The START signal is turned on for the duration of the machine use, and the heatseal element is constantly monitored and maintained at the pre-set temperature. During the sealing phases, the controller automatically compensates for the "lost" heat by adjusting the voltage supply. During pauses, only occasional controlling is necessary to maintain the preset temperature to offset the heat lost into the jaw and the environment.
When the jaws are open, the controller will compensate and overheating is not possible. The constant heat feature is usually used for speeds over 50-60 cycles/minute when the interval between seals is short and cool down time is minimal.
The advantage of this method is that the heatseal element is not constantly cycled. Therefore the power demand during the initial heating phase can be less than in the impulse mode. Also there is less demand upon the heatseal element (less expansion and contraction) the band remains in its expanded condition. The disadvantage of the constant heat is the loss of controlled coal down time while the jaws hold the seal. If the jaws open before the seal cools the seals may tend to shrink, pucker or deform. When operating with constant heat, the seal is frequently cooled immediately after jaw opening by use of cooling air or a subsequent quenching jaw.