Offset Overprinting Failure Analysis (1)

Imprinting is not one of the most typical failures in offset printing. Whether it is new or old, domestic or imported, monochrome or multi-color offset presses, these failures have occurred in varying degrees. In view of this, the causes and exclusions of overprint faults will be discussed in detail below.

Accurate printing is the common quality requirement for printing products. Normally people say that overprinting is accurate, but under different printing conditions, different types of printing products, the overprint error does not exceed the specified overprint tolerance, but is used as an offset press operator. To minimize the overprint tolerance. Theory and practice have proved that human eyes can detect normal lines with the ability to recognize subtle lines up to 0.1mm. The tone of print images is generally reproduced by dots. The diameter of each dot is actually less than 0.1mm. Therefore, our standards Overprint tolerance is 0.1mm, generally the high standard overprint tolerance is to be limited to within 0.05mm.

For printing products, overprinting is the first priority. If overprinting is not allowed, the work of an offset printing press will be meaningless. Overprinting errors are common in the offset printing process, and there is no offset printing error for an offset printing press.

Overprint faults are generally classified as "chronic" and "bursty". Chronic failures are manifested in the increase of rejection rate, product quality decline, etc. Sudden failures are manifested as sudden overprinting. No matter whether it is chronic or sudden, it can be solved by layering method.

There are many factors that affect overprinting, about hundreds of reasons, but the real factor that affects overprinting is offset press accuracy and adjustment control technology.

Reasons for Overprinting Misalignment Caused by Improper Adjustment of Offset Press Parts and Measures

(1) Breakage and wear of the cardan shaft or the coupling pin of the main shaft coupling and the main machine coupling. When the paper feeder feeds paper, the paper feed is unstable.

(2) The paper stopper, delivery nozzle, and paper take-up roller do not coordinate. The relationship between these three should be: When the paper-feeding nozzle just sucks the paper, the paper-stopping tongue starts to have a tendency to tilt backwards, the paper-feeding nozzle stops sucking the air, and the paper-feeding roller just presses the delivered paper on the paper-feeding roller. on. If this is not the case, there is no regular rule before the arrival of paper, and the speed of paper feeding is different. The relationship between the three is based on the paper-feed nozzle, and the control cams of the paper stopper and the paper-feeding roller are adjusted respectively.

(3) The feed roller pressure is not adjusted well. If the pressure of the two delivery rollers is different, the paper is skewed. If the lifting time is inconsistent, check whether the clearance of the adjusting screw is the same.

(4) The paper roller (ball) on the paperboard is improperly adjusted, and the paper is unstable, affecting overprinting. Should be adjusted depending on the weight of the paper.

(5) The cord is inconsistent, the paper is skewed, and the paper is affected, resulting in inaccurate overprinting. Retighten the thread.

(6) Before the regulation lifted (backwards) too early and too late. Generally, it will cause the paper to run out of control and the paper feed teeth and the front gauge to grab the paper. This phenomenon often occurs after the front machine of the offset press wears out or the machine overhauls. In this case, adjust according to the standard position specified in the manual.

(7) The paper stopper on the front gauge is not suitable for the clearance of the paper table. Paper does not appear in place, or under the phenomenon, this time should be adjusted in accordance with the relevant provisions of the operating instructions.

(8) The radius error of the front gauge cam is too large. An error in the cam surface of the cam that controls the front-swing oscillation causes a slight movement of the front gauge when the paper is stabilized on the table, resulting in inaccurate overprinting. This is usually the case when the machine is used for a long time or after an overhaul.

(9) The paper-handling teeth are closed prematurely or too late at the tooth table. The paper-handling teeth are closed prematurely at the tooth tray and the paper-grabbing phenomenon occurs. If the closed teeth are too late, the paper is out of control, and the closed-tooth time of the paper-feeding teeth must be performed at the “zero” speed of the paper-feeding teeth. It is not possible to register when the feeding tooth is at rest. For the upper oscillating paper-feeding tooth, the stationary state of the paper-feeding and cutting teeth is the lowest point of the swinging roller of the paper-transmitting tooth on the cam, and the teeth are closed after 2/3 of the time on both sides.

(10) Inconsistent size of brake force on both sides. Relying on the brake is the guarantee of holding the paper to rest on the dental platform. If the size of the brake is not consistent, there will be a twisting of the squeezing shaft of the delivery paper, which can easily cause overprinting and quasi-adjustment. Of course, there are many reasons for inaccurate overprinting and quasi-adjustment, and the inconsistency between the two sides depends on the inconsistent size of the brake force.

In addition, the two sides of the paper-delivery shaft eccentric sleeve do not make the two sides rely on the brake screw at the same time under the top by the brake, then adjust the eccentric sleeve to make it concentric.

(11) The delivery tooth is too long or too short on the dental table. Too long for the paper teeth to stay on the table, it means that the two sides rely on the brake to lift too much. On the contrary, the stay time is too short, that is, the two sides rely on the brake to lift too little. For the J2205 type machine, the top is too much, It not only affects the feeding of paper and scraping teeth on the table, but also greatly increases the impact noise caused by the brake, causing overprinting is not allowed, relying on the brake to raise the time often with the control of the paper transfer teeth swing cam and swing arm with the clearance adjustment of.

(12) The delivery time between the paper feed teeth and the embossing (pass) roller is too long or too short. This refers to the time that the paper handing and the embossing (passing) drum teeth control the paper together. When the two teeth are handed over, they must be synchronized. Otherwise, the transfer will be inaccurate. Generally, if the hand over time is too long, the cam is synchronized. The time is not easy to guarantee, so the handover time can not be too long, it can be stopped. The specific adjustment is as mentioned above.

(13) The transfer paper bumps against the front edge of the impression cylinder. If the dentition of the paper-feeding tooth bumps against the front edge, the sound of bumping can be heard during operation. The rim-mouth is inaccurate. Usually, the oscillating cam of the paper-transmitting tooth is adjusted clockwise. If there is less bump, you can also put the tooth. The film moves backwards a bit.

(14) Feeding teeth Rotate the cam roller to jump radially. The radial runout of the roller is generally a problem of the roller itself, and also the sum of the runout errors of the ball bearing. Therefore, it should be differentiated according to the specific circumstances.

(15) Worn arm pins or sleeves. After the pins and sleeves are worn, they become turbulent and overprinting is not allowed. At this time, the pins or sleeves should be promptly repaired and replaced.

(16) Side pull spring force is too large or too small. The overprint caused by the spring force is inaccurate. The compression spring should be replaced according to the thickness of the paper.

(17) If the lateral pressure platen and the paper table gap are not suitable, the paper may be pulled too far or not in place. Adjust according to operating instructions.

(18) Lateral roller radial runout. If the roller leaps radially, the pressure on the paper is so great that overprinting is not accurate.

(19) The drum axial movement is too large.

(20) The screws tightening the roller gear are loose, or the pins of the gland and hub are loosened. This situation changes the relative operation of the drum body and the drum gear, causing overprinting and irregularity. Check one by one from the transfer drum.

(21) The gripper slips. Reduce the paper force, resulting in inaccurate overprinting. Replace the dental pad or tooth chip.

(22) Grinding tooth height is inconsistent. This situation causes the paper to be uneven and directly affects the overprint accuracy. Replace the dental pad.

The above is the main reason for overprinting due to improper adjustment of printer components.