Exploring stand-alone coating techniques

Stand-alone coating is the use of a special coating machine to polish the printed surface, that is, printing and coating are performed on their own special equipment. Stand-alone coating has its own unique advantages in terms of light on the ground, and has a high degree of flexibility, which can be matched with several different types of printing presses. In addition, it can meet different coating requirements, provide a variety of special glazing effects, and has a wide range of applications. It can not only coat commercial prints such as corporate samples, calendars, and annual reports, but also can Packaged goods. Single machine polishing equipment investment is small, more suitable for professional use of post-press processing manufacturers.

Consumables and process parameters

In the glazing process, there are many factors that affect the quality of the single-machine coating, such as the light version, coating oil, printing materials, printing ink, coating temperature and so on.

1. The effect of glazing

The glazing is divided into overall glazing and partial glazing. The main function of the overall glazing is to protect the printed matter and improve the surface gloss of the printed matter. Generally, the glazing method is applied by roller coating; the partial glazing is generally emphasized on the printed matter. The graphic part of the glazing is polished, and the high-gloss picture of the glazing part is compared with the unglazed part, and wonderful artistic effects can be produced.

Since the overall glazing basically has no problem of registration, and the local glazing pattern must be accurately registered, the difficulty is high, so we talked more about the choice of the local glazing version. First of all, we should have a clear understanding of the structure of the glazing parts of local glazing products, based on the distribution of the local graphic text, the size of the text, the thickness of the lines, and the characteristics of the lacquer (most important for leveling). Material selection. If the partial fine pattern is applied, it is not suitable to use resin plate or rubber cloth as the glazing type. The number of different lines of the anilox roller should be selected according to the requirements of the product on the gloss, and the thickness of the varnish must be controlled.

2. Effect of Varnish

According to reports from everyone, the most important factor affecting the quality of glazing is the coating of varnishes. Therefore, we will start the discussion with Shanghai Shangguang Oil Co., Ltd., and put forward the matters that should be taken into consideration when selecting the coating oil for the purchase of the existing coating oil on the market.

For solvent-based varnishes and water-based varnishes, the current printing manufacturers use a relatively uniform process technology, relatively stable and mature; for UV Varnish, the problem is relatively more.

All three types of coating oils may have poor gloss, poor leveling, and poor drying. These phenomena have a lot to do with the operation, in addition to the quality of the coating. Poor gloss, should pay attention to the proportion of resin components of the light oil and the quality of plate and glazing roller of the oil machine during operation. The main reasons for poor leveling are too long drying time or too much light power, and the viscosity of the varnish is too strong. If the varnish is not dried well, the varnish will not dry completely and there will be no bottom. That is to say, the surface is dry and the inner layer is not dry. The drying time and light intensity need to be adjusted. At the same time, the viscosity of the varnish must be taken into account. Dilution. In addition, the phenomenon of crystallization of varnish oil also has a relatively large effect on the quality of coating. As the printed matter is left for a long time, the area of ​​the ink at the bottom of the printed product is too large, and the dry oil is excessively added, it will cause the lacquer hull not to go up or produce the “flower face” or “pitting” phenomenon. Generally, 5% lactic acid can be added to the coating oil and used after stirring.

And these three different coating oils also have different problems during the drying process. Solvent-based coating odors develop during drying. The oily solvent contains resin and toluene. After the heat-curing, the alcohol glazing paint and the solvent will have a chemical effect on the ink layer. The solution is to increase the intensity and speed of infrared and heat-sensitive curing. Appropriate thermal energy and control of the operating speed can promote solvent diluent volatilization, improve the drying effect, reduce the residual amount, and reduce the odor and discoloration. In the drying process, the aqueous coating oil should avoid the chemical effect of the thinner on the ink layer and the paper coating and the effect on the smoothness of the paper surface, and further avoid the pollution to the environment. UV Varnish uses the spectral range of the UV light source, the power of the light source, and the corresponding transmission width and effective curing speed to complete the drying. If there is solvent residue after curing, the oil will be lost. Therefore, it should be noticed when curing.

Currently, the following problems exist in the varnish sold on the market:

(1) The range of dilution ratio is too large, and it is difficult to grasp its best point in operation;

(2) The problem of unstable leveling is common, which is mainly manifested in the obvious oil marks on the edge of the coating.

(3) The gloss of gloss oil is not stable;

(4) The drying performance is difficult to control, and the same kind of varnish has different drying degrees in different batches;

(5) The hot stamping properties of varnish are not stable, especially UV varnish;

(6) Odor in varnish;

(7) Large viscosity (easy adhesion), the most important is that it can not pass the friction test;

(8) The environmental performance of varnish is not very good.

When selecting varnishes, note the following:

(1) Understand the type, nature and effect of coating oil, and the changes when preparing other solvents;

(2) Select suitable properties for the base material and ink surface properties of the substrate;

(3) Pay attention to the drying performance of the coating oil, and whether the drying temperature range and drying time have any influence on the substrate;

(4) to consider the post-processing adaptability of polishing materials, such as hot stamping performance, friction resistance, pressure resistance when die-cutting marks are cut, etc.;

(5) When testing a batch of varnish, check whether the varnish is sticky by heating, and whether the color of the printing pattern is changed;

(6) The transparent oil with high odor and low odor should be used.

3. Influence of printing materials

The impact of printing materials on glazing is reflected in two aspects. One is unprinted paper, which is mainly reflected in the effect of paper smoothness on the quality of glazing. The paper with high smoothness has a significant effect after polishing, while the paper with low smoothness has a poor effect after coating, because the coating oil is almost completely absorbed by the rough surface. To solve this problem, you can make a primer or light twice before you apply light. In addition, too thin paper or poor paper quality also has an effect on gloss. The paper is too thin to wrinkle easily, and the paper is too poor. Polishing oil will leak, and the gloss is not good. The second is printed paper, mainly in the following areas:

(1) If the surface of the printed matter contains talcum powder, additives, etc., it must be treated before it can be applied.

(2) When printing gold, silver ink or large-area on-site printing (such as black, dark brown, dark green, etc.), in order to avoid adhesion requires a lot of dusting, must be wiped before the stand-alone coating, clean the dusting powder before Light.

4. The effect of printing ink

The printing ink used for the products to be coated after printing must have solvent resistance and heat resistance, otherwise the graphics and texts will be discolored or wrinkled. The following principles should be adhered to when selecting printing inks: Use alcohols that are resistant to alcohols, ester-resistant solvents, acids and bases, durable colors, good gloss, and good adhesion to paper.

Human Factors

When operating the product, the operator must understand the coating requirements of the coating product, be familiar with the mechanical properties of the coating machine and strictly follow the operating procedures, and must understand the performance of the coating oil clearly. When the operation must reach the parameters of the coating oil before it can officially start production.

The correct operation method can increase the efficiency in the production process, reduce the waste of time and resources, and at the same time further guarantee the quality. The method of operation has a direct effect on the glazing effect:

(1) The temperature and time control of the polishing machine drying system does not meet the requirements, it will cause the coating oil to dry not completely, and the printed products will overlap after the paper overlaps; overdrying will make the surface of the printing product appear varnish. Bad or burst situation.

(2) When glazing, it is necessary to pay attention to the selection of plate materials. Improper selection of plates will directly affect the brightness and thickness of glazing, failing to meet product quality requirements.

(3) Diluting according to the dilution ratio of varnishes is not necessary. It has a direct influence on the drying performance, brightness, leveling, and wear resistance of varnishes.

Because the operator plays an important role in the operation process, the operator is required to:

(1) Must be rigorous, cautious, and meticulous in the operation process;

(2) The operation must be in place, and check the glossiness and color more;

(3) enhance product quality awareness;

(4) Problems that arise during the work process must be solved quickly and accurately.

In addition, the operator must take different measures according to the type, thickness and quality of the printed product paper, the depth of the ink, the degree of dryness of the surface, and the amount of dusting, in order to ensure the glazing effect. Any improper operation method will directly affect the glazing effect. For example, if the temperature is too high, the paper will be deformed, yellowed, brittle, or even vitrified. If the temperature is too low, the brightness will not be enough and it will stick. How much oil will affect the application The quality of light.

5. Influence of temperature

Glossing temperature is generally controlled at 18 ~ 20 °C, that can achieve better results. Excessive temperature on the glazing will cause waste of resources on the one hand, and on the other hand, it will also affect printing materials. However, if the temperature is too low, the varnish will be easily solidified, and the surface layer of the product after polishing will be uneven. Therefore, the varnish must be stored in a warm place; if the varnish temperature is too low, hot solvent can be added.

Unsolved issues

In addition to the problems mentioned above in the glazing process, front-line operators also encountered some problems that were difficult to solve, including on-line glazing and stand-alone glazing. They asked us to seek help from our readers. We hope that everyone will contribute ideas and solutions to existing problems, jointly promote the improvement of the quality of glazing, and produce more exquisite products.

Problem one: When the on-line laser coated card paper is glazing on line, the varnish is dry and severely sticky. At present, China's larger varnishes and agents are seeking solutions and have so far failed to resolve.

Problem 2: After a company's products are coated with light, their light film layer is slightly yellow and the gloss is not enough. After the coated UV light oil is solidified, the printed matter is unstable in color and discolored, which is very different from the original. For example: After 200g/m2 coated paper is coated with UV varnish, the color changes greatly after printing in less than a month, especially the blue and red colors are more obvious. This kind of UV glazing is based on the use of base oil followed by UV varnish, and the amount of sizing used during the crepe process is also moderate.