PSI - Issue 34

Veronika M. Miron et al. / Procedia Structural Integrity 34 (2021) 65–70 Miron et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Embedded in socio-economic needs to save resources for future generations, additive manufacturing can contribute by placing the minimal necessary amount of material to form the desired structure with maximum integrity. Additive manufacturing (AM), also known as 3D-printing, is to build up a structure layer by layer. This allows more design freedom than conventional manufacturing techniques. Allowing sometimes even the waste-free production of parts, to create complicated freeform shapes, and even to produce topology optimized parts, with the reduction of material where unnecessary – inside the part and outside. With lattice structures load paths can be optimized and structure mechanical properties selectively influenced. For AM of hard and stiff materials, there are established methods available, however for applications where softer and flexible materials are needed, there is still a gap in proven methods to fabricate these additively. With conventional fused-filament-fabrication and stereolithography printing techniques, only hardness levels down to around Shore A 85 are achievable. While PolyJet materials reach Shore A hardness levels down to 26, the material is less durable for long-term applications. Silicone elastomers can be tailored to a broad range of mechanical properties and find application in damping, sealing, biomedicine, mold making, or prototyping. Although, there are several silicone elastomer printing techniques in research, there are still only a few silicone 3D printers commercially available. The technologies discussed in literature for this purpose can be summarized in material extrusion, freeform reversible embedding, vat photopolymerization, and material jetting (Liravi and Toyserkani, 2018; Luis et al., 2018). Newer studies present also experiments with fiber reinforcement for enhancing mechanical (Koushki et al., 2020) and electrically conductive (Davoodi et al., 2020) properties. Amongst others, robotic soft actuators (Schaffner et al., 2018) and meniscus models (Luis et al., 2020b, 2020a) are found as 3D printed demonstrator cases in literature. The softness and durability of the silicone elastomers combined with the ability of producing individual freeform shapes is interesting also for creating anatomical models such as the human brain for surgery training models, where the haptic properties are relevant. The extrusion-based system allows printing with diverse off-the-market silicones. The printing speed is influenced largely by the possible extrusion speed and the crosslinking time. One challenge is the selection of suitable materials not only for the printing process in regard to viscosity and curing time but also in regard to the mechanical properties needed for the application in mind; the other challenge is, to adjust the processing parameters accordingly. Finding printable silicone elastomers, which are already used with other manufacturing techniques, will enable future product developers to have a broader selection of materials while not requiring to cost-intensively produce a batch of material specifically for AM. Currently there is only one silicone elastomer commercially advertised for extrusion-based 3D printing: Silastic TM 3D 3335 (Dow, Midland, Michigan) (further on referred to Silastic). For CAE in product development, it is fundamental to have material models describing the response of the material under different loading states. Putra et al. (2020) analyzed the biaxial behavior and fit material models for extrusion based 3D printed material 737 Neutral Cure Sealant (Dow, Midland, Michigan). They demonstrated that, even though, the homogenous printed films have isotropic properties, the mechanical properties in x- and y- direction can be different for the 3D printed anisotropic porous silicone elastomer parts (Putra et al., 2020). As for Silastic no material models could be found in literature to implement in finite element analyses (FEA) software. This work presents a detailed characterization including material data reduction for Silastic and Dragon Skin 10 Slow. Specimens were 3D printed and tested in uniaxial tension as well as compression, biaxial tension, and pure shear. Then hyperelastic material models were fitted to the experimental results in order to determine the material parameters for FEA The objective is also to compare and analyze the material behavior of these silicone elastomers in order to gain deep insights for product engineering. 2. Methods A customized 2-component silicone printer with the print head vipro-HEAD 3/3 (ViscoTec Pumpen- u. Dosiertechnik GmbH, Töging am Inn, Germany) was built with a print area of 210 mm x 270 mm x 300 mm, a 750 W print bed heating, automatic print bed measurement and levelling and a pneumatic material feed. The silicone rubber’s two components are dosed with a continuous piston system into a static mixer and extruded through a nozzle onto a heated build platform. After each layer, a 1000 W halogen lamp is moved back and forward over the print bed with an adjustable waiting period between and initiates the material crosslinking through heat. For Silastic TM 3D 3335