Main factors affecting the effect of post-press film
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The film coating effect is not only related to the film raw material, the film coating process, but more importantly, to the state of the ink layer of the adhered print. The ink layer condition of the printed matter is mainly determined by the nature of the paper, the ink properties, the thickness of the ink layer, the area of the graphic image, and the integrated density of the printed graphic. These factors affect the forming conditions of bonding mechanical bonding force, physical and chemical bonding force, etc. A change in the adhesion properties of the printed surface.
1. Printed ink layer thickness
Thick solid prints of ink layers are often difficult to bond with plastic films and will soon delaminate and foam. This is because the thick ink layer changes the porous surface characteristics of the paper, which closes the pores of the paper fibers and seriously hinders the penetration and diffusion of the adhesive. Adhesion of the adhesive to a certain extent is advantageous for film adhesion.
In addition, the difference in bonding wettability between the ink layer and the ink layer on the surface of the printed matter is different. Experiments have shown that as the thickness of the ink layer increases or the area of the image increases, the surface tension value decreases significantly. Therefore, whether it is monochrome printing or overlay printing, it is necessary to control the ink layer to a relatively small extent.
The thickness of the printing ink layer is also directly related to the printing method, and the printing method is different, and the thickness of the ink layer is also different. For example, the thickness of the ink layer of the lithographic product is about 1 to 2 μm, about 2 to 5 μm for embossing, and 10 μm for gravure printing. From the perspective of the film, the lithographic print is ideal and the ink layer is very thin.
2. Types of printing ink
The printed matter to be coated should be a fast-fixing glossy offset ink. The binder of the ink is composed of synthetic resin, dry vegetable oil, high-boiling kerosene and a small amount of colloid. The synthetic resin molecule contains a polar group, and the polar group is easy to diffuse and penetrate with the polar group in the binder molecule, and crosslinks to form a physical-chemical bonding force, thereby facilitating filming; fast fixing The photo-offset ink also has the advantage of rapidly drying the conjunctiva after printing, and is also very advantageous for the film. However, it is not advisable to use too much drier when using it. Otherwise, the surface of the ink layer will crystallize, which will affect the filming effect.
3. Use of ink thinner
The ink thinner is a kind of substance which can make the ink color lighter. The commonly used ink thinners are white ink, weiley oil and varnish oil.
The white ink is composed of white ink, binder and auxiliary materials, and is commonly used for light-colored solid printing, spot color printing and trademark printing. Inferior white ink has obvious silty particles, which are not tightly combined with the binder. After printing, the binder will quickly penetrate into the paper, and the pigment will float on the paper to form a barrier to the formation of the paper. This is why some light-colored solid prints are often covered with wood. The reason for the membrane.
Self-adhesive should be carefully selected before printing, and the self-ink which is even and delicate and has no obvious particles should be selected as a thinner.
Weili oil is a paste-like transparent body obtained by dispersing and rolling aluminum hydroxide and dry vegetable oil binder, which can be used to increase the gloss of the printed surface and has excellent printing performance. However, the aluminum hydroxide is light in weight and floats on the surface of the ink layer after printing. When the film is coated, an inconspicuous barrier layer is formed between the adhesive and the ink layer, resulting in no adhesion or foaming. It is slow to dry and has the property of suppressing ink drying, which is also difficult to adapt to the film.
Bright oil is a kind of quick-drying type thinner from the inside to the outside. It is a gel-like transparent material made of resin, flat vegetable oil and drier. It has fine texture, bright conjunctiva and good affinity. The polypropylene film can be firmly adsorbed on the surface of the ink layer. At the same time, the varnish can make the print more lustrous and the drying speed is faster, and the printing performance is good. Therefore, it is an ideal ink thinner.
4. Adding and discharging powder
In order to adapt to multi-color high-speed printing, the dust-spraying process is often used in offset printing to solve the drawbacks of back smudging.
Most of the dusting is composed of cereal starch and natural suspended matter. The anti-adhesive effect of dusting is mainly to form an irreversible mat on the surface of the ink layer to reduce adhesion. Due to the coarser particles, if the powder is excessively sprayed during the printing process, the particles float on the surface of the printed matter. When the film is coated, the adhesive is not bonded to the ink layer at every place, but is adhered to the powder, thereby causing a false The stickiness phenomenon seriously affects the quality of the film. Therefore, if the printed product needs to be processed by lamination, the amount of dust should be controlled as much as possible during printing.
Poor drying of the printed ink layer is extremely harmful to the quality of the film. Factors affecting the drying of the ink layer, in addition to the type of ink, the amount and type of drier in the printing process, and the ambient temperature and humidity between the printing and storage, the structure of the paper itself is also very important. If the coated paper and the offset paper have different structures, the drying condition of the ink layer is also different.
Whether it is coated paper or offset paper, the film is coated when the ink is not completely dried, which has an adverse effect on the quality of the film. The high-boiling solvent contained in the ink easily swells and elongates the plastic film, and the expansion and elongation of the plastic film is the most important cause of foaming and delamination of the film after lamination.