Struggling with stamped parts that keep cracking? A seasoned pro shares his expert solutions!

In metal stamping—whether for automotive parts, hardware, or home appliances—forming defects are the greatest challenge. Among these, "fracture" (or splitting) is the most troublesome, as a fractured part is typically rendered scrap. Today, let’s explore why steel sheets fracture during the stamping process and look at practical, on-the-floor solutions. Think of the stamping/drawing process like stretching dough: it is normal for the sheet metal to thin out during the operation—a reduction in thickness of 4% to 20% is generally considered within a safe range. However, if the material is stretched too aggressively, the resulting excessive thinning not only compromises the part's structural rigidity but, in the worst-case scenario, leads to outright fracture. Simply put, a fracture occurs when the local tensile strain becomes too great. When the material is stretched to its limit—to a point where work-hardening can no longer compensate for the reduction in load-bearing cross-sectional area—"necking" occurs at the material's weakest point, eventually leading to a complete break. This encompasses both macroscopic fractures (visible to the naked eye) and microscopic fractures (where the material has failed internally, even if not immediately apparent).

Experienced technicians, when conducting die trials, pay close attention to high-risk zones. Areas with small radii—such as those found in deep-drawing processes or at the rounded corners of a punch—are particularly prone to issues. Furthermore, the centers of sidewalls and areas where material flow is obstructed (for instance, as the sheet passes over draw beads before entering the die cavity) are also highly susceptible to fracturing. These areas require extra vigilance.

The core principle for addressing fractures is straightforward: alter the distribution of contact forces and friction so that the vulnerable areas are not subjected to excessive tensile strain. In practice, this can be achieved through a combination of adjustments. A good starting point is to modify the size or shape of the blank; sometimes, the root cause lies in the initial geometry of the raw material. For example, if the corners of a square-shaped part consistently fracture during deep drawing, trimming the corners of the square blank can often provide an effective solution. Next, you can make some adjustments to the die to facilitate the forming process. Without compromising the product's functionality, you might slightly increase the die's corner radii or reduce the draft angles to allow the material to flow more smoothly. In some cases, you can strategically incorporate "process notches"—cutouts that enable material from adjacent areas to flow into regions prone to tearing. Furthermore, fine-tuning the draw beads or the blank-holder force is crucial; while draw beads are highly effective, improper settings can create excessive resistance, causing the sheet metal to tear—so the force must be calibrated with precision. Finally, never overlook the importance of proper lubrication. Inadequate or incorrect lubrication significantly increases friction, which often leads to tearing; improving lubrication conditions can frequently yield remarkable results.