Gradation screen printing and quality control (below)

Ink thickness

The parameters that affect the ink layer thickness are: screen aperture, wire diameter, stencil thickness, and ink type and viscosity.
The thickness of the ink deposition is equal to the theoretical deposition of the screen, the theoretical deposition amount determines the final ink layer thickness, and the film thickness of the template has a significant effect on the deposition amount of the ink. Therefore, in order to print the halftone image better, it is equally important to select a smaller wire mesh screen and a thinner reticle screen. How to control the stencil thickness is an important technical detail of the gradation screen printing. In the commonly used screen printing plate making process, the film thickness of the indirect film and the direct film is certain, and it is relatively easy to establish a stable parameter system. The thickness of the sensitized coating on the coating machine is also relatively easy to control, which is also the current method commonly used for screen printing.
The use of thickness gauges to accurately measure stencil parameters, such as thickness and surface roughness, is an effective method for controlling the quality of platemaking and is an indispensable step in the standardization of platemaking processes.

Optical density

Tri-level printing adopts three primary color reproduction technologies, and the control of each color template is very important. Because primary color inks are substantially transparent (especially UV inks), small changes in the stencil can result in changes in the thickness of the printed ink film, and in turn lead to significant color shifts in the printed color.
This color shift can be described by the amount of light absorbed by the ink film. We can use the densitometer to detect the optical density of the ink layer to control the color deviation. Practice has proved that the use of density meter is the main means of obtaining accurate color control. The densitometer has two kinds of reflection densitometer and transmission densitometer. The density of the plate-making film is detected using a transmission densitometer, and the density of the printing ink layer is detected using a reflection densitometer.
If we use “standard density” to describe the density of the ink layer when printing normally on a specific substrate, the densitometer is used to measure the density of each print. Compared with the standard density, the color reproduction deviation can be obtained. Correcting this deviation within the range will standardize the color reproduction. By this method, the color change can be detected and controlled within the allowable latitude of the print.

Dot size

The change in the dot size is commonplace in the printing process, and any dot change before or in the printing will have a destructive effect on the color quality. For this reason, monitoring the dot size of the entire production process and correcting deviations in time is a central part of quality control of screen printing.
During the exposure process, after the dots on the printing film are transferred to the template, the dot size will increase or decrease, resulting in a change in the dot size printed on the substrate, and an unexpected increase between the dot size of the film and the final printing. Or reduction is called dot gain and dot loss. Loss of high-light outlets is usually called sharpening, and the loss of dark outlets is often referred to as paste.
In most cases, increase and loss are caused by the following reasons:
1) The ink film is too thick or too thin due to improper screen selection (incorrect proportion of screen aperture and screen wire diameter);
2) Problems with the platemaking process (low vacuum pressure during exposure, overexposure, and incomplete development of the stencil);
3) Excessive scraper pressure and squeegee speed;
4) Improper ink viscosity;
5) Material surface characteristics (absorption and non-absorption) do not match the process.
In order to ensure the accurate reproduction of the printing level, it is necessary to measure the dot gain and loss and make a reasonable deviation. Firstly, the number of screens and the matching relationship with the number of screen meshes of the film containing the halftone pattern are the basis for deciding whether or not the precise gradation range can be obtained. On this basis, it is necessary to use the densitometer to increase the expected dot number. Or reduce the precision of data.
Table 4 shows the percentage of dots taken from the film and print sample areas measured using a densitometer. Loss of gradation occurs when the size of a printed dot or a dot on a plate-making film (or gray scale) increases or decreases.
As can be seen from Table 4, the tone of 10% to 30% of the printed image is usually represented by the dot reduction, and the gradation of 40% to 90% is represented by dot gain, and the increase at 60% to 80% is the most serious.
The above data must be tested after the ink is completely cured to obtain accurate results, measure the percentage of dots in different printing areas, record well, and measure the percentage of corresponding dots in the same area on the film (step scale), so that records can be made. For comparison.
It should be noted that the above description of the dot change situation is obtained under the condition that the screen, film, printing ink, printing material, and printing press are of the same type. When any one of these factors changes, the results obtained will be different. . For example, using the same ink, the change in dot size will be different when the substrate is changed.

Ink overprint

Ink overprinting is the result of screen printing. It determines the success or failure of color reproduction.
There are two situations in which the dots are juxtaposed and the dot is superimposed on the printed matter. The overprinting effect of the ink is mainly related to the degree of drying of the ink layer and the thickness of the ink layer.
If the printing dots are juxtaposed at a certain distance, the color should be the final color according to the additive color mixing principle. When there is no cover between the inks, the resulting mixed color is ideal.
The phenomenon of overprinting is also common in print Internet sites. Incomplete ink transparency determines the loss of color. Therefore, it is necessary to control overprinting. Most printed test strips have overprint areas that can help find ink overprint problems. If the three-color overprint area of ​​the test strip, the yellow, magenta, and green overprinting effects are approximately neutral gray, indicating that the color mixing basically conforms to the principle of printing coloration, and the ink overprinting is normal; otherwise, it must be adjusted through the screen printing of the printing film. Angle or other measures taken in the prepress and production process to adjust the ink overlay results, control items in Table 5.

to sum up

Screen printing production requires data and standardized quality control. First, it is necessary to control the parameters of plate making, plate making, printing materials (inks, substrates), and printing processes. Screen printing companies that are setting up a quality control system should be controlled. Start with these parameters.
The implementation of digitization and standardization in the prepress and printing process requires rigorous work styles, and it is necessary to develop the habit of correctly recording data using testing instruments. Although the process of making careful analysis and establishing data systems after measuring platemaking and printing parameters is a long-term and tedious task, quality stability is the central task of screen printing enterprise management and is the most effective method for obtaining stable printing quality. , Dataization and standardization work are also effective measures to reduce quality problems. In today's increasingly fierce market competition, whoever wins the high ground of quality will eventually win the market.

Source: Gao Bao Printing Machinery Technology Co., Ltd.