A Computer Model And Experimental Study Of Laser Photopolymerisation: Application To Stereolithography

Introduction

Stereolithography is an established technique in the new and expanding field of
rapid prototyping. In this process an ultraviolet laser, driven by a 3D-CAD system, is
used to form a solid object by photopolymerizing an acrylate resin. The object is
constructed as a series of layers placed one upon another. The laser, which is focused at
the surface of a vat of liquid resin, traces the cross section of the object at each layer
causing rapid solidification (i.e., polymerization). After completion of a layer the object
is lowered into the liquid resin the amount of one layer thickness and the next layer is'
drawn on top of the previous layer.

One aspect of the stereolithography process that has not received much attention
during initial development of the overall procedure concerns the physical and chemical
changes that occur in and around the small zone of monomer/polymer mixture directly
exposed to laser irradiation, and the rates at which these changes occur. The work
discussed here addresses this aspect of the process.

In previous presentations [refs. 1,2] a mathematical model describing a stationary
laser spot contacting the surface of a photoreactive resin vat was presented. This allowed
computation of light intensity, absorbed light, photoinitiator concentration, mOnomer
concentration, and temperature profiles, as functions of time, for thà exposed reaction
zone and surrounding monomer/polymer mixture. In this paper we discuss a second
model, one capable Of handling the situation where the light spot moves across the
surface of the resin vat in a series of small discrete steps. Some preliminary results
obtained from the moving spot model are presented to illustrate some of its capabilities.
Finally a brief discussion of the experimental verification of the model is presented.