At present, the domestic cigarette pack printing process is more and more complex, especially some high-grade cigarette packs often use combination printing and processing techniques and anti-counterfeiting technology. Soft packs generally use 80g/m2 or 90g/m2 single-sided coated paper; cartons and hard packs often use 220g/m2 or 230g/m2 white cardboard and transfer laser paper, and some low-grade cigarette packs also use 250g/m2 Gray board white paper. In the printing process, the paper is affected by factors such as printing pressure, ambient temperature and humidity, etc. In particular, the comprehensive use of various printing methods, paper deformation is inevitable.
After the paper is deformed, it will affect the accuracy of registration between the front and back processes. It will directly produce quality defects and increase the reject rate. The finished product cigarette packs will be deformed due to changes in the ambient temperature and humidity, and will directly affect the packaging effect when they are packaged on the machine. Not only can not meet the flatness requirements of the package, but also when the machine package cigarette smoke will appear "card machine" phenomenon, resulting in reduced production efficiency of cigarette packaging machines, more serious and even hinder the normal adsorption of cigarette packaging machine suction nozzle , causing the packaging machine to not work at all. Therefore, the loss of the paper to the company is huge whether it occurs in the process of cigarette pack production or when it is packaged on the machine.
Factors affecting curl deformation
Paper is a very hydrophilic material. The change of paper moisture content is the main factor that causes paper deformation, and the change of temperature and humidity in the environment is the main factor affecting the paper's moisture content. Paper has a strong ability to absorb moisture in moist air, and it also easily loses moisture in dry air. After the paper absorbs water, it expands, and the area and dimensions in each direction increase. However, the mechanical strength decreases; the paper shrinks after water loss, the area and dimensions in each direction shrink, and the paper is stiff and brittle. Therefore, it is necessary to control the moisture content of the paper. One of the most direct and effective methods is to control the temperature and humidity of the paper environment (including storage warehouses, production plants, etc.).
1. Effect of relative humidity on moisture content of paper
Tests have shown that, at different relative humidity, the change in equilibrium water content of double-coated coated paper is also very different. When the relative humidity of the air is lower than 80%, the relative humidity of the air changes by 10% each time. 1%; when the relative humidity of the air exceeds 80%, the equilibrium moisture content of the paper changes greatly, generally about 1.5% to 20%. That is to say, the change in water content of paper caused by high humidity is far greater than the change in water content caused by medium humidity, that is, under conditions of high humidity, slight changes in humidity can cause large deformation of the paper.
Different papers do not have the same moisture content at the same relative humidity, so the amount of deformation of various papers at the same relative humidity is different (see Table 1).
2. The relationship between environmental temperature and humidity and paper deformation
The paper left from the paper mill is sent to the printing plant after long-distance transportation and stored in the warehouse. Due to the difference in temperature and humidity between regions, the paper after entering the warehouse may suffer from moisture absorption or moisture release. If the moisture content of the paper before storage is low, and the temperature in the paper store is low and the relative humidity is high, the paper will absorb hygroscopicity and cause the paper crepe to form a “foilâ€; if the paper has moisture content before entering the warehouse Higher, and the temperature in the paper store is very high, the relative humidity is very low, the paper will cause moisture release phenomenon, as a result of the paper around the rapid dehydration and tension, the middle of the paper remains the same size or only a little As a result of the changes, the "paper-edge" phenomenon has been formed in paper baskets. In addition, if the moisture content on both sides of the paper is not uniform, the dimensions of the two sides will be different from each other, and the paper will curl.
3. Hysteresis effect of paper moisture content
Figure 1 shows the relationship between moisture content and relative humidity at a certain temperature (t=20°C). The curve A in the figure indicates the moisture absorption process, the curve B indicates the moisture release process, and the curves A and B do not coincide, but constitute a closed loop. This shows that if there are two identical sheets, the balance is reached at the same relative humidity, but the original moisture content of the sheet A is relatively high (the relative humidity of the surrounding environment is also high), and the balance is reached through the dehumidification process; The original water content of B is relatively low (the relative humidity of the surrounding environment is also relatively low). To achieve a balance through the moisture absorption process, the water content of paper A is always higher than the water content of paper B after reaching equilibrium. This phenomenon is the hysteresis effect of the moisture content of the paper.
Because of the hysteresis effect of the moisture content of the paper, when the paper is relatively stable under a certain relative humidity, the moisture content of the paper is higher than that of the original when the moisture is absorbed and then released again before returning to the original relative humidity; When the humidity is released and the humidity is returned to the original relative humidity, the water content of the paper is lower than the original value. In other words, once the paper absorbs moisture, the water content will increase even if it returns to the original relative humidity; once the paper is released, the moisture content will decrease even if it returns to the original relative humidity.
Curl deformation solution
Tobacco packaging companies are faced with the problem of curling and deformation of paper or cigarette packets that affect production and packaging. The conventional approach is to improve the temperature and humidity of production and storage environments. Some qualified companies will install temperature and humidity control equipment in their workshops. The constant temperature and humidity workshop keeps the production site in line with the production temperature and humidity of the cigarette manufacturing enterprise. In addition, some auxiliary measures can be used to change the physical and mechanical properties of the paper and improve the paper's resistance to bending and moisture absorption.
1. Control of warehouse management
In order to ensure the stable quality of the paper, the warehouse where the paper is stored is required to be constant temperature, humidity, and ventilation. The reference temperature is 20°C to 25°C and the reference relative humidity is (60±5)%. At the same time, standardize warehouse management, follow the principles of first-in, first-out, and out-of-warehouse when producing and using paper. On the one hand, paper stored in a warehouse for too long will change color, and the color and gloss of prints will not meet quality requirements; on the other hand, After the paper is transported from the paper mill to the printing plant, it needs a certain preservation time to adapt to the temperature and humidity conditions of the printing plant. The storage time is too short, and the paper performance is also unstable. Therefore, following the principle of first-in, first-out can not only ensure the proper inventory time of paper, but also help ensure the stability of paper processing performance.
2. Process control in the production process
During the entire printing production process, the paper is taken out of the warehouse, cut, printed and printed and processed, and transferred from one workshop to another. It is necessary to consider that there may be a change in temperature and humidity due to the space change, resulting in paper Deformation. Can be improved from the following aspects.
First, from the point of view of process protection and environment, the semi-finished products between processes must be wrapped with a shrink film to protect them.
Second, scientifically arrange production plans, shorten the production cycle of orders, reduce the time for papers to be exposed to the poor environment, and reduce the number of times that the paper repeatedly absorbs moisture and releases moisture.
Third, by adjusting the process parameters, improve the paper's anti-bending performance; try to avoid the drying section temperature is too high, to maintain the stability of the paper moisture content.
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To this end, we have formulated the following options.
(1) Curl control of soft pack products
When printing soft package products, single-sided coated paper of 80 g/m2 or 90 g/m2 or 95 g/m2 of transfer paper is generally used. The paper is curled and deformed mainly in the gravure printing process, and the offset printing process is natural drying of the ink. The deformation of the paper is much smaller. The following are several ways to control the curl deformation of gravure soft packs.
1 back coating. In general, the last color group of the gravure printing machine is used to print the back coating liquid on the back of the paper. The key is to select a suitable back coating liquid, and the appropriate coating amount is also very important, and the general control is about 1.0 g/m2. However, using this method does not completely solve the problem because, under high humidity conditions, slight changes in humidity can cause large deformation of the paper. If the amount of coating is not controlled or the formula of the back coating solution is not stable, the moisture content on the front and back sides of the paper will be uneven, and the dimensions of the paper on both sides will be different. As a result, the curl of the paper may be more serious.
In particular, it is necessary to remind that the back coating solution needs to be specially modulated and no wax-like slip agent can be added. Otherwise, the cigarette package may not be firmly bonded. Some cigarette printing companies use ordinary water-based varnish diluted with water as the back. Liquid coating, it is prone to such quality problems. If the dryness of the back coating solution is not good, alcohol additives such as alcohol may be appropriately added.
2 coating anti-curling varnish. The choice of varnish type is important, and the thickness of the varnish coating must be moderate. Some of the varnish (mainly oily varnish) have good anti-curling effect, but the residual odor may be large and need to be carefully selected.
3 Adjust the temperature of each oven of the gravure printing machine to reduce the decrease of moisture content of the paper caused by high temperature drying. After testing, we found that this method works best, but it does not completely eliminate the paper deformation. After all, the temperature reduction limit is limited, otherwise the temperature will be too low to cause ink drying. The problem that easily arises in the initial adjustment is that the individual printing plates are not overprinted due to the decrease in the oven temperature, and at the same time, the indicators such as the drying speed and wear resistance of the inks need to be monitored, and the ink formulation must be adjusted to meet the quality requirements.
4 Replace the paper. If the above three measures cannot achieve the desired results, it is recommended to replace the paper for testing. Our company's "big harvest", "Bamboo plum", "Doubao" and other soft products, previously used a domestic paper company 90g/m2 single-sided coated paper, because the workshop is more decentralized, the ambient temperature and humidity Difficult-to-stable control Within the previously mentioned reference range, there is always a problem with curling in soft pack products. After adjusting for the above three measures, the degree of curling has been reduced, but it has not yet reached our expected effect. Later it was replaced with Taiwan's Yongfengyu 90g/m2 single-sided coated paper, with the previous three measures, basically solved this problem.
After testing, we found that the Yongfengyu 90g/m2 single-sided coated paper has more long fibers in the pulp, high pulp content, relatively small amount of filler, and relatively stable performance; however, the surface smoothness of this paper is not as good as the paper used before the replacement. Gravure printing on a large area of ​​the ground is relatively poor, need to adjust the parameters in the gravure plate, while adjusting the gravure ink to improve the leveling problem.
(2) Curl control of hard pack products
The curl of hard package products mainly occurs on composite gold paperboard, transfer gold paperboard, and transfer laser paper. White paperboard is much better than coated paper due to its thickness and stiffness. Whether offset or gravure printing is used, the paper is warped and curled. The situation is relatively less.
The structure of the composite paper we use is generally: through the wet composite method, a 12-μm, 16-μm or 18-μm thick aluminized film (or aluminized laser film) and 220-230 g/m 2 white card bottom paper are bonded together. Aluminum layer as the printing surface. Due to the layer of PET or BOPP film on the surface of the composite paper, its air permeability is limited. The moisture absorption and moisture release of the paper are mainly performed on the back of the composite paper. In addition, most of the wet compound uses water-based adhesives, which also increases the moisture content of the paper. For the transfer of aluminum spray paper, the surface is less of this layer of PET or BOPP film.
At present, most of cigarette paper printing composite paper and transfer paper use UV offset printing or gravure printing. If it is UV offset printing, the UV curing system of non-cold light source will emit a large amount of heat at the same time of curing, so the UV lamp will emit out of the printing press. The paper temperature is generally high, sometimes up to 50 °C. If the product still needs to use screen printing to print certain special inks (such as ice ink), it needs a relatively large curing energy, the drying tunnel temperature is relatively high, and the paper deformation is relatively large.
The curl control methods of hard package products mainly include the following.
1 Back-coating when producing base paper can control the moisture absorption and release rate of the fibers on the back side of the paper. The main component of the back coating solution is an aqueous resin solution, and the coating amount is generally 1.0 to 1.5 g/m2. For the selection of the back coating solution, reference may be made to the related introduction in the curl control of the front soft package product.
2 For double-coated gold cardboard, there are two issues to be noted in the production of base paper. First, the thickness of the film, generally using 16μm or 18μm film, try not to use 12μm film, so that although the cost will be higher, but when Change