Flexographic Closed Chamber Doctor Blade System 2


This article mainly focus on how to best use of doctor blade, including the setting of the blade paralleslism, blade angle, and blade change.

1.Blade Parallelism
According to the design principle of the closed ink chamber system, the upper and lower blades should be completely parallel to the centerline of the anilox roller and maintain the same or similar pressure to achieve uniform ink scratching effect.

To achieve this goal, first of all, the machining accuracy of the ink chamber must be high, and the datum plane for installing the blade must be precise enough.

Secondly, in the installation and adjustment of the closed chamber system, it is necessary to ensure that the ink chamber and the center line of the anilox roller remain parallel, and the blade pressure is adjusted to an appropriate level with a feeler gauge or other devices.

Finally, the straightness of the blade itself and the installation process are also more important. When installing the blade, make sure that the plane of the blade and the pressing plate are clean and free of foreign objects. While tightening the screws, press the blade to the back platform, and tighten the screws from the middle to both ends.

In addition, in the process of high-speed operation of the machine, it is not ruled out that the position of the ink chamber will slightly change after a period of time, resulting in a good blade effect if the blade is installed well. It is recommended to check the parallelism of the ink chamber and the anilox roller every three months or half a year to eliminate this problem.

2.Blade Angle
The blade angle is an important factor affecting the performance of the closed chamber doctor blade system. Many problems, such as unclean blade, rapid wear of the blade, and easy scratches on the anilox roller, are related to the blade angle to varying degrees.

The blade angle we often say usually refers to the blade setting angle. The setting angle is the angle when the pressure of the blade is not considered. However, in actual operation, the blade is under certain pressure under the push of the ink chamber. When the blade is under pressure, there will be a certain amount of deformation, resulting in a difference between the actual contact angle and the set angle of the anilox roll. For example, when the initial set angle of the blade is 31°, the actual contact angle becomes about 18° in the case of deformation under pressure.

The difference between the contact angle and the set angle is determined by the pressure and the strength of the blade. When the blade parameters are determined, the greater the pressure, the greater the difference between the contact angle and the set angle. That is, the smaller the contact angle, the larger the contact surface, which will cause the blade to wear faster, the blade is not clean, the blade and the foreign matter embedded between the rollers may cause the anilox roller to be scratched.
 
If the printing press operator can increase the printing color density by increasing the ink chamber pressure, it means that the contact point between the doctor blade and the anilox roller is no longer the top of the knife edge, which means the pressure is too high.

The problems of blade angle and parallelism are usually related. Generally, it is because the blade is installed unevenly or not parallel, which causes the light pressure to not achieve the normal ink scratching effect. In this case, the operator usually presses directly until the ink can be wiped uniformly. But at this time, the contact angle of the blade may have been much smaller than the set angle.

The US FTA recommends that the contact angle of the blade is about 30°. If the angle is too small, it will easily lead to dirty blade and dirty printing. If the angle is too large, it will easily cause vibration and print streaks. The contact angle can be calculated by measuring the cross-section of the blade edge after wear or by measuring the displacement change of the ink chamber before and after pressure.

3.Blade Change
Should I change the blade when changing the anilox roll? For most printing houses, this issue may be rarely considered. Generally, the blade will not be replaced by replacing the anilox roller.

As we see, the blade worn by the low lpi anilox roll and high lpi are different. The high lpi anilox roller does not have obvious wear on the surface of the blade edge, but the low lpi anilox roller can grind the surface of the blade edge into regular lines. It is not difficult to imagine that the width of these lines is the width of a single mesh. If the anilox roller is changed from a high lpi to a low lpi, the previous doctor blade can still fit on the surface of the anilox roller to achieve the ideal scraping effect. On the contrary, if the anilox roller is changed from a low lpi to a high lpi, since the surface of the blade edge of the doctor blade has been ground out by the anilox roller, it cannot fit well with the high lpi anilox roller. Therefore, it is recommended to replace with a new scraper in this case.



 

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