Printing quality is influenced by a variety of factors, including dot gain, print contrast, dot distortion, overprint, ink layer thickness, paper smoothness, printing speed, and plate wear. Even with the most skilled operators, these issues are inevitable in the production process. To maintain consistent print quality, operators must regularly sample printed materials, analyze changes, and make necessary adjustments.
In this article, we will discuss key parameters that should be measured during quality control, using a density meter like the X-Rit530 as an example to evaluate a sample.
1. **Solid Density (SolidInkDensity)**
Solid density refers to the density measured at 100% dot coverage on a solid color patch. It reflects how well the ink adheres to the paper. A darker, more vibrant color block indicates good color saturation and proper ink-to-paper bonding. Conversely, a lighter or dull color may suggest poor adhesion. Quantitatively, solid density can be measured using a densitometer. Two common methods include measuring different solid patches on the same sheet in various orientations and plotting the results into a density curve, or taking continuous or non-continuous samples during the printing process to assess the consistency of the ink distribution.
For fine prints, the solid density values should be within specific ranges: Yellow (Y) 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 prints, the tolerances are slightly higher: Y (0.80–1.10), M (1.15–1.45), C (1.25–1.55), BK (1.20–1.60), with a deviation not exceeding 0.07.
From the measurements, it appears that while yellow and black meet the general requirements, other colors fall short, indicating uneven ink distribution and larger color variations.
At lower solid density levels, the ink coverage is the main factor affecting the density. As the ink layer becomes thicker and more uniform, the density increases. However, when the density reaches high levels, the surface smoothness of the ink also plays a role in determining the final density.
2. **Dot Gain**
Dot gain refers to the increase in the size of printed dots compared to the original halftone image. This phenomenon occurs due to factors such as printing pressure, screen ruling, dot shape, and paper reflectivity. In fine printing, dot gain should generally be kept between 10% and 20%, while for general prints, it can range from 10% to 25%. The test results show that as the dot percentage increases, so does the dot gain, peaking at 70% due to dot overlap. The proof sheet meets the standard for fine printing in terms of dot gain control.
3. **Print Contrast (Relative Contrast)**
Print contrast measures the difference between mid-tones and dark tones. It is calculated as (Ds - D75)/Ds, where Ds is the solid density and D75 is the density of a 75% tone patch. As the solid density increases, so does the relative contrast. However, once the density reaches a certain point, dot gain begins to affect the contrast, causing it to decrease.
The experimental results align with standard printing requirements, showing that the print quality is acceptable. Additionally, when the ink layer reaches a thickness of around 10 micrometers, the solid density stabilizes, and further ink addition leads to increased dot gain rather than significant density improvement.
4. **Overprint Rate (Ink Trapping)**
The overprint rate measures how well one ink adheres to the previous color. It is typically tested for red (R), green (G), and blue (B). The formula used is T = (Dop - D1)/D2, where Dop is the density after overprinting, D1 is the density of the first color, and D2 is the density of the second color. A higher overprint rate indicates better ink adhesion. The overprint rate is closely related to the printing sequence, and different sequences can significantly impact the final print quality.
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