Doctor blades are essential components of the flexo printing process. When doctor blades are not optimized, the anilox roll cannot do it's job properly — that is, delivering a consistently metered, accurately measured, and evenly distributed amount of ink to the printing plate. In this blog post, we'll discuss blade selection and common factors contributing to blade wear in the flexographic printing environment.
Blade Selection
With so many options available on the market today, it can be confusing to know exactly which blade material and tip design will best fit your process and usage goals. APR has created a Doctor Blade Selection Tool to guide you in the right direction.
Click here to be directed to our Doctor Blade Selection Tool.
Blade Wear
Doctor blade wear is a common topic of discussion among flexo printers, as it strikes a chord in nearly any pressroom. Whether it's fighting a leaking chamber, obtaining proper metering, or avoiding costly damage to your anilox rolls, optimizing your doctor blades by checking for these common wear factors can greatly reduce print defects and extra work in the pressroom:
Wear Factor #1: Excessive blade loading pressure, caused by...
- Leaking end seals in chambers
- Improper blade mounting into the holder.
- Poor blade holder and/or chamber condition.
- Poor press loading mechanism condition.
- Using a too thick of a blade for the application.
Wear Factor #2: Using the wrong blade material or thickness for the application
- Example: An UHMW plastic blade for a corrugated non-process print job would be the right application, but for process printing the UHMW normally is the wrong material.
- Using white steel where the ink or coating has corrosive properties.
Stainless or plastic is needed depending on the required wiping quality.
- Using the wrong blade thickness for the application.
Example: Using a standard .006” blade on a coarse anilox (such as 120 LPI) will wear out quickly.
Wear Factor #3: Shop Practices
If the shop practices require changing the blade on a weekly basis for example, (not recommended), then a tool steel blade, a coated blade with a different metal, or a ceramic coated blade may be the right choice for the shop practice. Be aware that the shop practice may not be optimum for maintaining anilox life if the metering and containment blades are allowed to build up with dried ink. Reusing blades is acceptable as long as dried ink is not pushed back onto the anilox surface, which may cause polish lines. When reusing blades on different anilox rolls, blades should always be replaced when changing to a higher LPI anilox roll from an anilox of 330 LPI or less.
- Always wear cut resistant gloves when cleaning and changing doctor blades.
Wear Factor #4: Using the wrong tip for the application
- For flexo, .004", .005" or .006" tip lamella may be the right tip for situations where a 0.008" or 0.010" blade is needed (because of excessive blade extension for example), but the thicker tip is not desirable.
- A lamella tip in the thicknesses for gravure (0.003") will wear out will wear too quickly and will easily over-deflect for flexo applications, and should never be used.
- A radius tip (most common for wide web chambers) may not be appropriate in narrow web applications due to the excessive pressure that would be needed to wipe cleanly in a quick time frame. This is especially true for UV inks where ink spitting can occur if proper metering and blade seating is not obtained as quickly as possible.
Wear Factor #5: Ink Properties and Filtering
- Abrasive pigments like TiO² cause increased blade wear.
- Filter inks with magnets and strainers in chamber or circulating systems to remove metal debris.
- Regularly clean the filtering system to maintain performance.
- Filter bottled inks using cheesecloth when returning work-off ink to the bottle to capture debris and cured ink particles.
Finally, I would like to thank Mr. Tony Donato, Product Development Engineer at Harper Corporation of America, for contributing his vast experience and knowledge to this post. If you would like to contact Mr. Donato, he can be reached via email at: [email protected].
