3D printing design rules – what DfAM means
DfAM, design for additive manufacturing, means designing a part for the way it is built up. In FDM, molten plastic is laid down in strands, layer by layer. Each layer rests on the one below. That gives freedom to make shapes that are impossible to machine, but also limitations a CNC designer is not used to.
The good news is that complexity costs almost nothing extra. An internal cooling channel, an integrated hinge or an organic shape takes roughly as long to print as a simple box of the same volume. Time and price are driven mainly by the volume of material, the height and the amount of support. The guide values below apply to a standard 0.4 mm nozzle and layer heights of 0.1–0.3 mm. Other nozzles and materials give other values – treat them as a starting point.
Overhangs and bridges
An overhang up to about 45° from vertical can be printed without support material. Any steeper and the strands hang free and the surface is poor. Support material works, but leaves marks and costs time and material.
So design with 45° in mind. Replace horizontal ceilings with 45° chamfers and use chamfered edges instead of fillets on the underside. A fillet on the underside starts with an almost horizontal overhang – a chamfer prints cleanly.
A bridge is a horizontal strand between two supports. Short bridges of 10–20 mm usually work with the right part cooling; longer bridges need testing or support. Plan hole patterns and windows so the bridges stay short.
Wall thickness and detail
Wall thickness should be a multiple of the extrusion width. With a 0.4 mm nozzle and an extrusion width of about 0.42–0.45 mm, two perimeters give around 0.8–0.9 mm and three perimeters around 1.2–1.35 mm. A 1.0 mm wall instead gives two perimeters and a thin infill strand that can be weak.
A guide value is at least 1.2 mm for load-bearing walls and 0.8 mm for light covers. Small features under about 0.8 mm in XY come out blurred or disappear. Text meant to be readable should be at least 0.5 mm deep or raised and have strokes of at least 0.4–0.5 mm.
Infill percentage affects strength less than people think. More perimeters often give more strength per gram than denser infill.
Clearance, fits and holes
Parts that fit together need designed-in clearance. A guide value is 0.2–0.4 mm total clearance for parts that slide or are assembled by hand, and 0.1–0.2 mm for tighter fits. Moving parts printed assembled often need 0.4–0.5 mm. It is a good idea to print a small test piece with different clearances before printing the whole part.
Vertical holes typically come out 0.1–0.3 mm undersize, because the strand is pushed inwards on the curve. Add clearance in the model or drill the hole out afterwards. Horizontal holes have an overhang at the top. Give them a teardrop shape or a flat roof with 45° chamfers up to it, and they print without support.
Threads below M6 rarely turn out well printed directly in plastic. More on tolerances in the guide ISO tolerances and fits.
Screw joints and threaded inserts
For screws that will be done up and undone several times, brass threaded inserts are the best choice. They are heat-set into a prepared hole with a soldering iron or a dedicated tool. The hole diameter and depth should follow the insert manufacturer's dimensions, because they differ between brands.
Allow at least 1.5–2 mm of wall around the insert and a couple of extra perimeters in that area. Self-tapping plastic screws also work, with a hole slightly smaller than the screw's core diameter. For one-off assembly, a nut in a hexagonal pocket is enough – it is cheap and very strong.
Orientation and anisotropy
FDM parts are weakest between layers. If the part is loaded so that the layers are pulled apart – in Z – strength can be much lower than in the XY plane. So orient the part so the main load runs along the layers, not across them.
An example: an angle bracket printed standing up gets layer lines exactly in the corner where stress is highest. Printed lying on its side, it gets strands that run round the corner and becomes much stronger.
Orientation also affects surface finish, accuracy and the need for support. Faces against the build plate come out flat and accurate. Faces built vertically show layer lines. Sometimes it is best to split the part and print the halves in different orientations.
Choose the material for the use
PLA is stiff, easy to print and dimensionally accurate, but softens at around 55–60 °C. It suits prototypes, indoor fixtures and models. PETG is tougher, resists moisture and chemicals better and copes with about 70 °C. It is a good standard choice for functional parts indoors.
ASA resists UV light and weather and copes with around 90–95 °C. It suits parts outdoors and in vehicles. PA (nylon) is tough and has good wear resistance, but absorbs moisture and must be dried. PC withstands high temperatures, above 100 °C, and is stiff. TPU is flexible and is used for gaskets, dampers and grips.
Carbon-fibre-reinforced versions of PETG, PA and other materials give higher stiffness but are more brittle and wear the nozzle. The values are guide values and vary between manufacturers. Read more about design for additive manufacturing.
Common mistakes and large parts
The most common mistake is designing as if for moulding or machining: horizontal ceilings, sharp internal corners and tight fits without clearance. Another is thin 1.0 mm walls that do not match the extrusion width, or long thin pins that snap along the layers.
Other mistakes are putting text and logos on overhanging faces and forgetting how the support will be removed. Choosing PLA for a part that will sit in a car in summer is another. Go through the checklist below before sending a file for printing – it catches most problems.
Parts larger than the build volume are split into several pieces. Place the joint where it is least visible and least loaded. Use locating pins, dovetails or overlapping rebates so the pieces line up when glued or screwed.
Splitting can also be a way to get a better orientation and less support. We design for 3D printing every day and print the parts ourselves. Send us your model and we will review it, or read about 3D printed prototypes.
Materials for FDM – guide values
| Material | Properties | Heat resistance (guide value) | Typical use |
|---|---|---|---|
| PLA | Stiff, accurate, easy to print | approx. 55–60 °C | Prototypes, indoor fixtures, models |
| PETG | Tough, resists moisture and many chemicals | approx. 70 °C | Functional parts, enclosures, holders |
| ASA | UV and weather resistant | approx. 90–95 °C | Outdoor and vehicle parts |
| PA (nylon) | Tough, wear-resistant, moisture-sensitive | approx. 80–120 °C depending on type | Gears, sliding parts, clamps |
| PC | Stiff, impact-resistant, heat-resistant | above 100 °C | Parts near heat, engineering components |
| TPU | Flexible, rubber-like | approx. 60–80 °C | Gaskets, dampers, grips |
Heat resistance is given as the approximate softening temperature under light load. Values vary between manufacturers and print settings.
Checklist: designing for 3D printing
- Overhangs no more than about 45° without support
- Chamfers instead of fillets on the underside
- Walls as a multiple of the extrusion width, at least 1.2 mm for load
- Clearance of 0.2–0.4 mm for assembled parts
- Vertical holes compensated or drilled out
- Horizontal holes with a teardrop shape or flat roof
- Threaded inserts for screw joints that are opened often
- Orientation set by the main load – not across the layers
- Material chosen for temperature, UV and moisture
- Large parts split with location features at the joint
Frequently asked questions
How much clearance should I leave between 3D-printed parts?
A guide value is 0.2–0.4 mm total clearance for parts assembled by hand. Tighter fits can work with 0.1–0.2 mm. Print a test piece with different clearances first, because the result depends on material and settings.
What wall thickness is needed for 3D printing?
Make walls a multiple of the extrusion width, for example 0.8–0.9 mm or 1.2–1.35 mm with a 0.4 mm nozzle. Load-bearing walls should be at least 1.2 mm. More perimeters give more strength than denser infill.
Why do 3D-printed parts break between the layers?
The bond between layers is weaker than the strands themselves. If the part is loaded in Z, it can crack along a layer line. Orient the part so the load runs along the layers.
Which material should I choose for an outdoor part?
ASA is a good outdoor choice because it resists UV light and heat better than PLA and PETG. PLA softens at around 55–60 °C and is unsuitable in the sun. For flexible outdoor parts, TPU can work.