Determining the correct doctor blade for an application requires answering a number of questions to ensure the right blade is chosen to produce the needed outcomes of the application. Our previous post, “How to Choose a Roll Cleaning Blade” outlined some of the most important elements to consider when selecting a blade.

Once the correct blade is chosen, knowing when to replace the blade is important to ensure production issues related to roll hygiene or runnability are avoided. I sat down with Kadant Solutions’ Mike Six, Regional Applications Manager – Doctoring, to better understand the most appropriate time to change a doctor blade. Mike has worked for Kadant Solutions for over 14 years and currently supports the sales and service groups as a doctoring and roll cleaning expert.

V: Which positions does a mill need to watch for doctor blade changes?
M: It’s never a good idea to run a worn out doctor blade in any position, but it becomes especially important when the doctor is in a sheet shedding position or on a roll in the press that comes in direct contact with the sheet (e.g., a second top press roll).

V: How do you determine when a doctor blade should be changed based on these positions?
M: The blade should certainly be changed anytime the bevel on the topside of the blade has been worn through. However, it is best to develop a specific frequency for blade changes in critical positions based on previous observations of blade wear. This allows operators to know when a blade is due for a change.

V: What are the short term and long term benefits to changing doctor blades at regular intervals?
M: Benefits of doctor blade changes in critical positions at regular intervals typically include reduced roll wear, longer roll life, fewer sheet wraps, better press draws or sheet release, and increased production.

V: When is it appropriate to consider making a change in doctor blade material?
M: After first determining that there are no problems with the doctor in question, a change in blade material may be desirable if roll Ra can no longer be controlled to allow for acceptable sheet release, blade wear is too rapid, or the roll cannot be kept clean.

V: When should an upgrade be made to a doctor blade and/or doctor blade holder?
M: If there is an opportunity to increase production, decrease sheet wraps, give better press draws or sheet release, or increase roll life, an upgrade in holder or blade material should definitely be considered. Many of today’s modern blade holders can be easily retrofitted to existing doctors and upgrading doctor blade materials usually requires no change in doctoring equipment or setup.