Screen printing process is also known as screen printing process, screen printing is a main printing method in hole printing. Screen printing belongs to plate printing, which is known as the four printing methods together with lithography, letterpress and gravure. When printing, the extrusion of the scraper makes the ink transfer to the substrate through the mesh of the graphic part, forming the same graphic as the original manuscript. Screen printing equipment is simple, easy to operate, simple printing, plate making and low cost, strong adaptability. Screen printing has a wide range of common prints: color oil paintings, posters, business cards, bound covers, commodity signs and printing and dyeing textiles.
Screen is composed of five elements, namely screen plate, scraper, ink, printing table and substrate. The basic principle of screen printing is: the use of screen printing plate graphic part of the mesh through ink, the basic principle of non-graphic part of the mesh is not permeable. When the product is printed, ink is poured on one end of the screen plate, and the ink on the plate is applied with the scraper and moved to the other end. The ink is extruded by the scraper from the mesh of the graphic part to the substrate during the movement. In the printing process, the scraper is always in line contact with the screen plate and the substrate, and a certain gap must be maintained between the screen plate and the substrate, so that the screen plate produces a reaction force on the scraper through its own tension when printing, which is called the resilience. The action of resilience makes the screen plate and the substrate only in mobile line contact, while the other parts of the screen plate and the substrate are separated. Make the ink and the screen fracture movement, ensure the printing dimensional accuracy and avoid rubbing the substrate. When the scraper is lifted after scraping the entire page, the screen plate is also lifted and the ink is gently scraped back to the initial position. This is a printing run.
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Screen printing can be printed not only on a flat surface, but also on a curved surface, a spherical surface and a concave and convex surface substrate. At the same time, screen printing can be printed not only on hard objects, but also on soft objects, which is not limited by the texture of the substrate. In addition, in addition to direct printing, screen printing can also be printed by indirect printing method according to needs, that is, screen printing on gelatin or silicone plate, and then transfer to the substrate. This type of printing is also called pad printing. Equipment is as follows
Pad printing silicone
The ink layer is thick, three-dimensional, and rich in texture. The ink layer thickness of offset printing and letterpress printing is generally 5 microns, gravure printing is about 12 microns, and the ink layer thickness of screen printing is generally about 30 microns. Thick screen printing for printed circuit boards with ink layers up to 1000 microns thick. Screen printing can not only be monochrome printing, but also can be colored and screened color printing. Today's screen printing products can reach a maximum of 3 meters ×4 meters, or even larger
Screen printing can use many types of inks. Namely: oil, water, synthetic resin emulsion type, powder and other types of ink. Screen printing is soft and flexible, not only suitable for printing on soft items such as paper and cloth, but also suitable for printing on hard items, such as glass and ceramics. Printed circuit board, etc.
The main process of screen printing is: making the original (film), making screen plates, printing, ink drying, etc
The scraper pressure gauge is a display instrument to measure the friction between the scraper and the screen according to the scraper pressure. The unit of measurement is kg/cm. The test method is: the test strip is placed between the scraper and the wire mesh, and the pressure gauge is used to pull the test method. In the measurement, according to the number indicated on the dial, the friction value is obtained.
Mesh scale is also called warp and weft density meter. Mainly measure the mesh number of various wire mesh. There are two kinds of mesh ruler: glass plate and plastic plate. Measuring method: When measuring, first make the mesh in a transparent state or put on the table, put the mesh ruler on the mesh, and then slowly move the mesh ruler on the mesh, so that the vertical line on the mesh ruler is parallel to the warp or weft line of the mesh ruler, then due to the overlapping effect of the vertical line on the mesh ruler and the mesh ruler, the formation of a prism pattern on the mesh ruler, The corresponding scale number on the mesh scale pointed diagonally across the pattern is the mesh number (inches or centimeters) of the measured mesh. Mesh ruler is a common tool for measuring the mesh number, usually to maintain the accuracy of the measuring instrument, the accuracy of the measurement data, it is necessary to keep the mesh ruler clean, to prevent scratches and bruises on the mesh ruler, after use, the instrument should be wiped clean and properly kept.
The main materials used in the production of mesh frames are hollow aluminum profiles, wood, steel and other materials. The most commonly used are wooden mesh frames. In the selection, according to different circumstances, choose different material mesh frame, wooden mesh frame has the characteristics of simple production, light weight, convenient operation, low price, simple mesh method. This screen frame is suitable for hand printing. However, the screen frame of wooden materials is poor in solvent resistance and water resistance, and it is easy to deform after water immersion, which will affect the printing accuracy. Hollow aluminum mesh frame and cast aluminum molding mesh frame, with light operation, high strength of mesh frame, firm and not easy to deformation,
Central control aluminum mesh frame
Solvent and water resistance, beautiful features. Suitable for mechanical printing and manual printing. The steel mesh frame has the characteristics of firm, high strength and good solvent and water resistance, but it has the disadvantages of heavy and inconvenient operation, so the scope of use is very small.
The commonly used silk screens in screen printing are silk nets, nylon silk nets, polyester silk nets, stainless steel mesh
What problems should we pay attention to when choosing mesh?
① First consider the difficulty of the ink through the screen, and generally consider this aspect when selecting. Intermediate tone ink with high transparency ink, the pigment particles are finer, the ratio of the pigment carrier is more, the ink is better, this ink can also pass well when using high mesh screen; General plastic ink has good passability; Special purpose inks, inks with high pigment concentration, despite being fine particles, have relatively poor permeability.
If the substrate surface is rough, generally use a lower mesh screen.
Such as leather, canvas, foam sheet, wood and other materials printing. Due to the rough surface of the substrate, ink absorption is strong, so it is necessary to use a low mesh screen to ensure sufficient ink through
③ Select the mesh according to the fine degree of the original graphic lines. In contrast to the above, in general fine lines to choose high mesh mesh.
④ The selection of silk screen should also consider the cost, under the premise of meeting the printing requirements, try to choose the lower price of silk screen.
What are the common solvents for screen printing:
The solvent is mainly used in screen printing to adjust the viscosity of the ink, adjust the drying speed of the ink, and clean the screen plate and printing scraper after printing. The commonly used solvents are mainly aliphatic hydrocarbons, aromatics, ketenes, esters, and B
Alcohol, ethylene glycol ether and other six categories. The solvents in each category can be divided into several categories.
Screen printing ink to do the test.
① Adhesion test to determine the degree of adhesion of ink on different materials.
The adhesion firmness of the ink and the substrate material is tested by the tolerance test.
(3) Kneading resistance is mainly to carry out a strong operation test on the printed surface of cloth, paper, film and other materials to test the degree of adhesion and firmness of the ink to the substrate material.
④ After the printing surface of the friction-resistant cotton cloth is exposed to sunlight, the cotton cloth is rubbed a certain number of times to check the pollution of the cotton cloth.
⑤ Weather resistance Weather resistance has temperature, light, acid and alkali resistance, etc. To check the discoloration of the printing pigment, the degree of fading and the decomposition of the printing ink film, the degree of discoloration.
Heat resistant water is usually the unit area of the printing surface, adding a certain amount of warm water for a certain time to stir, to test the degree of water pollution. In particular, observe the degree of pigment solubility in water in the ink to determine whether the ink is suitable for the printing of food utensils and toys.
Oil resistance refers to the adaptability of grease, mechanical oil, etc., which is different from solvent resistance.
Why should there be a gap between the screen plate and the substrate: it is determined by the characteristics of the screen printing. The screen printing plate is based on the screen, the screen is stretched on the screen frame, and there is a certain sag when the screen is in a horizontal state. Especially in the scraping process, the sag value will increase, in order to make the substrate at the end of the scraping ink does not stick to ensure the printing quality, the lowest part of the screen plate must leave the substrate. In screen printing, the scraper and the screen plate are mobile line contact, screen plate and the scraper can not be in contact with the substrate before and after scraping, if the screen plate does not leave the graphic part before and after printing, the ink will continue to permeate and spread resulting in the size of the graphic line expansion, making the printing size accuracy decrease and the substrate distortion.
How to determine the gap between the screen plate and the substrate, and determine the amount of gap between the screen plate and the substrate is mainly based on:
① The size of the screen plate.
② The size of the tension of the mesh.
③ Center sag of screen printing plate.
④ The shape of the substrate.
⑤ The properties of the substrate material.
⑥ Substrate surface morphology.
⑦ ink viscosity, etc.
The gap between the screen plate and the substrate, due to different printing conditions, and the gap value is also different. Generally, for ceramics, glass, metal surface, hard plastic and other substrates with low ink absorption, the printing gap value is relatively high. On the contrary, for cloth, paper and soft plastic substrates with large ink absorption, the requirements for printing gap values are not so strict. Usually, prints with higher accuracy requirements require a gap value between l and 3 mm when printing. For ordinary printed matter, the gap can be 2-6 mm. The gap value is smaller when the surface is printed, and the gap value is larger when the plane is printed.
Why does the ink surface of screen printing produce bubbles? How to avoid bubbles? The main reasons for the formation of bubbles are as follows.
① The surface of the substrate is attached with dust and oil stains and other substances.
② The ink itself has bubbles or uneven viscosity when printing.
③ The printing speed is too fast or the printing speed is uneven.
The following measures can usually be used to prevent bubbles from forming in the ink layer.
① Try to use hydrophobic solvents.
When the ink itself produces bubbles, an appropriate amount of defoamer can be added to the ink, if the ink fluidity is poor, when the screen plate is lifted, the ink flow is relatively small, and the screen traces can not be filled, and the surface of the smooth and flat ink layer can not be obtained.
To prevent screen marks after printing. The following methods can be used.
1) Use ink with greater fluidity for printing.
2) The use of slow drying ink printing can be considered, and the flow time of the ink can be increased so that the ink is gradually cured.
3) Try to use a single diameter wire mesh with a smaller diameter when making plates. ,
Why does the surface of screen printing ink layer crack? How to prevent cracks on the surface of the ink layer?
The main reason for cracking is: due to the role of solvents and temperature changes caused by large. General styrene products using alcohol and petroleum solvents such as weak solvents will also occur cracking. Some substrate materials can also cause cracking due to their own factors. In order to prevent the occurrence of cracking, the nature of the ink and the solvent resistance of the substrate should be considered when selecting the solvent. Solvent resistance and oil resistance can be selected as the substrate material. Pay attention to keep the workshop temperature balanced. In multi-color overprinting, it is necessary to fully dry each color after printing, and strictly control the drying temperature. Generally speaking, paying attention to the above aspects can effectively control the cracking phenomenon of the ink layer.