Ensuring Rolls Run Right

FPInnovations Roll Testing Facility has provided a new opportunity for mills to improve their tissue making and converting operations.

GRAEME RODDEN 

FPInnovations developed its Roll Testing Facility (RTF) about 15 years ago to look at non-uniformity of paper rolls. One objective was to see if the root causes of problems seen in the pressroom, such as bagginess, could be explained.

We developed a tool to look at tension, moisture, paper properties, and roll structure quality to troubleshoot the problems with rolls rejected by pressrooms,” explains Frédéric Parent, research leader, web performance.

An important step in its evolution came in 2015 when a rebuild of the equipment allowed the RTF team to look at low basis weight paper rolls such as tissue (all grades).

As tissue is very weak, the RTF had to be able to reduce considerably the draw tension. A new distributed control system included the modification of the tension beam to avoid tears and breakage.

We recognized the decrease in demand for paper testing,” Parent adds. “At the same time, we saw the need in tissue testing, for example, uniformity as well as problems seen in embossing such as wrinkles. We are doing what we always did, but for tissue. We’re taking all the knowledge we learned from paper roll testing and applying it to tissue.”

This is not to say that the RTF has turned its back on other paper. “We are able to go from liner to tissue to flexible 

packaging, any web (including aluminum foil and plastic film), Parent says.

The major challenge the RTF team faced at the beginning was the inherent weakness of tissue, particularly one-ply bathroom tissue. “But now we can do it,” Parent says.

THE M-FACTOR

Analyzing strength uniformity (m-factor) is a useful tool in troubleshooting web breaks in tissue converting. In describing the work done at the RTF, Parent and co-authors Nina Deng, André Ménard, Jean-Francois Rousseau, Javad Saberian, Jimmy Jong, and Xuejun Zou wrote: “Low strength uniformity entails that there is a greater number of weak areas in the tissue; when the weak areas meet tension variations on the converting line, the risk for web breaks greatly increases. M-factor analysis consists in measuring the variations of MD tensile for a certain tissue length, and then calculates through software the Weibull modulus of the tensile distribution. The higher the Weibull modulus (or m-factor), the higher the strength uniformity and lower the risk of having web breaks.”

In North America, softness is key. Therefore, hardwood pulp is a desired furnish. But, with a higher amount of hardwood pulp, customers were facing runnability issues because of the loss in strength. This was happening at the converters as well as the mills.

Therefore, we need to get that perfect balance,” Parent says. “The industry is trying to get a more uniform product to be able to add more hardwood pulp.”

The softwood/hardwood issue is a relatively new area for the RTF. “We don’t know yet if furnish has an effect on CD profile or uniformity. This is in development,” Parent states.

Other parameters such as stretch, moisture, and bulk will all affect runnability.

Parent turns his attention to stretch, another vital parameter. “Tissue has a 20 times higher stretch than traditional paper grades,” Parent explains. “Low tensile, high stretch, and stretch is the key in many converting issues, so we are focusing on uniform stretch, finding the root causes of stretch issues that will lead to performance issues.”

PRODUCERS WANT THE SAME THINGS

Tissues producers are no different than other paper producers when it comes to what they want: high quality, no rejects, and the elimination of non-uniformity; everything related to efficiency. Wrinkles, bagginess, and web breaks are often at the origin of low efficiency. Parent adds, “It’s all related to the tissue itself, not the machinery.”

Often, the RTF sees problems the producer did not know existed because the rolls were rejected on the converting line. Sometimes, Parent says, the tissue makers do not know what they are looking for.

The equipment at the RTF is designed to test and quantify the uniformity of the web, especially CD tension profile. The web is gently deposited on a web tension beam made up of the Teflon pads that rest on the load cells. Measurement resolution (CD) is one inch. Each load cell locally records the pressure of the web, thereby generating the full width CD tension profile.

Other equipment includes scanners for moisture, caliper and basis weight. An Instron tester helps evaluate strength variations. A Tapio high-resolution tester looks at fiber orientation in both cross and machine direction.

Parent discusses some specific cases where the RTF helped mills. One mill had bagginess problems that led to speed

 limitations on the converting line. The RTF team was able to determine that it was due to misalignment between the yankee and the winder. There was more pulp draw on one side of the tissue machine than the other, making the tissue baggy.

In another, corrugations in the reel were caused by non-uniform bulk. Work was done on the calender and stack rolls were reground.

Parent also presented some case studies during his presentation at the 2017 TAPPI/RISI Tissue Conference in Miami. In one example, two rolls from the same parent reel behaved differently during converting. The RTF team looked at both CD and MD variability. The rejected roll showed a non-uniform CD tension profile.

In another case study, Parent showed how one mill had a non-uniform MD tensile across the CD. Lower tensile at the back may lead to more breaks and converting issues. It is linked to CD basis weight profiles. Basis weight decreases toward the back side could explain the lower MD tensile.

THE DOORS ARE OPEN

Parent says the RTF is open to all. “We get a lot of US customers,” he adds. “It is mostly a request type of operation.

We offer customers the opportunity to benchmark good rolls against bad ones, so they can see the difference, how to solve problems, and what a good roll should look like.”

The team is continually working on understanding the challenges of not only converting, but also tissue making overall, including calendering and winding—any issue that arises from non-uniformity.

Perhaps the conclusion to Parent’s paper in Miami also fits well here. With multiple complex operations, tissue converting is often the bottleneck in tissue production. There is significant opportunity for improvement.

Web uniformity is critical. Non-uniform tension causes bagginess. The RTF’s strength uniformity (m-factor) analysis provides new insights that can help a mill improve its efficiency.

There is an ongoing need to further understand the link between web uniformity and converting efficiency. This is a current focus of FPInnovations’ research effort.  

About Heidi Boe

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