3. Plastic film surface treatment. The plastic film is a general term for a flexible packaging material in which a polymer-based organic compound containing a synthetic resin as a basic component is formed into a flat shape and wound into a roll. Plastic film is the largest type of substrate used in gravure printing except for paper.
Commonly used plastic film materials for gravure printing include polyethylene (abbreviated as PE), polypropylene (PP), polyvinyl chloride (abbreviated as PVC), polystyrene (abbreviated as PS), polyester (abbreviated as PET), and polyacrylic adhesive. It is called nylon (abbreviated as PA), acetate (abbreviated as CA), and cellophane (abbreviated as PT). The molecular structure of most plastic films is non-polar, chemically stable, and has a low critical surface tension γc, which is a low-energy surface, and thus has poor affinity with inks and adhesives, and often the printing ink layer falls off. , compound layer peeling and other issues. At present, in addition to selecting or arranging inks and adhesives with good adhesion to plastic films, methods for treating the surface of plastic films are mainly used to improve the adhesion of plastic films to inks and adhesives.
Surface treatment of plastic films, commonly used methods are chemical oxidation, solvent, flame and corona discharge method.
The chemical oxidation method uses an oxidant to treat the surface of the polyolefin plastic to generate polar groups and increase the polarity of the surface of the plastic film.
The commonly used oxidants are anhydrous chromic acid-tetrachloroethane, chromic acid-acetic acid, potassium dichromate-sulfuric acid, chlorate and the like. The solvent treatment method is to use a surfactant or solvent to bleed out various additives such as antifreeze, plasticizer, lubricant, and anti-static agent that are oozing out of the plastic surface to improve the adhesion of the ink to the surface of the plastic film. The flame treatment method uses the high temperature effect of the flame to remove gas and oil adhered to the plastic surface and improve its wettability. Corona discharge method, also known as electronic shock, EDM treatment.
From the current production practice, corona discharge is a practical and simple method for the prepress pretreatment of plastic films (especially polyene plastic films).
The corona discharge treatment equipment mainly consists of a power supply and an electrode. Transistor medium frequency high voltage generator is generally used for power supply. The electrodes can be used in different forms such as blade shape, strip shape and needle shape. In order to protect personal safety, guards are also attached. In the corona treatment, under the action of a high voltage field, the plastic film generates displacements due to the molecular charge and forms an induced charge. The resulting positive and negative ions make it electrically effective, ie, the polarization effect occurs. Because the ions are of mass, when they hit the plastic during movement, the surface of the film will have micro-textures that are rough and uneven, which will make the surface coarse. The polarization effect and the roughened surface effectively increase the adhesion of the ink. At the same time, oxygen in the air quickly generates ozone. Ozone is a strong oxidant that promotes drying of oxidative polymerization of inks.
After corona discharge treatment, surface tension test pen or surface tension test can be used to determine the effect of plastic film pretreatment. The ratio of the surface tension test solution is shown in Table 6-3. During the test, the known surface tension measuring solution is applied on the surface of the treated film with a cotton wool ball. The coated surface is approximately 30 mm2. If it shrinks into a water particle shape within 2 s, the film surface tension is lower than that of the test liquid. Surface tension, need to improve the corona strength before processing. If the test solution does not shrink within 2 seconds, the film has reached the treatment effect. Under normal circumstances, the surface tension of the plastic film can reach 3.8×10-2N/m, which can be used for printing. When it reaches 4.0-4.2×10-2N/m, it can meet the compounding requirements.
Table 6-3 Surface Tension Determination Liquid Proportioning Table
0 100 3.0 67.5 32.5 4.1
2.5 97.5 3.1 71.5 28.5 4.2
10.5 89.5 3.2 74.5 25.3 4.3
19.0 1.0 3.3 78.0 22.0 4.4
26.5 73.5 3.4 80.3 19.7 4.5
35.0 65.0 3.5 83.0 17.0 4.6
42.5 57.5 3.6 87.0 13.0 4.8
48.5 51.5 37 90.7 9.3 5.0
54.0 40.0 3.8 93.7 6.3 5.2
59.9 41.0 3.9 96.5 3.5 5.4
63.5 36.5 4.0 99.0 1.0 5.6
4. Electrostatic blotting. Ink in the gravure network must be under greater pressure before it can be transferred to printing materials. Therefore, the load of the gravure printing press is greater than the load of the letterpress lithography printing press. In spite of this, when the surface of the paper is rough, the ink in the cells is also difficult to fully contact with the paper, and the ink does not transfer well to the substrate, resulting in a white dot in the printed graphic, which is commonly referred to as "frost." For the engraving intaglio, the ink transfer effect stored at the bottom of the cone tip is even worse. This is particularly noticeable in the highlight portion, so the subtle level of the highlight portion of the intaglio print is generally not reproducible. Although increasing the pressure can improve the transferability of the ink, the increase in the pressure has a certain limit, otherwise it is easy to burst the adhesive layer of the impression cylinder, or to extend the substrate. In order to increase the ink transfer rate of gravure printing, an electrostatic ink-absorbing device is installed on the gravure printing machine, and the ink on the printing plate is sucked onto the printing material by the principle of positive and negative charges attracting each other.
Figure 6-10 is a schematic diagram of an electrostatic ink-absorbing device. The imprinting cylinder of the printing press is conductive. When printing, an electrostatic high-voltage generator is used to generate an electric field between the printing plate or the embossing cylinder, and the ink in the printing plate cell is sucked onto the substrate by electrostatic attraction.
In designing and manufacturing an electrostatic ink-absorbing device, a voltage of 200 to 1000 V is applied to a sufficient amount of paper, and a voltage of 500 to 5000 V is applied to the printed thick paper.
The application of electrostatic ink-absorbing device will increase the ink transfer rate of gravure printing by 15% to 20% compared with that under normal pressure. This not only enables the reproduction of subtle levels of prints, but also reduces the printing pressure and prolongs the printing press. life span. However, in the printing process, it is necessary to keep the environment clean, to prevent the conductive rubber on the surface of the impression roller from adhering to the dust, and to prevent the uneven voltage from occurring at various parts of the impression cylinder, resulting in the problem of reduced ink absorption.
Figure 6-10 Electrostatic Absorption Device
Ball Game,Air Power Football For Kid,Tictac Tower Best Toys For Kids,Air Power Football
BELLMAY TOY CO., LTD. , http://www.bellmaytoys.com