1. The adverse effect of the surface tension of the plastic film
The most commonly used flexible packaging is polyethylene, polypropylene, etc .; abbreviated as: PE, PP, PVC, PS, PVA, PC, PET, CA, NY, etc. Most of them are non-polar materials, with dense, smooth surfaces and an inert surface. The surface energy is relatively low, such as polyethylene is only 2.9 ～ 3.0 × 10-6J / C, the surface energy is low, the transfer rate of the ink is not ideal, the surface is smooth, there is no capillary pores, the ink layer is not easy to fix or the fixation is not strong. After one color is printed, it is easy to be glued off by the ink that is overprinted in the next color, which makes the pattern and text incomplete and causes defects. In addition, if infrared spectroscopy is used to measure the spark-treated material, it can be found that there are carboxyl groups. If the film made of additives is added during the printing process, the additives will easily ooze out and form a layer of oil on the film surface. Floor. Ink layer coatings or other adhesives are not easy to adhere firmly to the surface of such films. This is also the root cause of the ink ink is not true. Therefore, it is necessary to increase the surface free energy of the plastic film. The most common method is corona treatment. After unwinding the plastic film, it passes between the high-frequency voltage electrode rollers, and the discharge device impacts the plastic film to make the pores appear uneven on the surface. Through the magnifying glass, we can see the original dense, smooth surface becomes smooth Rough, the ink adhesion is enhanced, the affinity and compatibility of the ink layer and the substrate are also increased, and the spots and poor reproduction of graphics and text also disappear. Therefore, as long as the surface energy of the substrate is only 738 dynes, its ink application and adhesion fastness have been significantly improved.
2. Blockage of the printing plate causes the fault of inking failure
In general, in gravure printing, only 1 / ～ 2/3 of the ink in the printing plate cell is transferred to the surface of the printing material, and the reasons for the spots on the printed product are:
(1) The ink dries on the surface of the printing plate.
Ink drying on the surface of the printing plate is one of the main reasons for blocking the plate. Generally, it is caused by the reason that the drying speed of the ink is too fast, the viscosity of the ink is high, or the printing speed is slow. Therefore, in actual printing, it should be noted that the viscosity control of the ink is extremely important. If the viscosity is too high, the solvent content in the ink is relatively small, and it is more volatile to dry the ink on the plate, so the viscosity of the ink must be properly reduced; the drying of the ink If the speed is too fast, it is easy to block the plate. Slow drying agent can be used appropriately to adjust the drying speed of the ink. Most of the composite inks are printed with BOPP film. This type of ink is mainly based on chloropropylene resin. Chloropropylene is dissolved in esters, ketones and aromatic hydrocarbons. Toluene xylene is added, and some esters are added, such as ethyl acetate for adjustment; The effect of printing speed: "The speed is too slow, which extends the evaporation time of the solvent in the ink, making the ink easy to dry in the mesh. This phenomenon is more common for printing plates with dots, and the doctor blade can be shortened according to the actual situation. The distance between the squeegee point and the embossing point, add a slow-drying agent to the ink or increase the printing speed appropriately. It should be noted that when the stoppage time is longer, it is best to raise the squeegee to let the plate cylinder in Run at low speed in the ink tank to prevent the ink from drying on the plate surface. Once the blockage phenomenon occurs, the plate should be washed with decontamination powder. If the printing is completed, the plate must be cleaned with a solvent and coated with a protective oil. Effectively place in a ventilated and dry place to avoid blockage in the next printing.
Environmental factors are very important. The temperature of the printing workshop should be 21 ℃ ～ 23 ℃, and the relative temperature should be below 60%. If the ambient temperature is too high and the solvent volatilizes quickly, causing blockage, it should be adapted to add a slow drying agent and speed up the machine.
(2) Poor performance of ink and the influence of quality problems of printing plates.
The ink pigment particles are large or not fully dissolved, the ink storage period is too long to cause deterioration, the solvent balance changes, resulting in reduced solubility of the ink, improper use of different types of ink formulation and chemical reactions. Poor fluidity, reduced ink transfer rate, etc. can cause blockage. Impurities mixed in the ink, such as scraper blades, undissolved ink residues, etc., can also cause blockage. It should be solved by filtering the ink. Now packaging When printing plastic films in color printing factories, most of the printing plates use electronic engraving plates. If the plate making process, the inner wall of the cell may be sheltered due to processes and other reasons, chrome plating leaves surface defects or the dot is lost, and the printing is incomplete. .
3. The fault of inking is not related to the surface aging, wear and slip of the impression cylinder:
The fineness and smoothness of the surface of the impression cylinder are closely related to the fault of inking failure. The surface of the impression cylinder is made of rubber or polyurethane material. Due to the high pressure during printing and long-term corrosion by the solvent, the surface is easily swollen, aging or worn. There are cracks or tiny pits on the surface, and the part of the pattern that is in contact with the defect position will not be inked during printing, resulting in incomplete graphics, especially when the printing plate is a gradation version, and the depth of the mesh is extremely shallow. Most of them can't be printed and are distributed in blocks. The solution is to replace the same type of impression cylinder. If dust or foreign objects adhering to the surface of the impression cylinder often appear as grainy spots in the picture and text, the voids of the spots are consistent with the adhesion of foreign objects.
4. Other factors that lead to inaccurate ink:
When plastic film is printed with dot products, the reproducibility of the printed layer is poor, and small tidbit-like spots usually appear, mainly because the dot depth of the dots is shallow, the transferability of the ink is poor, and the ink is easy to dry in the net holes. It is difficult to truly reflect the tone level of the pattern. Nylon NY is most susceptible to static electricity during printing, which causes spots on the printed matter. In addition to the use of static elimination agents, it can also be adjusted by physical methods, such as temperature and humidity in the printing workshop.
The function of the squeegee is to scrape off the ink without holes, so that the ink in the holes is clean and evenly printed on the substrate. If printing a gradation plate, the angle of the doctor blade is extremely important. If the angle is too large and the pressure is too high, it is easy to cause printed dots, incomplete printing of text lines, and plate wear. The angle of the doctor blade is too large, and the ink on the plate If the scraping angle is too small, it will affect the depth of the ink layer, which will affect the hiding power of the ink. During printing, the pressure of the doctor blade and the angle of the doctor blade are determined according to the depth of the graphic and the ink application of the printed product.
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