Plastic film gravure points and elimination of sticky faults

With the continuous expansion of the packaging and printing market, plastic films, which are important packaging materials, have been widely used. Therefore, plastic printing has become increasingly common. Its printing quality and adaptability are increasing. As an important printing method, gravure printing is now playing an important role in the printing of plastic films. However, due to some characteristics of plastic films and the characteristics of gravure printing itself, defects such as stickiness and the like often occur during printing, which affects the printing quality. Therefore, we must be familiar with the characteristics of plastic film gravure, master the printing technology, and constantly summarize the experience and lessons learned, we can ensure and stabilize the quality of printed matter.

The main point of film gravure

Compared with other printing methods, plastic film gravure printing has its different characteristics. For example, the amount of ink transferred by gravure printing is greater than that of lithographic printing and letterpress printing. Therefore, the product has a thick ink layer and a strong three-dimensional effect. The amount of gravure printing ink is determined by the depth of the recessed part of the printing plate graphic part. Because the depth of the concave part of the graphic part of the printing is not changed, the amount of printing ink remains relatively stable. Gravure printing ink is directly immersed in the ink tank to accept the ink, or by the transfer roller to the ink on the plate cylinder, eliminating the need for many transfer roller and ink volume adjustment device. Most gravure printing is rotary printing, with high speed and strong production capacity. The wide range of gravure printing materials has a good adaptability to materials such as plastic film and composite materials that are easily deformed. Because gravure printing uses volatile drying, it is more suitable for non-absorbent plastic film printing.

In the printing of plastic films, the following points should mainly be noted.

Ink change

The drying speed of the ink is proportional to the printing speed. If the drying speed of the ink is compatible with the printing speed, the printing quality is better. Grasp the ink thickness and drying speed and the law of printing, select the solvent that is compatible with the printing product and ink, and control the speed of ink evaporation to ensure that the product meets the quality standards.

Cold, hot air changes

According to the air and the temperature in the workshop, the cold and hot air intervals and heat are mastered. This is a key factor in printing quality products. Therefore, when the winter temperatures are low, hot air should be used. When the summer temperatures are high, hot air should be reduced and cold air should be increased. In addition, cold and heat at different times of the day should be different. For example, early morning open cold cars, printing rollers, embossing rollers, etc. have not yet been transferred by heat, and the printing speed will be appropriately slowed down. After about 1 hour, the vehicle speed will be gradually increased to adapt to the degree of ink drying. Correcting the distance of the electric wind can be adjusted according to the performance of the film and the size of the printing plate. Pay attention to the direction of the wind. Cold and hot air should be aligned with the printing surface. If you find that the air outlet is positive, you must correct it to avoid blowing the plate cylinder and affect the product quality. Because the cold and hot air adjustments are not well controlled, and the impact on printing speed and quality is great, it is better to have constant temperature equipment in the workshop.

Note the change in film tension

The tension is adjusted according to the type of film and its shrinkage. Such as PE, CPP and other high rate of stretch film, which itself is easy to deformation, so the tension should be smaller, such as PET, OPP and other shrinkage of the film is small, the tension can be correspondingly larger; film thickness and inherent quality, film on both sides of the elastic inconsistencies , Flatness is not good, the tension can be appropriately increased; if the film is of good quality, the tension can be reduced when the thickness is thin. When the temperature of the drying oven and the ambient temperature increase, the tension can be reduced accordingly because the film is easily stretched. Taking into account the product deformation, sticky, etc., the winding tension should not be too large, generally the product rolled neat, does not slip prevail. The constant tension that affects the accuracy of color registration, unwinding, and rewinding should be balanced. At present, foreign countries use electromagnetic powder clutch or DC motor to directly control the unwinding and winding tension. The color registration accuracy adopts the method of photoelectric control and automatic adjustment, and some unit type gravure rotary machines are equipped with a swing roller on the open reel to smoothly discharge the tension, etc. It's better.

Stick dirty troubleshooting

Viscosity is a phenomenon commonly encountered in the gravure printing of plastic films. There are many kinds of sticky phenomenon, such as adhesion, rubbing, and sticking after printing in the printing process. The occurrence time, phenomenon, and cause Differences. Therefore, according to different phenomena observed and analyzed, targeted solutions should be made.

Adhesion


Also known as anti-sticking, the printing surface is adhered to the back side of another layer between the top and bottom two layers of the printed roll. There are many causes of adhesions. To sum up, there are mainly: ink (solvent) is drying too slowly; there is residual solvent; the winding temperature of the rolled paper after printing is too high; the winding pressure (tension) is too large during printing; after printing Stacking pressure is too high, especially in hot weather; hot air drying in printing is too high, humidity is too high for ink drying; the moisture content of rolled paper is too large at the time of winding; improper processing of pre-press film, such as electronic The treatment is too large, and the single-sided printing film is subjected to double-sided electronic processing, or the local treatment is too strong. For the above reasons, we need to adjust and control accordingly, for example: use appropriate ink, use fast-drying and high-purity solvent; control the temperature of the roll paper and the gap between the rolls to 5 degrees Celsius when rewinding; adjust and reduce The tension of the roll makes the winding and pulling synchronous, the stacking of the printed matter can not be too high or stacking heavy objects on the surface; the cooling roller water temperature is higher than the indoor dew point temperature to prevent moisture condensation; the drying device is modified to control the hot air temperature, and the general requirement is at 50 degrees Celsius. Below, if necessary, cold air blowing should be added before rewinding to lower the film temperature. The relative humidity should be controlled below 90% to completely dry the ink; control the ambient temperature of printing sites and finished product warehouses to avoid heat sources and direct sunlight. Adhesion prevention agent is used for ink; check whether the surface treatment of the film is appropriate, such as electronic processing can meet the printing requirements at 38 dyne, and single-sided printing is not used for double-sided electronic processing.

Dirty and dirty

Dirty is a phenomenon in which the printing ink is transferred to the back surface of the base material. Viscous dirty refers to the place where the ink sticks in non-printing graphics or other printed graphics. The main reasons for this phenomenon are: the ink is not dried completely when winding, the solvent is drying too slowly, the residual solvent is too much, the adhesiveness of the ink is too large, the winding tension is too large, and the base material is treated with both sides. film. According to the above reasons, solve the problem one by one. Confirm the temperature and other factors of the dryer, increase the heat and air volume of the dryer, increase the temperature of the dryer, change the ink of the quick-drying solvent, reduce the printing speed of the printing, and use the appropriate ink to reduce the adhesion of the ink; Winding tension; reduce the rewind tension to a minimum value, and add an anti-dirty agent, an anti-adhesive agent, etc.; check that the film is processed correctly before printing.

Stick back

Back sticking refers to the fact that the imprint of the plastic film has dried after it has been printed. However, the phenomenon of back sticking occurs during storage after the roll or bag is printed. In severe cases, the back sheet cannot be torn between the prints. Dyeing, causing the product to be scrapped. This phenomenon is more likely to occur in the hot season, but the high temperature is not the only reason. The back sticking is also closely related to a series of factors such as the plastic, ink, solvent volatilization speed, operating environment, air volume, lighting, and so on. Reasons.

● The solvent in the ink does not completely evaporate after printing. Plastic film printing is to make the ink dry by evaporation of solvent, and its volatilization speed is an important factor that determines the drying condition of the ink layer and affects the printing quality. The slower the volatilization of the solvent, the better the reproducibility of the plate, the more beautiful the color of the print, but easy to adhere; on the contrary, it will produce a white blister and other images. Therefore, the choice of a suitable solvent for evaporation speed is the key to the gravure printing of plastic films. In continuous color printing, the solvent must be completely evaporated after the film is in operation between the two printing stations. Otherwise, the rollers will stick when printing at the next station.

After the film is printed, the organic solvent in the ink is not thoroughly evaporated in the drying system. After the film is wound, there is still residual heat, and the solvent remaining in the ink layer brings about the condition of continuing to volatilize, and the adhesion is formed. In the hot and humid season, after printing, the film is wound up tight or under pressure. For the above reasons, in order to avoid the phenomenon of back sticking, attention should be paid to the volatilization rate of the solvent in the ink, which depends not only on the boiling point of the solvent, vapor pressure, latent heat of vapor, etc., but also on the temperature, humidity, air volume, solute, and ink of the operating environment. Thickness and other conditions. Therefore, the volatilization rate of the solvent in the ink should be adjusted at any time according to the change of conditions. At normal temperatures, if the ink layer dries too fast, a slow-drying thin film of a slow-moving plastic film should be added; otherwise, a quick-drying thin film of a fast-evaporating plastic film is added. It can also be diluted with a mixed solvent of xylene, ethanol, and isopropanol (generally, it is better to mix the three solvents at a ratio of 1:1:1 each). If the drying is too fast, some butanol may be used in place of some of the ethanol (butanol has the effect of increasing the luster of the ink). But we must strictly control the amount of adding, so as not to affect the drying of the prints. Solvents for the gravure printing of plastic films, mainly alcohols, together with benzenes. Ethanol and isopropanol are more commonly used for alcohols, and toluene and xylene are more commonly used for benzenes. Due to the requirement of nitrocellulose, esters such as ethyl acetate and butyl acetate are also added. In short, the organic solvent in the ink is sought to completely evaporate before being wound up. Slow down the speed and increase the air volume. After the film is printed, it is loosely placed in a wire woven basket so that the ink layer can continue to be ventilated and dried. After solidification, the film is cut and bagged. After the film is printed and bagged, the film is wrapped and placed in the carton to reduce the film surface. The pressure between.

● Plastic gravure ink ink is poor. Some plastic gravure inks have a low melting point of the linking material and are prone to blocking when the operating temperature is high and the relative humidity is high. The solution should first be to control the indoor temperature (usually 18 to 20 degrees Celsius), and the relative humidity should be controlled below 65%. If possible, the printing shop should install a thermostat or install an air conditioner. Second, use good quality ink to ensure good ink.

● The poor suitability of plastic film printing and packaging. This mainly refers to the use of poor quality non-wrapped resin processed plastic film, resin opening agent is insufficient. The solution is to replace the plastic film with good printability.

● Electrostatic factors cause back sticking. Since the plastic film is very prone to static electricity, the films are formed to adhere to each other. Therefore, antistatic stings should be added to the plastic.
In short, for the phenomenon of sticking in plastic film gravure printing, it is necessary to analyze from various aspects such as ink, film, equipment, and the environment, and the right medicine can achieve the desired effect.

Source: Global Industry