Excessively large grains in hot runners lead to decreased strength, increased risk of cracking, poorer dimensional stability, and shortened service life, severely impacting mold performance and production quality.
1. Significantly Deteriorated Mechanical Properties
Reduced Strength and Fatigue Life: Excessively large grains reduce grain boundary area, and grain boundaries are key barriers hindering dislocation movement. Reduced barriers significantly decrease the tensile strength and fatigue strength of the material.
Deteriorated Plasticity and Toughness: Coarse grains increase the brittleness of the metal, significantly reducing elongation after fracture, making it more prone to brittle fracture under thermal cycling stress.
For example, the high-temperature strength of H13 mold steel can decrease by more than 20% after grain coarsening, directly affecting injection molding stability.
2. Increased Risk of Cracking and Early Failure
Stress Concentration at Grain Boundaries: Grain boundaries of coarse grains are more prone to sliding and migration, forming microcrack initiations, especially at the junctions of three grains, which easily trigger crack propagation.
Accelerated Creep Damage: Under long-term high-temperature service conditions, the nucleation and propagation of grain boundary voids accelerate, leading to rapid fracture after entering the third stage of creep.
If accompanied by grain boundary oxidation or localized melting, it has developed into "overheating," rendering the material irreparable and requiring scrapping.
3. Uncontrolled Dimensional Accuracy:
Uneven Thermal Expansion: Abnormal localized grain growth leads to uneven microstructure, and differences in thermal expansion coefficients cause mold deformation.
Impact on Injection Molding Quality: Changes in runner geometry cause uneven filling, flash, and material shortages, affecting product consistency.
4. Significantly Shortened Mold Life:
Fatigue Crack Initiation Point: Overheated areas become crack initiation zones, accelerating mold cracking.
Increased Maintenance Costs: Frequent downtime for inspection and component replacement reduces production efficiency and increases maintenance costs.
Practical Recommendation: Controlling the heating temperature to below the upper limit of mold steel tempering (e.g., H13 steel ≤ 600°C) can effectively inhibit grain growth.
