Impulse sealers work by applying a timed amount of heat to seal thermoplastic films such as polyethylene and polypropylene (poly bags, or poly lined bags) followed immediately by congealing or cooling. Power is only in use when the jaw is lowered. Cool down occurs fairly quickly so there is minimal exposure to any heated surfaces.
Sealers are available depending upon how automated you want the operation to be to have perfect seals. Standard arm sealers are the lowest cost but require the operator to hold the arm down for the seal to congeal (cool down). Table top units have congealing cycles as well as automatic repeat cycles.