Draft angle and surface roughness effects on stereolithography molds

Stereolithography (SL) is a rapid prototyping process, which allows one to build complex shapes quickly. Current research investigates the possibilities of using this process to make injection molds. This would allow designers to manufacture and test molds easily and rapidly. One of the main issues with this technique is the effects of its surface on the part. Molds built by SL have high roughness. This gives rise to a high friction force between the part and the mold, and increases the ejection force needed to eject the part from the mold. High ejection forces often lead to damage or breakage of the part and the mold. Research was undertaken on the effects of draft, angle and roughness on ejection forces. It was found that increasing the draft: angle does not necessary assist the ejection of the part. As the draft angle increases, the roughness and hence the friction force between the part and the mold also increase. There is a trade-off between draft angle and roughness. A model based on Glanvill's equation was developed to predict ejection force and was consistent with experimental results.