Vat photopolymerization builds a part by selectively exposing liquid photosensitive resin to light so that chosen regions polymerize into solid material.

Each layer requires two coupled operations:

1. define the spatial exposure pattern;
2. deliver enough optical dose to cure the intended depth without excessive cure beyond the boundary.

A simple exposure measure is energy per area,

$$E=It,$$

where $I$ is optical intensity and $t$ is exposure time.

If intensity is $8\ \mathrm{mW/cm^2}$ for $5$ s,

$$E=40\ \mathrm{mJ/cm^2}.$$

Higher exposure can increase cure depth but also broaden features or lock material outside the intended geometry. Insufficient exposure can leave weak or incomplete layers.

The process can achieve fine detail because energy is delivered optically rather than through a relatively large extrusion nozzle. However, cured parts usually require cleaning and additional post-cure to complete the desired polymer network.

Supports may be needed to hold islands or resist peel forces during layer separation. Resin chemistry also matters: the printed material is a crosslinked photopolymer whose stiffness, toughness, temperature resistance and aging behavior may differ greatly from common thermoplastics.

Vat photopolymerization therefore couples optics, reaction kinetics, layer separation and polymer-network properties.