Impulse sealing is a method of heat sealing thermoplastic. This process is just one of the many ways to heat seal plastics. Impulse sealing gets its name from the method used to create the heat. A large fixed coil has a plastic layer placed on it–it generates a burst of heat followed by a burst of cool. This will seal the thermoplastic and immediately cool it to create a long-lasting bond.
Bonding plastic is often easier than bonding other substances. Plastic has a high degree of uniformity, which means that one area of a plastic is nearly indistinguishable from another area of that same plastic. As a result, when plastic is bonded, it basically melts into itself. Since the melted zone bonds with itself, it often acts as though it were a solid piece, rather than a bonded joint.
With most plastics, heat is the preferred method of bonding. Chemical bonding agents often create toxic fumes and rarely work as well. Heat is simple to create and doesn’t produce any waste. Since heat is such a common agent, there are several methods for using it to bond plastics.
When used as part of an assembly process, there are three common types of heat sealers. A continuous heat sealer is an area in the assembly process that is always very hot. Plastics move through this area and bond as they move. This is a ‘no frills’ method of sealing, as there is little oversight and no guarantee that the process worked properly.
Another common method of manufacturing level heat sealing is called spot sealing. This uses a continuously hot metal rod. When the part moves by, the rod touches it in a specific spot. This is generally much more accurate that continuous sealing, but it only covers small areas at one time.
Impulse sealing is often used as part of a final production. This sealing process is often used to fuse multiple sheets of thermoplastic together. Sometimes the plastic sheets contain other substances that need to be sealed within the layers. In any case, impulse sealing will seal an entire area simultaneously, regardless of what ends up sealed.
A common impulse sealing machine looks similar to a press. The sheets of plastic enter the machine, sitting on top of an inactive heating coil. The top of the machine, which also contains a heating coil, comes down on top of the plastic. The coils go from inactive to fully heated within a few seconds, instantly bonding the plastic layers. Many impulse sealing machines then pump cool water through the coils, dropping their temperature and hardening the plastic.