Proofer quality control and maintenance

Proofing is a very important process in the prepress plate making process. Proofing can be used to check the quality of the previous steps. Proofing proofs at the same time is also a standard for color tracking of large numbers of prints. Therefore, proofing has played an important role in the quality control and control of various platemaking companies.

There are many factors that affect the quality of proofing, such as: imposition, printing, development, ink, wipe water, field density, dot gain, maintenance and maintenance of proofer, temperature and humidity of the environment and so on. Whichever of these factors occurs, it will have a great impact on the final proofing results. Therefore, only the standardized and data-oriented management and operation can make the copy and restore of the outlets meet the requirements in the proofing process, and provide qualified follow-up proofs for large-volume printing. Now I will discuss with readers how to effectively control the main factors and related data in the proofing process.
First, make, print and develop

Proofing imposition and printing imposition are very different. The printed layout must be folded in the order of folding according to the binding method, and the crosshairs and corners of some films should be cut off. Therefore, the imposition staff can only easily follow the edge of the image on the film or the lines and color blocks. Debate places and aligns. Proofing the imposition does not need to worry about the problem of hand-folding. Crosses and corners on the film will also be preserved. When imposition, cross lines and corner lines of colored films can be aligned. In general, the color sequence of proofing determines the color sequence of the imposition. Because it can improve work efficiency. In the fight for the first color, the film should be placed horizontally, and it should not be tilted. The four corners should be glued with transparent tape. The other three color plates are aligned on this basis. The standard of registration is that the cross and corner lines of the upper and lower films cannot be seen by the eyes.

The control of the printing and development is achieved through the UGRA control bar. The UGRA control strip has five functional segments, each with its own distinct role. as the picture shows:

Paragraph I: Used to control the development of the printing plate, it is also possible to evaluate the exposure of the printing plate. Eligible for the grade to begin with the third grade, ninth grade began to distinguish the grade.

Section II: Microcoils are used to control the exposure time. Passed to start clearly recognizing microcoils at 12 μm.

Section III: The network segment is divided into 10 blocks from 10% to 100% on the ground. It is used to check the exposure and development, and is also used as the standard for printing characteristic curve measurement. Qualified for 2% outlets, 3% outlets are clear.

Section IV: Fine mesh lines along different angles can be checked for dot distortion and ghosting.

Paragraph V: Fine dot plots are 0.5% to 5% for positive pictures and 95% to 99.5% for negative pictures, used to check whether or not the dots are reduced during the proofing process.

Although the UGRA control bar can be used to control the exposure and development more precisely, it is still necessary to make adjustments based on experience in some subtle areas to optimize the reproduction and restoration of dots. For example, the adjustment of the pumping time for different thicknesses of the plate, the adjustment of the temperature and speed of the developing machine, and the cleaning and maintenance of the printing plate glass, and the like.

Second, proofing

Proofers and presses are very different. The printing press uses a "circular press" printing method, while the proofer uses a "circular press" printing method. In addition, the degree of automation of ink and register printing presses is very high. Most of the proofing machines are adjusted by the operator according to personal experience, and the color version of the registration is also performed manually. Although during the proofing process, the operator must rely on the density meter to continuously measure the density of the spot and the dot density and dot gain at 80% to adjust the ink volume and the ink balance, but the process of inking must still use ink scoops. Scoop the ink onto the proofer roller. Therefore, the quality of proofing is closely related to the level of the operator.

The field density value is the most important technical parameter for control network copy and restoration. Different companies have their own different technical indicators and error ranges. This technical indicator is based on the company's specific circumstances. The solid density of each color version is Y: 1.30, M: 1.45, C: 1.50, K: 1.80. As for the margin of error, the European standard is ±0.07. It is said that the error range of a domestic company is controlled within ±0.05.

As with printing, the control of ink and water balance is also very important. The principle of printing is essentially the principle of oil and water incompatibility: the dots on the PS plate that are exposed to form images are not hydrophilic, and the unexposed blanks are not hydrophilic. Therefore, it is important to maintain a good ink balance. If the water is rubbed too little, it will result in a dry version, and reflecting on the proofs will be the level and level of the dark tone; if the water is rubbed too much, the dots will be deformed and the dots on the printed sheets will appear as “hollow dots”. So, how much should we wipe the water? In general, the less water you rub, the better. The mastery is that when water rolls over, there is no visible water on the platform.

The technical parameters of the fountain solution are often overlooked factors in the quality control of proofing. The technical parameters of fountain solution are mainly conductivity and pH. Their standards are: the conductivity is 800-1200, and the pH is 4.8-5.2.

The quality control of proofing also involves adjustments to the proofer. The main contents of the adjustment are the following:

1 The pressure of the blanket cylinder is ±0.03mm (measured once per rotation 120°, averaged), ie blanket thickness + liner thickness = (2.286 ± 0.03) mm.

The platen pressure (including the thickness of the PS plate) in the 2nd version is +0.12mm to +0.15mm, that is, the surface of the PS plate is higher than the height of the guide rail.

3 paper table pressure (including paper thickness) is +0.15mm ~ +0.18mm, that is, the surface of the paper is higher than the height of the guide rail.

4 There are two rollers on each side of the active rack. When the gantry is moved to the side of the ink distribution system and the machine is not pressurized at the same time, the pressure on the roller and the lower rail surface can be rotated just by hand. Too tight and too loose must be adjusted

5 even ink system: Ink roller roller width on the platen and embossing ink bar width on the paper table should be uniform.

Third, the proofing machine maintenance and maintenance

In the process of proofing, the operation of the proofing machine is the most important. Of course, what kind of working condition the proofer is in is also very important. Only by making the proofing machine in the best working condition can we ensure that the outlets maintain good quality during the delivery process, and finally obtain qualified proofs. Since the maintenance and maintenance of the proofer is a continuous process, it is necessary to establish a set of practical maintenance procedures and make it standardized and standardized.

1 ink roller maintenance

The dot change of the proofs will deteriorate with the hardening of the proofer's ink roller. The hardening of the ink roller means that the roller will be used for a long time and its surface will become light and hard until a "crystal layer" is formed. When the surface of the roller crystallizes severely, its ink transfer performance is affected, and the ink balance during the proofing process is difficult to maintain. Especially for magenta inks, due to the decrease of water absorption, hollow spots will appear. In this way, the dot area of ​​the sample will be smaller than the dot area on the film, and the color will become lighter. That is, saturation decreases. In some examples, when the color dots are changed, the color tone of the proof sheet will deviate from the expected effect, and the tone variation of the multicolor overprint will be greater. This shows how important it is for the maintenance of the ink roller. In addition to daily fixed weekly cleaning procedures, there are specific maintenance measures. That is, every two weeks, remove the ink roller from the proofer and take a rest. Replace the backup ink roller. In this way, the ink roller can maintain a good working condition, so that a satisfactory dot reduction quality can be obtained.

2 blanket maintenance

Blanket is the medium that transfers dots from printing plate to paper. Its performance directly affects the delivery quality of the outlets. After a long time, the blanket will have chemical residues attached to its surface, making the elasticity of the blanket smaller. This will make the edges of the dots that are transferred to the paper not sharp enough. Therefore, after the blanket has worked for a period of time, it must be allowed to “rest” so as not to fatigue it.

3 proofer's own daily maintenance and maintenance

The proofer is generally composed of a mechanical part and an electrical part. For the mechanical part, it is necessary to regularly add lubricants to the fixed oil holes and the transmission parts of the surface (such as the chain) to avoid excessive wear; for the electrical part, it is mainly to check and ensure the smoothness of the gas path, and to maintain The surface of the panel is cleaned and so on. During the proofing process, the oil black on the roller and the blanket should be cleaned after replacing the ink, and other colors of ink should be added later. Otherwise, the residual ink will be mixed with the added ink and affect the proofing quality. In addition, dust and paper powder near the platen and rules must also be erased on time, because excessive accumulation can affect the accuracy of registration.

Fourth, environmental factors

The influence of environmental factors on the quality of proofing is mainly temperature and relative humidity. If the temperature is too high or too low, the water on the PS plate will evaporate too quickly or too slowly, thus destroying the ink balance. If the relative humidity of the environment is too large or too small, the balance of water and ink will be damaged, and even ink emulsification will lead to the generation of hollow dots. In addition, the more serious is the instability of temperature and humidity will cause the paper to be deformed, resulting in decreased registration accuracy. Therefore, it is very important to maintain constant temperature and humidity in the proofing workshop. In general, the ambient temperature should be 22 °C ~ 23 °C, relative humidity of about 50% of the best.

V. Conclusion

There are many factors that need to be controlled to improve the quality of proofing. The above is only a brief analysis and introduction of some major control methods and empirical data. There are many subtle factors that are not listed one by one. In the proofing process, a small mistake can often lead to the production of scrap. Therefore, a complete and practical management system and operating procedures should be developed for the specific situation of the company or the department, so that the proofing can be on a standardized and data-oriented path. Only in this way can the quality of proofing be stabilized on a high level basis.

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