Analysis of Influencing Factors of Whiteboard Printing Quality (Part 2)

4 Paper Surface Strength

The surface strength of whiteboard paper refers to the combination of fibers, fillers, pigments, etc. in the paperboard pulp with the paperboard, and the firmness of the connection, that is, the strength of the bonding of the surface layers of the paperboard. In the offset printing process, the white paperboard is subjected to three forces in the z direction, namely, the adhesion of the ink to the paperboard, the adhesion of the blanket to the paperboard, and the centrifugal force generated by the printing press at high speed. The surface strength of the cardboard must resist the force of its three forces, otherwise it will fall out of powder and tearing will occur severely.

In order to prevent hair loss during printing, in addition to selecting cardboard with higher surface strength, it is also possible to solve the problem by reducing the viscosity of the ink, reducing the printing pressure and the amount of fountain solution under the precondition of ensuring the printing requirements. .

5 Moisture Content of White Paperboard

Refers to the ratio of the reduced mass to the original board quality when the board is baked to constant weight at the specified drying temperature, expressed as a percentage. The water content of the general paperboard is between 6% and 8%, and the water content is too low. During the printing process, electrostatic adsorption occurs, resulting in difficulties in paper feeding, pleating, overprinting, and other faults. Overprinting may also occur. Qualitative, pleated, curled, and back-smeared quality problems such as slow drying of printed products. The water content of the whiteboard varies with changes in the temperature and humidity of the environment, causing changes in size and shape. (Figure 2) Overprinting that causes multicolor printing is not allowed.

In order to reduce the sensitivity of the paperboard to moisture and maintain a stable moisture content, the white paperboard should be subjected to humidity conditioning (commonly known as paper paper) before printing. There are hygroscopic and dehumidifying methods. The commonly used method is to hang paper in the paper-making machine or the air-drying machine with a relative humidity of 10% to 15°C higher than the printing temperature, and the relative humidity is 10% to 20% for more than 24 hours. On the paper table with the same temperature and humidity in the printing shop, it was placed for more than ten hours. (The ideal moisture content of the paper is about 5.5% to 6.0%. The temperature of the printing shop should be controlled between 18°C ​​and 24°C, and the relative humidity should be between 60% and 65%).
The overprint accuracy in offset printing is generally controlled at ±0.2 mm for fine products and ±0.1 mm for fine products. The accuracy of overprinting accuracy for flexographic printing is generally controlled at ±0.3mm, and fine products are controlled within ±0.2mm.

6 Transverse or Lengthways Fibre

White cardboard refers to the direction in which most of the fibers are arranged in cardboard. In general, the direction of fiber alignment is parallel to the long side of the lithographic paper (based on the full sheet of paper) and the paper is called “longitudinal wire”. The cross direction is perpendicular to the long side of the lithographic paper. High strength, small deformation of the cardboard after pressure, is conducive to color overprint.

White cardboard expands and shrinks with temperature and humidity. Dry plant fibers, after wetting, can increase their diameter by 30%, and their length increases by only 1% to 2%. Therefore, the ratio of paper stretching at right angles to the silk file is greater than that of the parallel direction. many. In consideration of the above reasons, when printing white paperboard, the same product should be made of the same silk paperboard. Products with higher quality requirements should be selected with longitudinal silk paperboard, and the direction of paperboard silkworm should be perpendicular to the printing direction (roller circumference to). In the postpress processing, in order to ensure the flatness of the molded product, the silk defects of the cardboard should also be considered. The direction of the silk thread of the white paperboard should be parallel to the length of the die-cutting indentation line, and the formed carton carton is relatively smooth.

7 Absorbtivity

Absorption of paperboard refers to the ability of paperboard to absorb water or solvents. For the ink, it is the ability to absorb the ink, and it can also be said that the ink penetrates the paperboard. It is not only related to the looseness and capillary state of the cardboard, but also related to the surface properties of the fibers in the cardboard, the content of fillers, pigments, rubber compounds, the composition and characteristics of the ink, and also related to the printing method, the printing pressure, and the like.

Paperboard needs some absorbency to promote ink transfer. However, if the absorption is too high, the ink binder will be excessively penetrated into the interior of the paper, and the pigment particles will accumulate on the surface of the paper, making it difficult to form a film, resulting in a lack of gloss of the print and even the chalking of the ink layer. In general, the greater the absorption, the more obvious the dot expansion, and when the absorption is too strong, it will also cause the ink to appear through the printing, seriously affecting the quality of printed products. On the contrary, the cardboard absorbs less. In the multi-color overprinting, especially in the field printing, the ink dries slowly, and the backside of the printed matter may become dirty, and even agglomeration of the printed matter may occur.

Conclusion

In summary, with the rapid development of printing technology, the printing requirements for white cardboard are getting higher and higher. There are many factors affecting the quality of whiteboard printing. To ensure the printing quality of whiteboard, special attention should be paid to the whiteness, smoothness, glossiness, absorbency, moisture content, silk thread and surface strength of cardboard. Printability of performance indicators.

(Author/Li Rong Guangdong Vocational College of Light Industry)

Paper Science and Technology-No.3, 2005