There are many factors that affect the accuracy of overprinting in gravure printing, such as the stability and reliability of the main motor and its control system, the transmission form and accuracy of the gear box, the quality of the dynamic and static balance of the paper guide roller, the sensitivity and the electrical system Reliability, illegal operation of the equipment during use, wear caused by inadequate maintenance of mechanical systems, local deformation, etc. are all factors of inaccurate overprinting. Here we mainly analyze some factors related to the plate roller.
(1) Mechanical accuracy of the plate roller
At present, the mechanical accuracy of the plate rollers manufactured by the plate maker generally meets the following requirements:
Length limit deviation ± 0.2% of roller length
Diameter limit deviation ± 0.02mm
Wall thickness range 6mm-12mm
Keyway width depth limit deviation + 0.3mm
Taper hole taper limit deviation ± 1 °
Cylindricity is not greater than 0.03mm
Flange fillet 7mm ～ 11mm
Dynamic balance below 80g (1200r / m)
The excessive accuracy of the plate roller is one of the reasons for the inaccurate overprint. There are two types of plate making for single concave machine: one with shaft and one with hollow plate.
1. With shaft version
The cost of the shaft version is high, but the processing accuracy is also high. Because the plate-making manufacturer with shaft is processed by imported MDC lathe, on the one hand, the precision of the car mill is high, on the other hand, the reference point during the car grinding is the same as the reference point during printing, so the mechanical accuracy of the plate roller during printing high.
2. Hollow version
The cost of hollow version is slightly lower, but because the processing standard during plate making is difficult to be the same as the printing standard, secondly, even if the plate making is consistent with the printing standard, there are still irregularities when the operator wears the shaft during printing or long-term disassembly of the shaft, especially brutal disassembly Shaft damage caused that after the actual shaft wear, the plate roller jumped out of the left and right, and the out-of-roundness exceeded the standard, which were the main reasons for the overprint inaccuracy.
(2) The effect of paper thread direction on overprint
Generally speaking, the lateral expansion of paper fibers is much larger than the longitudinal expansion of paper fibers, generally 2 to 8 times. Therefore, the transverse elongation of paper fibers is much greater than the longitudinal elongation. Straight wire paper printing means that the direction of the printed paper fibers is consistent with the rotation direction of the cylinder. Horizontal thread paper printing means that the paper fiber direction is consistent with the cylinder axis direction. Straight silk paper from the start to the end of printing, the expansion and contraction of the paper is horizontal than vertical, after printing, there will be paper graphics and text is easy to produce horizontal overprint inaccurate. After the horizontal thread paper is printed, the paper's expansion and contraction change is that the longitudinal direction is greater than the horizontal direction. After printing, the graphics and text are prone to longitudinal overprinting. Therefore, what kind of silk paper is used for printing, the expansion and contraction of this silk paper should be provided to the plate making company.
(3) The plate-making company has an overall zoom function in the electric carving section
According to the fixed change amount of the paper thread, the horizontal overall zoom or the vertical overall zoom can be used to compensate for the change caused by the paper thread.
(4) Make use of trapping (missing) to compensate for overprinting errors
Fundamentally speaking, the purpose of capping technology is mainly to compensate for the overprint error of the printing press (because there are mechanical errors in the most accurate printing presses), and the capping method is divided into shrinking and expanding.
1. Inward shrinkage (color sequence of surface printing is generally from light to dark)
Mitre shrinkage is to shrink the previous color evenly within 0.05mm ～ 0.1mm of the next color. For example: under the big red, shrink the yellow by one line. The deep purple will shrink the red line. However, inward shrinkage is generally suitable for patterns with two colors overlapping in a larger area, not for small patterns.
2. External expansion (color sequence of surface printing is generally from light to dark)
There are many external expansion cover in practical use, all adjacent colors are generally expanded from 0.15mm to 0.3mm from light to deep, to compensate for the quality defects of the adjacent two-color exposed white seams caused by overprinting errors, and greatly improve the yield.
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