1. Introduction
No matter which kind of printed matter is produced, it takes several processes to transfer the graphic information on the original to the substrate. Due to the rapid development of printing technology, standardization and quality control have become a very important issue.
In scientific research, measuring and controlling quality is fundamental. In the printing process, the most important thing is to measure, improve and control the printing raw materials, the printing equipment used, the process used and the quality of the finished product. At present, there are a series of national standards consistent with international standards regarding the quality and testing methods of printed products. They require that each raw material used in printing, each printing process and the finished product in each process must be measured or tested according to this standard to determine whether it meets the requirements of technical standards.
It is difficult to control the quality of raw materials in printing. The main raw materials used in printing are paper and ink. As far as paper is concerned, there are no less than twenty experiments on the characteristics of paper printability and print quality, but no single instrument can measure the comprehensive quality of printed paper. The same is true for ink experiments. There are also some problems with the quality control of printed products. Many manuscripts will have different requirements in printing due to their different uses. In many cases, technical standards are developed by users in consultation with printers or sellers. If there is not an instrument and method approved by both parties to appraise this standard, this technical standard cannot be discussed.
At present, there are three methods for evaluating the quality of printing quality: subjective evaluation method, objective evaluation method and comprehensive evaluation method. The subjective evaluation method is based on the original manuscript against the proofs to evaluate. The artificial factors here, due to the different knowledge, skills, experience, hobbies and aesthetic viewpoints of the appraiser, will produce evaluation bias to a large extent. The objective evaluation method is to make a quantitative analysis of each quality link of the printed matter through special instruments and tools, and make an objective evaluation in combination with the copied quality standards. The comprehensive evaluation method combines subjective evaluation and objective evaluation to evaluate the printing quality. At present, this seems to be the best evaluation method.
Quality control is always an indispensable part of the printing process. Experience and superb skills were once the sole basis for quality control in the printing process. For example, the accuracy of ink viscosity or color is determined by sight and touch. Although the eyes are very sensitive to the difference between color and density, it cannot objectively, accurately, and quantitatively reflect the quality. Only by using special instruments and tools to test the basic performance of various raw materials and monitor the quality of the printed matter, can we produce stable and high-quality products.
Second, conventional control
An important part of printing quality management is routine control. The routine control of printing quality refers to the control work that the printing production personnel must directly engage in. The routine control of printing quality is mainly carried out in the printing workshop, and the implementation of the first-line printing operators is the routine control of one item or several items. It is actually an operating specification, which should normally include preparation for the early stage of printing, control of the printing process, and inspection of the finished product.
1. Pre-print preparation
The inspection of the graphic quality in the early stage of printing is the inspection of the graphic content. It needs to check the construction order and the proofs, and also check the images, text, symbols, lines, etc. for errors or defects. This kind of inspection is relatively simple, as long as the thickness is up to standard and smooth.
The second is the inspection of the image quality. The quality of the image on the printing plate is the basis of the printing quality. Without high-quality images on the printing plate, there would be no high-quality prints. The inspection of image quality is carried out by inspection of the dots. Because whether it is a monochrome continuous adjustment original or a color continuous adjustment original, the printed image is expressed through a halftone dot.
The content of the inspection is divided into two aspects.
One is the inspection of image imprints. Compare the corresponding parts of the monochromatic proofs and the printing plate by visual inspection, and observe whether the density of the printed product is consistent. In addition to the overall observation, you can also select the key area for observation. Use a magnifying glass to observe whether the dot size is consistent. The key areas should be selected in the deepest tone, the lightest tone, and the most important part of the image content. After printing, the color tone of the main part is suitable for the requirements, and the printed matter will not have a large deviation. Due to the spread of ink in printing and the double reflection of light, the printing dots must be expanded to a certain extent. Therefore, the halftone dots on the printing plate should be slightly smaller than the halftone dots on the monochrome proof, and must not be larger than the halftone dots on the proof. In addition, you can also use the measurement and control strip to check the depth of the printing plate.
The second is the quality inspection of outlets. The quality of the dots is an important guarantee for image quality. With high-quality outlets, there will be high-quality reproduction and high-quality color reproduction. The dots on the printing plate should be solid, the edges should be clear, and the surroundings should be smooth. The dots have false hairs at the edges, and the ink absorption performance will decrease during printing, and the dots themselves will be unstable. Usually use a magnifying glass with a magnification of 10 times or higher to check the formation of dots.
2. Control of printing process
In the lithographic printing process, paper, fountain solution, ink and printing plate are in contact with each other, and there are many changes. A little carelessness will cause uneven ink color and other disadvantages. This has a great impact on printing quality.
Before the printing machine is officially turned on, it must undergo trial printing and color correction to make the printed color consistent with the sample color. After signing the sample, the official printing can start. The signing work of the monochrome machine is more important, it is related to whether the color of the finished image can be faithfully restored. In actual production, although many products have been checked by the third-level signing, a considerable part of the finished images have color shift due to inaccurate signing. This requires the operator, especially the operator of the monochromator, to be able to use the control strip and other measuring instruments to accurately determine the color deviation after each monochromatic printing.
The printing process is completed in the process of high-speed operation, so the control of water-ink balance will be very critical. And practical experience also tells us that striving to achieve "three diligence" in printing production, that is, diligently sharing ink, diligently looking at water and diligently inspecting samples, is the most effective quality control measure in the printing process. Among them, "Let's check samples" item L requires diligently checking the quality of the sample during the printing process. During the printing process, the operator must often take out the printed sheet for inspection. It includes the inspection of the accuracy of the printing position and the inspection of the quality of the printed image. These two items.
3. Inspection of printed products
Every printing shop or printing factory will have a post for finished product inspection. The main contents of the printed product inspection are as follows:
One is to check the color and register of printed products by using color marks and rule lines.
The second is to judge the overall quality of the printed products (including color, gradation, registration, surface condition and dryness, etc.). Mark the defective products (foldings or clips, etc.) and draw them out in time. In addition, we must pay attention to the incorporation of other printed materials.
The third is the counting check, that is, checking whether the printed sheets are enough, usually a paper label is put every 500 sheets.
The routine control of printing quality is a very complicated, detailed and important task in the printing workshop. It directly affects the quality of printing products, affects the image of printing companies, and even affects the survival and development of printing companies. Therefore, the printing shop must persevere in this, and always work hard.
3. Conclusion
The digitalization of printing quality control means that the necessary control factors for printing quality are measured and recorded in the form of data through the necessary test methods as the basis for quality control. Standardization is to synthesize a series of laws that can guide production through a large amount of data, such as formulas, curves, charts, data, etc., so that there are rules to follow and rules to follow. At the same time, there are three standards for measuring the degree of data and standardization of printing quality control: first, good stability, second, good repeatability, and third, high efficiency.
Looking at the current status of China's printing industry, the data quality and standardization of printing quality control are still far from the international developed countries. Therefore, how to improve and strengthen the digitalization and standardization of printing quality control technically and administratively is an inescapable responsibility of the majority of printing workers, and will also be a key to whether printing companies can survive in the fierce competition in the future.
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