Demands for improved print quality at lighter weights drive printing and writing paper manufacturers to explore new finishing techniques

Blandin, Stora Mills Implement Steam Finishing For Better Gloss

BY ROBERT L. YEAGER

Which profile variables should a mill control to guarantee consistent, optimum sheet quality? For the answer, mills must turn to their customers. One group—catalog and magazine publishers—have repeatedly said they want better surface quality and a lighter sheet, two seemingly conflicting objectives.

“Anyone who mails catalogs or magazines wants lower basis weights to compensate for the steady rise in postal costs,” says Robert LaPlant, senior technical services representative at Blandin Paper, a large supplier of lightweight coated grades. “At the same time,” adds LaPlant, “their competition is keen, so they want print consistency that’s better than ever.”

A similar sentiment is expressed by Orvar Blomqvist, production manager at Stora’s big SCA mill in Kvarnsveden, Sweden. “Competition is fierce in the SC-A grades,” says Blomqvist, “so we must constantly look for ways to improve quality. Our customers want to see uniformity throughout the reel, week after week.”

To address these customer requirements, both of these mills have installed new precision steam finishing technology on their supercalenders, with excellent reported results. The systems installed are the Gloss Profilers from VIB Systems, Tucker, Ga.

A NEW LOOK AT CD CALENDER CONTROL. Cross-direction (CD) calender control has been around for almost two decades but has largely focused on controlling the caliper (thickness) of the sheet as it moves through the machine calender or supercalender. So, it’s not surprising that the influence of steam on smoothness and gloss has not been broadly discussed. Today, few papermakers are aware of the substantial benefits available from applying varying amounts of steam across the sheet as it enters the calender nip, improving overall gloss and reducing basis weight, moisture, and caliper profile variation as well.

The need for CD control is widely accepted, since systematic profile changes will occur if no corrective action is taken. These changes can come from the calender itself (i.e., varying nip width or pressure) or from variations in the paper as it enters the calender (i.e., basis weight, thickness, moisture, temperature, filler distribution, etc.).

In either case, some corrective cross-direction action is desired. To improve readability or printability, the CD actuator making the corrections must have some effect on smoothness and gloss. (Note: Smoothness and gloss are themselves so closely related that only gloss will be referred to in the remainder of this article.)

With these requirements in mind, it is appropriate to review the application of zone-controlled steam and its effect on the gloss of the sheet. A finely controlled actuator impinges steam onto the sheet immediately before calendering (Figure 1), placing a defined amount of condensed vapor on the surface. This increases both the temperature and moisture of the sheet, lowering its resistance to deformation and increasing the calender’s smoothening effect in the areas of steam application.

The lowering of the sheet’s physical resistance is limited to areas close to the surface, since the time is short between the sheet’s steaming and its travel through the nip. So, the changes in paper properties are more pronounced close to the surface. This results in improved gloss and decreased sheet two-sidedness. In addition, the thermal energy of condensation heats the sheet (by differing amounts in the cross direction), assisting the gloss profiling.

The temperature of the heated calender roll also increases, since the heated sheet takes less energy from the roll. The roll diameter enlarges, in varying amounts, as steam is applied across the CD increments of the sheet. The nip is reduced at each CD location, as a function of the amount of steam applied, also improving surface condition.

Thus, the Gloss Profiler not only profiles the paper sheet directly, but the heated rolls as well, similar to other roll profilers using thermal energy. However, this latter process is much slower and its effect lasts much longer than that of the steam on the sheet. Therefore, steam profiling is a much faster method for controlling the gloss levels across the sheet.

Another notable benefit is derived from sheet steaming. The thermal energy of the heated rolls also flows to zones where little or no steam is applied, increasing the total temperature level of the roll surface. Consequently, the temperature of the whole calendering process increases, further improving the gloss of the sheet.

Steam finishing also helps the sheet edges, which have always posed special problems, since both roll and sheet heat losses are higher at the edges than in the middle. Additionally, a loss of nip pressure generally occurs at the edges, making them thicker, which causes roll hardness problems when building the reel. As a solution, papermakers have traditionally run a lower weight at the edges and/or a higher incoming moisture profile (to reduce deformation resistance at the calender).

While moist edges are suitable for lowering deformation resistance and reducing thickness, thinner edges of the incoming profile cause loss in gloss because of insufficient deformation in the nip. Thus, satisfactory reel building is offset in these cases by losses of quality in the sheet surface. Fortunately, steam profiling can help solve this problem. By applying extra thermal energy and moisture to the edges, the Gloss Profiler can increase the sheet-edge gloss level to that of the remainder of the sheet. This also results in a flatter weight and moisture profile.

GLOSS PROFILING AT BLANDIN. The Blandin mill in Grand Rapids, Minn., produces about 1,200 tpd of 32-lb to 60-lb LWC paper, destined for use on the presses of high-end catalog and magazine publishers (e.g., Time Inc., J.C. Penney, Spiegel, Fingerhut). Blandin’s integrated pulp mill feeds bleached groundwood pulp to four paper machines—three fourdrinier units and one gap former machine—which are supported by four off-machine coaters and eight supercalenders.

In 1993, Blandin management saw the need to improve top- and bottom-side gloss uniformity on the sheet leaving the supercalenders. So, mill specialists began a search for the enabling technology.

“As our customers moved to lighter weights, they pressed us for greater gloss uniformity and less sheet two- sidedness so they could maintain print consistency,” says LaPlant. “That sparked us to look for technology to make this happen.” After a thorough evaluation of technologies and systems available, the mill chose the Gloss Profiler.

Soon after the selection, a small test unit was installed on the No. 5 supercalender, which produces world-class LWC paper. “We tried a four-foot, manually controlled actuator on the supercalender to determine the unit’s capability,” says Ken Warren, senior maintenance specialist at the Blandin mill. Visible results appeared as soon as they turned on the actuator.

“Even on the test unit, right from the start, the difference was incredible,” says LaPlant. As measured by the ABB online scanning systems, overall gloss improved by more than 15%. “We also manually profiled the steam actuator and saw that we could substantially improve the gloss profile,” he adds. “That convinced us that the unit could deliver the quality gains that we wanted to see.”

Four profilers (i.e., two actuators on each side of the sheet) were then installed on the No. 4 and No. 5 supercalenders, each nine-rolls high and 150-in. wide (Figure 2). Each actuator houses 25 precision steam nozzles, installed at 6-in. centers across the sheet for a total of 100 nozzles per supercalender. The Gloss Profilers are controlled by an ABB Model 1190 control system, which scans gloss on both the top and the bottom of the sheet.

PROFILER RESULTS. “We noticed the results immediately,” says LaPlant. “As soon as we put the units on control, we could visibly see the profile improvement.” This was verified by the online scanning system, which showed the difference in gloss variation before and after placing the units on control. The “before-and-after” tests were run for three different paper grades—i.e., litho, rotogravure, and a machine-finished grade (Figure 3)—and showed top gloss variation decreases of 51.3%, 64.2%, and 50%, respectively. Bottom gloss variation improved by 59.1%, 71.2%, and 50.0%.

Based on these results, additional systems were installed on four more supercalenders during the spring of 1995 and early 1996. “The gloss profiling system also showed that we had work to do upstream of the supercalenders,” says LaPlant. For example, the effects of upstream coat weight profiles, which were previously masked, could now be readily detected. Edge quality also increased.

“We also got a bonus from the profilers,” says Tom Nelson, paper machine superintendent. “As the gloss variation decreased, the caliper variation declined, as well. So, we’ve eliminated reel-building problems due to poor caliper profiles in the sheet.”

Mill operators justifiably worry about electronic systems in a steamy environment, so all profiler electronics are mounted remote from the on-sheet equipment. “To date, reliability has been excellent,” says Warren. Tom Nelson adds, “These systems just came in and worked, and we started getting results right away. That was really refreshing.” Based on these results, the mill plans to install similar systems on the two remaining supercalenders, which will put all the mill’s supercalenders under Gloss Profiler control.

NEWSPRINT/SC-A MILL EXPERIENCE. Gloss Profilers have also recently been installed at Stora’s Kvarnsveden mill, which produces 630,000 mtpy of newsprint and supercalendered paper (SC-A). The actuators reside on two supercalenders servicing PM 8, a 5.5-m, 110,000 mtpy gap former machine.

“Gloss profiling looked like a fine way to help meet our customers’ demanding quality requirements,” says Orvar Blomqvist. Before the profilers, the supercalenders each had a manually-controlled steam shower, top and bottom, applying a single level of steam across the sheet. “It was very difficult to control the gloss manually,” says Blomqvist, “and it had no profiling capability.”

The new actuators were installed on both top and bottom sides to profile steam across each of 38 zones (150-mm) across the sheet. Each profiler is followed by a Glosspipe, which controls the base gloss level, while the profiler adjusts the profile around this base. As the sheet leaves the supercalender, a scanner continually measures both gloss and caliper profiles.

“We could see the results right away,” adds Blomqvist. “Gloss increased and became more uniform across the sheet. Our customers have been very pleased as well.”

Since the installation, the two-sigma variation has been reduced from a level of 1.5 to 2.0 points to about 0.5 points—a reduction of 60% to 75%. “Our supercalender reject level is much lower as well,” notes Blomqvist, “due to the more uniform caliper profile that also results from the gloss profiling.”

CONCLUSION. Experience has shown the beneficial effects of applying zone- controlled steam across the sheet in the various calendering processes. Importantly, the steam adds both moisture and temperature to the sheet before it enters the calender. Varying amounts of steam, applied in the cross direction, significantly reduce gloss and smoothness variations, even if the variations are due to incorrect nip settings or bad incoming sheet profiles. Also, the steaming itself raises overall gloss levels and greatly reduces two-sidedness in the sheet.

Of course, the zone-controlled steam application needs a feedback to ensure proper results. The simplest feedback can be a laboratory measurement immediately after the sheet is finished. However, the best possible surface profiles are achieved when the setpoints for automatic control of the steam application are being generated from a scanning online gloss measurement.

ROBERT L. YEAGER is president of Integrated Marketing International Ltd., Wesport, Conn.