Currently, shrink-wrap labels are usually made of PP film. Since the shrinkage of PP film is not high, generally within 20%, it is more appropriate to use hot-melt adhesive when labeling. If a solvent-based adhesive is used, it may cause wrinkling of the PP film. Due to the higher shrinkage temperature of PP film, heat treatment is mainly carried out by hot air, which can reach a higher temperature than steam heating.
Heat shrinkable film labels are a part of the label market. They are currently undergoing rapid growth. The market share is constantly expanding. The expected annual growth rate is about 15%, which greatly exceeds the annual growth rate of about 5% in the ordinary label market. The development potential is huge. Become a major bright spot in the label printing industry. It is predicted that the domestic heat shrinkable film market will grow at a rate of more than 20% in the next 5 years.
The food industry is the largest market for heat shrink packaging. Heat shrinkable film is widely used in the packaging of various fast foods, lactic acid foods, beverages, snacks, beer cans, various alcohols, agricultural and sideline products, dry foods, local specialties, etc. At present, the customer base of the shrink film label market is mainly large fast-moving consumer goods companies, such as Procter & Gamble, Unilever, Shanghai Jahwa, etc., whose products are in large quantities and require long live printing. The initial investment of gravure printing is high, but the gravure printing plate has high durability and the relative low cost makes it the first choice for shrink film printing. Moreover, the graphic part of the printing plate is concave, so you can get a solid ink layer, bright colors and rich layers.
In recent years, with the promotion of flexographic printing, some shrink films are also printed by flexographic printing, especially PE materials that cannot withstand too much tension, and most of them use CI-type flexographic printing machines. In the non-food field, the application of heat shrinkable film labels is also increasing, such as labels and caps, sealing, cosmetics, pharmaceuticals, daily chemical products, stationery and sports supplies, kitchen supplies, daily sundries and so on. At the same time, it has been widely used in the fields of ceramic products, tea sets, mechanical parts, construction materials and transportation materials.
Pre-press production
Because the heat-shrinkable film is a thermoplastic film that is stretched and oriented during the production process, and is shrinked by heat during use. Therefore, no matter which printing method is used for printing, the horizontal and vertical shrinkage rate of the material and the allowable deformation errors in the decorative graphics after shrinkage must be considered before the design of the surface pattern to ensure that the pattern that shrinks onto the container, Accurate restoration of text and barcode.
1. The direction of the pattern. Regardless of whether the heat shrinkable film adopts gravure printing or flexographic printing, the printing is mainly in the inner printing method, and it should be positive with respect to the pattern direction on the printing plate. There are also shrink films for surface printing. If this happens, the direction of the pattern on the printing plate should be reversed.
2. The level of the pattern.
Due to the shortcomings of flexographic printing, if flexo printing is used for the shrink film, the image layer should not be too delicate, while using gravure printing can require a richer image layer.
3. Size design. The heat shrinkable film material used for printing has a lateral shrinkage rate of 50% to 52% and 60% to 62%, and can reach 90% in special cases, and a longitudinal shrinkage rate of 6% to 8%. However, when the film shrinks instantaneously, due to the limitation of the container, the horizontal and vertical directions cannot be completely contracted. To ensure the accurate restoration of the contracted patterns, characters and barcodes, the shape of the container must be taken into consideration, and the correct size and the Deformation rate. For heat-shrinkable labels that need to turn the sheet-like film into a cylindrical shape and use adhesives to seal the overlapping parts together, it should be noted that graphics are not generally designed at the sealing parts to avoid affecting the adhesion fastness.
4. Place the barcode. Generally, the bar code placement direction should be consistent with the printing direction, otherwise the lines of the bar code will be distorted, which will affect the scanning results and cause misreading. In addition, the color selection of label products should be based on spot colors as much as possible, and the production of the white version is necessary. It can be made into a full version or hollowed out according to the actual situation. The color of the bar code should follow the conventional requirements, that is, the color combination of the bar and the space should conform to the principle of bar code color matching. Print material selection. The printing of heat shrinkable labels has been briefly analyzed above. In addition to the quality of the printing process, the material plays a decisive role. Therefore, choosing the right material is the key. The thickness of the film material is determined according to the application field and cost of the heat-shrinkable label, the characteristics of the film, shrinkage performance, printing process and labeling process requirements. In general, the film thickness of shrink film labels should be 30 microns to 70 microns, 50 microns, 45 microns, 40 microns is more commonly used, depending on the labeling performance of the labeling equipment. For the selected label material, the shrinkage of the film material is generally required to be within the range of application, and the shrinkage in the transverse direction (TD) is higher than the shrinkage in the longitudinal direction (MD). The transverse shrinkage rates of commonly used materials are 50% to 52% and 60% to 62%, and can reach 90% in special cases. The longitudinal shrinkage rate is required to be 6% to 8%. In addition, because the shrink film is very sensitive to heat, it is necessary to avoid high temperatures during storage, printing and transportation. [next]
Printing essentials
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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