Toughening of Photopolymers for Stereolithography (SL)

Abstract

Additive Manufacturing (AM) received a lot of attention in the last years. Organizations are using AM systems for a range of applications such as prototypes for fitting an assembly, tooling components, patterns for prototype tooling, functional parts and many more. Nearly a third is applied for functional parts [1][2]. Hence, the SL method provides a smoother surface finish than other AMT[3]. Not only is the smoother surface a benefit but the good precision is also a positive feature. The ongoing development of new material systems and applications make them suitable alternatives for conventional series production like injection molding or machined-core fabrication for foundry use. Small to middle series cores for faucets with quantities from around 50,000 pieces produced using AM methods are already a reality [1]. From the economical point of view, the SL is a cheap and fast process in comparison to AM systems. The SL technology used in this work is based on an active mask exposure, the digital light processing (DLP). The term DLP refers to the digital mirror devices which are used for selectively tuning individual mirrors on and off in order to selectively expose a photosensitive resin with visible or ultraviolet light. The resin contains a photoinitiator which triggers radical polymerization when irradiated with light. The polymerization process leads to a solidification of the resin, leading finally to a solid polymer part [4]. A digital Mirror Device (DMD) chip acts as a dynamic mask to expose a defined area on the bottom of a transparent material vat above the optical system. The generated picture enables layer-wise polymerization of the photosensitive resin resulting in a 3-dimensional object. The light source radiates light with a wavelength of 460 nm which means blue visible light. At this wavelength the curing takes place. At the Institute of Materials Science and Technology at the Vienna University of Technology six generations of these Blueprinter machines have been developed and built to date. The largest parts that a Blueprinter can currently generate are 110 x 110 x 80 mm3 with a resolution of 25 x 25 x 25 μm3. The wall thickness can go down to four pixels which means one tenth of a millimetre.