Empirical analysis of wrinkling during offset printing
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Wrinkling (pleating or wrinkling) is a very serious quality failure in printing, and generally wrinkled prints are unacceptable. Sometimes wrinkling in printing is a very headache! The following is some analysis and discussion of the phenomenon of wrinkling.
The root cause of wrinkling is generally nothing more than the paper being compressed by the pressure of the impression cylinder after the uneven paper enters the embossing zone. When the amount of compression exceeds the compressibility limit of the paper, it will wrinkle. The paper is subjected to two forces during the imprinting process: the pressure perpendicular to the paper surface and the backward rolling force tangent to the paper surface, so that the flat paper may also bite the paper due to the drum biting during the imprinting process. Less than the rolling force causes the paper to slip back and wrinkle.
One is the flat suit of the material itself, and the otherwise plain paper becomes uneven before entering the embossing area due to the machine itself. Below we do some empirical analysis of the conditions that may cause wrinkles on printed products, and list some of the reasons why wrinkles are most likely to occur in actual printing:
First, the material:
1. The paper stiffness is too poor; most of the wrinkles are almost always based on the small stiffness of the paper and the small number of grams. That is to say, if the quality of the paper is good, even if there are some problems in the machine, wrinkles will not occur. In general, wrinkles are limited to papers of 90 G/m2 or less, especially papers with poor stiffness.
Root cause: Poor paper stiffness, can not effectively carry out rigid transmission when the front edge of the paper (at the mouth) or the side of the paper is stressed, especially in the case of partial force or sudden force The deformation is serious.
Solution:
(1) Paper change, <although thorough but often for cost reasons is not very practical>
(2) Doing paper indentation treatment, pressing the fan-shaped ridge line on the paper can appropriately improve the stiffness of the paper, <However, this will often affect the overprint>
2.the paper surface is not flat, there are ruffles (or tight edges)
Root cause: Due to the uneven water content on the surface of the paper, the surface is deformed, and the modified paper will cause wrinkles if it enters the imprint.
Solution:
(1) Cool paper, to make the surface of the paper humidity control.
(2) Try to drag the large part of the deformation as much as possible.
(3) Distributing the platen roller and the paper feeding belt on the paperboard into an inverted figure-eight shape, and trying to make the paper entering the embossed area as uniform as possible.
Second, machinery:
1. The paper that arrives at the front gauge is excessively skewed.
Cause: When the paper reaches the front gauge, when the front position is specified, the one side of the paper is too strong due to the skew, and if the stiffness of the paper is not high, the upward convexity of the middle of the paper will occur, and 叼The mouth is not in a straight line, so that the paper that is convex in the middle will remain in the embossed area and cause wrinkles!
Possible reasons:
(1) Feida's gas path is not smooth. <Sparse the ventilation road>
(2) Feida's adjustment is wrong. <correct adjustment>
(3) The pin on the side of the pendulum of the transfer nozzle is seriously worn (replacement pin)
(4) The falling time of the front-end paper-feeding balance of Feida is inconsistent, or the falling time is too early, and the paper-feeding nozzle is arguing. (to make it in the right state).
2. The parts on the cardboard are not properly adjusted.
(1) If the platen roller is too tight or the platen of the platen is inside, it will flatten the paper that was originally served.
(2) The brush wheel is too tight, causing the paper to exceed the pre-defined position line before the front gauge falls. When the current gauge falls, it cooperates with the brush wheel to squeeze the paper to deform it. In general, this situation is accompanied by severe overprinting or irregular alignment.
Solution: Correctly adjust the parts on the paperboard and the paper time.
3.the front rules, side rules:
(1)It is easy to produce when printing three-open (banner) thin paper, which is because the paper has poor stiffness and is easy to protrude in the middle of the front gauge.
(2) The front gauge, the side gauge line is not vertical. When this non-perpendicularity is severe, the paper is squeezed and deformed while the paper is being positioned.
Program:
①You can consider the paper indentation and increase the stiffness of the paper.
② It is recommended that if the front gauge is adjusted during the adjustment, the adjustment range should not exceed 0.3mm.
③Adjust the side gauge vertically when the front gauge is a zero position line.
(3) Transfer paper teeth, impression cylinders:
1. The dental pad is not flat, and it occurs mostly in long-term printing of small size. When the large-format is suddenly changed, the middle wear is serious, and the wrinkles are generated after the imprinting due to the unevenness at the mouth.
2. The transfer time between the transfer paper and the impression cylinder (or the transfer roller) is too large or too small.
3. The spacing between the transfer paper pad and the paperboard table is not suitable at the time of handover (general requirements: the hem type pad and the cardboard table are level or slightly lower; the pendulum type is thin paper thickness *3 or thick Paper thickness +0.2mm)
The above solution: Correctly adjust the timing and position of these teeth. Note: Generally, the impression cylinder is used as the reference (four-color machine is subject to the first group of imprints)! The order of adjustment is: embossing, transfer paper, and cardboard table. (The backwards are also adjusted backwards from the beginning of the imprint).
4. When adjusting the side gauge, the edge of the paper is caused by the paper-feeding at the corner of the paper. If the paper has a slight stiffness, or the paper is slightly curled, it will be handed over in the next step. It didn't go well, which caused wrinkling during imprinting!
Solution: At the same time, the side gauges and the printing plate are tilted to one side, so that the hanging paper corner can be bitten.
5, biting force and bite paper size is not uniform: especially in the middle of the large end small; or one side of the small side; even at the edge there is no paper teeth.
Possible reasons:
1 is improper adjustment of the teeth. ( Careful adjustment according to the steps can be avoided. Note: Make more than two-thirds of the teeth in the width direction bite the paper. Otherwise, the paper may slip backwards during the imprinting process.)
2 Closed springs may have fatigue or even break on one side (eg J2108; J2205)
3 The pin of the opener is worn.
6, the teeth are smooth; the pads are smooth. It can not effectively bite the paper (solution: replace the tooth pad and the tooth piece, and avoid changing only a few at a time).
other:
In the printing, sometimes the viscosity of the ink is too large, the pressure is too large, and the rubber is aging and sticky, and the pressing force of the embossing area to the back of the paper is too large, and the paper is partially backward when the embossing occurs. The slip produces wrinkles.
So far we have analyzed some of the main causes of wrinkling, but in actual printing, there are often several reasons that accompany it. This requires me to accumulate experience and comprehensive analysis and judgment in my work, and analyze and deal with it from many aspects. .