The key to preventing excessive grain growth in hot runner systems is controlling heating temperature, optimizing holding time, accelerating cooling rate, and selecting materials with strong resistance to grain growth. Through systematic process control, abnormal grain growth can be effectively suppressed, ensuring stable mold performance.
1. Precise Temperature Control: Avoiding Overheating
Setting a Safe Upper Temperature Limit: The operating temperature should be controlled below the tempering temperature of the mold steel, such as H13 steel, not exceeding 600°C, far below its austenitizing temperature (approximately 900°C);
Independent Zone Temperature Control: Implement precise temperature control for different areas such as nozzles and manifolds to prevent localized overheating;
Using a High-Precision Temperature Controller: Employ a controller with PID adjustment function to correct deviations in real time, ensuring temperature fluctuations ≤ ±5°C.
Practical Recommendation: Set parameters based on the material's heat treatment curve, leaving a safety margin of at least 50°C to avoid overheating due to misoperation.
2. Optimize Heating and Standby Systems
Avoid prolonged high-temperature holding: During non-production periods, lower the temperature to 100–150°C and hold to reduce ineffective high-temperature exposure;
Standardize start-up and shutdown procedures: Avoid frequent start-ups and shutdowns causing thermal shock; continuous production mode is recommended;
Monitor heating history: Record each heating curve to review for any over-temperature or over-time operation.
High-reliability data shows that prolonged residence time of metal in high-temperature zones is one of the main causes of grain coarsening.
3. Accelerate Cooling Rate
Adopt rapid cooling methods: After repair or replacement, prioritize air cooling or forced air cooling to shorten the material's residence time in high-temperature zones;
Prohibit slow cooling in the furnace: Especially after annealing, slow cooling easily leads to grain agglomeration and growth;
Localized enhanced cooling: Add cooling channels in hot spots to improve heat dissipation efficiency.
Excessively slow cooling rates significantly promote grain growth and are a key risk point in process control.
4. Material and Original Microstructure Control
Select inherently fine-grained steel: Prioritize the use of modified H13 steel containing microalloying elements such as Ti, Nb, and V, utilizing precipitates to pin grain boundaries;
Ensure raw material quality: Conduct metallographic testing before using new molds to confirm that the original grain size is within the range of 5–8;
Avoid using coarse-grained billets: If the forging billet itself has coarse grains (≤4 grade), subsequent heating easily inherits the coarse-grain tendency.
Steel treated with Al deoxidation or microalloying is more resistant to high temperatures and has stronger resistance to grain growth.
