On the most popular overprint error

2022-09-23
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On the overprint inaccuracy fault (Part 1)

overprint inaccuracy is one of the most common faults in offset lithography. There are various reasons for overprint inaccuracy fault. Below, the author makes a simple analysis and introduction on some methods and experience summarized in long-term work practice

overprint inaccuracy can be judged directly with the printing sheet. Generally, there are regular lines such as overprint cross and corner lines on the printing sheet. By comparing these regular lines, we can judge whether the overprint is accurate

there are two main reasons for inaccurate overprint: one is caused by materials; One is caused by printing machinery and operation

overprint inaccuracy caused by materials

water is involved in the offset printing process, and the paper has strong water absorption capacity. The expansion and deformation of the paper will affect the overprint accuracy. Due to external environmental reasons or local excessive water delivery of the machine, local heating or moisture will cause local expansion and deformation, resulting in inaccurate overprint

the film also has the characteristics similar to that of paper, which stretches after moisture and shrinks when dry. The printing plate is fastened on the printing plate cylinder with a plate clamp. During the calibration, the PS plate will produce tensile deformation due to the tension, forming a cylindrical shape from the original plane, resulting in bending deformation. In order to improve the printing resistance and baking treatment, if the baking time is too long or the temperature is uneven, the printing plate is also easy to stretch and deform. The above deformations are easy to cause inaccurate overprint

Troubleshooting: for the inaccurate overprint caused by the expansion and deformation of the paper, adjust the humidity of the paper before printing. The temperature of the printing workshop is controlled at 20 ~ 30 ℃ and the humidity is controlled at 0 ~ 60%. Close the doors and windows to prevent the workshop from convection with the outside air. The printed semi-finished products are sealed with paper or plastic film and isolated from the air

maintain the stability of the print film and printing plate, and control the temperature and humidity of the plate room. When installing and correcting the printing plate, we should master the correct drawing skills, prevent hard drawing and hard knocking, reduce the number of drawing plates, and try to avoid abnormal deformation

overprint inaccuracy caused by printing machinery and operation

overprint inaccuracy caused by printing machinery and operation mainly refers to overprint inaccuracy caused by improper adjustment of paper feeding parts and positioning parts and improper cylinder lining

1. Analysis of inaccurate overprinting caused by improper paper feeding

improper adjustment of paper feeding part and positioning part will make the position of printing plate text and text cannot be accurately transferred to the paper. In sheet fed offset printing, from the stack of the paper feeding table to the paper teeth of the imprint cylinder, any improper adjustment of any part will directly affect the accuracy of overprint. The main factors causing the misprint of the paper feeding parts are as follows

(1) common faults in the paper separation part

① relative to the paper pressing wheel, the flying paper feeding time is too early or too late

Troubleshooting: because the paper feeding time of the offset press is adjusted based on the paper pressing wheel. Turn on the air pump drive paper feeding Feida. When the paper feeding suction nozzle stops feeding the paper and the suction has not been completely stopped, the two paper pressing wheels on the swing shaft of the paper pressing wheel should fall on the paper feeding knurled shaft at the same time. At this time, the transmission cam of the swing shaft of the paper pressing wheel on the paper feeding table should be at the lowest point, and there is about 0.5% between the roller on the swing rod and the transmission cam 10mm gap to ensure smooth paper receiving. Adjust the relative position of the sprocket in the paper feeder Feida transmission box to adjust the paper feeding time of the paper feeder Feida

② the relative height of the presser foot and the paper separation suction nozzle is too small or the height of the paper stack is lower than the front paper stop tongue, and the conveyed paper is blocked by the front paper stop tongue, resulting in slow paper walking. Presser foot, paper separation suction nozzle and presser foot to the separated paper will also cause paper feeding failure

Troubleshooting: when the paper is flat, the fine-tuning Feida lifting screw can be charged at the same time, and the paper feeding Feida can be appropriately raised to make the paper stack 5 ~ 10mm lower than the paper stop tongue. When the paper is uneven. The presser foot is easier to step on the separated paper. At this time, pad the paper with paper tape or wedge. But don't forget to take out the paper tape or wedge in the printing process, so as not to crush the blanket and printing plate, causing quality accidents. When using paper tape and wedging, the eccentric cam of the presser foot and the low-pressure foot can be adjusted appropriately; Increase the height of the paper separation suction nozzle and increase the distance between the presser foot and the paper separation suction nozzle

③ the rubber band of the paper separation suction nozzle and the paper delivery suction nozzle is too often added, which leads to the whitening phenomenon caused by the bending and stretching of the product. It is small or too thin, and the suction is insufficient. The loose paper blowing nozzle and presser foot blowing wind are too strong, which leads to the paper can not be sent out normally

Troubleshooting: appropriately increase the suction capacity of the paper separation suction nozzle and the paper delivery suction nozzle, and replace the rubber bands of the paper separation suction nozzle and the paper delivery suction nozzle according to the thickness of the paper. Properly reduce the blowing volume of the loose paper blowing nozzle and presser foot blowing. Regularly clean and dredge the air path, fly the paper to the suction chamber and air pipe, and regularly maintain the air pump to ensure that the air path is smooth

④ improper coordination between the paper feeding time of the paper feeder and the falling time of the front gauge causes the paper to lead or lag behind the positioning time of the front gauge

Troubleshooting: the matching time should be about 3 ~ 5mm from the front gauge to the position of the paper feeding table, and the position of the front edge of the paper on the paper feeding board should be about 3 ~ 5mm from the front gauge. Beiren series models can adjust the matching time of relative movement by adjusting the method blue plate under the transmission surface of the paperboard conveyor; Medium range models can adjust the handover time between the cosmetic paper conveyor and the front gauge by changing the gear meshing relationship in the transmission box of the paper conveyor

⑤ the tension of the wire belt is too small or the pressure of the paper pressing wheel is too light, the paper pressing wheel is seriously worn, and the pressure on both sides of the paper pressing wheel is asymmetric, resulting in the paper not reaching the front gauge when positioning the side gauge

Troubleshooting: loosen the tension line pulley, check whether the tension wheel and tension shaft can operate freely compared with foreign products, and then evenly tension each line belt, appropriately increase the pressure of the paper pressing wheel, so that the pressure of the paper pressing wheels on both sides is balanced, and replace the paper pressing wheel with serious wear in time

⑥ the cardan ball on the paper feeding table and the paper feeding Feida drive shaft is seriously worn, and the cone pin is broken, resulting in the good and bad of the printing sleeve on time

Troubleshooting: add oil to the cardan ball many times a day. The cardan ball with serious wear should be replaced in time. When replacing the broken taper pin, do not leave too long at both ends to avoid injury

when dealing with the inaccurate overprinting caused by poor paper feeding, attention should be paid to strengthening the maintenance of machinery and equipment at ordinary times, refueling frequently, cleaning the Feida suction chamber and air circuit in time, maintaining the air pump in time, maintaining the constant temperature and humidity of the printing workshop, and improving the ability of analyzing and troubleshooting

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