Glass screen printing refers to the screen printing method using glass plates or glass containers as the main product. The use of glass has a long history in China. Even today, glass still occupies an important position in our lives. At present, the financial crisis that originated on Wall Street in the United States, like the "tsunami", has generally impacted the nerves of the world and has had a certain impact on the economies of all countries in the world.
Since 2008, the surge in international oil prices, the appreciation of the renminbi, and the reduction of export tax rebates have negatively affected the export-oriented enterprises of China ’s screen printing industry. For enterprises, it is even worse, especially for small and medium-sized packaging and printing enterprises. In addition, after the implementation of the new labor law, the increase in labor costs has increased the operating costs of screen printing enterprises, coupled with serious downstream customers' bets, resulting in tight or even broken capital chains, increasing the difficulty of operating screen printing enterprises. Therefore, screen printing enterprises should rationally combine the actual situation and product characteristics of the enterprise, and serve customers with the lowest production cost and the highest production efficiency. Under the conditions of both opportunities and challenges, applying screen printing to glass products is undoubtedly a very effective response measure, which can increase market development efforts and open up a variety of marketing channels.
As a building decoration material, glass has the advantages of being flat and transparent, being able to block wind and rain, and being easy to scrub. The screen-printed glass is more popular for indoor and outdoor decoration, which greatly meets people's life and aesthetic needs. A variety of beautiful patterns are striking and beautiful, and they are perfect. Now that glass containers have begun to develop into medical supplies and industrial supplies, the high quality of glass products decorated with screen printing technology will become a new growth point for glass printing.
1. The screen printing process of glass
According to the principle of screen printing, the ink is printed on the surface of the glass, and then the curing measures of the ink are adopted, and the printed pattern is firm and durable. The technological process is as follows:
Stretching net → sizing → drying → printing plate → developing → drying ↓
Flat glass → Cutting → Edge grinding → Cleaning and drying → Printing ↓
sintering
1. Glass suitable for printing
According to the user's requirements, the glass can be cut in a regular shape or an irregular shape, then beveled and ground, then washed and dried for use. Special care should be taken here that there are no traces of water on the glass surface.
2. The choice of wire mesh
The screen printing plate for glass printing is the same as the general screen printing plate. The glass screen printing plate uses synthetic fiber screen, stainless steel screen, and natural fiber screen. When printing general color materials, the above-mentioned screens are acceptable. Generally, cheap synthetic fiber screens are used; when used as gold and silver decoration, stainless steel screens cannot be selected. The specifications of wire mesh are generally 270 ～ 300 mesh.
3. The choice of network frame
At present, the more regular and less deformed mesh frame is an aluminum alloy mesh frame, its size should be larger than the pattern, the specific size should be the outer edge of the pattern from the mesh frame should be between 70 ~ 100mm. In addition, the strength of the frame is very important. The key is that the horizontal rigidity is sufficient. 4. Stretch net chooses wire mesh and net frame, combine the two, this is stretch net. There are many ways to stretch the net, and manual, motorized, and pneumatic methods can be used. At present, the more advanced one that can stretch the high-quality screen is the pneumatic stretch machine. The requirements for stretching the net are that the tension is uniform, that the warp and weft of the net are kept vertical, and that the mesh adhesive is firm and not slack.
5. Choose photosensitive glue
Photoresists currently on the market include dichromate, diazonium, and iron salts. The requirements for photoresist in silk-screen printing are: good plate-making performance and easy coating. The photosensitive spectrum range is 340 ～ 440nm, with good developing performance, high resolution, good stability, easy storage, economical and sanitary, non-toxic and pollution-free. The requirements of the printing on the photosensitive material are: the plate film formed by the photosensitive material adapts to the performance requirements of different types of inks, has considerable durability, and can withstand the scraping pressure of the scraping board for a considerable number of times; No film release failure; easy to peel, which is conducive to the recycling of screen materials.
6. Exposure
After selecting the above materials, apply the photoresist in the dark room; it needs to be uniform and then dry. Generally, it needs to be applied twice. Exposure requires exposure on a cold light source exposure machine. The exposure time is determined according to the pattern, generally between 12 and 30 minutes. After exposure and development, a silk screen is produced.
7. Printing
There are several types of flat glass printing machines: manual, semi-automatic, and fully automatic. The manual printing machine is suitable for flat glass in small batches and small specifications. Semi-automatic and full-automatic printing machines are suitable for high-volume, large-size multi-color printing, which has the advantages of accuracy and speed. The largest semi-automatic printing machine can print 2000 × 1500mm glass. Fully automatic printing machines are generally used for the printing of automobile windshields. Several foreign companies can provide such equipment.
8. Ink
Two inks are used for inorganic glass screen printing. One is that inorganic pigments are ground to a certain fineness after grinding. Adding acrylic resin, screen printing on the glass surface, it needs to be fired at high temperature (above 600 ℃) for 1 ~ 2min, the ink layer and the glass surface layer are melted together, and the fastness is excellent. The heat resistance temperature of this ink is above 600 ℃. However, because this screen printing process is expensive and complicated, it is rarely used in the screen printing industry. The other type is inorganic glass ink that uses high molecular compound as a binder. Both amino type and epoxy type need to be baked, and the domestic market is extensive. The applied ink is baked at l300 ~ l 400 ℃ for 30min after screen printing, the heat resistance temperature of this ink is relatively low. However, the baking time is longer, the fastness and hardness of the ink are good, and the brightness is also very good. It can withstand the corrosion of low-concentration electrolytes and solvents such as ethanol, acetone, benzene, and cyclohexanone.
9. Sintering
Glass is an amorphous inorganic material that softens with increasing temperature. When heated to the softening temperature of glass, the glass will deform. The glass glaze is decorated on the surface of the glass by silk screen, and it needs to be consolidated on the glass surface by high temperature sintering, but the sintering temperature must be lower than the softening temperature of the decorated glass to ensure that the decorative glass does not deform. Generally, the sintering temperature of glass glaze is not higher than 520 ℃, usually controlled at 480 ~ 520 ℃. There are several domestic manufacturers of low-temperature glass glazes, and the prices are relatively low. Several foreign companies can also provide similar products, but the price differs from domestic products by a factor of 20. The domestic consumption level has not yet reached the level of using this kind of glaze, but the glaze provided by foreign countries is indeed much better than the domestic color and level.
To improve the quality of decorative glass, it is inevitable to improve the grade of glaze. The sintering equipment includes a flame furnace and a resistance furnace. The resistance furnace is divided into a vertical hanging type and a horizontal roller table type according to the hanging method of glass. The vertical hanging sintering furnace has the advantages of small sintering deformation and yield rate of more than 95%, but the equipment is complicated. If the horizontal roller table adjustment is not good, it will be easy to cause large deformation and low yield. In addition, the use of horizontal tempering furnaces to produce decorative glass is also feasible. The grade of the product is improved and the safety is also guaranteed. It is an ideal sintering method. [next]
Second, the glass screen printing process is prone to failures
Some of the silk screen faults are caused by a single reason, but more are the result of the cross-effect of various reasons. This is what the operator should pay special attention to when judging the cause of the failure of the glass screen printing:
1. paste version
The paste version is also called blocking version, which refers to the phenomenon that the screen through-hole portion of the screen printing plate cannot transfer ink to the substrate during printing. The appearance of this phenomenon will affect the printing quality, even in serious cases, it will not be able to print normally.
The cause of the scumming phenomenon that occurs during the screen printing process is complex. The reasons for lay-up can be analyzed from the following aspects:
â‘ The reason of the glass: the surface of the glass is not cleaned, and there are watermarks, paper marks, oil marks, handprints, dust particles and other dirt. Thus causing lay-up;
② Reasons for workshop temperature, humidity and ink properties. The screen printing workshop requires a certain temperature of about 20 ° C and a relative humidity of about 50%. If the temperature is high and the relative humidity is low, the volatile solvent in the ink will quickly evaporate, and the viscosity of the ink on the screen will become high, thereby blocking Live mesh. Another point that should be noted is that if the downtime is too long, it will also cause the phenomenon of pasting, the longer the time, the more serious the pasting. Secondly, if the ambient temperature is low, the poor fluidity of the ink is also easy to produce paste;
â‘¢ The reason for the screen printing plate. The prepared screen printing plate should be washed with water and dried before use. If it is placed for a long time after making a plate, it will not be printed in time. In the storage process, more or less dust will adhere. If it is not cleaned during printing, it will cause paste.
â‘£The reason of printing pressure. Excessive embossing force during the printing process will cause the squeegee to bend. The squeegee is not in line contact with the screen plate and glass, but is in surface contact, so that each time the squeegee can wipe the ink clean, leaving residual ink After a certain period of time, the conjunctiva will cause paste.
⑤ Reason for improper gap between screen printing plate and glass. The gap between the screen printing plate and the glass should not be too small. If the gap is too small, the screen cannot be separated from the glass in time after scraping. When the screen printing plate is lifted, a certain amount of ink is stuck on the bottom of the plate, which is also easy to cause paste board.
â‘¥ The reason for the ink is that when the pigments and other solid particles in the screen printing ink are large, the phenomenon of blocking the mesh is likely to occur.
In addition, the number of screen meshes and the area of ​​the through holes selected are smaller than the particle size of the ink, making it easier for coarser particles of ink to pass through the mesh and cause the screen sealing phenomenon. For paste plates caused by large particles of ink, it can be resolved from the time of manufacturing the ink. The main method is to strictly control the fineness of the ink.
During the printing process, the increased viscosity of the ink causes paste printing. The main reason is that the ink solvent on the screen printing plate evaporates, resulting in an increase in the viscosity of the ink and a screen sealing phenomenon. If the area of ​​the printed image and text is relatively large, the ink consumption on the screen printing plate is large, and the phenomenon of paste-up is less. If the graphic area is small and the ink consumption on the screen printing plate is low, it is easy to cause scumming. The countermeasure is to use a small number of times to add ink. Poor fluidity of the ink will cause the ink to paste when it does not pass through the screen. This situation can be solved by reducing the viscosity of the ink and improving the fluidity of the ink without affecting the printing quality. After the pasting failure occurs, you can use an appropriate solvent to scrub the ink on the plate. The main point of scrubbing is to start from the printing surface and gently wipe from the middle to the periphery. Check the printing plate after wiping, if any defects should be repaired in time, and printing can be restarted after repairing. It should be noted that every time the plate film is scrubbed, it becomes thinner. If the plate film is damaged during the wiping, it has to be replaced with a new version.
2. The ink is not firmly fixed on the glass
â‘ When printing on glass, it is important to strictly degrease and inspect the glass before printing. When oils, adhesives, dust, etc. are attached to the glass surface, it will cause poor adhesion between the ink and the glass.
â‘¡ Insufficient adhesion of the ink itself causes the ink film to be not firmly fixed. It is better to replace other types of ink for printing. Improper selection of the dilution solvent will also result in the phenomenon of poor adhesion of the ink film. When selecting the dilution solvent, the nature of the ink should be considered to avoid the phenomenon that the adhesion of the ink and the substrate is not strong.
3. Uneven ink
The thickness of the ink film is uneven due to various reasons. In terms of ink, the ink is poorly formulated, or the normally formulated ink is mixed into the ink skin. During printing, due to the expansion and softening of the solvent, the ink should pass through. The mesh is blocked, which acts as a plate film, making the ink unable to pass through. In order to prevent such malfunctions, the ink after preparation (especially the old ink) should be filtered with a net before use. When reusing an already used printing plate, the old ink attached to the frame must be completely removed. When storing the printing plate after printing, sufficient washing (including squeegee) is required. If the tip of the ink return plate is damaged, a trace will appear in the direction of the blade movement. When printing glass, there will be obvious ink unevenness. Therefore, the front end of the squeegee must be well protected from damage. If it is damaged, it must be carefully ground with a grinder. In addition, the unevenness of the printing table will also affect the ink uniformity. The ink layer on the convex part is thin, and the ink layer on the concave part is thick. This phenomenon is also called unevenness in ink. In addition, if dust adheres to the back of the substrate or the printing table, the above failures will also occur.
4. Pinhole
The pinhole phenomenon is the most headache problem for the staff engaged in glass screen printing. The reasons for the occurrence of pinholes are also varied, many of which are currently unexplained, and some are quality management problems. Pinhole is one of the most important inspection items in the inspection of printed products.
â‘ Dust and foreign matter attached to the plate. When making a plate, some sol will be mixed into it by washing with water. In addition, when the emulsion is applied, dust is also mixed in, and pinholes are generated when it adheres to the screen. During the test, if you pay attention to inspection, you can find and make timely repairs. If dust and foreign objects adhere to the screen, blocking the screen opening will also cause pinholes. Before the official printing, you should carefully check the screen version to remove the dirt on the screen;
â‘¡Cleaning of glass surface. The glass plate should be pre-treated before printing to make its surface clean and then printed immediately. If the glass is not printed immediately after being processed, it will be contaminated again. After pretreatment, it can remove grease and other dirt, and at the same time, it can also remove dust attached to the surface. Pay special attention to the fact that when you move the glass by hand, the fingerprints of your hands will also adhere to the printed surface, forming pinholes during printing.
5. Bubbles
Bubbles sometimes appear on the ink of the glass after printing. The main reasons for the bubbles are as follows: â‘ The substrate is not well processed before printing. Dust and oil stains are attached to the surface of the substrate; â‘¡ bubbles in the ink. In order to adjust the ink, when adding solvents and additives to stir, some bubbles will be mixed into the ink. If left untouched, the ink with low viscosity will naturally defoam, while the ink with high viscosity may not defoam naturally. Some of these bubbles are naturally eliminated in the printing process due to the transfer of ink, while others have become larger and larger. In order to remove these bubbles, an anti-foaming agent is used. The amount of anti-foaming agent added to the ink is generally about 0.1 to 1%. If it exceeds the specified amount, it will have a foaming effect. Even if foaming occurs after the transfer of ink, as long as the wettability of the substrate and the fluidity of the ink are good, the bubbles on the surface of the printing ink film will be gradually eliminated, and the ink forms a flat printing ink film. If the ink bubbles are not eliminated, the ink film will form a ring-shaped uneven film surface. In general, the bubbles in the ink can be deaerated when passing through the screen; â‘¢ bubbles can also be generated if the printing speed is too fast or the printing speed is uneven. The printing speed should be properly reduced to maintain the uniformity of the printing speed.
6. Static breakdown
Electrostatic current is generally very small, but the potential difference is very large. At first, phenomena such as attraction, repulsion, conduction, and discharge can occur. ①Adverse effects on screen printing. During printing, the rubber part and the screen are electrified due to the pressure of the scraper rubber. If the screen is electrified, it will affect the normal inking and cause blockage faults; it will be attracted by the screen at the moment the substrate is output; ②A method to prevent static electricity. The methods to prevent static electricity are: adjust the ambient temperature and increase the air humidity. The appropriate temperature is generally about 20 ℃, the relative humidity is about 60%; the static electricity can be transmitted in the humid air; reduce the mesh distance and reduce the printing speed.
7. Expanded ink film size
After screen printing, the print size sometimes expands. The main reason for the enlargement of the printing size is that the ink viscosity is relatively low and the fluidity is too large; the enlargement of the size of the screen printing plate at the time of production is also the cause of the enlargement of the printing size. In order to avoid the excessive fluidity of the ink and cause the ink to overflow to the surroundings after printing, resulting in a larger printing size, it may be considered to increase the viscosity of the ink to reduce the fluidity of the ink. When making screen printing plates, we must strictly guarantee the quality of screen printing plates and the tension of screen printing plates. [next]
8. The printing plate leaks ink
A part of the plate film is leaking ink, which is called a leaking ink failure. The reasons are: a part of the scraper is damaged; the pressure of the scraping ink is large; the gap between the plate and the glass is too large; the plate frame is deformed and the local printing pressure is not enough: Ink is uneven; screen is too fine; printing speed is too fast. If dust is mixed on the glass and the ink, and printing is performed without treatment, the plate film will be damaged due to the pressure of the squeegee; if the exposure is insufficient during plate making, pinholes, etc., will cause the plate film to leak ink. At this time, you can use adhesive tape to paste from the back of the plate for emergency treatment. If this operation is not very rapid, the ink on the plate surface will be dried, and the entire plate will have to be wiped with a solvent. Wiping the plate is also the cause of the peeling of the plate film, so it is best to avoid it. The ink leakage of the plate often occurs in the part where the ink stays, so it is better to strengthen this part when making a plate.
9. Zimo
Moisture refers to the appearance of speckled marks on the graphic and dark parts of the glass surface, which damages the printing effect. Glass screen printing is easy, causing this phenomenon. The reasons are as follows: the printing speed and the drying of the ink are too slow; the ink layer is too thin; the ink is thixotropic; the effect of static electricity; the pigment dispersion in the ink is poor, due to the polar effect of the pigment particles, the particles agglomerate with each other and appear color Dot blot. The improved method is: improve the fluidity of the ink; use fast-drying flux; print with high viscosity ink as much as possible: increase the thickness of the wet film of the ink, try to use ink made of pigments with small oil absorption: minimize the effect of static electricity .
10. Image distortion
The printing pressure added to the printing plate by the squeegee during printing can make the printing plate and the printed object be in line contact, it should not exceed. If the printing pressure is too large, the contact between the printing plate and the substrate will cause the screen to expand and contract, causing the printed image to deform. Screen printing is the printing with the least pressure among various printing methods. If we forget this, we can not print good prints. If you cannot print without increasing the pressure, you should reduce the gap between the plate and the glass surface, so that the pressure of the scraper can be reduced.
3. Application and market prospects of screen printing flat glass
Screen-printed flat glass is widely used in all corners of people's lives: you can easily find them in display windows, building space partitions, building beautification, car windshields and mosaic glass. Simultaneously. They are also widely used in furniture decoration and game tools, as well as countless decoration and artistic creation. Screen printing has subsequently become part of the creative design work of glass manufacturing. One can print color images directly onto the glass surface by screen printing. In addition, it can also be used to temporarily cover the non-graphic parts of the glass surface, or print auxiliary agents such as preservatives on some areas of the glass surface, so that it will not be affected by the etching or sanding process. Relatively, special inks can also be screen printed directly to create an etching or matte effect. sometimes. People will also take advantage of the strong covering ability of screen printing, such as printing a thin layer of conductive material on the glass, which is used to make anti-icing (frost) devices for windows.
Screen printing continues to expand in the field of flat glass decoration. In addition to being suitable for printing materials of various materials, another important factor is that it can meet flat glass substrates of different sizes and different weights in a wide range. Moreover, due to the absolute advantages of screen printing in ultra-large format image printing, it has further strengthened its leading position in leading the development of the entire flat glass printing industry. Especially in recent years, the demand for large-format flat glass printing has risen sharply. In fact, due to the rapid development of screen printing in the field of large-format flat glass decoration, printing equipment manufacturers are constantly improving to meet the more demanding printing requirements, especially in areas such as flat glass, resulting in many of the most flexible in the world. The equipment is widely used in the printing of building boards.
The application of screen printing in flat glass decoration will continue to develop and continue to expand its application in glass decoration. The screen printing machine has a faster printing material conveying system, and a more advanced power supply system, combined with a powerful PLC (logically controllable device), which improves productivity and image quality and guarantees it, ensuring it in the decorative flat glass market The continuous expansion.
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