(Dr. Jonathan Colton, advisor)
"The Effect of Feature Geometry on the Life of Stereolithography Molds"
Stereolithography tooling is a form of rapid tooling that has been used to injection mold limited runs of prototype parts. However, the process is not well understood, and feature life is difficult to predict. Processing conditions, mold design, and geometry affect the number of parts that can be made before a mold fails. The effect of feature geometry on mold life is studied in this research. The experiments in this work used molds of Dupont Somos 7110 stereolithography resin, general purpose polystyrene for the part, and a Sumitomo SG75 Injection Molding Machine to inject the parts.
Initial experiments conducted in molds with multiple features found
that part life increased with smaller heights, larger draft angles, and
lower flow velocity into the cavity. Typical failure mechanisms of
raised features in the molds were flow, pullout, and chipping. A
simple mold with only one feature was created in order to examine theoretical
models developed for the flow and pullout failures. The experimental
results were compared to these models. Flow failures occurred in
taller features due to the force of flow encountered by the feature and
the feature’s geometry. Pullout failure occurred in the shorter features
when the stress from the ejection force (distributed over the bond area)
exceeded the yield strength of the material. Due to the cyclic nature
of the ejection force on a feature, the number of parts that can be made
before failure depends on the stress seen by a feature. In order
to predict the number of parts that a feature could create before breaking
off, a failure factor was created and validated through experimental results.