Ceramic press roll conditioning tips – when, how and why
Mar 1, 2022
Ceramic press rolls offer a host of benefits over other types of roll covers. Cover life and need to change them are greatly extended, with most ceramic rolls lasting several years in the machine.
Incomplete doctoring as well as the types of filler, fines, and chemicals used can fill the ceramic surface or change the surface chemistry of the ceramic cover. When the surface smoothness gets too low (Ra < 20 μinches), some type of action may be required to return the roll surface back to target range of 35-55 μinches.
Conditioning a ceramic roll
The fastest and least expensive way to increase the surface roughness is to use a full width grinding strip equipped with an abrasive diamond paper (normally 74-125 micron grit diamond paper). This special conditioning strip can be placed over a new doctor blade and loaded against a slowly turning ceramic roll, normally at 150 fpm. Duration ranges from 5-30 minutes per pass, depending on micron paper used (rougher micron paper = shorter time on roll). Always use the finest micron that is successful in doing the job and check with your roll supplier on how to make up the abrasive strip. Turn on the doctor oscillator and lube showers and even the mating press felt cleaning showers to help flush grinding debris from the roll. Typical in-machine conditioning operation will include 30 minutes with 74 micron diamond paper followed by 5 minutes with the 125 micron diamond paper. Typical surface Ra after this procedure will be from 35-40 μinches. It is to be noted that it is very hard to achieve higher Ra with this procedure.
Onsite superfinishing removes about 0.0004” of ceramic material, so a cover with a usable thickness of 0.020” can handle many such onsite and grinder superfinishings. When finished, install a new doctor blade and allow it to clean the roll for 5-10 minutes, and then recheck roll smoothness.
For a complete analysis, surface smoothness should be checked every 20” across roll face. Use a three measurement average for each station. Plot the Ra vs. roll position.
Another way to treat the surface roughness is to remove the roll from the machine and superfinishing the roll on a grinder using special diamond grit paper. This process removes about 0.0008” of material.
Always contact your roll cover supplier to determine what type of superfinishing paper is recommended. The correct paper may not be the cheapest paper but has been found to do the best job for your application. Again, the surface roughness target is about 35-45 μinches for most ceramic rolls.
After checking the smoothness profile, save grinding information along with other process observations such as machine runabillity, press draws, release point, shape of release point and cleanliness of roll surface. Combining this information can establish a baseline to compare to if there are any future problems.
Cleaning a ceramic roll
If needed, the ceramic roll surface can be cleaned with a mild detergent (CitraKlean), turpentine or alcohol. Do not use any petroleum based solvents as they will leave an oily film on the cover.
Ceramic rolls place no extra limitations on the overall cleaning of the machine. For optimal performance, rinse the cover well after exposing it to cleaning chemicals. This is essential if using chemical solutions below pH 3.5 or above pH 10.5. The exposure of the roll coating to chemicals should be minimized during felt cleaning: doctor showers must be on, doctors must be in contact with the roll, and nips must be opened, if possible. An easily evaporating detergent, such as xylene, acetone, ethanol or pine turpentine, is recommended for local cleaning. The permitted pH range is 4 to 10. Do not use solvents that leave the roll surface oily, such as kerosene. Do not scratch the coating with any sharp, hard objects.
Contact your Valmet representative for more information on roll conditioning options.