In the past, the primary techniques employed to join thermoplastic packaging materials or multi-layer packaging materials with a thermoplastic sealing layer involved direct heat contact. Alternatively, the bonding zone can be heated and melted through use of ultrasonics. In this sealing process, the packaging materials with the sealing layers to be joined are placed between the sonotrode (also known as horn) and a so-called anvil, which serves as a passive tool. When the ultrasonic energy is no longer introduced, the material in the sealing layer solidifies. Pressure is maintained as the melt cools for a material- and machine-dependent holding period. Ultrasonically created seams are narrower than heat contact seams, but are just as tight and as strong and conserve material as well.
A further benefit of ultrasonic sealing methods is that the heat needed for melting is generated in the sealing zone only in the interior of the seam (sealing layer). The temperature of the substrate (carrier) layer remains practically unchanged. As soon as energy is no longer being introduced, the heat dissipates to the outside more quickly because of the temperature difference between the substrate and sealing layer. The hot tack (strength of the warm seal) is considerably higher, so that the packaging can be filled immediately after the seal has been formed.
Ultrasonic methods ensure a tight and strong seam even when the sealing layers are wetted by the product, since the mechanical vibrations drive any possible contamination of the seam by the package contents out of the sealing zone. Reliable sealing of surfaces wetted by the product translates into considerably fewer leaky packages.
Since the tools are cold, the packaging material and package contents are subject to almost no thermal load. The time and effort required for cleaning is minimal. Use of nonstick coatings can be dispensed with. Moreover, ultrasonic sealing systems are characterized by high energy efficiency, since energy is needed only while introducing the ultrasonic vibrations and cycle times are short. These benefits become obvious, above all, when thick packaging materials or bonding between injection molded packaging materials/packaging aids and film are involved.
Ultrasonic sealing systems are ready to operate as soon as they are switched on. The ability to store sealing parameters in the memory of the ultrasonic generators permits automatic switching between parameter sets. Changes take effect immediately with the start of the next sealing operation, which means no lost production time.