Designing Parts for Outsourced SLS 3D Printing Services: Draft Angles, Wall Thickness, and Nesting
Sending a design to an SLS printing service (Shapeways, Sculpteo, or a regional print bureau) is a fundamentally different design exercise than printing it yourself on an FDM machine at home, even though the end goal — a functional nylon or polymer part — looks similar. SLS builds parts by sintering powder layer by layer inside a sealed chamber, with no build plate adhesion to fight and (unlike FDM) no need for support structures under overhangs, since unsintered powder itself supports everything above it. That freedom comes with its own rules, and designing as if SLS were "FDM without supports" leaves real capability on the table and can produce parts that cost more, take longer, or fail to depowder properly.
No Supports Needed, But Powder Removal Is the New Constraint
Because loose powder fills every cavity during the build, any enclosed or deeply recessed internal void in your design needs an escape path for that powder after printing — a part with a fully sealed internal cavity and no drainage hole will come back from the printer with trapped powder inside, adding unwanted weight and potentially compromising the part depending on how much is trapped. Add at least one, ideally two, small drainage holes (3-5mm diameter is a common minimum, check your specific service's guidelines) to any enclosed cavity so compressed air or gravity can clear trapped powder during post-processing. This is the single most common reason a first SLS design comes back from the printer heavier and dirtier than expected, and it's invisible in a slicer preview the way FDM support material is, because there's no equivalent warning — the part simply prints "successfully" with powder trapped inside.
Wall Thickness Rules Are Different Too
SLS parts can run thinner walls than most FDM parts while remaining structurally sound, because there's no layer-adhesion weak axis the way FDM has between printed layers — a typical SLS service's minimum recommended wall thickness sits around 0.8-1mm for a small, well-supported wall, versus FDM where getting reliable results much below 1.2-1.5mm often requires careful slicer tuning. But thinner isn't free: very thin SLS walls (especially large, unsupported flat panels) are prone to warping slightly during the post-print cooling cycle, since the whole powder bed cools together and large thin sections can bow if nothing constrains them. Add gentle ribbing or a slight dome/curve to large flat panels rather than leaving them perfectly flat and thin, which gives the geometry some resistance to that cooling-stage warp without adding much material.
Design ElementFDM Rule of ThumbSLS Rule of Thumb Minimum wall thickness1.2-1.5mm (printer/slicer dependent)0.8-1mm, service-dependent OverhangsNeed supports past ~45-60°No supports needed at any angle — powder bed supports everything Enclosed cavitiesFine if sealed, no removal neededMust include powder drainage holes Fine detail / small textLimited by nozzle diameterGenerally finer detail achievable, limited by laser spot size and powder grain Living hingesPossible in flexible filamentsPossible and often more durable due to nylon's inherent flexibilityNesting and Build Volume Economics
Most SLS services price primarily by the bounding-box volume your part occupies in their build chamber (since that volume of powder gets consumed/processed per job) rather than purely by material mass the way a simple "grams of filament" estimate works for FDM — a hollow part with a large bounding box can cost more than a smaller solid part despite using less actual material, which surprises people used to thinking about cost in terms of material weight alone. If a design has flexibility in its overall footprint (several small parts that could nest together, or an assembly that could be reoriented to occupy a smaller overall bounding volume), check your chosen service's specific pricing model before finalizing dimensions, since the "obvious" orientation isn't always the cheapest one to submit.
Draft Angles and Tolerances for Assembled Parts
SLS parts don't need draft angles for mold release the way injection-molded or vacuum-formed parts do, since there's no mold to pull away from — but if your SLS part is going to mate with a separately manufactured part (an injection-molded enclosure, a machined bracket), design that mating interface with whichever process's tolerance rules apply to the other part, not SLS's own looser tolerance band. SLS typically holds dimensional accuracy in the range of a few tenths of a millimeter to roughly half a percent of the feature's size for larger parts, which is usually fine standalone but worth confirming against your specific service's published tolerance spec before designing a tight press-fit or precision mating feature that assumes better accuracy than the process reliably delivers.
Material and Finish Expectations
Standard SLS nylon (commonly PA12) comes back from most services with a slightly grainy, matte surface texture from the sintered powder — this is normal and not a defect, though it does mean features relying on a glass-smooth surface (optical components, tight-tolerance bearing surfaces) need post-processing (vapor smoothing, bead blasting, or hand finishing) that the base process alone won't deliver. Most services offer a dyeing or vapor-smoothing finish option at extra cost, worth considering upfront if surface finish matters for your application rather than treating it as an afterthought once the raw part arrives.
Designing specifically for SLS rather than translating an FDM mindset over to it unlocks the process's real advantages — thinner walls, support-free complex geometry, genuinely functional living hinges and interlocking assemblies printed in one piece — and avoids the two most common first-timer mistakes: trapped powder in sealed cavities, and a bounding-box cost surprise on a part that looked cheap by weight alone.