3D Printing Transparent Photosensitive Resin vs Plexiglass (Acrylic)

A photosensitive resin starts as a liquid: oligomers, a photoinitiator, and reactive diluents. A laser or LCD projector cures it layer by layer, typically 25 to 100 μm per pass. The part comes off the build plate, gets washed in IPA, then post-cured under UV. Clear grades are formulated so the cured polymer scatters as little light as possible, but layer lines and residual photoinitiator still cost you some transmission.

Acrylic sheet is polymerized in a mold at the factory, so the optical quality is set before you ever touch the part. Cast sheet has the best clarity and the lowest internal stress. Extruded sheet is cheaper and more dimensionally consistent, but it machines with more risk of stress crazing. You buy a panel, then cut, drill, or mill it to shape.

That difference decides the geometry you can make. Additive builds internal channels, undercuts, and lattice without extra setup. Subtractive needs a tool to reach the surface, so a hollow sphere or a curved light pipe is hard and often impossible in one piece.

• 1
  Additive routeGood for organic shapes, internal cavities, one-off prototypes.
• 2
  Sheet routeGood for flat panels, windows, lenses, and any prismatic part.
• 3
  HybridPrint near-net, then machine the critical faces to tolerance.

Clarity in a resin print is a finishing problem more than a material problem. As-printed surfaces show layer stair-stepping, which scatters light at every step. Sanding from 400 grit up to 2000 grit, then a polishing compound, brings transmission close to acrylic on flat or gently curved faces. Deep internal features stay hazy because you cannot reach them.

Acrylic arrives clear and stays clear if you protect the surface. Machined edges come out frosted, so you flame-polish or vapor-polish them when the edge is in the light path. Cast sheet gives 92% light transmission across the visible range, and it holds that number over years of indoor UV exposure better than most clear resins, which yellow as unreacted photoinitiator breaks down.

For a light pipe or a lens, acrylic wins on transmission and on scratch resistance. For a hollow diffuser with internal ribs, the resin print is the only practical option. Ask what the light path looks like, not just what the part looks like.

A well-tuned resin printer holds about ±0.1 mm on a 100 mm part, and shrinkage during post-cure can pull that wider. Thin walls warp. Long unsupported spans sag. You compensate by orienting the part and adding supports, which means the same file can print differently on two machines.

Machined acrylic is predictable. We hold ±0.005 mm on critical features, and flatness on a 300 mm panel stays within a few hundredths of a millimeter if the sheet is stress-relieved before cutting. Acrylic has a high coefficient of thermal expansion, roughly 70 × 10⁻⁶ /°C, so a 500 mm panel moves about 0.35 mm over a 10 °C swing. Design a slotted mount instead of a bolted hard point.

Both materials creep under sustained load. Acrylic crazes around a tight fastener or a press-fit insert. Resin cracks at support nubs if you load them in tension. Neither is a structural plastic, so treat both as cosmetic and optical parts.

For a single part, printing wins on setup. There is no sheet to buy, no fixture to make, and no programming time. The cost is machine time plus hand polishing, and polishing is where the hours go on a clear part. A palm-sized lens housing can take 30 to 60 minutes of wet sanding per unit.

Machined acrylic wins once the geometry repeats. Programming and fixturing are one-time costs, then each part is fast cycle time. A flat window or a simple bracket gets cheap by the tenth piece. Complex 3D contours bring the cost back up because you pay for 5-axis time.

There is a middle path that often beats both: print the shape, then machine the optical face. The print gives you the internal geometry for free, and the mill gives you a flat, polished surface that meets a real tolerance. We run both processes under one roof, so the handoff is a work order, not a shipping label.

Pick the resin print when the part has internal channels, undercuts, or a shape no cutter can reach. Pick it when you need one or two units tomorrow and the clarity target is cosmetic. Pick it when the part is small and the wall is thin.

Pick machined acrylic when the part is prismatic, when flatness matters, or when you need a tolerance tighter than ±0.05 mm. Pick it for outdoor or UV-exposed parts, and for anything that gets handled, since acrylic resists scratching far better.

Pick neither when the part carries load, sees high temperature, or needs solvent resistance. PMMA and clear resin both fall short there. Polycarbonate machines well and takes impact, though it is softer and scratches more easily than acrylic.

If you are unsure, send the STEP file and describe the light path. We will tell you which route holds the tolerance you need, and where the cost sits.