Heat-sealing method for heat-seal bags

　　 Heat-seal bag Heat-sealing method

　　Sealing methods for heat-sealable plastic bags include knotting, heat sealing, stapling, and adhesive sealing. Among these, heat sealing is simple, reliable, and widely used. In heat sealing, thermoplastic materials are employed to join the sealed portions of the plastic film by heating and applying pressure to achieve adhesion. Various heat-sealing techniques are available, including hot-plate sealing, melt-sealing, high-frequency sealing, ultrasonic sealing, electromagnetic induction sealing, and infrared sealing.

　　In heat-sealing bags, a heating plate is heated to a specified temperature, and the sealing plastic films are pressed together. This is the basic principle and structure of heat sealing: the sealing process is relatively fast, temperature control is precise, and this method is commonly used for sealing polyethylene composite films. However, it is not suitable for heat-shrinkable and decomposable films, such as various heat-shrink films and PVC. In another approach, high-frequency current passing through a coil generates a magnetic field in the surrounding high-frequency magnetic field; magnetic materials heat up due to hysteresis losses. If magnetic material films or thin magnetic layers are sandwiched between plastic layers, or if magnetic iron oxide powder is added to the plastic, the plastic or the adhesive layer will melt instantaneously. The heating element does not need to be in direct contact with the plastic bag, allowing continuous high-speed sealing—making this method well suited for production lines. This represents a new development in plastic heat sealing in recent years. A third method involves using infrared radiation to directly heat and melt specific areas of the film for sealing. The radiation source delivers intense heat, and darker-colored materials absorb heat more readily. To implement this technique, simply cover the transparent film beneath the sealing layer with a black cloth. This method can seal materials that are difficult to seal using conventional heating, such as polytetrafluoroethylene and polyethylene sheets 5–6 mm thick. It too is a novel plastic heat-sealing method that has been developed both domestically and internationally in recent years.

　　 Heat-seal bag This is often a critical issue in the design of packaging machinery. The quality of heat sealing directly affects the machine’s long-term stable operation and its ability to operate at higher speeds. Many factors influence heat-sealing quality, primarily the melting point, thermal stability (including thermal decomposition and thermal shrinkage), and flow characteristics of the packaging material. Once the packaging material has been selected, the key factor determining heat-sealing quality is the heat-sealing time: 5×10, 12.9×10, 15.2×10. Clearly, double-sided heating reduces the time required to heat the film on both sides to only one-third of that needed with single-sided metal plates, thereby virtually eliminating overheating. Bag-making packaging machines commonly employ bar-type and roller-type sealers. Heat-seal bag After the resistance-heating sealing machine heats the material to a specified temperature, it is then pressed to form a 2–4-layer sealed film; after a certain dwell time, a strong, hermetic seal is achieved. The quality of the seal depends on the proper selection of sealing temperature, pressure, and time. When using single-layer films as packaging materials—such as polyethylene—the sealing device’s surface is coated with a layer of non‑ethoxylated methyl resin, titanium ester, or other similar coatings, or the film is used in conjunction with PTFE fabric. When adhesion occurs between the heat-sealable material and the sealing device, low-speed packaging machines employ pulse heating to achieve thermal sealing; the sealing device is then cooled to a temperature below the plastic melt point, with a typical cooling time of 2–3 seconds. Consequently, the production rate rarely exceeds 20 bags per minute.