

Supports touch the part, so choose which face they touch.

A resin part is grown hanging off supports and every contact point leaves a small witness mark that is sanded off. Saying which face is the show face means the marks go somewhere that does not matter.

From how a part is grown off the build plate.

What this page does not tell you

There is no tolerance table here and no strength figure, and that is deliberate. A printed part does not have the properties of the spool it came from, so the only honest source for those numbers is a measured part rather than a datasheet. What is missing, specifically:

• Dimensional tolerance actually held, per process, as a plus/minus figure or a percentage of nominal.
• Minimum feature size that survives on a real part, per process.
• Minimum wall thickness that comes out solid rather than gappy, per material.
• Tensile strength and heat deflection on printed test coupons, per material, per print orientation.

Where a part needs one of them, say so on the quote and it is measured on a real printed sample rather than quoted from a table.

The machines these rules come from

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Terms used above are defined in the glossary. Which process a part belongs on is covered under processes.