Analysis of Insufficient Bonding of Plastic Film Gravure Printing Inks

Abstract: This paper discusses the ink adhesion problem in printing in many aspects. Mainly from the substrate, and from shallow to deep impact on the printing process to analyze and find solutions. Practical practicality, for reference by peers.

Keywords: plastic film, ink, causes and countermeasures

With the rapid development of composite flexible packaging, the gravure printing technology of plastic films has been continuously improved. The quality of printed products has a direct impact on the status of products, affecting the efficiency of enterprises and the future of development. Plastic film gravure printing is a professional technique and quality failures often occur in actual use. Such as: printed products have cut lines, printed text is not complete, printed products have smell and so on. This article focuses on the analysis of a phenomenon that occurs during printing—“ink is not secure” and discusses and exchanges with everyone.

The insufficiency in ink formation caused by the gravure printing of plastic films is mainly manifested in spotting, graphics, lines, repetitive layers, or missing dots in printed blocks. The color patches appear and the level of reproducibility is poor, and often occurs in the level of 30um below the majority of the above. To sum up, there are many factors:

First, adverse effects of plastic film surface tension

The most commonly used flexible packaging is polyethylene, polypropylene; code shown as: PE, PP, PVC, PS, PVA, PC, PET, CA, NY and so on. Its non-polar thin film belongs to non-polar material, and its surface is compact, smooth, and it is an inert surface. The surface energy is relatively low, but the polyethylene is only 2.9~3.0*10-6J/cm2, the surface energy is low, the ink transfer rate is not ideal, the surface is smooth, there are no pores, the ink layer is not easy to fix or not firmly fixed, After one color is printed, it is easy to be stained by the ink of the next color overprint so that the pattern and text are incomplete and cause defects. In addition, if the point-spark treated material is measured by infrared spectroscopy, the presence of carboxyl groups may be found. If the film made of an additive is added during the printing process, the additive part may ooze out and form a layer of oil on the surface of the film. Layer, ink layer coating or other adhesive. It is not easy to firmly bond on the surface of such films. This is also the root cause of the ink's inaccurate ink. Frequently, spots appear on the solid parts of the prints. The level and level of the images are poor in appearance, and the phenomenon of missing dots is observed. It is judged whether the dot is lost or not. Often, a magnifying glass is used to observe the impression of the ink. Including the appearance of spots in the printed product and distribution, as well as the extent of missing dots. In order to improve this phenomenon, the only way is to increase the surface free energy of the plastic film. The most commonly used method is corona treatment. The so-called corona treatment refers to that after the plastic film is unwound and passes through the high-frequency voltage between the electrode rollers, it discharges the impact on the plastic film of the discharge device, so that the surface has uneven pores. Through the magnifying glass, we can see the original closeness. The smooth and smooth surface becomes rough, and the dyne pen detection also increases from the original 2.9-3.0x10-6J/cm2 to about 40dp/m2, the ink adhesion is enhanced, and the ink layer and substrate are compatible and compatible. Sexuality has also increased, and spots and graphic reproduction have disappeared. Therefore, as long as the substrate surface energy as long as 738 Dyne, the ink situation and the attachment fastness are significantly improved.

Second, printing plate blocking caused ink inaccurate fault phenomenon

Generally speaking, in gravure printing, only 1/3~2/3 of the ink in the printing plate's cell site is transferred to the surface of the printing material, resulting in spotted prints.

1. The ink dries on the surface of the plate. The drying of ink on the surface of the plate is one of the main reasons for blockage. Generally due to ink drying speed, ink viscosity or slow printing speed and other reasons. So in the actual printing should pay attention to several points: the viscosity of the ink. The viscosity control of the ink is extremely important and there are many print failures due to improper viscosity control. If the viscosity is too high, the solvent content in the ink is relatively small, and it is easier to volatilize to make the ink dry on the layout. Therefore, the viscosity of the ink must be properly reduced, and there are various methods for reducing the viscosity of the ink; the drying speed of the ink is too fast and is easy to produce. Blocking plates can be suitably used with a slow drying agent to adjust the drying speed of the ink. Among the composite inks, the majority of printed BOPP films are chlorinated polypropylene resins. Chlorinated polypropylene is dissolved in esters, ketones and aromatics. Toluene, xylene, and some esters, such as ester acids, are added. Ethylene and other adjustments; printing speed effects: "The speed is too slow, prolonged the solvent evaporation time in the ink, so that the ink is easy to dry in the cell, for the printing plate with dots, this phenomenon is more common, according to The actual situation shortens the distance between the squeegee squeegee and the imprinting point, adds slow-drying agent to the ink or appropriately increases the printing speed. It should be noted that it is better to raise the squeegee when the downtime is longer in the middle. Allow the plate cylinder to run at a low speed in the ink tank to prevent the ink from drying on the plate. Once a plate jam occurs, the plate must be washed with a decontaminating powder. If printing is completed, the plate must be scrubbed with solvent and protected by painting. The oil is wrapped in a blanket and placed in a ventilated and dry place to avoid blocking of the plate in the next printing.

The environmental factors are very important. The temperature of the printing shop should be 21~23°C, and the relative temperature should be less than 60%. If the ambient temperature is too high, the solvent volatilizes quickly and the plate is blocked, the slow-drying agent should be added and the machine speed should be increased.

2. The poor performance of ink and the effect of printing plate quality problems

The performance of the ink itself is poor, the pigment particles are large or not fully dissolved, the storage period of the ink is too long to cause deterioration, the solvent balance changes, and the solubility of the ink is reduced, and the use of different types of inks is improperly used to cause a chemical reaction. Poor flowability, reduced ink transfer rate, etc. will all result in blockage, which can be solved by replacing inks that meet the requirements for gravure printing. The ink contains impurities such as scraper blades, undissolved ink residue, etc. Lead to block printing, it should be through the filter ink to solve, and now the printing color printing factory printing plastic film most of the electronic engraving version, if the process of plate making, may be due to process and other reasons to make the inner wall of the site to create points, chrome retention Defects in the surface or in the occurrence of missing dots, printing incomplete and other failures.

Third, the phenomenon of ink failure is related to the surface aging, wear, and slip of the impression cylinder :

The fineness of the surface of the embossing cylinder, the smoothness and the lack of ink on the fault are closely related. The surface of the impression cylinder is made of rubber or polyurethane. As the printing is under great pressure, it is generally about 4kg, and it is eroded by solvents for a long time. It is very easy for the surface to swell, deteriorate or wear out. The appearance of cracks or tiny pits on the surface, the part of the pattern in contact with the location of the defect during printing will not be inked, resulting in the incomplete graphics, especially when the printing plate is a layered version and the depth of the cell depth is extremely shallow. Most of the cases in which the ink is not stuck are printed and printed in blocks. The solution is to replace the same type of impression cylinder. The surface must be smooth and tidy and exquisite. If dust or foreign matter adheres to the surface of the impression roller, the phenomenon of ink insufficiency is also caused. This condition is often manifested as grainy spots at the graphic and textural points, and the voids at the spots coincide with the adhesion of foreign matter. The solution is to wipe off dust and foreign objects.

Fourth, other factors lead to ink failures

When the plastic film is printed with dot products, the reproducibility of the print layer is poor, and fine floc-like spots usually appear. This is mainly due to the shallow depth of the dots in the dots and the poor transferability of the ink, and the ink is easily dried in the cell. It is difficult to truly reflect the tone level of the pattern, which is related to the role and influence of static electricity. Nylon (NY) is most likely to generate static electricity during printing. The static electricity of the material is not conducive to printing. It directly affects the quality of the printed product and causes the printed product to appear spotted. In addition to the use of static eliminators, it is adjusted by physical methods, such as adjusting the temperature and humidity of the printing shop.

The effect of the squeegee is that the squeegee is used to clean the ink without the net, so that the ink in the net is clean and evenly printed on the substrate. If printing the gradation version, the angle of the squeegee is extremely important. The angle is too large and the pressure is too high, which can easily cause the printing product dots, the incomplete printing of the character lines, the wear of the printing plate, the angle of the squeegee is too large, and the ink on the printing plate. The scraping is not clean, and the squeegee angle is too small to affect the depth of the ink layer, thereby affecting the hiding power of the ink. When printing, the pressure of the squeegee and the size of the squeegee angle are determined according to the depth of the printing plate graphic and the ink situation of the printing product.

To sum up, there are still many factors that cause inaccurate inks. It is necessary to make adjustments and solve them according to the actual situation.

Source: China Plastic Packaging Information Network Author: Chen Quandong