Some problems and solutions in the film
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First,Introduction
Filming is a comprehensive technology that involves many factors such as plastic film, adhesives, solvents, ink layer conditions on the print surface, mechanical control, and environmental conditions. In order to obtain a high quality film product, it is necessary to control the changes of the above factors in the process and coordinate the relationship between them.
Second, the processing technology
The process of laminating is: process preparation → installation of plastic film roller → coating adhesive → drying → setting process parameters (bakeway temperature and hot pressing temperature, pressure, speed) → test film → sampling test → official Laminating → rewinding or finalizing.
1. Process preparation work. Whether the preparation work is sufficient or not has a great influence on ensuring the normal operation of the film production and improving the production efficiency and product quality. Preparation for film production should generally include: inspection of the printed matter to be covered, selection of plastic film, and preparation of the adhesive.
(1) Inspection of the printed matter to be covered. The inspection of the film print is different from the quality inspection of the ordinary print. It should mainly be applied to projects with large influence on the film, such as whether the surface is sprayed, whether the ink is sufficiently dry, and whether the printed matter is flat. If problems are found, timely measures should be taken.
(2) Selection of plastic film. The selection of plastic film includes the selection and quality inspection of plastic film and slitting. Whether the film material is appropriate is an important factor in the quality of the product.
Commonly used plastic films are: polyvinyl chloride (PVC), polypropylene (BOPP) and polyester (PET) films. Among them, BOPP film (15～20μm) is flexible, non-toxic, and has good flatness, high transparency, good brightness, abrasion resistance, water resistance, heat resistance and chemical resistance. In addition, it is cheap and is covered. An ideal composite material in the membrane process.
The quality requirement of the plastic film for the film coating process is that the thickness directly affects the transmittance, refractive index, film fastness and mechanical strength of the film. According to the properties and the purpose of the film itself, the thickness of the film is 0.01 to 0.02 mm. It is appropriate between. It must be treated by corona or other methods. The surface tension of the treated surface should reach 4Pa, so as to have better wettability and adhesive properties, and the corona treated surface should be uniform. The higher the transparency, the better, to ensure that the printed matter has the best definition. Transparency is expressed as the transmittance, which is the percentage of transmitted light and projected light; the transmittance of PET film is generally 88 to 90%, and the transmittance of other films is usually between 92 and 93%. Good light resistance, that is, it is not easy to change color under long-time illumination, and has certain mechanical strength and flexibility. The mechanical strength of the film includes tensile strength, elongation at break, elastic modulus, impact strength and folding resistance. index. The geometric size should be stable, and it is usually expressed by indicators such as hygroscopic expansion coefficient, thermal expansion coefficient, and heat distortion temperature. The film should be in contact with solvents, adhesives, inks, etc., and must have certain chemical stability. The appearance of the film surface should be flat, no unevenness and wrinkles, and the film is required to have no bubbles, shrinkage holes, pinholes and pitting, etc., and the film surface is free from dust, impurities, grease and the like. Uniform thickness, small longitudinal and lateral thickness deviation; due to the limited adjustment ability of the laminating machine, it is also required to be rewound and tidy, and the ends are elastic and uniform to ensure uniform coating. Of course, the cost is low.
The quality inspection method of the plastic film can be solved by hand or visual inspection, except for the above-mentioned film surface treatment. The wide-film film roll must also be cut into narrow rolls according to the required width to be used for laminating. The narrow-cut rolls after slitting require flush edges, flush ends, and uniform curl tension.
(3) Preparation of the binder. There are many types of adhesives used in China, mainly polyurethanes, rubbers, and thermoplastic polymer resins. Among them, thermoplastic polymer adhesives have the best effect.
Solvent-based adhesives are widely used. Commonly used solvents are vinegars (such as ethyl acetate), alcohols, and benzenes (such as toluene). Solvent-based adhesives are available in one-component and two-component parts. One-component adhesives are easy to use and low in cost. Although the bonding strength is slightly lower than that of the two-component adhesives, other process measures can be taken to make up and the use is more common. Although the two-component adhesive has high bonding strength, it is costly and cumbersome to use.
All kinds of adhesives should meet the following requirements: light color, high transparency; no precipitation impurities; good dispersing performance, easy to flow, good drying; solvent non-toxic or less toxic; good adhesion performance, good for ink, paper Plastic film has good affinity; the film product is not yellowed, wrinkle-free, non-foaming and non-delamination for a long time; and the adhesive is required to have high temperature resistance, low temperature resistance, acid and alkali resistance and easy operation. , the price is cheap and so on.
One-component adhesives can be used directly. Two-component or multi-component adhesives are reacted once mixed, and the adhesive strength of the adhesive decreases with surface aging as the storage time increases. Therefore, the prepared adhesive should not be stored for a long time. Should be used up in the day. The quality of the film is closely related to the quality of the adhesive. The preparation should be careful.
2. Install a plastic film reel. After the selected film is cut to a suitable width according to the web of the print, it is mounted on the unwinding device of the laminator and the plastic film is passed to the coating mechanism. The film is required to be flat and wrinkle-free, and the tension is even and moderate. If the film print is to be made into a carton, it is necessary to consider leaving a gap in the interface, otherwise the bond is not strong.
3. Coating the 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 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 achieve uniformity. The thickness of the coating should be determined according to the quality of the paper and the thickness of the ink layer: the coated paper with a smooth surface is generally 3 to 5 g/m (about 5 μm thick); the offset paper, the ink with a large amount of ink, and the whiteboard. The paper was applied in an amount of 7 to 8 g/m (about 8 μm thick). Of course, the ink layer is thick, and the coating amount should be slightly larger, and vice versa. However, the coating is too thick, easy to foam, wrinkle, and vice versa.
4. Drying. The purpose is to remove the solvent from the binder and retain the solids content of the binder. The temperature of the drying tunnel should be between 40 and 60 ° C, which is mainly determined by the volatility of the solvent in the plastic binder. The dryness of the rubber layer is generally controlled at 90 to 95%. At this time, the adhesion is large, and the paper-plastic composite is the strongest. If the coating is uneven or too dry, the adhesion will be reduced, causing foaming and delamination of the film.
5. Adjust the hot pressing temperature and the pressure between the rolls. The hot pressing temperature is adjusted according to the thickness of the printed ink layer, the quality of the paper, the climate change, etc., and should generally be controlled at 60 to 80 °C. If the temperature is too high, it will exceed the film receiving range, and the film will be deformed by high heat, which will easily cause the product to curl, blister, wrinkle, etc., and the surface of the rubber roller is easily burned and deformed; if the temperature is too low, the film is not strong and easy to take off. Floor. The hot stamping temperature of the coated paper is low, and the hot pressing temperature of the offset paper, the whiteboard paper and the thick ink layer is high.
The pressure between the rolls should be adjusted correctly depending on the paper quality and paper thickness. If the pressure is too large, the paper surface is slightly uneven or the film tension is not completely uniform, wrinkles or streaks may occur; if the pressure is too long, the rubber roller will be deformed, and the roller bearing will be worn due to excessive force. . If the pressure is too high or uneven, the film will be weak and delaminated. Generally, the surface of the film is smooth, flat and strong, and the pressure is 19-20 mPa; the film is rough and soft, and the pressure is 22-23 mPa.
6. Machine 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. The machine speed is generally controlled at 6~10m/min, and the machine speed is too fast or too slow will affect the film quality.
7. Sample testing. After the film is tested, the sample is taken out, and the key performance test of the sampled product is carried out according to the product standard, and the surface is required to be bright and smooth, and no wrinkles, bubbles, delamination, and the like.
8. Stereotype segmentation. If the product after lamination is white paper, it should be divided immediately and the film side should be placed upwards; the thin coated paper and offset paper should be re-rolled and placed for 24 hours before being divided, so as to improve the adhesion. , can prevent the sheet of paper from curling.
Third, the impact of printing on the film
The film coating effect is related not only 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 bonding conditions such as bonding mechanical bonding force and physical and chemical bonding force, thereby causing the printed matter. A change in surface adhesion properties.
1. Print 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, when the ink layer and the ink layer area of the printed matter are different, the adhesive wettability is also 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. The type of printing ink. The printed matter to be coated should be a fast-fixing glossy offset printing ink. The bonding material 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 physicochemical bonding force, thereby facilitating filming; fast fixing Bright offset printing inks also have the advantage of fast drying of the conjunctiva after printing, which is also very beneficial to 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. The 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, weili 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 powdery particles, which is 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 difficult to cover. s reason. White ink should be carefully selected before printing, and white ink with uniform and fine particles without 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 itself 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 from resin, dry vegetable oil and drier. The texture is fine: the conjunctiva is bright and has a 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 shiny and dry faster, and the printing performance is good. Therefore, it is an ideal ink thinner.
4. Add and discharge 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 all places, but is bonded to the powder layer, thereby causing 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.
5. Drying of the ink layer on the surface of the printed matter. 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 of the printing and storage room, the structure of the paper itself is also very important. If the structure of the coated paper and the offset paper are different, 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.
Fourth, common faults and solutions
There are many factors affecting the quality of the film. In addition to objective factors such as paper, ink layer, film and adhesive, it is also affected by subjective factors such as temperature, pressure velocity and glue amount. When these factors are not handled properly, various film quality problems will arise.
1. Poor adhesion.
A. Due to improper selection of adhesive, improper setting of glue amount, and poor adhesion of the film due to incorrect metering, the adhesive grade and coating amount should be re-selected and accurately matched;
B. If the surface condition of the printed matter is not good, such as dusting, too thick ink layer, ink is not dry or not dry thoroughly, etc., the adhesive may be lightly wiped off with a dry cloth, or the adhesive may be coated. Amount, increase the pressure, and use a hot press to re-glue, or use a binder with a high solid content, or increase the thickness of the adhesive coating, or increase the temperature of the drying tunnel;
C. If the coating is insufficient due to the absorption of the ink by the printing ink and paper, consider re-setting the formulation and coating amount.
D. Check if the film corona treatment reaches 36-40 dynes.
E. The bonding at the edge is not true. This is mainly manifested in thick paper. The paper will have some curl deformation when it is wound. And the smaller the winding diameter, the more obvious. A film that has not been dried before the pressing roller is often opened at the edge of the curled paper, causing the bonding to be unsound. In order to improve the yield, the width of the lap should be minimized to ensure that the width of the bond is not within the die-cutting mouth.
2. Foaming.
A. If the printing ink layer is not dry, it should be hot pressed once and then glued, or the filming date can be postponed to make it dry thoroughly;
B. The pressure of the pressure roller is too small
There are bubbles on the surface of the entire print because the pressure is too small, and the pressure of the pressure roller can be appropriately increased.
C. If the printing ink layer is too thick, the amount of adhesive coating can be appropriately increased to increase the pressure and the composite temperature;
D. If the surface temperature of the composite roll is too high, heat dissipation measures such as air cooling and heating of the heating wire should be taken to reduce the temperature of the composite roll as soon as possible;
E. If the drying temperature of the film is too high, it will cause the surface of the adhesive to crust and cause foaming failure. At this time, the drying temperature should be appropriately lowered;
F. Foaming failure caused by wrinkles or slacks of the film, uneven film or curling, can be solved by adjusting the tension or replacing the qualified film;
G. High adhesive concentration, high viscosity or uneven coating, and a small amount is also one of the reasons. At this time, the diluent should be used to reduce the binder concentration, or to appropriately increase the coating amount and uniformity.
H. There are bubbles in the overlap of the two sheets of paper because the top and bottom of the paper are together when the film is covered. This is mainly manifested in thick paper. If the print has a large bite, the paper can be adjusted to make the two sheets have a certain amount of overlap; when the bite is small, the two sheets can be properly pulled apart, but the tail film will appear after slitting. Therefore, the distance to be opened should not be too large.
I. The storage of printed matter is generally tens of thousands of stacked. This causes the moisture absorption of the paper to not be balanced over the entire area, and the edge area is faster, which is why the ruffles are produced. Prints with ruffles will flatten under the lamination pressure, and will return to their original shape after the pressure roller is applied. This creates bubbles. The solution is to humidify or spread the print on the ruffle.
3. Uneven coating.
A. The plastic film is uneven in thickness, the composite pressure is too small, the film is slack, some of the adhesive in the glue tank is solidified, and the rubber roller is swollen or deformed, which may cause uneven coating;
B. After the fault occurs, find out the cause and take corresponding measures as soon as possible, or adjust the traction, increase the composite pressure, or replace the film, rubber roller or adhesive.
4. Wrinkle film.
The reason is generally that the film conveying roller is unbalanced, the two ends of the film are inconsistent or wavy, the rubber layer is too thick, or the electric heating roller and the rubber roller are uneven at both ends, the pressure is inconsistent, and the line speed is not equal, which can be adjusted and transmitted separately. Measures such as rolling to balance, changing the film, adjusting the amount of glue, increasing the temperature of the drying tunnel, adjusting the position of the heating roller and the rubber roller, and the process parameters.
5. The film product is warped.
A. The reason is that the printed matter is too thin, the tension is unbalanced, the film is pulled too tightly, the composite pressure is too large or the temperature is too high;
B. The corresponding treatment method is to avoid laminating the thin paper as much as possible;
C. Adjust the film tension to make it balanced; appropriately reduce the composite pressure and reduce the composite temperature.
6. The product has a snowflake point.
A. If the printed matter of the printed matter is too large, it cannot be completely dissolved by the glue, and there will be no large-area snowflake. In this case, the amount of glue should be appropriately increased or the printed matter should be cleaned before the film is covered.
B. If the amount of glue is too small, snow will appear on the entire surface of the print. The solution is to increase the amount of glue.
C. If the pressure of the pressure roller is too large, the glue at the edge of the print will be squeezed out, resulting in snow on the edge of the print; otherwise, the snow will appear on the pressure. The solution is to adjust the pressure of the pressure roller.
D. The rubberized roller has a small amount of glue in the dry rubber, which will cause the film product to appear snowflake here. The solution is to wipe the applicator roller clean.
E. The glue extruded from the edge of the print or the glue extruded from the needle hole of the film sticks to the pressing roller. After a long time, a dry apron is formed, and a small snowflake appears here at the back of the printed film. The solution is to wipe the pressure roller in time.
F. If there is too much dust in the surrounding environment or there is dry rubber in the glue and cut film fragments, the film product will have snow, so you should pay attention to the sanitation, the glue can not be used, it should be sealed back to the rubber barrel. Or take the method of filtering before sizing.
7. The product is curled after slitting.
A. If the film pulls too much, it will cause elongation deformation. After the product is cut, the external force will be removed, and the film will be restored to the original state, causing the product to curl to one side of the film. The solution is to adjust the screwing depth of the film tensioning screw to reduce the braking force. .
B. The too large pulling force will cause the film and the printed matter to be deformed at the same time, but the deformation amount is different. After the external force is removed, the product will curl toward one side of the paper, especially thin paper. The solution is to reduce the friction of the winding power wheel (friction sprocket).
C. After the film product is fully dried, the paper will be dehumidified and the finished product will be curled to one side of the paper. The solution is to control the humidity in the workshop.
D. Short drying time
8. The product turns black.
This is mainly reflected in large-area gold and hot stamping products. This is due to the chemical reaction between the chemically active elements of the glue and the gold powder. The solution is to use special gold ink or special glue for the gold products, or bronzing the products after the film is coated.