Some discussion about calendering process
(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
In modern printing, the surface finishing of printed products is paid more and more attention, and the glazing and calendering processes are the highlight of surface finishing.
People often refer to the glazing and calendering process as a glazing process, but in fact, glazing and calendering are two completely different processes in terms of processing technology, processing equipment or principle. The surface of the printed product is calendered, which can significantly improve the glossiness of the surface of the printed product that has been coated.
Calendering changes the surface state of the glazed coating after drying under the combined effect of the temperature and pressure of the calender, making it an ideal mirror surface.
Process factors affecting the quality of calendering. In calendering, the main factors affecting the calendering quality of the printed matter are the calendering temperature, pressure and machine speed.
During the calendering process, there should be a suitable temperature in each stage of hot pressing, glazing and cooling peeling to facilitate the secondary wetting, adhesion and penetration of the main agent molecules in the coating on the surface of the printed matter and enhance the contact effect between the two. Under certain temperature conditions, the plasticity of the coating film can be improved, and the smoothness of the calendered surface of the printed matter can be greatly improved under the action of pressure. If the temperature is too high, the adhesion strength of the coating layer will decrease, the deformation value will increase, and the water content of the printed matter will decrease sharply. This is detrimental to the glazing and peeling process. The secondary wetting, adhesion and penetration capabilities of the printed matter are 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 film layer after coating and drying, the arrangement of paint molecules is relatively loose, and there are many tiny holes in between. During calendering, under a certain temperature and pressure, the movement between molecules intensifies, which is manifested by the volume change of the film layer. The percentage change of this volume is called compression rate, which is the ratio of the volume reduced when the paint layer is pressed to the original volume before pressure. The high compression rate is beneficial to the formation of a smooth film on the surface of the coating; on the contrary, the amount of compression of the paint layer is small, and the surface is not easy to form a smooth film. However, when the pressure is too high, the extensibility and plasticity of the printed matter will be reduced, resulting in fracture.
The surface smoothness and adhesion strength of the glazed film layer generally increase with the increase of curing time, but the rate of increase is getting smaller and smaller, and no longer increases after reaching a certain value. The curing time depends on the calendering speed. When the glazing paint is in contact with the glazing belt, its molecular activity gradually weakens as the coating temperature decreases. If the curing time is short and the attenuation rate is fast, the coating molecules and the ink layer on the surface of the printed matter cannot fully function, the adhesion strength of the coating layer to the ink layer is poor, and the surface smoothness of the film layer after drying and cooling is low.
In calendering, the calendering temperature, pressure and machine speed should also be reasonably determined after comprehensive consideration based on the type of glazing paint and the glazing suitability of the printed matter.
Calender. The calender is the supporting equipment of the coating machine. After the paint layer coated on the surface of the printed matter is dried and then calendered by a calender, the smoothness and gloss of the glazed coating can be greatly improved.
The calender is mainly composed of printed matter conveying mechanism, mechanical transmission, electrical control system and other parts. Its basic structure is shown in the figure
1-Printed product conveying table 2-Manual high-pressure oil pump 3—Hot pressure roller 4—Pressure roller 5—Speed-adjustable drive motor 6—Bright steel belt 7—Cooling box 8—Observation door 9—Cooling water tank 10—Ventilation circulation system 11 A transfer roller 12 a printed matter collection station
The working mode of the calender is usually continuous rolling. The printed products are transported from the paper conveying table to the calender belt between the heat press roller and the pressure roller. Under the effect of temperature and pressure, the coating is attached to the surface of the calender belt and calendered. The calendered paint layer gradually cools to form a bright surface layer. The calender belt is a specially treated stainless steel ring-shaped steel belt. There are multiple sets of far-infrared heating sources inside the heat pressing roller to provide the heat required for calendering. The pressure of the pressure roller mostly adopts an electro-hydraulic pressure regulating system, which can accurately meet the pressure requirements in calendering. The calendering speed can be controlled by a speed-controlled drive motor or a slip motor.
Calendering is the post-process of printing and glazing. The quality of the finished product is directly related to the glazing and printing process. It is not correct to discard glazing and printing when analyzing the quality of the product after calendering.
Below we have some discussion on the problems encountered in calendering and glazing, hoping to play a role.
1. Streaks or wrinkles appear on the film surface
The reason may be: the viscosity value of the glazing paint is high, the paint is too late to level, and the streak is easy to appear. According to the different suitability of the printed matter, choose a glazing paint with good leveling and wetting properties; or add an appropriate amount of thinner to reduce the viscosity value of the paint. If the coating amount is too large, it can be adjusted to reduce the coating amount; sometimes the wettability of the glazing paint on the ink layer on the surface of the printed product is not good, which affects the smoothness of the drying film formation, or the leveling of the paint is poor, the process conditions It does not match the suitability of the paint. You can choose other types of glazing paint or change the process conditions to match the performance of the paint.
Second, the printed matter sticks to each other
The reason is: the volatility of the solvent in the glazing paint is not good, and the drying performance of the paint is not good. It may be considered to use a solvent with a high volatility rate or replace the type of glazing paint; The solvent is not completely volatilized, and the residual amount is high, which can reduce the coating amount and the thickness of the film layer; or the working temperature is low in glazing coating or calendering, and the drying time is short, which makes the coating drying poor, and can improve the glazing coating. And the working temperature in calendering, reduce the machine speed, and make the coating dry completely.
3. Poor gloss of film-forming film
If the quality of the glazing coating is a problem, you should consider using a better quality glazing coating according to the process and economic requirements; if the coating is too thin, the coating amount is insufficient or the coating concentration is small, the roughness is high, the absorbency If the surface of the strong printed matter is not easy to fill and fill up, you should increase the amount of coating to increase the concentration of the coating, or apply a layer of glaze primer before processing, and then apply glaze coating; it may also be glaze coating drying When the temperature during calendering is low and the calendering pressure is small, the process parameters should be adjusted during operation to increase the temperature and increase the pressure; it is also estimated that it is the cause of the equipment itself, such as the wear of the calendered steel belt and the gloss smoothness This requires repairs to improve the surface condition of the calendered steel strip.
4. The blank part of the printed product is light-colored after calendering, and the light-colored part is discolored
The reason may be: the glazing paint solvent has a certain dissolution effect on the ink layer, which needs to change the paint type or change the solvent composition; if the ink is poorly dried and the ink layer has poor solvent resistance, it is necessary to improve the ink drying situation. Then glaze coating to reduce the amount of solvent that has a dissolving effect on the ink in the glazing coating; the coating layer is not completely dried, and the residual amount of solvent in the film layer is high, then the drying temperature during glazing coating should be increased or the machine speed should be reduced Prolong the drying time and reduce the amount of residual solvent in the coating.
5. Uneven coating, bubbles, pits, etc.
The reason is: the surface tension value of the glazing paint 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 to improve the wetting property of the ink layer of the paint; If the residual amount of the solvent is high, you can use a solvent with a high volatilization rate or dry the coating thoroughly before calendering; it may also be that the speed of the coating is too fast, the drying temperature is low, the coating is not completely dried, and the solvent is volatilized Incomplete coating, the process conditions of glazing coating should be adjusted; the ink layer on the surface of the printed matter will also produce crystallization, which must take effective measures, such as removing the oil quality of the crystallization surface of the ink layer or performing deburring treatment to improve the coating Wetting effect on the ink layer to reduce or eliminate the adverse effect of crystallization of the ink layer on the glazing coating.
Six, the film surface blisters
The reason is: during the calendering process, the pressure is too high, and the temperature of the calendered steel belt is too high, which partially softens the coating film. This requires appropriate reduction of the pressure and temperature of the calendering; it may also be glazing coating and calendering process The conditions do not match, after the coating layer on the surface of the printed product is cooled, the peeling force of the coating is poor, and the process conditions need to be changed to match the coating, reduce the speed of the calender, and improve the coating layer between the coating and the coating Peel force.
7. In the calendering, the adhesion between the printed matter and the glazing belt is poor
The reason is: the coating is too thin, the coating amount should be increased; the viscosity of the coating is too low, the viscosity value of the coating should be increased; the pressure of the calendering is too small, the calendering temperature is insufficient, and the calendering temperature should be increased to increase the pressure Light pressure.
8. After printing, the surface is easy to crack
The reason is: the temperature is too high, so that the printed matter is dehydrated too much during calendering, and the water content is reduced, so the paper fiber becomes brittle. The working temperature during calendering should be reduced, and effective measures should be taken to maintain a certain moisture content in the printed product; Excessive pressure during calendering will reduce the extensibility and plasticity of the printed matter, and the toughness will be reduced, which can reduce the pressure during calendering; if the post-processing suitability of the glazing coating is poor, the post-processing suitability should be reselected. The glazing coating; the post-processing technology conditions are not suitable for selection, and the post-processing technology conditions can be adjusted to match the suitability of the printed product after calendering.
Nine, the brightness of the film on both sides is inconsistent after calendering
The reason is: during calendering, the pressure on both sides of the glazing belt is not equal, or the wear on both sides of the glazing belt is inconsistent, the pressure on both sides of the glazing belt should be adjusted to be equal, and the parallelism between the heat pressing roller and the conveying roller should be adjusted so that The two sides of the glazing belt are uniformly tensioned and evenly worn; it may also be uneven thickness on both sides of the glazing coating. This requires adjusting the glazing coating mechanism and checking the parallelism and gap between the metering roller and the coating roller to make The thickness of the coating on both sides should be as consistent as possible.
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