The main forms of heat-sealed bags are hot plate type, pulse type, high frequency type and ultrasonic type.
(1) Hot plate type heat sealing. This is the most common heat sealing method in which the film 3 to be sealed is pressed between the heat-resistant rubber 4 and a heating plate heated to a certain temperature (constant temperature control) to achieve heat sealing. The heat sealing machine has the advantages of simple structure and principle, and fast sealing speed, so it is widely used for heat sealing of polyethylene film and polyethylene composite film. However, it is not suitable for heat sealing of a film which is easily contracted or decomposed by heat.
(2) Pulse heat sealing. For example, a nickel-chromium alloy strip film is pressed on a heat-resistant rubber, and a nickel-chromium alloy strip is instantaneously applied by a large current, and the heat is used for sealing. It is characterized in that the nickel-chromium alloy strip leaves the heat-sealed portion after being cooled, so that even a film that is easily deformed can be heat-sealed by this method. This method is suitable for the sealing of products requiring high sealing strength and sealing properties. For example, the packaging of items with a lot of water and the sealing of vacuum packaging. This method is generally suitable for intermittent sealing.
(3) High frequency heat sealing. It is suitable for high-frequency electrode pressing film, plus high-frequency voltage, heat-sealing by dielectric loss of polymer. The sealing portion has the highest sealing surface, so the film does not overheat and the sealing strength is high.

(4) Ultrasonic hot air. This hot air mechanism is composed of a high frequency oscillator, a magnetostrictive vibrator that converts high frequency electric energy into longitudinal vibration, and an exponential curve type amplitude amplifier that transmits longitudinal vibration to the film. The ultrasonic vibration transmitted by the exponential curve type amplitude amplifier during heat sealing causes the laminated surface of the film to generate heat and melt and bond. This heat sealing method is characterized by heat generation at the center of the film lamination, and is suitable for continuous sealing of heat-shrinkable films such as biaxially oriented films.