Monochromatic machine multi-color overprinting failure analysis

Overprint accuracy is an important indicator for evaluating the quality of printed products. Among them, the overlay accuracy is affected by various factors such as the precision of the machine, the performance of the raw materials, and the technical level of the operator. In order to control the accuracy of overprinting and produce fine prints, many printing companies have introduced four-color or four-color multi-color printing presses. However, because of its own strength and other reasons, the main equipment used in our factory is still the domestic J2108 monochrome offset press. How to use and make good use of the existing equipment, to print well each batch of color printing products, to return customers with qualified printing products, and win credibility, is the pursuit of the printing plant in our factory.
Misregistration is one of the common faults in offset printing, and most offset prints are multicolor overprinted. In the process of multi-color printing, all parts of the paper feeding system and the printing system have precise time control and strict position restrictions, so that the printed sheets can reach the end of the delivery period from the paper feeding end, and finally the printed matter with uniform graphics and specifications can be obtained. When the paper is being transported and imprinted, the failure or misadjustment of any of the control components can cause misprints. Misregistration is generally not allowed due to overprinting in the longitudinal direction, inaccurate horizontal overprinting, and inaccurate vertical and horizontal misregistration. Therefore, if overprinting is found to be inaccurate during printing, it is necessary to stop the machine decisively, check it in time and take corresponding measures to eliminate it. The author combines many years of production practices, carefully analyzed and summarized, and now summarizes some of the causes of failures and their solutions as follows for reference by peers.

The overall overprint is not allowed

1. Inaccurate positioning of rules results in inaccurate overprinting. Solution: Carefully check the working conditions of the front and side rules, such as the front rules, side rules, and what time to position the paper, adjust it according to the requirements of the specification, and pay attention to the rules and regulations. The height of the side gauge should meet the requirements.

2. Incorrect paper-pulling caused by misfed paper (1) Dizzling when paper-feeding paper is pulled, causing unsettled paper solution. Solution: Adjust the paper-passing stopper to keep it stationary on the paper-feeding plate, and the transition is gentle. .
(2) Axial movement when paper is pulled by the paper-feeding solution: Adjust the axial lock nut to ensure that it has a turbulence of 0.03mm.
(3) Insufficient strength of paper-feeding teeth, slippage of paper solution: Increase the force of paper-feeding teeth, especially at high-speed printing (mainly the inertial effect of paper).
(4) The opening and closing time of the paper-handling teeth is inaccurate. Solution: Re-adjust the opening and closing time of the paper-feeding teeth.

3. Mismatching between paper handing and rules, resulting in incorrect overprinting. (1) When the paper hand has not been taken, the rules have begun to put paper solution: adjust the rules so that the paper handing and the rules have a common paper holding time.
(2) The rules have not yet completed the positioning, and the paper-feeding teeth begin to pick up. Solution: Same as 3(1).
(3) Delivery Paper When the paper leaves the feeding board, the rules have not been released. Solution: Adjust the rules so that the paper is discharged in advance.
(4) The height of the upper-end delivery paper tooth pad is not suitable. Consequences and solutions: too low, it is easy to cause impact on the paper, making it shift; too high, the paper will be pulled up when closing the teeth, the paper's Location will change. The height of the general delivery pad should be the same as the front gauge, plus 0.2mm for the paper thickness.

4. Improper fit between the paper feed teeth and the impression cylinder, resulting in overprinting is not allowed (1) The transfer time is too short Solution: Adjust the position of the corresponding open and closed teeth cam.
(2) Improper distance between tooth pads Solution: Re-adjust the height of their respective tooth pads to a distance of 0.2 mm from the thickness of the paper.

5. Insufficient enamel force of the impression cylinder, the paper sliding in the tusks, resulting in overprinting is not allowed (1) The pressure of the dental piece is too small. Solution: Increase the pressure of the dental piece.
(2) The friction coefficient of the dental pad is too small Solution: Replace the dental pad.

6. After the transfer roller, the buckling force of the teeth is insufficient, and the paper slides in the tusks. Solution: Same as 5.

7. Axial tamping of the roller caused misalignment Solution: Replace the pad.

8. Inadequate overprinting caused by other reasons (1) The viscosity of the ink is too large Solution: Add an ink or retreat agent to reduce the viscosity of the ink.
(2) The surface of the blanket is sticky. Solution: Replace the blanket.
(3) Printing pressure is too large Solution: Adjust printing pressure.

Local overprinting is not allowed

　　 Local misprinting refers to the relative slipping of the local position of the paper during the printing process.
1. When there are only two front gauges, when the paper arrives at the front gauge, the middle part protrudes forward. The softer the paper is, the worse the overprint is, the more serious the problem is. Solution: Reduce the speed of paper feeding or increase the number of front gauges, so that the front of the paper is in order Accurate positioning.
2. The lack of strength of the individual teeth on the paper handing tooth causes the position of the front mouth of the paper to change at this location, resulting in a solution that cannot be overprinted: replacing the tooth pad or increasing the force of individual tooth decay.
3. The amount of paper feeding the paper teeth is not uniform. On the one hand, there are many sides, and there is a relative slippage in the parts where the amount of paper is small. This causes the overprinting to be inaccurate. The solution is to adjust the front gauge so that the amount of paper is even.
4. Insufficient tamping force of the individual impression teeth in the impression cylinder causes the overprint to be inaccurate. Solution: Replace the dental pad or increase the force of the individual tooth.
5. Excessive partial force, resulting in increased resistance to paper advancement, causing the paper to slide in place, resulting in overprinting inaccurate solution: check the blanket and liner, and recalibrate the printing pressure.
6. The paper swells with water, causing the second half of the paper to come out of the corner. Solution: The paper is dampened before printing.

Inaccurate overprint due to platemaking

1. Imposition accuracy is not high, resulting in inaccurate overprinting

Solution: The following word line or corner line is used as a reference line when printing, and the same applies when imposition. If the position of the graphic or rule line is not properly matched, it is difficult to achieve overprinting accuracy when printing. Therefore, it is necessary to maximize the imposition accuracy, and it is particularly necessary for the imposition staff to use the high magnification magnifier.

2. The number of prints is too large, resulting in inaccurate registration

Only once in the sun, few overprints are allowed, and multiple exposures are prone to occur. If you must repeatedly print, you must do a partitioned exposure on the stencil sheet.

3. When the plate is deformed when the plate is pulled, the overprint cannot be solved. The solution is that when the plate is subjected to a large force, it is prone to deformation or cracks. When drawing a plate, it should not be used too hard. If you do not pull it, look it up carefully.

4. The problem of craftsmanship is not allowed.

(1) Misalignment of multi-colored jobs. Solution: The higher the number of overprints, the greater the possibility of errors. Therefore, the number of overprints should be minimized.
(2) Overprinting of multi-colored overprints or lines appear to be overprinted. Solution: Try to use spot colors instead of overprinting. It is also possible to use reasonable processes to reduce the number of overprints and minimize the possibility of error.

Overprint inspection during printing

　　 Accurate printing is the basic requirement for the quality of printed products, so the content is essential for every inspection.
(1) Check the situation of the registration of the regulatory line and check the registration of the inner chart. Some small-scale impositions of printed products, there is a hollow overprinted prints, or paper, there are local deformation, there will be a rule line has been registered and the case is not allowed to set the situation. At this time, according to the situation in the map, readjust the rules.
(2) When printing the first color, according to the requirements of printing and cutting, check whether the position of the whole graphic on the printed sheet is correct, and often pick a stack of printed sheets, open them after checking and check whether the gauge line is in a straight line. .
(3) For the prints printed on the front and back, check the registration on both sides.
(4) During normal printing, the high speed machine can generally be tested once every 500 samples. When taking a sample, pay attention to the pinch of the finger in the blank area of ​​the printed sheet and take it out quickly and smoothly. The inspection will be conducted in two parts. The first inspection: take out the proofs and place them on the sample stage to do a comprehensive inspection. If there is a big line in the gauge line and if it is in a straight line, whether the paper is wrinkled or not. Fine inspection: Magnifier can be used to check the accuracy of the gauge and the overprint of the inner graph.
(5) During the printing process, ensure that the machine runs at a constant speed. Otherwise, it is easy to cause the paper to have different magnitudes of impact at the front gauge, so that the paper cannot be positioned correctly.