Design Tips for 3D Printed Parts

FDM is not a magic box that makes any shape. It builds a part upwards in stacked layers of squashed plastic, and everything about designing for it follows from that one fact. Learn the handful of rules below and your parts get cheaper, faster and better in one go.

Wall thickness

Walls are extruded in beads, so useful thicknesses are multiples of the bead width. With a common 0.4 mm nozzle, that means thinking in steps of roughly 0.4 mm.

• Absolute minimum: about 0.8 mm — two beads. Below that, walls come out fragile or print as a gap.
• Sensible minimum for anything structural: 1.6 to 2 mm.
• Load-bearing brackets: 3 mm and up.
• Avoid awkward in-between numbers. A 1.0 mm wall wastes the extra 0.2 mm — the slicer cannot fit a third bead in, so you get two beads and a gap. 0.8 or 1.2 is better than 1.0.
• Keep it consistent. A part that jumps from 2 mm to 8 mm cools unevenly and warps. Hollow out thick sections rather than leaving a solid lump.

Overhangs and the 45 degree rule

Each layer needs something underneath to sit on. A wall that leans out at 45 degrees from vertical still has roughly half of each bead resting on the layer below — that prints fine. Steeper than that and the plastic has nothing to hold it, so it droops, and the slicer adds supports.

Supports are not a disaster. They are extra material, extra time, extra manual cleanup, and they leave a rougher surface where they touched. All of that lands on your invoice.

Design them out where you can:

• Chamfer instead of overhanging. A 45 degree chamfer under a boss or lip does the same job with no support.
• Turn a horizontal hole into a teardrop shape, so its top is a point rather than a flat ceiling.
• Split a part into two support-free halves and join them.
• Think about which way up it will print — and tell us if you have a preference.

Bridging

A flat roof spanning a gap can print unsupported if the span is short — the plastic stretches across between two anchor points. Short bridges come out well; long ones sag. Under about 20 mm is usually fine. Beyond that, expect droop or supports.

Holes

• Printed holes come out undersized. The plastic pulls in slightly as it cools, and the bead on the inside of a curve squeezes inward. A 5 mm hole often measures nearer 4.8 mm.
• Design clearance in deliberately, or plan to drill it out afterwards. For anything that has to fit accurately, drilling a printed pilot hole is quicker and better than fighting the printer.
• Vertical holes print far better than horizontal ones. A hole with its axis vertical is a clean circle on every layer. Horizontal, and the top half is an overhang.
• Small holes below about 2 mm are unreliable. Print them solid and drill.

There is more on this in our guide to tolerances and fit.

Fillets and chamfers

• Fillet internal corners. Where two arms meet is where a part cracks. A radius spreads the stress. This is the single highest-value change on most brackets.
• Chamfer the bottom edge. A small chamfer around the base reduces the "elephant's foot" bulge from the squashed first layer, and helps the part sit flat afterwards.
• Chamfer anything that has to slide or insert. A lead-in makes assembly forgiving.

Threads and fasteners

Threads printed directly into plastic work, but they strip. If a fastener is ever undone and redone:

• Heat-set inserts — a brass insert melted into a printed hole. The strong, tidy option.
• Captive nuts — design a hexagonal pocket and drop a nut in mid-print or from the side. Cheap and very effective.
• Self-tapping screws into a plain hole — fine for something assembled once.
• Printed threads below about M6 are rarely worth it. Above that, they can be.

Tolerances

Nothing fits at nominal size. Parts that mate need clearance designed in — a few tenths of a millimetre for a sliding fit, less for a press fit. Ask for exact and you will get a part you cannot assemble. Our tolerances guide has the numbers.

Text and fine detail

Raised text prints better than recessed. Keep it above about 1 mm stroke width and 0.5 mm proud. Anything finer turns to mush on FDM — if you need crisp small text, that is a job for a different process.

Think about the layers

Everything above is detail. The big one is this: your part is weakest across its layers. Design and orient so the load runs along them, not across a joint. A hook, a clip, a bracket arm — all of them have an obviously right and an obviously wrong print direction. See how strong printed parts are.

You do not have to get this right

If designing is not your job, it does not have to be. Upload your file and we will check it, tell you plainly what we found, and suggest fixes. Or send photos and measurements and our design help service will do the modelling.

Get an estimate · see the STL printing service.