Evaluation of UV post-curing depth for homogenous cross-linking of stereolithography parts

Abstract

Purpose During post-processing of stereolithography photopolymers, the limited penetration depth of ultraviolet (UV) light can lead to inhomogeneous cross-linking. This is a major problem in part design for industrial applications as this creates uncertainty regarding the mechanical load capacity. Therefore, this paper aims to present an experimental method to measure the post-curing depth in stereolithography photopolymers. Design/methodology/approach Printed specimens made from urethane acrylate photopolymers are placed in a protective housing and are exposed on one side to UV light during post-processing. A depth profile of the hardness according to ASTM D2240 Shore D is determined alongside the specimens. UVA,-B and -C spectra are investigated and the dependence on exposure dose and pigmentation is studied. The results are directly linked to the mechanical properties via tensile tests and validated on an automotive trim part. Findings Exposure with a 405 nm light-emitting diode provides the deepest homogenous post-curing depth of 10.5 mm, which depends on the overall exposure dose and pigmentation. If the initially transparent photopolymer is colored with black pigments, post-curing depth is significantly reduced and no homogenous post-curing can be achieved. To obtain comparable mechanical properties by tensile tests, complete cross-linking of the specimen cross-section has to be ensured. Research limitations/implications The spatial resolution of the presented measurement method depends on the indenter size and sample hardness. As a result, the resolution of the used setup is limited in the area close to the edges of the specimen. Originality/value This paper shows that the spatially resolved hardness measurement provides more information on the post-curing influence than the evaluation of global mechanical properties. The presented method can be used to ensure homogenous cross-linking of stereolithography parts.