In the field of sheet metal stamping, one particularly troublesome quality defect is "wrinkling." While wrinkling may not compromise a part's structural strength or rigidity—as fracturing would—it severely impacts both the surface appearance and dimensional accuracy. Consider, for instance, automotive exterior body panels: a wrinkled surface is clearly unacceptable, and such defects are a common cause of poor quality. Furthermore, if wrinkling occurs during an intermediate stage, it can disrupt subsequent manufacturing processes. In the worst-case scenario, if wrinkled sheet metal is forcibly "ironed out" by the die, it can damage the workpiece and even scratch the expensive die itself, leading to significant production losses.

Why does a perfectly sound sheet of metal wrinkle? Simply put, it is primarily due to excessive localized compressive stress, which causes the sheet to undergo compressive instability in the direction of its thickness. Typically, the orientation of the resulting wrinkles is perpendicular to the direction of the compressive stress—though it is important to note that not all wrinkling is caused solely by compressive stress. Generally, wrinkling can be categorized into two types: "accumulation-based" wrinkling, which occurs when an excess of material is forced into the die cavity with nowhere else to go; and "instability-based" wrinkling, which happens when insufficient constraint in the thickness direction leads to instability in the flange, or when uneven tensile stress causes the material to become unstable and form wrinkles.

Given that wrinkling is driven by excessive localized compressive stress—occurring most readily when the material is subjected to a combination of tensile and compressive forces—the core strategy for addressing it involves accurately predicting material flow and increasing the normal contact force at the site of the wrinkle to flatten the material. In a production environment, several practical adjustments can be employed. The most direct approach is to increase the blank holder force; doing so raises the resistance to material flow into the die cavity, effectively mitigating wrinkling along the flange edges. If this proves insufficient, one can also increase the number of draw beads or adjust their height to provide additional control. Draw beads are categorized into round beads, square beads, and draw bars, with material flow resistance increasing in that order. By comprehensively considering factors such as the drawing depth of the part, material properties, and product geometry—and subsequently selecting and appropriately configuring the suitable draw beads—one can systematically control material flow resistance, alter the internal stress state of the material, and adjust the direction of material flow. In this way, wrinkling defects can be effectively mitigated.