Design for 3D printing: overhangs, bridges and supports
FDM builds the part layer by layer, and every layer needs something to rest on. The guide value is that overhangs up to about 45° from vertical print without supports. Steeper surfaces need support material, which costs time, material and post-processing and leaves a poorer surface.
So we remodel the geometry so supports are not needed. 45° chamfers replace fillets on the underside, horizontal holes get a teardrop shape at the top and large flat ceilings are split up with ribs. Straight bridges between two supports typically manage 10–20 mm without problems. Longer bridges are possible, but with some sagging. The right orientation on the build plate often solves more than any change to the geometry.
Where supports are still needed, we place them on faces that are neither visible nor functional. Supports can also be modelled into the part as thin break-away ribs. That gives better control than the slicer's automatic supports and leaves cleaner surfaces.
Wall thickness and feature size
The minimum wall thickness is set by the nozzle. With a 0.4 mm nozzle, a line is about 0.4–0.45 mm wide, and a durable wall should be 2–3 line widths, i.e. 0.8–1.2 mm. Loaded parts get 1.6–2.4 mm, which corresponds to four to six perimeters. Walls designed as exact multiples of the line width print more cleanly, without thin infill strands.
Vertically, resolution is set by the layer height, typically 0.1–0.3 mm. A height of 5.1 mm therefore becomes 5.0 or 5.2 mm with 0.2 mm layers. Small holes under 1–2 mm and text with line widths below about 0.5 mm come out blurred. Functional features that must be sharp are therefore drawn with a margin or drilled out afterwards.
Thin, tall features can also vibrate as the nozzle passes. Slender towers and pins therefore get a wider base or a helper structure that is broken off after printing. That gives sharper details and fewer failed prints.
Tolerances and clearance for fits
An FDM print typically holds a few tenths of a millimetre, depending on material, size and direction. Holes often come out slightly smaller than drawn, partly because the circle is built from short straight segments and partly because the plastic contracts. The first layer is squeezed out slightly, the so-called elephant's foot, so we chamfer the bottom edge of parts that must fit against something.
For moving fits we design with a clearance of 0.2–0.4 mm per side as a guide value, and 0–0.1 mm for press fits. Joints printed fully assembled, print-in-place, get 0.3–0.5 mm. Materials that shrink, such as ASA and PA, need more. For critical fits we first print a small test piece with several clearances and pick the one that works. The background is in the guide to tolerances.
Dimensions are also affected by how the part is oriented. A hole standing vertically comes out rounder and more accurate than one lying horizontally. Parts with important holes are therefore oriented with the holes pointing upwards, where that is compatible with the strength requirements.
Threads, inserts and screw joints
Screws straight into plastic work for occasional assembly, but the thread wears quickly. For joints that are opened often we use brass heat-set inserts, pressed in with a soldering iron. The hole is drawn to the insert manufacturer's dimensions, and the wall around the insert should be at least 1.5–2 mm so it does not crack.
The alternatives are hexagonal pockets for standard nuts or modelled threads. Modelled threads work well from about M6 upwards, and for coarse threads on lids and jars. Smaller threads we prefer to cut afterwards with a tap. Self-tapping screws for plastic also work, if the hole is drawn to the screw manufacturer's recommendation and the screw boss is braced to the wall with ribs. In jigs and fixtures we often combine inserts with metal drill bushes where wear is high.
Orientation and anisotropy
An FDM part is not equally strong in all directions. The bond between layers is weaker than the strand itself, and strength in the vertical direction is often half or less of that in the plane of the layers. So we decide the print direction early and design the part accordingly. Snap fits lie in the plane of the layers, and screw bosses are braced to the walls with ribs.
Sometimes it is better to split a part in two and screw it together than to force a poor orientation. We add locating pins and joints so the parts are assembled correctly. If you have an existing file to adapt, we often start by converting STL to STEP. More on the method and materials is in [[g_3dutskrift|the guide to 3D printing]].
Infill matters less than many people think. Once infill is around 20–40 %, more perimeters usually add more strength per gram than denser infill. We send recommended settings with the file, so the part comes out the same every time it is printed.
DfAM guide values for FDM (0.4 mm nozzle)
| Parameter | Guide value |
|---|---|
| Overhang without supports | Up to approx. 45° from vertical |
| Bridge without supports | Approx. 10–20 mm without problems, longer with some sagging |
| Minimum wall | 0.8–1.2 mm (2–3 line widths) |
| Wall for loaded parts | 1.6–2.4 mm (4–6 perimeters) |
| Layer height | 0.1–0.3 mm, typically 0.2 mm |
| Clearance in a moving fit | 0.2–0.4 mm per side |
| Press fit | 0–0.1 mm, verify with a test piece |
| Print-in-place joint | 0.3–0.5 mm clearance |
| Brass heat-set insert | Hole to manufacturer's spec, surrounding wall at least 1.5–2 mm |
| Strength in the vertical direction | Often half or less of that in the plane of the layers |
Guide values for FDM – they depend on material, print settings and the size of the part.
What we need from you
- Existing file (STEP, STL or 3MF) or a sketch with dimensions
- How the part is loaded and in which direction
- Which dimensions and fits are critical
- Environment: temperature, sun, moisture and chemicals
- Quantity and whether we print the part or you do
- If you have your own printer – nozzle, material and build volume
Typical projects
- Part redesigned to print without supports
- Hinge printed fully assembled
- Enclosure with heat-set inserts for screws
- Lid and jar with a modelled thread
- Large part split with locating pins and dovetails
- Snap fit oriented for maximum toughness
- Lightweight part with ribs instead of solid material
- Fixture with pressed-in metal bushes
- Test piece with several clearances for a fit test
- Multi-part assembly replaced by a single print
What affects price and lead time
- Whether the part is designed from scratch or adapted from an existing file
- Number of fits, threads and moving parts
- Whether the part has to be split and given joints
- Number of test prints for clearance and fit
- Strength and weight requirements
- Whether finished prints are to be delivered
What you get
- STL and 3MF with recommended orientation
- STEP for future changes
- Recommendation of material and print settings
- List of inserts and screws
- Finished prints from NordiTeq (optional)
Frequently asked questions
What tolerance can a 3D print hold?
An FDM print typically holds a few tenths of a millimetre, depending on material, size and direction. For fits we design with clearance and test with a sample when the dimension is critical.
How much clearance should there be between two 3D-printed parts?
The guide value is 0.2–0.4 mm per side for parts that must move and 0–0.1 mm for a press fit. It depends on the material and printer, so a test piece with several clearances is the safest option.
What angle can be printed without support material?
About 45° from vertical is a safe guide value. Steeper surfaces we redesign with chamfers, teardrop holes or a different orientation.
How do you fix screws into a 3D print?
Brass heat-set inserts are the most durable option for joints that are opened often. Hexagonal nut pockets and modelled threads from about M6 are alternatives.
Why do 3D-printed parts break along the layers?
The bond between layers is weaker than the plastic strand itself. So we orient the part so the load runs along the layers and reinforce sharp transitions with radii.
Can you print the part for me as well?
Yes. NordiTeq manufactures with FDM in materials including PLA, PETG, ASA, PA, PC and TPU. You can also have the files and print them yourself.