Page 176 - IJB-7-4

3D Printing Custom Shoe Sole

Figure 2. A schematic workflow of custom 3D-printed midsole production.

process starts with receiving the foot shape of an s n
individual using scan or even the shoe size. Then, the W = ∑ i i 1 − 3) 3 + ∑ DI( i 1 − 3) (1)
CI(
i
lattice of different shapes, for example, three here, are = i 0 is
= +1
designed and generated in computer-aided design (CAD)
software. It should be noted that the cell size of lattices
is chosen arbitrarily while their effect is considerable W = aI + bI 4 + cI (2)
for a further study to deliver more regional stiffness in 1 1 2
sole as per individual specifications, such as diabetic
patients. Next, a simulation is carried out in an FEA The viscoelastic behavior of EPU40 for 3D printing
platform called ABAQUS, considering the nonlinearity of midsole is characterized at different strain rates and
and viscoelastic properties of the 3D printing material it was used in this work to simulate the results having
to reflect the stress distribution on the midsole surface in validated using FEA in ABAQUS. To find the stress-
contact with plantar subjected to increasing, downward- strain relationship in quasi-static state, homogeneous
37]
directed displacement, which leads to contact with uniaxial tensile test with low strain rate were conducted .
the rigid ground surface and compression of the Furthermore, to confirm the ability of traction, that is,
lattice. Finally, the desired lattice providing less stress high elongation in the failure of EUP40 in the set of
compatible to the user application, that is, walking or experiments, the strain rates of 0.032/s, 0.128/s, and
running, are suggested for 3D printing. 0.576/s at speeds of 50 mm/min, 200 mm/min, and
900 mm/min were conducted, respectively, to achieve
2.2. Materials preparation and characteristics high elongation.
DLS technology is a new printing method for 3D printing The validation of stress-strain results at different
of soft polymers. Elastomeric polyurethane (EUP40) is a strain rates are shown in Figure 3. According to this
type of soft polymer that can be printed by this method. figure, increasing the strain rate has increased the
This material has an elongation length of about 275%, stress in the same strain by 100%. This behavior of
shear strength 23 kN.m, shore hardness 68A, and T (glass material indicates that models (1) and (2) are suitable
g
transition temperature) 8°C . These properties have led to use to understand the behavior of the midsole
[37]
to EUP40 being classified as a rubber-like viscoelastic material. Finally, after performing quasi-static tests,
material. For this reason, the neo-Hookean and Yeoh’s cyclic tests, relaxation tests, and experimental study of
rubber-like model as well as Carol are used to study the material behavior, the parameters of the two models are
behavior of the material as: presented in Table 1.

172 International Journal of Bioprinting (2021)–Volume 7, Issue 4