Understand the quality control in screen printing

With the increase in the level of packaging products, packaging screen printing has emerged in a market, the market share in the packaging industry is expanding. The process of screen printing and packaging is relatively simple, the basic principle is: the use of printing plate (stencil printing screen printing), with the help of printing equipment (automatic or semi-automatic screen printing machine) or artificial, through the squeegee The pressure allows the ink to penetrate through the openings (mesh holes) of the screen and adhere to the substrate to complete the printing process. However, there are many factors affecting product quality in the entire screen printing process that directly affect the quality of the final product. Improper equipment, printing plates, inks, post-processing and operating skills can cause printing failures, and most of them are Due to the cross-effects of various factors. Packaging products have high requirements for printing quality. Therefore, the quality control of packaging screen printing is more important. The following is a brief analysis of the quality control of screen printing packaging products as follows.

Prepress carefully selected

Box

In general, the screen frames used in screen printing and packaging are mostly aluminum alloy frames. The aluminum frames are highly praised by users for their resistance to stretching, strength, good quality, light weight, and ease of use. The size and material of the frame play an important role in the quality of the screen.

2. Selection network

Silk screens are divided into polyester meshes, nylon meshes, and stainless steel meshes. They are divided into multifilament meshes and monofilament meshes. Depending on the accuracy of the printed pattern, the quality of the printed matter, and the requirements of the customer, in general, single-filament screens are used for fine products.

3. Stretching

Aluminum alloy frame is usually used pneumatic stretching machine stretch net to ensure the screen tension. In order to achieve the best print quality, the tension of the screen must be uniform. If the tension is too high, the screen will be damaged and the printing will not be tolerated; if the tension is too low, the printing quality will be low and the overprinting will be inaccurate. The tension of the screen depends on the screen printing pressure, the printing accuracy, and the stretch resistance of the screen.

4. Printing

(1) The first-strength screen plate should be degreasing to enhance the affinity of the photosensitive adhesive and the screen.

(2) In order to improve the screen printing quality, the screen printing requirements for the printing plate are positive and positive. The photosensitive material is selected according to the printing material. The thickness of the printing film depends on the requirements of the printing product.

(3) Exposure time: The exposure time depends on the film quality, the number of screen meshes, the thickness of the photopolymer, and the power of the exposure light source. The correct exposure time can improve the quality of screen printing.

(4) Overhaul and double exposure: After the screen is dried after the first exposure, the quality of the screen printing should be checked and repaired. After the correctness is determined, double exposure is required to enhance the printing resistance of the screen.

(5) Substrate: According to the product and customer's requirements to determine the required printing materials, if the customer provides the substrate, to check the substrate material, color, thickness uniformity, surface texture, etc., according to their different requirements Choose the right printing ink.

(6) Ink: The physical properties of printing inks include density, fineness, flowability, and lightfastness, which have a great influence on the quality and special effects of printed matter. If the density is moderate, the fineness meets the requirements, the ink flowability is better, the light resistance is better, and the product can be printed to achieve the desired effect. Ink type solvent ink (natural drying) and UV light curing ink, according to the requirements of equipment and printing methods, select the matching ink.

Printing control variables

Influencing factors

Printing is the last process. In the printing process, if it is found that the printing effect does not meet the requirements, adjustments can be made to remedy the problems that have already occurred. Obviously, if you use a defective plate, squeegee, or other equipment, you will certainly not achieve the desired printing effect. At the end of printing, the drying equipment is usually used for drying. Two drying machines are commonly used for screen printing. One is drying with IR heat source; the other is curing with UV light source. With regard to the printing and drying section, there are 8 major variables.

1) Network distance

Non-contact printing is a prominent feature of screen printing. That is, the screen printing plate keeps a small distance above the substrate. This distance is called the web distance. The pressure of the screen printing against the squeegee brings the printing plate in contact with the substrate and completes the transfer of the graphic. The net distance should be kept properly, otherwise, the printing effect is not good.

2) Peeling distance (off-grid distance)

Peeling distance refers to the distance that the squeegee is separated from the substrate after the squeegee has printed an area of ​​the image of the printing plate. In the ideal case, the peel distance does not need to be adjusted, and the distance between the printing plate and the substrate can be easily separated by using the mesh distance. However, if the ink is too thick or printed on a large area, the printing plate and the substrate may not be separated quickly, resulting in blurred images. In these cases, the distance to the off-grid must be fine-tuned, but the amount of adjustment must not be too large. Otherwise, the image may be distorted.

3) Print plate flatness

To make the print quality consistent, the flatness of the printing plate is very important. Any depressions and projections on the table have an adverse effect on the print quality.

4) Degree of parallelism between printing platen and screen plate

The distance between the screen and the entire platen surface must be the same. When measuring the distance before printing, it is necessary to measure more points.

5) Color sequence

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

6) Curing temperature

For solvent 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 inks are dried using UV light. The UV light intensity is high, the curing speed is fast, the strength is low, and the curing speed is slow. Therefore, we must 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 drying time will be longer and the ink layer will not be completely dried.

8) curing time

The curing speed of the ink affects the quality of the ink after curing, and the curing speed must be adjusted according to the curing temperature and the light intensity of the UV lamp so that the ink film is completely cured. The curing quality of the printed product depends on the curing speed, the curing temperature and the power adjustment of the UV lamp.

surroundings

Screen printing work environment has a direct impact on the printing results, especially high-quality printed matter must be printed and dried under a stable temperature and humidity environment. If constant temperature and humidity cannot be guaranteed, measures must be taken to monitor environmental changes and compensate them. There are two variables that affect the printing environment.

1) Contaminants in the air

Screen printing workshops have various contaminants, such as dust and fluff, which have an impact on print quality. A clean environment can eliminate these problems, but it requires an investment.

2) Ambient temperature and humidity

In the platemaking and printing workshops, any change in temperature and humidity will affect the print quality. If it does not reach constant temperature and humidity, it is necessary to flexibly adjust other variables according to changes in temperature and humidity. Although it is cumbersome, it can save expenses.

In addition to effectively controlling and handling many of the factors that affect the quality of screen printing as described in this paper, training high-quality operators, establishing sound process management procedures, and quantitatively controlling each variable can effectively improve the quality of printed products. The company brings economic benefits.