Discussion on the performance requirements of paper and ink printing raw materials
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First, the aesthetic and functional requirements of the quality of printed products
The design of modern packaging and decoration products is becoming more and more high-end, paying more attention to the aesthetic requirements of products. To this end, a number of high-grade materials and new printing processes have been developed to meet aesthetic requirements. The choice of substrate and ink before printing determines the aesthetic quality of the printed product. The choice of materials includes: the artistic and technical aspects of raw materials. The design of print product designers is always subject to certain conditions. Unlike newspapers and magazines, raw materials have been identified, and the choice of materials for packaging and printing should not only consider aesthetic requirements, but also consider its structure and physical properties. When the choice of materials is not limited, only the artistry of the product is considered, without having to consider technical issues. The artistic choice of the product is not a question for the printer to consider, but the discussion here will involve technical factors that produce visual effects and serve as artistic components. From the perspective of raw materials themselves, the technical aspects of aesthetics can be divided into three parts: apparent, chemical and physical.
1. Apparent factors and their quality requirements
(1) color
The overall choice of ink and substrate should be such that the color representation of key areas of the print is as large as possible (especially saturation and brightness). In production practice, it is sometimes necessary to choose outside of the standard four-color ink. The color of the substrate should be neutral, ie white. However, in reality, the human eye can conform to a slightly color-changing substrate (such as a newspaper) and use it as a neutral white. Then the human eye uses this "white" as a benchmark to perceive other colors.
(2) gloss
For the gloss of paper and ink in image reproduction, it is required to be as high as possible in order to maximize the color saturation and brightness of the printed matter. For text supplies, the gloss requirements for paper and ink are lower. However, drafts and suede photo originals have no certain requirements for the gloss of the paper base and ink.
(3) Lightness
The brightness of the substrate should be as high as possible in order to obtain the maximum contrast. The so-called lightness refers to the total reflectance of all visible spectra, and the brightness of the black ink should be as low as possible. We generally use optical density to describe the "lightness" of the ink. The higher the density, the lower the brightness.
(4) opacity
The opacity of the substrate is the ability to prevent images from being printed on the other side of the sheet. It should be as high as possible, especially for publications and magazines.
2. Chemical properties and quality requirements
(1) Light resistance
Ink and substrate should have the ability to resist discoloration. This is important for outdoor advertising posters, but not for magazines, newspapers, and catalogs.
(2) Chemical resistance
Inks and substrates should be resistant to acids, alcohols, and alkalis. This is important for labels and packaging, but not for other prints.
3. Physical properties and quality requirements
(1) Intensity
The tensile strength, tear resistance, crack resistance and folding resistance of the printing material should be as high as possible. In production practice, some of the above-mentioned properties are very important, such as the crack resistance of corrugated boxes, the crack resistance of children's books or banknotes, and the folding resistance of maps. The stiffness of the cardboard is also important to the carton. It must have a high enough stiffness, but it cannot be too high to bend.
(2) Smoothness
For most text and image prints, the substrate is required to be as smooth as possible.
For embossing and gravure, the smoothness is particularly high. For lithographic offset and filter printing, high quality prints do not require much smoothness. Textured paper or non-coated paper is sometimes used to obtain the surface effect or texture of the original.
(3) Texture
The texture of the substrate should be as uniform as possible to facilitate the printing of a uniform image.
(4) Quantitative and thickness
Generally, as the basis weight of the paper increases, the translucency of the paper also increases, and the user generally does not want the weight of the printed matter to be mailed by magazines or the like to increase.
(5) wear resistance
The wear resistance of the ink should be considered when designing the print. This indicator should be as high as possible. Especially for those who are prone to friction between the prints during transportation, such as cartons or catalogs.
Second, the process requirements of image quality
For raw materials, the process factors that affect the quality of printed products are those that affect the image quality of the substrate. They include dot size, uniformity of opacity, embossing and smudges. The above influencing factors are analyzed from three aspects of apparent, chemical and texture.
1. Apparent factors and their quality requirements
(1) Viscosity of ink
Multicolor offset printing uses a wet-wet printing method, so the viscosity of the ink affects the overprinting of the ink film. In terms of process requirements, the ink before the printing color sequence should have a higher viscosity than the ink after it, otherwise there will be a reverse pulling phenomenon.
(2) Sticky
For lithographic and embossing processes, low viscous inks are generally not as clear as highly viscous prints. For gravure and flexographic printing, the ink viscosity is too low to reduce the print density.
(3) Particle fineness or dispersibility
Large agglomerates or other particles of pigment in the ink can cause uneven printing images or additional plate wear, which can result in image damage.
(4) Concentration of pigment
When the same ink film thickness is formed, the ink density of the low pigment concentration is much lower than that of the high pigment concentration, and the thickness of the transfer ink film must be increased. However, low pigment concentration inks can form higher gloss than high pigment inks.
(5) Emulsifying properties
In the lithographic offset printing process, the degree of emulsification of the ink affects other printing qualities such as dot enlargement rate, overprinting, and drying time. The volume percentage of water contained in the offset ink after emulsification cannot exceed 50%. Otherwise, it is difficult to achieve the balance of ink and wash.
(6) Absorptive
Highly absorbable paper has problems such as transparency, low gloss, and chalking. Conversely, low-absorbent paper can have problems such as excessive drying time, smearing on the back, or crusting.
In addition, the sensitivity characteristics of film and plate emulsions affect the resolution of the printing process.
2. Chemical properties and quality requirements
(1) pH (pH)
The pH of the paper should be 7.0, which is neutral, otherwise it will affect the drying time and cause the back to be dirty. (2) Temperature and relative humidity
The relative humidity (RH) and temperature of the paper should be consistent with the temperature and humidity of the workshop. Serious temperature and humidity differences between the workshop and the paper can cause problems such as inaccurate overprinting and wrinkling.
(3) Drying time
The performance of the ink during drying must be considered comprehensively, neither dry on the press roll nor on the paper.
3. Physical properties and quality requirements
(1) Dropping powder and falling hair
The bristles can cause defects in the stains or parts of the non-image parts. The printed sheet shall be protected from falling lint on the paper side.
(2) Paper strength
When the paper having a low surface strength uses a high-viscosity ink, the paper may be pulled or even broken, and the surface strength of the paper is required to be high.
(3) Ink length
If the ink is too short, it will cause pile paper hair. If the ink is too long, it will cause ink mist to splash and affect the image quality.
Third, the quality is consistent
The key to evaluating raw materials is to consider the extent to which raw material properties can vary within the limits of print quality. It is also important to consider that this range of variation should not affect the printing speed or cause other problems during the production process.
The properties of the raw materials described above require further testing. If a specific requirement value can be proposed and the control range of the data can be determined, then the performance of the ink or paper can be measured and controlled. However, it is difficult to determine the scope of control.
The final method of testing the raw materials in the printing process is used under actual printing production conditions. However, this method is only suitable for large paper mills or ink factories, and most printers generally cannot afford such tests. Most laboratory tests are more or less lacking predictive of actual print production conditions.