Film coating process

(Figure I: Laminating process flow chart)

1. Process preparation. The preparatory work is sufficient to ensure the normal production of the film production and improve production efficiency and product quality. Preparations for film production should generally include inspection of pending prints, selection of plastic films, and preparation of adhesives.

(1) Inspection of pending prints. The inspection of coated films is different from the quality inspection of ordinary printed materials. Mainly for projects with large film coverage, such as whether the surface is sprayed, whether the ink is fully dried, and whether the printed matter is even. If problems are found, measures should be taken in time.

(2) Selection of plastic film. The selection of plastic films includes the selection and quality inspection of plastic films and slitting and reeling. Whether the coating material is appropriate or not is an important factor that affects product quality.

Commonly used plastic films are: polyvinyl chloride (PVC), polypropylene (BOPP) and polyester (PET) films. The BOPP film (15-20pm) is flexible, non-toxic, and has good flatness, high transparency, good lightness, and is resistant to abrasion, water, heat and chemicals. In addition, it is cheap and The ideal composite material in the membrane process.

The quality requirements for the plastic film of the film lamination process are: the thickness directly affects the transparency, refractive index, film fastness, and mechanical strength of the film. According to the film's own performance and purpose of use, the thickness of the film is 0.01 to Between 0.02mm is appropriate. To be treated by corona or other methods, the surface tension of the treated surface should reach 4 Pa ​​in order to have good wettability and adhesion performance, and the corona treatment surface should be uniform. The higher the transparency, the better so as to ensure the best clarity of the printed material.

Transparency is expressed as the percentage of transmitted light, ie, transmitted light and projected light; the transmittance of PET film is generally 88-90%, and the transmittance of other types of films is usually between 92-93%. Good light resistance, that is, it is not easy to discolor under long-term irradiation of light, and has certain mechanical strength and flexibility characteristics. The mechanical strength of the film includes tensile strength, elongation at break, modulus of elasticity, impact strength, and the number of folding times and other technologies. index. The geometric dimensions should be stable, and commonly used indicators such as moisture absorption expansion coefficient, thermal expansion coefficient, heat distortion temperature, and the like. Coated film should be in contact with solvents, adhesives, inks, etc., must have a certain degree of chemical stability. The appearance of the film surface should be smooth, without unevenness and wrinkles, and the film should be free from bubbles, shrinkage holes, pinholes, and pits, etc. The film surface should be free from contamination such as dust, impurities, and grease. Uniform thickness, small deviation in vertical and horizontal thickness; due to the limited adjustment ability of the laminating machine, it is also required to be rewinder and neat, and the two ends should be loose and uniform to ensure uniform coating. Of course, the cost is low.

The quality inspection method of plastic film, in addition to the above-mentioned several kinds of surface treatment such as the stock surface, most of them can be handled by hand or visually. The wide-barrel film roll must also be slit into narrow rolls in accordance with the required width in order to be used for film coating. After slitting, the narrow roll requires that the edges are flush, the two terminals are aligned, and the curling tension is uniform.

(3) Preparation of adhesives. There are many kinds of adhesives used in the country, including polyurethanes, rubbers, and thermoplastic polymer resins. Among them, thermoplastic polymer adhesives work best.

Solvent adhesives are widely used. Commonly used solvents include esters (such as ethyl acetate), alcohols, and benzenes (such as toluene). Solvent-based adhesives have one-component and two-component components. One-component adhesives are easy to use and lower in cost. Although the adhesive strength is slightly lower than that of two-component adhesives, other process measures can be used to make up for them and use them more commonly. Although the two-component adhesive has high adhesive strength, it has high cost and is troublesome to use.

All kinds of adhesives should meet the following requirements: light color, high transparency; no precipitate impurities; good dispersion when used, easy to flow, good drying; no toxicity or low toxicity of solvents; long lasting adhesion performance, ink, paper , plastic film has good pro-adhesion; film products are not yellowing, wrinkle, foaming, and non-delamination for a long time; and require high temperature resistance, low temperature resistance, acid and alkali resistance, and easy operation. , Cheap and so on.

One-component adhesives can be used directly. Two-component or multi-component adhesives react once they are mixed, and the adhesive strength decreases with the increase of storage time due to surface aging. Therefore, the prepared adhesive should not be stored for a long time. Should be used up. The quality of the coating film is closely related to the quality of the preparation of the adhesive, and careful attention must be paid when preparing the film.

2. Install a plastic film roll. After the selected film is cut to an appropriate width according to the web of the printed product, it is mounted on the take-up device of the laminator, and the plastic film is put on the coating mechanism. The film is required to be smooth and wrinkle-free with moderate tension. If the laminated film is to be made into a paper box, consideration must be given to leaving the interface void, otherwise the bonding is not secure.

3. Coating adhesive. First of all, the viscosity of the adhesive should be determined by factors such as the quality of the paper, the thickness of the ink layer, the temperature of the drying tunnel, the length of the drying tunnel, and the speed of rotation of the machine. When the ink layer is thick, the drying tunnel temperature is low, the drying tunnel is short, and the machine speed is fast, the viscosity of the adhesive should be appropriately increased, and vice versa. Secondly, the thickness of the coating layer should be mastered to make it uniform. The thickness of the coating should be based on the quality of the paper and the thickness of the ink layer: the coated paper with a smooth surface, the coating weight is generally 3 ~ 5g/m (thickness of about 5μm); the rough surface, offset printing paper, whiteboard The paper has a coating weight of 7 to 8 g/m (thickness about 8 μm). Of course, the thickness of the ink layer should be slightly larger, and vice versa. However, if the coating is too thick, it may cause blistering and wrinkling. If not, the coating may not be strong.

4. Drying. The purpose is to remove the solvent from the adhesive and retain the solid content of the adhesive. Drying temperature should be controlled between 40 ~ 60 °C, mainly determined by the volatility of the solvent in the plastic adhesive. The dryness of the adhesive layer is generally controlled at 90 to 95%. At this time, the adhesive force is large, and the paper-plastic composite is the most secure. If the coating is uneven or too dry, the adhesion force will be reduced, resulting in blistering and delamination of the coating.

5. Adjust the hot pressing temperature and the pressure between rollers. The hot pressing temperature is adjusted according to the thickness of the ink on the printed matter, the quality of the paper, and the climate change, and generally should be controlled at 60 to 80C. If the temperature is too high, it will exceed the acceptable range of the film, and the film will be deformed by high heat, which can easily cause the product to curl, blisters, wrinkles, etc., and the surface of the rubber roller is easy to burn and deform; the temperature is too low, the film is not strong, and is easy to remove Floor. In general, the hot press temperature of coated paper is relatively low, and the hot press temperature of offset printing paper, whiteboard paper, and ink layer thick print is high.

The pressure between rolls should be adjusted correctly depending on the paper and paper thickness. If the pressure is too large, if the paper surface is slightly uneven or the film tension is not completely uniform, creasing or streaking will occur. If the pressure is too long, the rubber roller will be deformed and the bearing of the roller will be worn out due to excessive force. . If the pressure is too high or inhomogeneous, the film will not be firm and the delamination phenomenon will occur. Generally, the surface of the film is smooth, smooth, and strong. The pressure is 19-20 mPa. The film is rough and soft, and the pressure is 22-23 mPa.

6. Speed ​​control. The faster the machine speed, the shorter the hot-pressing time, so the temperature can be adjusted higher, the pressure can be increased, and the viscosity of the adhesive should be larger; vice versa. Speed ​​is generally controlled at 6 ~ 10m/min, speed is too fast or too slow will affect the quality of the film.

7. Sample test. After the test film is pulled out proofs, in accordance with product standards, the key performance testing of the sampled products is required to achieve surface light, smooth, and no wrinkles, bubbles, delamination and so on.

8. Stereotyped segmentation. If the coated product is a whiteboard printed product, it should be immediately divided and the coated paper and offset paper should be placed with the film facing up. The paper should be rewinded and placed for 24 hours before it can be divided. This can increase the degree of adhesion and prevent The sheet curls.