Photopolymerization in Liquid-based 3D Printing

Photopolymerization is a chemical reaction in which a liquid photosensitive resin (monomers and oligomers with a photoinitiator) solidifies into a cross-linked polymer when exposed to light of a suitable wavelength (usually UV, about 355 to 405 nm).

Process steps

1. The platform is placed one layer height below the resin surface.
2. A UV laser (SLA) or projector (DLP, LCD) cures the cross-section of the layer.
3. The platform moves by one layer thickness and fresh resin flows over the part.
4. The steps repeat until the part is complete.
5. The part is washed (isopropyl alcohol) and post-cured under UV.

Processes using it

Process	Light source
SLA	UV laser scanning point by point
DLP	Projector, whole layer at once
LCD / MSLA	LED with LCD mask
PolyJet	Jetted droplets cured by UV lamp

Key factors

Exposure energy, cure depth ($C_d = D_p \ln(E/E_c)$), layer thickness, resin viscosity and wavelength.

Features

High accuracy and smooth finish; brittle parts, resin toxicity, supports and post-curing needed.

Exam tip

Explain the chemistry in 2 lines, give the layer cycle and list SLA, DLP, PolyJet.