Invented in the late 1980s, Vat Polymerization (VP) was another new development that led to more 3D printing. Charles Hull discovered that using a scanning laser could create solid patterns in liquid polymer.
Vat Polymerization uses radiation (usually UV light) to harden/cure liquid resins. These resins are called photopolymers.
There are three major configurations for VP, as far as we’re concerned. They are:
Vector Scan - use a mirror, direct a laser, trace a pattern. This becomes a layer.
Mask Projection - use a optic system to shine the pattern on the surface. This becomes a layer.
Two Photon -use two lasers, when they point and meet at a spot on the liquid surface, polymerization happens, this can eventually develop a layer (for high resolution)
The type of materials used in VP are a combination of epoxies and acrylates.
Acrylates are used due to their high reactivity to light radiation. However, acrylates also shrink around 5–20% (bad part size), tend to only partially cure (a layer above means you’re still cooking the polymer layer underneath), and are bad with oxygen inhibition (more shrinking, and residual stresses can form).
Epoxies cross-link as polymers, so they are already stronger. They also will shrink less than acrylates (epoxies shrink around 2%). For an entire layer, this means less curling up at part corners. However, epoxies are brittle relative to acrylates, and epoxies don’t react as well to the UV (or other) light to become solid polymer. Epoxies are sensitive to humidity.
The idea, then, is to mix the two. Epoxy + Acrylate gives a better composition that has a little bit of both material pros.
Photoinitiators
Photoinitiators increase the build speed because they are intermediates (not the polymer) used to get the resin to solidify. Too much photoinitiator chemicals and chains will have trouble forming (lack of polymer molecules!) and too little is wasted time that adding a photoinitiator intermediate would have saved.