Resin and Powder Processes
Resin (MSLA) and powder (SLS, MJF) printing: how they work, settings, washing and curing, and what they cost.
How resin (MSLA) printing works
A resin printer builds a part upside down from a plate that dips into a vat of liquid photopolymer. A UV light (405 nm) behind an LCD screen that shows each layer as a black-and-white mask cures the resin where the pixels are clear; the plate then lifts, peeling the layer from the FEP film at the bottom of the vat, and drops to cure the next layer. Because a whole layer is exposed at once, the print time depends on the part’s height, not its area, and the detail depends on the pixel size of the screen.
Worked example: resolution and time
- XY resolution. A 4K mono screen of 3 840 pixels across a 192 mm width has a pixel of 192 ÷ 3 840 = 50 µm. Features finer than two or three pixels (0.1 to 0.15 mm) do not print reliably.
- Layers and time. A 50 mm tall print at 0.05 mm layers has 1 000 layers. With an exposure of 2.5 s and about 9 s of lift, peel and retract per layer (11.5 s in total), the print takes 1 000 × 11.5 ÷ 3 600 = 3.2 h however many parts fill the plate. Packing the plate with parts makes resin printing cheap per part.
Exposure, cure depth and layer height
Cured depth follows the working curve of the resin: Cd = Dp ln(E ÷ Ec), where E is the exposure (light intensity times time, in mJ/cm²), Ec the critical exposure below which the resin stays liquid, and Dp the penetration depth of the light into the resin. A layer must be cured deeper than its thickness (typically 1.5 to 2 times) to bond to the layer below.
Worked example: setting the exposure
- The resin has Dp = 0.15 mm and Ec = 6 mJ/cm² (typical orders of magnitude); the printer delivers I = 4 mW/cm² at the film. An exposure of 2.5 s gives E = 4 × 2.5 = 10 mJ/cm², so Cd = 0.15 × ln(10 ÷ 6) = 0.077 mm.
- A 0.05 mm layer needs Cd ≥ 0.05 mm at the very least, which gives t = (Ec ÷ I) × e(layer ÷ Dp) = (6 ÷ 4) × e(0.05 ÷ 0.15) = 2.09 s; for good bonding Cd = 2 × layer gives 2.92 s. The setting of 2.5 s gives 1.5 times the layer thickness, between the two.
- Overexposure makes features swell, holes close and fine details fill in; underexposure makes weak, soft layers that delaminate and prints that fail to stick to the plate. A calibration print (an exposure test with tiny holes and pins) finds the right value for each resin.
- Bottom layers get an exposure of about 8 times the normal exposure (here 20 s) for 4 to 8 layers, to make them stick to the plate.
| Setting | Typical value | Notes |
|---|---|---|
| Layer height | 0.025 to 0.1 mm (0.05 mm common) | Thinner layers give a smoother surface and take more time |
| Normal exposure | 1.5 to 4 s per layer | Depends on the resin and the light power: always calibrate |
| Bottom layers and exposure | 4 to 8 layers; 20 to 60 s | For adhesion to the plate |
| Lift distance and speed | 5 to 8 mm; 1 to 3 mm/s on the first part of the lift | Slow lift reduces peel force (suction); the film must release |
| Temperature | 25 to 30 °C resin | Cold resin is thick and fails to flow; a warm room or a heater helps |
The resin workflow: print, wash, cure
- 1. Print. Filter the resin through a paper or mesh filter when it is returned to the bottle (cured bits cause failures). Check that the plate and film are clean and the vat is free of hardened pieces.
- 2. Remove the part with a scraper, wearing nitrile gloves; the part is still coated with uncured resin.
- 3. Wash in isopropyl alcohol (IPA, 90 percent or higher) or a water-washable resin wash, for 3 to 10 minutes in a washing station or with agitation, in two baths (a dirty then a clean one). Do not leave a part in IPA for long: it makes thin parts brittle and cloudy.
- 4. Dry and remove supports (they come off more easily before the final cure).
- 5. Cure under a UV lamp (405 nm) for 2 to 15 minutes (follow the resin maker’s time), turning the part. Under-cured parts are sticky and weak; over-cured parts yellow and become brittle.
- 6. Dispose of waste safely. Cure leftover resin and wash fluid in sunlight or under UV, then dispose of the solid as the maker directs; do not pour liquid into a drain. Used IPA with dissolved resin is a hazardous waste in many places.
Designing and orienting for resin
- Tilt the part by 30 to 45° and print it on supports: each layer then has a small cross-section, the peel force is lower, and suction cups and flat faces are avoided.
- Supports: light supports at overhangs and islands (the first layer of a new feature); medium supports for heavy parts; keep supports away from critical surfaces and on the lower side.
- Hollow large parts and add drain holes (at least 2 to 3 mm diameter, two holes at opposite ends and at the lowest and highest points) so that trapped resin escapes and the vacuum does not cup and burst the part. A solid large part costs more resin and may crack from internal stress.
- Avoid large flat horizontal faces on the plate or at the top: they cause large suction and peel forces; tilt them.
- Wall thickness: at least 0.6 to 1 mm for strength; thin walls under 0.4 mm break. Resin parts are strong in compression but brittle in tension and impact.
- Accuracy: parts are accurate to ±0.05 to ±0.15 mm after calibration, but shrink slightly (about 0.5 to 3 percent, mostly during the cure) and can warp if cured too long.
- Holes and fits: print 0.1 to 0.2 mm oversize clearances; hole diameters are typically 0.05 to 0.1 mm undersize (overexposure).
Resin types
| Resin | Properties | Use |
|---|---|---|
| Standard | Fine detail, rigid and brittle | Models, miniatures, prototypes |
| ABS-like / tough | Less brittle, higher impact strength | Functional parts, snap fits |
| Water-washable | Washes in water; usually a little weaker | Simpler cleaning (still a skin irritant) |
| Flexible / elastic | Rubber-like | Gaskets, grips, soft models |
| Castable | Burns out cleanly in investment casting | Jewelry and metal casting patterns |
| High-temperature and ceramic-filled | Withstands 150 to 250 °C (after post-cure) | Molds, heat-resistant parts |
| Dental and biocompatible | Certified for specific medical uses | Only for approved printers and processes |
Worked example: resin cost
A part of 20 cm³ plus 30 % for supports and the raft is 26 cm³, or 26 × 1.1 = 28.6 g. At $30 per kg that is $0.86 of resin, plus the wash fluid, gloves, FEP film and IPA, which usually cost about as much again. The plate holds many parts, so print several at once.
Resin safety
- Uncured resin irritates the skin, can cause allergic sensitization after repeated contact, and is toxic to aquatic life. Wear nitrile gloves (change them if contaminated), eye protection and a lab coat or apron; never touch resin with bare hands.
- Ventilate. Resins and IPA emit vapors: print in a ventilated room or a vented enclosure, not a bedroom or a living space, and keep the printer away from food.
- Spills: wipe with paper towels and IPA, cure the towels in sunlight or UV, then bin them; do not wash resin down a drain.
- Eyes and UV: never look at the UV light, the lamp or a curing station without the lid or protective eyewear.
- IPA is flammable: keep it away from sparks and heat, store it sealed, and use the smallest quantity.
- Read the safety data sheet (SDS) of the resin and follow its first aid advice.
- See Workshop Safety and Troubleshooting and Safety.
Powder processes: SLS and MJF
Selective laser sintering (SLS) sweeps a thin layer of nylon powder (usually PA12, layers of about 0.1 mm) over a heated bed and a laser sinters the part’s cross-section. Multi Jet Fusion (MJF) from HP prints a fusing agent where the part is, and infrared lamps fuse it. In both, the surrounding unsintered powder supports the part, so there are no supports, the parts can be nested in three dimensions, and the strength is nearly the same in all directions. After printing the parts are dug out, cleaned by blasting, and optionally dyed, tumbled or vapor smoothed.
- Strength and detail: nylon parts with 40 to 50 MPa tensile strength, good toughness and elongation, a slightly grainy surface (Ra 6 to 12 µm) and tolerances of ±0.2 to ±0.3 mm (or ±0.3 % on larger parts).
- Design rules: walls at least 0.7 to 1 mm; escape holes (at least 4 to 5 mm) so that powder can be removed from hollow parts; clearance of 0.5 mm or more between moving parts that print assembled; no trapped volumes.
- Powder reuse: unsintered powder ages with heat, so each build uses a mix of fresh and recycled powder; the refresh rate is typically 30 to 50 percent fresh powder, which sets the material cost.
- Safety: fine nylon powder is a dust and combustible hazard, so the machines need closed handling, extraction and respiratory protection: this is why SLS is rarely a home process.
- Metal powder processes (DMLS, SLM) fuse metal powder with a laser in an inert atmosphere: used for aerospace and medical parts, with supports to anchor the part to the plate and conduct heat, and heat treatment afterwards.
Ordering parts from a service
| Need | Order | Notes |
|---|---|---|
| Strong functional nylon parts in small batches | SLS or MJF PA12 (or filled PA) | No supports; good surface; typical lead time of days; cost scales with volume and nesting |
| Fine detail or a smooth surface | SLA / MSLA resin from a service | Choose a tough or a rigid resin; check the post-cure |
| Metal parts | DMLS / SLM or binder jet | Costly; plan for machining of critical faces and heat treatment |
| Large FDM parts or engineering plastics (PEEK, ULTEM) | Industrial FDM service | Large build volumes, certified materials |
| Cast or molded parts later | Print a pattern and cast or mold | Printed patterns are good for urethane casting and investment casting |
- Send a good model: a watertight STEP or STL in millimeters, with a drawing for critical dimensions and tolerances (see GD&T).
- Quotes depend on volume, bounding box, material, finish, quantity and delivery; compare services on the same basis.
- Ask for the process capability (tolerances, wall thickness limits, surface finish) before you design to it.
FDM, resin and powder compared
| FDM | Resin (MSLA) | SLS / MJF | |
|---|---|---|---|
| Machine cost | Low | Low to medium | High (service) |
| Detail | Medium (0.1 to 0.3 mm layers) | High (pixel 0.03 to 0.05 mm) | Medium to high |
| Strength | Good, directional | Brittle | Good, near-isotropic |
| Surface | Layer lines | Smooth | Slightly grainy |
| Supports | Often | Always | None |
| Post-processing | Light | Wash and cure; messy | Depowdering, dyeing |
| Best for | Functional prototypes, large parts, low cost | Detailed models, small parts, molds | Production-grade nylon parts |
Related
- 3D Printing and Additive Manufacturing: the overview of this guide.
- Processes and Workflow: the processes compared.
- Designing for 3D Printing: design rules for FDM; many apply to resin too.
- Post-processing and Finishing: sanding, painting and smoothing printed parts.
- Workshop Safety: gloves, ventilation and chemical handling.
FAQ
Is resin printing safe at home?
It can be, with gloves, eye protection, good ventilation, a proper wash and cure station and safe disposal, but uncured resin is a skin irritant and sensitizer and the vapors are unpleasant. Keep it out of living areas and away from children and pets.
Can I wash resin parts with water?
Only water-washable resins. Standard resins need IPA (90 % or higher) or a dedicated cleaner, and washing waste must be cured and disposed of correctly.
Why did my resin print fail?
Common causes: too little exposure on the bottom layers, a dirty or damaged film, cold resin, too-fast lift, an unsupported island, or a large flat surface causing suction. A calibration print and a clean vat fix most failures.
When should I order SLS instead of printing myself?
When you need strong, near-isotropic nylon parts, complex shapes with no supports, or batches of a few to hundreds, and the machine cost of SLS is not justified.
References
- Jacobs, P. F., Rapid Prototyping and Manufacturing: Fundamentals of Stereolithography, SME (working curve).
- Resin makers’ technical data sheets and safety data sheets (Elegoo, Anycubic, Formlabs, Siraya Tech) for exposure and cure data.
- HP Multi Jet Fusion and EOS / Formlabs SLS design guides (wall, escape-hole and clearance rules).
- ISO/ASTM 52900, Additive manufacturing: general principles and terminology.
Last reviewed: October 2026. Resin constants, exposure times and costs are illustrative typical values: calibrate for your own printer and resin, and read the resin’s safety data sheet.