Influence of the post-curing treatment on the mechanical properties in the vat photopolymerization technique

Among all additive manufacturing processes, vat photopolymerization methods are, by component number, one of the most widely used methods for production of the end use components. The process in Denmark is especially driven by the hearing aid industry (Hopkinson et al., 2006) and has gained the attention of many other fields such as biotechnology and the medical industry (Davoudinejad et al., 2018). Despite the advantages of being highly digitally driven with high precision and providing functional surfaces with fine details, the post-treatment of the vat polymerization process is often manual. This introduces high variability to the end product, depending on how the post-treatment is conducted (Thompson, 2015). Although the individual components from the same batch are commonly handled by the same operator and treated following the similar procedure, the industrial partners have reported a noticeable variability between batches.
One of the predominantly defining steps of post-treatment is post-curing. Unless a complete curing is ensured, a reduction in the crosslink density within the cured photopolymer results in a change in properties over time (Ligon-Auer, 2016). As a result, the long-term stability of the printed parts will be reduced, which influences its efficient use in industrial applications. It is therefore crucial to comprehend the post-curing parameters and their effect on the final part properties. The intent of this work is to demonstrate the effect of the post-treatment in the form of post-curing on the mechanical performance of components manufactured by vat photopolymerization. This study emphasizes the argument of how important it is to apply a stringent methodology for the post-treatment of the additively manufactured components in order for these to meet the necessary qualifications. The investigation of the influence of post-curing to the performance of photopolymer components was carried out by employing two post-curing schemes; thermally and applying UV flood light. The mechanical evaluation was carried out with DMA analysis. It was observed that the final mechanical performance is directly related to the applied method and that the performance is sensitive to variability in post-curing time for both the thermally and UV light-cured components.