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Welcome to  ‘Ask The Expert’.  This page is moderated by industry consultant, Steve Wilkinson.  This page features the answers to your questions. Our visitors can also post comments to these questions/answers as well.  You can learn more about Steve’s background and our ‘Ask The Expert’ page by Clicking Here.

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July 19, 2010

We are about to switch from wiper rolls to a doctor bladed system, what are the pro’s and cons between a bladed and two roll system?

There is no doubt that if you want to achieve cleaner more consistent print quality then a doctor bladed system is the way to go.

That said, if you have been running for a long time with a rubber wiper roll and your operators have little or no experience with using doctor blades then you need to understand there will be a learning curve as with all new practices and equipment.

You did not mention what type of doctor blade system you have adopted or what materials you will be using so I will briefly cover the basics of the available systems.

When doctor blades were introduced, a positive angle (trailing blade) was used but today the reverse angle blade is used in most all flexo applications. Unlike the trailing blade the reverse angle blade works against the rotation of the anilox roll as it meters the excess ink from the surface of the anilox.

Using a doctor blade will dramatically improve your control over the ink film thickness that you lay down and will allow you to run faster as, unlike your old rubber wiper roll, the reverse angle blade will not be pushed away from the anilox surface as you try to run faster. This means however that you must maintain your anilox cleanliness and be more certain that you have the correct line count and volume engraving on your anilox as a blade will not allow you to open up the nip impression between, as you will have gotten used to with your wiper roll system.

The blade angle and pressure are critical factors to maintaining a good clean wipe of your anilox surface. Blades can be made from a multitude of materials ranging from blue/white carbon steel, stainless steel, plastics, laminates, composites so there is a lot of choice, although carbon steel is by far the most common. Blade thicknesses and blade tip configurations also vary, although the beveled tip blade is still the most common with thicknesses generally varying dependenton the ink system being used, generally the more viscous the ink the thicker the blade.

Inks need to be filtered in particular when using blades as any metal or hard particles that get into the ink can become trapped between the blade and anilox resulting in scoring of the anilox engraving. Blades must not be run out of alignment or dry as scoring of the anilox will also occur.
If you have selected a chamber blade system then you will have two blades, a metering blade and a retaining blade, so alignment, blade angle and pressure are even more critical.

Seals also become a factor with chambered systems in retaining the ink in the chamber; make the wrong choice and you will have a lot of ink all over your press. That said. the chamber system gives you the best control over your ink chemistry, its viscosity and will dramatically reduce your ink consumption as well as further improve the consistency of your print quality.

Rubber wiper rolls have a tendency to wear unevenly, swell, shrink, crack and allow for excessive wiping pressure and exaggerated alignment issues so you have made the right choice whichever blade system you have chosen but you will have a learning curve and should ask for as much support from your vendor as you think you will or might need.

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