What are the specific manifestations of excessively large grains in hot runner systems?

Excessively big grains in hot runner systems are identified by a coarsening of the grains, a deterioration in mechanical characteristics, stone-like fracture surfaces and localised deformation in the mould. These behaviours can be described by direct or indirect metallographic observation and performance testing.

 

1. Direct manifestation in metallographic structure:

Significantly coarse grains: The grain size is well over the typical range (grain size ≤ 4 grade) under a 100× or 200× microscope, with an inhomogeneous structure of "large grains engulfing smaller ones";

Sparse and clear grain boundary: The number of grain boundaries is less, the bond force between grains is weak and it is easy to generate the concentration areas of stress;

Possible Widmanstätten structure or secondary recrystallisation: In some regions, needle-like ferrite (Widmanstätten) is formed, or a very small number of grains become unusually big, swallowing up surrounding tiny grains.

The rating is based on the GB/T 6394-2017 standard. The grain size is too big below grade 5.

 

2. Mechanical properties Indirect manifestations

Performance Indicators Specific Expressions

Tensile Strength: Much lower, especially at elevated temperatures, resulting in reduced load-bearing capacity.

Elongation after Fracture: Lower plasticity, greater brittleness and earlier fracture of tensile specimens.

Impact Toughness: Greatly reduced; fracture more probable at equal impact energy.

Fatigue Life: Shorter life due to heat cycle stress, quicker crack initiation.

Reliability studies reveal coarse grains can greatly lower the impact toughness and fatigue strength of metals and are a major cause of early failure.

 

3. Abnormal Manifestations in Macroscopic and Use

Local Overheating or Discolouration of the Mould: There may be oxidation (bluish purple or greyish white) in the overheated spots, showing microstructural degradation.

Lower Injection Moulding Stability: Runner deformation often leads to defects, including uneven filling, flash and material scarcity.

Frequent Cracking or Crackling: High temperature zones such as hot runner nozzles and manifolds experience intergranular fracture, which reduces the number of repair cycles."Stone-like Fracture": The fracture surface has nonmetallic lustre and appears as greyish white granules, which is a typical feature of grain boundary weakening.

If intergranular fracture or remelting spheres are observed, it means that the material may have grown into "overheating", it is irreparable.

 

4. Identification Recommendations in the Field

Routine Metallographic Inspection: Microstructural examination of critical components should be performed every 6-12 months;

Establish Comparison Standards: Conduct first metallographic inspection on new moulds so that subsequent comparisons can be made;

Review heating curves and temperature records to establish if the machine has been working at a temperature that is too high for too long (e.g., H13 steel > 600°C).