Dr E. Ramanathan PhD
How to apply the pigment paste on Grind Gauge
The grind gauge being used with the scraper should be held at a 90° angle to the surface of the gauge. The viewing angle is 45 degree.
- 90° angle refers to the scraper orientation, i.e., perpendicular to the grind gauge surface while drawing down the paste.
- This is standard practice: holding the scraper vertically or nearly vertical (80°–90°) allows uniform pressure and better control during drawdown.
Correct Practice:
| Aspect | Angle | Purpose |
|---|---|---|
| Scraper Application | ~90° | Consistent paste application |
| Viewing Angle (Observation) | ~45° or low angle | Detect particle streaks clearly |
NOTE: 90° for application, 45° for visual inspection.
The angle to be maintained while checking the grinding level of a ground pigment paste using a grind gauge (Hegman gauge) is approximately 45 degrees.
Reason:
- This angle ensures that the paste flows gradually from the deep end to the shallow end of the gauge, allowing particle agglomerates or coarse pigments to be visually detected as streaks.
- A steeper angle may cause the paste to run too quickly; a shallower angle may prevent proper filling of the groove depth.
Additional Best Practices:
- Use a consistent drawdown speed (typically using a steel scraper).
- Clean the gauge immediately after use to avoid dried residues.
This method helps determine the fineness of grind or dispersion quality of the pigment in the paste.
How to Inspect the Ground Paste on Grind Gauge
After applying the paste on the grind gauge at ~45° using a scraper:
- Inspect Immediately:
- Look along the length of the groove under good light.
- Begin from the deep end (starting point) toward the shallow end (zero point).
- Observe for Streaks or Specks:
- Coarse particles appear as visible streaks or scratches where the pigment stops filling the groove.
- The point where continuous smooth paste breaks down into specks or scratches indicates the fineness level.
- Determine the Grind Level:
- Read the number marking on the gauge at the point where specks begin to appear.
- This value represents the fineness of grind in microns or Hegman units, depending on the gauge type.
- Lighting Tip:
- Use angled lighting or side lighting for better visibility of particles.
- Repeat for Accuracy:
- Perform the test 2–3 times to ensure repeatability. Clean the gauge between trials.
This process gives a reliable indication of whether the pigment dispersion is adequately ground.
Interpretation
Left image: 4 Hegman, Right image: 4.75 H
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Should identify the front line with maximum number of coarse particles.
What is Hegman unit?
The Hegman unit is a scale used to measure the fineness of grind or particle size in a liquid dispersion, especially in paints, inks, and coatings.
Key Points:
- Range: 0 to 8 Hegman units.
- Interpretation:
- 0 Hegman = 100 microns (coarse)
- 8 Hegman = 0 microns (very fine)
How It Works:
- A Hegman grind gauge has grooves of decreasing depth from 100 µm (at 0) to 0 µm (at 8).
- After applying and drawing down the paste:
- The point where coarse particles start to appear determines the Hegman value.
- Higher Hegman = finer grind = better dispersion.
Example Conversion Table:
| Hegman Unit | Microns (µm) |
|---|---|
| 0 | 100 |
| 2 | 50 |
| 4 | 25 |
| 6 | 12.5 |
| 8 | 0 |
Used widely in quality control to ensure pigment dispersions are within specification.
Recommended Hegman Value for different types of paints
Here is the recommended fineness of grind (FOG) in Hegman units and corresponding micron ranges for various types of paints and coatings:
| Type of Paint / Coating | Recommended Hegman Value | Micron Equivalent |
|---|---|---|
| Primer (Red Oxide, Epoxy, etc.) | 3.0 – 4.0 | ~25–37 µm |
| General Industrial Paints | 4.0 – 6.0 | ~12–25 µm |
| Enamels (Synthetic, Alkyd) | 5.0 – 6.5 | ~8–20 µm |
| Automotive Topcoats | 6.5 – 7.5 | ~5–8 µm |
| Clear Coats (PU, NC, Acrylic) | 7.0 – 8.0 | ~0–5 µm |
| Inks (Offset, Flexo, Gravure) | 7.0 – 8.0 | ~0–5 µm |
| High-Performance Coatings | 6.5 – 8.0 | ~0–8 µm |
| Emulsion Paints (Architectural) | 3.0 – 5.0 | ~20–37 µm |
Notes:
- Higher Hegman = finer grind and better optical, flow, and gloss properties.
- Primers tolerate coarser grind; topcoats and clear finishes require finer dispersion to avoid surface defects.
Use this as a QC guideline to match grind quality to paint application.
For more technical guidelines contact Dr E. Ramanathan in https://t.me/saitechinfo
