Heat-sealing process for heat-seal bags

　　 Heat-seal bag Heat-sealing process

　　Heat-seal bags are a type of adhesive tape used to seal waterproof and airtight products such as raincoats, tents, and balloons. They are applied by heating the material with specialized equipment—such as hot-air seam sealers or high-frequency heat sealers—and then sealing it over pinholes to achieve a hermetic seal that prevents water or air leakage. Heat-seal bags are widely used in packaging for daily chemical products, food, and pharmaceuticals. However, since heat sealing can lead to leaks during product filling and most damage during actual use occurs at the sealed joints, selecting appropriate heat-seal materials and process parameters can reduce defect rates on the production line and significantly enhance the overall barrier performance of the packaging.

　　Heat-sealing involves heating the sealing material to a viscous-flow state by a specific method and then applying pressure to achieve a seal. This process is typically carried out using a heat-sealing machine or a heat-sealing unit. The heat-sealing head serves as the actuating mechanism for heat sealing. Depending on the design of the heat-sealing head and the heating method employed, heat-sealing techniques can be classified into conventional heat sealing, fusible-seal sealing, pulse sealing, ultrasonic sealing, high-frequency heat sealing, and induction heat sealing. For instance, ultrasonic sealing and high-frequency sealing are particularly suitable for thin films that are prone to thermal deformation. However, the most commonly used heat-sealing method is conventional heat sealing. Common forms of conventional heat sealing include plate-type sealing, disc-type sealing, belt-type sealing, and slide-clamp sealing, with plate-type sealing being the most widely applied. The heat-sealability of a material is assessed by measuring both the residual strength of the seal while it is still hot (before it has cooled to ambient temperature) and the ultimate seal strength after the seal has cooled and stabilized. To evaluate a material’s heat-sealing performance, it is necessary to conduct comprehensive tests covering both of these aspects. It is generally accepted that the heat-sealing performance of packaging materials is primarily determined by the heat-sealing temperature, heat-sealing pressure, and heat-sealing time, with heat-sealing temperature being the critical parameter and heat-seal strength serving as the basis for assessing the material’s heat-sealing performance.

　　In Heat-seal bag On packaging production lines, the short time interval between heat-sealing the pouch and filling it often results in many materials being filled while the seal has not yet cooled to ambient temperature. The heat-sealed portion then experiences rupture forces caused by the filling process; if the strength of the heat seal is insufficient to withstand these forces, bag failure can occur during the manufacturing process. Bag breakage is particularly pronounced in high-speed vertical form-fill-seal machines, though it can also occur in low-speed packaging equipment where the heat seal has not fully cooled. The ability of the heat-sealed portion of a material to maintain consistent integrity under external force for a very short period after heat sealing (while still warm) is referred to as its hot tack. Technically speaking, a material’s hot tack is the sum of its adhesive performance within the heat-sealing temperature range and the adhesive strength of the sealant to other components in multilayer structures. In general, a material’s hot tack is significantly lower than its post-cooling heat-seal strength, as clearly illustrated by ASTM F 2029-00. The curve labeled “Equil” shows the heat-seal strength–temperature relationship measured after the film heat-seal has completely cooled (with an equilibrium holding time of 1,000 ms), while the curve on the right depicts the “100 ms dwell” condition.

　　 Heat-seal bag The curves depict the heat-seal strength and temperature of the test material measured just 100 ms after heat sealing (before sufficient cooling). Although both curves exhibit the same trend with increasing temperature, at any given temperature the apparent heat-seal strength measured after only 100 ms is significantly lower than that measured after complete cooling.