Determination and adjustment of printing pressure of sheetfed offset press
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The printing pressure of a sheetfed offset press refers to the pressure between the rollers, that is, the pressure between the impression cylinder and the blanket cylinder and between the blanket cylinder and the plate cylinder. It refers to the portion of the sum of the radii of the two cylinders, which is also called the extrusion amount. Generally, the compression amount generated by the pressure of the roller liner is taken as the standard, and the unit is mm. If the printing pressure is too large, it will cause the dot to increase, the deformation is serious, and even damage the accuracy of the roller and the gear; if the printing pressure is too small, the printed product will be unreal and faint; if the printing pressure on both sides of the roller is inconsistent, the overprint will be inaccurate and heavy. Printing and other printing failures. The size of the printing pressure depends on the thickness of the paper, the center distance of the two rollers, the thickness of the blanket cylinder liner, and the like. This article will briefly explain the determination and adjustment of the printing pressure of the sheetfed offset press.
Determination and adjustment of pressure between impression cylinder and blanket cylinder
The unit type multi-color printing machine generally adopts a medium-hard lining, and the selected substrate is generally a special whiteboard paper or a relatively flat coated paper and white cardboard. Under such conditions, the printing pressure is preferably about 0.20 mm when printing whiteboard paper, and the printing pressure is about 0.15 mm when printing coated paper and white cardboard.
Take our company's XJ142 series full-sheet offset press (roller liner as shown in Figure 1) as an example. The diameter of the impression cylinder is 800.00mm, the diameter of the blanket cylinder is 393.50mm, and the thickness of the blanket is 3.30mm. . When printing white paper with a thickness of 0.25mm, when the distance between the two cylinders is adjusted to 3.35mm, the printing pressure will be:
Printing pressure = liner thickness + paper thickness - barrel distance: 3.30mm + 0.25mm - 3.35mm = 0.20mm
Among them, the two drum cylinders can be measured by the method of pressing the lead wire, and can also be directly measured by the feeler gauge.
Since the blanket cylinder is provided with a liner and a blanket, it is troublesome to remove the printing pressure during the normal printing process, so the method of measuring the shoulder iron (roller) can be used to determine the printing pressure. For example, the XJ412 series offset printing machine rubber roller shoulder iron diameter is 400.00mm, the impression roller shoulder iron diameter is 799.30mm, when the rubber roller lining (3.30mm) is wrapped, the rubber cloth just touches the surface of the impression cylinder, the two roller shoulders The distance of iron is:
Shoulder iron distance = (imprint roller body diameter - shoulder iron diameter) / 2+ (rubber roller package lining diameter - shoulder iron diameter) / 2
Namely: (800.00mm-799.30mm) / 2+ [(393.50mm + 3.30mm × 2) - 400.00mm] / 2 = 0.40mm
That is to say, when the blanket cylinder just touches the surface of the impression cylinder, the shoulder rollers of the two rollers are separated by 0.40 mm, which is the zero distance of the shoulder iron.
If you want to print paper with a thickness of 0.25mm and the printing pressure is 0.20mm, the distance between the shoulders of the two rollers should be:
Shoulder iron distance = shoulder iron zero distance + paper thickness - printing pressure is: 0.40mm + 0.25mm-0.20mm = 0.45mm
It can be seen that by adjusting the shoulder iron distance to the paper thickness plus 0.20 mm, the printing pressure of 0.20 mm can be achieved.
The barrel, shoulder iron and lining of different models are different in size, but this method can be used to determine the printing pressure.
Determination and adjustment of pressure between blanket cylinder and plate cylinder
The current offset presses are mostly shoulder-iron structures. The pressure between the plate cylinder and the blanket cylinder refers to the interference between the blanket and the plate when the two shoulders are pressed together, that is, the sum of the distance between the shoulder and the blanket and the shoulder. Under normal circumstances, the pressure between the plate cylinder and the blanket cylinder of the full sheet offset printing machine is about 0.13mm, and the pressure of the following offset printing machine is about 0.11mm.
The XJ142 series offset printing machine is taken as an example to describe the method of determining and adjusting the pressure between the blanket cylinder and the plate cylinder.
The plate cylinder and the blanket cylinder have a shoulder iron diameter of 400.00 mm, a plate cylinder diameter of 399.00 mm, a blanket cylinder diameter of 393.50 mm, and a blanket cylinder liner thickness of 3.30 mm.
When the two roller shoulder irons are just in contact, the center distance between the two rollers is 400.00 mm. There is still a gap between the two rollers at this time.
Roller gap = (the sum of the diameters of the two rollers and the diameter of the two rollers) / 2 ie: (400.00mm + 400.00mm - 399.00mm - 393.50mm) / 2 = 3.75mm
After the blanket cylinder has a thickness of 3.30mm, the gap is reduced to: 3.75mm-3.30mm=0.45mm
After the plate cylinder has a thickness of 0.28mm, the gap is reduced to 0.45mm-0.28mm=0.17mm.
At this time, the rubber lining and the printing plate cannot be contacted, and no printing pressure is generated. To achieve a pressure of 0.13 mm, it is necessary to add a gasket between the printing plate and the plate cylinder.
Pad thickness = printing pressure + roller gap: 0.13mm + 0.17mm = 0.30mm
The rubber roller shoulder iron and the plate cylinder shoulder iron should have a certain negative pressure, which can make the drum movement smooth and reduce the ink stick.
The adjustment procedure for the negative pressure is as follows.
(1) The plate cylinder is added to the liner as required, and the blanket cylinder and the plate cylinder are installed.
(2) Adjust the pressure (the distance between the two rollers and shoulders at the time of pressure) to more than 0.05 mm.
(3) Evenly apply 5 points on the rubber roller shoulder iron, one of which is in the neutral position, the coating can use ink or lipstick, the coating should be thin, and the thickness is not more than 0.01mm.
(4) Driving at low speed (compressed state).
(5) Simultaneously pressurize both sides at the same time, add to the first point contact. If there is one side to contact first, it means that the two sides are not parallel, and slowly add the other side to the first point contact until the two sides are in contact at the same time.
(6) Adjust the effect to be contacted at 5 o'clock and record the scale.
(7) Add a pressure of 0.05 mm to make the negative pressure reach 0.05 mm.
It is worth noting that no matter which two rollers are pressed, the difference between the two sides should not exceed 0.02 mm, and the pressure of each color group should be the same.
In short, the printing equipment must work under the pressure of appropriate conditions, so that not only can obtain better quality prints, but also extend the life of the press.