SLA 3D Printing

How SLA works

SLA builds a part from a tank of liquid resin. When the resin is exposed to light of the right wavelength, its short molecular chains link into a solid polymer. The printer cures one thin cross-section, moves the build platform by one layer, and repeats until the part is complete.

Many desktop machines work upside down. The light source sits below a tank with a clear bottom, each layer is cured against that bottom, and the platform lifts as the part grows, so the finished part hangs from the platform.

Resin printers differ mainly in their light source. Classic SLA traces each layer with a focused ultraviolet beam. DLP printers project a whole layer at once, and MSLA printers, also called LCD printers, shine light through a screen that masks each layer. The steps after printing are the same for all of them. Chuck Hull coined the word stereolithography and patented the process in 1986, and the STL file format was first developed for these machines.

From print to finished part

1. Prepare: print-preparation software sets the part's orientation and adds thin supports under overhangs.
2. Print: the part is built layer by layer on the platform.
3. Wash: the part is rinsed in isopropyl alcohol (IPA) or a similar solvent to remove uncured resin from its surface.
4. Dry: the solvent evaporates before the next step.
5. Post-cure: parts leave the printer in a partly reacted green state, and extra light and heat complete the reaction so the material reaches its full properties.
6. Remove supports: small marks remain where each support touched the part.

While in the green state, the resin can still bond across layers, so a fully cured SLA part is close to equally strong in every direction and is watertight. That is a real difference from FDM, where the bonds between layers stay the weak point.

Layer height and feature size

SLA layers are usually 0.025 to 0.1 mm thick; 0.1 mm is about the thickness of a sheet of paper. Thinner layers give smoother curves and finer detail but take longer: a print at 0.025 mm takes roughly four times as long as the same print at 0.1 mm.

Other printers and resins give somewhat different results, so treat these figures as a guide. For larger features, Formlabs quotes an XY tolerance of about ±0.3%, with a lower limit of ±0.15 mm, on 81 to 150 mm features printed in Grey Resin.

Designing parts for SLA

• Decide which faces matter. Supports leave small marks, so orient the part to keep them off cosmetic and mating surfaces.
• Give fully enclosed hollow parts drain holes so resin and air can escape. A sealed cavity can make the print fail.
• Angle overhangs more than 10 degrees above horizontal, or plan for supports under them.
• Use very thin walls sparingly. They can absorb solvent during washing and deform.
• Leave at least 0.4 mm between surfaces that must move or come apart.

When to choose SLA

Choose SLA when the surface or the small features are what you are testing: presentation models, fine text and texture, small snap features, and smooth housings. Choose FDM for rugged, low-cost functional prototypes. Choose CNC machining when the part must be metal or hold tighter tolerances than a print can.