# Intermediate Model Design in the Progressive Stamping Process of a Truss Core Lightweight Panel
## 2 Materials and Methods
### Verification of the Press-Forming Performance  
To verify the forming performance of the intermediate model of the regular hexagonal pyramid proposed in the previous section when applied to the progressive press-forming method, the FEM (finite element method) analysis model created is shown in Figure 8. The analytical model comprises three parts: the punch (red), die (green), and formed plate material (light blue). Figure 7: Intermediate model in progressive stamping method- the process of parametric design. Three rows of punches and dies, including the pre-forming, positioning, and final forming rows, were set up along the feeding direction of the plate. The punches and dies were modeled using 4-node rigid shell elements with an average side length of 1.0 mm, and the formed plate was modeled using 4-node elastic-plastic shell elements with an average side length of 1.0 mm. The number of elements in the plate was 42,000 and the number of nodes was 42,411. The plate thickness is 1.0 mm, yield stress is 235 MPa, [PERSON]'s modulus is 186.5 GPa, and Poisson's ratio is 0.3. The true stress and plastic strain diagram is shown in Figure 9. The forming load was applied by forced displacement. Conversely, to verify the formability of truss core panels using the proposed intermediate model of regular hexagonal pyramids, the progressive press-forming die shown in Figure 10 was fabricated and actual forming experiments of truss core panels were conducted. Figure 10 shows a trial-forming test using a manual 30-ton hydraulic press (KPA-30, Osaka Jack, Japan) with a progressive press-forming die, and Figure 10 shows a trial-forming test using the progressive press-forming die. Figure 10 shows the forming die set fixed to the bolster of the press machine, and Figure 10 shows the punch set attached to the piston of the press machine. In the forming die shown on the right side of Figure 10, three rows of forming die pairs are lined up from left to right, the first (black dotted line) and third (red dotted line) of which are for the preliminary intermediate model forming and final truss core forming, respectively. The second row (dotted blue line) is for positioning, and the mold block below is set higher than the other two rows by attaching a spring under it; thus, when the upper punch presses down, the positioning punch and mold of the second row first come into contact with each other. By clamping the intermediate core formed in the previous process, the plate is fixed firmly. As the upper punch presses dow  
Figure 8: FEM model of progressive press forming by using hexagonal frustum intermediate model. Figure 9: True stress–strain diagram of forming plate material. of punches and die moved downward together. With the compression of the spring, the other two rows of punches and dies contacted and pressed the plate into a designed shape. At the end of each molding process, the spring under the second row of molds lifts the molded part back and separates the shaped plate from the mold. In addition, a mold set with cylindrical guides on both sides was utilized to maintain a constant and accurate positional relationship between the upper punch and lower forming die. In progressive press forming using the proposed intermediate model of regular hexagonal pyramids, the thickness distribution of the formed truss core panels was used as an important index to evaluate the forming stability of the truss core panel. It is ideal to form a truss core panel with a uniform thickness distribution. To measure the thickness of the formed truss core panel, the panel was cut with a wire electrical discharge machine along the area to be measured, as shown in Figure 10(b), and the thickness was measured using a dial caliper gauge (BO-1, [PERSON] of Japan, measurement accuracy 0.025 mm), as shown in Figure 10(c). As shown in Figure 10(a), it was possible to compare the thickness distribution of the truss core panel with the FEM analysis results and measured values of the thickness at the same site.