Thermoplastic polyurethane (TPU) is a promising polymer with a wide range of practical applications. It was demonstrated that the 3D printing of TPU provides a unique possibility to manufacture customizable, flexible structures that can be designed and optimized for specific energy absorbing applications [12].

TPU is generally a 3D printing material. Its linear polymer chains make it in the form of wire, with flexible and rigid chains connected through covalent bonding. Therefore, it displays flexibility, elasticity, and shock resistance like rubber and thermoplastic. This study used PolyFlex TPU 90 A with a diameter of 1.75 mm, as shown in Figure 2. Because of its properties which are suitable to produce the shoe insole.

Generally, TPU has a high elongation and tensile strength. Compared with other rubber-like elastic materials, TPU has an excellent compressive strength. Like ordinary thermoplastic, they can be used in injection, blow, and compression moulding applications. These elastomers have low-temperature flexibility. It has excellent abrasion and tear resistance and high hardness [13]. In addition, they are highly resistant to ozone, aliphatic solvents, and petroleum-based fuels and oils.

Additive manufacturing refers to a group of technologies that can quickly and

easily convert virtual solid model data into physical models by adding material. AM will make benefit in many ways. The manufacturing process can be carried out straight from the 3D design, eliminating the need for any intermediate process or tooling, such as the usage of injection moulds. This process shortens the production cycle and enables design teams to create high-quality prototypes in a matter of hours [14]. It also makes it easier to produce small-batch series or customized products, such as insoles. The unique capability of AM includes shape complexity which is easy to produce virtually any shape by avoiding the approximations imposed by the machining process [15].

3D printing using Fused Deposition Modelling (FDM) is one of the most used additive manufacturing techniques [16] [17]. These printers can produce items by heating and extruding a thermoplastic filament that deposits layer upon layer. The widespread usage of this system has been recently boosted by the expiration of the FDM patent and the subsequent worldwide development of low-cost machines by a huge number of companies [18].

The FDM-printer used in this study is a “Creality ender 3V2” shown in Figure 3. It is equipped with a carborundum glass platform. This platform enables the build plate to be heated up quickly, and the molten filaments to adhere to the build plate well without warping. The printing size of this 3D printer is 220 × 220 × 250 mm, and the main advantage of this FDM 3D printer is that it is user-friendly [19].

Slicing is one of the main steps in FDM where 3D CAD model is virtually converted into layers and then G-codes. The quality of the sample is based upon the layer height. To achieve the smoothest walls, a fine print setting of 0.12-mm layers

[20] were used in this study, perimeter as “1”, top and bottom layers were taken as “0”. According to the compressive strength, different infill patterns were taken at different regions of the insole, as shown in Figure 4.

In this study, four infill patterns (A-Triangle, B-Rectilinear, C-Gyroid, and D-Cubic) were used, as shown in Figure 5. Generally, the higher the infill percentage, the higher the object’s strength. According to the requirement, an infill percentage should be chosen. This study focuses on three infill percentages (25%, 40%, and 60%) used at different insole locations. Infill has various factors in the slicer. Among those factors, fill patterns, fill density, and fill angle are very important.

In this study, the design of the insole was taken from the “Gensole Software”. In Gensole, modification of the shoe insole is easy. By just dragging the points, modification can be done. Height, length, and width of the insole can be modified easily according to the size of the leg. These insoles are generally for the person with all type of feet. Some people have problems (flat feet, high arch, and very high arch) with the foot’s shape. This insole can be modified easily by placing

the STL file of the leg on the insole. After inserting the leg file on the insole, the pressure of the foot can be seen in the form of colors (red, orange, and yellow). This can be seen in Figure 6. The red color indicates more pressure, and this

region can be divided into parts as shown in Figure 4. The infill patterns and infill percentages in this region can be decided with the better compressive strengths which can be taken from the compressive test. Table 1 shows the specifications of the insole before and after the placement of the STL file leg.

Compressive testing demonstrates how a material will react when compressed. Compression testing can identify a material’s behaviour or response under crushing loads and evaluate its plastic flow behaviour and ductile fracture limits [21] [22]. Compression tests are important for determining brittle or low-ductility materials’ elastic and compressive fracture properties.

Compression tests are also used to determine the modulus of elasticity, the proportional limit, the compressive yield point, the compressive yield strength, and the compressive strength. These qualities are vital in determining whether a material is suitable for a particular application or fails under specific stresses. Compression tests were done on the Universal Testing Machine (UTM) from Zwick. The following compression tests were conducted according to DIN ISO 604.

Here, with high compression modulus values infill percentages were selected as compared with the weight.

60% Infill

A standard cylindrical specimen of 20 mm diameter and 30 mm height was taken. First, 60% infill was used for the test and then compared among four infill patterns. Triangle and rectilinear have better results when compared to other patterns. (Graph 1)

40% Infill

Now, 40% infill was used for the test and compared all the patterns. Gyroid and triangle infill patterns have better results when compared to other patterns. (Graph 2)

25% Infill

For 25% infill density, gyroid and triangle infill patterns showed better properties in terms of compressive modulus and weight. (Graph 3)

Here, with high compression strength values infill patterns were chosen as compared

Graph 1. Comparison of compressive modulus of different patterns with 60% infill.

Graph 2. Comparison of compressive modulus of different patterns with 40% infill.

with compression strain.

60% Infill

A standard cylindrical specimen with a diameter of 20 mm and a height of 30 mm was used. In this case, 60% infill was utilized for the test and compared with all the patterns. Rectilinear and triangle infill patterns gave better results. (Graph 4).

40% Infill

Here, 40% infill was used for the test and then compared among four infill patterns. Comparing all the patterns, triangle and rectilinear were best with this infill. (Graph 5)

25% Infill

In this test, all patterns were compared using a 25% infill. Finally, with this infill,

Graph 3. Comparison of compressive modulus of different patterns with 25% infill.

Graph 4. Comparison of compressive strength and compressive strain with 60% infill.

Graph 5. Comparison of compressive strength and compressive strain with 40% infill.

Graph 6. Comparison of compressive strength and compressive strain with 25% infill.

triangle, gyroid, and rectilinear were the best among all the patterns. (Graph 6)