Roll paper rotogravure printing and post-processing tips

Paper packaging occupies a considerable proportion in the packaging industry. In a sense, the products packaged with coated copperplate paper generally have relatively high grades and quality, and they require higher requirements for printing. Therefore, it is necessary to conduct detailed studies on the printing and post-processing processes of paper.

At present, paper printing methods mainly include offset printing, flexo printing (including UV) and gravure printing. From the standpoint of product exquisiteness, offset printing and gravure printing are more prominent, and offset printing is more frequently used for printing flat sheets. The famous ROLAND and HEIDELBERG are The field almost dominates the world; in gravure printing, gravure printing is applied more widely. CERRUTI, W&H, and BOBST all lead the way. The author shared some of his experiences in coated paper and kraft paper gravitational printing with readers in order to reach a valuable conclusion. the goal of.

The application of gravure printing on paper is actually very extensive. However, in contrast, gravure printing is used more in the plastic printing industry, especially in the flexible packaging industry. In recent years, with the rapid development of flexible packaging, gravure printing is almost linked with plastic printing. In fact, gravure printing was first applied to paper. In modern paper gravure printing technology, the use of bronzing, embossing, die-cutting, and other technologies in tandem has made its performance means richer and its application range more extensive.

Paper and plastic films will exhibit very different properties in the same gravure printing due to their different properties. In the early days of flexible packaging, the equipment used for plastic gravure printing and paper gravure printing was completely different and incompatible. With the development of modern technology, paper and plastic dual-use gravure printing machines have been developed, which greatly reduces the investment cost of paper gravure printing equipment. The application of paper gravure printing is also more popular. In order to simultaneously take into consideration the different material printing on one device, it is necessary to distinguish from the device configuration, raw material control, and plate rollers in order to obtain exquisite prints at the same time.

The main raw material of raw paper is raw wood pulp. According to the different types of pulp and whether the surface is coated with calendering, it can be divided into different types of coated paper, offset paper, moisture-proof paper, kraft paper, and medical paper. But its basis is the natural polymer of cellulose molecular structure. Unlike synthetic polymers such as plastics, cellulose is a hydrophilic polymer, while the latter is completely hydrophobic. Secondly, as far as the commonly used paper is concerned, the mechanical properties of the paper are much higher than those of plastic films, and the longitudinal elastic modulus of the tensile modulus is small. It is these two points that have caused the unique process details in the gravure printing of paper.

Cellulose has a certain degree of water absorption. Therefore, for the commonly used coated paper, offset paper, and kraft paper, the moisture content is an important factor in indicating the quality of the paper. The paper will swell after absorbing water, and will shrink after losing water, which is a characteristic that plastic film does not have.

The common substation gravure printing machine adopts the automatic color registration system for tracking each color of the overprinting marks, and performs vertical and horizontal overprinting by means of following colors to track the previous color or fixedly track the principle of the first color; after each color printing, it enters the high temperature airflow drying. The ink layer is dried in the drying tunnel to prevent the printed ink from being contaminated and dissolved by the latter color to cause so-called "biting". Such a process is suitable for a non-absorbent plastic film, and for a sheet whose external dimensions are affected by the moisture content, the negative influence thereof must be carefully considered and eliminated. The author in the actual production, often found that the former color of the overprint mark in the width direction will be less than about 0.5-0.8 mm after the color, resulting in horizontal overprint deviation of defective products. From the point of view of occurrence, the deviation of the second to third colors from the first color transverse overprint is greater, and the deviation of the second colors from the other colors is smaller.

The fundamental reason for the above-mentioned unfavorable phenomena is that after the printing is finished, the paper is drying itself while it enters the drying path of the drying ink, and the water contained in the base paper is also heated and dried, and shrinkage occurs after the water is dehydrated. In the longitudinal direction, due to the presence of tension, the contraction amplitude is relatively small, while the lateral direction is relatively large. Since the size has become smaller after the latter color has been reached in the width direction, the overprint system cannot accomplish the goal of automatic correction and will continuously alarm. The reason for the inconsistent shrinkage before and after the appearance is that the first color is not yet heated, the water content is high, and the degree of shrinkage is large; while the latter colors have been removed due to the previous heating, and the degree of shrinkage is relatively low. Once you understand this principle, you can make targeted adjustments.

Control inspection of incoming paper quality

Compared with the plastic film, the content of the moisture content was increased in the inspection of the incoming paper. Because of the different kinds of paper, the inspection standards for water content are not all the same. Coated paper is usually used for fine printing, and its water content index is generally less than 4%. Test methods include weighing method and conductivity method. Both methods have their advantages and disadvantages.

The weighing method is the most direct method. The moisture content is calculated using the weight difference of the oven-dried sample. No special testing equipment is required, but there is a certain requirement for the skill of the surveyor. From the detection results, although the detection speed is relatively slow, the accuracy of the measurement results is high because multiple lateral sampling is possible.

Conductivity method is fast and simple. The sample can get results in one minute, but it needs higher investment. Currently, the paper moisture meter sold on the market adopts digital display technology and is very convenient. However, since the equipment is tested by the conductive resistivity and is carried out on the large-volume paper storage site, the test results will be affected by the humidity of the storage location. The instrument also needs regular calibration, and the accuracy of the results is not as high as the weighing method.

Adjust gravure printing equipment

It is ideal to control the moisture content of the paper, but at present the level of control of this indicator varies from supplier to supplier. At the same time, different papers are also very different: the surface coated and calendered coated paper has a much lower moisture content than kraft paper, which is another reason why kraft paper cannot achieve the printing effect of coated paper. At the same time, if the moisture content in the paper is high, the solvent contained in the ink after printing will evaporate together with the moisture in the fiber of the paper in the drying tunnel to form a common boiling system, which will affect the normal drying of the organic solvent and cause organic residual solvents in the printed matter. The increase will affect the safety performance and functional quality of the final product. Therefore, the paper must be pre-heated before gravure printing to remove as much moisture from the paper as possible to achieve a new, lower level of water-vapour balance, so as to reduce the loss of water shrinkage of the paper during the printing and drying process.

At present, the gravure printing machine for paper is basically equipped with an infrared pre-drying device, and the paper is subjected to a preheating drying path during the unwinding process to achieve the purpose of drying. For products with a small number of colors, the first and second color spare units can be used for secondary drying to achieve better results. Only the pre-drying effect after the paper is unwound can be guaranteed to achieve perfect printing overprinting.

Adjust the increment of the printing plate roller

The same moisture content does not mean that the same shrinkage rate occurs during printing. In general, the higher the weight of the coated paper of the same manufacturer is, the wider the width is, the higher the absolute moisture content is, and the higher the possibility and magnitude of the dimensional shrinkage after water loss occurs. For some gravure printing machines with relatively early investment, since there is no heating system with far-infrared heating, the heating performance of the equipment is not high. In addition to investing more funds for equipment modification, incremental adjustments on the printing plate roll can be considered. The so-called increment refers to the fact that in the production process of gravure, considering that the film will have very slight stretching during the printing process, the circumference of the latter color roll will increase more than the previous color, so as to meet the requirement of color registration, increasing The amount is generally controlled between 0.01-0.05 mm.

Because the paper has almost no stretching during the printing process, and the phenomenon of shrinkage due to water loss occurs at the same time as the plastic film, in the process of plate making, the increment can be reduced, even without increment or negative increment. At the same time, make a proper size reduction in the horizontal direction to facilitate color registration. Of course, these tasks must be carried out without changing the customer's requirements on the dimensions of the product.

Controlling the moisture in the paper is particularly important for fine registration and is a key control point. However, due to the hydrophilic properties of paper cellulose, it is not possible to reduce the moisture content of the paper completely to zero, and it is also unrealistic and unnecessary. Too dry paper becomes very fragile, loses its original toughness, and is prone to breakage and is not easy to process. In fact, everything has positive and negative sides. In the intaglio printing, we hope to reduce the moisture content of the paper as much as possible, but at the same time, we can use the special properties of paper, which is different from the plastic film, to correct its deformation in the subsequent processing. The author will introduce you in the next period. (Chen Manli)

Source: Shanghai Packaging