When packaging liquid products, how should the edge seal structure of a three-side seal bag be designed to prevent leakage?
Publish Time: 2025-09-25
Due to its mature manufacturing process, low cost, smooth appearance, and excellent sealing performance, three-side seal bags are widely used in packaging liquid products in various sectors, including food, daily chemicals, and pharmaceuticals. These include sauces, shampoo samples, disinfectants, jellies, and dairy products. However, liquids are highly fluid and easily permeable. Improper edge seal design or process control can easily lead to leakage during transportation, storage, or use. This not only causes product loss, but can also negatively impact brand image and even cause safety issues. Therefore, ensuring the absolute sealing performance of three-side seal bags for liquid packaging through the coordinated design of scientific edge seals and material processing becomes a key challenge in packaging technology.
1. Optimizing Heat Seal Layer Materials: Ensuring Melt Bond Strength
The sealing performance of the edge seal depends primarily on the choice of the inner heat seal material. Three-side seal bags for liquid packaging typically utilize a multi-layer composite structure. The inner layer, which comes into direct contact with the contents, requires excellent heat sealing, chemical resistance, and contamination resistance. Common heat-seal materials include PE, CPP, or specially modified co-extruded films. For liquid packaging, a heat-seal resin with a high flow index should be selected to ensure it fully melts and spreads evenly during the heat-sealing process, forming a continuous, dense adhesive layer. Furthermore, the heat-seal layer must possess a certain degree of toughness to withstand the static pressure of the liquid and external impact, preventing the seal edge from cracking due to microcrack propagation.
2. Wider Sealing Width to Improve Sealing Redundancy
The wider sealing edge of a three-side seal bag means a larger bonding area, effectively distributing the pressure of the liquid on the seal and improving overall pressure resistance. Furthermore, the wider sealing edge provides a greater "safety margin." Even if the bond weakens slightly in some areas due to uneven heat-sealing temperature or contamination, the seal can still be maintained in other areas, significantly reducing the risk of leakage. Furthermore, the wider sealing edge facilitates subsequent heat-seal strength testing and quality control.
3. Multi-pass Heat Sealing and Stepped Sealing: Enhancing Sealing Reliability
To ensure complete fusion of the sealing edges, three-side seal bags for liquid packaging often utilize a "double-pass heat-sealing" or "stepped heat-sealing" process. The first heat seal initially presses the materials together to expel air; the second heat seal, performed at a higher temperature or pressure, ensures the inner layer is fully melted and securely bonded. Some high-end equipment also uses a stepped seal blade, where the temperature inside the seal edge is slightly higher than the outside. This causes the molten material to contract inward, preventing the formation of "burrs" that curl outward. This reduces stress concentration points and improves seal smoothness and sealing performance.
4. Introducing Leak-Proof Ribs and Grooves
Designing raised "leak-proof ribs" or recessed "guide grooves" in the seal area is an innovative approach to improving the sealing performance of liquid packaging. The ribs create multiple barriers within the seal. Even if there is a slight leak in the outer layer, the liquid is trapped between the ribs, preventing further spread. The guide grooves, on the other hand, guide any remaining liquid back into the bag, preventing it from accumulating at the seal edge. These microstructures are typically formed in a single step during heat sealing using precision molds, eliminating the need for additional steps and significantly improving leak-proof performance.
5. Strengthen Corner Seals and Eliminate Weak Links
The four corners of a three-side seal bag are stress concentration areas and are most susceptible to leakage. After being filled with liquid, the bag expands, subjecting the corners to significant tensile forces. Therefore, special attention must be paid to the heat seal quality of these corners. This can be achieved by optimizing the corner arc of the heat-sealing mold, extending the corner heat-sealing time, or using localized reinforcement strips to ensure complete fusion and a flawless seal. Some high-end packaging even adds additional dot or short line heat seals at the corners for further reinforcement.
6. Strict Process Control and Quality Inspection
In addition to structural design, precise control of heat-sealing temperature, pressure, time, and cooling efficiency is also crucial. Too low a temperature results in a weak bond, while too high a temperature may burn through the film. Heat-sealing parameters must be monitored in real time during production, and regular heat-seal strength testing, drop testing, and pressure-holding testing are conducted to simulate actual transportation conditions to ensure the seal reliability of each batch.
The leak-proof performance of three-side seal bags in liquid product packaging relies on systematic optimization from material selection, structural design, to process control. By selecting a high-performance heat-sealing layer, widening the edge seal, employing multiple heat seals, introducing leak-proof structures, strengthening corner seals, and implementing strict quality management, we can effectively eliminate the risk of leakage. As consumers' demands for packaging safety increase, refined and functional edge seal design will become the core competitiveness of the three-side seal bag in the continued development of the liquid packaging field.
