## 1 Introduction
### Deep Drawing Behaviour of Sandwich Materials  
In order to ensure the industrial applicability of sandwich materials, reliable and efficient sheet-forming techniques (herein, deep drawing) need to be developed and characterized. Deep drawing of monolithic sheet materials is well-established and widely used in the automotive and aerospace industries. In this case, an optimum holding force is applied in the flange region for the purpose of reaching the maximum drawing ratio. The stress-strain evolution is concentrated on the edge of the punch and the die, while the flange area is subject to a circumferential stress compression, which can lead to wrinkling or tearing in the case of unsuitable drawing parameters. Avoiding these defects can be achieved by adjusting the friction conditions, the drawing ratio, the die radius, the gap size, the process temperature, and the blank holding force [11]. In the case of deep drawing of sandwich materials, the situations are more complex. For MPM sandwich materials, studies have shown that deep drawing limits the decrease progressively with increasing core layer thickness [12; 13; 14]. This is due to the high tensile stresses exerted by the soft polymer core on the outer metal cover near the edge of the punch. This high tensile stress leads to an increase in the void volume fraction within the metal, which, in turn, eventually leads to earlier large crack formation and a reduction in the limit drawing ratio [13]. However, this is only a reduction relative to the monolithic sheet metal, and the formability of the MPM is still satisfactory at room temperature (RT) [14]. It is well-known that at RT, the mobility of the fibre reinforcement in the FML core layer is severely restricted in the matrix polymer, and that the low plastic deformation of the core layer can lead directly to a sharp increase in tensile stress on the outer steel sheet and premature cracking. Therefore, deep drawing using thermoforming (i.e., forming under controlled heating and cooling conditions) is considered an innovative approach for processing thermoplastic-based FML. Previous research has been limited to deep drawing of thermoset-based FML under semi-cured or non-cured conditions of the core layer in order to enable the fibre mobility, as reheating of the cured resin for forming is not possible. For instance, using a combination of metal deep-drawing and core curing during forming, the semi-cured FML exhibits better formability, where both the cover and core are simultaneously stamped and then joined [15]. However, due to the viscous flow nature of the resin, a stable flow condition during the forming process was not possible, resulting in irregular thickness distribution of the formed part. In addition, the support of the core layer for the metal cover during the deep-drawing process is also low due to the resin being in a liquid state, which can lead to wrinkling of the metal cover under circumferential compressive stresses. Therefore, a method of applying holding force in a stepwise manner to avoid such defects is introduced. Using semi-cured GLARE(r) panels in this method ensured a better holding force effect in the initial step to reduce wrinkling, because in the earlier stage, the circumferential compressive stresses were mainly distributed in the flange area, which is prone to wrinkling. In the later stages, however, the radial tensile stresses dominated mainly in the wall area, which was susceptible to tearing. Thus, the pressure needs to be varied and adjusted during the deep-drawing process rather than remaining constant. For a semi-cured GLARE(r) sheet with a blank diameter of 140 mm and a punch diameter of 75 mm, it can be deep-drawn to 30 mm without wrinkling or fracture by varying the holding force from 110 kN to 40 kN in two steps [16].