Screen Printing 150 Questions (57-64)

â–¼57. What preparations must I make before printing?

Preparing for printing is an indispensable task for ensuring the smooth progress of printing. Before printing production, the following basic preparations must be made.

1 Clear up the printing site to ensure that there is a certain operating space around the printing table or printing machine to prevent other objects from obstructing work. Keep the site clean and avoid the effects of dust, etc. on the print quality.

2 Pay attention to adjust the humidity and temperature of the printing shop to meet the printing requirements.

3 Prepare a suitable squeegee and inspect the squeegee for any bruises. Grind the squeegee if there is a scratch to prevent it from affecting the print quality.

4 Check whether the screen printing plate is in good condition, and find that the problem is corrected in time to ensure that the screen printing plate is surrounded by the screen and text to avoid the phenomenon of ink leakage in the non-graphic part. Wipe the plate with a damp cloth to remove any dust on the plate.

5 According to the characteristics of the substrate, adjust the appropriate gap between the screen printing plate and the substrate.

6 Check that the color and viscosity of the ink used meet the requirements.

7 According to the size of the substrate, determine its position on the printing table. Fix the rules for printing auxiliary equipment.

8 Make preparations for using drying racks and other drying equipment. If using a dryer, adjust the drying temperature in advance. Prepare for the above inspections and start printing.

In addition, according to actual needs, we must pay attention to other related preparations.

â–¼58. What issues should be noted before printing?

Apart from making preparations before screen printing, other related issues should also be noted. Precautions should be taken from the following aspects before printing.

1 Printing site cleaning Screen printing With the continuous improvement of printing accuracy requirements, especially high-precision printing of printed circuit boards, electrical components, and so on, the environment has also put forward higher requirements. General requirements The printing shop and plate making shop must be clean and there is no dust in the air, otherwise it will affect the printing quality. For workshops with high requirements for printing accuracy, air purification equipment must also be installed if necessary to keep the printing and plate making workshops clean.

2 Temperature and Humidity In spring, summer, autumn and winter, the temperature and humidity in the four seasons change greatly, and the impact on printing accuracy is also relatively large. Especially for multi-color overprinting, the effect is more obvious. Therefore, indoor humidity and temperature must be relatively stable. And according to the seasonal changes in the indoor temperature and humidity properly adjusted to ensure the print size accuracy.

3 fully understand the nature of the substrate due to a wide range of screen printing applications, a wide range of printing materials, and the shape of the substrate is also different, according to the different requirements in the choice of appropriate ink and solvent and scraper.

â–¼59. What aspects of screen printing inks are usually tested?

Due to the wide range of screen printing applications, the requirements for screen printing inks are also stricter. Therefore, in order to meet the different requirements of printing products for inks, the following aspects of screen printing inks should be tested.

1 Adhesion Tested to determine the degree of ink adhesion on different materials.

2 Flexibility The fastness of adhesion of the ink to the substrate material is checked by the resistance test.

3) Resistance to rubbing and rubbing The rubbing resistance is mainly performed on the printed surface of materials such as cloth, paper and film to test the degree of adhesion and firmness of the ink to the printed material.

4 After the printing surface of the abrasion-resistant cotton cloth is exposed to sunlight, the cotton cloth is rubbed for a certain number of times to check the degree of contamination of the cotton cloth.

5 Weather resistance Weather resistance also contains light resistance. Weather resistance is usually placed outside the print, the wind and sun, rain and other tests to test the printed pigment discoloration, bleaching degree and decomposition of the printing ink film, discoloration and so on.

6 heat resistance, cold resistance Because some prints to use at different temperatures. Therefore, some prints are processed by high temperature after printing, and some prints are subjected to low temperature processing after printing. Through the heat resistance and cold resistance test to test the ink at different temperatures.

7 Hot water resistance The hot water resistance test is usually to add a certain amount of warm water to the printing surface per unit area for a certain period of time to test the degree of contamination of the water. In particular, the extent to which the pigment in the ink is soluble in water is observed to determine whether the ink is suitable for the printing of food utensils and toys.

8 Oil resistance Oil resistance refers to the suitability of oils and fats, mechanical oils, etc., which is different from solvent resistance.

â–¼60. What are the characteristics of color screen printing?

Compared with gravure printing and offset printing, color screen printing has the following features in addition to the characteristics of screen printing itself.
1 has a very strong expression, colorful, strong three-dimensional.

2 has a strong light resistance, drug resistance, durability, long-lasting use, without fading, especially in the screen printing on plastic sheet substrates this feature is particularly prominent.

3 The cost is relatively low.

In addition to the above advantages, color screen printing is also unsatisfactory. This is the reason for considering the factors such as matching of photographic screen lines and screen meshes, screen angles, etc. in color screen printing. A certain degree of difficulty.

â–¼61. Color screen printing color sequence is how to arrange?

Color screen printing color sequence arrangement is generally yellow, cyan, magenta, and black. Taking into account the process requirements, the order of the colors is also cyan, magenta, yellow and black. In foreign countries, black, yellow, magenta and cyan are also used for printing color sequences. Since the shapes of the substrates in screen printing are different (planes and curved surfaces) and the nature of substrates is different, the color sequence is also different. For example, When printing on transparent objects, the color sequence should be black, yellow, cyan, magenta. The arrangement of the color sequence has an important restriction relationship to reflect the original color. How to ensure that the color of the copy is not distorted, in addition to the correct use of the ink, it also requires long-term work practices and experience accumulation, to constantly explore and summarize. Determine the screen printing color sequence to consider two factors, one of which is the transparency of the ink on the substrate, which is the main factor in the two factors. Since in the screen printing, ink of a new color is produced by mixing or superimposing inks of various colors, if the transparency of the ink on the substrate is poor, the ink of the first color will be covered when the second color is printed. It cannot be combined with the second color to create a new color. The second factor is that people’s eyes have different abilities to experience various colors. In general, people's eyes are most sensitive to magenta, followed by cyan and least sensitive to yellow. Because people have different color sensitivities, it often results in the expansion or reduction of yellow dots, the loss and the change in the size of ink, which can affect the printing quality.

The correct screen printing procedure and requirements are that the first color should be printed with a cyan version, and after printing, a contrast check of the gradation reproduction shall be performed with a step scale and a color scale. The second color is printed in yellow, so that the two colors are stacked in green and then the human eye is green

The characteristics of strong tone recognition ability, check the print quality with reference to the scale and color mark. The third color is printed in red, and the black version is used last. The black version enhances contours and dark levels. After the four-color version is printed, the same color copy as the original is obtained.

â–¼62. Color screen printing should pay attention to what issues?

Color screen printing should pay attention to several issues, as described below.

1 Use a strong, non-deformable metal mesh frame for color screen printing. This is because if an unsturdy frame or a wood frame plate is used, skew or deformation is likely to occur during printing, the quality of the screen plate will be reduced, and accuracy will be difficult to guarantee. When the deformation is severe, the screen plate cannot be used. This will affect the print quality. So you must use a sturdy metal mesh frame. The frame size of a set of color screen printing plates should be the same. Mainly for printing with a unified positioning rules, and easy to color printing. The number of screen meshes of a set of color screen printing plates should be the same as that of the screen materials, and the tension of the stretch net and the thickness of the screen plate should be the same. It is mainly to ensure that the ink layers after printing of different colors are uniform and have the same effect, and the original colors and graphics are perfectly reproduced.

2 The use of scraper hardness is generally 70 degrees Shore hardness. Scraper edge accuracy is high. Due to its moderate hardness, it is conducive to meeting printing requirements, facilitating the improvement of printing accuracy and ensuring the printing quality.

3 The screen printing ink with high viscosity should be used as much as possible. The ink with higher viscosity will not cause the problem of thickening of the graphic lines due to the diffusion of oil, which will help to improve the fineness and sharpness of the lines.

4 Photo-screen plate screen number (line / inch), usually between 55 ~ 120 lines. This can reduce or avoid moire.
The general relationship between the number of screens and the number of screens is: screen (number of lines): 4.2X screen number.

â–¼63. Why is there a certain gap between the screen printing plate and the substrate?

The certain gap between the screen plate and the substrate is determined by the characteristics of the screen printing.
The screen printing plate is based on a screen, and the screen is stretched on the screen frame. When the screen is in a horizontal state, a certain degree of sag occurs. Especially in the squeegee process, the sag value will also increase. In order to ensure that the substrate will not stick to the ink during final squeegee printing and thus ensure the printing quality, the lowest part of the screen printing plate must leave the substrate. In screen printing, the squeegee and the screen plate are in contact with the movable line. The screen plate and the squeegee cannot contact with the substrate before and after the squeegee is printed. In particular, the screen plate moves along with the squeegee. And leaving the substrate, if the screen printing plate does not leave the graphic part before and after printing, the ink will continue to infiltrate and spread, causing the size of the graphic line to expand, which will cause the printing dimensional accuracy to decrease and the substrate to be distorted.
There must be a certain gap between the screen printing plate and the substrate. This is also closely related to the form of movement of the substrate. When the substrate is printed, especially when the surface of the forming surface (such as a container) is printed, the substrate is printed at the same time. Also make a circular motion. There is a certain gap will not result in the vagueness of the substrate graphic or text distortion.

â–¼64. How to determine the gap between the screen plate and the substrate?

It has been mentioned above why there must be a certain gap between the screen printing plate and the substrate when printing, but the size of the gap can not be arbitrary. If the gap between the screen printing plate and the substrate is too small, penetration and sticking may easily occur in the direction of the movement of the squeegee. If the gap is too large, the screen printing plate can not be restored after printing, loose and sagging, so that the screen size after printing is smaller than the screen printing plate screen size. In severe cases, the screen plate can be damaged and cannot be printed. If it is the use of stainless steel wire with less elasticity, the layout will not touch the surface of the substrate and printing cannot be performed.

The main basis for determining the amount of clearance between the screen plate and the substrate is:

1 The size of the screen printing plate.

2 Stretch the tension of the mesh.

3 The center sag of the screen plate.

4 the shape of the substrate.

5 The nature of the substrate material.

6 substrate surface morphology.

7 ink viscosity and so on.

The gap between the screen printing plate and the substrate is different depending on the printing conditions. In general, objects with relatively low ink absorptivity, such as ceramics, glass, metal surfaces, and hard plastics, have relatively high requirements for printing gap values. However, the requirements for the print gap value of objects with softer surface, such as cloth, paper, and soft plastics, which absorb ink more, are less stringent. In general, high-precision prints require a clearance of between l and 3 mm when printed. For ordinary prints, the gap can be 2-6 mm. When the surface is printed, the gap value is smaller, and the gap value in the plane printing is larger.