Nuclear web gauging; keeps pace with processor needs.

Wider and thinner films, faster line speeds, more frequent product changes, and ever-stricter quality requirements are forcing evolutionary changes in thickness gauging technology.

As film production environments evolve, so do the demands placed on nuclear thickness gauges. Flexible manufacturing operations need sensors that can gauge multiple products. Other sensors must meet the tightened accuracy and precision requirements of today's quality-driven manufacturing. Often, nuclear gauges must address both demands simultaneously, providing heightened accuracy across a range of products. What's more, the increasingly common need to certify quality means that thickness sensing must fit into enhanced SPC frameworks.

In response to these sorts of pressures, gauge suppliers have had to broaden product lines by adding new sensors, developing more automatic calibration routines, and beefing up documentation capabilities. The bottom line for processors is the availability of more accurate, repeatable thickness measurements on wider, thinner, and faster-moving webs.

It's true that all gauging needs do not apply to every processor equally. "High-value films for products like videotape or capacitor film are definitely the most critical applications today, and they drive most of the developments," says Measurex marketing manager Patrick O'Neill.

Yet requirements for leading-edge applications often herald tomorrow's standard technology as quality demands strengthen across the board. Witness, for example, the waning tendency to buy packaging film merely by the pound. "More custom film producers used to sell by weight alone, but now their customers demand uniform rolls," says NDC Systems v.p. and general manager Yudie Fishman.

SHIFTING NEEDS

The experience of Beaulieu of America, a polypropylene film producer, under-scores some of the issues surrounding nuclear gauging today. While starting up two PP lines in Bridgeport, Ala., the company ran a long trial comparing nuclear gauges from Ohmart, NDC, and Barber-Colman. Over a period of six months, Beaulieu validated the performance of each gauge by comparing on-line data with off-line laboratory measurements.

In this case, the Ohmart gauge won out, says extrusion department foreman Brian Smith.

But manufacturing needs change over time, and Smith indicates that he might make a different choice today. Since the start-up two years ago, his company's requirements have grown to include ease-of-use features, calibration by plant personnel, and enhanced SPC capabilities.

"In my view, recipe set-up on the Ohmart is more involved and technical than it needs to be," says Smith, noting that the same held true for the Barber-Colman unit. By contrast, the NDC settings could be changed by Beaulieu's staff. And the unit provided more in the way of historical SPC information, according to Smith.

Barber-Colman also exemplifies how a supplier has tried to keep pace with these needs. Since the trial, it has added software enhancements that allow users to easily calibrate their own gauges. "Everybody has been pushed in the same way," says product manager Bruce Martin. "The end user doesn't want to come back to the manufacturer and say 'modify this.'"

THE SOURCE MATTERS

One way sensor offerings reflect more processing diversity than in the past has been through the emergence of new sensors and shifts in isotope choices.

NDC, for instance, has broken out of its 15-year role as a supplier only of gamma backscatter sensors and has come out with its first beta gauge. According to Fishman, a beta gauge becomes "significantly superior" as web width and speed increase. "Gamma gauges can't scan as fast as beta," says Fishman, comparing gamma's 1-2 in./sec speeds with beta's 3-12 in./sec range. Beta gauges also outperform gamma backscatter in precision of measurement. Fishman cites a repeatability of|+ or -~0.5% of film thickness for a gamma gauge, compared with |+ or -~0.2% or better for a typical krypton-85 beta gauge.

Responding to current regulatory pressures in Europe and anticipated ones here, NDC designed its new krypton-85 beta gauge around a 200 milli-Curie (mCi) source--roughly one-fifth the size of most beta gauges, says Fish-man. To ensure the smaller source would provide an adequate signal-to-noise ratio, NDC developed a proprietary detector chamber. Fishman claims this new design increases detection efficiency by a factor of six, facilitating the smaller source and related reductions in sensor size.

Even among beta gauges, source choices can vary. For films between 0.1-3 mils thick, Measurex sells a promethium-147 gauge in place of krypton-85. O'Neill explains that promethium inherently responds to changes in thickness with more sensitivity, making it more appropriate on thin films. For the same reason, NDC offers this source choice as part of its new beta gauge line.

Though the promethium source has been around for nearly five years, its use has grown sharply with the popularity of thinner films, according to O'Neill. "Downgauging by 10-20% isn't necessarily a huge shift, but repeatability becomes more of an issue," he says. Measurex places the repeatability of its Pm147 sensor at 0.004 mils or better--about twice as precise as its krypton gauge on thin films.

Though LFE once developed a promethium-based sensor, the company now reports that mechanical design innovations have enabled its year-old krypton gauge to compete in thin-film applications. One LFE customer has even found the company's latest krypton gauge equal to earlier promethium sensors in a 1/2-mil film application, says product manager Stephen Eakman. More important, Pm147's comparatively short half-life means source replacement roughly every three years while krypton can work for 10 years or more, Eakman adds. And Pm147's quicker decay can also eat up any sensitivity advantages. "After a year, it's worse than the krypton," Eakman says.

Meanwhile, gamma backscatter gauges continue to offer the most compact, cost-effective solution to gauging problems that fall within its limitations. And because many gamma applications don't require a scanning action, the lack of a frame can result in significant savings in purchase and operating costs.

CALIBRATION WOES 

As essential as manufacturing flexibility can be for some operations, it can engender gauging problems of its own. As film compositions vary in a multi-product setting, accuracy can drop off as a result of calibration errors, says Ohmart senior application engineer Bob Yowell. These errors spring from the tendency of each composition to cause a different degree of "forward scatter," or stray particles that throw off a detector's signal, Yowell explains.

Coextrusion presents an especially tough challenge because calibration errors can arise from changes in the layer ratio--not to mention the composition of one or more layers. Dense pigments or other additives can induce further errors, explains Fishman, because beta gauges become more composition-sensitive as the atomic weight of the target material goes up. Here, too, is another area where gamma gauges can shine. According to Fishman, they are less sensitive to atomic weight, allowing them to tolerate a wider variety of composition changes. For example, 3M Co. can run a wide variety of adhesive-tape film using a single gamma-gauge calibration, Fishman notes. "The most precise measurement device in the world is worthless if it won't hold calibration with grade or color changes," he says.

Suppliers agree the fundamental basis of calibration--setting up the factors that relate a sensor's thickness measurements to a standard--must be incorporated in a gauge and should remain "transparent" to the user. Ohmart says its Unity Response feature conforms to this approach. According to Yowell, the company's lab develops a single calibration curve that describes the full range of products a processor plans to run. It does so with the help of a proprietary "filter" that reportedly eliminates the effects of forward scatter. Since a range of products can share calibration parameters, coextruded products are viewed as a single structure, says Yowell. Otherwise, each layer-ratio change would be viewed as a new product requiring new calibration settings. Similarly, Measurex and LFE say they have always been able to offer "composition-insensitive gauging" by carefully matching the isotope and detector design to a given range of applications.

Making the underlying physics of calibration invisible doesn't necessarily preclude user choice among different calibrations. NDC, for instance, prefers to let processors choose from a suppler-installed menu of calibration parameters. Touchscreen instructions enable users to alter calibrations when appropriate to recalibrate their own lines--much in the way they call up product recipes. "It really shouldn't be necessary to call in your gauge vendor," Fishman Says.

And thanks to the software upgrades of the past year, users of Barber-Colman systems can store calibrations for groups of products. Martin says two of the calibration set-ups would be of the users own choosing while a third would come with the system. Moreover, users can now view graphical representations of the calibration factors. "It's definitely easier to calibrate the gauge now," Martin says. "The software allows users to make changes they couldn't make before."

In operations making only one product, O'Neill notes that validating a calibrated sensor's performance against a film standard would be more common than recalibration. "If you're doing anything more than verification, something's definitely wrong," O'Neill warns. And here, inconsistent placement of a film standard in the sensor gap can create errors--a problem LFE solves with a jig that can hold film standards within 5-10 mils of the same position, Eakman notes.

ASSAULTS ON ACCURACY

Other well-known threats to accuracy become more important as films thin out. "Once you get around half a mil, external factors of all sorts become more critical than ever," O'Neill notes, listing temperature, pressure, and sensor alignment as just a few of the factors that can lessen accuracy. In fact, Measurex's main R&D thrust has lately been focused on eliminating these environmental and mechanical influences. With the Precision Plus series, recently adapted for plastics applications after debuting in the paper industry, Measurex made electronic corrections for environmental factors more automatic and damped temperature effects with a new "isothermal" frame design.

The company also found a new way to house its Pm147 source. In place of a sliding shutter, the new design places the source in a rotating housing. By bringing the promethium capsule closer to its target than in earlier designs, the new set-up reduces errors caused by a relatively large air column that would otherwise sit between the source and target material. The result, says O'Neill, has been a stronger signal that shows less sensitivity to changes in air temperature, air pressure, and film movement.

Ohmart, meanwhile, applies its Unity Response to film movement as well. Yowell notes that web flutter in the sensor gap changes the amount of forward scatter, throwing off accuracy. By filtering out unwanted forward scatter, however, Ohmart gauges can tolerate the effects of flutter--even quarter-inch web movements would throw off accuracy only by 0.2%, Yowell notes.

BEEFING UP SPC

Some information related to sensor operation can even fit into enhanced SPC infrastructure. "A lot of documentation capabilities that were buried in the system have become more accessible to the user," says Martin, citing Barber-Colman's ability to provide calibration trending as one example.

And Measurex's relatively new MXOpen system can automatically furnish trend plots of calibration changes or validation activities. O'Neill notes that such plots of sensor performance are based information that formerly remained internal to the sensor. Now, they can be easily displayed--for example to find out if each check against the standard yields the same value.

Likewise, NDC can provide historical calibration data, and Fishman points out that the rigors of increasingly popular ISO 9000 certification make that ability essential. "Gauge suppliers must provide the tools not only to calibrate the unit, and to do so easily, but to document the integrity of the measurement system itself."

"There is a new interest in traceable calibration accuracy," agrees LFE's Eakman, noting that LFE can provide systems with a long-term memory of standardization and sample-check values.

Last, some news comes in the form of a new domestic supplier. Betacontrol, a German gauge manufacturer, recently set up shop in New Jersey. Sales manager George Stoddard says the company furnishes a range of thickness sensors with the bulk being nuclear gauges. The company can offer all common isotopes, including the two mentioned here.