Common Problems in Screen Printing

Seven, net marks

Screen marks sometimes appear on the surface of the ink film. The main cause of 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 printing material fills the screen traces with its own flow, making the surface of the ink film smooth. If the ink has poor fluidity, when the screen printing plate is lifted, the ink flow is relatively small, and the screen marks cannot be filled, and a smooth and smooth ink film cannot be obtained. To prevent the appearance of screen marks on prints, the following method can be used.

1 Use a highly fluid ink for printing.

2 can consider the use of slow drying ink printing, increase the ink flow time so that the ink gradually exhibited and cured.

(3) Use silk finer wire mesh when making plates.

Eight, the printing position is not accurate

Even if there are no problems in the screen size, printing machine, etc., the inconsistent shape of the printing material, the excessive shrinkage of the material, and the inconsistency will cause the printing position to be inaccurate. For example, paper type printing, drying after a color printing, changes in temperature, causing a change in its size, results in the second and third printing, it will appear overprint failure. When the printed material is a plastic product, changes in the temperature and humidity of the printing place can cause changes in its size, affect the printing accuracy, and the shapes, molding conditions (such as temperature, time) of plastic products are not exactly the same, when printing All must be considered, take appropriate measures, and make corrections as much as possible in advance.

9. Overprint fault

Overlapping ink film is called overprinting. In the case of multi-color printing, ink printed on the previous ink cannot be clearly printed on the ink film of the previous printing. This phenomenon is easily caused by some kinds of ink, and some are not easily produced. For example, an oxidative polymerization type ink has a large amount of desiccant added to promote drying, and when the ink film is oxidized and hardened excessively, two colors of ink repel each other. In addition, if an antifoaming agent is added in excess to the volatile ink, the antifoaming agent forms a thin film on the surface of the ink strand, preventing the overprinting. The remedy is to use inks with good overprint properties to reduce the viscosity of the ink, add additives to the ink, and reduce the drying speed of the ink.

X. Expanded size of finished ink strands

After screen printing, the print size sometimes increases. The main reason for the increase in printing size is the relatively low viscosity of the ink and excessive fluidity; the size of the screen printing plate is also the cause of the increase in the printing size.

In order to avoid the ink flowability caused by the overflow of the ink after printing to the surrounding, resulting in larger print size, consider adding a certain amount of thickener in the ink with excessive fluidity, in order to reduce the flowability of the ink, can also be used Fast-drying inks speed up the drying of ink after printing and reduce the flow of ink. When making screen printing plates, the quality of the screen printing plates must be strictly guaranteed.

Eleventh, ink film cracking <br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br Substrate material itself can also cause ink film cracking.

In order to prevent the occurrence of ink film cracking, the nature of the ink and the solvent resistance of the substrate should be taken into consideration when selecting the solvent. Use solvent-resistant and oil-resistant materials as substrates, and pay attention to maintain a balanced shop temperature. In multi-color overprinting, it should be fully dried in each color, and strictly control the drying temperature, which can effectively prevent ink film turtles Cracking occurs.

Twelve, ink ink <br> ink ink refers to the phenomenon of ink overflow on the outside of the printed line. When printing a line, the ink overflows on the side of the direction of the squeegee and affects the neatness of the line. This phenomenon is called squeaking. The ink can be solved by adjusting the relationship between the printing plate and the ink, the operation of the squeegee, and the relationship of the screen stretching angle. The shape of the through hole portion of the screen printing plate varies depending on the plate making method (indirect method, direct method, straight line method). The ideal through hole should be able to seal with the surface of the substrate when it is scratched. With the ideal through hole shape of the printing plate, the printed ink film is sharp, neat, and accurate in size, without ink. In order to prevent the occurrence of smoke and ink phenomenon, the film should have appropriate thickness, elasticity, and smoothness. For this purpose, a soft nylon mesh and a high-precision polyester mesh plate can be used. In order to prevent blemishes, it is best to use diagonal stretching in the plate making process.

Thirteen, the back of the dirty <br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br If this phenomenon is not controlled, it will cause the page to stick and affect the printing of the other side of the double-sided print. The main reason for the back stickiness is poor ink drying.

The solution to overcome the stickiness on the backside is to adjust the ink viscosity, use faster than the ink, add driers in the ink, or spray powder on the surface of the semi-finished products, or add backing paper.

XIV. Sticky pages

In the accumulation process of screen printing products, adhesion failures may occur between the printing sheets, which is also called adhesion failure. Adhesion can cause print quality problems and even scrap. The main reasons for the occurrence of adhesions are the following:

1 After printing, the printing ink is not dry enough. When the ink is not dried, the printed matter is piled up and piled up, resulting in dirt and adhesion.

(2) The improper selection of the constituent materials of screen printing ink is also the cause of adhesion between printed materials. When the softening point of the film-forming material of the synthetic resin in the ink is relatively low or the ink is not good in volatility, blocking occurs. In general, a thermoplastic resin is used for the evaporative drying ink, and the resin has poor heat resistance. If the solvent remains on the ink film after printing, the ink film will soften, causing the print to stick. Especially in the summer, due to the relatively high temperature, the adhesion between prints is likely to occur.

3 The ink used for printing has a certain dissolution effect on the substrate, and it also causes adhesion between the printed materials. Some of the solvents used in printing are very soluble to the substrate. When printing, the ink produced a certain amount of dissolution on the surface of the substrate. At this time, although the surface of the ink has dried, the contact portion between the ink and the substrate has not yet dried out, and adhesion phenomenon will occur under the action of the gravity. When the substrate is a soft vinyl material, part of the plasticizer in the vinyl material after printing is transferred to the ink film, resulting in the softening of the ink film and the phenomenon of blocking.

4 In order to prevent adhesion phenomenon, we must first select the ink and solvent suitable for printing materials. Secondly, ink with faster drying speed is selected, and sufficient drying is taken care of. Strictly follow the process requirements. General glossy inks are prone to blocking, so pay full attention.

5 scraper plastic strip wear, knife-shaped round, resulting in thickened ink film scraping, or printing pressure is too large, ink film thickening, it will lead to adhesion failure.

Fifteen, printing ink leak

Part of the printing ink leakage, known as the ink leakage failure, the reasons are: a part of the scraper is injured; the pressure of the scraping ink is large; the gap between the printing plate and the printing material is too large; the plate frame is deformed and the printing pressure is insufficient. The viscosity of the ink is too high; the ink is not uniform; the screen is too fine; the printing speed is too fast.

If the printing material and the ink are mixed with dust and printed without processing, the printing plate will be damaged due to the pressure of the squeegee; when the plate is exposed, pinholes are generated in the wooden foot, and the printing film will leak ink. . At this time, adhesive tapes, etc. can be used for emergency handling from the back of the plate. If this operation is not very fast, the ink on the layout will be dried, and the whole of the plate will have to be wiped with solvent. Wipe plates are also the cause of delamination of the plate, so it is best avoided. The ink leakage of the printing plate often occurs in the ink staying part, so it is better to strengthen this part in the plate making.

In the process of manually printing the binder, when inserting a metal plate and a rigid plastic plate into the printing table, it is easy to puncture the plate with a sharp corner, 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 begins: the pinholes of the printing plate should be filled, the printing plate must be fully reinforced, and printing should begin.

Sixteen, image distortion

The printing pressure applied to the printing plate by the squeegee during printing can bring the line contact between the printing plate and the printed material. It should not exceed. If the printing pressure is too high, the printing plate contacts with the printing material and the screen may expand and contract, causing the printing image to be deformed. Screen printing is the smallest type of printing in various printing methods. If we forget this, we cannot print good prints.

If you cannot print without increasing the pressure, reduce the gap between the plate and the surface of the substrate so that the pressure on the blade can be reduced.

Seventeen, AIDS

Ink-wetting refers to the dot-like blotting of the graphics and dark-sounding parts of the substrate, which impairs the printing effect. In particular, the use of transparent ink is more likely to produce this phenomenon. The reasons are as follows: printing speed and ink drying are too slow; ink layer is too thin; ink thixotropic; electrostatic effects; Dot blots.

The improved methods are: improving the fluidity of the ink; using a fast-drying flux; printing with a high-viscosity ink as much as possible; using as much ink as possible with a small amount of oil; minimizing the influence of static electricity.

Eighteen, flying ink

Ink flying is the phenomenon of ink drawing. The causes are: the ink is unevenly grinded; the squeegee is slow from the print during printing; the white space around the printed image is less; static electricity is generated, resulting in the angle of the ink pulling squeegee being too small.

19. Electrostatic fault

Electrostatic current is generally very small, the potential difference is very large, and there may be the phenomenon of attraction, repulsion, conduction, and discharge. These phenomena will lead to deterioration of the product, degraded performance, fire, fire and human body and other adverse consequences.

1 adverse effects on screen printing. In the screen during printing, the rubber portion and the screen are charged by the pressurization of the blade rubber. If the screen itself is charged, it will affect the normal inking and produce blockage failures; it will be absorbed by the screen at the moment when the substrate is output.

a. Synthetic resin inks are easily charged.

b. Substrate is water-absorbent even though it is papery, but static electricity can also be generated when the air is dry. Plastic substrates have good insulation properties, are not affected by temperature, and are prone to static electricity.

c. The printing area is large and the charging is also large, which is easy to produce bad effects.

d. Since spark discharge can cause flames, be very careful when using flammable solvents.

e. The human body's electric shock due to static electricity is caused by contact with a charged object, or by electrostatic discharges that generate spark discharges when grounded. Although the electric current generated by electric shock is small, no danger will occur, but electric shocks often occur, which will have an adverse effect on the operator's psychology.

2 method to prevent static electricity. The methods for preventing the generation of static electricity include: adjusting the ambient temperature and increasing the air humidity. The appropriate temperature is generally about 20° C. and the relative humidity is about 60%. A small amount of antistatic agent is put into the alcohol used for scrubbing printing materials; the friction pressure and the speed are reduced; Minimize the friction, pressure and impact of substrates; install general grounding devices; use ionization of flame, infrared, and ultraviolet rays; and use ionization of corona discharge with high-voltage currents.