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.
Eleven, ink film cracking
The cracking of the ink strand is caused by the effect of the solvent and a large temperature change. 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 is the phenomenon of ink overflowing outside the printed lines. 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
Dirty on the back refers to the phenomenon in which the ink on the lower print sticks to the back of the upper print when the printed product is stacked. 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.
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.
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.
(to be continued)