Doctor blades are the right Rx for gravure
- Published: October 01, 1995, By Weeks, John A.
A new doctor blade design, offering spring load tension and high contact angle, reportedly makes the quality printing afforded by gravure even better.
The Angle Blade was designed for gravure printing, where the quality of the product is dependent on a clean wipe with no streaks or hazing. The Angle Blade is now being adopted by converters who need to control quality and coating weight.
The design of this new blade is unique in that it provides a spring load tension while maintaining a high contact angle. These qualities allow the printer and the converter to wipe the cylinder cleanly with a light spring load and preserve the cylinder while improving the quality.
The volumetric quality of the gravure process is attractive to converters, because they need to control coating weight in order to control cost. However, in order to take advantage of the intrinsic nature of a gravure coating or printing application, the cylinder must be cleanly wiped without being damaged.
The cleanest wipe is achieved when the contact angle is about 80 [degrees]. Doctoring a cylinder with a conventional flat doctor blade at the 80 [degrees] angle does present problems. The press geometry may not be capable of doctoring large cylinders at that high angle, particularly when the doctoring must take place close to the nip point.
With a conventional blade, chatter may be a limiting factor. The web path may restrict the high contact angle on some equipment. However, the major obstacle of the high contact angle with a conventional blade is total indicated runout (T.I.R.) that may result from an out-of-round cylinder or a worn bearing.
Any T.I.R. measurement presents a wiping problem for a conventional flat blade when the contact angle approaches 80 [degrees]. Since the conventional flat blade has little or no resiliency at high contact angles, machine operators resolve the problem by flattening the blade. For the printer this remedy results in hazing and streaking; the converter loses control of his add-on (coating weight).
Premise is Documented
The premise - high contact angle and a clean wipe - is well documented, as is the improvement in blade and cylinder life when T.I.R. is negligible. However, mechanical excellence is a prerequisite to successful wiping when the conventional flat doctor blade is used. Ideally, all presses should run without any measurable T.I.R., and all oscillators should move flawlessly.
In reality, even the best production equipment will eventually exhibit enough normal machine wear to result in T.I.R. and limit the ability to wipe the cylinder with a precise blade load at the correct angle.
Doctor blade loading systems have been included on most presses since the mid-1950s. They usually incorporate air cylinders that work through a pivot deck where the blade holder is mounted. While these systems do provide a fast and convenient method of changing blades, they are limited in their ability to provide the precise blade load necessary even when T.I.R. is not a factor.
The air pressure, in P.S.I., must first overcome friction in the pivot bearing and air cylinder before the load at the blade is achieved. Conversely, as the blade wears, the friction must be overcome before the system can respond, and the hazing and streaking may appear first.
Controlled air pressure, expressed in P.S.I., should not be construed as controlled blade load. If it were, press operators would not have to "tweak" doctor blades during a production run in order to combat hazing and streaking.
Figure 1 represents the typical doctor blade configuration where air pressure is applied, resulting in torque around the pivot bearing, then load at the blade.
In Figure 2, the Angle Blade is installed and the blade load is maintained by the support portion of the Angle Blade and is not dependent on the mechanics of the press.
The Angle Blade tip can be a lamella or beveled blade, but the ultimate wiping is achieved with a 10-mm, 55-micron, hardened precision strip steel, backed with an 8-mm, .006-in. backup.
The hardened steel, also known as reed steel, has a Rockwell C Scale rating of 57 and is used in the looms of the textile industry where the requirements for straightness, purity and tensile strength must meet higher standards. It is durable and very kind to cylinders.
When quality steel is used at the correct contact angle in a system that has been designed to handle the dynamics of a rotating cylinder, proper wiping takes place, and printing excellence will be achieved.
It is the recognition of these mechanics that fostered the development of the Angle Blade concept: maintaining the high contact angle and precise blade load with the simplicity of a spring provided by the support portion of the blade.
John A. Weeks is president of Angle Blade Co. He has 20 years of print management experience in decorative and label gravure printing. He has been an active member of the Gravure Assn. of America and has authored a number of papers on print technology.
