Oversized grain in hot runner systems is mainly detected through metallographic examination combined with grain size rating standards. This method can directly and accurately determine whether the grains are abnormally coarsened.
1. Metallographic Examination (Most Reliable Method)
Sampling Location: Cut samples from the most severely heated area of the hot runner mold (such as nozzles or manifolds), avoiding the surface decarburized layer;
Sample Preparation Process:
Cutting: Use water-cooled cutting to prevent high temperatures from altering the original microstructure;
Inlaying: Hot-mounting is recommended to protect the edges;
Grinding and Polishing: Grind sequentially with 240# to 2000# sandpaper, then polish with diamond to a mirror finish;
Etching: For carbon steel or low-alloy steel, etch with a 4% nitric acid alcohol solution for 5–15 seconds to reveal grain boundaries.
Microscopic observation: Observe using an optical microscope at 100× or 200× magnification;
Compare with the grain size chart of GB/T 6394-2017 or ASTM E112 standard for rating.
Judgment criterion: If the grain size grade is ≤4, it is considered that the grains are too large (coarse grains), and there is a risk of performance degradation.
2. Testing Basis and Standards
|
Standard Number |
Name |
Applicability |
|
GB/T 6394-2017 |
Method for Determination of Average Grain Size of Metals |
Domestically Used Arbitration Standard |
|
ASTM E112-2013 |
Test Method for Determination of Average Grain Size |
Internationally Used, Applicable to Scientific Research and Export Projects |
|
GB/T 24177-2009 |
Dual Grain Size Characterization and Determination Method |
Used for cases of uneven grain distribution (mixed crystals) |
High-reliability data shows that GB/T 6394-2017 is currently the most authoritative grain size determination standard in China and is widely used in arbitration testing.
3. Auxiliary Detection Methods (Indirect Judgment)
|
Method |
Principle |
Limitations |
|
Mechanical Property Testing |
Excessively large grains lead to decreased strength and elongation, verifiable through tensile testing. |
Cannot pinpoint specific areas; for reference only. |
|
Heating History Analysis |
Long-term operation at excessively high temperatures (e.g., >600°C) easily induces grain coarsening. |
A risk warning, not direct evidence. |
|
Fracture Analysis |
The appearance of a "stone-like fracture" or intergranular fracture indicates grain boundary weakening. |
Requires metallographic confirmation. |
Hardness Testing and Eddy Current Testing : Cannot effectively reflect grain size; prone to misjudgment due to interference from various factors.
4. Practical Operation Recommendations
Regular Sampling Inspection: Conduct metallographic inspections of key components every 6–12 months.
Establish Benchmark Files: Complete the initial grain size test before putting a new mold into use as a subsequent comparison benchmark.
Combined with Process Control: If a grain growth trend is found, immediately check whether the heating temperature setting and holding time are compliant.
