SLA and DLP printers cure liquid photopolymer layer by layer, so they resolve detail that fused-filament machines cannot touch. That resolution comes with a different failure mode: uncured resin trapped inside a part, suction forces that tear geometry off supports, and a green print that keeps shrinking and embrittling until it is washed and cured correctly. Most resin failures are designed in long before the vat is filled. This guide covers the handful of rules that separate a clean pull from a scrap pile. If you are still choosing between resin and powder or filament, start with our process selection guide.
Hollowing and the suction problem
Solid resin parts are expensive, heavy, and prone to shrinkage stress as the interior cures unevenly. Hollowing solves cost and warp, but it creates two new hazards. The first is trapped resin: an unvented cavity fills with liquid that never drains, adding weight, wasting material, and leaching out later. The second is the suction cup effect. As the build platform peels off the FEP film between layers, a sealed internal volume resists that pull like a plunger. The force can delaminate layers, snap supports, or drag the whole part loose.
The fix is always the same: give every enclosed cavity at least two holes, one to drain and one to vent. A single hole traps air and still forms a partial vacuum. Place one hole low in the build orientation so resin runs out under gravity, and one high so air can enter behind it. For large hollows on DLP machines with strong peel forces, err toward larger and more numerous holes.
| Feature | Typical minimum | Notes |
|---|---|---|
| Wall thickness (unsupported) | 1.0–2.0 mm | Thin walls cup and warp during cure |
| Wall thickness (supported/rigid resin) | 0.6–1.0 mm | Depends on resin modulus and span |
| Drain/vent hole diameter | 2–4 mm | Two per cavity minimum; scale up for big volumes |
| Embossed detail height | ≥ 0.4 mm | Below this, anti-aliasing blurs edges |
| Engraved detail width | ≥ 0.4 mm | Narrower channels fill with cured skin |
Every fully enclosed cavity needs at least two holes – one drain, one vent. One hole traps air and still pulls suction against the FEP.
Wall thickness and thin features
Resin walls that are too thin do not fail by breaking; they fail by curling. A large, thin, flat panel cures faster on the laser-facing side and shrinks against the still-liquid side, bowing the wall before it fully solidifies. Keep unsupported walls around 1–2 mm and break up large flat spans with ribs, curvature, or gussets. Standard rigid resins tolerate thinner walls than tough or flexible formulations, which stay compliant and sag under their own weight while green.
- Add a small fillet where thin walls meet thick bosses to reduce the shrinkage step that causes visible sink and internal stress.
- Avoid abrupt transitions from a 1 mm wall to a 5 mm solid block; the mass difference cures and shrinks at different rates.
- For miniatures and jewelry, thin blades and spokes print fine but need dense support, because they have almost no rigidity while curing.
Orientation and supports
Orientation drives surface quality, support-mark placement, and peel force all at once. Angle the part so the cross-sectional area per layer stays small – a large flat face printed parallel to the platform maximizes suction and almost always warps. Tilting a part 20–45° reduces peel force, breaks up layer lines on visible faces, and lets resin drain during the print.
Supports leave witness marks wherever their tips touch. Orient the part so tips land on non-cosmetic surfaces, then let the slicer bias support density toward overhangs and the lowest points, which anchor first. Islands – features that start mid-air with no connection to the layer below – must be supported or they cure floating in the vat and drop. If tolerances on a mating face matter, keep supports off it entirely and read our tolerances guide before committing an orientation.
Tilt parts 20–45° off the platform. It cuts peel force, drains trapped resin, and moves layer lines off the faces you care about.
Detail limits and anti-aliasing
DLP resolution is set by pixel size (often 35–100 µm), while laser SLA is set by spot diameter. Anti-aliasing smooths jagged edges by partially curing boundary pixels, which is great for cosmetic surfaces but softens fine text and sharp corners. Keep embossed and engraved detail at or above roughly 0.4 mm so it survives both the optics and the wash. Text reads best embossed rather than engraved, because engraved channels tend to fill with a cured skin during rinsing.
Wash, post-cure, and long-term aging
A part leaving the printer is green: dimensionally close but mechanically weak and coated in sticky monomer. Wash it in IPA or the resin maker's solvent to strip uncured resin from surfaces and drain holes, then let solvent evaporate fully before curing – trapped IPA leaves a cloudy, tacky finish. UV post-cure then drives the polymer to full strength. Over-curing is real: too long under UV makes many resins brittle and yellowed. Follow the datasheet time and temperature rather than maximizing exposure.
Finally, design around the material's lifespan. Most standard photopolymers keep aging under sunlight, growing more brittle and discoloring over months of UV exposure. For load-bearing or outdoor parts, choose an engineering resin rated for it, or plan a pigmented coating. Compare cured resin against nylon and filament properties in our materials comparison.
Turn these rules into muscle memory. Foundations walks through hollowing, venting, and orientation on real parts, then the process-specific tracks go deeper.
Take Foundations free See Pro pricingPre-print checklist
- Hollow solid parts and add at least two holes per cavity – one low to drain, one high to vent.
- Confirm unsupported walls are roughly 1–2 mm and large flat panels are ribbed or curved.
- Tilt the part 20–45° to cut peel force and drain resin during the print.
- Support every island and overhang; keep support tips off cosmetic and mating faces.
- Keep embossed/engraved detail at or above 0.4 mm; prefer embossed text.
- Wash thoroughly, dry off all solvent, then UV cure to the datasheet time – no more.
- Select a UV-stable engineering resin for any part exposed to sunlight or sustained load.