3D Printing Design Guidelines: Wall Thickness, Overhangs, Holes and Fits

Most 3D printing problems start in CAD, not on the printer. A wall that is too thin, a hole with no clearance or an overhang with nothing under it can turn a good idea into a failed part, a support-scarred surface or a higher quote. This guide gives you practical design rules for the four processes you will meet most often: FDM, SLA, SLS and MJF.

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The numbers below are typical starting values used across the industry. Machines, materials and part sizes vary, so treat them as safe defaults and check anything critical with your print service before you order.

Quick reference: design rules by process

Rule	FDM	SLA (resin)	SLS (nylon powder)	MJF (nylon powder)
Minimum wall (supported)	0.8 mm	0.5 mm	0.7 mm	0.5 mm
Recommended wall	1.2–2 mm	1–2 mm	1–2 mm	1–2 mm
Minimum hole diameter	2 mm	0.5–1 mm	1.5 mm	1 mm
Unsupported overhang	About 45°	Needs supports below about 30°	No supports needed	No supports needed
Clearance between moving parts	0.3–0.5 mm	0.2 mm	0.4–0.5 mm	0.4–0.5 mm
Escape holes for hollow parts	Not needed	3.5 mm or more	5 mm or more	5 mm or more

1. Wall thickness

Thin walls are the most common reason a file is rejected or a part breaks. In FDM, a wall is built from extruded lines (perimeters). With a standard 0.4 mm nozzle, a 0.8 mm wall is just two lines, which is fine for a cosmetic shell but weak for anything that takes load. For functional parts, aim for 1.2–2 mm.

In SLA, very thin walls can warp as the resin cures, so keep walls of large flat areas at 1 mm or more. In SLS and MJF, thin walls survive printing but can bend during powder removal and cooling, so 1 mm is a safer minimum for anything bigger than your hand.

Tip: keep wall thickness as even as possible. Sudden jumps from thin to very thick sections cause uneven cooling, which leads to warping and sink marks, especially in SLS and MJF.

2. Overhangs and supports

FDM and SLA build parts layer by layer on top of the layer below. When a surface leans out too far, there is nothing to print on, so the printer adds supports. Supports add cost and leave marks where they are removed.

• FDM: overhangs up to about 45° from vertical usually print cleanly without supports. Short horizontal bridges (under about 10 mm) are often fine too.
• SLA: almost every part needs some supports. Design so the support marks land on surfaces that do not matter.
• SLS and MJF: the loose powder supports the part, so overhangs are free. This is a big reason to choose powder processes for complex shapes.

Simple ways to avoid supports in FDM: replace flat ceilings with a 45° chamfer, turn round horizontal holes into a teardrop shape, and split very complex parts into two pieces that print flat and bolt together.

3. Holes, pins and fits

Printed holes almost always come out slightly smaller than designed, especially in FDM, because the plastic spreads a little on each layer. If a hole must fit a bolt, pin or bearing, either add clearance in CAD or plan to drill or ream it after printing.

• For a bolt to slide through, design the hole 0.2–0.4 mm larger than the bolt in FDM.
• For parts that must move against each other (hinges, sliders), leave the clearance values from the table above on each side.
• For threads, small threads print poorly. Use heat-set brass inserts or a nut trap for M2–M6 screws, and only model threads larger than about M8.

4. Small details and text

Fine details need to be bigger than the smallest thing the process can make. As a rule of thumb, keep embossed or engraved text at least 0.5 mm deep and use letters at least 5 mm tall for FDM. SLA can reproduce much finer detail, which is why it is used for jewellery, dental and miniature work.

5. Hollow parts and escape holes

Hollowing a large part saves material, weight and money. In SLA, SLS and MJF, a hollow part traps uncured resin or loose powder inside, so you must add at least two escape holes so it can be cleaned out. In FDM, hollow is handled by the infill setting instead, so you design the part solid and let the slicer decide.

6. Print orientation

The direction a part is printed changes its strength, surface finish and cost. FDM parts are weakest between layers, so a part that takes a bending load should be oriented so the load runs along the layers, not across them. Taller orientations also take longer and cost more. If strength or appearance matters in one direction, tell your print service when you order.

7. Large flat parts and warping

Big flat plates are the hardest shapes to print straight. Plastic shrinks as it cools, and a large flat area pulls its edges up. Add ribs instead of making the plate thicker, round sharp corners, and avoid long thin flat parts in ABS, which warps more than PLA or PETG.

Checklist before you upload your file

• Walls are at least the recommended thickness for the process.
• Holes and moving parts have clearance.
• Overhangs are under 45° (FDM) or you accept supports.
• Hollow parts have escape holes (SLA, SLS, MJF).
• The file is a closed, watertight mesh or a clean STEP file in millimetres.

For more on preparing your file, read our CAD file preparation guide. Not sure which process fits your part? See FDM vs SLS vs SLA vs MJF, then upload your file for an instant quote.