Designing for 3D Printing

Design principles for printed parts

A good printed design works with the process instead of fighting it. The rules come from how the part is built: layer by layer from the bottom, with lines of a fixed width, bonded weakly between layers, and shrinking as they cool. 1. Orient the part so the load follows the layers and the overhangs are few. 2. Keep walls and features at multiples of the line width. 3. Avoid steep overhangs with chamfers and teardrops. 4. Round the corners where stress concentrates. 5. Leave clearance for fits, and expect to finish critical holes. 6. Design for the screws, inserts and bearings you will actually use. A design that prints well the first time is cheaper than one that needs three iterations.

Orientation: the first decision

• Layer lines are the weak planes. The bond between layers is weaker than the lines themselves, so a part is strongest along the layers (X and Y) and weakest in tension across them (Z). A hook printed standing up snaps at a layer line; printed lying down, it holds.
• Put the largest flat face on the bed for adhesion, accuracy and the best surface; the bottom will be glossy or textured according to the bed surface.
• Think about which surfaces matter. Holes with axes in Z are the most circular; holes lying in the XY plane are slightly flattened on the top. Mating surfaces and cosmetic faces should be vertical or on the bed, not on a slope.
• Minimize supports and warping. A long, thin flat part warps at its ends; a tall thin part wobbles: sometimes a diagonal layout, or splitting it, is best.
• Orient for fatigue, too: cyclic loads open the layer bonds first (see Fatigue and Stress Concentration).

Overhangs, bridges and supports

• Overhangs. Surfaces up to about 45° from vertical usually print without support, and cooling and a slower speed can stretch this to 60° for short spans. Beyond that they sag unless supported. Use chamfers rather than horizontal ceilings.
• Bridges. A flat span of a few centimeters can print unsupported with good cooling and a slower bridging speed; longer ones droop. Break a long span with a rib, or print a thin sacrificial layer.
• Teardrop holes. A horizontal round hole has a flat, sagging top; replace the top by a point (a teardrop of 45° sides) or a flat bridge so that it prints without support and the hole stays round.
• Hole in the side wall: a diamond-shaped or teardrop opening needs no support; a square hole has a bridge at its top that is easy to print.
• Chamfer the base. A 0.5 to 1 mm chamfer at the bottom edge avoids “elephant foot” where the first layers squash outward.
• Supports leave marks and cost material; design to need few of them, and set the support settings in the slicer (see Slicer Settings and Print Quality).

Walls, ribs and minimum features

The printer builds walls from whole lines, so a wall thickness that is a multiple of the line width prints cleanly, and one that is not leaves a gap the slicer must fill or skip. With a 0.45 mm line:

Wall thickness in the model	Whole lines	Thickness actually printed	Result
0.8 mm	2	0.90 mm	fits well
1.0 mm	2	0.90 mm	fits well
1.2 mm	3	1.35 mm	slicer thins or fills the gap: change the model to 1.35 mm
1.5 mm	3	1.35 mm	slicer thins or fills the gap: change the model to 1.35 mm
2.0 mm	4	1.80 mm	slicer thins or fills the gap: change the model to 1.80 mm

• Minimum wall: at least two line widths (about 0.8 to 1.0 mm); for strength 1.5 to 3 mm. Walls thinner than one line are skipped.
• Ribs and gussets stiffen thin walls: height up to 3 times the wall thickness, thickness about 60 percent of the wall (to avoid sink), and a fillet at the base.
• Pins and posts: at least 2 to 3 mm diameter; thinner ones are fragile and heat up (see the cooling example in Slicer Settings and Print Quality).
• Text and fine details: raised or engraved features at least 0.8 mm wide and 0.4 to 0.6 mm deep; embossed text reads better than engraved in coarse layers.
• Fillets and chamfers: round tension corners with a radius of at least 1 mm (a sharp inside corner concentrates stress and starts a crack), and chamfer rather than fillet where a surface would overhang.
• Avoid large flat areas on a base that is likely to warp: split them with slots or ribs, or use a brim.

Holes, shafts and clearances

A printed hole is a stack of slightly rounded lines pulled inward by the nozzle path and the plastic’s shrinkage, so holes come out 0.1 to 0.3 mm undersize and slightly out of round, and shafts come out slightly large. The fix is to compensate in the model, or to drill or ream the hole after printing.

Threads, screws and inserts

Method	How	Strength and use
Self-tapping screw into a pilot hole	Pilot hole about 80 % of the screw diameter: 2.4 mm for an M3 screw	Fine for a few assemblies; the thread wears in plastic. Keep 3 to 4 times the screw diameter in depth and a wall of at least one diameter around the hole
Heat-set brass insert	Press the insert in with a soldering iron: hole slightly smaller than the insert (about 4.0 mm for an M3 insert; follow the insert maker’s figure)	Strong, reusable threads; the best option for repeated assembly and for loads
Nut trap	A hexagonal pocket for a nut, printed into the part (add 0.2 to 0.3 mm of clearance), with a pause to insert the nut	Strong and cheap; a bolt can be tightened many times
Printed thread	Model an external or internal thread with extra clearance (0.2 to 0.4 mm per flank)	Large pitches only (for example a lid or a jar); small threads are weak and fragile
Clamping and sandwich	A bolt passes through clearance holes and clamps parts between plates or with a nut	Best strength; use washers to spread the load on plastic

• Tighten gently. Plastic crushes and creeps: a bolt in a printed part should be tightened to a light torque, and a washer spreads the load. Use the Bolt Torque and Thread Engagement tool with a low friction-limited torque and see Bolted and Welded Joints for preload.
• Direction of load. A screw loaded along the layers (axis in the XY plane, in the thick wall) holds better than one pulled across the layers.
• Wall around holes: at least 3 or 4 walls, so the hole sits in solid shell, not sparse infill.
• Bearings and press-fit parts: add a lead-in chamfer, and keep the layer direction so the loaded side is along the layers.

Snap fits and living hinges

A cantilever snap fit is a flexible arm with a hook that deflects during assembly. The strain at the root of the arm sets whether it survives: for a rectangular arm of thickness t and length L deflected by y, ε = 1.5 t y ÷ L² (the maximum strain at the surface at the root). It must stay below the allowable strain of the material, which for printed plastic across the layers is lower than the datasheet value.

Living hinges (thin, flexible strips) work in polypropylene and, less well, nylon and TPU; a printed PLA hinge fatigues quickly. Use a hinge of 0.4 to 0.8 mm and print it with the layers parallel to the hinge’s length, or make a separate hinge with a pin.

Print-in-place and moving parts

• Clearance between moving parts: 0.3 to 0.5 mm per side for print-in-place hinges and gears; less (0.2 mm) after loosening them by hand. Too little and the parts fuse.
• Orientation: let the gaps be vertical gaps between vertical walls, not horizontal gaps that bridge.
• Gears: print them flat for the best tooth form; use larger modules (1.0 or more) and allow extra backlash (about 0.2 to 0.3 mm) in the gear design (see Gears and Power Transmission).
• Bearings and bushings: use real bearings for precision; printed bushings in nylon or PETG work for light loads with lubrication.
• Support removal inside gaps is difficult: design so the moving parts need no support.

Splitting parts and joining them

• Split large parts that exceed the build volume, need many supports or would warp, and print each half flat.
• Alignment features: pins and sockets (with 0.2 mm clearance), tongue and groove, dovetails or a stepped joint keep halves aligned for gluing.
• Glue: cyanoacrylate (superglue) for PLA and PETG, solvent welding with acetone for ABS and ASA, epoxy for nylon and mixed materials.
• Mechanical joints: screws with inserts or nut traps allow disassembly.
• Hide the joint in a feature line or a step, and plan the split so that the glued surface is large.

Design checklist

Check	Rule
Orientation	Loads along the layers; large flat face on the bed; few supports
Overhangs	Up to 45° from vertical without support; chamfers and teardrops
Walls	Multiples of the line width; at least 0.8 to 1.0 mm (functional parts 1.5 to 3 mm)
Corners	Fillet radius at least 1 mm on tension corners; chamfer the base edge
Holes	Model 0.1 to 0.3 mm larger; drill or ream critical holes; test a ladder of sizes
Fasteners	Inserts or nut traps for repeated assembly; walls of 3 to 4 lines around holes; washers
Snap fits	Strain ε = 1.5 t y ÷ L² within the material limit; long, tapered arms; layers along the arm
Moving parts	Clearance 0.3 to 0.5 mm; vertical gaps; test before relying on the fit
Strength	Check heat, creep and load before use; see the strength page

Related

• 3D Printing and Additive Manufacturing: the overview of this guide.
• Strength, Accuracy and Fits: how strong the part really is and how fits behave.
• Slicer Settings and Print Quality: supports, walls and infill.
• GD&T and Fits and Tolerances: specifying fits on drawings.
• 3D Printing Tolerancing Guide and the Bolt Torque and Thread Engagement tool.

FAQ

How thick should a printed wall be?

At least two line widths (about 0.8 to 1.0 mm) for a cosmetic part, 1.5 to 3 mm for functional parts, in whole multiples of the line width for a clean print.

Why is my hole too small?

Printed holes shrink by 0.1 to 0.3 mm because of the line paths and the plastic’s shrinkage. Make the hole larger in the model, or drill it to size.

Should I use heat-set inserts?

For any part you will assemble more than a few times or load with a bolt, yes: brass inserts give a durable thread in plastic, and they are cheap.

Can I print gears?

Yes, with a coarse module (1.0 or more), a tough plastic such as nylon or PETG, a wall of several lines around the teeth and a bit of added backlash. Lubricate and expect wear.

References

• Gibson, I., Rosen, D. and Stucker, B., Additive Manufacturing Technologies, Springer (design for additive manufacturing).
• BASF, Snap-fit design manual; Bayer MaterialScience, Snap-fit joints for plastics (cantilever snap-fit strain formula).
• Heat-set insert and fastener manufacturers’ application notes (hole sizes).

Last reviewed: October 2026. Allowable strains and allowances are typical: test on your own printer, material and orientation.