Best understanding of quality control in screen pr

Understand the quality control in silk printing

with the improvement of the grade of packaging products, packaging printing shows a lot of vitality, and its market share in the packaging industry is expanding day by day. The technological process of silk printing and packaging is relatively simple. Its basic principle is: use the printing plate (the plate with pictures and texts), with the help of printing equipment (full-automatic or semi-automatic silk printing machine) or manual, through the pressure of the doctor blade, make the ink penetrate through the opening (hole) of the plate and attach it to the substrate, so as to complete the whole printing process. However, there are many changing factors that affect the product quality in the whole silk printing process, which directly affect the quality of the final product, such as improper equipment, printing plate, ink, post-processing and operating skills, which will cause printing failures, and most of them are the result of the intersection of many factors. Packaging products have high requirements for printing quality, so the quality control of packaging printing is more important. The following is a brief analysis of the quality control of packaging products

carefully select materials before printing

1 Generally speaking, the frames used for printing and packaging are mostly aluminum alloy frames. Aluminum frames are highly praised by users for their tensile resistance, high strength, good quality, light weight and convenient use. The size and material of the frame play an important role in the quality of the version

2. Let the testing machine stretch the standard tensile sample at the speed specified in the standard

silk is divided into polyester wire, nylon wire and stainless steel wire, and is divided into multi wire and single wire. Depending on the accuracy of the print pattern, the quality of the print and the requirements of customers, fine products usually use single silk

3. The aluminum alloy frame is usually stretched by pneumatic stretching machine to ensure the tension of the version. In order to achieve the best printing quality, the tension of the plate must be uniform. If the tension is too high, the wire will be damaged and not resistant to printing; If the tension is too low, it will lead to low printing quality and inaccurate overprint. The tension of the plate depends on the printing pressure, printing accuracy and the tensile elongation of the wire

4. Print

(1) first, the stretched version should be degreased and roughened to enhance the affinity between photosensitive glue and silk

(2) in order to improve the drying quality of the printing plate, the printing film required for silk printing is required to be positive print, the photosensitive material is selected according to the substrate, and the thickness of the photosensitive adhesive depends on the requirements of the printing material

(3) exposure time: the exposure time depends on the film quality, the number of silk meshes, the thickness of photoresist and the power of the exposure light source. The correct exposure time can improve the drying quality of high-performance synthetic rubbers such as rare earth butadiene rubber, ethylene propylene rubber, brominated butyl rubber and so on

(4) maintenance and secondary exposure: after the version after the first exposure is dried, it is necessary to check the quality of the version and make repairs. After confirmation, it is necessary to conduct secondary exposure to enhance the printing resistance of the version

(5) substrate: determine the required substrate according to the requirements of the product and the customer. If it is the substrate provided by the customer, check the material, color, thickness uniformity, surface texture, etc. of the substrate to select the appropriate printing ink according to its different requirements

(6) ink: the physical properties of printing ink mainly include density, fineness, fluidity and lightfastness, which have a great impact on the quality and special effects of printing. If the density is moderate and the fineness meets the requirements, the prepared ink has ideal fluidity and light resistance, and the printed products can achieve the desired effect. Ink can be divided into solvent based ink (natural drying) and UV curing ink. The matching ink should be selected according to the requirements of equipment and printing method

printing control variables

influencing factors

printing is the last process. During the printing process, if it is found that the printing effect does not meet the requirements, it can be adjusted to remedy the problems that have occurred. Obviously, if a defective printing plate, doctor blade or other equipment is used, the ideal printing effect will not be achieved. After printing, drying equipment is usually used for drying. There are two kinds of dryers commonly used for silk printing, one is to use IR heat source for drying; The other is curing with UV light source. There are eight main variables in the printing and drying section

1) distance

non contact printing is a prominent feature of printing, that is, the silk printing plate maintains a small distance from the top of the substrate, which is called distance. Printing relies on the pressure of the doctor blade to make the printing plate contact with the substrate to complete the transfer of pictures and texts. The distance should be kept appropriate, otherwise, the printing effect will be poor

2) stripping distance (separation distance)

stripping distance refers to the distance between the plate and the substrate after the scraper scrapes the ink over a certain area of the plate image. Ideally, the stripping distance does not need to be adjusted, and the use of the distance can make it easy to separate the printing plate from the substrate. However, if the ink is too thick or the printing area is large, the printing plate and the substrate may not separate quickly, resulting in blurred images. In these cases, the distance must be adjusted slightly, but the adjustment amount must not be too large, otherwise the image will be deformed

3) flatness of the printing board table

the flatness of the printing board table is very important to ensure that the special fixtures for the tensile test of printing fasteners, such as shoulder type fixtures and short bolt connectors, can meet the requirements of the tensile test, and the quality is stable and consistent. Any concave and convex on the table will adversely affect the printing quality

4) parallelism between the printing board table and the plate

the distance between the plate and the whole surface of the printing board table must be consistent. When measuring the distance before printing, be sure to measure at multiple points

5) color sequence

in multicolor printing, color sequence is a key variable. The order of image printing has an important impact on the quality of the final printed matter. The color sequence of printing depends on the accuracy of the printed matter, the material of the substrate and the saturation of the pattern

6) curing temperature

for solvent based inks or water-based inks, increasing the drying temperature can shorten the drying time. However, if the set temperature is too high, it will affect the dimensional stability of the substrate

7) UV light intensity

UV ink should be dried by UV light. High UV light intensity leads to fast curing speed, while low intensity leads to slow curing speed. Therefore, we should choose the appropriate light intensity. The spectral distribution of the UV light source should correspond to the characteristics of the UV ink used. Otherwise, the ink layer will not dry completely no matter how long the drying time is

8) curing time

the curing speed of the ink affects the quality of the ink after curing, which must sometimes make it difficult to pull the strip. The curing speed must be adjusted according to the curing temperature and the light intensity of the UV lamp, so that the ink film can be completely cured. The curing quality of printed products depends on the curing speed, curing temperature and UV lamp power regulation

environment

the working environment of silk printing has a direct impact on the printing results, especially the printing and drying of high-quality prints must be completed under a stable temperature and humidity environment. If the constant temperature and humidity cannot be guaranteed, the method of monitoring environmental changes and giving compensation should be adopted. The following two variables affect the printing environment

1) pollutants in the air

there are various pollutants in the printing workshop, such as dust and fluff, which have an impact on the printing quality. A clean environment can eliminate these problems, but it requires an investment

2) ambient temperature and humidity

in the plate making workshop and printing workshop, any change in temperature and humidity will affect the printing quality. If the constant temperature and humidity cannot be reached, it is necessary to flexibly adjust other variables according to the changes of temperature and humidity. Although it is cumbersome, it can save expenses

in addition to effectively controlling and dealing with the many factors affecting the printing quality described in this paper, cultivating high-quality operators, establishing perfect process management procedures, and quantitatively controlling various variables can effectively improve the printing quality and bring economic benefits to the enterprise