Precise control improves heat sealing

Plastics can be sealed, or sealed and cut, at high speeds with virtually zero rejects. By providing precise control of all parameters, high-quality, repeatable seals can be made rapidly, cost-effectively and consistently.

Polymer producers continue to create previously unimaginable new plastics with superior properties for production, protection and display. Many of the newer polymeric structures require greater precision in the joining or sealing process to ensure both package integrity and tamper-evidence. So the objective is to produce high-integrity seals on production lines operating at maximum speed. The optimum seal:

Heat sealing is somewhat of a misnomer: The bond is not created until the structure subsequently solidifies during the process's cooling phase. Continued pressure application also is essential during cooling, to maintain absolute control over the work piece and to produce a seal of good appearance and high integrity. With constantly heated sealing bars, the cartridge heaters' location and consistency of contact with the seal bar must be very carefully maintained to assure uniformity of heating, particularly on longer bars. The thermocouple will only monitor the temperature at one location, with no assurance of sealing-surface temperature uniformity. Uncontrolled impulse sealing, using a fixed amount of power for a predetermined time, can be problematic as sealing-surface temperature creeps upward with each heating cycle. Consequently, manual adjustment of the sealing parameters must be made during each work day.

Readily available, precision heat sealing by controlled impulse's advantages include higher-quality seals with greater productivity due to fewer rejects.

The sealing process begins with a heated sealing element that engages the work piece at an accurately preset temperature and pressure—thus, a high-response temperature controller is needed. Then seal surfaces can be heated to a commingling state without temperature overshoot deforming and damaging the work piece's external surfaces. This is followed by a controlled cooling cycle, under pressure until the material is reformulated, regaining strength. The entire process can be accomplished in milliseconds.

Heating element temperature must be controlled to ensure identical heating cycles. Cooling also must be consistent, with the jaws opening at the preset cooling temperature. Heating cycle duration can be a function of time, but ideally, cooling is correlated to the preset cooling temperature. To assure seal integrity, it is essential to retain control of the work piece until the cooling cycle is completed. A dependable seal cannot be made without some residual pressure holding the seal in place during cooling.

Absolute control must be maintained regardless of environmental temperature fluctuations. Heat-seal controllers are now available to both monitor and energize the heating elements uniformly over their entire surface. When energized, the elements' resistance will change slightly and uniformly. The controller can monitor the resistance change, correlating measured resistance to temperature. This is the fundamental design principle of resistance temperature detectors (RTDs). Every time the controller is activated, it will automatically repeat a complete, preprogrammed sealing cycle.

Experts agree on the uncertainty of localized temperature sensing, such as with thermocouples. Heat-seal temperature controllers drive the heat-seal band temperature from the standby preheat level to sealing temperature in 30 to 50 milisec, maintaining the temperature at the preset level with repeatable accuracy of under 2 percent. The controller will also signal an alarm and shut down the system due to a fault.

Sealing element design is important for optimum performance. Cycle time should be minimized to maximize production as well as to conserve energy. To heat and cool the heating element rapidly, its key attributes must be low mass thinness and proper mounting.

When sealing films, the heat-seal band is designed with tapered edges that eliminate thinning and tearing along the edge of the seal. To assure uniform contact and application of uniform pressure across the face of the seal jaw, bars must be straight and true and platens flat.

Besides temperature management, heat-seal perfection requires pressure control. Pressure monitors—sensors or strain gauges—should be installed with high and low limits to monitor each cycle. Failure to fall within the prescribed limits should activate a fault response, sound an alarm and shut down the process. Some heat-seal temperature controllers include this feature.

Careful synchronization of temperature, time and pressure are very important. High performance and productivity require close coordination between the heat-seal impulse and the jaw action. The most important timing steps are: cycle start, heat-seal band at temperature, heat-seal jaw action and power output. Additionally, it is possible to energize the heat-seal band prior to jaw closing, because the temperature controller eliminates the risk of overheating. After de-energization, the jaws absorb most of the excess heat. Contrary to constant-heat-mode operation, good thermal conductivity between the sealing element and the jaws is assured using a relatively thin, thermally conductive, backup material behind the heat-seal band.

Rest-heat mode is a variation of the impulse-sealing mode. For effectiveness, the heat-seal band must have enough mass to store the heat required for the sealing process. Power is cut off when the jaws close, and the heat-seal band's retained heat is unloaded rapidly into the work piece, resulting in a shorter cooling phase and the shortest overall cycle time.

This article appears courtesy of Packworld USA Ltd.; www.packworldusa.com