Coating process in cardboard packaging (below)

The coating type water-based coating is applied on-line on a printing press. Offset presses, gravure presses or flexo presses can all be equipped with coating units; their type depends on the type of press. Gravure and flexo press coating technologies are the same as ink coating technologies. Coating devices on offset presses generally fall into two categories: conventional roll coaters and anilox roll coaters. Common roll coaters come in a variety of different configurations. What's common is that a roller pulls paint from the paint tank. Then another roller measures the amount of coating. The coating is then transferred to a blanket or plate cylinder and finally coated on the substrate. The amount of coating is controlled by the printing press (for example, the relative speed between rolls and the pressure between rolls). The viscosity of the coating has a great influence on the result of the job. The greater the viscosity, the more often the coating is transferred to the substrate, which further increases the amount of coating. In addition, the speed of the printing press has a certain influence on the coating amount. The faster the speed, the greater the coating amount.
The anilox roller has a few small cavities above it, turning the paint to the plate. The size of the cavity directly affects the size of the coating amount. The void coating capacity is measured in terms of cubic centimeters per square meter of area (cm3/m2), or in hundred cubic microns (BCM) per square inch of area. About 19-25% of the coating contained in the cavity on the surface of the anilox roll is transferred to the substrate.
The coating amount of a common roll coater is affected by the printing press settings, the job speed, and the viscosity of the coating. The amount of coating on the anilox coater is largely influenced by the volume of the voids. The operator should use an anilox roll with an appropriate void volume to ensure that the desired coating weight is achieved. Using formula 1, the dr coating weight can be estimated.
1 type: CW=TE/100·CV·P·S/100
In the formula, CW represents the coating amount (grams per square meter g/m2). Note: pounds/1000 square feet = 4.8823 g/m2);
TE stands for transfer rate (in percent); CV stands for anilox roll cavity volume (cm3/m2, note: 1 BCM = 1.55 cm3/m2);
P represents the specific gravity of the coating (g/cm3), which is usually about 1.0-1.05 g/cm3. P value of 1 has little effect on accuracy; S represents the percentage of paint solids.
Metric units make it easy to calculate the results. It is also possible to use imperial units, but it is more troublesome to perform unit conversion. If necessary, convert BCM to cm3/m2 and g/m2 to 16/1000 sg ft (pounds per thousand square feet).
Formula 2: Dry coating weight = 0.25 x 22 cm3/m2 x 1 g/cm3 x 0.40 = 2.2 g/m2 (0.451b/1,000 sg ft)
Anilox roller void volume controls the amount of wet coating, so the higher the solid content of the coating that remains stable, the greater the amount of dry coating.
The same coating amount analysis is also suitable for calculating the gravure coating amount. In this case, the transfer rate of the paint is twice that of flexo, which is about 40-50%. Although the transfer rate is also affected to some extent by the geometry of the cavity and the viscosity of the coating, the volume of the cavity is the most influential.
The importance of the viscosity of the paint The paint must be applied to the selected press at the appropriate viscosity. If the viscosity is too high, the coating on the applicator roll will be too much, and the paint will likely splash on the press. This is especially true at high speeds. In addition, if too much paint is transferred to the substrate, it may cause poor drying and sticking problems. In order to reduce the viscosity of the paint, operators sometimes add water to the paint. If the viscosity of the paint is too low, the amount of paint transferred to the substrate will be too low to meet the operational requirements. The relative content of coating solids decreases, and the coating amount will decrease. Therefore, in order to reduce the viscosity of the paint, it is necessary to add water to the paint, which can reduce the percentage content of the paint solids. When the paint viscosity is adjusted improperly, there may be some problems with the coating amount and the surface characteristics.
All of the paint on the press will lose some water due to the evaporation of water, resulting in an increase in the percentage of cured paint content; the viscosity of the paint will also increase. The situation on the roll coater is more prominent and the thin layer of paint is constantly in contact with the air. According to operating personnel, they often find some deposits on the bottom of the paint tank. This is because the evaporation process for more than 8 hours increases the viscosity of the coating, leaving some soft deposits. In addition, the temperature also affects the viscosity of the paint. The higher the temperature, the greater the viscosity of the paint.
Effect of Paint Viscosity on Work Results Coating viscosity can affect the paint transfer status and effectiveness on a roll coater. The viscosity of the paint is low and the amount of coating is also reduced. If the viscosity of the coating is too high, problems such as poor drying and sticking may occur. Improper control of the dilution process can affect the percentage of solids in the coating and the amount of coating, resulting in various changes in the surface characteristics of the package:
● When processing the same batch of job, the viscosity of the coating will increase due to moisture evaporation, which will affect the quality of the job. Adding water to the paint can make a big difference in the amount of coating.
● The viscosity of the paint may be too high when each team is handing over jobs or handling different jobs. Each group of operators adds water to reduce the viscosity of the paint. Because each person's viscosity standards are not the same, or the degree of control at the time of adding water is not in place, it is possible that the percentage of solids content of the coating and the final coating will also be different.
When working in different seasons, the paint viscosity changes with temperature. Some operators change the percentage of solids content of the coating to maintain the desired coating viscosity.
● The presses are different from each other. Due to the different design structures of the coater, the viscosity of the paint required by each press is also inconsistent. The amount of dilution for each paint is different, and the amount of coating required is also different from each other. This means that the standard percentage of paint solids per printer is not the same. Therefore, even if the operators work, the amount of wet coating is the same, and the dry coating amount of each printing press is not the same. In sum, if the coating viscosity changes, the quality of the work, appearance and performance of the packaging will be subject to different degrees. influences. (USA) Paul E. Glo Johnson