The causes of screen printing failures are multi-faceted, involving many factors such as screen printing plates, screen printing squeegees, screen printing inks, screen printing equipment, screen printing materials, and operating techniques. There are single reasons for the silk screen failure, but more are the result of the cross-influence of intricate causes. This is what the operator should pay special attention to when determining the cause of the failure and taking corresponding countermeasures.
1. Confused
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 the 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 the lay-up can be analyzed from the following aspects.
â‘ The reason for the printed matter. There are many kinds of screen printing substrates, and the texture characteristics of the substrates are also a factor in the pasting phenomenon. For example: paper, wood board. The surface smoothness of textiles and other substrates is low, and the surface strength is poor. During the printing process, it is easier to produce powder and hair loss, which causes paste.
â‘¡ Reasons for workshop temperature, humidity and ink properties. The screen printing workshop requires maintaining a certain temperature and relative humidity. 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 becomes high, thereby blocking the 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 prone to 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 left too long after the plate is made and is not printed in time, more or less dust will adhere during the preservation process. 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 the substrate, but is in surface contact. The residual ink, after a certain time, will cause conjunctiva to cause paste.
⑤ Reasons for improper gap between screen printing plate and substrate. The gap between the screen printing plate and the substrate should not be too small. If the gap is too small, the screen printing plate cannot be separated from the substrate after scraping. When the screen printing plate is lifted, some ink adheres to the bottom of the plate, which is also easy Cause paste.
⑥ The cause of ink. When the particles of the pigment and other solid materials in the screen printing ink are large, the phenomenon of clogging the mesh is prone 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.
The ink dries too quickly during the printing process, which can easily cause paste-plate failure. In particular, this phenomenon is more prominent when using volatile drying ink, so you must choose the appropriate solvent to control the drying speed during printing. When selecting inks, the influence of climate should be considered. Generally, quick-drying inks are used in winter, and late-drying agents should be added to the inks in summer. If the late-drying agent still pastes, other types of inks must be used.
With oxidative drying inks, the paste phenomenon does not appear very much, but if excessive use of desiccant in the summer, the paste phenomenon will also occur. Generally, the use of desiccant should be controlled in summer.
When using two-liquid reaction ink, the ink itself dries slowly, so the scumming phenomenon hardly occurs, but occasionally the scumming phenomenon occurs.
In the printing process, the viscosity of the ink increases to cause paste, the main reason is: the ink solvent on the plate evaporates, causing the viscosity of the ink to increase, and the phenomenon of network sealing occurs. 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 frequently replace with new ink. The poor fluidity of the ink will cause paste printing when the ink does not pass through the screen. This situation can be solved by reducing the viscosity of the ink and increasing the fluidity of the ink.
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.
Second, the ink is not firmly fixed on the substrate
â‘ When printing on printing materials, it is very important to perform strict degreasing and pre-treatment inspection on the printing materials before printing. When oils, adhesives, dust, etc. are attached to the surface of the substrate, it will cause poor adhesion between the ink and the substrate. Insufficient surface treatment of plastic products before printing will also cause the failure of ink fixation.
â‘¡ As a printing material, the polyethylene strands must be surface flame treated in order to improve the adhesion to the ink during printing. For metal materials, they must be degreased and dedusted before printing. After printing, they should be baked at the temperature required by the ink Dry treatment, if the improper drying treatment will also cause ink film peeling failure. In addition, in order to make textiles waterproof in textile printing, silicon processing is generally required, so that the phenomenon of poor ink adhesion is not easy to occur during printing.
â‘¢ Articles such as glass and ceramics should be sintered at high temperature after printing, so as long as the temperature treatment is appropriate, the adhesion will be good. A simple method to test the good and bad fastness of the ink film: When the printed object is paper, you can bend the printing surface repeatedly to see if the ink at the crease peels off. If the ink peels off, then its bonding strength is weak. In addition, exposing the printed matter to rain and dew to see if the ink is easy to peel off is also a method to check the fastness of the ink film.
â‘£Insufficient adhesion of the ink itself causes the ink film to be unsightly. It is best to replace other types of ink for printing. Improper selection of the dilution solvent will also result in the phenomenon of poor fixation of the ink film. When selecting the dilution solvent, the nature of the ink should be considered to avoid the phenomenon of the adhesion of the ink and the printing material.
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3. Ink film edge defects
In screen-printed products, a common problem is the appearance of jagged edges (including incomplete or broken lines) on the edges of the printed ink film. There are many reasons for the glitch, but the main reason is the quality of the screen-printed version.
â‘ The resolution of the photoresist is not high, causing the fine lines to be broken or incomplete.
â‘¡ The exposure time is insufficient or the exposure time is too long, and the development is insufficient, and the edges of the screen-printed graphics and text are uneven and appear jagged. A good screen printing plate should have smooth and neat edges.
â‘¢ The surface of the screen printing plate is uneven. When printing, there is still a gap between the screen printing plate and the substrate. Due to the penetrating ink penetration, burrs appear on the edges of the printing ink.
â‘£During the printing process, due to the expansion of the plate film after contact with the solvent, and the degree of expansion in the warp and weft directions, the surface of the plate film is uneven, and the contact surface of the screen plate and the substrate appears locally during printing; Penetration, the ink film will appear burrs.
In order to prevent the appearance of jagged hairs, we can consider solutions from the following aspects:
â‘ Select high mesh screen plate;
â‘¡Use photosensitive materials with high resolution to make plates;
â‘¢Make a screen printing plate with a certain thickness to reduce expansion deformation;
④ Try to use oblique cross net method to stretch the net, the best angle is 22.5 °;
⑤ Fine line printing, try to use indirect plate-making method to make plate, because the indirect method plate-making may have less glitches;
â‘¥ In the process of plate making and printing, try to control the temperature expansion factor, and use photosensitive materials with small expansion coefficients;
⑦Improve the quality of plate-making, ensure that the surface of the screen printing plate is smooth and smooth, and the edges of the screen lines should be neat;
⑧Apply the screen printing plate with water spray gun to improve the developing effect;
⑨The distance between the screen and the substrate, the angle of the scraper, and the printing pressure should be appropriate.
Fourth, 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 this kind of malfunction, the prepared ink (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, it should be washed thoroughly (including the squeegee). If the above requirements are followed, the accident of Momu Yun will not happen.
If the tip of the front edge of the sharp plate is damaged, a trace will appear along the direction of the scraper's movement. Especially when printing transparent materials, obvious ink unevenness will appear. Therefore, the front end of the sharp plate must be well protected so that it will not be damaged. If it is damaged, it must be carefully ground with a grinder.
The unevenness of the printing table will also affect the inking uniformity. The convex ink layer is thin, and the concave ink layer is thick. This phenomenon is also called uneven ink. In addition, if dust adheres to the back of the substrate or the printing table, the above failures will also occur.
5. Pinhole
The pinhole phenomenon is the most headache problem for the staff engaged in screen printing. In the case of printing opaque objects such as billboards and thick paper, such small holes that are not easily observed are generally not a problem. However, when precision printing is performed on aluminum, glass, and acrylic plates, pinholes are not allowed when post-processing and corrosion processing are required. In addition, there are many reasons for the occurrence of pinholes, 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, if you use ink-absorbing paper, after several sheets of printing, you can remove these dusts from the plate.
â‘¡Clean the surface of the substrate. Aluminum plates, glass plates, acrylic plates, etc. should be pre-treated to make their surfaces clean before printing. After the substrate has been pretreated, it should be printed immediately. In multi-color printing, the method of rubbing with alcohol before printing is generally used. In addition, semi-automatic and fully automatic ultrasonic cleaning machines can also be used. After pretreatment, it can remove grease and other dirt, and at the same time, it can also remove dust attached to the surface.
The detergent used in the washing machine is often mixed with chopped fiber. This detergent is soluble in alcohol. When cleaning the surface of the substrate, a thin surfactant film is often formed, and pinholes occur when printing ink on the film. Therefore, pay special attention when using cleaning agents and alcohol. When carrying the substrate by hand, the fingerprint of the hand will also adhere to the printing surface, forming a pinhole during printing.
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Six, bubbles
Bubbles may sometimes appear on the ink after printing on the substrate. The main reasons for the bubbles are as follows:
â‘ The substrate is not well processed before printing. Dust and oil stains adhere 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. Generally, the bubbles in the ink can be defoamed by the action of the screen when passing through the screen. In addition, when the ink is mixed and stirred, hot water or boiling water will have a good defoaming effect.
Red, blue, green and other transparent inks, because of the small proportion of fine particles of organic pigments, the bonding material of these inks has the characteristics of easy foaming. If the corresponding thinner, tackifier or debonding agent is added, the ink can also be turned into a stable ink with good printability.
â‘¢ The printing speed is too fast or the printing speed is uneven. Properly reduce the printing speed to maintain the uniformity of the printing speed.
If the above measures can not eliminate bubbles in the printed matter, you can consider using other types of ink.
Seven, net marks
Screen prints sometimes have screen marks on the surface of the ink film. The main reason for the screen marks is the poor fluidity of the ink. During the screen printing process, when the printing plate is lifted, the ink transferred to the substrate fills the screen by its own flow, so that the surface of the ink film is smooth and neat. If the ink flow is poor, when the screen printing plate is lifted, the ink flow is relatively small, and the screen traces cannot be filled up, and the smooth and flat ink film cannot be obtained. In order to prevent screen marks from appearing on the printed matter, the following method can be used.
â‘ Use ink with high fluidity for printing.
â‘¡It can be considered to use ink with slow drying speed to increase the ink flow time to gradually expand and solidify the ink.
â‘¢ When making plates, use monofilament screens with finer filaments.
8. Inaccurate printing position
Even if there are no problems with screen size, printing machine, etc., the shape of the printing material is inconsistent, the material shrinkage is too large and inconsistent, etc. will cause the printing position to be inaccurate. Such as paper color printing, drying after a color printing, the temperature changes, causing its size to change, as a result, when the second and third printing, the fault of inaccurate overprinting will occur. When the printing material is a plastic product, changes in the temperature and humidity of the printing place can cause changes in its size, which affects the printing accuracy. Moreover, the shape of the plastic product and the conditions during molding (such as temperature and time) are not completely consistent. All must consider, take corresponding measures, and make corrections in advance as much as possible.
9. Poor overprinting
Overlapping ink film is called overprinting. In multi-color printing, the ink of the previous printing cannot be clearly printed on the ink film of the previous printing. This phenomenon is easy to occur due to different types of ink, and some are not easy to produce. For example, if the amount of desiccant added to the oxidative polymerization type ink is too large to promote drying, when the ink film is excessively oxidized and hardened, the two-color inks will repel each other. In addition, if the anti-foaming agent is added excessively in the volatile ink, the anti-foaming agent forms a film on the surface of the ink strand, which prevents overprinting. The remedy is to use an ink with good overprinting performance, reduce the viscosity of the ink, add additives to the ink, and reduce the drying speed of the ink.
10. Expanded size of finished ink stocks
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 prevent the ink from flowing too much and causing the ink to overflow around after printing, resulting in a larger printing size, it may be considered to add a certain amount of thickener to the ink with excessive fluidity to reduce the fluidity of the ink, and it can also be used Quick-drying ink, to speed up the drying speed of the ink after printing, reduce the flow of ink. When making a screen printing plate, we must strictly guarantee the quality of the screen printing plate.
Eleven, cracked ink film
The cracking of the ink strands is caused by the effect of the solvent and the large temperature change. Factors of the substrate material itself can also cause cracking of the ink film.
In order to prevent the occurrence of cracks in the ink film, the nature of the ink and the solvent resistance of the substrate must be considered when selecting the solvent. Use solvent-resistant and oil-resistant materials as the printing materials, and pay attention to maintaining a balanced temperature in the workshop. In multi-color overprinting, it should be fully dried in each color printing, and the drying temperature should be strictly controlled to effectively prevent ink film turtles. The occurrence of cracking.
Twelve, ink
Blotting refers to the phenomenon of ink spillage outside the printed lines. When printing a line, on the side of the squeegee's direction of movement, the ink overflows and affects the neatness of the line. This phenomenon is called blotting. Blushing can be solved by adjusting the relationship between the printing plate and the ink, the relationship between the operation of the squeegee and the angle of the screen stretch. The shape of the through-hole portion of the screen printing plate differs depending on the plate-making method (indirect method, direct method, and direct method). The ideal through hole should be able to be in close contact with the surface of the substrate when printing. The printing plate with the ideal through-hole shape, the printed ink film is sharp, neat, accurate in size, and does not bleed. In order to prevent the occurrence of smoke and ink, the plate film should have an appropriate thickness, elasticity and smoothness. For this purpose, a soft nylon screen and a polyester screen with high dimensional accuracy can be used to make the plate. In order to prevent ink bleeding, it is best to use oblique stretching in the plate-making process.
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Thirteen, the back is sticky and dirty
Dirty on the back side refers to the phenomenon that the ink on the lower printed matter sticks to the back of the upper printed matter when the printed products are stacked. If this phenomenon is not controlled, it will cause sticking and affect the printing on the other side of the double-sided print. The main reason for the backside to be dirty is poor ink drying.
The solution to the sticky back is to adjust the ink viscosity, use faster than the ink, add a drier to the ink, or spray powder on the surface of the semi-finished product, or add liner paper.
14. Sticky pages
During the stacking of screen-printed products, there will be adhesion failure between printed pages, also known as adhesion failure. The phenomenon of sticking will cause quality problems in the printed matter, and even scrap. The main reasons for the occurrence of adhesion phenomena are as follows:
â‘ After printing, the ink of the printed matter is insufficiently dried. When the ink is not completely dry, the printed materials are stacked and piled up, causing dirt and adhesion.
â‘¡ Improper selection of the constituent materials of screen printing ink is also the cause of adhesion between printed materials. Blocking occurs when the softening point of the synthetic resin film-forming substance in the ink is relatively low or when the volatility of the ink is not good. Normally, evaporative drying ink uses thermoplastic resin, which has poor heat resistance. If solvent remains on the ink film after printing, the ink film will soften, causing the printed matter to stick. Especially in summer, due to the relatively high temperature, it is also easy to cause adhesion between printed materials.
â‘¢ The ink used for printing has a certain dissolving effect on the substrate, and it will also cause adhesion between the printed materials. Among the solvents used in printing, some solvents are very soluble in the substrate. After printing, the ink produces a certain amount of dissolution on the surface of the substrate. At this time, although the surface of the ink has been dried, the contact part of the ink and the substrate is not yet dry, and adhesion will occur under the action of the gravity of the stack. When the substrate is a soft vinyl material, part of the plasticizer in the vinyl material is transferred to the ink film after printing, which causes the ink film to soften and also causes blocking.
â‘£In order to prevent the phenomenon of sticking, the ink and solvent suitable for the printing material must be selected first. Secondly, choose the ink with faster drying speed, and pay attention to fully drying. Operate strictly in accordance with technological requirements. In general, glossy inks are easy to cause blocking, so be careful.
⑤ The scraper rubber strip is worn, and the blade is round, resulting in thickening of the ink film for scraping, or thickening of the ink film due to excessive printing pressure, which can also cause adhesion failure.
Fifteen, printing plate leaking ink
A part of the plate film is leaking ink, which is called an ink leakage 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 substrate is too large; ; The viscosity of the ink is too high; The ink is uneven; The screen is too fine; The printing speed is too fast.
If dust is mixed on the substrate and the ink, and printing is performed without treatment, the plate film will be damaged due to the pressure of the squeegee; the pinholes caused by the exposed wooden feet during plate making 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.
In the process of manual printing, when inserting metal plates and hard plastic plates into the printing table, it is easy to puncture the printing film with sharp corners, so be very careful. It is generally best to check and reinforce before printing. In order to prevent ink leakage, the printing plate should be inspected or repaired before printing starts: the pinhole of the plate film should be repaired, and the surroundings of the printing plate should be fully strengthened before printing.
16. 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 layout and the surface of the substrate, so that the pressure of the scraper can be reduced.
17. Zi Mo
Moisture refers to the appearance of speckled prints on the graphic and dark parts of the substrate, which damages the printing effect. Especially the use of transparent ink is more prone to 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 quick-drying flux; print with high viscosity ink as much as possible; use ink made of pigment with small oil absorption as much as possible; minimize the effect of static electricity.
18. Flying Ink
Ink flying is the phenomenon of ink drawing, which is caused by: uneven grinding of ink; slow squeegee release during printing; less white space around the printed image; static electricity is generated, which causes the angle of ink squeegee to be too small.
Nineteenth, static electricity failure
Electrostatic current is generally very small, but the potential difference is very large, and can attract, repel, conduct, discharge and other phenomena. These phenomena will lead to product degradation, performance degradation, fire, and human body electrification.
â‘ Adverse 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 itself 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.
a. Synthetic resin-based inks are easily charged.
b. Even if the substrate is as absorbent as paper, static electricity will be generated when the air is dry. Plastic substrates have good insulation, are not affected by temperature, and are also prone to static electricity.
c. The larger the printing area, the greater the charging, which is easy to produce adverse effects.
d. Since spark discharge can cause flames, be very careful when using flammable solvents.
e. Electric shock to the human body caused by static electricity is caused by contact with charged objects or spark discharge when the static electricity is grounded before accumulation. Although the electric current produced by electric shock is very small, it will not be dangerous, but frequent electric shock will bring adverse effects to the operator's psychology.
② Method to prevent static electricity. The methods to prevent the generation of static electricity are: adjust the ambient temperature, increase the air humidity, the appropriate temperature is generally about 20 ℃, the relative humidity is about 60%; put a small amount of antistatic agent in the alcohol used to scrub the printing material; reduce the friction pressure and speed Reduce the friction, pressure, and impact of the substrate as much as possible; install a general grounding device; use the ionization of flame, infrared, and ultraviolet; use the ionization of corona discharge with high-voltage current.
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