Plastic film concave printing "inking is not true" fault 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. Such as: India crystal knife line, printed crystal text is not complete, India crystal smell and so on. This article analyzes a phenomenon that occurs during printing—“ink is not true” and explores and communicates with everyone.

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 majority of color patches appear and the level reproducibility is poor, which often occurs in a gradation version of 30Un or less. To sum up, there are many factors:

1 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. Surface energy is relatively low, such as polyethylene is only 2.9-3.0 x 106j / 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 After the first color is printed, it is easy to be stained by the ink of the next color overprinting, so that the pattern and the characters are incomplete, resulting in 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 lack of ink and 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 and passed through, the high-frequency voltage electrode roller is used to discharge its impact on the plastic film by the discharge device, so that the surface has pores showing irregularities. Through the release lens, we can see the original dense, smooth surface. The smoothness becomes rough, the adhesion of the ink is increased, the affinity of the ink layer and the substrate, the state of the state are also increased, and the defective reproduction of spots and images disappears. Therefore, as long as the surface of the substrate can be as high as 738, the ink condition and the adhesion of ten degrees are significantly improved.

2 printing plate blocked caused false ink failure

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

1) The ink dries on the surface of the printing 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. 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. Such inks are mainly based on chloroprene resin. Chloropropion is dissolved in esters, ketones, and aromatics. Toluene xylene is the main component. Some esters, such as ethyl acetate, are added for adjustment. The effect of printing speed is: “The speed is too slow, prolonging the evaporation time of the solvent in the ink. The ink is easy to dry in the cell, this phenomenon is more common for printing plates with dots, according to the actual situation, the distance between the squeegee squeegee and the imprinting point can be shortened, and the slow-drying agent can be added to the ink. Appropriately increase the printing speed, it should be noted that, when the downtime is longer in the middle, it is best to raise the squeegee to allow the plate cylinder to run at low speed in the ink tank to prevent the ink from drying on the plate. Smear is washed and plated. If printing is completed, the plate must be scrubbed with solvent, coated with protective oil and blanketed, and ventilated and dry in order to avoid the occurrence of plate jamming in the next printing.

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

2) Insufficient ink performance and 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 solubility of the ink is reduced, and improper use of different types of inks causes chemical reaction. Diffuse flow; good, ink transfer rate, etc. will cause blockage, ink is called: Ye Kun has impurities, such as scraper knife chips, undissolved ink residue, etc. will also lead to blocking plate, you should filter through the ink To solve, now the printing plate printing color printing factory when the printing plate people use electronic engraving version, if the process of plate making, due to process and other reasons to make the inner wall of the site to generate points, chrome plated surface defects or in the network site Loss, incomplete printing, etc.

3 Imprinting failures are 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 more pressure and is eroded by solvents for a long time, the surface is easily swollen, aged or worn. 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.

4 Other factors that led to inaccurate ink:

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 fact that the dot depth of part of the dot network is shallow and the transfer of the ink is poor, and the ink is easily dried in the cell It is difficult to truly reflect the order of the patterns. 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 such as temperature and humidity in the press room can also be used.

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, and the printing ink on the printing plate. The scratch is not clean, and too small an angle of the ink will 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. (Text / Chen Xiaoshun)

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