Discussion on the failure of printing ghost

Ghosting is a common printing failure, with the three major failures of overprinting and taping together as offset printing.

Ghosting refers to the appearance of double footprints on imprints. In the printing process, ghosting generally has three forms: axial ghosting, circumferential ghosting, and irregular ghosting. The danger of ghosting damage to printed matter is well-known, and it will lead to the entire picture of the printed product being blurred and distorted. Therefore, in offset printing production, measures must be taken to avoid the occurrence of ghosting failures. The causes of offset ghosting are process operations, printed materials, and equipment adjustments. This article mainly analyzes the ghosting caused by the printing equipment.

1. Ghost image caused by presses

We know that paper is choked by the first tooth, and it must be transferred several times after it is printed out. If there is a shift during the transfer process, wrinkles, ghosting, rejection of corners, overprinting, etc. will occur. . When printing with a multicolor machine, ghosting failures are often associated with overprinting. When the impression force of the impression roller is not uniform, the paper is deformed, so that the previous color print on the paper cannot be accurately overlapped with the ink on the rear color blanket, causing ghost; and the force when the transfer cylinder is buckling teeth When there is not enough inconsistency or lack of strength, the paper will shift during the transfer process, which will also result in overprinting and ghosting. The majority of dental caries problems are caused by wear of dental pads and dental discs, unevenness, and fatigue failure of dental springs. Nowadays, many manufacturers have designed a “netted” surface on the tooth chip and tooth pad in order to increase the strength of tooth decay. For example, the tooth pattern and tooth pad twill of Heidelberg, the sandblasting tooth pad of manroland, and the high Bao's rubber-containing dental pads and so on, their materials are quite good, but after a period of use will also be due to wear and failure. Once the printing machine appears the above situation, it must pay attention, because in this case it is impossible to print a good product.

If there is a ghost image or a non-ghost image, you need to consider the structure of the machine. Today's multi-color machines use double-diameter impression cylinders and transfer cylinders, such as Komori, Korbor, Manroland, etc., which occur due to the inconsistent or abrasive wear of two teeth on one cylinder. Different due to. In Heidelberg's CP 2000 and CD 102, the transfer roller is a triple-diameter triangular air-pass transfer roller. If such printing presses are overprinted or ghosted, this may be due to the impression of the impression roller; If there are two ghosts or three ghosts in each of the two, you need to check the transfer roller.

In addition to the ghosting caused by the above reasons, the turbulence of the dental shaft can also cause ghosting. For example, a domestic four-color machine in our factory had a ghosting phenomenon after three years of use. The first color group was particularly bad, and even the black version had to be placed in the fourth color group, putting the yellow version forward. Affect the quality of the product. After inspection, the replacement of the pad, the tooth chip, and cleaning, adjusting, tightening the drum still cannot solve the problem. Finally, it is suspected that the double-diameter impression roller shaft is loose. After checking the tooth axis with a short crowbar, it was found that the axial turbulence of the tooth axis was as much as 0.23mm, and almost every group had it. Finally, after taking measures such as dismantling the tooth shaft, inserting sleeves, and placing washers, ghosting disappeared and overprinting was more accurate than before.

2. Grinding caused by uneven sheet feeder

As we all know, multi-color machine printing is wet and wet printing, and the first color is mostly black, because the black version of the dot area is small, the ink viscosity is high. However, this color arrangement has put strict requirements on overprinting. For example, a Heidelberg 102V four-color printing press, which was put into operation in 1993 by our factory, suffered from black version ghosting and overprinting failures in 1999, especially on both sides of the paper. After replacing parts such as cavities, gum pads, open teeth, and bearings, there are still black ghosts. Later, we suspected that there was a problem with the transfer board. After careful examination, it was found that the paper conveying plate was low in the middle and high on both sides. Originally, we used to print products with a format of 787mm x 542mm. The middle of the transfer platen was worn, but the two sides were relatively flat. Now, when we print a large number of products with a format of 880mm x 620mm, we deliver the paper to the handle. When the paper is choked to the internal pressure of the impression cylinder, the two sides of the paper are bowed because of the unevenness of the paper feed table. After the first color is printed and then transferred to the transfer cylinder, the paper is flattened again. One-color and second-color blots do not overlap accurately and cause ghosting. After we replaced the transfer board, the problem was solved.

3. Ghost caused by roller bearing damage

This kind of ghosting phenomenon is generally a circumferential ghosting, and in our factory, it mostly occurs on offset printing presses and flat offset printing presses, especially offset printing rotary presses. High-speed web presses have printing speeds of more than 25,000 prints/hour. The high speeds of the machines place high demands on the bearings, bushings and lubrication of the drums. Although most of these machines are equipped with double bearings and double bushings, if the bushes wear out and the bearings do not fit well, or if the bearing wears loose and causes the drum to shake or vibrate during high-speed operation, it will inevitably cause ghost images. This situation has been common in our factory. The solution is to replace worn bearings or bushings, and reinstall, inspect and adjust the printing pressure (note that the oil circuit is unblocked). However, it does not necessarily solve the problem of ghosting. We have found that the gap between the roller bushing and the wall hole increases with the age of the engine. Some of them are 0.03mm, and some have reached 0.07mm or even larger. When this clearance is large enough, the bearing of the printing equipment will jump in the sleeve, and the sleeve will shake in the hole of the wall. In this case, how can the product printed by the machine not be ghosted? In this regard, our solution is to insert the wall hole and insert it to ensure the accuracy of the bushing in the wall hole. Of course, the difficulty and workload of doing so is relatively large, and there is no certain technical force and processing equipment is not feasible, but this method is feasible and effective.

4. Ghost caused by wear of roller gear

We can judge the grade of the machine from the material and processing precision of the press roller gear. Roller gears are made of durable materials. High-precision roller gears can not only print good products, but also avoid many printing failures. These products have strong prints, good dot reproduction, accurate registration, no striations, and no ghosting. However, we must see that we still have a part of the printing machine's roller gear in wear, especially some older printing equipment. In the normal center distance of the printing machine, the gear side clearance of the drum should be controlled within 0.15mm, otherwise it will cause the drum vibration, especially when the printing speed is not stable, it is especially easy to cause graphic ghost phenomenon, and this kind of weight The shadow is mostly circumferential. Therefore, if you encounter such a situation, one must check the center distance of the roller and the printing pressure, and the second is to check the wear of the gear. The change must be changed so as not to affect the product quality.

In short, ghosting is a common fault in offset printing production. The mechanical causes of ghosting are not only those mentioned above, but also due to various reasons such as roller tumbling, ink roller, and clutch pressure. We will continue in future articles. Discuss with everyone.


Author: Gordon Lang
Source: Guangxi National Printing Factory