Comprehensive Guide to Heat Sealing Machines: Methods, Types, and Applications

Introduction

Heat sealing is a crucial process in the packaging industry, used to bond plastic films using heat and pressure. Different heat sealing methods offer different advantages depending on the type of material and specific packaging requirements. Whether you're using polyethylene film, composite materials, or specialized films for sensitive products, choosing the right heat sealing method can ensure a strong, durable seal that maintains product integrity. This guide explores the most common heat sealing methods, including plate, roller, pulse, and ultrasonic sealing, along with their applications and benefits in packaging.

According to different heating and pressure methods, the heat-sealing methods of heat-sealing machines can be divided into the following types.

Plate heat seal

Plate heat sealing is the most common heat sealing method. It uses a heating plate to heat and press intermittently to achieve plastic film sealing. The principle of heat sealing is shown in Figure 6-2. The two layers of film 3 to be sealed are transported between heating plate 1 and worktable 5, and then pressed tightly against the non-stick material 4. Heating plate 1 and worktable 5 are heated to a specific temperature (constant temperature control), pressurized, and then cooled to achieve a tight seal.
The commonly used anti-stick material is high-temperature-resistant polytetrafluoroethylene or fiberglass cloth, especially to prevent sticking of the T-table and film material layers, so that the bag opening is neatly sealed and the worktable can be kept clean. Electric heating wire 2 mounted on heating plate 1 heats the heating plate. The heating temperature is usually detected by a resistance temperature element and displayed by a temperature display device. The temperature is adjusted by a voltage regulator or a resistor. To ensure a high-quality sealing weld, the heat-sealing surface of the heating plate should be flat, and the supporting plane should be flat or cushioned with heat-resistant rubber. The pressure-moving mechanism for the heat-sealing plate 1 can be pneumatic, hydraulic, cam-driven, electromagnetic, etc.

This slat-type hot-melt sealing device features a simple structure and principle and offers fast sealing speed. It is widely used in intermittent automatic packaging machines. It is primarily used for sealing polyethylene films and polyethylene composite films, but not for films that shrink easily or decompose under heat.

Heat roller

The roll heat sealer is used to seal plastic film by applying pressure with continuously rotating rollers and heating with one or two rollers in a pair.
The principle of roller heat sealing is shown in Figure 6-3. When two layers of film 2 to be sealed are drawn through a pair of heating rollers 1 (or just a single heating roller), they are heated and pressed, and then cooled to achieve a tight seal. Resistance heaters are installed in the heating roller, and current is supplied to the heating wire through wiring devices (such as brushes, slip rings, and wires) that are shielded from the machine. The temperature adjustment devices, as well as the instruments for detecting and displaying the temperature of the heat-sealing roller, are located externally.
The feature of the heat sealing roller is a continuous seal, which is suitable for heat sealing composite films consisting of a base film (cellophane) and a heat sealing film (polyethylene); In some continuous automatic packaging machines with multiple functions such as bag making, filling, and sealing, the heat-sealing roller not only completes the longitudinal heat sealing of the bag-forming packaging film material but also plays a role in guiding and conveying the packaging film web. For single-layer film, it is easily deformed by heat, which will affect the sealing quality, so it is not suitable for use.

Belt heat seal

As shown in Figure 6-4, it clamps two film layers 2 between a pair of thin, round belts 1 (such as polyethylene belts, steel belts, stainless steel belts, or nylon textile belts) rotating in opposite directions, and heating it in the heating section 4 provided on both sides of the belt to cool the two film-bonding layers, then the pressing section. 5 to avoid film 2. Before the seal is fully formed, it is embossed by a pair of embossing rollers with pre-adjusted pressure, and then the production date is printed by a printing code roller, and finally the seal is completed.

This method is suitable for heat sealing composite film materials. Even shape-memory films can be directly heated with this machine, and the sealing speed is quite high, making it widely used.

Sliding Roller Heat Sealer

As shown in Figure 6-5, two overlapping layers of film 1 are passed between a pair of heating plates 4 to be heated to a soft, molten state, and as they pass between closely pressed heat-sealing rollers 3, they are squeezed and welded. The features of this type of sealing device are that the resistance heater and the heat-sealing roller are each independent components, which simplifies the roller's structure. The overall design is straightforward and offers a wide range of applications.
This method can be used for the continuous heat sealing of film packaging materials, and can also be used for the continuous heat sealing of films with large thermal deformations. If the machine stops unexpectedly, the film material between the electric heating plates will overheat and be lost, so the heater must be able to automatically vent to prevent film overheating.

Sliding Heat Seal

The sliding heat seal is used for packaging, as shown in Figure 6-6. It uses film 3 to wrap the packaged object 2 so that it slides on heating plate 4, and relies on the slight pressure exerted by the packaged object and the heat of heating plate 4 to bond the overlapping portions of the two film layers.

Pulse heat seal

As shown in Figure 6-7, the nickel-chromium alloy strip 2 presses film 4 onto heat-resistant rubber 5, and the nickel-chromium alloy strip 2 is immediately heated by a large current, with the heat used for thermal sealing. Its feature is that the alloy strip 2 leaves the heat-sealed portion only after it has cooled, so easily deformable films can be sealed using this method. This method is suitable for sealing products with high requirements for sealing strength and sealing, such as liquid packaging and vacuum packaging. This method is generally suitable for intermittent sealing and is widely used in bag-making machines or automatic packaging machines.

Liquid-cut thermal sealant

As shown in Figure 6-8, heating knife 5 (or steel wire) is used to melt and cut film 2 and to peel it off at the same time. This sealing mechanism has a simple structure, fast sealing speed, and can simultaneously complete melting, cutting, and sealing of the film. However, due to the limitations of the welded joint area, the sealing strength is relatively low and it is prone to opening. It is only suitable for small-quantity packaging of fine powders and granular items.

Pulse-melting heat seal

Melt-sealing heat seals require frequent heating of the steel blades and wires, whereas pulse-cutting heat seals are the opposite, as shown in Figure 6-9. When pressure plate 6 drives casting-alloy wire 1 to press film 2 onto heat-resistant rubber 3, nickel-chromium alloy wire 1 is immediately energized and continues to press the heated, molten sealing portion until it is released after cooling. This method can also complete the melting and film sealing simultaneously.

Melting heat seal

As shown in Figure 6-10, it involves bringing heating plate 1 or the flame to one end of the overlapping film 4 to melt and bond it. This method can produce a biaxially oriented, heat-shrinkable polypropylene film with strong sealing strength.

Ultrasonic heat seal

As shown in Figure 6-11, this thermal sealing mechanism consists of a high-frequency oscillator, a magnetostrictive vibrator 1 that converts high-frequency electrical energy into longitudinal vibrations, and an exponential-curve amplitude amplifier 2 that transmits the longitudinal vibrations to the film. During thermal sealing, the ultrasonic vibration delivered by the exponential-curve amplitude amplifier causes the overlapping surface of film 4 to heat up, melt, and bond.
This sealing method is characterized by heat generation at the center of the film overlap, making it suitable for continuous sealing of heat-shrinkable films, such as biaxially stretched films. It can heat-seal a variety of plastic film materials (such as polypropylene, nylon, aluminum-plastic composites, polyoxyethylene tins, etc.), and can seal effectively even if the packaging material is accidentally contaminated with water, oil, and so on during filling. It also has high sealing quality for plastics that are prone to heat-shrinkage deformation or thermal decomposition. It is particularly suitable for heat sealing in food, medicine, radio, and electronic components sensitive to heat radiation, and is widely used in bag-making machines or automatic packaging machines.

High-frequency thermal sealer

As shown in Figure 6-12, the high-frequency thermal sealing applies high-frequency voltage between the high-frequency electrode 2 and the film 4, and heats the film by polymer dielectric loss. The sealing section's temperature is highest at the sealing surface, so the film won't overheat and the sealing strength is high.

The heat sealing time for heat-sealing film is related to the heating temperature, heating method, film material, film thickness, and sealing pressure. For a film material of the same material and thickness, when the sealing pressure is constant, the higher the heating temperature, the shorter the heat sealing time. The heat sealing temperature, pressure, time, and other parameters should be determined by experimental methods based on the mechanical and physical properties of the material being sealed.

Conclusion

A heat sealing machine is a machine that seals packaging containers with a heat seal. It is widely used to heat-seal various plastic bags. Understanding the various heat sealing methods—from the simplicity of plate heat sealing to the advanced capabilities of ultrasonic sealing—allows manufacturers to choose the most efficient and suitable solution for their packaging needs. Each sealing method offers specific advantages based on the material, production speed, and required quality. By choosing the right technique, companies can improve packaging efficiency, reduce waste, and ensure high-quality seals that meet industry standards.