Influence And Countermeasures Of Paper Shrinkage Change in UV Printing

Influence and Countermeasures of Paper Shrinkage Change in UV Printing

In recent years, with the intensified commercial competition, in order to expand the market, merchants hope to increase the attractiveness of products and achieve anti-counterfeiting effects through the beautiful design and exquisite craftsmanship of product packaging. This promotes the continuous development of printing and decoration technology, and the industry's exquisiteness of product packaging quality and design has also improved.

In recent years, with the intensified commercial competition, in order to expand the market, merchants hope to increase the attractiveness of products and achieve anti-counterfeiting effects through the beautiful design and exquisite craftsmanship of product packaging. This promotes the continuous development of printing and decoration technology, and the industry's exquisiteness of product packaging quality and design has also improved.

In order to improve the production cycle and production capacity, when the printing factory produces and processes long-term products (such as cigarette packaging, daily necessities, toothpaste boxes, etc.) The method makes the printed matter dry in a short time and then carries out subsequent processing. The paper must be subjected to a high temperature baking process during the printing process of the printing machine. When the printed paper is baked, it will cause paper moisture loss and paper fiber in Shrinkage under high temperature, so that the paper is deformed (reduced length and width), which seriously affects the positioning accuracy of the post-processing process of the product and reduces the production qualification rate of the product.

So how to deal with the problem of paper deformation? Due to the characteristics of paper fibers and the direction of paper grains, the change size of paper during product printing and processing is directly related to the direction of paper fibers. Usually: the direction of the fibers along the paper changes greatly, and the direction of the longitudinal grain changes less. Under normal circumstances, the change rule is that the larger the paper format size, the larger the shrinkage change error will be. If you pay attention to the overprinting characteristics of the product to determine the direction of the silk grain of the paper and adjust the size of the paper change, it can also reduce the difficulty of post-processing.

According to the structural requirements of packaging products, some processing processes are necessary processes, such as die cutting, indentation, etc. Some products have bronzing, embossing, partial screen printing and other processes due to design or customer requirements. At this time, the deformation of the printing paper will have a serious impact on the quality of these post-press processes. It directly leads to the inaccurate positioning of these processes, which reduces the product qualification rate and affects the product material cost. Since the beginning of UV printing, paper deformation has always been a focal problem that many manufacturers want to solve.

In order to control the deformation of paper, many manufacturers use process protective film to protect the product to reduce the change of paper caused by moisture absorption and dehumidification of the paper when the product is to be processed. This method can play a certain role in a sense, but it can only control the printing within a certain range after the change of the machine, and there are many human factors, so there is still a certain risk.

For example, when a worker wraps a protective film, the result of the film may be different for different people, and the protective effect is different. In addition, there is a restoration process after the printed product is cooled after it is off the machine. If the protection is different, it will affect the different deformation of the printed product, which will increase the difficulty of post-processing overprinting.

After a long-term analysis of the deformation data of similar products in different regions, we have come up with the following solutions:

Scheme 1, prepress typesetting processing

That is, according to the different paper grain direction and deformation law when the product is typesetting, the product typesetting spacing is pre-pressed incrementally, and the gap between the shrinkage caused by the product printing process and the post-press restoration is placed in the imposition spacing, so as to control the product. The variation error of the whole processing process is within the controllable range. The controllable range refers to the range of overprinting and processing accuracy during production and processing of post-processing equipment, tools, and plates (including the range of overprinting accuracy required by processes such as bronzing, die-cutting, and embossing).

The specific treatment method is to send the Yunnan cigarette label product Hongmeichun. The size of the imposition is 520*730mm, and the number of impositions is 3 horizontally and 5 vertically. The 730mm changes greatly due to the paper grain direction. The design pattern and box-shaped structure of this cigarette pack must be arranged horizontally. The hot stamping of the plum blossom of the hot stamping pattern requires high overprinting accuracy (the error must be below 0.20mm> so the deformation of the paper seriously affects the quality of the product.

Suppose we measure that under normal conditions of the printed product, the shrinkage data of the paper after printing is 0.5-0.7mm horizontally and 0.15-0.25mm vertically. In this way, we can specifically enlarge the distance between the three products in the horizontal row during typesetting. When enlarging, it is required to make adjustments based on the center line. The enlargement distance is determined according to the distance between the post-processing overprint position and the center line. .

The plum blossom as shown in the picture is difficult to overprint, and the positions of the plum blossom patterns on both sides are different from the center line. A total of 0.45mm is enlarged, and the number of changes in the printing machine is 0.5-0.7mm minus 0.45mm, so there is only a change error of about 0.25mm. Usually, the processing accuracy of our post-processing equipment is about 0.2mm. At the same time, there is also an overprint error space of about 0.15mm in the post-processing printing plate (bronzing plate, embossing plate and die-cutting plate), so that after removing the positive and negative errors, the quality requirements of the post-processing technology can be fully met.

This solution is suitable for areas with dry climate, such as: Northwest, Northeast, North China and Yunnan-Guizhou Plateau. In these areas, due to geographical and climatic factors, general printing companies do not have central adjustment systems installed, so they cannot manage the production workshop at constant temperature and humidity, and cannot control the printing process. The moisture absorption and desorption stability of the product during processing. Special attention is that when adopting, it is necessary to carry out a random check of the printing change data to find out the corresponding enlarged data, which requires relatively high technology and operation precision of the staff.

Option 2: Process Control Method

Strictly control the unevenness of moisture release and moisture absorption caused by the products during the printing process. According to the different water content of the paper, the baking temperature of the printing process is controlled to ensure that all the printed products are controlled and released relatively uniformly during the printing process, and the printing process environment is kept at a constant temperature. Wetness, strengthen the protection and control of the printing process, the upper and lower moisture absorption and restoration are uniform, measure the moisture absorption and restoration law and time of the printing process, deal with the production arrangement of the front and back processes, shorten the processing cycle of the front and back processes, and reduce the moisture absorption process. The protection method of the production environment with central air conditioning is different from the environmental protection method of production without central air conditioning.

Option 2 is suitable for East China, Central China and South China, where the climate is hot, the relative humidity in the air is high, and the printed products absorb moisture quickly during the processing process, and generally large and medium printing companies are equipped with central air conditioners to control the temperature of the production workshop. humidity. Of course, scheme 1 can also be used, especially for some products with poor paper quality (such as gray bottom cardboard), due to the large change in the thickness of the paper fiber.

The above plan is one of the experiences that the author has long been engaged in the printing process to solve the relevant quality problems in the production example. Of course, there is still technical space for discussion for the reference of the same industry.