Plastic film gravure printing ink false analysis

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 problems often occur in practical use. The inaccurate ink produced during the gravure printing of plastic films is mainly manifested in spots, lines and lines of graphics, reproducibility of layers, or missing dots. The color patches often appear on the gradation board that is less than 30 μm in size due to the appearance of spots and poor reproducibility of the gradation. To sum up, there are many factors:

First, adverse effects of plastic film surface tension

The most commonly used flexible packaging polyethylene, polypropylene, etc.; abbreviations: PE, PP, PVC, PS, PVA, PC, PET, CA, NY and so on. Mostly non-polar materials, the surface is dense, smooth, is an inert surface. Relatively low surface energy, such as polyethylene is only 2.9 ~ 3.0 × 10-6J / C, 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, the first 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. Floor. Ink layer coatings or other adhesives are not easily bonded on the surface of such films. This is also the root cause of the ink's inaccurate ink. Therefore, it is necessary to increase the surface free energy of the plastic film. The most commonly used method is the corona treatment method. After the plastic film is unwound, it passes through the high-frequency voltage between the electrode rollers, and discharges the impact of the plastic film on the discharge device, causing the surface to have pores that are uneven. Through the magnifying glass, we can see the original compactness, and the smooth surface becomes smooth. Roughness increases the adhesion of the ink, and the affinity and compatibility of the ink layer with the substrate also increase, and the poor reproduction of spots and images disappears. 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 failure

In general, in gravure printing, only 1/ to 2/3 of the ink in the substrate of the printing plate is transferred to the surface of the printing material, resulting in spots on the printing product:

(a) The ink dries on the plate surface.

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. Therefore, the actual printing should pay attention to: the viscosity of the ink control is very important, if the viscosity is too high, the solvent content of the ink is relatively small, more volatile and the ink dry in the layout, so we must properly reduce the viscosity of the ink; ink drying If the speed is too fast, it is easy to produce a blockage plate, and a slow drying agent may be appropriately used to adjust the drying speed of the ink. Among the composite inks, the majority of BOPP films are printed. This type of ink is mainly composed of chlorinated acrylic resin, chloropropylene dissolved in esters, ketones and aromatics, mainly toluene-xylene, and some esters such as ethyl acetate are added for adjustment. The effect of printing speed: "The speed is too slow, prolongs the time of volatilization of the solvent in the ink, so that the ink can easily dry in the cell. For the printing plate with dots, this phenomenon is more common, and the squeegee can be shortened according to the actual situation. The distance between the squeegee and the embossing point, adding a slow-drying agent to the ink or appropriately increasing the printing speed, it should be noted that when the downtime is long in the middle, it is best to raise the squeegee to allow the plate cylinder to The ink tank runs at a low speed to prevent the ink from drying on the plate. Once a plate jam occurs, it needs to be washed with a decontamination powder. If the printing is completed, the plate must be scrubbed with solvent and coated with a protective oil blanket. Efficient ventilated and dry place, so as to avoid the phenomenon of blockage in the next printing.

The environmental factors are very important. The temperature of the printing shop should be 21°C~23°C, the relative temperature should be less than 60%, 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.





(b) The poor performance of the ink and the effect of printing plate quality issues.

Ink pigment particles are large or have not been fully dissolved, the storage period of the ink is too long to cause deterioration, the solvent balance changes, and the ink solubility decreases. Different types of inks are used improperly to cause chemical reactions. Poor flowability, reduced ink transfer rate, etc. will all result in blockages. Immersion of impurities in the ink, such as the scraper blade, undissolved ink residue, etc., will also result in blockage, which should be solved by filtering the ink. Most of the printing plates used in printing plants are printed on plastic plates. If plates are processed during the platemaking process, the inner walls of the cell walls may be exposed, and chrome plating may leave surface defects or network dots missing. Printing is incomplete. .

Third, inaccurate ink indentation is related to the aging, wear, and slippage of the surface of the impression cylinder:

The fineness of the surface of the embossing cylinder, and its smoothness are closely related to the fact that the ink is not practical. The surface of the impression cylinder is made of rubber or polyurethane. As the printing is subjected to great pressure and is eroded by solvents for a long time, the surface is likely to swell, deteriorate or wear out. The appearance of cracks or tiny pits on the surface, the part of the pattern that is in contact with the location of the defect during printing will not be inked, resulting in the incomplete picture, especially when the printing plate is a layered version and the depth of the cell depth is extremely shallow. Most of them cannot be printed and they are distributed in blocks. The solution is to replace the same type of impression cylinder. If dust or foreign matter adheres to the surface of the impression cylinder, it often appears as grainy spots at the graphic and textural points, and the voids of the spots coincide with the adherence of foreign matter.

Fourth, other factors that lead to ink is not true:

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. Nylon NY is most likely to produce static electricity when printing, causing the print to appear spotted. In addition to the use of static eliminators, physical methods can be used to adjust, such as the temperature and humidity in the press room.

The role of the squeegee is to scrape off the ink at the cells, so that the ink in the cells 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 many factors that cause inaccurate ink, and this should be adjusted and resolved according to the actual situation.


Reprinted from: Plastics Industry Network

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