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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Satisfying fits for material models were achieved with the second- and third-degree Ogden models (R2=0.952) as well as the Van der Waals model (R2=0.949). The material model parameters are given in Table 1 and can be used as input in Abaqus using the mm/N/s/K unit system. While the Van der Waals model is stable for all strains, the Ogden models are stable for strains for uniaxial loading between -0.42 and 0.63, for biaxial loading between -0.22 and 0.31, and for planar loading between -0.35 and 0.55.

Table 1: Hyperelastic material model parameters for 3D printed Silastic. Material Model R2 Fitness Material Parameters Ogden, N=2 0.952 µ 1 = 1.1838 α 1 = 2.3585

D 1 = 0 D 2 = 0

µ 2 = -0.3658 µ 1 = 0.94705 µ 2 = 0.236737 µ 3 = -0.36577 µ = 0.80846

α 2 =-2.472

Ogden, N=3

0.952

α 1 = 2.358312 D 1 = 0 α 2 = 2.359143 D 2 = 0

α 3 = -2.47222

D 3 = 0

Van der Waals

0.949

λ M = 12.56182 α = -0.03117 β = -0.60985 D = 0

4. Conclusion and Outlook Extrusion based 3D printing of temperature initiated crosslinking silicone rubber allows to create parts in a wide variety of apparent hardness . Apparent hardness is a combined effect of the material’s intrinsic hardness and its porosity, thus reflecting the component hardness that reaches beyond the material hardness. The adjustability of the infill amount enables engineers to select a material normally applied only in a small sector for parts with specific hardness requirements (e.g. Silastic with Shore A 50) and apply it over a broader range towards lower hardness (e.g. down to Shore 00) (Figure 3).

Figure 3 : The application range of silicone elastomers connected to a materials’ intrinsic hardness and the apparent hardness due to adjustable infill amount of 3D printed parts.

Utilizing lattice structures and load path optimization the desired structural compliance can be achieved. As design limitation it should be kept in mind that the lower apparent hardness limit is reached, when the infill structures are insufficient to carry the outside perimeters and top layers (e.g., in our experience to ensure that the top layers are closing with Silastic, the grid infill amount should not be lower than 15%, at 0.25 mm layer height printed with a 0.4 mm nozzle). By selectively extruding material, where it is necessary to ensure structural integrity of the part, the advantage of 3D printing is owned also for saving material. Not only can the inner and outer topology of parts be optimized regarding the overall mechanical properties of parts but minimizing the use of material allows to save resources. For engineering applications this opens up the possibilities to use stiffer materials and create parts with lower infill density to achieve the same overall part performance that otherwise would require a more compliant material. Further on, the independency of the mechanical properties of strain rate at ambient temperature is useful to predict the reaction of the part for applications where the loading rates are unknown or range from low to high loading rates.