What to do if registration is inaccurate? Pay attention to avoiding these two points!

Misregistration is one of the common faults in the gravure printing process, especially when printing products with high registration accuracy requirements on multi-color high-speed gravure presses. The misregistration problem becomes particularly noticeable and is one of the key factors affecting product quality and yield.

There are various factors that cause misregistration, such as material factors; equipment factors, such as faults in the web guide system or tension control system, registration signal faults or registration drive control board faults, damage to bearings of rollers in the paper path such as feed rollers, impression rollers, guide rollers, or cooling rollers, faults in the plate drive motor, or severe wear of drive gears; improper operation, such as incorrect parameter settings or insufficient equipment adjustments.

The author now takes an example of a misregistration problem encountered during the company's previous production process to analyze each key factor causing misregistration, eliminate numerous interfering factors, identify the root cause, and completely solve the problem.

Description of the Quality Problem

Earlier, when printing a certain white cardboard cigarette sleeve box on a reel-fed multi-color gravure press, we found that two base colors showed single-sided printing, that is, intermittent misregistration on the operator side, as shown in Figure 1. Consequently, when the two base colors were overprinted, small shaded letters on the base color intermittently showed ghosting. This issue was discovered during the restart and plate registration process after a product change. Even after the plates were correctly registered (two base colors properly aligned), sampling again revealed significant registration errors between the two base colors (the registration lines of the two base colors continuously changed).

Figure 1 "Filter Cigarettes" and "Filater Cigarettes" are not overprinted

Cause analysis and troubleshooting

In order to facilitate you to understand the various measures taken in the troubleshooting process, I will briefly introduce the printing process of the product: gravure printing 7-color printing (of which there is a transition net in the middle of two blue background colors) slitting large sheet → hot stamping 2 passes + embossing → cold transfer (corresponding to the blue background color position to adopt a cold transfer process to make the surface of the product present a laser effect) → die cutting forming, the printing material is 225g/m2 white cardboard, of which the gravure printing process is completed on the 9-color unit gravure printing machine, and the two blue background colors occupy two adjacent units.

There are many reasons for gravure overprinting inaccuracy, but the quality problem is found in the process of starting the machine after the product is replaced, and there is no overprinting problem in the printed product before the order change, theoretically the equipment problem (including the registration system) should be ruled out, so we first suspect the change elements when the product is changed. The change elements when changing products include paper, ink, printing plate, and embossing rubber roller, and the other 3 of the 4 change elements have a direct impact on overprinting except ink. Therefore, we first troubleshoot from three aspects: paper, printing plate, and imprint rubber roller.

01 Paper problems

The unstable tension of the paper roll exceeds the adjustment limit of the equipment, and the roll is not round or the tension is unstable due to factors such as collision and fall during the handling process, which may lead to inaccurate overprinting. So we first carried out a trial printing of the roll, cut off the paper roll being printed and replaced it with another roll, there was still an intermittent overprinting inaccuracy, and then we replaced different grades of paper for verification, and there was still an intermittent overprinting inaccuracy, and then the paper factor was ruled out.

02 Printing plate problem

Because the plate shaft head with shaft plate used is welded together with the plate cylinder, there have been occasional quality problems caused by the opening welding of the plate shaft head in the past, so we first check whether there is a shaft head open welding break phenomenon on the two blue printing plates. We adopt the method of allowing the plate to idle at high speed to observe the plate runout and listen to the sound of plate rotation, which eliminates the problem of plate shaft head opening weld breakage.

Although the diameter tolerance, straightness tolerance, radial runout tolerance, dynamic balance tolerance, eccentricity and other indicators of the printing plate also have a greater impact on overprinting, because the gravure printing plate is outsourced, the technical indicators in the quality inspection report of the outsourcing party meet the quality requirements, and the two-color printing plates are the plates used in the previous batch of production, and there is no problem of inaccurate overprinting in the previous batch printing process, so the printing plate problem is excluded.

03 Problem with the embossing adhesive roller

After eliminating the paper and plate factors, we focused on the two embossing rubber rollers, first checked whether the embossing rubber roller bearings were damaged, and after confirming that the imprint adhesive roller bearings were no problem, we disassembled the two imprint adhesive rollers, measured them accurately with calipers and dial indicators, and found that the two imprinting adhesive rollers had different diameters at the left and right ends, and the diameters of the left and right ends were 0.5mm and 0.6mm respectively.

After finding the problem, we are ready to replace two spare embossing rubber rollers, and after testing, we found that there is a certain error in the diameter of both ends of the spare imprinting rubber roller, so we picked two rubber rollers with a small error, and staggered the size of the two rubber rollers, one small head of the rubber roller is on the operating surface, and the small head of the other rubber roller is on the transmission surface, and then the machine is turned on again to print, and the problem of inaccurate overprinting is eliminated.

After the problem was solved, we re-analyzed and researched, and found that there were two main factors causing the problem of inaccurate overprinting: first, there was a large error in the diameter of both ends of the imprint roller; Second, the small heads of the two embossed rubber rollers are installed on the same side when the rubber rollers are installed, resulting in cumulative errors.

According to common sense, the error of a rubber roller has little impact on the accuracy of overprinting, but if the small head of the two rubber rollers is installed on the same side when installing the rubber roller, the error of the two units is superimposed, resulting in the continuous accumulation of error. The maximum error can be found to be: (0.5+0.6)×3.14=3.454mm.

After finding the root cause of the problem, we made corresponding technical requirements for replacing the rubber roller, one is to measure the diameter error of both ends before replacing the rubber roller, and the error exceeds 0.5mm is prohibited from being used on the machine, and the rubber roller needs to be re-ground and the large and small head is marked; the second is to avoid the size of the rubber roller of the two adjacent units to face the same side. Through the above measures, the problem of inaccurate overprinting caused by the different diameters of the two ends of the rubber roller is basically eliminated.