Doctor blade angle control for wide-web cylinder metering (Part 1)
von Bill Warner, Vice President of Allison Systems,
For gravure applications, doctor-blade control needs to be both fast and foolproof with no guesswork, mechanical problems, or startup waste that is attributable to doctor-blade performance. While this statement is true for any gravure press, it is particularly important for wide-web applications including décor printing, coating, and hot-melt gravure. These applications can have doctor blades with lengths greater than 100 in. (254 cm). When doctor blades are that long, setting them correctly and consistently to achieve effective metering can be more difficult.
Often the doctor blade systems found in these very-wide applications do not offer the doctor-blade position and angle control needed to provide consistent and uniform metering of the gravure cylinder. The blade system must be adjustable so that the blade contacts the cylinder at a position on the cylinder relative to the web nip point that doesn’t allow the ink or coating to dry on the cylinder before it is transferred to the web. The blade angle must also be set to an appropriate angle for gravure doctoring.
Again, many blade systems do not have the appropriate adjustments or, if adjustment mechanisms are available, they are either damaged or completely nonfunctioning. Even if the adjustment mechanisms are in good working order, they can still be problematic because often there is no way of indicating the positions so that the setup can be repeated the next time the job is run. Assuming the blade system can achieve a nominal good blade position and angle, there is no guarantee that the same blade angle will be achieved along the entire doctor blade.
A small amount of blade-holder deflection can have a big effect on blade angle from one side of the press to the other. Additionally, if the doctor blade is not installed precisely in the blade holder, slight blade-position variation will also be magnified on wide-web presses. For these reasons, it is extremely important to have and use the capability to adjust the doctor blade so that it is running parallel to the cylinder every time a new setup is made. If not, cross-web print variations will likely be produced.
A good doctor-blade system will have multiple degrees of adjustment mechanisms that work smoothly, include position indicators, and can apply the doctor blade at industry-acceptable blade angles and positions for all cylinder sizes that are used. If your blade system doesn’t currently have all those capabilities, it is likely time to start investigating how to achieve those functions.