There are many factors that affect the quality of printed images. Common ones include: dot gain, printing contrast, dot distortion, overprint, ink layer thickness, paper smoothness, printing speed, plate wear, etc. In printing production, no matter how carefully the operator of the printing press works, the above phenomenon will always happen. In order to ensure the stability of the printing quality, the operator must sample the printed products, make judgments on the changes, and take corresponding adjustment measures.
Here we introduce some main characteristic parameters that need to be measured, and use a density meter X-Rit530 and other instruments to measure and evaluate a sample as an experimental example.
1. Solid density (SolidInkDensity)
The solid density refers to the density value at 100% dots, which is measured by the density meter on the solid color patch. Solid density can be understood as a qualitative or quantitative description of the matching between printing paper and printing ink. Qualitatively speaking, if the solid color block is darker and more vivid, and the dots observed with a magnifying glass are fuller, it means that the color of the image in the proof is relatively saturated, which basically meets the requirements of printing. On the contrary, it shows that the paper and the ink have poor bonding performance. Quantitatively speaking, we can use the density meter to measure the existing solid color patch to get the solid density D. There are two measurement methods: first, in the same printing sheet, select different solid color patches in different orientations (horizontal / vertical) for measurement, and draw the results as a solid density state curve; second, in a certain state Choose continuous or non-continuous sampling on the printing machine in the printing machine for measurement, and also draw a curve to get the solid density state curve in the printing state. According to the drawn solid density curve, we can find the difference between the solid density at different locations on the same sheet and the regularity of the solid density between continuous or discontinuous samples. We can adjust the printing process through such a rule.
In general, in order to effectively reproduce the tone value of the image, we need to control the solid density of each color. For fine prints, the solid density of yellow (Y) needs to be controlled at 0.85-1.15, magenta (M) The field density should be controlled at 1.25-1.55, the green (C) should be controlled at 1.30-1.60, and the black (BK) should be controlled at 1.40-1.80, and the deviation of the same color density should not be greater than 0.05. For the printed products with general requirements, the solid density of yellow (Y) should be controlled at 0.80-1.10, magenta (M) must be controlled at 1.15-1.45, and cyan (C) must be controlled at 1.25-1.55, black (BK) It needs to be controlled between 1.20 and 1.60, and the deviation of the same color density is not greater than 0.07.
It can be seen from the measurement results that, as a general printed matter, except for yellow and black, which meet the general printing requirements, other general requirements for printing quality cannot be met. In addition, the color difference of each color block is large, which shows that the ink supply is uneven.
In the range of low field density values, field coverage is the main factor that affects field density. If the coverage of the field reaches saturation, the influence of the thickness of the ink layer should be highlighted. As the thickness of the thinner portion of the ink layer gradually increases, the thickness of the ink layer tends to be uniform, so the density of the field increases. After further reaching the range of high solid density value, the smoothness of the ink surface will also affect the solid density value.
2. Expansion of outlets (DotGain)
The dot expansion refers to the difference between the dot size and the dot size on the color separation film. In printing production, dot expansion is normal due to factors such as printing pressure, number of screen lines, dot shape, and paper's dual reflectivity to light, but it should be controlled within a certain range. Generally speaking, the expansion of fine printing dots should be controlled at 10%-20%, while the expansion of dots for general printed products should be controlled at 10%-25%.
It can be seen from the test results that with the continuous increase of the percentage of dots, the dot increase rate continues to increase, reaching a maximum at 70%, which is caused by the overlap of dots. And the proof sheet basically meets the requirements of fine printing in terms of control network expansion.
3. Relative contrast (PrintContrast)
The relative contrast is used to reflect the transition between the middle tone and the dark tone. The calculation formula is as follows: Ds-D75Ds where Pc is the relative contrast, Ds is the solid density, and D75 is the 75% color patch density.
Generally speaking, as the solid density of the image increases, the relative contrast of the image also gradually increases. However, when the density of the field increases to a certain value, the expansion of the outlets occurs, and the relative contrast begins to decline, the level decreases, and the contrast decreases.
It can be seen from the experimental results and printing standard data that the printing requirements are basically met. In addition, what needs to be known is that when the amount of ink reaches a thickness of 10 Î¼m, the ink reaches its solid density, and then increase the amount of ink, the solid density of the ink increases slowly or almost no longer, but will cause the dots to increase continuously.
4. Overprint rate (InkTrapping)
Overprint rate is used to describe the ability of an ink to adhere to the printing surface of the previous color. Generally only red (R), green (G), and blue (B) are tested. The calculation formula is as follows: T = Dop-D1D2 where T is the overprint rate, Dop is the density after overprinting, D1 is the density of the first primary color, and D2 is the density of the second primary color.
The larger the value of overprint rate, the better the effect of overprint. The overprint rate is closely related to the printing color sequence. In the actual printing production process, the print quality reflected by different overprint rates will be different accordingly.
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