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D1316-20 – Standard Test Method Technical Guide
📐 Test Method Overview and Scope
This test method, designated D1316 – 20, describes the procedure for determining the fineness of grind of printing inks using an NPIRI Grindometer. It evaluates the size of the largest particles in a finished dispersion but not the average particle size or concentration of sizes. The method covers both manual and automatic drawdowns using an A1 scraper and is applicable to any dispersion fine enough to fall within the 0 to 25 µm (0 to 1 mil) range of the specified grind gauge.
With a minor variation in procedure, this standard is applicable to both paste (nonvolatile) and liquid (volatile) inks. It is important to note that the 0 to 25 µm gauge is similar in principle to the Hegman gauge described in Test Method D1210 and the various gauges described in ISO 1524:2000. For analysis of coarse particles above 45 µm, users should refer to Test Method D2067.
📌 Scope Clarification: This test method is designed to identify the largest particles present in a dispersion. It does not provide data on the average particle size or the distribution of particle sizes within the ink.
📏 Key Terminology and Drawdown Anomalies
The standard defines specific terms to describe the quality of the grind and the appearance of the drawdown. A scratch is a depression at least 10 mm in length in the surface of the drawdown, caused when a particle is trapped between the blade and the groove. A speckle is a protuberance of particles above the drawdown surface, typically indicative of hard or large particles found at shallower gauge depths.
| 🟦 Term | ⚡ Definition |
|---|---|
| Fineness of Grind | A measure of the size and prevalence of oversize particles in a printing ink dispersion. |
| NPIRI Grind Unit (gu) | The distance equal to 2.5 µm (0.1 mils) on the grind gauge. |
| Scratch | A depression at least 10 mm in length in the drawdown surface caused by a trapped particle. |
| Speckle | A protuberance of particles above the surface of the drawdown. |
💡 Technical Distinction: Scratches and speckles often appear at different points on the gauge. Scratches form in the deeper (wider) portion of the groove, while speckles tend to appear at the shallower (finer) end, representing particles that protrude from the film rather than being dragged.
⚙️ Test Procedure and Speed Selection
The NPIRI Grindometer features two precision machined grooves, each 25.4 mm (1 in.) wide, with a taper from 0 to 25 µm (0 to 1 mil). The test specimen is drawn down the length of the paths using the A1 scraper. The speed of the drawdown is a critical variable and depends entirely on the type of ink being tested.
| 📂 Ink Type | ⚡ Scraper Speed | 📏 Rationale |
|---|---|---|
| Paste (Nonvolatile) Ink | Slow | Allows for a consistent, undisturbed film to be drawn without trapping air. |
| Liquid (Volatile) Ink | Brisk | Minimizes solvent evaporation during the drawdown, which could otherwise cause false readings by solidifying the ink prematurely. |
Following the drawdown, the film is examined for scratches and speckles. The end point is determined by noting the exact scale value (in µm or NPIRI units) where a specified concentration of these anomalies first appears. This value represents the fineness of grind of the ink sample.
⚠️ Critical Procedural Note: Using an incorrect drawdown speed is a common source of error. A volatile liquid ink drawn too slowly will dry on the gauge, creating false scratches and a poor correlation with the true grind quality of the dispersion.
❓ Frequently Asked Questions
🔍 What does the NPIRI Grindometer measure?
It measures the fineness of grind, specifically the size of the largest particles in a printing ink dispersion. It does not measure the average particle size or the concentration of sizes.
💡 How is an NPIRI Grind Unit (gu) defined?
One NPIRI Grind Unit (gu) is defined as the distance equal to 2.5 µm or 0.1 mils on the grind gauge. A standard gauge with a 0 to 25 µm range therefore spans 0 to 10 NPIRI units.
⚡ What is the difference between a scratch and a speckle?
A scratch is a depression in the film surface at least 10 mm long, caused by a particle being dragged by the blade. A speckle is a protuberance of a particle standing above the drawdown surface, typically seen at higher (finer) grind levels.
📌 Is D1316-20 applicable to both paste and liquid inks?
Yes. With a minor procedural variation—the speed of the drawdown (slow for paste inks, brisk for liquid inks)—this test method is applicable to both types of printing inks.