Medical Device Link.

Originally Published PMPN June 2004

Heat Sealers

Charles Trillich, president, PackworldUSA

PackworldUSA PW3016 with TOSS technology employs thermal coefficient of resistance for precise, validatable temperature control.

Experienced users of heat-sealing equipment seem to agree that it is difficult to accurately measure the temperature of the sealing device at the point where it engages the workpiece. Wafer-thin thermocouples can be used as a referee for monitoring, but it is a challenge to have them produce accurate measurement. When inserted between the workpiece and the heater, the thermocouple measures a temperature that is somewhere between the heat source temperature and the heat sink temperature.

The task is further complicated by the fact that some heating systems are not uniform; the measurement taken at one time or place is not the same throughout the length of the seal. In addition, increased productivity requires greater speed and shorter sealing cycle times. With heating cycle times of less than 0.5 seconds, the peak temperature of the workpiece may be maintained for only 20 milliseconds. This leaves insufficient time to obtain an accurate measure of temperature.

Resistance temperature detectors (RTDs) face the same obstacles, and optical pyrometry is useless because the site to be measured cannot be observed. 

There is, however, a way that this task can be accomplished. It requires the ability to measure the internal molecular activity within the heating element itself. This may seem to be complex, but it is not. The level of molecular activity is directly related to the temperature of the heating element. This activity is manifested by a proportional increase in the electrical resistance of the heating element. If the thermal coefficient of resistance (TCR) of the heating element is known, the electrical resistance of the heating element can be precisely measured. An electronic monitor then translates it into a digital display of heating element temperature. 

There is little magic here. This technique uses the same physical principles that are used in the design of conventional RTDs. There is, however, a critical consideration. The TCR of the heating element must be known, and it must be quality controlled to ensure that each and every heating element conforms to the design specifications and prescribed TCR. If the heating element is a heat-seal band of uniform shape and cross-section, the temperature along the length of the heat-seal band will be absolutely uniform. It will also cool rapidly to create a secure seal while the workpiece is held firmly between the sealing jaws.

These things done, a high response controller can simultaneously monitor the temperature and control the electrical current to the heating element, thereby attaining and maintaining the heating-element temperature throughout a preprogrammed heat-sealing cycle. Not only can the temperature be monitored and controlled, but controllers can also be set with a variable time/temperature profile and high/low limits that, if violated, will activate an alarm. Controllers are also fitted with system diagnostics that detect, report, and activate the alarm if any inconsistency in controller performance occurs.

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