Continuous heating vs. impulse sealing
The impulse sealing process is a good alternative to the classic, continuously heated sealing tools in widespread use which provides very efficient solutions and multiple benefits in complex heat sealing applications.
Continuously heated sealing tools are used in the majority of situations where plastic films have to be thermally bonded. This technology involves heating metal sealing jaws electrically and then maintaining them permanently at a settable temperature. This is generally the easiest way to bond plastic films together. Unfortunately, however, this technology may be simple but it also has drawbacks for the quality of the sealed seam or the productivity of the machine which are very difficult to compensate. In particular, the fact that the fused film cannot be cooled down directly in the closed tool, so that a strong and durable seal is obtained as soon as the two halves are opened, is a major disadvantage. When processing films where high elastic forces result following the sealing process (thick films or aluminum composites) owing to the pouch or bag geometry or the packaged goods, additional cooling stations or processes may have to be incorporated in the machine to enable the seam to be actively cooled after the sealing tool is opened. That means more challenges for the packaging machine manufacturer’s design engineers as well as higher running costs, for instance for the compressed air that is necessary to cool the seam. The heat which is emitted by the continuously heated tools can likewise be detrimental both to the product to be packed and to the film itself. Once again, appropriate design measures must be integrated in the packaging machine to compensate for this undesirable influence of the hot sealing tools on the packaged goods. Problems such as inadmissible heat stress or hot seams that are liable to come open can be avoided with the impulse sealing process. The heat of fusion for the film is only actually produced when, and for as long as, it is actually needed for the sealing process. The current which is passed through a resistor for this purpose causes a temperature rise there. As soon as the current flow is interrupted, the resistor starts to cool down as the generated heat is dissipated into the tool or the environment. The heat of fusion which was introduced into the film is thus removed again and the seam is cooled.
Impulse sealing offers several other advantages compared to continuous heating:
Lower power consumption
Electrical energy is only required during the heating phase of the sealing cycle. No current is fed to the heat sealing band during the cooling phase. Continuously heated sealing tools are heated throughout regardless of whether or not any heat is actually needed.
No heating-up or cooling-down times
Impulse controlled sealing tools have only negligible heating-up and cooling-down times when the machine is started or stopped compared to continuously heated alternatives. Continuously heated tools consist of solid metal parts which have to be heated before the machine is ready to use. If a sealing tool is due for maintenance, it has to be cooled down first. Delays of several minutes are not uncommon. Heat sealing bands heat up and cool down in just a few hundred milliseconds, or in the worst case a few seconds.
Impulse sealing – perfect control of the sealing process!
With classic time control, current is fed to a heat sealing band for a settable time. The band heats up continuously during this period; at the end of the heating cycle, the heating is switched off and the heat sealing band cools down again by dissipating heat into the environment. This method of controlling the heating element only remotely resembles impulse sealing with a heat sealing band, and the sealing process is extremely difficult to manage because there is absolutely no way of controlling the sealing temperature. Although it is theoretically possible to cool the seam under pressure, since the sealing temperature cannot be controlled, it could vary from one heating impulse to the next. Even though a constant heating power is supplied for a constant sealing time, the final temperature differs according to the start temperature of the heat sealing band. This is particularly true if the packaging machine does not operate either continuously or periodically. To avoid this, it is vital to control not only the sealing time but also the sealing temperature. Only then can we speak of a genuine impulse sealing process because in this case an ideal square-wave impulse which is more or less salient, depending on the rate at which the heat sealing band heats up and cools down, can be generated for the temperature curve over time. This rectangular shape ensures a repeatable seal quality and high machine output for a given sealing cycle time, with the optimal ratio of sealing time to cooling time. With our RESISTRON temperature controllers for heat sealing bands and our CIRUS UPT impulse sealing system, ROPEX has come up with the perfect solution.
RESISTRON or CIRUS? Two alternative impulse sealing processes for plastic films
Each of these technologies represents an excellent alternative for sealing thermoplastics using the impulse method. The efficient interaction of the RESISTRON temperature controller with the heat sealing band and the fact that all components in the control loop are individually specified by ROPEX guarantees the optimal ratio of sealing time to cooling time for a given sealing temperature. Owing to the special design of our CIRUS tools in combination with a UPT temperature controller that is fine tuned to this technology, the heating impulses which are generated are almost ideal. The CIRUS technology, in particular, achieves very fast heating-up and cooling-down rates, leading to extremely short process times when plastic films are thermally bonded. You can find a more detailed description of the RESISTRON and CIRUS technologies on the RESISTRON and CIRUS pages of our website.