Unlike paper labels, heat-shrinkable films use PVC, PP, PETG, OPS, OPP, and various multi-layer coextruded films and other non-absorbent printing materials. The nature of such materials determines their printing process and paper labels different. In traditional lithographic offset printing, relief printing (flexo printing), gravure printing, and silk printing, the current printing method of heat shrinkable film labels is still mainly gravure printing. The main reason is that the domestic gravure printing machine has a large amount of printing, and the printing cost is competitive Fierce, and gravure printed products have the characteristics of thick ink layer, bright colors and rich layers, and such labels are mostly based on long version, and the durability of gravure printing plates can reach millions of sheets, so for large print volumes In terms of jobs, it is undoubtedly the most cost-effective. However, with the increasing market competition and the development of flexo platemaking, machinery, ink and other technologies, the proportion of flexographic printing is increasing year by year. However, from the customer's point of view, it is more concerned with satisfying quality and reducing costs, which printing method is followed.
1. Tension control
Since the film is more susceptible to changes in tension during printing, which leads to inaccurate overprinting, it is necessary to pay great attention to the control of tension during printing to maintain the stability and balance of tension. The size of the tension adjustment should be determined according to the type and tensile strength of the film. For example, the tensile strength of the film is weak and it is easy to be stretched and deformed, and the tension should be smaller. For the film with strong tensile strength, The tension can be increased accordingly. In the case of a certain type of film, the width and thickness of the film are also important factors in determining the tension. The wide film should have a higher tension than the narrow film, and the thicker film has a higher tension than the thin film.
Gravure heat shrinkable film mainly uses unit type gravure printing machine. Nowadays, these machines are equipped with automatic tension control system and automatic register control system, which can automatically adjust the unwinding area, printing area and rewinding according to the error between the measured register marks. Zone tension to ensure the stability of the tension in the printing process and the accuracy of the final overprint.
Compared with laminated and unit-type flexographic printing presses, flexographic heat shrinkable film is more suitable for CI-type flexographic printing presses, because in the printing process, each color group shares an impression cylinder, the printing material and the impression cylinder are tight Close fit, small change in tension, so that the material's tensile deformation is small, and the registration accuracy is high.
2. Selection of ink
There are four main types of inks used for shrink film printing: solvent-based inks, water-based inks, cationic UV inks and free radical UV inks. As far as the current application situation is concerned, in the field of shrink film label printing, solvent-based inks dominate, followed by water-based inks and free radical UV inks, while cationic UV inks are relatively expensive and difficult to print, There are not many applications in the field of shrink film. The solvent-based inks mainly used for gravure and flexo heat shrinkable films. Different films should use special inks and cannot be mixed. Ink companies generally provide different solvent ratios of quick-drying, medium-drying, and slow-drying for different materials. The printing plant can choose the appropriate solvent ratio according to the actual production conditions such as workshop temperature and printing speed. In addition, water-based inks and UV inks can also be used. But no matter what type of ink is selected, the performance index of the ink must be fully considered and must meet the requirements. For example, the shrinkage of the ink must match the shrinkage characteristics of the heat-shrinkable film, otherwise it will cause the ink layer to split or even deink.
3. Control of drying temperature
When printing heat shrinkable film, how to control the drying temperature is very important. If the drying temperature is too high, the material will thermally shrink; if the temperature is too low, the ink will not dry thoroughly, resulting in the final adhesion and dirt on the back. Both the gravure printing machine and the flexo printing machine are equipped with an inter-color drying device to ensure the complete drying of each color ink. At the same time, in order to prevent the material from deforming during the drying process, it is required to provide a cold air channel between the color groups to control the influence of the residual heat. Today's printing presses use freezing drums, which can quickly reduce the temperature of the material during the printing process. The shrink film has common printing suitability, such as strong chemical stability, low surface energy, smooth surface without absorption, and poor affinity with printing ink. Therefore, no matter what printing method is used, the film needs to be corona discharged on the surface to improve its surface energy and roughness, and improve the adhesion of ink to the surface of the material.
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