High-Temperature Grain Growth in Hot Runner Systems

High temperature grain growth in hot runner systems is described as "overheating". Overheating is the abnormal coalescing of grains due to extended heating at high temperature which results in a deterioration in the plasticity and mechanical properties of the material.

 

1. Grain Growth in Hot Runner Systems

Continuous High Temperature Environment: Hot runner systems are generally operated at temperatures higher than 200~300°C, leading to localised heating of die steel in long-term operation.

protracted Holding Time Continuous heating during processing is analogous to protracted "holding", and encourages grain boundary migration.

Material Sensitivity Common die steels (e.g. P20, H13) are sensitive to grain growth at elevated temperatures, particularly at or above their tempering temperature.

According to the references, the long-term heating of steel billets at high temperature causes the continuous expansion of grains, which causes the reduced bonding and worsened plasticity. This phenomenon is called as "overheating".

 

2. Negative Effects of Grain Growth on Hot Runner Systems

Damage

Description:

Lower power

Grain coarsening leads to reduced grain boundary area, so tensile strength and fatigue life are much reduced.

Higher Possibility of Cracking

Large grains make the material more brittle and it may shatter under the stress of thermal cycling.

Diminished Dimensional Stability

Injection moulding accuracy can be affected by localised deformation caused by inhomogeneous microstructure.

Reduced Service Life

Hot spots create failure starting points and accelerate mould scrapping.

Special Note: Excessive temperatures can produce grain boundary oxidation or localised melting (a more serious form of "overheating") which is unrecoverable to the material.

 

3. How to Control Grain Growth in Hot Runners

Control heating temperature: Do not exceed the maximum tempering temperature of the mould steel (e.g. for H13 steel, not higher than 600°C);

Optimise Heating Cycle: Shut off heat during non-production periods to reduce ineffective holding time;

Choose Fine-Grained Steel - Use steels with microalloying elements like Nb, V, and Ti added to take advantage of second-phase particle size to pin grain boundaries.