SLA 3D Printing Resin Design Guide

• 1
  What decides a resin part
• 2
  Wall thickness, ribs and bosses in SLA resin
• 3
  Drain holes and hollowing for SLA parts
• 4
  Orientation, supports and layer height
• 5
  Shrinkage, fit and feature limits
• 6
  When resin is the wrong choice
• 7
  Seven steps to a printable resin part
• 8
  Resin design values by feature

Resin cures under a laser, so every wall is built as a stack of thin layers rather than cut from solid stock. A wall under 0.8 mm usually survives the print, then warps during the UV post-cure bath and the final bake. The part looks fine on the platform and bends a day later. Keep cosmetic shells at 1.5 to 2.5 mm and you get a surface that stays flat.

Ribs follow a different rule. A rib at 0.6 to 0.8 mm is flexible in a good way, which suits snap clips and living hinges on prototypes. Once you pass 3 mm, the rib behaves like a solid block and pulls the adjacent wall as it shrinks. If you need stiffness, add height or a second rib rather than thickness.

Bosses for threaded inserts need more care than walls. A boss wall of 1.5 to 2 mm around a brass insert is enough for M3 heat-set hardware. Leave a clearance of 0.1 to 0.2 mm on the insert diameter so the plastic can flow when you press it in. Tight bosses crack at the root, because the corner acts as a stress riser.

Fillets do the heavy lifting on internal corners. A 0.5 mm radius where a rib meets a wall removes most of the peel stress and makes the part easier to drain. Sharp internal corners also trap resin, which stays liquid and slowly attacks the cured surface.

• 1
  Shells1.5 to 2.5 mm for flat panels, housings and covers.
• 2
  Ribs0.8 to 1.5 mm, with a 0.5 mm root fillet.
• 3
  Bosses1.5 to 2 mm wall, 0.1 to 0.2 mm insert clearance.
• 4
  AvoidWalls over 4 mm. Hollow them and add drain holes instead.

A solid block of resin is expensive and slow, and it keeps shrinking for hours after the print. Hollow anything thicker than about 4 mm. A 2 mm shell keeps the outer surface accurate while cutting build time and resin use. The catch is that trapped liquid resin will keep curing inside the cavity and can split the part weeks later.

Give every cavity two openings. A drain hole at the lowest point lets uncured resin and IPA run out, and a vent hole at the highest point lets air in so the liquid can actually move. A single hole creates a vacuum and the wash cycle never clears the inside. Ø2 to 4 mm works for most parts. Use 2 mm for small cavities and 4 mm where the resin is thick.

Place the holes where they will not sit on a sealing face, a bearing bore or a visible A-surface. On a manifold or a fluid housing, plan the hole position early so you can plug it with a set screw or a bonded plug. Designers who leave drain holes to the last minute usually end up with one on a critical face.

Wall thickness inside the cavity matters as much as outside. Keep the shell uniform at 2 to 3 mm. A cavity wall that jumps from 1 mm to 5 mm creates a hard shrink gradient, and the part distorts at that step even if the outer surface looked correct.

• 1
  Two holes minimumOne low to drain, one high to vent.
• 2
  Hole sizeØ2 mm for small cavities, Ø4 mm for thick sections.
• 3
  Keep it clearStay off sealing faces, bores and A-surfaces.

Orientation decides more about the final part than any printer setting. Tilt a flat face 15 to 30° from the build plate. The peel force drops, the support contact area drops, and the face that was going to be a witness mark now prints clean. Flat surfaces parallel to the plate are the classic failure, because every layer peels the whole cross-section at once.

Put supports on the non-critical side. Support tips leave small witness marks, and they are hard to sand out of a fine texture or a polished face. On a housing, support the inside. On a jig, support the base. If both sides are cosmetic, rotate the part so the marks land on an edge.

Layer height sets finish and time together. At 0.05 mm you get a surface that reads as smooth under raking light and a build that takes roughly twice as long as 0.1 mm. Most engineering prototypes run at 0.05 to 0.1 mm, then get bead blasted or primed. If the part needs a gasket seal, choose the finer layer and allow a light sand on the mating face.

Resolution is not the same as accuracy. A 0.05 mm layer on a poorly oriented part still curls at the corners. Fix orientation first, then spend time on layer height.

• 1
  Tilt 15 to 30°Off the build plate for large flat faces.
• 2
  Supports on hidden facesKeep witness marks off sealing and cosmetic surfaces.
• 3
  Layer height0.05 mm for finish, 0.1 mm for speed.

SLA resin shrinks as it cures, usually 0.5 to 2% depending on the formulation and the post-cure schedule. That number is not noise. On a 100 mm part, 1% is a full millimeter, which is the difference between a slip fit and a press fit. Scale the model in CAD or in the slicer before printing, and measure a test coupon the first time you run a new resin.

Compensate fits with clearance rather than a scale factor where you can. Add 0.15 to 0.3 mm per side for a sliding fit, and 0.05 to 0.1 mm per side for a light press. Resin is more brittle than machined plastic, so a press fit that works in POM will crack a resin boss. If the joint carries load, design a mechanical feature instead of relying on interference.

Small features have a floor too. Holes below Ø1 mm tend to close or print oval, and text below 1.5 mm character height loses its edges. Both are easy to add later with a drill or a laser, so leave them out of the resin print and finish them in a second operation if the part is a functional prototype.

Threads are the classic trap. Printed threads at M6 and below are rough and weak, and they gall when you run a screw in. Print a pilot hole and tap it, or use a heat-set insert. That one change removes most of the assembly problems engineers hit on resin prototypes.

• 1
  Shrinkage0.5 to 2%, scale before printing.
• 2
  Sliding fit0.15 to 0.3 mm per side.
• 3
  Press fit0.05 to 0.1 mm per side, or use an insert.
• 4
  Small holesBelow Ø1 mm, drill after printing.

SLA wins on surface finish, fine detail and small feature size. It loses on impact strength, heat resistance and long-term UV stability. Standard resin softens near 60 °C, so a part that sits in a hot engine bay or under direct sun will creep. Engineering and high-temperature resins improve this, but they still do not match a machined polymer.

If the part carries a load, holds a thread under torque, or sees repeated impact, move it to machining. A CNC prototype in ABS, POM, PEEK or aluminium gives you real material properties and a tolerance of ±0.005 mm when the geometry needs it. Resin is best used to prove the shape, then the same CAD goes to the mill for the functional test.

The usual split is fast and simple. Use SLA when the question is what the part looks like or whether it fits. Use machining when the question is whether the part survives. Sending the wrong question to the wrong process is how a project loses a week.

There is a middle path. Print the resin part for form and fit, then machine only the critical interface, a bearing bore or a sealing face, as a second operation. That keeps the cost low and puts real metal or engineering plastic exactly where the tolerance matters.

• 1
  Use SLA forForm, fit, fine detail, small runs.
• 2
  Use machining forLoad, heat, threads under torque, ±0.005 mm.
• 3
  HybridResin body plus a machined critical interface.