Quality control of the hottest tone screen printin

2022-08-19
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Quality control of gradient printing based on main variables (2)

ink layer thickness

parameters affecting ink layer thickness include: wire diameter, wire diameter, template thickness, ink type and viscosity

the thickness of ink deposition is equal to the theoretical deposition of silk, which determines the final ink layer thickness, and the film thickness of the template has a significant impact on the deposition of ink. Therefore, in order to better print the eye tone image, it is equally important to choose a smaller wire diameter and a thinner template. How to control the template thickness is an important technical detail of step tone printing. In the common printing process, the film thickness of indirect diaphragm and direct diaphragm is certain, which is relatively easy to establish a stable parameter system; The thickness of photosensitive adhesive coated on the coater is also relatively easy to control, which is also the common method of step printing at present

using a thickness gauge to accurately measure template parameters, such as thickness and surface roughness, is an effective method to control the quality of plate making and an indispensable step in the standardization of plate making process

optical density

gradient printing adopts three primary color reproduction technology, and the control of various color templates is very important. Because the primary color ink is basically transparent (especially UV ink), small changes in the template can lead to changes in the thickness of the printing ink film, and then lead to significant color deviation of the printing color

this color deviation can be described by the light absorption of the ink film. We can control the color deviation by detecting the optical density of the ink layer with a densitometer. Practice has proved that the use of densitometer is the main means to obtain accurate color control. There are two kinds of densitometers: reflection densitometer and transmission densitometer. Transmission densitometer is used to detect the density of plate making film, and reflection densitometer is used to detect the density of printing ink layer

if we use "standard density" to describe the density of the ink layer during normal printing on a specific substrate, then measure the density of each print with a densimeter, and then compare it with the standard density to obtain the deviation of color reproduction. Correcting this deviation within the allowable range will realize the standardization of color reproduction. Through this method, color changes can be detected and controlled within the allowable tolerance range of printed matter

dot size

the change of dot size is common in the printing process, and any dot change before and during printing will have a destructive impact on the color quality. For this reason, monitoring the point size of the whole production process and correcting the deviation in time is the central link of the quality control of step adjustment printing

in the process of exposure, the dot turning on the print film means that the carton fully meets the requirements. After moving to the template, the dot size will increase or decrease, resulting in the change of the dot size printed on the substrate. This unexpected expansion or decrease between the dot size of the film and the final print is called dot increase and dot loss. The loss of highlights is usually called sharpening, and the loss of dark spots is usually called paste

in most cases, the increase and loss are caused by the following reasons:

1) the ink film is too thick or thin due to the improper selection of wire (the ratio of wire aperture to wire diameter is improper)

2) there are problems in the plate making process (low vacuum pressure in the exposure process, overexposure and incomplete template development)

3) the scraper pressure and scraping speed are too large

4) the ink viscosity is inappropriate

5) material surface characteristics (absorbent and non absorbent) do not match the process

in order to ensure the accurate reproduction of the printing level, it is necessary to measure the point increase and loss and formulate a reasonable deviation. First of all, the number of lines added to the film containing the eye tone pattern and the matching relationship between the number of wires and the number of wires are the basis for determining whether to obtain the accurate tone range. On this basis, it is necessary to use the densimeter to accurately increase or reduce the expected point data

Table 4 shows the percentage of points in the film and print sampling area measured with a densimeter. When the dot size of the printed matter or the dot on the plate making film (or gray scale) increases or decreases, the step loss will occur

it can be seen from table 4 that 10% - 30% of the gradient of the printed image is usually shown, especially in intelligent manufacturing, interconnection + now the point is reduced, 40% - 90% of the gradient is shown as the point is increased, and the increase at 60% - 80% is the most serious

the above data must be tested after the ink is completely cured to get accurate results. Measure the point percentage of different printing areas and make records. Measure the corresponding point percentage in the same area (ladder) on the film and make records for comparison

it should be noted that the above description of point changes is obtained under the condition that the types of plates, films, inks, printing materials and printing machines are consistent. When any one of these factors changes, the results will be different. For example, using the same ink, when the substrate changes, the change law of dot size will be different

ink overprint

ink overprint is the result of various hardness values of metal materials, which determines the success or failure of color reproduction

there are two cases of point juxtaposition and point superposition on printed matter. The overprint effect of ink is mainly related to the drying degree of ink layer and the thickness of ink layer

if the printing points are juxtaposed at a certain distance, the final color should be obtained according to the additive mixing principle of color light. When there is no masking where the average import price of ink in January 2016 is between 6779.8 yuan/ton, the mixed color produced is ideal

overprint is still common on the top of printed matter. The incomplete transparency of ink determines that the loss of color is inevitable, so it is necessary to control overprint. Most printing test strips have overprint areas, which can help find ink overprint problems. If the overprint effect of yellow, magenta and cyan is similar to neutral gray in the three color overprint area of the test strip, it indicates that the color mixing basically conforms to the printing color principle, and the ink overprint is normal; Otherwise, it is necessary to adjust the ink stacking results by adjusting the addition angle of the print film or taking other measures before printing and during the production process. See Table 5 for the control items

summary

printing production needs to carry out data-based and standardized quality control. First of all, it is necessary to control the parameters of plate making film, plate making process, printing materials (ink, substrate) and printing process. Printing enterprises that are starting to establish a quality control system should start with controlling these parameters

the implementation of digitization and standardization before and during printing requires a rigorous work style, and the habit of using detection instruments to correctly record data must be formed. Although the process of conducting detailed analysis and establishing data system after measuring plate making and printing parameters is a long-term and tedious work, quality stability is the central task of printing enterprise management and the most effective method to obtain stable printing quality. Data and standardization work is also an effective measure to reduce quality problems. In today's increasingly fierce market competition, whoever wins the commanding height of quality will eventually win the market

(author/Dongguan wotian printing equipment Co., Ltd. daytime)

source: Printing Technology

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