SLA and DLP: How the Process Shapes the Design Rules
Stereolithography (SLA) uses a UV laser to cure liquid resin point by point on each layer. Digital Light Processing (DLP) uses a UV projector to cure an entire layer at once. Both produce the highest dimensional accuracy and finest feature resolution of all polymer 3D printing technologies — but their liquid-resin photopolymer chemistry creates design constraints that differ significantly from powder-bed or extrusion processes.
The most critical constraint: every layer is attached to the build plate above it and submerged in liquid resin below. Peel forces act on the part with every layer, making SLA/DLP more sensitive to part geometry than FDM or SLS. Good design for resin printing means minimising peel forces while maximising feature accuracy.
Wall Thickness Guidelines
| Feature Type | Minimum | Recommended |
|---|---|---|
| Supported walls | 0.4 mm | 1.0 mm |
| Unsupported vertical walls | 0.6 mm | 1.5 mm |
| Horizontal spanning features | 0.5 mm | 1.0 mm |
| Pins and rods (vertical) | 0.5 mm diameter | 1.0 mm |
| Pins and rods (horizontal) | 1.0 mm diameter | 1.5 mm (add support below) |
| Text (embossed) | 0.3 mm depth, 0.5 mm width | 0.5 mm depth |
| Holes (vertical axis) | 0.5 mm diameter | 1.0 mm |
These values apply to standard engineering resins. Castable jewellery resins and dental resins have slightly different minimum feature sizes due to their specific viscosity and cure chemistry.
Tolerances and Dimensional Accuracy
SLA and DLP are the most accurate polymer AM processes:
- Typical tolerance (XY): ±0.1–0.15 mm
- Typical tolerance (Z): ±0.1 mm (layer height dependent — 25–100 µm layers)
- Hole undersizing: Add 0.1–0.2 mm to nominal diameter for clearance fits
- Post-cure shrinkage: 0.1–0.3% isotropic — scale-up parts by 0.2% in all axes if absolute dimensions are critical
SLA achieves better accuracy than FDM because there is no nozzle diameter constraint, no filament pressure variation, and no inter-layer adhesion uncertainty. The main accuracy risk in SLA is resin photopolymer shrinkage during curing — minimised by proper post-cure protocols (45–60 min in UV chamber, 35–45°C).
Overhang and Support Design
SLA/DLP require supports for overhangs beyond approximately 19° from vertical (much stricter than FDM's 45° rule) because liquid resin cannot support uncured geometry below it. However, the support contact points are much smaller and more precise than FDM supports:
- Support tip diameter: 0.5–0.8 mm (fine point minimises surface scarring)
- Support spacing: Every 3–5 mm along unsupported edges
- Support placement: On non-critical surfaces only — support contact points leave ~0.3 mm surface marks that require sanding
- Support removal: Twist and snap, or needle-nose pliers — far easier than FDM supports
Design strategies to minimise SLA supports:
- Tilt the part at 45° to the build plate — this converts horizontal features into angled ones that need fewer supports
- Design hollow parts as shells rather than solids — reduces peel forces and resin cost
- Replace flat undersides with chamfered or domed geometry where cosmetics permit
- Use drain holes in hollow parts (2–3 mm) to allow uncured resin to escape — trapped liquid resin inside hollow parts will crack during post-cure UV exposure
Hollow Parts and Drain Holes
Solid SLA parts are expensive and heavy — most designs benefit from hollowing. Key rules for hollow SLA/DLP parts:
- Minimum wall for hollow parts: 2.0 mm (thinner walls risk distortion during peel forces)
- Drain hole minimum: 2 mm diameter, placed at the lowest point in the print orientation — allows liquid resin to drain during printing and prevents hydrostatic pressure buildup
- For completely sealed hollow parts (no drain holes): risk of explosive cracking during post-cure UV exposure when trapped resin expands. Always include at least one drain hole.
- Layer X can hollow solid files automatically — mention this when requesting a quote via our shop.
Resin Types and Their Design Implications
| Resin Type | Key Properties | Design Notes |
|---|---|---|
| Standard engineering resin | Rigid, brittle, smooth surface | No special rules — use tables above |
| Tough / ABS-like resin | Higher impact, less brittle | Can reduce wall thickness 10–15% |
| Flexible / rubber-like resin | Shore 40A–80A, elastic | Min 1.5 mm walls; support removal harder |
| Castable / wax resin | Burns clean for investment casting | Min 0.8 mm walls; add casting sprues |
| Dental resin (Formlabs) | Class II biocompatible, ISO 10993 | Min 0.5 mm; post-cure critical for biocomp |
| High-temp resin (HDT > 200°C) | Rigid at elevated temperature | More brittle — increase wall thickness 20% |
Post-Processing Design Considerations
SLA/DLP parts require isopropyl alcohol (IPA) washing to remove uncured surface resin, followed by UV post-cure. Design for post-processing:
- Complex internal geometries must allow IPA to reach and drain from all surfaces — use wash ports 2–3 mm in size
- Very long thin features (aspect ratio > 15:1) will warp during IPA washing or UV cure — redesign with intermediate ribs or reduce aspect ratio
- Painting and priming: SLA surfaces are already smooth (Ra 1–3 µm) and accept paint without primer; light 400-grit sanding before priming improves adhesion
Layer X operates industrial Formlabs Form 3L printers for large-format SLA and Carbon M2 for Engineering and Dental resins. See our full resin capabilities or upload your STL for an instant SLA quote.