Seven Popular Heat Sealing Methods for Packaging Bags: Which One Suits You?

There are many heat sealing methods, commonly used ones include manual heat sealing, high-frequency heat sealing, hot plate heat sealing, pulse heat sealing, infrared radiation heat sealing, ultrasonic heat sealing, and hot air heat sealing. Different plastic packaging bags should use corresponding heat sealing methods to achieve good heat sealing effects.

Below, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will explain the seven major heat sealing methods for packaging bags.

1. Manual Heat Sealing Method

The manual heat sealing method is the simplest heat sealing method, often used for PE and PP single-film heat sealing. Heat sealing requires the use of heat-resistant films (parchment paper, polyester film, etc.). First, bend the cutting edge of an ordinary electric soldering iron into an "L" shape, with a blade width of generally 3mm, and then connect the wire to the voltage regulator for constant temperature control. Leaving an edge at the sealing part of the film will result in greater heat sealing strength.

Before heat sealing, cover the heat sealing line of the film to be heat sealed with parchment paper. Then, hold the soldering iron handle firmly and move the heat sealing blade along the heat sealing line continuously to fuse the two melted surfaces together. During actual operation, it is best to lay 2 layers of thin flat cloth, and then lay a layer of khaki cloth that is not easily deformed. The weave should be at a 45° angle or straight to the heat sealing line. Its function is to create a uniform pattern at the heat sealing part to strengthen the pattern (playing a supporting role) structure to achieve the purpose of mutual penetration between molecules and improve the sealing of the sealing interface.

2. High-Frequency Heat Sealing

High-frequency heat sealing uses a high-frequency electric field generated by an electron tube self-excited oscillator to generate heat in the plastic dielectric molecules and apply a certain amount of pressure to achieve heat sealing. The basic principle of high-frequency heat sealing is that under the action of a high-frequency electric field, polar molecules are arranged in the opposite direction to the electrodes, with positive charges moving towards the negative electrode and negative charges moving towards the positive electrode.

If the direction of the electric field changes, the charges will also change position. This displacement of charge is called "polarization." "Polarization" is the process of making molecules orient themselves regularly. To do this, frictional resistance must be overcome, consuming a certain amount of energy, and this energy consumption is released in the form of heat, which is the reason for heat generation. Under the appropriate pressure of the bottom plate and mold, the two layers of film can be combined together.

The heat generated by the plastic film is proportional to the electric field frequency, the square of the electric field strength, and the loss factor. The frequency generally used for high-frequency heat sealing is about 50-80 MHz (20-45 MHz for PVC film). For thinner films, higher frequencies and lower voltages should be used to prevent the film from being punctured and producing electric sparks. High-frequency heat sealing machines include three parts: high-frequency oscillation, mechanical transmission, and electrical control.

3. Hot Plate Heat Sealing Method

The hot plate heat sealing method relies on external heating to achieve heat sealing. First, two overlapping layers of film are placed between two heated (or one heated) metal plates until the surface melts, then the melted surfaces are quickly combined under pressure, and the pressure is maintained until cooling. The hot plate heat sealing device has a simple structure, low cost, long life, and is not easily damaged. Therefore, it is widely used in the sealing of polyethylene bags and other flexible packaging.

4. Pulse Heat Sealing Method

The pulse heat sealing method, also known as heat pulse welding, first melts the heat-sealed part under pressure, then cools and releases the pressure. Usually, a nichrome strip is used instead of a metal plate as a heating element, and a transient current is passed through the nichrome strip to heat it.

During operation, the overlapping plastic film is placed on the workbench. Through manual or automatic control, the upper heating element presses the plastic film, and the current automatically flows into the nichrome strip. Then, a strong heat energy pulse acts on the heat sealing surface. After a short time, the current is cut off by a time relay, then cooled, the pressure is removed, and the heat-sealed part is taken out.

5. Infrared Radiation Heat Sealing Method

The infrared radiation heat sealing method can be used for heat sealing PVC and LDPE films. The heater is a silicon carbide clay rod with metal plating at the end to facilitate electrical contact. Electric quartz tubes can also be used as heaters. Most plastics absorb the most energy at a radiation wavelength of 3μm, and the corresponding surface temperature is about 700℃, and the heat sealing effect is also the best.

6. Ultrasonic Heat Sealing Method

Sound is the result of elastic vibration of a medium and can be transmitted in the medium. The frequency of sound audible to humans is approximately in the range of 16-20 kHz. If the frequency exceeds 20 kHz, it is called ultrasonic vibration, which is inaudible. The frequency used for ultrasonic heat sealing is 20-40 kHz.

The main component of the heat sealing machine is a transducer. A coil is wound on the transducer, and the coil is connected to an oscillator. Using the magnetostrictive effect, the high-frequency current is converted into mechanical vibration. Mechanical energy is emitted from the vibration head, and then transmitted to the film in the form of waves, causing an effect force inside the film, and forming a collision phenomenon between the two layers of film on the vibrating interface. With heating and temperature increase, the film softens and melts, and together with the reaction force of the supporting table below and the clamping force of the vibration head, the polymer molecules on the heat sealing surface diffuse into each other. Therefore, the molecules between the heat-sealed films have a strong binding force.

7. Hot Air Heat Sealing Method

The hot air heat sealing method uses hot air to blow it onto the surface of the plastic film for heat sealing. This method can be used for heat sealing heavy packaging.