Analysis and elimination of common faults in glass screen printing

　

There are many reasons for the failure of glass screen printing, involving many factors such as screen printing, screen printing scraper, screen printing ink, screen printing equipment, screen printing materials and operation technology. There is a single reason for the occurrence of screen printing failures, but more is the result of the cross-influence of various reasons. This is what the operator should pay special attention to when determining the cause of the failure and taking corresponding countermeasures.

1. Paste version

Paste plate, also known as blocking plate, refers to the phenomenon that the through-hole part of screen printing cannot transfer the ink to the substrate during printing. The appearance of this phenomenon will affect the printing quality, and even normal printing will not be possible in severe cases.

The causes of the pasting phenomenon in the process of screen printing are complicated. The reasons for pasting can be analyzed from the following aspects.

1. The reason for the substrate. The glass surface is not cleaned, and there are still watermarks, paper prints, mimeographs, fingerprints, dust particles and other dirt. As a result, paste is caused.

2. The reason for the temperature, humidity and ink properties in the workshop. The screen printing workshop requires to maintain a certain temperature of about 20 degrees 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 evaporate quickly, and the viscosity of the ink on the Internet will become high, thereby blocking the mesh. Another point that should be noted is that if the shutdown time is too long, there will also be a paste phenomenon, and the longer the time, the more serious the paste will be. Secondly, if the ambient temperature is low, the ink fluidity is poor and it is easy to produce paste.
3. The reason for the screen printing plate. The prepared screen printing plate can be used after rinsing with water and drying before use. If the plate is not printed in time after being placed for too long, it will adhere to dust more or less during the preservation process. If it is not cleaned during printing, it will cause paste.

4. The reason for the printing pressure. Excessive embossing force during the printing process will cause the scraper to bend. The scraper is not in line contact with the screen printing plate and the substrate, but in surface contact, so that the ink cannot be scraped clean every time it is scraped, leaving residual ink, and after a certain period of time, the conjunctiva will cause paste.

5. The reason for the improper gap between screen printing and the 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 in time after scratching. When the screen printing plate is lifted, a certain amount of ink is attached to the bottom of the printing plate, which can easily cause paste.

6. The reason for the ink. When the particles of pigments and other solids in the screen printing ink are large, it is easy to block the mesh.

In addition, the number of mesh and through-hole area selected are smaller than the granularity of the ink, which makes it difficult for coarser ink particles to pass through the mesh and cause the phenomenon of mesh sealing. For the paste caused by the large particle size of the ink, it can be solved from the time of manufacturing the ink. The main method is to strictly control the fineness of the ink.

During the printing process, the increase of ink viscosity causes paste. The main reason is that the ink solvent on the screen printing plate evaporates, resulting in the increase of ink viscosity and the occurrence of screen sealing. If the printed graphic area is relatively large, the ink consumption on the screen printing plate is more, and the paste phenomenon is less. If the graphic area is small and the ink consumption on the screen printing plate is less, it is easy to cause paste. The countermeasure is to use the principle of adding ink a few times. The 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 failure of the paste plate, it can be scrubbed with an appropriate solvent according to the nature of the ink on the plate. The key to scrubbing is to start from the printed surface and gently wipe from the middle to the periphery. Check the printing plate after wiping. If there is any defect, it should be repaired in time, and printing can be restarted after repairing. It should be noted that the plate film becomes thinner every time it is scrubbed. If the plate film is seriously damaged during wiping, it has to be updated.

2. The ink is not firmly fixed on the substrate

1. When printing glass, it is very important to carry out strict degreasing and pretreatment inspection of the glass before printing. When grease, binder, dust and other substances are attached to the glass surface, it will cause poor bonding between ink and glass.

2. The bonding force of the ink itself is not enough to cause the ink film to not stick firmly. It is best to replace other types of inks for printing. Improper selection of dilution solvent will also cause the phenomenon of weak ink film adhesion. The properties of the ink should be considered when selecting dilution solvent to avoid the phenomenon of weak adhesion between the ink and the substrate.

3. Ink film edge defects

In glass screen printing products, the common problem is that jagged burrs (including incomplete or broken lines) appear on the edge of the printing ink film. There are many reasons for burrs, but the main reason is the quality problem of the screen printing plate itself.

1. The resolution of the photosensitive glue is not high, resulting in broken or incomplete fine lines.

2. Insufficient exposure time or too long exposure time, insufficient development, screen printing graphic edge is not neat, jagged. Good screen printing, the edge of the graphic should be smooth and neat.

3. When the photosensitive glue coating on the surface of the screen printing plate is uneven, there is still a gap between the screen printing plate and the substrate during printing. Due to the suspended penetration of the ink, burrs appear on the edge of the printing ink.

4. During the printing process, due to the expansion of the plate film after contacting the solvent, the surface of the plate film is uneven. During printing, there is a local gap on the contact surface between the screen plate and the substrate, and the ink is suspended and penetrates, and the ink film will appear. Burr.

In order to prevent the appearance of serrated burrs, solutions can be considered from the following aspects:

1) Choose high mesh screen plate making;

2) Use photosensitive materials with high resolution to make plates. To use photosensitive glue with bridge type and good resolution;

3) Make a screen printing plate with a certain photosensitive film thickness to reduce expansion deformation;

4) The oblique stretching method can also be used to stretch the net, and the best angle is 22.5;

5) Fine line printing, indirect plate making can also be used, because the possibility of burrs in indirect plate making is small;

6) In the process of plate making and printing, try to control the temperature expansion factor, and use photosensitive materials with small expansion coefficient;

　7) Improve 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;

8) Use a water spray gun to spray and wash the screen printing plate to improve the development effect;

　　9) The distance between the screen plate and the substrate, the angle of the scraper, and the printing pressure should be appropriate.

4. Uneven inking

The thickness of the ink film is uneven due to various reasons. As far as the ink is concerned, the ink is not well blended, or the ink that is normally blended is mixed with the ink skin. During printing, due to the expansion and softening of the solvent, the ink-permeable The mesh is blocked, which acts as a plate film, so that the ink cannot pass through.

In order to prevent this kind of failure, the prepared ink (especially the old ink) should be filtered with a mesh before use and reused. When reusing the already used printing plate, the old ink attached to the plate frame must be completely removed. When storing the printing plate after printing, wash it well (including scrapers).

If the above requirements are done, the accident of uneven inking will not occur.

If the tip of the front end of the ink return plate is damaged, there will be traces along the direction of movement of the scraper. When printing glass, there will be obvious uneven inking. Therefore, the front section of the scraper must be well protected from damage. If it is damaged, it must be carefully ground with a grinder.

The unevenness of the printing table also affects the ink uniformity. The ink layer in the convex part is thin and the ink layer in the concave part is thick, which is also called uneven inking. In addition, if there is dust on the back of the substrate or on the printing table, the above failure will also occur.

5. Pinhole

Pinhole phenomenon is the most headache problem for staff engaged in glass screen printing. There are various reasons for pinhole occurrence, many of which are currently unexplained, and some are quality management problems. Pinhole is one of the most important inspection items in printing product inspection.

1) Dust and foreign objects attached to the plate. When making the plate, some sol will be mixed in during washing and developing. In addition, when the emulsion is coated, dust will also be mixed in, and pinholes will be generated when attached to the screen. These can be found and repaired in time if you pay attention to inspection during the test. If dust and foreign objects adhere to the screen, blocking the opening of the screen will also cause pinholes. Before formal printing, check the screen carefully to eliminate dirt on the plate.

2) Cleaning of the surface of the substrate. The glass plate should be pretreated before printing to make its surface clean, and then printed immediately. If the glass is not printed immediately after being treated, it will be polluted again. After pretreatment, dirt such as grease can be removed, and at the same time, dust adhering to the surface can also be removed.

Special attention should be paid to the fact that when the glass is carried by hand, the fingerprints of the hand will also adhere to the printing surface, and pinholes will be formed during printing.

6. Bubbles

Bubbles sometimes appear on the ink after printing. The main reasons for the bubbles are as follows:

1) The prepress treatment of the substrate is poor. Dust and oil stains are attached to the surface of the substrate.

2) Bubbles in the ink. In order to adjust the ink, add flux and additives for stirring, some air bubbles will be mixed in the ink. If you leave it alone, the ink with low viscosity will naturally defoam, and some inks with high viscosity cannot naturally defoam. Some of these bubbles are naturally eliminated during printing due to the transfer of ink, and some become larger and larger. In order to remove these bubbles, defoamers should be used. The amount of defoamer added in the ink is generally about 0.1-1%. If it exceeds the specified amount, it will play a foaming effect.

Even if the ink foams after transfer, as long as the wetness of the substrate and the fluidity of the ink are good, the bubbles on the surface of the printing ink film will gradually be eliminated, and the ink will form 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 air bubbles in the ink can also be defoamed due to the action of the screen when passing through the screen.

3) Bubbles will also be generated if the printing speed is too fast or the printing speed is uneven. The printing speed should be appropriately reduced to maintain the uniformity of the printing speed.

If none of the above measures can eliminate air bubbles in the printed matter, other types of inks can be considered.

7. Net marks

Screen traces sometimes appear on the surface of the ink film of screen printing products. The main reason for the appearance of screen traces 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 traces by its own flow, so that the surface of the ink film is smooth and flat. If the ink has poor fluidity, when the screen printing plate is lifted, the ink flow is relatively small, and the screen traces cannot be filled, and an ink film with a smooth and flat surface cannot be obtained. In order to prevent screen traces from appearing on the printed matter, the following methods can be used:

1) Use ink with good leveling for printing;

2) Consider using ink with slow drying speed to increase the flow time of the ink to gradually level and cure;

3) When making plates, try to use a monofilament mesh with finer silk.

8. The printing position is inaccurate

It is related to the screen size, the repeated positioning accuracy of the printing machine and the glass size.

9. The size of the finished ink film is enlarged

After screen printing, sometimes the printing size is enlarged. The main reason for the expansion of the printing size is the relatively low viscosity of the ink and the excessive fluidity; the expansion of the size of the screen printing plate during production is also the reason for the expansion of the printing size.

In order to avoid excessive ink fluidity and cause the ink to overflow around after printing, resulting in a larger printing size, consider increasing the ink viscosity to reduce the fluidity of the ink. When making screen printing plates, it is necessary to strictly ensure the quality of the screen plate and ensure the tension of the screen plate.

10. Sticky back

Back sticking refers to the phenomenon that the ink on the lower glass sticks to the back of the upper glass when the glass is stacked. The printed glass can be isolated using isolation pads, isolation strips, and blow molding paper.

11. Ink leakage from printing plates

Ink leakage from part of the plate film is called ink leakage failure. The reasons are: part of the scraper is damaged; the pressure of scratching ink is large; the gap between the plate and the substrate is too large; the plate frame is deformed greatly, and the partial printing pressure is not enough; The ink is uneven; the screen is too thin; the printing speed is too fast, etc.

　　If dust is mixed on the substrate and in the ink, and printing is carried out without treatment, the plate film will be damaged due to the action of scraper pressure; pinholes caused by insufficient exposure during plate making will cause the plate film to leak ink. At this time, it can be pasted from the back of the plate with adhesive tape or the like for emergency treatment.

If this operation is not very fast, it will dry the ink on the layout and have to wipe the whole plate with solvent. Wiping plates are also the cause of peeling of the plate film, so it is best to avoid them. Ink leakage of the plate often occurs in the part where the ink stays, so it is best to strengthen this part when making plates.

12. Image warping

When printing, the impression pressure added by the scraper to the printing plate can make a linear contact between the printing plate and the object to be printed, and should not exceed it. If the impression 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 be deformed. Screen printing is the printing with the smallest impression among various printing methods. If we forget this, we will not be able to print good prints.

If printing cannot be done without increasing the pressure, the gap between the plate and the substrate surface should be reduced, so that the pressure of the scraper can be reduced.

13. Zimo

Ink-raising refers to the speckled imprints on the graphic and text parts and dark parts of the substrate, which damages the printing effect. Glass screen printing is 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 has high thixotropy; the influence of static electricity; the pigment dispersion in the ink is poor. Due to the polarity of the pigment particles, the particles agglomerate with each other, resulting in color spots.

The improved methods are: improve the fluidity of the ink; use fast-drying solvents; use ink with high viscosity as much as possible; increase the wet film thickness of the ink, and try to use ink made of pigments with low oil absorption; minimize the influence of static electricity.

14. Flying ink

Flying ink is the phenomenon of ink wire drawing. The reasons are: the ink is not polished evenly; the scraper is slow to leave the plate during printing; there is too little space around the printed image; static electricity is generated, resulting in the ink off-screen angle being too small.

15. Static failure

Electrostatic current is generally very small, but the potential difference is very large, and phenomena such as attraction, repulsion, conduction, and discharge can occur at the same time.

1) Adverse effects on screen printing. The screen during printing, the rubber part and the screen are electrified due to the pressurized scraping of the scraper rubber. The screen itself is electrified, which will affect the normal inking and cause a blocking failure; it will be sucked by the screen at the moment when the substrate is output.

A. The glass is easily charged when the heating wire is printed.

B. Static electricity is easy to absorb ink particles on the glass surface, making the glass surface unclean.

C. Electric shock to the human body caused by static electricity is caused by contact with something with a spot, or the static electricity generates spark discharge in the grounded city before accumulation. Although the current generated by electric shock is very small and will not be dangerous, electric shocks often occur, which will bring adverse effects to the operator's mind.

2) Methods 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 ° C and the relative humidity is about 60%; static electricity can be transmitted in wet air; reduce the screen distance and reduce the printing speed