Is the cigarette pack prone to curling and deformation? Teach you how to easily crack it!
After summarizing the quality and yield of different brand cigarette packaging products in the previous year, as well as tracking the adaptability of cigarette packaging on the machine, our company organized relevant technical personnel to establish a special team to conduct a special study on enhancing the weather resistance of cigarette packaging products. The team aims to solve the problem of paper curling and deformation caused by time, temperature, humidity, and other factors after the completion of gravure printing, in order to better adapt to the adaptability of cigarette packaging on the machine and reduce the high rate of paper cutting edge waste caused by paper curling and deformation.
Analysis of the reasons for weak weather resistance
Poor weather resistance refers to the characteristic of cigarette packaging products being easily affected by environmental temperature and humidity after printing due to the special nature of the paper, resulting in paper curling and deformation, as well as the appearance of paper cutting waste.
Cigarette pack products generally use a fixed weight of 90g/m2 copperplate paper. Compared with other 210g/m2, 220g/m2, and 225g/m2 cigarette white cardboard, the weight is smaller, the paper is relatively soft, and it is easily affected by environmental temperature and humidity to produce curling deformation. Especially when printing on gravure machines with different color sequences, after ink coating and drying for each color sequence (the specific ink coating and drying process for gravure printing on paper is shown in Figure 1), the difference in dehydration rate between the front and back sides of the paper causes curling deformation. With the changes of time and environment, the deformation gradually becomes more severe, and the product's weather resistance is not strong, so it is prone to high scrap rate of paper cutting edges and poor adaptability of cigarette packs on the machine.
Figure 1: The specific process of ink coating and drying during gravure printing on paper with connected lines
During the gravure printing process of cigarette packaging products, the main water absorption process of the paper is the ink application process of semi matte oil. Among them, in the production and ink blending process of semi matte oil, water gloss oil is used, with a semi matte oil ratio of 1:2 and a water to alcohol ratio of 3:7. This process contains a large amount of water, and the viscosity of the color sequence after ink blending is 18-21mPa · s. Other color sequences use the original ink and the solvent is n-propyl ester. Therefore, the main water absorption process occurs during the ink application process of semi matte oil.
On the other hand, there are also differences in the drying temperature during the process of line intaglio printing for each color sequence of paper. The drying temperature for other color sequences is between 55-80 ℃, while the production drying temperature for the second to last color semi matte oil is 100-125 ℃. The drying temperature is higher, and after the paper is ink coated and dried, the weather resistance of the paper will be worse than that of the original paper before printing. After completing line intaglio printing, the cigarette pack products are mainly printed on the front side of the paper with ink, and the moisture is not easily lost after drying. The back side is not ink coated, which is affected by the paper fibers and drying temperature. This will make the paper more prone to poor weather resistance after high-temperature drying, resulting in cigarette pack machine impact and scrap rate.
The current batch production situation of cigarette pack products with cut paper edges caused by deformation due to weak weather resistance of the investigation company is shown in Table 1.
Table 1 Comparison of Batch Waste Categories Caused by Deformation of Cigarette Pack Products
From Table 1, it can be seen that paper cutting waste is the main proportion of total waste. Therefore, further classification analysis and statistics of paper cutting waste show that the size and edge of paper cutting are the main proportion of paper cutting waste, with an average of 92.91%, as shown in Table 2.
Table 2 Comparison of Types of Paper Cutting Waste in Batches
From this, it can be seen that due to the poor weather resistance of paper, after being subjected to line gravure printing and the influence of environmental temperature and humidity, paper is prone to produce waste products with cut edges. Therefore, it is urgent to solve the problem of high scrap rate of paper cutting edges caused by the weather resistance of cigarette packaging products and the reduction of paper deformation.
Design experimental plan
In order to reduce the difference in dehydration rate between the front and back sides of the paper caused by dehydration during drying, which affects the weather resistance of the paper, the company's technical personnel designed relevant schemes and conducted actual tests to observe the differences in weather resistance of the paper under different testing schemes and measure the deformation of the paper.
In response to the deformation of paper, the company's technical personnel have carried out corresponding improvement designs on the production process points of gravure printing on both sides of the paper. The main idea is to reduce the water absorption rate on the front side of the paper and reduce the dehydration rate on the back side of the paper through printing and back coating. The specific steps are as follows.
Step 1
Firstly, in order to reduce the absorption of moisture by the non graphic parts on the front side of the paper and reduce the difference in moisture content between the front and back sides of the cigarette pack product paper, a 24 pack cigarette pack product first color base coat full plate solid printing is added to form a layer of isolation film or "protective film" on the front side of the paper. In the subsequent process of connecting different color groups for gravure printing, the absorption of moisture by the front side of the cigarette pack product paper is reduced. The plate depth is set to 23 µ m, the drying temperature is 65-85 ℃, the material uses a diluent, and the viscosity is set to 17-20mPa · s.
Step 2
After analyzing the water absorption and loss of ink on the paper after being connected for gravure printing, the project team decided to add water to the back printing of the paper to alleviate or reduce the poor weather resistance caused by factors such as production, ink application, and drying, which can lead to paper deformation and die-cutting of large and small edge waste. Generally, paper is printed with ink on the front side, while ink on the back side is opposite to the actual printing situation. Therefore, based on the specific production situation of the printing color sequence and the corresponding reversing mechanism characteristics of the connected gravure printing equipment, the eighth color group has a reversing mechanism that can perform back printing with water replenishment. Therefore, the project team sets the eighth color group as the color sequence for ink replenishment on the back, and sets and installs the reversing mechanism.
Step 3
Develop a plan to replenish water on the back of cigarette packaging products during the gravure printing process. One is to use a humidifier for physical hydration and humidification. After testing, there was no significant improvement effect, so it will not be adopted or further tested. The second is to use the method of printing water on the back of the paper to reduce the difference in moisture content between the front and back sides, thereby enhancing the weather resistance of the paper.
Firstly, the depth selection of the back coating plate is carried out. Based on the bottom coating plate depth of 23 µ m, a back coating plate with the same plate depth is used for back printing. In addition, a plate with a plate depth of 14 µ m and a mesh hanging method (i.e. a plate depth of 14 µ m and a dot area ratio of 50%) are selected for experimental testing.
Three testing schemes were used to produce 72000 sheets for tracking tests, to check the weather resistance and deformation of the paper. Before production testing, the length and width of the paper cutter used for production were tested for accuracy, and it was found that the length and width accuracy deviations were all less than 0.3mm, which complies with company regulations. Continue with the experimental testing, and the relevant waste data and product deformation of the testing plan are shown in Table 3.
Table 3 Comparison of scrap rates for paper cutting edges of different printing plate depths and sizes
According to the test plan results, it can be seen that there is a significant improvement in the experimental results of Plan 2, but it still does not meet the company's expectations. Therefore, the company will further optimize the testing for Plan 2. Firstly, confirm that the back coating plate is printed with a back coating solution at a plate depth of 14 µ m to reduce back water loss, and change the ratio of back coating solution to solvent (solvent is a mixture of water and alcohol) for testing. The specific results are shown in Table 4.
Table 4: New Scheme Testing for Changing the Ratio of Back Coating Liquid and Solvent for Scheme 2
Based on the results of test plan five, the testing experiment will continue. Combining the test results of plan two, the technical personnel have decided to change the ratio of water and alcohol in the solvent based on plan two and plan five for further testing experiments to study the deformation of the paper in this state, as shown in Table 5.
Table 5 Change the ratio of water and alcohol in the solvent for further testing
After testing from plan six to plan nine, the company tracked and counted the cutting size and edges of the paper according to plan nine, which had the smallest appearance deformation of the final cigarette pack product. Before production, the corresponding length and width accuracy of the paper cutting machine were tested, and it was found that the length and width accuracy deviation was less than 0.3mm, which meets the company's regulations. After completing the accuracy testing of the paper cutter, the project team conducted corresponding scrap statistics for the cigarette pack products in Scheme 9, and the specific results are shown in Table 6.
Table 6: Statistical Analysis of Waste Products for Cigarette Pack Products in Plan Nine
From the above data, it can be seen that with the continuous promotion and optimization of the testing plan, the weather resistance of cigarette pack products has become stronger, resulting in a decrease in the scrap rate of paper cutting edges, achieving the goal of the testing plan.
Step Four
After the completion of the experimental plan, the project team will track and track the specific waste situation of production related batches, conduct effect tracking to verify whether the weather resistance of cigarette packaging products has been enhanced, and whether the scrap rate of paper cutting edges has decreased.
According to test data, printing waste has become the main waste in batch production, with paper cutting and breakage accounting for the largest proportion of paper cutting waste. The proportion of paper cutting size and edge has decreased from 92.91% to 30.32%, and the waste rate has decreased compared to before the experiment. The weather resistance of paper has been effectively enhanced.