How to choose printing ink for thin film adhesive materials? What should be paid attention to in printing? These key points must be remembered!

Film adhesive materials are a type of polymer whose surface does not absorb printing ink. The drying methods of printing ink on the surface of film adhesive materials include evaporation, coalescence, crosslinking, and curing, with almost no penetration. This is significantly different from paper-based adhesive materials. The surface energy of film adhesive materials is low, so pretreatment is necessary before printing to meet the wetting and leveling requirements of the printing ink; otherwise, issues such as poor ink adhesion may occur.

Selection and Application of Printing Ink

01
Use printing inks, varnishes, and matched additives specifically designed for film printing

There are many types of ink, and some are specifically designed for film adhesive materials or even for certain types of film adhesive materials. Therefore, inks must be specialized and used together with matched varnishes and additives; otherwise, issues of printing quality due to incompatibility between the ink and the substrate may occur, leading to ink layer peeling.

02
Choose ink that matches the coating of the film material to increase ink layer adhesion

For film materials with surface coatings, it is essential to choose ink whose performance matches that of the coating; otherwise, ink drop-off problems may occur. If the ink is properly chosen, the resin in the ink will firmly bond with the resin in the coating, and the ink layer will not fall off even if the ink is not fully dried. For example, a certain label printing company experienced poor results using Hanghua 161 ink when producing a particular item, but after switching to VP ink of the same brand, the results improved and the ink drop-off issue was resolved.

03Control the ink layer thickness; if necessary, you can use the method of overprinting.

Some printed materials require the ink layer to have a certain thickness. When the ink layer is thick, incomplete drying can cause ink loss issues. The best solution is to use screen printing and gravure printing methods. For printing factories without these conditions, printing the same color ink twice 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 its surface. After two prints, the ink layer thickness is improved and the ink adhesion problem is solved, meeting the process requirements.

04

Control the ink drying speed properly

Pay attention to the ink's shelf life, and when the ink dries slowly, a small amount of drier can be added to increase the drying speed.

05

Ensure the energy for ink drying or curing

Regular cleaning of the ink curing equipment and periodic replacement of UV lamps are key to ensuring that UV ink fully cures. Use instruments or test papers to regularly check the efficiency and status of the UV curing device, and adjust the ink layer thickness and printing speed based on this data to achieve optimal printing results.

06

For multi-color printing, control the printing speed to ensure the ink layer is thoroughly dried

For multi-color printed labels, high-speed printing is not recommended. It is suggested to control the printing speed at 30–50 m/min to ensure the ink layer is thoroughly dried and reduce waste.

Elimination of Static Electricity

Static electricity is one of the main factors causing quality problems when printing on film adhesive materials. Eliminating static and controlling it within a certain range is essential in such printing. For printing companies, eliminating static requires work on two fronts: first, environmental requirements, and second, adding corresponding static elimination equipment.

01

Temperature and humidity control

Low temperature and dryness are the main causes of static electricity. When the printing workshop temperature is around 22°C and the relative humidity exceeds 50%, the static electricity on the surface of film adhesive materials will disappear or be minimized. Therefore, controlling temperature and humidity in the production workshop is necessary. The workshop should be equipped with temperature and humidity control devices. For companies without such conditions, it is possible to isolate the printing equipment, such as setting up a glass enclosure to create an ideal printing environment within a small area. Currently, some small and medium label printing companies in northern regions commonly adopt this method to control static, with good results.

02

Install static elimination devices

There are two common types of static elimination devices. One is electric, which relies on a large number of ions in the generated electric field to absorb, neutralize, and eliminate free electrons on the film surface, thus eliminating static. The other is discharge type, which can work without any energy, as shown in Figure 1. When the film passes over a metal wire (also called a static discharge cord), excess electrons on the film surface are transferred to the ground and disappear. Because the potential of the metal wire is lower than that of free electrons on the film surface, the film can achieve static elimination after discharging through the metal wire.

Figure 1 Schematic diagram of a discharge-type static eliminator

Static eliminators are very practical devices in the printing process of film self-adhesive materials. They can be placed in multiple positions on the printing equipment as needed. As shown in Figure 2, static eliminators are installed before and after the die-cutting station. When installing static eliminators on a printing machine, the following issues should be noted: the equipment must be grounded, as grounding allows static electricity to be conducted to the ground; the static eliminator itself must also be grounded, and it is required to be grounded separately to achieve better static elimination; static electricity is generated continuously, so if conditions allow, all stations of the printing machine should be equipped, that is, the paper feeding station, each printing processing station, and the paper receiving station should all be equipped to achieve the best effect; the static elimination effect should be checked regularly to ensure the reliability of the static eliminator.

Figure 2 Die-cutting and waste disposal unit with static electricity treatment device installed

Printing Precautions for Film Adhesive Materials

01

Proper Control of Printing Tension

For intermittent paper feeding and reciprocating paper feeding printing equipment, paper feeding tension is easy to control. For example, the B-100 label printing machine and the 300 type rotary-to-rotary label printing machine from Lindco, Japan. These types of equipment belong to constant tension printing; the tension is stable and does not affect print registration. However, for continuous paper feeding rotary label printing machines, such as flexographic machines and satellite label printing machines, it is necessary to strictly control and adjust the printing tension; otherwise, registration errors will occur.

The tension control on the printing machine is divided into feed tension and take-up tension control, and needs to be adjusted accordingly for different types and widths of materials to ensure proper printing.

(1) Feed Tension Control

Proper feed tension ensures the accurate registration of film adhesive materials and that the material does not deform during printing, meaning the edges of the material remain straight. If the edges of the material rise during printing, it indicates that the rewinding tension is too loose and the taper setting is unreasonable; the material needs to be rewound before printing again. If rewinding equipment is not available, a temporary remedy is to install a clamp on the unwind shaft to prevent the material edges from rising, but registration errors may still occur under this condition.

(2) Take-up Tension Control

Take-up tension is also related to print registration and determines the neatness of the wound material edges. If the take-up tension is too low, it not only affects registration but results in uneven roll edges, causing problems for the subsequent process. Excessive take-up tension can cause adhesive seepage, label misalignment, or release liner breakage in the printed material, affecting printing efficiency. Therefore, suitable take-up tension is crucial, and generally, the optimal tension value should be determined through testing.

For continuous paper feeding label machines, it is essential to control both the feed tension and take-up tension. After successfully printing different types and widths of materials, it is important to summarize and record this data to improve efficiency and ensure quality. Typically, take-up tension should be equal to or greater than the unwind tension, and not exceed twice the unwind tension.

02

Using Coating Process Instead of Lamination Process

Each film material has its own special physical and chemical properties, and some are not suitable for lamination. If laminated after printing, the material's characteristics are compromised. PE materials have a soft surface and can be applied to curved or spherical objects with a small radius. Commonly used lamination materials are stiff BOPP films; once laminated, the label becomes a PE and BOPP composite material, and its properties change, making it unsuitable for curved or spherical surfaces.

In the composite process, tension mismatch between the two materials may occur, causing label deformation, which can result in edge lifting or detachment during application. Therefore, for large-area labels or labels applied to curved surfaces, it is recommended to use UV coating instead of lamination. The coating process not only protects the ink layer but also maximizes the performance characteristics of the material for optimal application outcomes.

03

It is recommended to use 150 lpi for color halftone printing on labels; 175 lpi is not recommended.

Labels ultimately attract consumers' attention through shelf appeal. For printing, the higher the screen ruling, the clearer the image and text, but the less ink is used, making the colors lighter; the lower the screen ruling, the more ink is used, creating a better visual effect, but the image and text become rougher.

The screen ruling is also directly related to printing quality, printing waste rate, the precision of printing equipment, and the printing materials. A higher screen ruling requires higher material smoothness and equipment precision; otherwise, quality requirements cannot be met. A lower screen ruling has much looser requirements. Therefore, the best choice for film labels is 150 lpi color halftone, which satisfies visual effects, has a certain shelf appeal, ensures printing quality, and keeps consumption within a reasonable range, achieving a balanced compromise.