AN-045 Build guides · cluster
Design for Additive Manufacturing (DfAM)
Design for additive manufacturing rules by process: wall, hole, overhang and clearance numbers for FDM, SLA, SLS and metal, plus when to stop printing.
by the HardwareMap editors4 min read
Contents
Design for additive manufacturing (DfAM) is designing a part for the way a 3D printer builds it: layer by layer, with process limits on walls, holes, overhangs and trapped material, and strength that changes with build direction. It is the printing branch of design for manufacturing. The rules below come from process vendors' published guides, with the numbers attached.
Two things make DfAM different from molding or machining DFM. First, the limits change completely between print processes, so the process has to be picked before the geometry. Second, printing removes constraints that drive part count in every other process: no draft, no tool access, no parting line. A good DfAM pass uses both.
Design for additive manufacturing rules by process
Sources: Protolabs Network design guides for FDM, SLA and SLS; Renishaw's design for metal AM guide for powder bed fusion.
Clearances matter as much as walls. The SLA guide gives 0.5 mm between moving parts, 0.2 mm for assembly connections and 0.1 mm for a snug fit. The SLS guide gives 0.3 mm for running axles and 0.2 mm for ball-and-socket hinges. Print a clearance test coupon on the actual machine before trusting any of these.
Pick the process before the geometry
Desktop machines have moved this decision in-house for most startups. Formlabs sells resin and SLS powder printers to engineering teams. Prusa Research and Bambu Lab make the FDM printers on many hardware benches. Snapmaker combines printing with laser and CNC heads. A bench printer turns a looks-like prototype into an overnight job, which is the stage the prototype to production note calls proof of concept.
Orientation, supports and strength
Orientation decides three things at once: where supports go, which surfaces are rough, and which direction is weak.
- FDM. Put the main load along the layers, not across them. Chamfer edges that touch the build plate at 45° so first-layer squish does not change outside dimensions. Bridges under 5 mm print clean; longer ones sag or need support. Pins under 5 mm diameter need a base fillet, or a printed hole with a metal pin pressed in.
- SLA. Minimize the cross-section in each layer, not the support count. The peel force that tears a part off the build plate scales with layer area, so parts print at an angle with more support and fewer failures.
- SLS and MJF. Nest parts in three dimensions to fill the build volume. Hollow thick sections and add escape holes, which saves powder and cost.
- Metal. Renishaw recommends rotating scan direction between layers to spread residual stress, and treats any surface under 45° to the plate as needing support. Supports conduct heat as well as hold the part, so designers place them on faces that will be machined anyway.
What DfAM allows that molding does not
The second half of DfAM is using freedoms other processes lack.
Part consolidation. An assembly of brackets, fasteners and manifolds becomes one printed part. Fewer parts means fewer joints to leak, fewer suppliers and fewer lines in the bill of materials. The trade is that one printed part replaces several cheap ones, so consolidation only pays when the joints, fasteners and assembly time it removes cost more than the print. Agnikul Cosmos describes its rocket engine as a single-piece 3D-printed semi-cryogenic engine, which removes the welded and brazed joints of a conventionally built engine.
Generated structure. Topology optimization and lattices put material only where load runs. Divergent Technologies pairs generative design and metal additive manufacturing with fixtureless robotic assembly in its DAPS platform, which underpins the Czinger 21C hypercar's structure.
A print farm instead of a mold. Prusa says its farm of around 700 printers produces the plastic parts for its printers, and a part improvement can reach shipping units within hours. That is a choice to keep iterating rather than freeze geometry in steel.
When to stop printing and cut a mold
Printing costs roughly the same per part at unit one and unit one thousand. Molding costs a lot at unit one and very little at unit ten thousand. The crossover depends on part size, material and finish. For small plastic parts it often lands in the hundreds to low thousands, against a prototype aluminum mold at $3,000 to $25,000est. The injection molding cost note works through the break-even.
Hybrid is common. Many teams mold the large cosmetic housing and keep printing small internal brackets that still change with every board spin. Others print in SLS or MJF nylon through the first production run, accept a higher unit cost, and tool only once sales justify it. Either way, the decision belongs in the 18-month budget of how to start a hardware company, next to tooling lead time.
Before switching, redesign the part for molding. A printed part has no draft, uneven walls and undercuts the printer did not care about. Those all have to change, which is why a part that printed well for EVT often needs a DFM pass before it can be tooled.
Frequently asked questions
What is design for additive manufacturing (DfAM)?
DfAM is designing a part for the way a 3D printer builds it: layer by layer, with limits on overhangs, wall thickness, hole size and trapped material, and with strength that depends on build direction. It also covers the opposite side: using freedoms printing allows, such as consolidating assemblies into one part, internal channels and lattices that no mold or cutter could make.
What is the minimum wall thickness for 3D printing?
It depends on the process. Protolabs Network says all printers in its network can print walls of 0.8 mm and over. SLA can hold supported walls down to about 0.4 mm. SLS in PA12 needs about 0.8 mm, and carbon-filled nylon closer to 2 mm. Metal laser powder bed fusion is sensible from about 0.4 mm on most machines, per Renishaw.
What overhang angle can a 3D printer handle without support?
FDM and SLA print overhangs up to roughly 45 degrees from vertical without support. Metal laser powder bed fusion needs support on surfaces less than 45 degrees to the build plate, per Renishaw's design guide. SLS and multi jet fusion need no supports at all, because unfused powder holds the part, which is why they suit complex production parts.
Are 3D-printed parts as strong as molded parts?
Not in every direction. FDM parts are weakest across layers, because bonding between layers is weaker than the extruded line itself. SLS and multi jet fusion nylon parts are closer to isotropic and are used as end-use parts. Metal powder bed parts can approach wrought properties after heat treatment, depending on alloy and process. Test printed parts in the orientation they will be loaded.
When should a startup switch from 3D printing to injection molding?
When the design is stable and the quantity makes tooling cheaper per part than printing. For small plastic parts that crossover often falls somewhere in the hundreds to low thousands of units, but get quotes for both at your volume. Keep printing parts that still change every build, or where printing removes an assembly.
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