Paper Coating Technology (2)

Control points for laminating process operations

Paper-plastic lamination is the combined result of thermal compression and physical effects. Film lamination temperature, film lamination pressure, and lamination speed are the basic conditions for realizing this physical integration. Correctly handling the film temperature, film pressure, and film speed is critical to ensure the quality of the film and bring a good interface for the next process.

1, film temperature control

That is, the temperature of the coating film drying tunnel can not be too high or too low, generally controlled at about 50-60 degrees. If the temperature is too low, the solvent in the adhesive cannot be fully exerted, which will affect the fastness of the film, and bubbles are likely to appear. If the temperature of the drying tunnel is too high, many wrinkles will be generated on the surface after coating.

Under heat pressure, the laminated adhesive layer is in a molten state, molecules increase the number of molecules participating in bonding, and the maximum adhesive force is achieved between the interfaces of the plastic film, the printed matter, and the adhesive layer. The increase in film temperature contributes to the increase in bond strength, but it must be controlled within a reasonable range. That is, the film coating temperature is generally controlled at 80 - 90 degrees, pre-coated film temperature is generally controlled at 80 - 100 degrees. If the temperature is too high, the plastic film shrinks and deforms when it is laminated, resulting in wrinkling, curling, and local blistering after the film is covered.

The correct temperature control can completely melt the adhesive layer on the plastic film to a viscous state with proper fluidity to achieve the wet adhesion of the surface of the printed material, and at the same time, ensure that the surface of the overlay is cured instantly. The film product has a smooth, bright appearance, a good melting degree of the adhesive layer, no crease, and peeling of the ink. The temperature control of the lamination process also influences and determines the resistance of the hot melt when subjected to heat and the new thermal resistance that will be tolerated during subsequent operations (such as bronzing). Therefore, the film temperature must be strictly controlled.

2, film pressure control

The seemingly smooth surface of the paper prints is not very smooth. Only under the ideal pressure, the hot melt glue in the viscous flow state will fully wet the surface of the paper prints in the process of driving away the air on the surface of the prints, thus generating colloidal molecules. Ink layer, paper molecules spread, dislocation, to achieve a good bond, the formation of a complete coverage of the print surface, so that the printed surface of the paper light, no fog, glue layer leveling, no traces, to achieve a good bonding effect .

If the covering pressure is too large, the film is easily deformed to generate wrinkles, and the rubber roller is deformed by pressure. If the lamination pressure is too small, the adhesive fastness is poor. If the paper prints are not wrinkled and the lamination pressure is properly increased, the thermoplasticity and curing performance of the hot melt adhesive can be fully expressed, so that the film products will be heated during the subsequent heating (such as bronzing and indentation). It has strong resistance to various physical peelings, impacts, etc., making the internal structure and surface state of the laminated paper prints perfect and harmonious.

In actual production, the pressure of the film is also adjusted according to the variables of the process conditions. Paper prints have a rough surface, a loose texture, and a high permeability of the pores, and the pressure for lamination must be increased in a timely manner. In actual operation, the pressure of printed paper with tight structure, tightness, and high degree of smoothness of the paper is generally controlled between 180 kg and 200 kg. The printed paper with loose fiber structure, tightness and rough surface The lamination pressure is generally controlled between 190 kg and 220 kg.

3, film speed control

The film of the paper print is completed under the movement of the conveyor belt, completing the paper print and plastic film (BOPP)

Lapping. In this exercise process, the transfer speed of the laminator determines the length of time that the paper and the plastic film assembly stay on the hot-pressing work interface. When the lamination temperature and the lamination pressure are fixed, the laminating machine will change the transport speed. The effect of lamination changes.

The laminating speed of the lamination film is too fast, and the binding between the printed matter of the paper and the plastic film is not firm, and the haze may easily occur. The film lamination speed is too slow, not only in the production process efficiency but also the product is easy to blistering.

Film lamination rate determines the bonding time between plastic film and paper prints. If the paper has a high basis weight, low ambient temperature, high humidity, and poor brightness of the printed paper, the speed of the laminating machine should be appropriately reduced. In order to meet the best effect of the film is good. According to the actual work experience, the laminating machine's laminating speed range is generally controlled between 6 and 12 meters per minute, which is about 20 sheets of facing paper.

Factors affecting film quality

Factors affecting the quality of the film In addition to the plastic film, adhesive properties, pressure of the film, speed of the film, temperature of the film, factors affecting the quality of the film are also related to the printed paper, the state of the printed ink layer, and the viscosity. The coating conditions of the mixture, the environmental humidity of the film, and the printing process conditions of the paper print have a close relationship.

1, the impact of printing paper on the film

The coated paper is mostly offset paper, cardboard, and coated paper. Different papers will have different quality effects after being coated. Paper should be selected smooth, high tightness, lint-free, can not afford the paper, so as to ensure that the paper evenly adsorb adhesive, improve bond strength.

When the surface of the paper is low in smoothness and the tightness of the paper is covered, the structure of the paper is rough and loose during the printing process.

When an excessive amount of oily substance is absorbed, the pigment component in the ink floats on the surface of the paper and cannot be strongly adsorbed. When the film is coated, the adhesive cannot evenly and uniformly penetrate between the paper surface of the printed matter and the plastic film. Drying can easily cause defects such as degumming and foaming of the film products.

Most of the film-covered products are offset printing products. Offset printing uses water-oil repellency to complete the printing. After the printing, the moisture content of the paper increases, and it is prone to elongation and deformation. During the film coating process, the paper loses moisture due to heat and pressure, resulting in shrinkage, curling after coating, and the like.

2, the ink on the film

The performance of the ink, the thickness of the printed ink layer, the printed image area, and the printed graphic density have a great influence on the adhesive strength of the film, and should cause a great deal of attention. When the printed ink layer is thick, the graphic area is large, and the graphic density is high, the ink changes the porous surface characteristics of the paper, and the use of a chemical liquid during printing also hinders the adhesion and affects the penetration and diffusion of the adhesive. Therefore, it is difficult to combine plastic film and paper prints, and delamination and foaming occur.

Coated products in the ink should not be excessive plus dry oil and sprayed too thick to the print, too thick spray will be due to plastic film pressure is not true and lead to coated products appear fine fog-like spots. It is even more important to note that white ink cannot be used as a diluting agent in a film-covered printing product, and white ink has significant silt and particles that can cause incompatibility with the binder.

The printed film should be allowed to be stored for a period of time after printing to facilitate the volatilization of paper moisture. Otherwise, the water will evaporate after the film is coated, and the shrinkage of the paper fibers will cause the product to wrinkle after being coated.

3, the impact of adhesive coating on the film

Coating process is to apply adhesive to plastic film (BOPP), the amount of coating depends on the thickness of the paper and ink layer of the print, the paper has a high degree of smoothness (such as coated paper) coating should be less . The coating amount of the adhesive is generally about 5 to 10 grams per square meter, and the thickness of the adhesive layer is about 8 to 10 microns.

The amount of coating is also related to the speed of the coater and the parallel gap between the coating roll and the doctor roll. The coater has a high rotation speed and the coating amount is large. During the operation, the speed of the host and the speed of the coater are The ratio should be 1:1·5, and the adjustment range of the parallel gap between the coating roller and the squeegee roller should be controlled between 5 and 6 μm.

If the coating amount of the coating agent is too small, the thickness of the coating layer is too thin, which may affect the adhesive strength. Since the adhesive strength is insufficient, the degree of the coating film is not firm, and the plastic film can easily separate and fall off from the paper. If the coating amount is too large and the coating layer is too thick, the effect will be more severe, which will cause the solvent in the adhesive to evaporate slowly, and the phenomenon of foaming and wrinkling after the product is placed.

4, the impact of environmental humidity on the film coating site

In a relatively high relative humidity environment, the equilibrium moisture content of the printed film will change, absorbing a large amount of water from the air and releasing it in the filming process will form a non-adhesive phenomenon at the adhesive interface. In addition, the edge of the paper print will also form a wavy shape, which cannot be well adhered to the plastic film when laminated. Therefore, the humidity of the site environment should be kept relatively stable, and the relative humidity of the site environment should be strictly controlled between 45 and 50%.

5, the impact of printing process on the film

Paper prints use a variety of printing processes during the printing process. In general, there are flexo printing, gravure printing, and screen printing, and their ink layers are thicker. Offset printing products have the thinnest ink layer. Therefore, offset printing products are more suitable for laminating operations than other types of printing products. In prints, prints of gold and silver inks are often involved. Gold, silver, and ink prints are not suitable for the lamination process. Because the distribution of metal powder in the binder is very poor in uniformity and fixation, it can be easily separated during the drying process of the ink layer; if the coating process is adopted, the separated metal powder will be in the ink layer and the adhesive. A barrier is formed to affect the effective integration of the interface; and after such products are stored for a period of time, there will be blistering and wrinkling, etc., and careful consideration and attention must be paid to the product process design.