A functional prototype has to behave as closely as possible to the final part. It has to fit into the assembly, hold a screw, survive a drop and snap shut. Nylon SLS suits this work because the material is an engineering thermoplastic. A photopolymer resin behaves differently under load, and in FDM the deposited layers are only weakly bonded to each other. An SLS part behaves fairly uniformly in all directions, so the test results mostly reflect the design of the part.
The unsintered powder supports the part during printing, so there are no supports to remove and no support marks on functional surfaces. You can put several variants of the same part in a single build (with different clearances or with clips of different thicknesses) and pick the right one after trying them. For features close to the limits, Formlabs recommends a small test part printed in the same orientation as the real part.
For clips and living hinges, choose the material by how far the part has to flex. PA12 is stiff and dimensionally stable and suits most enclosures and brackets. PA11 has an elongation at break of 40%, against 11% for PA12 (Formlabs data sheets), so a PA11 clip bends much further before it cracks. Polypropylene is used for hinges that flex often, for parts in contact with chemicals and for parts that have to be welded to other PP parts.
Why SLS
- Parts print without supports, so internal channels and print-in-place assemblies come out with no support marks.
- The materials are engineering thermoplastics (PA12, PA11, PP, TPU), from the same families as many injection-moulded parts.
- Properties are more uniform across directions than in an FDM part. Elongation is lower in Z (for PA12, 6% in Z against 11% in XY, according to the Formlabs data sheet).
- Several variants of the same part fit in a single build, with no tooling cost for each of them.
Parts we often print
- Clips and clamps for testing assembly and disassembly forces
- Enclosures with snap-fit lids
- Living hinges for lids and boxes, in PP or PA11
- Gears and mechanisms printed fully assembled
- Handles and ergonomic parts for user testing
- Fit-check parts for assemblies with bought-in components
- Fittings and ducts for low-pressure air
Design tips
- Orient clips and flexing features so that they work in the XY plane, because SLS parts are tougher in XY than in Z. If a particular orientation matters, state it in your request.
- For print-in-place assemblies in PA12, leave at least 0.3 mm clearance on small features (under 20 mm²) and 0.6 mm on large ones. In PA11 and TPU 90A, start from 1 mm.
- If you don't know which clearance or thickness will work, put 3–4 variants with different dimensions in the same file. The extra variants add a little to the cost and usually save you a design iteration.
- Living hinges have to be thicker and longer than those on injection-moulded parts. Test them in the final material, because a hinge tested in PA12 does not tell you how the same hinge will behave in PP.
- The minimum wall thickness is 0.6 mm for vertical walls, but that is only the limit at which the wall forms. Size the walls of parts you handle often for the load they carry.
The full rules are in the SLS design guide.