Innovative polyurethane replacement of traditional rubber cover eliminates chemical cleaning costs, reduces breaks, and improves machine operation

New Lump Breaker Roll Cover Improves Runnability at Smurfit-Stone Container

Smurfit-Stone Container’s mill in upper Michigan produces recycled content corrugating medium.

The two containerboard machines at the Ontonagon mill each trim approximately 160 inches and produce corrugating medium with a recycled content of 33%. Both paperboard machines have conventional fourdrinier wet ends with a steam box located approximately three feet ahead of the lump breaker roll position. Stock temperature is 170°F to 190°F out of the steam box. The mill loads its lump breaker rolls to between 40 and 90 pli.

In general most mills that make this grade, as well as most brown paper grades, traditionally use a two-inch-thick, 200 ± 50 P&J rubber cover on the lump breaker roll. The mill had tried to alleviate the buildup problems by use of a number of different cover compounds supplied from a variety of cover manufacturers but with little success. All of these rubber covers would pick at one time or another. The mill, along with many others, used showers and chemicals to limit or prevent stickeys from building up on the cover.

Buildup on the covers caused the rolls to vibrate and bounce, leading to breaks and subsequent machine downtime to clean them. The chemical addition to the cleaning shower was also adding cost. The cleaning showers were also causing wet end breaks and housekeeping problems.

New polyurethane lump breaker roll cover has eliminated stickeys buildup problems and improved machine runnability.

In July of 1997, at the request of Joe Asiala and Eugene Lewis, Assistant Superintendents, Scapa Rolls (now Voith Sulzer Paper Technology Service Division) agreed to supply a polyurethane lump breaker roll cover for a trial. Polyurethane was never known to have been used for this application before.

Application. The decision to try a poly cover in this position was based on results the mill had seen when rubber covers in the first and second press top roll positions on the No. 2 papermachine were switched to poly covers. These rolls are located just downstream from the lump breaker roll on the fourdrinier and are in direct contact with the sheet. Previously, the rubber covered press rolls would pick and cause runnability problems. Switching to poly covers in the first and second press positions eliminated buildup and picking problems.

The first consideration for using polyurethane in the lump breaker position was that the cover be soft enough to work in this application but hard enough to resist water permeation. A 50 P & J Polymax cover was chosen as the best choice for a roll cover running in this position. Installing a 50 P & J cover where a 200 P & J cover had been running would require a reduction in load and roll crown adjustment to maintain the same nip pressure. The effects of reducing the nip dwell time were an unknown at this point in the project.

The surface finish of the Polymax roll cover was also changed to a very smooth finish compared with the rather rough finish typical of a rubber lumpbreaker cover. Cover thickness was designed at 1-in. instead of the traditional 2-in.-thick soft rubber cover. This was done for cost savings only at this point. The last consideration was the time interval between grinds. The mill agreed to a one-month interval until the effects of water permeation were determined. Keeping the poly cover roll out of the machine for this time would allow the cover to dry out before it was reinstalled in the machine.

Start-up. On November 13, 1997, the first Polymax lump breaker cover was installed on No. 2 papermachine. The roll, two inches smaller in diameter than it was when covered with rubber, required a spacer to be installed under the bearing housing. Dynamic nip impressions were taken with Fuji Prescale film and scanned with a nip-scan unit to confirm uniform loading at the lower pli. Because of problems with the spare rubber covered roll, the Polymax remained in the machine for three months before it was removed for regrind and inspection.

Results. The machine started up with no cleaning shower and no chemical addition to the lumpbreaker roll. No downtime to clean buildup off of the Polymax lumpbreaker roll cover has been needed. The reduced dwell time in the nip has not affected sheet quality and high off-couch sheet dryness has been achieved. Machine breaks were reduced considerably and the No. 2 machine is now producing record tons at record speeds.

The grind interval of the polyurethane roll cover has remained at three months with no water permeation issues. The machine continues to run with no chemical addition and no shower on the lumpbreaker roll.

With the overwhelming success on the poly roll on No. 2 PM documented, the No. 1 PM lumpbreaker rolls were covered in Polymax. To-date, all lumpbreaker rolls at the Ontonagon mill are covered in Polymax and are achieving similar results. Ron Howard, General Manager, said “Both Mike Woller, Voith-Sulzer, and the mill supervision are to be commended for their innovations and development of new lumpbreaker operation technology. Few people in the industry supported this concept initially, but many mills will take advantage of it in the future based on the Ontonagon Mill’s success.”

Cost and Operational Considerations. Detailed, specific cost savings data cannot be released. However, the following outlines some of the cost issues and benefits. First, a poly roll similar to a traditional rubber roll would likely be about twice as expensive as the rubber cover. However, since the poly cover was only 1-inch thick, vs. the two-inch thick rubber cover, the actual cover costs were very close. With only a very minor cost penalty for using a poly covered roll vs. a rubber one, cost savings were realized in increased speed, reduced breaks, elimination of the cleaning shower and shower maintenance. The cleaning chemical and chemical introduction system were also eliminated. The break frequency on the machine when stickeys were bad had been between 6 to 8 breaks per day. This has been eliminated since the new covers were installed.

Couch solids did increase with the use of the poly covers. This was caused by: (1) elimination of the cleaning shower and chemical use reduced the amount of water out of this nip; (2) cover hardness was increased from 200 P&J to 50 P&J, so the actual nip pressure was substantially increased, and thus likely removed more water from the sheets.

All polyurethane covers absorb water to one degree or another, while rubber covers typically do not absorb water. Typically, the softer the poly cover, the quicker it will absorb water. Along the same lines, the heavier a poly cover is loaded the quicker water will be absorbed into the cover. If a polyurethane cover is left in the paper machine too long, moisture will penetrate to its bond layer, and the cover will become loose. Leaving the roll out of the machine for four weeks before re-installing it will give the roll sufficient time to dry out and prevent this.

This concept—the use of this material in this position—has had little support because it was a big step from what was considered to be typical. First, the change from a 200 P&J cover hardness to a 50 P&J cover raised issues such as what effect the increased pressure would have on the sheet and drying properties. To address this issue, as noted, the mill reduced load and crown to start with to keep the actual nip pressure the same. It should also be noted that the nip width was reduced with the use of a poly cover. After successfully running with no buildup, the loading and crown were brought back to where they were when the mill ran rubber covers. The nip width was still only a fraction of what it was before, but this had no ill effect. Increasing nip pressure actually helped to increase machine speed without negative effect on sheet properties. Fears of sheet crushing and the possibility of sealing the sheet too early, preventing proper dying, weren’t encountered. Water permeation into the poly cover is still a concern, as the cover could come loose, but with proper maintenance this limitation can be managed.

Mr. Asiala is assistant superintendent, Smurfit-Stone Container, Ontonagon, Mich. Mr. Woller is technical sales and service representative, Voith Sulzer Paper Technology, Service Division.