Paper Printing Common Problems and Solutions

The printing of paper and board includes processes such as pressing (except for non-contact printing such as ink jet printing), ink application, ink absorption, and drying. The various properties of the paper that are suitable for printing conditions in these processes, namely the printability of the paper, and the performance of the printing ink and the printing machine, are the keys to determining the print quality of the paper.

The printability of paper mainly includes: surface strength, ink absorption, smoothness, moisture content, pH, elastoplasticity, whiteness, dustiness, and so on. The printing problems caused by the printability of paper and its solution can be summarized as follows:

(1) The surface strength of the paper is too low, resulting in the appearance of paper surface picking and de-powdering due to the adhesion of the ink during the printing process, and the printing quality is poor.

The surface strength of the paper refers to the degree of bonding between the fibers of the paper and the filler and the glue. The higher the degree of bonding, the higher the surface strength of the paper. Paper surface strength test method: Under a certain pressure, a certain viscosity of ink to make paper under accelerated conditions, the maximum speed of the surface of the paper when fluffing, that is, the speed of pulling hair, in meters / second (m / s) . Or sticking with different viscosity wax sticks, the surface of the paper begins to appear as the level of viscosity of the wax stick.

Solution: Make the paper have a higher surface strength in order to obtain better print quality. In order to ensure good printing quality, paper with higher surface strength is used, or the printing pressure is appropriately reduced, or the ink viscosity is appropriately adjusted. The main reasons affecting the surface strength of paper are: paper fibers have more impurities and reduce the surface strength of the paper; the degree of hydration of the pulp is too low, too much short fibers, and the ability to interweave fibers is reduced, resulting in loose texture of the surface of the paper; If the dry temperature is too high, the surface strength of the paper may also be decreased. When the amount of filler is too large and the amount of the rubber material is too small, the paper surface has a poor bonding force. The surface of the paper is insufficient in calendering and the tightness is low, thereby reducing the surface strength of the paper. Therefore, it is necessary to put forward specific requirements in the paper related standards. For example, in the technical standard QB1011-1991 for single-side coated white paperboard, it is stipulated that the surface strengths of coated white paperboards such as A, B, etc. and C, etc. shall be 2.0 in order. More than m/s, 1.2m/s or more, and 0.8m/s or more are used as the basis for use to ensure print quality.

(2) The ink absorption of the paper is too strong, resulting in the loss of paper transparency and gloss during the printing process; or because the ink absorption of the paper is too weak, the ink is not adhered tightly and sticky during the printing process.

The absorption of ink by paper is mainly related to the state of interweaving between the fibers on the surface of the paper and the use of fillers and compounds. The voids between the paper fibers have a capillary sorption effect on the binder in the ink, and the more porous voids, the stronger the adsorption capacity. The ink layer on the surface of the paper, except for a small part of the dry paper absorption, is mostly dried by the oxidative polymerized conjunctiva of the binder in the ink. If the ink absorption of the paper is too strong, the binder in the ink is absorbed in a large amount by the paper, and the ink film on the surface of the paper is powdered, resulting in incomplete printing, dim luster and penetration. If the ink absorption of the paper is too weak, the ink layer on the surface of the paper may not be dried in time, resulting in smearing; or the ink layer on the surface of the paper may be dry but the ink may not adhere securely.

Solution: All kinds of paper according to actual needs, to maintain a certain ink absorption performance. For example, the ink absorption of coated white paperboard is generally controlled at 15.0-30.0%. Correctly master the paper's ink absorption performance, reasonably deploy the ink suitable for paper, and reasonably grasp the printing pressure and the imprinting time to ensure the improvement of printing quality. In general, the ink has the following relationship between the penetration depth d of the paper surface, the printing pressure P, the imprinting time t, the ink viscosity η, and the capillary radius r of the paper surface.

d=(Pr2t/4η)1/2

(3) The smoothness of the paper is too low, resulting in broken prints, blurred print screens, empty dots, and poorly patterned layers.

Smoothness is the physical quantity that represents the level, uniformity, and smoothness of the paper surface. Paper smoothness test method: under a certain vacuum conditions, the time required for a certain volume of air to flow between the surface of a paper sample under a certain pressure and the smooth glass surface, in seconds (s). Paper with low smoothness has strong ink absorption, and the graphic blots have poor gloss after drying. On paper with high smoothness, the paper and the graphic part of the plate are evenly and tightly contacted during printing, and the ink is evenly distributed. Transfer to the surface of the paper to obtain a clear and complete copy of the text.

Solution: Pure pulp material is used in the papermaking process, and fillers and compounds are added appropriately to fill the gaps between the fibers of the paper. Calendering or supercalendering is used to achieve higher smoothness.

(4) When the moisture content of the paper is too high, the paper will become soft, expand, the tensile strength will decrease, the plasticity will increase, the imprinting will not be dried in time during the printing process, or the moisture content will be too low, the paper will be brittle, shrink, and it will be easily broken and broken. And easy to generate static electricity, affect the normal printing. The expansion and contraction of the paper caused misprints.

Solution: The moisture content of paper is affected not only by the temperature and humidity of the environment, but also by the nature and materials of the paper. In general, under the same temperature and humidity conditions, the greater the tightness of the paper, the greater the amount of sizing, the more long fibers in the slurry, the lower the moisture content. In the storage and printing of paper, controlling the proper moisture content is the key. For example, to control the moisture content of the paper at 6.0-9.0%, the ambient temperature is controlled at 18-24°C and the relative humidity is 40-55% RH.

(5) The pH of the paper is not suitable (paper's pH is the degree of acidity or alkalinity of the paper). Too strong acidity of the paper will inhibit the drying of the prints and the color-printing effect; the over-strength of the paper will accelerate the drying of the prints.

Solution: The first is to strengthen the management of papermaking technology. In the papermaking process, the chemical materials used must be properly treated. Generally, the pH of the paper must be controlled to be 6 or less; secondly, in the printing production, the prediction of the paper performance is emphasized and the appropriate Ink to meet the printing needs.

(6) The elastoplastic problem of paper (paper will produce compression deformation under the pressure of printing, the deformation that can completely disappear when the pressure stops, called the elastic deformation; the deformation that cannot completely disappear when the pressure stops, called plastic deformation). During the printing process, due to the elasticity of the paper, the printing pressure is insufficient to cause the printing to be fuzzy, the printing is not true, or the amount of ink is used to cause paste printing; the printing pressure is caused due to the good plasticity of the paper and excessive printing pressure or ink usage.

Solution: Elasto-plasticity of paper is closely related to the smoothness and surface strength of the paper. Paper with good smoothness and surface strength has better elasticity; paper with poor smoothness and surface strength has better plasticity. During the printing process, the appropriate printing process must be selected according to the different elasto-plastic properties of different papers. The paper with better elasticity will be printed with higher printing pressure and appropriately reduced ink volume. The plasticity is better. When printing, the printing pressure is reduced and the ink volume is increased appropriately to improve the printing quality.

(7) Whiteness and dustiness of paper, whiteness refers to the degree of whiteness on the surface of the paper, and dustiness refers to the degree and size of spots on the surface of the paper. Low whiteness and high dust will reduce print quality. In the printing of coated and uncoated white paper, especially in color printing, the requirements are higher. In general, whiteness is above 78%, and at least not less than 75%. For dust degree, 0.3-1.5mm2 spots, no more than 20 A per square meter, B no more than 60, C no more than 80, and 1.0-1.5mm2 spots, A per square meter, etc. One, B, etc. shall not exceed 2, C, etc. shall not exceed 4, and no more than 1.5mm2 spots shall be allowed.

In the printing process of paper, combining every factor of human, machine, ink and paper, each step of printing is well controlled to achieve good results.