Measuring Heat Sealers
Process control and validation are the cornerstones of successful heat sealing.
By David Vaczek
Although companies’ requirements for heat sealers vary, all systems must support the ability to set precise sealing parameters for producing consistent seals. Using technology such as programmable logic controllers (PLCs) and proportional integral derivative (PID) controllers, heat sealers typically control sealing values, such as temperature, within tight ranges and within sealing windows tested and set by end users.
Often, machine and supplier selection hinges on the ease with which machines and their sealing processes can be validated. In validation, companies establish documented evidence that processes perform consistently with a high degree of assurance.
“The big issue with our customers is the validation process. They want the ability to recalibrate and validate machines for product changeover. Another major concern is how long the machine is going to work before requiring maintenance,” says Garry Jones, export sales manager, Fischbein-Saxon (Statesville, NC).
To support validation, Sencorp (Hyannis, MA) supplies customers free sample IQ and OQ protocol documents. “This documentation is designed to allow someone to follow the process step by step. IQ addresses the documentation of the machine identity and its conformance to the purchaser’s PO and to the equipment manufacturer’s specifications. OQ verifies that everything functions as designed,” says Kent Hevenor, sales manager CeraTek, Sencorp.
Sencorp’s documentation was a factor in Boston Scientific’s decision to expand its use of the company’s medical hot-bar sealers, says Tim Early, senior manager, packaging R&D Endovations division, Boston Scientific.
“We have standardized on Ceratek and are in the process of upgrading our pouch sealer equipment,” Early says. “Medical device suppliers have to champion their equipment through to the customer. Sencorp seemed to have a better understanding of the validation requirements. They are telling us they understand FDA’s expectations, and they are helping us comply by providing template documents for equipment validation,” he says.
Belco's hot-bar pouch and tray sealers control bar and platen temperatures within ±1°.
Belco Packaging Systems Inc. (Monrovia, CA) gives customers a starting point for validation of its hot-bar pouch sealers and tray sealers. “We discover the initial parameters for the customer. We are making excellent seals by the time the sealer leaves the factory, as defined by 100% adhesive transfer between the tray and the lid,” says Tom Misik, vice president, sales and marketing.
“The customer then has a benchmark on what the machine can do using the process conditions we have established. The customer destructively challenges the package by deviating high and low from a known value that works, to define the sealing window. There is a ±5° alarm process window preset at the factory. The customer has to validate not only the temperature, but also the time and pressure settings. We are typically holding a temperature tolerance of ±1°,” says Misik.
To ensure that machines will produce a compliant package, Misik notes that Belco evaluates customers’ tray and lidding material for irregularities.
O/K International (Marlborough, MA) has launched a medical sealer with an Ethernet connection for capturing pouch seal data to a PC. “You can use one PC to monitor multiple sealers,” says Ann Marie Kellett, product manager. “Also, we don’t use Teflon-coated bands, reducing costs and downtime associated with changing bands and revalidating equipment.”
Belco Packaging and SCA Consumer Packaging Inc. (DeKalb, IL) feature tray-sealing validation ports that users can tap into to measure results.
While many sealers feature alarm functions and inherent systems for measuring machine performance, process validation practices require not only the calibration of the controlling components, but also independent verification of performance, such as with NIST-certified measuring tools. Yet firms have taken different approaches for proving machine compliance.
“In most cases, when you are validating temperature with validation ports, the same thermocouple that is supporting the temperature controller is [also] supplying the output to the validation port. This begs the question: Are you really validating the actual temperature of the sealing element, or simply the accuracy of the signal [that] the thermocouple is sending to both the controller and to the independent calibration meter?” says Charles Trillich, president, TOSS Machine Components (Nazareth, PA).
For validating sealer temperature, PackworldUSA (Nazareth, PA) in its TOSS-based impulse sealers mounts a NIST-calibrated Omega thermocouple to the machine. Since the temperature varies across all impulse heat-seal bands, the thermocouple is dialed incrementally across the width of the element until the temperature indication matches the controller’s set point. With the thermocouple in a position to measure the average temperature across the band’s width, the machine is cycled repeatedly at different set points to confirm that the measured temperature matches the controller settings.
“With this, we can demonstrate temperature accuracy and repeatability throughout the temperature range, cycle after cycle,” says Jeff Quinto, general manager, PackworldUSA.
Charlie Webb, president of van der Stahl Scientific Inc. (Wrightwood, CA), agrees that machine-independent testing is critical. “It is not good science to ask a machine to wholly monitor itself with watchdog equipment intrinsic in the machine. People lean too much on their equipment to be automatic validation boxes. The technology that is performing the sealing should not be relied upon for determining whether the machine is engendering compliant seals,” Webb says.
Webb says that firms need to focus on the end result. “Nothing can replace in-process seal peel and burst testing. You need a quality team, a cogent preventative maintenance program, and seal testing,” he says.
Van der Stahl's impulse sealers hit heat-up and cool-down temperatures consistently.
Van der Stahl will launch this year a patented peel-testing system integrated into its impulse sealers for seal-strength testing following ASTM F88 standards. A separate microprocessor controls the peel testing. Operators are prompted to stop at intervals and to peel bags on the machine. The unit locks down if minimum seal test volumes are not accomplished. Also integrated will be the firm’s Visual Inspection Unit (VIU), which uses polychromatic side lighting and 3× magnification for highlighting pouch seal topography.
“This is a paradigm shift in our industry. The machine checks to make sure the end product is compliant,” Webb says.
PLCS AND PROCESSORS
Some machines use PLCs, and others discrete microcontroller-based designs dedicated to specific tasks.
PLCs support data acquisition and programming versatility. “The PLC gives our machines enhanced process control capabilities, and it is incrementally more accurate. Using a PLC, we provide as a standard feature a system for validating air pressure,” says Misik.
“Typically, makers of disposable products are looking for repeatability, confidence in their high-volume output, and systems for auditing all phases of the process. Makers of implantable products want the highest levels of process control, so that each and every time they cycle the machine, they get exactly what they expect,” says Misik.
“With a non-PLC machine, there is no way of setting out a range of pressure parameters, and you won’t have a visual indicator. You can set independent and unique parameter settings for pressure and time,” says Tom Aidich, design engineer, SCA Consumer Packaging (DeKalb, IL).
This flexibility is useful when two operators are using a dual-shuttle machine. “You can use different values for pressure and time [on each side], while sharing the same platen dimensions and temperature,” says Aidich.
PLC units also support recipe management. “With a PLC, recipes can be stored and easily pulled up to match whatever product you are running that day, so you have a more accurate and more repeatable set up,” says Kenneth Sullivan, director of marketing, SCA.
Yet John Lewitt, vice president of sales, Emplex Bag Sealing Solutions, PlexPack (Toronto), says that users of its continuous band sealers “have shied away from technology that makes their validation process too long.”
“People are looking for a product that can be up and running quickly. Normally we put one controller on each parameter. We offered a PLC, but no one ever bought it,” he says.
|See Sidebar: The Ergonomics and Measurement of Shuttle Sealers|
For narrow-range control of temperature, PID controllers are used in many heat sealer types. PIDs calculate the heating input required when machine temperature set points vary from the measured temperature of the heating element. PID algorithms reference past deviations and anticipate future deviations in adjusting process inputs to keep the temperature stable.
“We use a stand-alone solid-state relay PID controller for maintaining constant platen temperature. The PID communicates with the PLC and the thermocouple.
We do not rely on the PLC to do PID control. The PLC tells the PID what set point to go to and what alarm values to use. The PID controller reports back to the PLC on the temperature values and power output,” Aidich says.
“The consistency of PID controllers is excellent. As long as you don’t change any machine components used in deriving your program, you can transplant the programmed controller to any machine,” says Tom Volby, design engineer, Belco Packaging.
“PIDs hold temperatures within ±2°C in our continuous rotary and hot-air sealers. If the temperature fluxuates more than 5°, the unit shuts down,” says Fischbein-Saxon’s Jones.
Volby notes that seal bar and platen configurations influence the heating controls employed. “We have a very large thermal mass in our tray sealer platens capable of storing the heat energy—20 sq in. and 2 in. thick. So you don’t have to constantly adjust heater output to maintain your temperature when you have repeated surface contact with your trays.”
In Precise-Seal medical impulse sealers from Aline Medical Packaging, Aline Heat Seal Corp. (Cerritos, CA), a temperature impulse controller (TIC) is integrated with and controlled by the PLC. The PLC-PID-based system reads and updates heating element power every 18 milliseconds as the impulse band heats and cools.
“The PLC handles all the machine functions, including temperature control. We know that we can avoid overshooting and stay within ±1° of the set point very easily,” says Julio Gonzales, vice president, manufacturing.
The PLC units support on-the-fly creation of custom applications and recipes can be password protected. “Operators can call up six different screens programmed for sealing up to six different materials, without the ability to change the settings,” says Gonzalez.
STURDY IMPULSE SEALING?
Aline Heat Seal’s president Charles Schapira says that temperature variance is partly a function of ramp-up speed. Aline sealers can be programmed for different requirements.
“There is a trade off between speed and accuracy that can be calculated in milliseconds. If you are ramping up to temperatures in one second, we provide accuracy within a degree or two of the set point. If you are ramping up in a half-second, you will have an initial overshoot by as much as 10°. That is not an issue for certain customers,” Schapira says.
Impulse sealer vendors say that the technology provides high accuracy and consistency for many applications. Yet some users are concerned about impulse sealers’ relative variability and the difficulty this presents in equipment validation.
Webb at van der Stahl says that impulse sealers meet most of its customers’ needs for sealing dual-substrate packages that have narrow heat-seal windows.
“Impulse sealing is sturdy, proven technology, with a long successful history of providing good repeatable seals,” Webb says. “A lot of companies miss the fact that sealing a pouch is a molding process. You are liquefying the clear film and inducing it to flow into the fibrous back of the medical-grade paper or Tyvek. Then you are setting it in place as the seal cools under pressure. Having a thin element with little mass supports the quick cooldown that is essential for sealing the film.”
When tested in auto cycle mode with tight dwell cycles of 1.4 sec, the company’s MP 350 NP Medical Pouch Sealer hits high and low temperatures consistently.
“This is much faster than the sealer would be used in most applications. We are not getting any kind of temperature build up that would affect the cool down,” he says.
Quinto says that PackworldUSA impulse sealers respond to band temperature changes every 17 milliseconds, employing its TOSS sealing system, which is based on RTD (resistance temperature detector) technology. “Using older impulse technology familiar to many, temperature control is often an issue. With TOSS, our tolerances are tight from seal to seal, and any band temperature loss caused by contact with the work piece is adjusted in milliseconds. Using a high-response controller, we can support accurate ramp up to temperature at rates as fast as 1° C per millisecond,” Quinto says.
For some applications, however, companies have favored hot bar sealers as technology that provides tighter temperature control, such as in high-capacity pouch production.
“Impulse sealers are not as repeatable and accurate in the delivery of temperature. The cooler the bar is, the more it acts as heat sink. The amount of heat that is going into the pouch changes in direct correlation to the amount of heat sinking into the bar,” says Sencorp’s Hevenor.
“It is much more difficult for a package engineer to demonstrate a thorough process validation with an impulse sealer,” says Early, who describes most of Boston Scientific’s pouches as requiring peelable seals.
Early explains that in PQ, engineers first perform a range study, varying the parameters to establish the sealing window. Design-of-experiment testing is then used to center the process window and define the optimal sealing point. Using a process capability study, the repeatability and consistency of the process is defined and expressed as a CPK value. “The CPK describes how capable your sealing process is within the number of standard deviations from your minimum specification requirements. If your sealer’s temperature varies significantly, it’s eating into your sealing window, and the net result is a lower CPK number that may not meet your company’s requirements,” he says.