Larger jumbo rolls increase pressroom breaks, a case study

Note: This case study, though several years old, illustrates how sources of print house production problems can come from multiple locations on a paper machine, from wet end to finishing. Diagnosis requires a comprehensive view of the papermaking process.

A paper mill delivered its first lot of LWC 45-inch diameter rolls, a 5 to 6-inch increase from a long-standing order of 39-40-inch rolls. Its customer used this order on its new rotogravure, 84-inch-wide press designed for 50-inch diameter rolls. The press replaced a 64-inch press that ran 40-inch diameter rolls.

The old press averaged 1-3 breaks per 100 rolls, but with the new press and larger rolls, this excellent record changed dramatically to 10-12 breaks. What was the problem, and how could it be fixed - especially in the short run to avoid losing the customer?

Investigation at the customer's press

The mill’s service technician gathered information on the customer's press to build a statistical database. Her analysis identified two break characteristics.

1. One characteristic was near the core of the ongoing roll where breaks appeared at random in the cross machine direction.
2. The other characteristic was on the incoming roll where breaks were confined, almost exclusively, to the roll’s ‘B’ position.

Valmet joins the mill team for further analysis

The mill called a Valmet field service papermaking specialist in to help, and together with the mill's technician they analyzed the winder and paper machine situation. Investigative trials and tests of the mill’s winder and rolls revealed a very low wound-in tension profile, particularly around the core – a profile that did not lend itself to the large diameter rolls. T

Before a long-term solution requiring equipment upgrades could be reached, Valmet recommended immediate steps to get operations running satisfactorily:

• A mechanical condition audit was needed.
• Winder operating parameters needed to be checked.
• Caliper variations in the papermaking process should be investigated.

As it turns out, these three steps made it possible to increase roll diameter and maintain roll quality until a full rebuild could be made.

Immediate, inexpensive actions allowed renewed operations

The audit revealed that the front drum, originally friction-coated to facilitate variable torque differentials, had lost most of its coating and slipped easily. This explained the low wound-in tension.

Second, the rider roll’s nip control was improperly set. Because of a rotationally slipped cam, it showed a very high nip load on the outside of the roll contrasted by an inadequate nip at the core. This added nip-induced tension to the high-caliper area and promoted fiber separations leading to the breaks on the full roll. The low nip around the core allowed even more slippage, resulting in very low wound-in tension. A brief two-hour repair job corrected this problem.

Meanwhile, at the wet end, a worn slice lip was replaced to eliminate lip deterioration in the area where the most ridging appeared. Also, the majority of operating parameters were preset with a limited adjustment range to ensure controls stayed within the criteria required for the larger diameter rolls. These were relatively small fixes.

In addition, to ensure a reduction of web breaks, the mill asked the printer for permission to increase roll diameter gradually with the winder’s current configuration. Within three months, the mill’s rolls grew from 39 to 44 inches in diameter. After a 100-percent quality inspection of each roll, the mill selected only the top-quality rolls for this particular customer. Sometimes, up to 20 percent of the rolls were rejected because the winder’s traditional configuration could not consistently wind larger rolls.

Short term vs. long term problem resolution

The improved runnability achieved through the greater rejection rate exacted a high price. However, the mill planned to install new technology within five months to remove the roll-size limitations. This investment to equip the winder for the new jumbo roll market would yield a return in less than a year. Therefore, a short term sacrifice could be and was made to meet customer needs, retain their business and eventually expand jumbo roll market share.

For assistance with papermaking problem diagnosis and roll quality improvement, contact your Valmet representative.