Process Control Improves Heat Seal Quality

Sealing Methods

In recent years, the process of joining plastic materials by the use of heat and force has undergone a major evolution. Where the sealing operation was basically the application of a fixed amount of heat for a fixed amount of time without monitoring or feedback, it is now becoming a precise process of heating, pressing, and cooling under closely controlled conditions. The creation of nominal seals is a relatively simple task, but perfection requires close control over every aspect of the process. We begin with the recognition that so-called heat-sealing is somewhat of a misnomer. Whereas heat and pressure must be applied to induce commingling of the materials to be joined, the bond is not created until the subsequent solidification of the structure occurs during the cooling phase of the process. The continued application of pressure is also essential during cooling in order to maintain absolute control over the workpiece and produce a seal of good appearance and high integrity. Constantly heated sealing bars have long been regarded as the most reliable means of maintaining uniform temperature. With this method, the location of the cartridge heaters and consistency of contact between the heaters and seal bar must be very carefully designed to assure uniformity of heating, particularly on longer bars. This is equally true of thermocouple installation, because the thermocouple will only monitor the temperature at one location and cannot provide assurance of temperature uniformity over the entire sealing surface. Uncontrolled impulse sealing, using a fixed amount of power for a predetermined time, is also problematical because the temperature of the sealing surface gradually creeps upward with each successive heating cycle. Consequently, manual adjustment of the sealing parameters must be made during each working day.

Precision heat-sealing by the controlled impulse method is now readily available. Users recognize its advantages and are demanding higher-quality seals with the advantage of greater productivity because of fewer rejects. If perfection is required, nothing can be random or left to chance. The key word is control—over every aspect of the sealing process. The optimum sealing process begins with a heated sealing element that engages the workpiece at an accurately preset temperature and pressure. Pressure is relatively easy to control, but temperature requires a high-response temperature controller to heat the seal surfaces to a commingling state without temperature overshoot that can deform and damage the external surfaces of the workpiece. This is followed by a controlled cooling cycle, used under pressure until the material is reformulated and regains strength. The entire process can be accomplished in milliseconds.

Sealing Temperatures

The temperature of the heating element must be constantly monitored and controlled to ensure that each and every heating cycle is the same. Equally important, the cooling cycle must be consistent; with the jaws opening when the preset cooling temperature has been attained. The duration of the heating cycle can be a function of time, but the duration of the cooling cycle is ideally a function of the preset cooling temperature. In the case of some automated processes, the cooling cycle may be uniform in duration, dictated by the uniform speed of the production line. However, it must be at least long enough that the ideal cooling temperature is attained. For assurance that the seal has the desired integrity, it is essential to retain control of the workpiece until the cooling cycle is complete. A dependable seal cannot be made unless a modicum of pressure remains to hold the seal in place while cooling.

Temperature Control

Temperature control means the ability to maintain a predetermined, uniform temperature over the full length and breadth of the seal area from cycle to cycle without fail. Absolute control must be maintained regardless of changes in the environmental temperature. Heat-seal controllers are now available to both monitor and energize the heating elements uniformly over their entire surface. When the heating element is a band of uniform cross section, it will heat up uniformly over its full sealing surface when energized. When heated, its resistance will change slightly and uniformly as well. The controller can then monitor the change in resistance and interpret the measured resistance in terms of temperature. This is the fundamental principle employed in the design of resistance temperature detectors (RTDs). Every time the controller is activated, it will automatically repeat the complete sealing cycle as preprogrammed. Experts tend to agree that localized temperature sensing, such as with thermocouples, is uncertain. This is because the resulting temperature sample provides no assurance that the thermocouple contact surface temperature is the same as the heat source or is uniform over the entire seal area. (See “Can Heat Seal Temperature be Measured?“)

Controllers

Heat-seal temperature controllers are designed for high response. They drive the heat-seal band temperature from the standby preheat level to sealing temperature in 30 to 50 milliseconds, and maintain the temperature at the preset level with repeatable accuracy of less than two percent. In addition, the controller will signal an alarm and shut down the system in the event of a fault. The system can be programmed to display error messages in the event that the prescribed temperature is not attained or is exceeded. Other features include timed relays to activate jaw bars, indicate a completed cycle, or activate the next process. For the cooling cycle, the controller can be programmed to open the jaws after a predetermined duration or alternately monitor the heat-seal band temperature so as to open the jaws when the temperature has dropped to a predetermined level.

Bar and Platen Design Design of the sealing element is an important part of optimum performance. In order to minimize cycle time and maximize production, as well as to conserve energy, the heating element must be properly mounted, be thin, and have minimum mass so that it can be heated and cooled rapidly. When sealing films, the heat-seal band is designed with tapered edges that eliminate thinning and tearing along the edge of the seal. In addition, care must be taken to ensure that jaw bars are straight and true and platens are flat to assure uniform contact and application of uniform pressure across the face of the seal. Aluminum jaw bars and platens are preferred because the higher thermal conductivity aids in the cooling process.

Jaw Bar/Platen Pressure

In addition to temperature control, heat-seal perfection requires pressure control. Pressure or force can be monitored by pressure sensors or strain gauges, and 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 have this feature included.

Timing-Impulse Sealing Mode

It is very important that temperature, time, and pressure are very carefully synchronized. The figure on page 64 shows an example of the timely setting of temperature and jaw movement with the aid of a heat-seal controller. High performance and productivity with perfection require close coordination between the heat-seal impulse and the jaw action. Specifically, cycle start, heat-seal band at temperature, heat-seal jaw action, and power output are the most important timing steps. In addition, it is possible to energize the heat-seal band prior to jaw closing because the temperature controller eliminates the risk of overheating. After turning off the energy, most of the excess heat is absorbed by the jaws. Cooling of the jaws is sometimes recommended so that heat can be absorbed from the seal and the heatseal band more quickly. Contrary to operation in constant-heat mode, good thermal conductivity between the sealing element and the jaws must be assured by using a relatively thin, thermally conductive, backup material behind the heat-seal band.

Rest-Heat Mode

The rest-heat mode is a variation on the impulse sealing mode. For this mode to be effective, the heat-seal band must have enough mass to store the heat required for the sealing process. The power is cut off when the jaws close, and the retained heat in the heat-seal band is unloaded rapidly into the workpiece. This results in a shorter cooling phase and the shortest overall cycle time.

Preheat

To create a good seal, machines must be timed to allow enough time for the heating cycle to be effective, followed by time for adequate cooling before the jaws open. At high speeds, the jaws will be activated at a residual temperature somewhat higher than the ambient temperature. Since the first cycle is started at ambient temperature, there may not be sufficient time for the band to reach the sealing temperature on the first cycle or two. By using the preheat feature of the controller, a minimum temperature level can be set for the heat-seal band before production begins. A minimum temperature threshold assures faultless sealing the first seal and every seal, even when the allowable sealing and cooling times have been reduced to the bare minimum or when the machine is run intermittently.

Summary

Plastic materials can be sealed, or sealed and cut, at high speed with virtually zero rejects. By providing precise control of all parameters, high-quality, repeatable seals can now be made rapidly and cost-effectively with assurance of consistency.