Editor's Note: The printing of paper packages tends to have lamination after printing. The coated package not only enhances the surface strength and gloss, but also improves the grade of the product. Therefore, the coating process is common in paper packaging, especially carton packaging. This article briefly introduced some basic elements of the film: film, adhesives, process, equipment, and influencing factors, hoping to give readers some help.
The film is coated with a plastic film adhesive, and it is pressed together with the paper by a rubber roller and a heating roller to form a paper-plastic product. Cover film can be divided into two cases, one is to print the plastic film on the graphic and then laminated with the paper, the other is to print the paper on the paper, and then overlay a layer of plastic film, this article Refers to the second case. After the paper is printed on the paper, the plastic film is laminated thereon, the material film and the adhesive are used, and the equipment is a laminating machine.
First, the plastic film for plastic film covered with plastic film are: polypropylene film, polyvinyl chloride film, polyethylene film, polyester film and polycarbonate film. The thickness of the film used for the film is generally 0.01-0.02mm, the higher the transparency, the better, the light transmittance should be more than 90%, while having light resistance, chemical resistance, geometrical stability, a certain degree of mechanical strength and other properties .
Most of the plastic films used for laminating are non-polar, low-surface-energy hard-to-adhere materials. In addition, due to weak surface layers, poor cleanliness, and other factors, it is difficult for plastic films to be well covered with the substrates to be covered. Bonding. Therefore, the plastic film is subjected to surface treatment in advance to oxidize the film to generate polar groups such as carbonyl groups, carboxyl groups, etc., and the surface of the film is cleaned, so that the free surface energy of the film surface is increased, and the adhesive is wetted against the film surface. Ability and adhesion to meet the process requirements of the film. The methods for surface treatment of plastic films include chemical oxidation, drug treatment, solvent treatment, flame treatment, and corona discharge treatment. Currently, the commonly used method is corona discharge treatment.
Second, the film adhesive adhesive generally consists of the main material and shop help materials. The main material is the main component of the drilling compound and can play an adhesive role. The materials of the main body materials include synthetic resins, synthetic rubbers, natural macromolecular materials, and non-polar compounds. Auxiliary materials are substances used in adhesives to improve the properties of the main materials or to facilitate the application. They include curing agents, plasticizers, fillers, and solvents. The adhesive used for the film is generally a solvent-based adhesive. With EVA as the main resin, it is of the hot-melt type. When the EVA resin is in the molten state, its surface tension is low, and it has better adhesion with the non-polar polyolefin, and it is transparent and has good fluidity and is easy to apply. However, the aromatics, petroleum hydrocarbons and the like are used as the solvent alone or in admixture, which is slightly toxic and has poor ink resistance. Water-based adhesives have been developed. They are non-toxic, non-pollution, and non-flammable. However, the adhesive's main material is limited to various types of polymer resins that can be dispersed in water, Membrane equipment and other requirements are high, and the stability of adhesion performance is not ideal.
Third, the laminating process The plastic film used for laminating: There are two types of precoated film coated with adhesive and coated film coated with adhesive. The process flow of the pre-coating film is: paper feeding at the feeding station—printed film—to take the finished product. That is, the process flow of the coating film is: paper feeding at the paper feeding station—film coating adhesive—printing film pressing—receiving the finished product.
Fourth, laminating equipment laminating machine is divided into two types of coating type laminating machine and pre-coated laminating machine. The coating type laminating machine includes three parts of sizing, drying and hot pressing. The gluing part is equipped with an adjusting device to adjust the amount of glue to meet the requirements of different printing parts. The rubber roller or the steel roller is used for the rubber roller. The drying section of the supply channel is generally 2-3m long. The drying crossing is equipped with infrared lamps and electric fans to dry the adhesive and remove the exhaust gas. The hot-pressing part is the key to the film coating and directly relates to the adhesive fastness of the film-coated product. The hot-pressing part is composed of a set of rollers. One of the hollow metal rollers has a built-in electric heating tube and the temperature can be automatically controlled. The pressure of this set of rollers can be adjusted. The coating type laminating machine has a wide range of application and stable and reliable processing performance. It is currently a widely used film coating equipment in China. Precoat laminating machine, no glue and dry part, small body size, low cost, flexible and convenient operation, not only for the processing of large quantities of printed films, but also for the coating of small batches and scattered prints such as automated office systems. Processing.
Fifth, the factors affecting film quality In addition to plastic film, adhesive properties, quality and other factors, the factors that affect the quality of the film, but also with the printing ink layer conditions, coating temperature, bonding pressure, adhesive The coating conditions, environmental humidity and so on.
1. The condition of the ink layer of the printed product: The condition of the ink layer of the printed product mainly refers to the nature of the paper, the ink properties, the thickness of the ink layer, the area of ​​the graphic and the density of the printed graphic, etc. Their influence on the film is mainly for the print and the film. Effect of adhesive strength. When the printed ink is thick and the area of ​​the graphic is large, these inks change the porous surface characteristics of the paper, hindering the penetration and diffusion of the adhesive, making it difficult for the printed matter and the plastic film to adhere, and causing delamination. Bubbles and other faults. In addition, white ink can not be used as a diluting agent in the ink, because there are obvious silt particles in the white ink, which is not tightly combined with the linking material and has an obstructive effect on adhesion. Excessive dry and dry oil and ink spray on the print will affect the film's fastness. Incomplete drying of the ink will also cause blistering and delamination of the product after lamination.
2. Lamination temperature: When laminated under hot pressure, the adhesive layer is in the molten state, the molecular motion is intensified, and the number of molecules participating in the bond formation is increased, so that the interface between the plastic film, the printed matter, and the adhesive layer reaches the maximum viscosity. Together force. Although the increase in lamination temperature contributes to the increase in bond strength, the control range must be reasonable, with empirical values ​​between 70-90°C. If the temperature is too high, the film will shrink and deform, causing wrinkles, curls, and local blisters.
3. Lamination pressure: Larger lamination pressure helps to improve the adhesion of the film and the print. However, if the pressure is too large, the film is easily deformed and wrinkles are generated, and the rubber roller is deformed by pressure. If the pressure is too low, the adhesion is not strong. In actual production, the pressure is to be adjusted according to the process conditions. For example, the pressure on the paper with rough surface, loose texture, and high permeability is higher. The empirical value of lamination pressure is generally in the range of 15-25 MPa.
4, the adhesive coating conditions: the coating state of the adhesive refers to the uniformity of the coating and the amount of coating, the adhesive in the printed matter and the plastic film surface requires thin and uniform coating. The coating should be as thin as possible while guaranteeing the firmness of the coating. The adhesive coating had an empirical value of 3.8 g/m 2 and the adhesive layer had a thickness of 5-8 μm.
5. Ambient humidity: In the environment of relatively high relative humidity, the balance moisture value of the printed matter will change, and a large amount of water absorbed from the air will be released during the lamination process, forming a non-adhesive phenomenon at the adhesive interface. Moreover, the edge of the printed product will form a wavy shape, and when laminated, it cannot bond well with the cover film. Therefore, the relative humidity of the environment can be kept stable.