DESIGNING FOR SLA ON THE FORM 4
SLA resin captures detail FDM can't touch, but the design rules are different. This guide covers the feature sizes and dimensional tolerances we hold on the Formlabs Form 4, so your part prints right first time. Figures are drawn from the Formlabs Form 4 datasheets and our own experience — send us your model and we'll flag anything that needs a second look before printing.
MINIMUM FEATURE RULES
The smallest features the process reliably reproduces, per the Formlabs Form 4 Design Guide. Treat these as floors, not targets — a little more material always prints more reliably.
| FEATURE | MINIMUM | NOTES |
|---|---|---|
| Wall thickness | 0.2 mm | Thinner walls print but grow fragile; use 1mm+ on parts that get handled. |
| Embossed (raised) detail | 0.1 mm high | Raised text and logos read cleanly at this height. |
| Engraved (recessed) detail | 0.15 mm | Recessed text and grooves — go slightly wider than embossed. |
| Hole diameter | 0.5 mm | Small holes can seal with resin; drain holes help clear them. |
| Clearance between mating parts | 0.4 mm | Gap for parts that must slide, snap or assemble after cure. |
| Unsupported bridge | up to 29 mm | Spans beyond this need a support or a redesign. |
| Free-standing wire / pin | 0.3 mm dia | Thinner pins tend to bend or break in wash and cure. |
TOLERANCE BY PART SIZE
SLA tolerance scales with part size — a percentage of the dimension, not a fixed number. On the Form 4 we hold the following bands. A 20mm feature lands inside ±0.05mm; a 120mm feature inside ±0.36mm.
| LARGEST DIMENSION | TOLERANCE |
|---|---|
| Up to 30 mm | ±0.15% (min ±0.05 mm) |
| 31 – 80 mm | ±0.2% |
| 81 – 150 mm | ±0.3% |
Bands are the measured Formlabs Form 4 tolerances. The measurement floor is around ±0.02mm, but we quote to ±0.05mm to stay on the safe side. Real-world tolerance depends on geometry, orientation and post-cure — tolerance-critical parts should be flagged so we can advise on orientation and design-for-print before we print.
ORIENTATION, SUPPORTS & CURE
Parts print at an angle to cut suction forces and hide layer transitions. The down-facing side takes the support marks — tell us which face is cosmetic or critical and we'll orient around it.
Overhangs need support touchpoints, which leave small nubs we clip and sand. Design in flat bases or sacrificial tabs where you can, and keep critical surfaces off the support side.
Large solid parts can be hollowed with 2mm+ walls and a pair of drain holes — saves resin, cuts cost, and reduces the internal stress that causes warping.
Give assemblies at least 0.4mm of clearance. Resin can bond mating faces if they touch, and a light post-cure shrink tightens close fits.
Every part is washed and UV-cured as standard. Curing brings full hardness but adds slight shrinkage — account for it on press-fits and threads.
Not sure a wall or pin will survive? Upload the model for an instant quote and we'll review the geometry, or ask us first — we'd rather fix it in CAD than in the bin.
QUOTE YOUR SLA PART
Upload your STL or STEP, select SLA, and get an instant price. We'll flag any features below the rules above before anything prints.