Aiming at the "spot spot" problem often encountered by packaging and printing companies, the printing materials are mainly composed of three main elements: paper, oil, and ink jet machine, and the relationship between the three.
The cause of the appearance of India's "silkworm spots" was studied, and a collision method was proposed. Printing spot analysis and drawing packaging printing will be discussed below. 2 "Ink spot" cause analysis and control The so-called printing "ink spot", which is on the surface of the printed matter, especially in the solid area due to uneven ink distribution and color irregularities , Uneven spots or stripes.
As shown in the figure, several forms of ink spot printing are normal images. The appearance of ink spot images seriously affects the appearance of the package due to the appearance of ink spots. Therefore, the current packaging and printing companies, especially cigarette packaging (analysis of the use of constant packaging devices) performance companies are very strict control of this. The national standard stipulates that "ink blots, no more than qualified, companies must not exceed in the specific implementation of this standard, cigarette pack companies even as long as the products with ink spots are regarded as unqualified products Sichuan. Its appearance is in many ways, we The non-uniformity of the paper manufacturing process, the main components of cellulose and hemicellulose are extremely hydrophilic substances, and the contact angle with water is almost. Sexual droplets will completely diffuse on this paper surface and penetrate quickly. This kind of performance is very unfavorable for some printing methods such as the use of wetting fluid in offset printing, and the use of water-based ink printing in flexographic printing. It will reduce the paper's directional strength and produce a roughening phenomenon during printing. Where there are inks, white spots are left. In multi-color printing, the appearance of the printed matter on the larger ground is uneven, and spots appear. Adding various auxiliary materials to the finished pulp can change this performance of the paper. Sizing can improve the quality indicators related to pulp and paper from different angles, increase the resistance of paper to liquid diffusion and penetration, and improve the printability of paper. Stop the fluffing of the base paper during the printing process. Experiments show that the appearance of printing ink spots mainly depends on the compactness of the film formed by the surface sizing. The base paper sizing by the synthetic surface sizing agent has strong resistance to penetration. The film is dense and has few pores. The coating can be evenly distributed in the thickness direction. The points of the coating are uniformly pulled by the ink adhesive during the printing process, and the printing ink spots are not easy to appear. At the same time, the main components of the paper are relatively Other non-fibrous components are relatively large.
Due to the lack of papermaking parts, the uniformity of the paper obtained by the rotary screen paper machine and the long screen paper machine is poor, and the vertical and horizontal strength of the paper varies greatly. The arrangement and positioning of the fruit fibers in the space during the papermaking process are quite different. That is, the local distribution of fibers is not uniform. In the "flattening" process performed in the later stage of the paper, the paper will be partitioned due to the pressure, that is, the densely dense areas are formed in the areas with many fibers, and the fibers are distributed. Less areas form loose areas with lower density. This kind of partitioning phenomenon will change the paper's acceptance and absorption performance of ink. When printing, there will be differences in the absorption and penetration of ink in the loose and tight areas of the paper. The loose area will receive more ink and the ink film will be thicker. The compact area is the opposite. Therefore, uneven reflection characteristics will be formed on the entire surface of the printed matter. The reaction to the human eye is that the printing surface has ink shifts and stripes.
The paper mesh paper machine currently advocated for use is because the paper sheet is forcibly formed between the two webs, the paper is dehydrated on both sides at the same time, the dehydration speed is fast, there is no difference between the two sides of the paper, the vertical and horizontal strength is close, the paper quality is uniform and stable Avoid the phenomenon of paper partitioning. Xu Bu is to apply pigments and binders on the surface of the base paper, and through super calendering to improve the surface structure of the paper, to obtain paper with good ink absorption, uniformity and smoothness, to meet fine prints Requirements.
The choice of coating and coating method are related to the quality of the coated paper and also affect the quality of the printed product. If a large amount of ultra-fine calcium carbonate and kaolin are used as pigments in the paint, and synthetic latex is used as the adhesive, the porosity of the paper will be too poor. As a result, the penetration speed of the ink binder in the paper becomes slow, and a large amount of pigment particles and binder are accumulated on the surface of the paper. In multi-color printing, when the previous color group enters the next color group, the ink of the previous color on the printed product will be peeled off by the blanket or the ink sticky on the plate cylinder in the subsequent color group to produce reverse ink spots. Appropriate viscosity and leveling can ensure that the coating can be smoothly applied to the paper, and it will quickly level on the paper to make the coating even and the surface smooth. Different coating methods also affect the quality of the paper. Squeegee coating, partial overheating of the blade during continuous work will consolidate part of the material at the overheated area and scratch out the impurities in the coating to damage the blade, causing the blade to have gaps and leaving the coating convex Dao, if this happens, replace the scraper in time. At the same time, the soft blade is better than the rigid blade.
The uniformity and temperature of compressed air jet in air knife coating will also affect the quality of the coating. In roll coating, the properties of the coating and the Xubu condition determine the leveling of the Xu layer. The leveling performance affects the appearance of orange peel on the paper.
The influence of printing technology and ink Pre-press control Improper processing of pre-press graphic information will also lead to the appearance of ink spots on later prints. In the process of multi-color copying and screening,
Improper four-color screen angle will cause the dot pattern to overlap each other. The magenta can reduce the moire caused by the dot overlap. At the same time, the FM screening technology is used. The FM screening is not periodic due to the randomly distributed FM dots. There is no restriction on the angle of the mesh, there will be no moire caused by mutual interference.
At the same time, there is no net line on the color separation film, and no rose pattern will appear during printing. When dealing with multi-color overprinting, the trapping parameters should be set correctly to compensate for errors that occur when the printing press overprints. When designing multi-color overprinting, arrange the order of overprinting correctly to prevent the adhesion of subsequent color groups to the previous color group, which will reduce the formation of "ink spots". Process control The reasons for the appearance of ink spots in different printing processes are different. Gravure printing adopts short ink way to convey ink
In direct printing, the ink is directly imprinted from the ink bearing unit on the plate cylinder and fixed on the surface of the paper. Gravure inks are basically Newtonian fluids. The viscosity affects the fluidity of the ink and determines the transfer effect of the ink.
Low viscosity will make the ink dry slower, make the ink flow on the surface of the substrate, irregularities appear in the dark tone part of the printed product, uneven screws, streaks, reduce the clarity, black virtual and unrealistic viscosity, its flow The performance deteriorates accordingly, it is not easy to fill every mesh hole on the plate cylinder, the dots cannot be completely transferred to the substrate, and scratches and white spots appear on the surface of the printed product. The impact force on the squeegee blade increases, which makes it difficult for the squeegee blade to scrape the ink, causing printing problems such as knife lines and the like. Since gravure inks are diluted with volatile solvents, care must be taken whether the solvents used are compatible with the ink.
In the printing process, the viscosity of the ink must be detected at all times to prevent excessive volatilization of the solvent, resulting in uneven ink viscosity during the printing process. The doctor blade plays an important role in the gravure printing machine, and the quality of its blade largely determines the quality of the printing quality. If the blade is too thick, its elasticity will be poor, affecting the thixotropic fruit, and the unevenness of the high-profile part of the printed product will appear.
The blade is too thin, although it can improve the scraping effect, but the blade is easily damaged by ink or "hard objects" on the paper, resulting in a gap in the blade and a very thin track and stripes on the printed matter. Many experiments have proved that the angle of the scraper blade grinding should be used in flexographic printing because of its high printing speed, high degree of printing integration, and strong environmental protection, which has become the preferred method for many packaging companies.
The flexographic plate material currently used is composed of photosensitive resin, and has the advantages of excellent ink transfer performance, high graphic reproduction accuracy, short production time, and good wear resistance.
At the same time, the plate hardness and flatness should be selected. The hardness of the plate material is determined by the photopolymerization components contained in the flexographic plate itself, the plate-making process, and the control of temperature and time during the production process.
If a rigid plate is used on a rough substrate, if there is insufficient contact between the two, the ink transfer will be insufficient, and quality problems such as ink spots with different ink shades will be formed on the printed product.
Such problems can also occur when using plates with low hardness on substrates with smooth surfaces. The selection of plates of different hardness must be based on the type of printing machine, printing conditions, the nature of the printing material, etc.
For example, the Shore hardness of the packaging printing material plate is about an extra-thick material with a rough surface for the elastic substrate. In order to ensure good ink transfer, the Shore hardness of the printing plate should be relatively low.
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