Analysis of scratch marks of offset printing (1)
The streak is commonly called " bar " , dark streak is called " ink streak " , and light streak is called " water streak. "
Streaks are a type of failure that is more difficult to solve in offset printing, especially when printing solid or spot colors. The appearance of streaks directly affects the print quality.
One trace of features, causes and exclusion
The occurrence of streak marks is caused by the instantaneous friction and sliding of the blanket, ink roller, water roller, etc. after contact with the surface of the printing plate, resulting in the destruction of the balance of the ink and ink relationship, and a straight line parallel to the axis line of the plate cylinder. Graphic and textual changes in the shape of the dots and the thickness of ink deposits have resulted in streaks.
According to the obvious features of the streak marks, ink marks and white streaks can be distinguished.
The dots in the ink streaks are irregularly enlarged, the dots are elongated, the text is heavy, and the ink becomes heavier, forming a pronounced deep streaks on the layout.
White streak marks, also called water streak marks, refer to the occurrence of irregular narrowing in the streak area, which constitutes an obvious shallow streak.
According to the category of the streak fault, it can be divided into a streak mark, a vibration streak mark, a roller streak mark and other factors.
The method of checking the striae is: using a half-dot mesh version, print on a coated paper with thin ink and observe whether or not streaks appear.
Streak failures often occur when the machine is in the stage of trial installation or the machine is in disrepair for a long time. Modern high-speed multi-color offset presses have few streaks in the printing process. Most streaks are caused by improper adjustment ( see the table for details). 8-1) .
Table 8-1 Characteristics and identification of striations, causes and solutions
category
Features and identification
Causes and Solutions
Rack marks
1 , in the entire layout appears streak, the shape of the plaque, the first streak from the mouth value = "10" unitname = "mm"> 10mm , fixed position on each sheet.
( 1 ) The pressure of the ink roller on the printing plate and the ink roller is too large.
( 1 ) The pitch of the rack marks is the same as the pitch of the ink roller gear.
( 2 ) The accuracy of the uniform ink gear is poor, and the part that affects the vibration is detected.
( 2 ) The rack mark pitch is the same as the roller gear pitch.
( 3 ) The installation of the impression cylinder and the excessive pressure.
( 4 ) The roller gear has poor accuracy. Change gear position.
( 3 ) There are 5 to 8 rack marks with a fixed position.
( 5 ) Inaccurate matching of the reciprocating motion and printing time of the ink-transmitting and ink-sinking mechanism or the clearance at the shaft end.
( 4 ) There are 2 to 3 rack marks and the position is fixed.
( 6 ) Impact caused by cam mechanism or other problems. There are problems with individual teeth or inclusions.
Roll marks
2. After the roller is pressed off, there are ink streaks.
( 1 ) Large drum journal and bearing clearance.
( 1 ) Inconsistent ink color before and after printing.
( 2 ) The drum is unbalanced and eccentric.
( 2 ) In the ink traces of the printed product, the left and right sides do not agree with the middle thickness.
( 3 ) The drum is not balanced.
3 , there are still ink marks after the roller is pressed.
( 1 ) There are problems with the arrangement of the rubber rollers and the diameter of the ink roller.
( 1 ) Inconsistent ink color before and after printing.
( 2 ) Heavier ink, wider striated lines, and small and uniform ink gaps. The position is not fixed. Place the paper on the impression cylinder and press the position where the striated mark was generated. Correspondingly find the occurrence on the machine. The cause of the mark).
( 3 ) Large ink roller shaft and bearing clearance.
( 4 ) Ink roller accuracy: eccentricity, bending, deformation. The ink roller balance is not good.
( 3 ) Ink marks appear on the leading edge of the blanket. The width of the strip is equal to the circumference of the rubber roller.
( 4 ) The pressure of the ink roller is large.
( 4 ) The position of the streaks is fixed, and the ink first weight gradually decreases.
( 5 ) Hardness of the roller
4 , wet system problems
( 1 ) The contact pressure between the water roller and the printing plate is too large.
( 1 ) The white streaks mostly appear on the image of the mouth and are relatively fixed in position.
( 2 ) The string water roller drive gear is seriously worn.
( 2 ) Produce white streaks in a certain area when printing
( 3 ) Improper adjustment of the water transfer roller.
( 3 ) White streaks appear in the print area and are fixed.
Vibration streak
5. The position of the streak on each sheet is fixed, and the width of the streak is the circumference of the rubber roller.
( 1 ) There is an impact when the plate cylinder and the blanket cylinder come into contact.
( 1 ) Appears later in the middle of the page.
a pad is too hard.
b There is a raised plate at the roller contact, or a transitional ramp at neutral.
( 2 ) appears in the front half of the paper. The distance from the tip of the streak to the gills corresponds to the distance from the impact of the blanket cylinder and the impression cylinder to the start of embossing.
( 2 ) Large drum journal and bearing clearance.
a Adjust the center distance between the rollers.
b Printed with a rolling pillow.
( 3 ) There are ink streaks in the local area of ​​the print, and the location rules are uncertain.
( 3 ) The impact of the mechanism that is prone to impact is transmitted to the impression cylinder, and vibration occurs in the printing part, so that the printing pressure changes directly. a Eliminate irregular vibrations in some institutions. b Printed with contact roll.
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