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Structural Characteristics and Forming Process of Three-Side Sealed Food Bags
Release Date:
2026-04-03
Summary
In the field of flexible food packaging, the three-side-sealed pouch is a widely used pouch type. Its name derives from the sealing method—the three edges of the pouch are heat-sealed, leaving a single opening for filling. Understanding the structural characteristics and manufacturing processes of this pouch type is essential for making the right selection.
I. Structural Definition: Sealed on three sides, open on one side
Three-side sealed bags are made by laminating two or more layers of film; three of the four sides are heat-sealed to form a space for holding contents:
Seal Configuration: The left, right, and bottom sides form the sealed area, while the top remains open. The opening can be resealed after filling with food (e.g., with a zipper or tear-open tab), or it can be permanently sealed to create a four-side sealed bag
Seal width: The width of the seal varies depending on the packaging machine and bag size, typically ranging from a few millimeters to over one centimeter, to ensure seal strength.
Corner treatment: Some three-side sealed bags feature rounded or chamfered corners at the seal junctions to reduce the risk of puncture from sharp edges and enhance safety.
II. Forming Process: From Film Rolls to Finished Bags
The production of three-side sealed bags is carried out on a bag-making machine. The main processes include film unwinding, folding, heat sealing, cooling, and cutting:
Unwinding and Folding: The composite film roll is unwound by the unwinding device and folded in half by the forming plate to create two layers, with the printed side facing outward and the heat-sealable sides facing each other.
Bottom Sealing: The bottom of the folded film is heat-sealed horizontally to form the bag’s bottom seal.
Side Sealing: According to the preset bag width, the left and right sides are heat-sealed vertically to form two side-sealed areas.
Cooling and Setting: The areas that have just been heat-sealed pass through a cooling unit to rapidly lower the temperature, solidifying the seals and preventing wrinkling or shrinkage at the heat-sealed areas.
Cutting and Separation: The continuous heat-sealed film strip is cut into individual bags by a cross-cutting blade. After collection and sorting, they become finished products.
III. Factors Affecting Sealing Quality
The heat-sealing quality determines the reliability of the seal and the load-bearing capacity of three-side sealed bags. It is primarily influenced by several factors:
Heat-sealing temperature: The sealing blade temperature must be compatible with the film’s heat-seal layer material. If the temperature is too low, the seal will not be secure; if it is too high, the film may burn through or wrinkle.
Heat-sealing pressure: Sufficient pressure ensures that the two layers of film adhere tightly, expelling air from the interface and forming a uniform seal.
Heat-sealing time: The duration of contact between the sealing blades must be controlled within a reasonable range to ensure sufficient heat transfer without excessive melting.
Cooling effect: Rapid cooling after sealing helps maintain a flat seal and reduces shrinkage and deformation in the heat-sealed area.
IV. Common Specifications and Compatible Packaging Machines
The dimensions of three-side sealed bags can be customized to meet product requirements, and they typically have a rectangular shape. These bags are compatible with a variety of packaging machinery:
Pre-made bag packaging machines: Pre-made three-side sealed bags are picked up by a robotic arm, opened, filled, and sealed. This method is suitable for small-batch, multi-variety production.
Continuous bag-forming and filling machines: Film rolls are converted into three-side sealed bags on the production line, with filling and sealing completed simultaneously. This method offers high efficiency and is suitable for large-scale production.
Manual filling: Some three-side sealed bags feature a self-sealing zipper at the opening, allowing for manual filling followed by pressing to seal. This is suitable for home use or small-scale workshops.
With their neat appearance, excellent sealing properties, and mature production processes, three-side sealed bags hold an important position in the food packaging industry. Understanding their structural characteristics and forming process provides a foundational understanding for selecting the appropriate packaging.
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