Postpress calendering process

In modern printing, the surface finishing of the printed matter is getting more and more attention, and the finishing and calendering process is the highlight of surface finishing.
People often refer to glazing and calendering processes as glazing processes, but in fact, glazing and calendering are completely different processes, whether from the processing technology or processing equipment or principles. The calendered surface of the printed matter can significantly improve the gloss of the printed surface that has been coated.
The calendering is to change the surface state of the glazing coating after drying by the action of the temperature and pressure of the calender, making it an ideal mirror.
The process factors that affect calendering quality. In calendering, the main factors influencing the calender quality of the printed matter are calendering temperature, pressure and machine speed.
During the calendering process, the appropriate temperatures should be provided for each stage of hot pressing, glazing, and cooling in order to facilitate the secondary wetting, adhesion and penetration of the main agent molecules in the coating on the surface of the printed matter, and to enhance the contact effect between the two. Under certain temperature conditions, the plasticity of the coating film layer can be improved, and the smoothness of the calendered surface of the printed matter can be greatly improved under pressure. When the temperature is too high, the adhesion strength of the coating layer decreases, the deformation value increases, and the water content of the printed matter decreases rapidly. This is disadvantageous for the coating and stripping process. On the contrary, the hot pressing temperature is too low, and the coating layer is not completely plasticized. The secondary wetting, adhesion and penetration of the printed matter is insufficient, and the adhesion of the coating layer is poor, so the calendering effect is poor, and it is not easy to form an ideal film layer with high smoothness after calendering.
In the coated and dried film layer, the arrangement of the coating molecules is relatively loose, with many tiny holes in between. When calendering, under certain temperature and pressure, the intermolecular movement is intensified, showing the volume change of the film. The percent change in this volume is called the compression ratio, which is the ratio of the reduced volume of the paint layer upon pressurization to the original volume prior to pressurization. The large compression rate is favorable for the formation of a smooth film layer on the coating surface; conversely, the coating layer is compressed to a small amount, and a smooth film layer is not easily formed on the surface. However, if the pressure is too high, the printability and plasticity of the printed matter will be reduced, resulting in breakage.
The surface smoothness and adhesive strength of the coating film generally increase with the increase of the curing time, but the rate of increase is getting smaller and smaller, and does not increase after reaching a certain value. The length of the curing time depends on the calendering speed. When the glazing coating is in contact with the glazing tape, its molecular mobility gradually decreases as the coating temperature decreases. For example, if the curing time is short and the weakening rate is fast, the coating molecules can not fully function with the ink layer on the surface of the print. The adhesion of the paint layer to the ink layer is poor, and the surface smoothness of the film after drying and cooling is low.
In calendering, it is also necessary to reasonably determine the calendering temperature, pressure, and machine speed based on a comprehensive consideration of the type of glazing coating and the glazing suitability of the printed matter.
Calender. The calender is an accessory for coating coaters. After the paint layer coated on the surface of the print is dried and then calendered by a calender, the smoothness and gloss of the gloss coat can be greatly improved.
The calender is mainly composed of a print conveying mechanism, a mechanical transmission, and an electrical control system. The basic structure is shown in the figure.

1- Printed material conveying table 2 - Manual high pressure oil pump 3 A hot press roller 4 A pressure roller 5 A speed control drive motor 6 A calendered steel strip 7 A cooling box 8 An observation door 9 A cooling water tank 10 A ventilation circulation system 11 A transfer roller 12 and a print collection station calender generally operate in a continuous roll type. The printed matter is conveyed from the transfer table to the calender belt between the hot roller and the pressure roller. Under the effect of temperature and pressure, the coating is attached to the surface of the calender belt and is calendered. The calendered coating layer gradually cools to form a shiny surface layer. The calender belt is a specially treated stainless steel endless steel belt. The heat roller is internally equipped with a plurality of sets of far-infrared heating sources to provide the required heat for calendering. The pressure of the pressure roller adopts the electrohydraulic pressure regulating system, which can accurately meet the pressure requirements in the pressure. The calendering speed can be controlled by a speed-regulating drive motor or a slip motor.
Calendering is the later process of printing and polishing. The quality of finished products is directly related to coating and printing processes. It is not correct to analyze the quality of products after calendering and to put aside polishing and printing.
Here we will discuss some of the problems encountered in calendering and glazing, hoping to play a role in attracting suspicion.
First, the film surface appears streak or wrinkle may be due to: the viscosity of the coating material is high, the paint is too late, and prone to streaks. According to the printability of different prints, choose a good leveling, wettability good glazing coating; or add an appropriate amount of thinner to reduce the viscosity of the paint. If the coating amount is too large, the coating amount can be reduced by adjusting; sometimes, the coating property of the coating coating on the surface of the printed matter is not good, the smoothness of the dried coating is affected, or the leveling property of the coating is poor, and the processing conditions are Unlike the suitability of the coating, other types of coating or other process conditions may be selected to match the properties of the coating.
Second, the reasons for the adhesion of the prints are: solvent in the paint coating is not good volatility, poor drying performance of the paint, you can consider changing to a high volatility solvent or replace the coating type; may also be too thick coating film The internal solvent of the coating is not completely volatilized, and the residual amount is high, the coating amount can be reduced, the thickness of the film layer can be reduced, or the working temperature is low in the coating or calendering, and the drying time is short, so that the coating is poorly dried. Improve the working temperature in coating and calendering, reduce the speed, and thoroughly dry the coating.
Third, the film coating film gloss difference If it is the quality of the coating coating, you should consider changing the process and economic requirements to use better quality coating coating; if the coating is too thin, insufficient coating or coating concentration Small, high roughness, strong absorption of the printed surface is not easy to fill up, you should increase the amount of coating, improve the coating concentration, or a layer of light before the end of processing primer, and then coating May also be coating coating drying and calendering when the temperature is low, calendering pressure is small, the operation should adjust the process parameters, increase the temperature, increase the pressure; also estimated to be the device itself, such as pressure The optical steel belt wears, and the smoothness of the gloss decreases, which is to repair and improve the surface condition of the calendered steel strip.
Fourth, the blank part of the printed product is light-colored after calendering, and the reason for the discoloration of the light-colored part may be: the coating solvent on the coating has a certain degree of dissolution of the ink layer, which will change the type of coating or change the solvent composition; if the ink is poorly dried, the ink The layer is not good solvent resistance, it is necessary to improve the ink drying conditions, such as the ink after drying and coating, reducing the coating solvent in the coating solvent dosage; coating layer is not completely dry, residual solvent in the film layer High, it is necessary to improve the coating drying temperature or reduce the speed, extend the drying time, reduce the coating residual solvent.
5. The reasons for uneven coating, air bubbles, and pitting are: the surface tension value of glazing coating is large, and the wetting effect on the ink layer on the surface of the printed matter is not good, the surface tension value should be reduced, and the coating should be improved on the ink layer. Wet performance; poor solvent volatilization in coating, high residual solvent content in coating, which can be changed to solvent with high volatilization rate or the coating is thoroughly dried and then calendered; or it may be too fast in coating application. Drying temperature is low, so that the coating is not completely dry, solvent evaporation is not complete, you should adjust the glazing coating process conditions; printed surface of the ink layer will also produce crystallization phenomenon, which should take effective measures, such as removing the ink crystal The surface of the oil or hair treatment, to improve the wetting of the coating on the ink layer, in order to weaken or eliminate the adverse effects of the crystallization of the ink coating on the coating.
Sixth, the membrane surface blisters cause is: In the calendering process, the pressure is too great, the temperature of the calendered steel strip is too high, causes the coating membrane layer to soften partially, this needs to reduce the pressure and the temperature of calendering appropriately; May also be The light coating and the calendering process conditions do not match, so that after the paint layer on the surface of the print is cooled, the peeling force with the light strip is poor, the process conditions need to be changed to match the glaze coating, the calender speed is reduced, and the paint layer is improved The peel force between the same band.
Seven, in the light, the bad adhesion between the printed matter and the glazing tape is: the coating is too thin, the coating amount should be increased; the viscosity of the coating is too low to increase the viscosity value of the coating; Too small, the calendering temperature is insufficient, the calendering temperature must be increased, and the calendering pressure must be increased.
Eight, after calendering of the printed matter, the surface is easily cracked due to the fact that the temperature is too high, the printed matter is dehydrated too much in the calendering, the water content is reduced, and the paper fiber becomes brittle, and the working temperature in calendering should be reduced and taken Effective measures to maintain a certain amount of moisture in the printed matter; Excessive pressure in the calendering reduces the extensibility and plasticity of the printed matter, deteriorating the toughness and reducing the pressure in the calendering; for example, poor post-processing suitability for glazing coatings , you should re-select the coating after processing better adaptability; after the selection of the processing conditions are not suitable, you can adjust the post-processing conditions to make it suitable for printing calendered match.
9. The reasons for the inconsistency of brightness on both sides of the film after calendering are: in the calendering, the pressure on both sides of the glazing band is not equal, or the wear on both sides of the glazing band is inconsistent, and the pressure on both sides of the glazing band should be adjusted to make them equal. The parallelism between the hot roller and the transfer roller makes the both sides of the glazing tape even and evenly worn. It may also be uneven on both sides of the glazing coating. It is necessary to adjust the glazing unit to check the measuring roller and coating. The degree of parallelism and gap between the cloth rolls make the coating thickness on both sides as uniform as possible.