The front and back overprinting is not accurate:
(1) Printing press offset printing
a. The locking screw of the lid and base adjustment plate is not fixed, or the M5 hexagon socket head cap screw on the paper feeding rocker of the offset machine such as the tin printing machine is not fixed tightly;
b. Whether the copper sleeve of the paper pushing holder is worn or not;
c. The paper feeding teeth are worn;
d. The printing pressure is too small;
e. The gears are worn;
f. The blanket is too loose and is not tied tightly.
(2) Paper a. The cutting precision of the paper is not high; b. The amount of water during printing is too large, and the paper is stretched.
(3) Other factors a. There is a problem with the printing plate or with the manuscript itself; b. The paper jam was piled too high or the pressure of the paper platen is too heavy, which causes the paper jam to go obliquely. The left and right printing position deviates:
a. The left and right baffles are too tight or the left and right baffle screws are not fixed;
b. The length of the paper jam varies;
c. The drum sways from side to side;
Oil stains can be divided into two categories: one appears in a greasy state, commonly known as “greasy”, which is due to the greasy state of the blank part of the layout by the printing ink; the other is due to the ink emulsion with very fine particles evenly distributed in the layout, commonly known as “floating dirt”. The causes of oily dirt are:
(1). Printing plate
a. The quality of the printing plate itself is not high (the plate base is not good or it is the regenerated base);
b. The exposure time is insufficient and developer concentration is too low during printing;
c. The protective glue on the printing plate layout is not cleaned properly;
d. The blank part of the printing plate is oxidized;
a. The pressure on the plate is too light;
b. The pressure between the water rollers is too light;
c. The viscosity of the ink is too low;
d. The surface of the blanket is aging;
e. Paper powder and paper fiber wear the printing plate;
f. The pH value of fountain solution is improper.
In actual printing, sometimes it is found that the ink color on the same screen is different in the front and back, and often times the ink color is dark in the front and light in the back. The opposite is less common. The reason for the inconsistent ink color in the front and back：
(1). In terms of the roller
a. The roller gears or bearing journals are worn out over time.
b. The drum journals are not concentric with the drum shell.
c. The pressure between the plate roller and the plate is too heavy, which results in jumping.
(2). In terms of the ink roller a. The surface of the ink roller is aging after long-term use. b. The ink roller has not been cleaned for a long time, so the surface is covered with ink, which makes it impossible to apply ink. c. The viscosity of the ink is too large. 12. How does the ink bar come about and how can it be eliminated?
Ink bar refers to the obvious deep streaks in the axial direction on the printing plate layout and printed pieces when the metal printing press prints in flat screen or in the field, which are commonly known as “black bars”. The causes and elimination methods of ink bars are:
(1). The roller gears have been worn after long-term use, and a new set of gears should be replaced;
(2). The printing pressure is too large or the pressure between the plate cylinder and the rubber roller is too large. The metal sheet printing machine should be adjusted to appropriate pressure;
(3). Paper fiber, ink skin and other dirt are embedded between the gears. All kinds of dirt between the gears should be removed, new gear lubricants should be added, and the cleaning and maintenance of the multi color offset printing machine should be strengthened;
(4). The pressure between the printing plate and the plate roller is too large. The pressure between the plate roller and the plate should be adjusted;
(5). The surface of the plate roller is uneven or the roller core is bent. The ink roller should be replaced with a new one;
(6). The ink is too thin. An appropriate amount of 0# varnish should be added to enhance the viscosity of the ink;
(7). The paper is rough and the surface strength is poor. The blanket should be washed frequently, and 6# varnish should be added to the ink to reduce the viscosity of the ink and reduce the degree of de-powdering and hair loss of the paper.