The impulse-heat sealing method occurs when the start signal is synchronized with the machine rhythm, every heatsealing cycle is followed by a cooling phase with jaws closed.
Cooling is effected with jaws closed so that the seal has already set and has good strength and appearance when the jaws are opened. This method is preferred when sealing time is not critical since seam strength and appearance are better.
Since the heat sealing is a thermodynamic reversible procedure, and often influenced by time restraints, it is very important that the relevant parameters: temperature, time and pressure are very carefully synchronized with regards to time and temperature cycle.
The following diagram shows an example of the timely setting of temperature and jaw movement.
The preset temperature should always be attained when the sealing jaws are still open. This allows the sealing element to expand without interference and avoids overstressing of the ends (phase 2).
The system should be designed to drive the heatseal band to the preset temperature in minimum time. However, the maximum rate of temperature rise should not exceed ,1°C./mSec. The total system, principally the secondary voltage, must be optimised to achieve the best result. (Our application service will give you the necessary data for your special application.)
The cool down of the heatseal band while still under the force of the jaws is the main advantage of impulse sealing (phase 5). After turning off the energy most of the excess heat is absorbed by the jaws.
Cooling of the jaws is sometimes recommended so that they can absorb enough heat from the heatseal band in less time.
Contrary to the constant heat feature, a constant heat flow between the sealing element and the jaws must be assured by using a thin backup material with good thermal conductivity behind the heatseal band.