Meaning
Mechanical fatigue generated by the repetitive application and release of high tonnage load during the molding cycle affects the structural integrity of tool steels. Exposure to cyclic clamping stress occurs with every machine cycle as the press moves from clamp-up to opening. Over thousands of cycles, this repetitive load induces micro-cracking in sharp internal corners or around deep pockets.
Stress Cycle
Plastic injection molding demands that the clamp force exceeds the pressure of the flowing polymer inside the cavities. The resulting cyclic clamping stress subjects the core and cavity blocks to alternating compressive and tensile forces during the process. When the melt is injected, the mold tries to force itself open, which causes the steel to flex slightly.
As the part cools and pressure drops, the clamp relaxes and the steel returns to its original state.
Steel Degradation
High strength tool steels like H13 or P20 are selected to withstand these rapid load changes without immediate deformation. However, prolonged cyclic clamping stress eventually initiates cracks at stress concentration points such as cooling channel transitions or ejector pin holes. Once a crack begins, the continuous cycling drives it deeper until a catastrophic plate fracture occurs.
Mitigation Practice
Machining generous radii in all internal pockets represents the most effective defense against localized stress concentrations. This design choice disperses the cyclic clamping stress across a larger volume of steel, which prevents early cracks and extends tool life.
