M.S. Thesis Presentation by Thomas Cedorge
Friday, June 4, 1999

(Dr. Jonathan Colton, advisor)

"Stereolitography Mold Design"

Abstract

 The stereolithography (SL) process is a new, yet proven, technique for the rapid manufacture of parts made from CAD.  Compared to traditional methods, SL can build very complex geometries easily and rapidly.  Some trials with SL mold have been made successfully in the injection molding field.  But the feature geometries of the mold were very basic.

The aim of the thesis was to establish SL design rules similar to the ones that already exist for metal molds.  This research investigated mold life as a function of part geometries.  It focused mainly on the study of the feature geometry (draft angle, mold roughness) and SL machine parameters (layer thickness, build style).  As one of the properties of a good mold can be defined by the capacity to eject easily a part from a mold, the ejection force was measured.  Shot after shot, the wear of the mold and its influence on the molded part were observed.

It was shown that the build style had no effect on the surface roughness of the mold.  There was no evolution of the mold roughness as the number of shots increases.  Mold and part deformed elastically during knock out to get rid of the mechanical interlocking.

The part was a perfect print of the mold feature.  The exact same surface roughness of the mold appeared on the part.  The overall quality of the part was fully dependent on the mold and therefore a low roughness mold inputted a smoother surface on the part.

There was a trade-off between draft angle and mold roughness.  As draft angle increased, roughness also increased which led to untraditional results.  For a thin layer thickness, the ejection force decreased as the draft angle increased.  This was an expected result and was consistent with traditional metal tooling.  However, for larger layer thickness (above 102 ?m), the ejection force increased as draft angle increased.  Therefore, two trends existed.  For large layer thicknesses, the ejection force increased as the draft angle increased and for thin layer thicknesses, the ejection force decreased as the draft angle increased.

A mechanical analysis confirmed the experimental trends.  It showed opposite trends between 51 ?m (0.002 in.) and 203 ?m (0.008 in.) whereas 102 ?m (0.004 in.) appeared to be between the two trends.