Printing quality is influenced by a variety of factors, such as dot gain, printing contrast, dot distortion, overprint, ink layer thickness, paper smoothness, printing speed, and plate wear. Despite the operator's best efforts, these issues are almost inevitable during the printing process. To maintain consistent print quality, operators must regularly sample printed materials, analyze changes, and make necessary adjustments.
In this article, we will introduce key parameters that should be measured and use a X-Rit530 densitometer as an example to evaluate a sample.
1. **Solid Density (SolidInkDensity)**
Solid density refers to the density value at 100% dot coverage, measured on solid color patches. It provides a qualitative or quantitative assessment of how well the ink adheres to the paper. A darker and more vivid solid area with full dots under magnification indicates good color saturation and meets most printing standards. Conversely, a dull appearance suggests poor ink-paper adhesion. Quantitatively, the solid density (D) can be measured using a densitometer. Two common methods include measuring different solid patches in various orientations on the same sheet to create a density curve, or sampling continuously or intermittently on the press to track changes in real-time. These curves help identify variations across the sheet and between samples, enabling process adjustments.
For fine prints, the solid density for yellow (Y) should be between 0.85–1.15, magenta (M) between 1.25–1.55, cyan (C) between 1.30–1.60, and black (BK) between 1.40–1.80, with a maximum deviation of 0.05 per color. For general print jobs, Y should be 0.80–1.10, M 1.15–1.45, C 1.25–1.55, and BK 1.20–1.60, with a deviation not exceeding 0.07. From our measurements, it was observed that except for yellow and black, other colors did not meet general quality standards, and significant color differences indicated uneven ink distribution.
At low solid density levels, dot coverage is the main factor affecting density. As coverage increases, ink layer thickness becomes more influential. As the ink layer becomes more uniform, the field density rises. At higher solid density values, surface smoothness also starts to impact the readings.
2. **Dot Gain**
Dot gain refers to the increase in dot size compared to the original film. It is a natural occurrence due to factors like printing pressure, screen ruling, dot shape, and paper reflectivity. However, it should remain within acceptable limits. For fine prints, dot gain is typically controlled between 10% and 20%, while for general prints, it may range from 10% to 25%. Our test results showed that as dot percentage increased, the dot gain rate also rose, peaking at 70% due to dot overlap. The proof sheet met the requirements for fine printing in terms of dot gain control.
3. **Relative Contrast (PrintContrast)**
Relative contrast measures the tonal transition between midtones and shadows. The formula is:
**Pc = (Ds - D75) / Ds**, where Ds is the solid density and D75 is the density at 75% dot coverage.
As solid density increases, so does the relative contrast. However, once the ink reaches a certain density, dot gain begins to reduce the contrast, leading to a loss of tonal depth.
From the experimental data and industry standards, the print quality met the required criteria. It’s also important to note that when the ink layer reaches about 10 μm, it achieves solid density, and further increasing the ink amount leads to minimal density gain but increased dot expansion.
4. **Overprint Rate (InkTrapping)**
The overprint rate measures how well one ink adheres to a previously printed color. It is commonly tested on red (R), green (G), and blue (B). The formula is:
**T = (Dop - D1) / D2**, where Dop is the density after overprinting, D1 is the first color density, and D2 is the second color density.
A higher overprint rate indicates better adhesion. This parameter is closely tied to the printing sequence, and different overprint rates can significantly affect the final print quality.
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