# Intermediate Model Design in the Progressive Stamping Process of a Truss Core Lightweight Panel
## 1 Introduction  
Lightweight design has always been an important target in automobiles, aircraft, and many other manufacturing industries, and to achieve this target, various lightweight structures have been proposed and studied in the past several years [1; 2; 3; 4; 5].  
The honeycomb core panel, which is a lightweight and rigid structural panel, is relatively easy to produce with various core sizes and panel dimensions [6; 7], and is widely used in industries because of its excellent sound insulation property and bending rigidity [8; 9; 10; 11; 12]. However, because the regular hexagonal cross-sectional core and the surface plate are bonded by adhesive, it is difficult to apply the structure to an environment where the temperature changes rapidly, vibration is high, or shear load is applied [13; 14; 15]. Therefore, various sandwich core structures with higher strength and bending rigidity than honeychoms have been proposed [16; 17; 18]. Among them, the octet truss core structure has been extensively studied for its high strength and rigidity [19; 20; 21]. In addition, a lightweight truss core structure consisting of truss core panels has been proposed based on origami engineering [22], the static bending stiffness and collision energy absorption performance of truss core panels have been investigated, and the mechanical advantage of truss core panels as lightweight structures has also been proven [23; 24; 25; 26] by simulation.  
However, it is difficult to manufacture truss core panels with high quality and low-cost owing to their complex three-dimensional shape. Because of the characteristics of thin-walled panels made of thin steel plates, it seems appropriate to use the sheet metal press method to fabricate truss core panels; however, the convex part of the truss core panel must be formed as high as possible using the deep-drawing method to increase the bending rigidity [27; 28]. However, by forming the truss core as much as possible, the problem of extreme thinning of the plate can be observed locally. Because there is a limit to the height at which thin steel plates can be deep drawn [29; 30], a multi-step deep-drawing formingmethod has been devised, and the results of a study on processing truss core panels using a progressive press-forming method with a hemispherical intermediate model have been published [31]. However, a stable press-forming method for truss core panels has not yet reached a practical level.  
In this study, the author handles the problem of intermediate model design, which is the most important issue when processing truss core panels using the progressive press-forming method and proposes a design method that replaces the hexagonal pyramidal intermediate model with the hemispherical intermediate model. Prototype experiments and numerical analyses were conducted to verify the formation characteristics of the proposed hexagonal pyramidal intermediate model. In comparison with the conventional hemispherical intermediate model, it has been verified that the problem of extreme thinning in the local thickness of truss core panels can be improved by using the proposed hexagonal pyramidal intermediate model in progressive press forming.