Common faults and countermeasures of screen printing

1. Paste

Paste (blocking) means that the screen printing graphics and text cannot be printed with ink during printing. to the material. The reasons for silk screen printing are complicated.

1. Material reasons

The material has low smoothness and poor surface strength. Powder and lint loss occur during printing, which will lead to plate paste.

2. Reasons for workshop temperature, humidity and ink properties

The screen printing workshop must maintain a certain temperature and relative humidity. When the temperature is high and the relative humidity is low, the ink solvent will evaporate quickly, the ink viscosity will increase, and the mesh will be blocked. If the shutdown time is too long, the plate will be smeared. The longer the time, the more serious the smearing will be. Low ambient temperature and poor ink fluidity can easily cause paste.

3. Reasons for screen printing plates

The screen plates must be clean and dry before use. If the plate is left for a long time without being printed in time, dust will more or less adhere to it during the storage process. If it is not cleaned during printing, it will cause paste.

4. Reasons for printing pressure

If the pressure is too high, the scraper will not be in line contact with the screen and material but in surface contact. In this way, the ink cannot be scraped clean every time. The residual oil left behind will form film and paste after a certain period of time.

5. Reasons for improper gap between the screen printing plate and the material

The gap between the screen printing plate and the material is too small. If the gap is too small, the screen plate cannot be separated from the material after scraping. The screen plate is lifted. When printing, the ink adheres to the bottom of the printing plate, which makes it easy to paste and stick to the plate.

6. Causes of ink

When the ink particles are larger, it is easy to block the mesh. So filter the ink.

The ink dries too quickly and is easy to paste. Especially when using volatile drying ink, it is more prominent.

When using oxidative drying ink, the paste phenomenon does not occur much. If the desiccant is used excessively, paste will also occur.

The evaporation of the ink solvent causes the viscosity of the ink to increase, resulting in network blocking. If the area is relatively large and the screen ink consumption is low, it is easy to cause the screen to be pasted (use new ink frequently).

The fluidity of the ink is poor, and the ink will be pasted when it does not pass through the screen (this can be solved by reducing the viscosity of the ink and increasing the fluidity of the ink).

After a paste failure occurs, scrub with an appropriate solvent based on the nature of the ink. The key to scrubbing is to start from the printing surface and wipe gently from the center to the periphery. Check the printing plate after wiping. If there is any defect, repair it in time. After repair, printing can be restarted. It should be noted that the plate film becomes thinner every time it is wiped.

2. The ink is not firmly fixed on the material

1. The printed materials should be printed before printing. Pre-processing inspection. When grease, adhesive, dust and other substances adhere to the surface of the material, it will cause poor adhesion between the ink and the material.

2 In order to improve the adhesion with the ink during printing, the material must be surface treated and dust must be removed before printing.

3. Zigzag

The appearance of silk screen printing Sawtooth (including chipped or broken wires). The main problem lies in the quality of the screen version.

1. The resolution of photoresist is not high, causing fine lines to be broken or incomplete.

2. If the exposure time is insufficient or too long, the development will be insufficient, and the edges of the screen image and text will be uneven and jagged. For a good screen version, the edges of graphics and text should be smooth and neat.

3. The surface of the screen is uneven. When printing, there is still a gap between the screen and the material, and the ink is suspended in the air and penetrates, causing burrs on the edge of the ink.

4. The screen swells when it comes into contact with the solvent, and the degree of expansion is different in the longitudinal and latitudinal directions, causing unevenness on the surface of the screen. There are gaps between the screen and the material, and burrs appear when the ink is suspended and penetrated.

Solution: A. High-mesh screen printing plate

B. Photosensitive material printing plate with high resolution

C. Screen printing plate with certain film thickness , to reduce expansion and deformation (coat photosensitive paste and add water film)

D. Line printing, try to use indirect plate making method to make plates (the possibility of burrs in indirect plate making is smaller)

E .During the plate making and printing process, try to control the temperature expansion factor and use photosensitive materials with small expansion coefficients

F. Improve the quality of plate making and ensure that the surface of the screen printing plate is flat and smooth, and the edges of the screen lines should be neat< /p>

G. Use a water spray gun to spray the screen printing plate to improve the development effect

H. The distance between the screen plate and the material, the angle of the scraper, and the printing pressure must be appropriate.

4. Pinholes and foaming phenomena

1. Dust and foreign matter attached to the plate

During plate making, some sol will be mixed in during water washing and development. . In addition, when the emulsion is coated, dust is also mixed in, which will cause pinholes when adhering to the screen (it must be inspected and repaired before printing). If dust and foreign matter adhere to the screen, blocking the opening of the screen will also cause pinholes. Before formal printing, if you use highly ink-absorbent paper (such as newspaper), the dust can be removed after several printings.

2. Cleaning of material surface

The surface of aluminum plate and PVC must be clean before printing. After the material is processed, it is printed immediately. When printing in multiple colors, wipe with alcohol or facial cleanser before printing (dirt and dust such as grease and oil can be removed after treatment).

3. Bubbles in the ink

When the ink is stirred with solvents and additives, some bubbles will be mixed in. If left alone, the ink with low viscosity will naturally degas, and the ink with high viscosity will naturally degas. Some cannot defoam naturally. Some of these bubbles are naturally eliminated due to the transfer of ink during printing, while others become larger and larger. In order to remove these bubbles, defoamer should be used (defoamer has two types: quick-acting and slow-acting). It is added to the ink. The amount is generally about 0.1%-1%. If it exceeds the specified amount, it will have a foaming effect. .

Even if the ink foams after transfer, as long as the wetness of the material and the fluidity of the ink are good, the bubbles on the surface will gradually be eliminated. If the ink bubbles are not eliminated, the ink film will form a ring-shaped uneven surface. Printing side. Generally, when the bubbles in the ink pass through the screen, they can be defoamed due to the action of the screen. In addition, using hot water or boiling water when mixing ink will have a better defoaming effect.

Transparent inks such as red, blue, and green are prone to foaming. If the corresponding diluent, tackifier or tackifier is added, the ink can also be transformed into a stable ink with good printability.

Printing too fast or uneven printing speed can also produce bubbles. Reduce the printing speed appropriately to maintain the uniformity of the printing speed.

5. Uneven inking

The thickness of the black film is uneven. 1. The ink is poorly blended. 2. The blended ink is mixed into the ink skin. (The action of the solvent causes expansion and softening. Blocking the mesh that should be permeable to ink acts as a plate film so that ink cannot pass through).

Prevention: The prepared ink (especially old ink) must be filtered through a mesh before use. When reusing a used printing plate, the old ink attached to the plate frame must be completely removed. When storing the printing plate after printing, wash it thoroughly (including the scraper). If you do it as required, accidents like uneven inking will not happen.

The tip of the front end of the scraper is damaged, and there will be traces. Especially when using PC transparent materials, the inking is obviously uneven. Therefore, the front end of the scraper must be well protected from damage. If it is damaged, it must be ground.

The unevenness of the printing table will also affect the evenness of inking. The ink layer is thin in the convex parts and thick in the concave parts. This phenomenon is also called uneven inking. In addition, if there is dust on the back of the material or the printing table, uneven inking will also occur.

6. Screen marks

Sometimes screen marks appear on the surface of the product. The main reason is that the ink has poor fluidity. During the screen printing process, when the screen is lifted, the ink transferred to the material fills the screen traces by its own flow, making the surface of the ink film smooth and flat. If the ink fluidity is poor, when the screen is lifted, the ink flow is small and cannot fill the screen traces, and an ink film with a smooth surface will not be obtained.

Improvement methods:

A. Use ink with high fluidity for printing;

B. Consider printing with ink that has a slow drying speed and increase the amount of ink. The flow time allows the ink to gradually flatten and solidify;

C. Try to use a single-diameter screen with a smaller wire diameter when making plates.

7. Inaccurate printing position

Even if there are no problems with screen size, printing machine, etc., inconsistent material shape and inaccurate registration will cause inaccurate printing position. (The material is required to be accurate to the plate glue)

8. The size of the finished ink film is enlarged

After screen printing, the printing size may sometimes be enlarged (1. The viscosity of the ink is relatively low and the fluidity is Too large, 2. The size of the screen is enlarged during production).

Method: In order to avoid excessive ink fluidity causing ink to overflow to all directions after printing, causing the printing size to become larger, the fluidity can be considered Add a certain amount of thickener to oversized ink to reduce the fluidity of the ink. Quick-drying ink can also be used to speed up the drying speed of the ink after printing and reduce the flow of the ink. When making a screen version, the quality of the screen version must be strictly guaranteed.

10. Ink film cracks

Ink film cracks (caused by the action of solvents and large temperature changes). The material itself can also cause cracks in the ink film.

To prevent the occurrence of ink film cracks, pay attention to maintaining a balanced temperature in the workshop. When multi-color overprinting, fully dry each color after printing, and strictly control the drying temperature.

Ten 1. Ink bleeding

Ink bleeding refers to the phenomenon of ink overflowing outside the printed lines. When printing a line, ink overflows on one side of the squeegee movement direction, affecting the neatness of the line. This phenomenon is called ink bleeding. Ink bleeding can be solved by adjusting the relationship between the printing plate and the ink, the operation of the scraper and the angle of the screen stretching. The shape of the through hole part of the screen printing plate varies depending on the plate making method (indirect method, direct method, direct method). The ideal through hole should be able to closely contact the surface of the substrate when scraping and printing. A printing plate with an ideal through-hole shape produces sharp, neat and precise ink films without ink bleeding. In order to prevent the occurrence of ink bleeding, the plate film should have appropriate thickness, elasticity and smoothness.

12. Backside smearing

Backside smearing refers to the accumulation on the printed product. When the ink on the lower print sticks to the back of the upper print. If this phenomenon is not controlled, it will cause page sticking and affect the printing of the other side of the double-sided print. The main reason for the sticky back is poor ink drying.

The solution to the sticky back is to adjust the viscosity of the ink, use quick-drying ink, add catalysts to the ink, or spray powder on the surface of the semi-finished product, or add lining paper.

13. Sticky Pages

During the accumulation process of screen printing products, adhesion failure will occur between the printed pages, which is also called sticky page failure. Adhesion will cause quality problems in the printed matter and may even cause it to be scrapped. The main reasons for the occurrence of adhesion are as follows.

1. After printing, the printed ink is not fully dried. When the ink is not dry, the printed matter is stacked and piled up, causing smudging and adhesion.

2. Improper selection of the composition materials of screen printing ink is also the cause of adhesion between printed matter. When the softening point of the synthetic resin film-forming substance in the ink is relatively low or the volatility of the ink is not good, adhesion will occur. Usually, evaporative drying ink films use thermoplastic resin, which has poor heat resistance. If there is solvent remaining on the ink film after printing, the ink film will soften, causing printing sticking. Especially in summer, the relatively high temperature can easily cause adhesion between printed materials.

3. The ink used in printing has a certain dissolving effect on the substrate, and will also cause adhesion between the printed materials. Some of the solvents used in printing are highly soluble in the substrate. After printing, the ink will dissolve to a certain extent on the surface of the substrate. Although the surface of the ink has dried, the part in contact with the substrate has not yet dried completely, and adhesion will occur under the gravity of the stack. When the substrate is soft vinyl material, part of the plasticizer in the vinyl material will transfer to the ink film after printing, causing the ink film to soften and causing adhesion.

4. In order to prevent adhesion, you must first select inks and solvents suitable for the substrate material. Secondly, choose ink that dries faster and pay attention to drying it thoroughly. Operate strictly according to process requirements. Generally, glossy inks tend to cause adhesion, so be careful.

5. The scraper strip is worn and the blade is rounded, causing the scraped ink film to thicken, or the printing pressure to be too large and the ink film to thicken, which can also cause adhesion failure.

14. The printing plate leaks ink

If dust is mixed on the substrate and in the ink and is printed without treatment, the plate film will be damaged due to the pressure of the scraper. Damage, pinholes caused by insufficient exposure during plate making, etc. will cause ink leakage from the plate film. At this time, tape or other tape can be used to affix it from the back of the plate for emergency treatment. If this operation is not performed very quickly, the ink on the plate will dry out and the entire plate will have to be wiped with solvent. Wiping the plate is also a cause of plate film peeling, so it is best avoided. In addition, ink leakage of the plate often occurs in the part where the ink stays, so it is best to strengthen this part when making the plate.

During the feeding process of manual printing, when inserting metal plates and hard plastic plates into the printing table, sharp corners may easily puncture the plate film, so be very careful. It is generally best to check and reinforce before printing. In addition, text errors and pinholes in the plate film should be fully corrected, and the surroundings of the plate should be reinforced to avoid downtime during printing due to plate, ink, scraper, etc. failures.

15. Ink staining and flying ink

Ink staining refers to the appearance of spotty marks on the graphic and dark parts of the substrate, which damages the printing effect. This phenomenon is more likely to occur especially when using transparent ink. 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 influence of static electricity; the pigments in the ink are poorly dispersed. Due to the polarity of the pigment particles, the particles agglomerate with each other and appear color. Dot blot.

The improvement methods are: improve the fluidity of the ink; use quick-drying solvents; print with inks with high viscosity as much as possible; it is recommended to use inks made of pigments with small oil absorption; and try to reduce the impact of static electricity.

Ink flying is the phenomenon of ink drawing, which is caused by: uneven grinding of ink; slow scraper separation during printing; less white space around the printed image; generation of static electricity, resulting in ink drawing; excessive scraper angle Small.

16. Electrostatic Failure

The electrostatic current is generally very small, but the potential difference is very large, and phenomena such as attraction, repulsion, conduction, and discharge can occur. These phenomena can lead to product deterioration, performance degradation, flames, human body electrification and other adverse consequences.

1. Adverse effects on screen printing

When printing the screen, the rubber part and the screen are charged due to the pressure and scraping of the scraper rubber. The screen itself is charged, which will affect normal inking and cause blockage; it will be sucked by the screen at the moment when the substrate is output.

A. Synthetic resin-based inks are easily charged.

B. Even if the printing substrate is as absorbent as paper, static electricity will be generated when the air dries. Plastic substrates have good insulation, are not affected by humidity, and are prone to static electricity.

C. The larger the printing area, the greater the charge, which can easily produce adverse effects.

D. Since spark discharge can bring sparks, be very careful when using flammable solvents.

E. Human body electric shock caused by static electricity is caused by contact with charged objects, or the accumulated static electricity produces spark discharge when grounded. Although the current generated by an electric shock is very small and will not be dangerous, frequent electric shocks will have adverse psychological effects on the operator.

2. Methods to prevent static electricity

Methods to prevent static electricity include: adjusting the ambient temperature and increasing air humidity. The appropriate temperature is generally around 20°C and the relative humidity is around 60%; Put a small amount of anti-static agent into the alcohol used to scrub the printing material; reduce the friction pressure and speed; reduce the friction, pressure, and impact of the printing material as much as possible; install a general grounding device; use the ionization effect of flame, infrared rays, and ultraviolet rays; Utilizes the ionization effect of corona discharge with high voltage current.

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