Challenges in Various Thermal Sealing Methods for Heat-Seal Bags

　　 Heat-seal bag Challenges of Various Thin-Film Heat-Sealing Methods

　　There are numerous methods for heat sealing plastic films used in heat-seal bags, but each method is suitable only for specific types of plastic films, facilitating selection during the design phase. Factors such as temperature, pressure, seal-head geometry, and heating method all play a role. In general, lower temperatures, reduced pressure, and longer sealing times yield better seal quality. However, if the sealing temperature is too high, the coating layer may soften or shrink and deform, compromising the appearance of the seal and even leading to burn-through; prolonged sealing can cause thermal decomposition in some plastics. Conversely, excessive pressure increases seal distortion and reduces seal strength. The upper limit for dimensional shrinkage is 3%, while the lower limit for seal strength is 1 kg/cm. The wider the range between these limits, the better the heat-sealing performance. The concept of thermal shrinkage in plastic films was first introduced as early as 1936, when rubber films were used to shrink-wrap and package perishable foods. Since then, shrink-wrap technology has evolved to the point where nearly all types of products can be packaged using plastic shrink film.

　　For single-layer films, the sealing head surface typically uses a lightweight panel, with the upper panel made of stainless steel and the lower panel made of silicone rubber. For aesthetic reasons, the seal width is generally 2–3 mm. For composite films, to enhance seal strength and improve appearance, the sealing head surface is often embossed with horizontal patterns, and the seal width is typically 10 mm. Heat-seal bag The production processes and characteristics of shrink film typically involve extrusion blow molding or extrusion casting to produce thick films, which are then subjected to longitudinal and transverse stretching at temperatures above the softening point and melt temperature, while maintaining a highly elastic state. The stretching may be unidirectional—either in one direction only, with the other direction left unstretched—or bidirectional, in which case the material is referred to as biaxially oriented shrink film; if stretching occurs in only one direction, it is called uniaxially oriented shrink film. When used for heat-seal bags, such films exhibit reliable shrinkage force at or above the stretching temperature, effectively securing packaged items.

　　Additionally Heat-seal bag When high-frequency and ultrasonic internal heating are employed, the temperature at the contact surface with other heating plates is consistently higher than that at the sealing interface between the sealing films. The shorter the sealing time and the thicker the sealing film, the greater this temperature difference becomes, making it easier for the film in contact with the heating plate surface to overheat. Therefore, it is recommended to use a double-sided heating approach. For example, when sealing a 0.05 mm-thick PVC film at 204°C under ambient conditions of 27°C, three different heating configurations can be tested: single-sided silicone rubber heating, single-sided metal plate heating, and double-sided heating. Calculations indicate that the sealing interface temperature can reach 104°C in all cases. Heat-shrink packaging is currently one of the more advanced packaging methods on the market. It involves wrapping products or packages with shrink film; after heating, the shrink film conforms tightly to the product or package, effectively highlighting its appearance, enhancing its marketability, and increasing its aesthetic appeal and perceived value. At the same time, packaged items are sealed, moisture-proof, and protected from contamination, while also being cushioned against external impacts—particularly beneficial for fragile goods, as it helps prevent broken items from scattering. In addition, heat-shrink packaging reduces the likelihood of products being torn open or stolen.