Specific manifestations of excessively large grains in hot runner systems include significantly coarsened grains, decreased mechanical properties, stone-like fracture surfaces, and localized mold deformation. These phenomena can be identified directly or indirectly through metallographic observation and performance testing.
1. Direct manifestations in metallographic structure:
Significantly coarse grains: Under a 100× or 200× microscope, the grain size far exceeds the normal range (grain size ≤ 4 grade), exhibiting an inhomogeneous structure of "large grains engulfing smaller ones";
Clear but sparse grain boundaries: The number of grain boundaries is reduced, the intergranular bonding force is weakened, and stress concentration points are easily formed;
Possible Widmanstätten structure or secondary recrystallization: Needle-like ferrite (Widmanstätten) appears in local areas, or a very small number of grains grow abnormally large, engulfing surrounding fine grains.
Rating is performed according to standard GB/T 6394-2017. If the grain size is below grade 5, it is considered excessively large.
2. Indirect Manifestations of Mechanical Properties
Performance Indicators Specific Manifestations
Tensile Strength: Significantly decreased, especially under high-temperature conditions, reducing load-bearing capacity.
Elongation after Fracture: Decreased plasticity, increased brittleness, and earlier fracture in tensile specimens.
Impact Toughness: Significantly reduced; more prone to fracture under the same impact energy.
Fatigue Life: Shortened life under thermal cycling stress; earlier onset of cracks.
High-reliability data shows that coarse grains significantly reduce the impact toughness and fatigue strength of metals, and are one of the main causes of early failure.
3. Abnormal Manifestations in Macroscopic and Usage
Local Overheating or Discoloration of the Mold: Overheated areas may exhibit oxidation (bluish-purple or grayish-white), indicating microstructural degradation.
Decreased Injection Molding Stability: Defects such as uneven filling, flash, and material shortages frequently occur due to runner deformation.
Frequent Cracking or Crackling: Intergranular fracture occurs in high-temperature areas such as hot runner nozzles and manifolds, shortening repair cycles.
"Stone-like Fracture": The fracture surface lacks metallic luster and appears as grayish-white granules, a typical characteristic of grain boundary weakening.
If intergranular fracture or remelting spheres are found, it indicates that the material may have developed into "overheating," making it irreparable.
4. Practical Identification Recommendations
Regular Metallographic Inspection: Perform microstructural analysis on key components every 6–12 months;
Establish Comparison Benchmarks: Complete the initial metallographic inspection before putting new molds into use to facilitate subsequent comparisons;
Combined with Temperature Records: Review heating curves to determine if the machine has been operating at excessively high temperatures for an extended period (e.g., H13 steel > 600°C).
