1. Wear Failure
Wear is a common failure mode, characterized by the gradual loss of material from the working surfaces of the die (e.g., punch cutting edge, die cavity), leading to decreased dimensional accuracy and scratches or burrs on the workpiece surface.
Severe Friction Conditions: During the stretching process, the relative sliding between the die and the workpiece generates intense friction. Insufficient lubrication (e.g., dried-out lubricant, incorrect lubricant selection) or improper lubrication methods (e.g., failure to cover critical contact areas) will accelerate abrasive wear.
Insufficient Material Hardness: The die steel has not undergone sufficient quenching or tempering treatment, resulting in a surface hardness lower than the workpiece material hardness (e.g., when stretching high-strength steel plates, insufficient die hardness leads to easy wear).
Surface Treatment Defects: Insufficient nitriding layer thickness, coating peeling, or uneven carburizing layer result in a loss of protection for the die surface.
Inappropriate Clearance Design: Insufficient punch-die clearance increases contact pressure and friction area, exacerbating wear.
2. Fatigue Fracture Under cyclic loading, microcracks may initiate on the surface or inside the mold, gradually proliferating until fracture. This is commonly seen in stress concentration areas such as the root of the punch and the fillet of the die.
Stress Concentration: Sharp corners, small fillets, and grooves in the mold structure easily become stress concentration points, acting as crack initiation points.
Improper Heat Treatment: Excessive residual stress after quenching or insufficient tempering increases material brittleness.
Load Fluctuations: Uneven workpiece material thickness and unstable tensile speed cause fluctuating stress on the mold, accelerating fatigue crack propagation.
Material Defects: Inclusions, porosity, and other internal defects in the mold steel easily trigger crack initiation.
3. Plastic Deformation Deformation of the working parts of the mold (such as punch bending, die hole enlargement, and cavity collapse) leads to dimensional deviations in the workpiece.
Insufficient Material Strength: The die steel has low yield strength and cannot withstand tensile loads.
Excessive Operating Temperature: Frictional heat generated during stretching raises the die temperature (especially when stretching high-carbon steel or thick plates), causing material softening.
Overload: The tensile pressure exceeds the die material's bearing capacity limit (e.g., incorrect equipment parameter settings).
Heat Treatment Defects: Insufficient quenching hardness or excessively high tempering temperature leads to a decrease in material hardness and strength.
4. Chipping and Fracture: Sudden breakage of the die cutting edge or localized areas often occurs under impact loads, commonly seen in thin-edged dies or dies made of brittle materials.
High Material Brittleness: The die steel has not undergone proper tempering (e.g., high-carbon tool steel not tempered after quenching), or brittle materials (e.g., unalloyed carbon tool steel) have been used.
Structural Design Defects: The cutting edge is too thin, the transition radius is too small, or the cavity wall is too thin.
Improper Installation and Adjustment: Poor die alignment leads to uneven loading, or loose fixing bolts cause impact.
Workpiece Abnormalities: Hard particles (e.g., oxide scale, inclusions) exist in the workpiece, impacting the die cutting edge.
5. Corrosion Failure
The mold surface is corroded by chemical or electrochemical processes, manifesting as rust spots, pitting, or surface peeling, affecting precision and lifespan.
Environmental factors: Humid workshop environments or materials that are corrosive to the tensile material (e.g., stainless steel, aluminum alloy); Lubricant corrosion: Some lubricants contain acidic components (e.g., residual cutting fluid), which can corrode the mold surface upon prolonged contact; Poor material corrosion resistance: The mold steel has not undergone anti-corrosion treatment (e.g., chrome plating, nitriding), or the selected steel has insufficient corrosion resistance.
