How to choose printing ink for film adhesive materials? What should be paid attention to during printing? Remember these key points!

Film adhesive materials are a type of polymer whose surface does not absorb printing ink. The drying methods for printing ink on the surface of film adhesive materials include evaporation, coalescence, cross-linking, and curing, with almost no penetration, which is quite different from paper-based adhesive materials. The surface energy of film adhesive materials is low, and pretreatment is required before printing to meet the wetting and leveling requirements of the ink; otherwise, problems such as poor ink adhesion may occur.Selection and Application of Printing Ink01Use inks, varnishes, and additives specifically designed for film printingThere are many types of inks, and some are specifically designed for film adhesive materials, or even for a particular type of film adhesive material. Therefore, the ink must be specialized and used with compatible varnishes and additives; otherwise, mismatches between the ink and the substrate can lead to printing quality issues and cause the ink layer to peel off.02Use inks that match the coating of the film material to increase ink layer adhesionFor coated film materials, inks with compatible properties must be chosen; otherwise, ink may peel off. If the ink is properly chosen, the resins in the ink will bond firmly with the resins in the coating, and even if the ink has not fully dried, the ink layer will not peel off. For example, a label printing company found poor results when using Hanghua 161 ink for a certain product, but after switching to VP ink of the same brand, the results improved and the ink peeling problem was resolved.

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Control the ink layer thickness, and if necessary, use the overprinting method.Some printed materials require a certain thickness of the ink layer. When the ink layer is thick, incomplete drying of the ink can cause ink shedding. The best solution is to use screen printing and gravure printing. For printing factories without these conditions, using the same-color ink for double printing can be considered. The usual practice is to first print a very thin layer of ink, and after it is completely dry, print again on the surface. After the two printings, it not only improves the ink layer thickness but also solves the ink adhesion problem, meeting the process requirements.04Control the ink drying speedPay attention to the ink's shelf life. When the ink dries slowly, a small amount of drier can be added to increase the drying speed.05Ensure the ink drying or curing energyRegular cleaning of the ink curing device and periodic replacement of UV lamps are key to ensuring complete curing of UV ink. Regularly check the efficiency and condition of the UV curing device with instruments or test paper, and adjust the ink layer thickness and printing speed according to these data to achieve the best printing results.06Control printing speed for multicolor printing and ensure ink layer is fully driedFor multicolor printed labels, high-speed printing is not recommended. It is suggested to control the printing speed within 30–50 m/min to ensure the ink layer is completely dried and to reduce waste.Elimination of static electricityStatic electricity is one of the main factors causing quality problems during printing on film self-adhesive materials. Eliminating static and keeping it under control is necessary in the printing of film self-adhesive materials. Eliminating static has two requirements for printing companies: one is environmental, and the other is adding corresponding static elimination devices to the equipment.01Temperature and humidity controlLow temperature and dryness are the main causes of static. When the printing workshop temperature is around 22°C and relative humidity is above 50%, the surface static electricity of film self-adhesive materials will disappear or reduce to a minimum. Therefore, temperature and humidity must be controlled in the production workshop. The workshop should be equipped with constant temperature and humidity devices. Companies without such conditions can consider isolating printing equipment, such as establishing a small glass room to create an ideal printing environment. Currently, some small and medium-sized label printing companies in northern China commonly use this method to control static with good results.02Install static elimination devicesThere are two common types of static elimination devices. The first type is electric, which uses a large number of ions generated in the electric field to absorb, neutralize, and eliminate free electrons on the film surface to remove static. The second type is discharge type, which can work without any power, as shown in Figure 1. When the film passes through a metal wire (also called an anti-static rope), the excess electrons on the film surface are discharged into the ground via the metal wire. Because the potential of the metal wire is lower than the potential of the free electrons on the film surface, passing through the metal wire discharge can eliminate the static electricity on the film.

Figure 1 Schematic diagram of a discharge-type static eliminatorStatic eliminators are very practical devices used in the printing process of thin film self-adhesive materials. They can be placed at multiple positions on printing equipment as required. As shown in Figure 2, static eliminators are installed before and after the die-cutting station. When installing static eliminators on a printing press, the following issues should be noted: The equipment must be grounded, as grounding allows static electricity to be conducted into the earth; the static eliminator itself must also be grounded, and it is required to be grounded separately to achieve a better static elimination effect; since static electricity is generated continuously, if conditions permit, static eliminators should be installed at all workstations of the printing press, including the paper feeding station, each printing and processing station, and the paper collecting station, for the best result; the effectiveness of static elimination should be checked regularly to ensure the reliability of the static eliminator.

Figure 2 Die-cutting and waste disposal units with electrostatic treatment devices installedPrecautions for Printing on Film Adhesive Materials01Properly Control Printing TensionFor intermittent and reciprocating paper-feeding printing equipment, it is easy to control the paper-feeding tension. Examples include Japan Lindetec's B-100 label printing machine and the 300-type cylinder-to-cylinder label printing machine. These types of equipment are constant-tension printing machines with stable tension, which does not affect print registration. However, for continuous paper-feeding rotary label printing machines, such as flexo machines and satellite label printing machines, it is necessary to strictly control and adjust the printing tension, otherwise misregistration may occur.Tension control on the printing machine includes controlling both the paper-feeding tension and the rewinding tension. These must be adjusted according to different types and widths of materials to ensure normal printing.(1) Paper-Feeding Tension ControlProper paper-feeding tension ensures accurate registration of film adhesive materials and prevents material deformation during printing, meaning the material edges do not tilt. If the edges of the material rise during printing, it indicates that the rewinding tension is too loose or the taper is set improperly. The material needs to be rewound and reprinted. If rewinding equipment is not available, a temporary solution is to install a clamp on the unwinding shaft to prevent the material edges from rising. However, registration issues may still occur in this case.(2) Rewinding Tension ControlRewinding tension is also related to printing registration and determines the neatness of the rewound edges. If the rewinding tension is too low, it affects registration and causes uneven rewound edges, which could cause problems in subsequent processes. Excessive rewinding tension may lead to adhesive seepage on the material edges, label displacement, or liner breakage after printing, which impacts printing efficiency. Therefore, appropriate rewinding tension is crucial, and the optimal tension value should generally be determined through experimentation.For continuous paper-feeding label machines, both paper-feeding and rewinding tensions must be well controlled. After achieving qualified printing with different materials and widths, these data should be promptly summarized and recorded to improve efficiency and ensure quality. Generally, rewinding tension should be equal to or greater than the unwinding tension, but not exceed twice the unwinding tension.02Use Varnishing Instead of LaminatingEach type of film material has unique physical and chemical characteristics; some are not suitable for lamination. Laminating after printing can destroy the material's properties. PE materials have a soft surface, suitable for objects with large curvature radii or spherical surfaces. Laminating materials commonly used are stiff BOPP films, and once a laminated label combines PE and BOPP, its properties are altered, rendering it unsuitable for curved or spherical surfaces.The combination of the two materials may also lead to tension mismatch, causing label deformation and issues such as edge lifting or detachment during application. Therefore, for large-area labels and labels applied to curved surfaces, it is recommended to use UV varnishing instead of lamination. Varnishing not only protects the ink layer but also maximizes the material's performance for optimal application.

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It is recommended to use 150 lpi for printing color halftone labels, and 175 lpi is not recommended.

Labels ultimately attract consumers' attention through shelf appeal. For printed materials, the higher the halftone screen frequency, the clearer the images and text, but the ink coverage is less, resulting in lighter colors; the lower the halftone screen frequency, the more ink is used, giving a better visual effect, but the images and text appear rougher.

The halftone screen frequency is also directly related to printing quality, printing waste rate, the precision of printing equipment, and the printing materials. A higher screen frequency requires smoother materials and high-precision equipment to meet quality requirements; a lower screen frequency has much more relaxed conditions. Therefore, the best choice for film labels is 150 lpi color halftone. This not only meets visual appeal and shelf impact requirements but also ensures printing quality, controls consumption within a reasonable range, and achieves a comprehensive balance.