Argument for consistency heats up sealing process

By Daphne Allen in Sealers on June 17, 2002

Suppliers debate the means to maintaining consistent temperatures.

 

Talk to any provider of heat sealers for medical device packages, and chances are you will hear the term validatable used several times. Most heat sealers today are equipped with a number of features to help users validate the heat-sealing process, such as digital temperature controllers and alarms for unmet parameters.

While these features certainly help packagers set, verify, and maintain sealing temperatures for an accurate, repeatable process, they may not be enough. Curt Larsen, a packaging consultant for DuPont Medical Packaging (Wilmington, DE), argues that one of the most important attributes of a heat sealer is its ability to maintain heat consistency across the seal bar or platen. "Some blister-sealing equipment I have seen has 10° to 20°F temperature variations across a seal platen. Such variation can cause sealing anomalies, such as extreme seal-strength variations, incomplete seals, and tray seal flange warpage. It may also transparentize Tyvek, which does not negatively affect package integrity, but may affect the consistency or smoothness of the opening peel forces and affect the look of the package."

Tom Misik, vice president of sales and marketing for Belco Packaging Systems Inc. (Monrovia, CA), agrees that platen temperature consistency is critical to package integrity. "Temperature control is important, but delivering that temperature is what it is all about. You may be able to control the heat to ± ½°F, but can you deliver it according to the same tolerance?"

A number of heat sealer suppliers have responded by adding continuous monitoring devices to their medical sealers. Others promote models that rely on impulses or hot air instead of bars or platens as the heat source. Finally, some say that heat consistency is not so critical, but if it is a problem, it may have more to do with a lack of preventive maintenance than it does with machine capability.

WATCH THE HEAT

To ensure that heat is delivered uniformly to the machine elements responsible for sealing, suppliers place thermocouples or other devices in sealing areas. In some models, if the detected temperature is above or below set parameters, alarms sound and sealing ceases. In other models that employ a closed-loop process, if temperature deviation is detected, the monitoring device will feed that information back to the controller, which will then adjust the temperature to meet specifications.

For instance, temperatures of the sealing elements in models from Packaging Aids Corp. (PAC; San Rafael, CA) and Accu-Seal Corp. (San Marcos, CA) are constantly monitored by single thermocouples placed along the sealing elements. "The thermocouple then feeds that information back to the temperature controller. If the seal temperature doesn't conform to set parameters, the seal is aborted and the machine will alarm," explains Russ Perrone, PAC's national sales and marketing manager.

Emplex Systems Inc. (Toronto, ON, Canada) offers two levels of monitoring on its band sealers, says John Lewitt, vice president of sales. "Our standard validatable package has one temperature controller controlling three heater bars and one thermocouple reading back the temperature. On our upgraded package, we supply a temperature controller and thermocouple for each heater bar."

On two of its medical pouch sealers, Belco Packaging Systems has built four temperature-validation ports to allow users to plug-in their own devices for temperature profiling and monitoring across the entire seal bar. On its tray sealers, it has built into the platen twelve thermocouples to automatically ensure heat delivery. "We are pretty proud of our incorporation of thermocouples into the platen," says Belco's Misik. "Platen design was quite a challenge— the thermocouples are invasive to the platen, so we needed to ensure that their placement didn't interfere with heat distribution. Our goal is to deliver consistent platen temperature and then monitor it to prove that we have done it."

NO NEED FOR ALARM

Constant monitoring through multiple thermocouples placed along sealing areas may be informative, but it may also be overkill, say some. "Most medical packaging materials are forgiving," says Accu-Seal's Roger Lakey, and as a result, they can handle minor inconsistencies in sealing temperatures.

Dave Bornhuetter of Pack Rite, a division of Mettler-Toledo Inc. (Racine, WI), says there is little need for such additions. The firm's model RTP relies on digital control of temperature and other parameters, which Bornhuetter says maintains a "tolerance of plus or minus a couple of degrees across the entire sealing area. Users shouldn't run into problems."

So the investment in extra sensors, which could triple sealer cost, may not be a prudent one for everyone. And for firms that equip each assembly station with a tabletop sealer, costs may be prohibitive. Bornhuetter's advice is to "keep it simple to keep costs down."

Kent A. Hevenor, product manager, laboratory machinery, for Sencorp Systems Inc. (Hyannis, MA), a division of DT Industries, does see a benefit to using multiple thermocouples along a seal die to monitor and verify the temperature profile along the die. However, he argues that such elements play no role in the actual process of providing consistent heat distribution. Instead, sealing dies can be designed to provide even heat distribution when used with custom-made heaters.

"The heaters that we use in our medical tray sealers are all one-piece heaters that generate heat across the entire seal area, as opposed to the traditional multiple-cartridge heaters used throughout industry that generate heat in lines across the die with the hope that it distributes evenly into the areas between the heaters," he says. "The one-piece heaters are designed with a varying watt-density layout to compensate for different heat-loss areas on the die." On Sencorp's MD-16x12 and MD-24x20 tray sealers, the average temperature profile across the entire 16x12-in. and 24x20-in. seal areas is ± 2°F when operating at about 265°F. The firm has also designed its Series-AS/1 and Series-AS/2 pouch sealers with a varying watt-density layout to compensate for different heat-loss areas on the die. On the 12-in. and 24-in. sealers, the average temperature profile is ± 1.5°F when operating at 285°F and ± 2.5°F when operating at 285°F, respectively.

Charles Trillich, president of Packworld USA and TOSS Machine Components Inc. (Nazareth, PA), says that the ability to make a consistently validatable seal depends upon the ability to control the temperature profile during both heating and cooling. "New technology employed in impulse heat sealing can ensure a repeatable sealing cycle while gripping the work piece until the seal is properly set," he says. TOSS and Packworld offer RES controllers that use the Alloy-20 heatseal element as the temperature monitoring device by measuring the changes in resistance of the element as its temperature changes, providing instant feedback and precise control. "The determined temperature is displayed and compared with the preset temperature target. The primary voltage of the power transformer is then automatically adjusted to achieve the desired temperature level. This closed-loop system constantly monitors and instantly corrects the temperature so that the actual temperature always equals the preset temperature."

METAL DEFECTORS

If all this metal weighs you down, you might want to try hot air. With a hot-air sealer, pouches are held in place with bands that press their two sides together after the pouches move past a stream of hot air. "The temperature of the hot air is monitored constantly," says Anne Marie Kellett, marketing manager for O/K International (Marlborough, MA), which markets the O/K Medical Supersealer. "There is much more uniformity in air than there is in metal bars covered with Teflon tape." She adds that another benefit to a hot-air sealer is that it provides a continuous stream of air held at a constant temperature.

"Another valuable addition to hot-air sealing," says Stan Hall, sales manager of Fischbein, maker of the Saxon hot-air and band medical sealers, "is the ability to reject bags that do not meet the specified sealing temperature." Saxon sealers are equipped with a reverse function tied to the temperature controller. If the set point goes outside the specified range, the belts reverse direction, rejecting the bag.

There may be concerns with constant heat, though, explains Charlie Webb, president of Van der Stahl Scientific Inc. (Wrightwood, CA). "Heat sealing is almost like injection molding. While under dynamic force, a polymer is exposed to heat and changes molecularization. To maintain that change, it needs to cool while under pressure." He points to the heat-up and cool-down processes that pouches are exposed to during sealing with an impulse sealer.

KEEP THE FIRES BURNING

Webb, who markets the MS-350 impulse sealers and other models considers the Nichrome wire typically used in impulse sealers pretty reliable. "Its like a lightbulb filament—temperature inconsistency is not an issue."

Instead, if operators are detecting uneven temperatures across their sealing bars, the problem may be a lack of proper preventive maintenance. "Operators are getting marginal service out of machines that are not maintained," Webb explains. For instance, if the Teflon tape used to cover the Nichrome wire is not changed regularly, burn spots may occur on pouches during sealing.

Webb says that many medical device manufacturer's engineering departments put a lot of time and money into testing and validating top-of-the-line heat sealers and designing the heat sealing processes to be used with those models. But, "if operations staff don't perform preventive maintenance, it is all for naught."

Proper machine calibration is also essential, says Emplex's Lewitt. "Every customer is different, but it is essential to calibrate the machine on an ongoing basis—anywhere from 6–12 months—to ensure that everything is working properly. All the alarms in the world can't help you avoid a poor seal if something becomes uncalibrated."

Controllers and thermocouples aside, temperature consistency may depend upon the simplest element of all—consistent housekeeping.

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