Quality control of raw materials in the hottest pr

2022-08-12
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Quality control of raw materials in the printing process

the selection of raw materials in the printing process will have an impact on the artistic effect and technical performance of the printed image, and also affect the stability of the quality of the printed image. In addition, the performance of raw materials will also affect production efficiency. Raw materials that fail to meet the quality requirements will reduce production efficiency and waste the extra energy of operators

the most important raw materials in the printing process are paper or other substrate materials and ink. Of course, it also includes less important materials such as viscose, hot stamping foil, inking oil and binding materials

the purpose of using other materials in the printing process is to finally form auxiliary materials of images on the substrate, such as developer, proofing materials, PS plates and other materials as image carriers; Materials used for printing machine assisted image formation, such as components in offset fountain solution; There are also some consumable materials, such as tape, polyester film base, lubricant and some developing and fixing drugs; These can be classified as consumables and will not be discussed here

many properties of raw materials can be quantitatively expressed through appropriate testing, as plastic cracks and embrittlement will occur in advance. But these tests sometimes require sophisticated equipment and skilled operators. It takes a lot of time to test the raw materials, analyze the data and interpret the results. In addition, the test results may not be used as a means of predicting the quality of printed products. Therefore, how to carry out selective detection of raw materials has become an important issue. The managers of printing enterprises must master the key issues of raw material quality control: which important printing properties should be monitored for raw materials; Which monitoring items should be selected as a means of predicting the printing performance of raw materials; How much investment should be made in testing in order to achieve the maximum benefit of printing enterprises

next, we will discuss the printing performance of raw materials, the detection of raw materials, and some related detection indicators, methods and principles

I. printing performance of printing raw materials

(I) aesthetic and functional requirements of printing quality

the design of modern packaging and decoration products is becoming increasingly high-end, paying more attention to the aesthetic requirements of products. For this reason, many high-end materials and new printing processes have been developed to meet the aesthetic requirements. The choice of substrate and ink before printing determines the aesthetic effect of printing product quality. The choice of materials includes two aspects: artistry and technicality of raw materials. The design of printing product designers is always limited by certain conditions. Unlike newspapers and magazines, raw materials have been determined, and the selection of packaging and printing materials should consider not only the aesthetic requirements, but also its structure and physical properties. When the choice of materials is not limited, only the artistry of the product is considered, without considering technical issues. The artistic choice of products is not a problem for printers to consider, but the discussion here involves those technical factors that produce visual effects and serve as artistic elements. From the perspective of raw materials themselves, the technical factors of aesthetics can be divided into three parts: apparent, chemical and physical

1. Apparent factors and their quality requirements

(1) color: the comprehensive selection of ink and substrate should make the color representation domain of key areas of the printing piece as large as possible (especially saturation and brightness). In production practice, sometimes we have to choose from standard four-color inks. The color of the printing material should be neutral, that is, white. But in fact, the human eye can adapt to slightly colored printing materials (such as newspapers) as neutral white. Then the human eye perceives other colors based on such "white"

(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 can reach the maximum. For writing supplies, the gloss requirements of paper and ink are lower. However, there are no definite requirements for the gloss of the paper base and ink of the manuscript and suede photo manuscript

(3) lightness: the lightness of the printing material 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 lightness of black ink should be as low as possible. We usually use optical density to describe the "Brightness" of printing ink. The higher the density, the lower the brightness

(4) opacity: the opacity of printing materials is the ability to prevent the image on the other side of the printing sheet from being reflected. It should be as high as possible, especially for printed materials 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 poster prints, but not for magazines, newspapers and product catalogs

(2) chemical resistance: ink and printing materials should have the ability to withstand acids, alcohols and alkalis. This is very important for labels, packaging and printing, but not for other prints

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 production practice, some of the above 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 cardboard is also very important for cartons. It should be high enough, 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, the smoothness requirements are special. 7. Diameter of sample holder: ф 50mm ф 60mm ф 89mm high. For offset lithography and filter printing, high-quality prints do not need much smoothness. Sometimes, textured paper or non coated paper is specially used to make prints obtain the surface effect of the original or produce texture

(3) texture: the texture of the printing material should be as uniform as possible in order to print a uniform image

(4) quantity and thickness: generally, with the industrialization of paper quantitative graphene, once its output value increases by at least trillion yuan, the translucency of paper also increases, and users generally do not want the weight of printed matter such as magazines to be mailed to increase

(5) wear resistance: the wear resistance of ink should be considered in the design of printing. This indicator should be as high as possible. Especially for those that are prone to friction between prints during transportation, such as cartons or product catalogs

(II) process requirements for image quality

for raw materials, the process factors that affect the quality of printing products refer to those factors that affect the image effect of the substrate. They include dot size, uniformity of coverage, blotting and smudging. This paper analyzes the above influencing factors from the three aspects of appearance, chemistry and texture

1. Apparent factors and their quality requirements

(1) viscosity of ink: multicolor offset printing adopts wet pressing, so the ink viscosity affects the overprint of 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 it will be pulled back

(2) viscosity: for lithography and embossing processes, low viscosity inks are generally not as clear as high viscosity inks. For gravure and flexographic printing, low ink viscosity will reduce the density of printed matter

(3) particle fineness or dispersion: large particles of pigment or other particles in the ink will cause uneven printing image or additional plate wear, which may lead to image damage

(4) pigment concentration: when forming the same ink film thickness, the ink density of low pigment concentration is much lower than that of high pigment concentration, so the thickness of transfer ink film must be increased. However, low pigment concentration inks can form higher gloss than high pigment inks

(5) emulsifying property: in offset lithography, the emulsifying degree of ink affects some other printing quality, such as dot enlargement rate, overprint, drying time, etc. The volume percentage of water in offset printing ink after emulsification shall not exceed 50%. Otherwise, it is difficult to achieve the balance of ink and water

(6) absorbency: highly absorbent paper will have problems such as reflection, low gloss, chalking, etc. On the contrary, low absorption paper will have problems such as long drying time, dirty back or scab

in addition, the sensitivity characteristics of film and plate emulsion affect the resolution of the printing process

2. Chemical properties and quality requirements

(1) pH value: the pH value of the paper should be 7.0, which is neutral, otherwise the drying time will be affected and the back will be sticky and 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. The serious temperature and humidity difference between the workshop and the paper will cause problems such as inaccurate overprint and wrinkling

(3) drying time: the performance of the ink during drying must be comprehensively considered. It must not dry on the rubber roller of the printing machine, but must be quickly fixed on the paper

3. Physical properties and quality requirements

(1) powder and hair loss: powder and hair will cause spots in non graphic parts or defects in graphic parts. The printing sheet must avoid the powder and wool falling off the edge and surface of the paper

(2) paper surface strength: when using high viscosity ink, the paper surface with low surface strength will be roughened or even broken, so the surface strength of the paper is required to be high

(3) ink length: too short ink will produce paper bristles, and too long ink will produce ink mist splash, which will affect the image quality

(III) stability and consistency of quality

the key to evaluating raw materials is to consider the extent to which the performance of raw materials can change when the printing quality does not exceed the required limit. In addition, it is also necessary to consider that this change range should not affect the printing speed or cause other problems in the production process

the properties of the raw materials mentioned above need further testing. If a specific requirement value can be proposed and the control range of the data can be determined, these properties of ink or paper can be measured and controlled. However, it is difficult to determine the control range

the final detection method of raw materials in the printing process is used under the actual printing production conditions. However, this method is only suitable for large paper mills or ink mills, and most printing mills cannot afford this test. Most laboratory tests lack the ability to predict the actual printing production conditions more or less

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Second, the phenomenon of repeated research and repeated investment is relatively prominent. The detection of raw materials

the standard test methods of raw materials in the printing industry are mostly focused on the test of paper mimeograph ink. The literature workers of the International Association of printing research institutes have compiled the printing standards of 15 countries and two organizations into a volume. This publication lists 88 standards or specifications on paper and other printing materials issued by Germany and 66 standards on ink and other related materials issued by Poland. Some standards are about adhesives and other book binding materials, as well as photosensitive materials. But in general, the printing industry does not pay enough attention to materials other than ink and paper

a large number of testing methods, testing instruments and testing images can be evaluated by the printing factory, which means that almost all the properties of paper or ink can be tested to comply with the set standards. The key question now is what detection methods should be used and what should be done in various occasions

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