1. Aesthetic and functional requirements for print quality
The design of modern packaging and decoration products is becoming more and more high-end, and more attention is paid to the aesthetic requirements of the products. To this end, many high-end materials and new printing processes that meet aesthetic requirements have been developed. The choice of substrate and ink before printing determines the aesthetic effect of the quality of the printed product. The choice of materials includes: the artistic and technical aspects of raw materials. The design of printed product designers is always restricted by certain conditions. Unlike newspapers and magazines, the raw materials have been determined, and the choice of packaging and printing materials must not only consider the 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, and no technical issues need to be considered. The artistic choice of products is not a matter for printers to consider, but the discussion here will involve those technical factors that produce visual effects and serve as artistic elements. From the perspective of raw materials, aesthetic technical factors can be divided into three parts: apparent, chemical and physical.
1. Apparent factors and their quality requirements
(1) Color
The comprehensive selection of inks and substrates should make the color representation of the key areas of the print as large as possible (especially saturation and brightness). In production practice, sometimes it is necessary to choose outside the standard four-color ink. The color of the printing material should be neutral, that is, white. But in fact, the human eye can adapt to the printing material with slight color deviation (such as newspaper), and use it as 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, so that the color saturation and brightness of the printed matter reach the maximum. For writing supplies, the gloss requirements of paper and ink are lower. There are no definite requirements for the glossiness of the paper base and ink for the drafts and suede photo originals.
(3) Brightness
The brightness of the printing material should be as high as possible in order to obtain the maximum contrast. The so-called brightness refers to the total reflectance of all visible spectrum, and the brightness of black ink should be as low as possible. We generally use optical density to describe the "brightness" of printing ink. The higher the density, the lower the brightness.
(4) Opacity
The opacity of the printing material is the ability to prevent the image on the other side of the printed sheet from being reflected. It should be as high as possible, especially for printed matters such as books and magazines.
2. Chemical properties and quality requirements
(1) Light resistance
Ink and printing materials should have the ability to prevent fading and discoloration. This is very important for outdoor advertising posters, but not for magazines, newspapers, and product catalogs.
(2) Chemical resistance
Ink and printing materials should have resistance to acid, alcohol and alkali. This is very important for labels and packaging printing, but less important for other printed products.
3. Physical properties and quality requirements
(1) Strength
The tensile strength, tear resistance, crack resistance and folding resistance of the printing material should be as high as possible. In the 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 very important for the carton. It must have a sufficiently high stiffness, but not so high that it is difficult to bend and form.
(2) Smoothness
For most text and image prints, the printing materials are required to be as smooth as possible.
For embossing and gravure printing, smoothness requirements are particularly high. For offset printing and filtered printing, high-quality printing does not require much smoothness. Sometimes, in order to obtain the surface effect or texture of the original manuscript, specially used textured paper or uncoated paper.
(3) Texture
The texture of the printing material should be as uniform as possible to facilitate the printing of a uniform image.
(4) Quantitative and thickness
Generally, as the basis weight of paper increases, the translucency of paper also increases, and users generally do not want the weight of printed matters such as magazines to be mailed to increase.
(5) Wear resistance
The wear resistance of the ink should be considered when designing the printed product. This indicator should be as high as possible. Especially for those that are prone to friction between printed products during transportation, such as cartons or product catalogs. [next]
Second, the image quality process requirements
For raw materials, the process factors that affect the quality of printing are those that affect the image of the substrate. They include the size of the dots, the uniformity of the coverage rate, the faintness of the print, and smudges. The above-mentioned influencing factors are analyzed from three aspects: apparent, chemical and texture.
1. Apparent factors and their quality requirements
(1) Viscosity of ink
Multi-color offset printing uses wet press wet printing, so the viscosity of the ink affects the overprinting of the ink film. In terms of process requirements, the ink before printing the color sequence should have a higher viscosity than the ink after it, otherwise there will be a reverse pull phenomenon.
(2) Stickiness
For lithography and relief printing processes, low-viscosity inks are generally not as clear as high-viscosity prints. For gravure printing and flexographic printing, the ink viscosity is too low will reduce the density of printed matter.
(3) Particle fineness or dispersion
Large agglomerates of pigments or other particles in the ink can cause unevenness of the printed image or additional plate wear, which can lead to image destruction.
(4) Pigment concentration
When forming the same ink film thickness, 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, inks with low pigment concentration can form higher gloss than high pigment inks.
(5) Emulsification
In the lithographic offset printing process, the degree of emulsification of the ink affects some other printing qualities such as dot expansion rate, overprinting, and drying time. The volume percentage of water after emulsification of offset printing ink cannot exceed 50%. Otherwise, it is difficult to achieve the balance of ink and wash.
(6) Absorbability
Papers with high absorbency will have problems of translucency, low gloss, and chalking. On the contrary, the low absorption paper will have problems such as long drying time, dirty or scab on the back.
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 value)
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. It cannot be dried on the rubber roller of the printing press, and must be fastened on the paper.
3. Physical properties and quality requirements
(1) Powder and hair loss
Pink hair can cause spots or defects in non-graphic parts. The printed sheet should avoid dust and lint on the paper side of the paper.
(2) Paper strength
Paper with low surface strength will produce paper surface roughness or even break when using high viscosity ink, so the surface strength of the paper is required to be high.
(3) Ink filament length
If the ink filament is too short, a pile of paper wool will occur, and if the ink filament is too long, ink mist will splash, which will affect the image quality.
3. Stable and consistent quality
The key to evaluating raw materials is to consider the extent to which the raw material properties can change within the print quality not exceeding the required limits. In addition, it should also be considered that this variation range should not affect the printing speed or cause other problems during the production process.
The performance of the raw materials mentioned above requires further testing. If a specific required value can be proposed and the control range of the data can be determined, then these properties of ink or paper can be measured and controlled. However, it is difficult to determine the control range.
The final inspection method for raw materials in the printing process is used under actual printing production conditions. But this method is only suitable for large-scale paper mills or ink factories, most printing companies can not do this kind of test. Most laboratory tests lack more or less the ability to predict actual print production conditions.
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