How to Make Three-Side Seal Bags

A Comprehensive Analysis of Three-Side Seal Bag Manufacturing: Professional Process from Raw Materials to Finished Products

Abstract:

Three-side seal bags, as a widely used and structurally simple form of flexible packaging, play a significant role in industries such as food, daily chemicals, pharmaceuticals, and industrial goods. The name derives from their unique sealing method—the left, right, and bottom edges of the bag are heat-sealed, leaving only the top open. This article systematically explains the manufacturing process of three-side seal bags, covering core steps such as material selection, printing techniques, lamination and curing, and bag formation, aiming to provide industry professionals with a rigorous technical reference.

I. Preliminary Preparation and Design

Before production begins, thorough planning is the cornerstone of ensuring product quality and efficiency.

Clarify Product Requirements:

Contents: What will the bag contain? Powder, granules, liquids, or solids? The characteristics of the contents (e.g., acidity/alkalinity, oil content, volatility, or moisture absorption) directly determine the choice of packaging materials.

Shelf Life: How long does the product need to last? This affects the required barrier properties (e.g., oxygen or water vapor resistance) of the materials.

Packaging Specifications: Determine the final dimensions (length, width, thickness) and filling capacity of the bag.

Special Features: Are features like light resistance, anti-counterfeiting, easy-tear openings, zippers, or stand-up functionality needed?

Structural Design and Material Selection:

Three-side seal bags are typically composed of multiple laminated film materials to achieve the desired physical strength, barrier properties, and heat-sealing

performance. Common material combinations include:

Basic Structure: Outer/printing layer + middle barrier layer + inner/heat-sealing layer.

Common Materials:

Outer Layer: Usually materials with good mechanical properties and printability, such as BOPP (biaxially oriented polypropylene), PET (polyester), or paper. These provide the bag’s primary appearance and rigidity.

Barrier Layer: Selected based on requirements, such as aluminum foil (AL), metallized film (VMPET/VMCPP), EVOH (ethylene-vinyl alcohol copolymer), or PA (nylon), to block oxygen, moisture, or light.

Inner/Heat-Sealing Layer: Must have excellent heat-sealing properties and compatibility with the contents, such as CPP (cast polypropylene) or PE (polyethylene).

Pre-Printing Design and Plate Making:

Using professional software (e.g., Adobe Illustrator), design the layout based on the bag’s dimensions and shape. Key considerations include:

Bleed: The design must extend at least 3mm beyond the finished edge to prevent white borders during cutting.

Sealing Zones: Avoid placing critical graphics in the left, right, and bottom heat-sealing areas (typically 10–15mm wide) to prevent distortion during sealing.

Color Management: Perform color separation and plate making according to the printing method (e.g., gravure or flexographic printing) to ensure accurate color reproduction.

II. Core Manufacturing Process

The production of three-side seal bags is a continuous, multi-step integrated process.

Film Printing:

Primary Method: Gravure printing, the most widely used technique in flexible packaging, is renowned for its high color saturation, durable plates, and suitability for mass production.

Process: Install the engraved gravure cylinder on the printing press. A doctor blade fills the cylinder’s cells with ink, which is then transferred under pressure to the substrate film (e.g., BOPP, PET). After multi-color registration, a complete colored pattern is formed. The ink is dried in a curing oven.

Material Lamination:

Purpose: Combine the printed outer layer with barrier and heat-sealing layers using adhesives to create a composite material with integrated properties.

Common Methods:

Dry Lamination: Apply adhesive to the printed side of the outer layer (or barrier layer), dry the solvent in an oven, and press it onto another substrate. This is the most widely used method.

Solvent-Free Lamination: Use 100% solid adhesives to bond layers at room temperature. This eco-friendly, safe, and efficient method is an industry trend.

Extrusion Lamination: Use molten resin (e.g., PE) as an adhesive layer, extruded directly between two substrates. This combines lamination with adding a heat-sealing layer.

Curing:

After lamination, the roll material is placed in a curing chamber (oven) at specific temperatures (typically 40–55°C) for 24–72 hours to allow the adhesive to fully react and achieve final bond strength. Conditions depend on the adhesive type.

Slitting and Rewinding:

The cured composite film is slit into narrower rolls of specified widths using a slitting machine and rewound for bag-making.

Bag Formation (Key Step):

This critical step transforms flat film into three-dimensional bags, typically performed on a fully automatic three-side seal bag-making machine.

Unwinding and Registration: The film roll is mounted on the machine, guided through rollers, and aligned using photoelectric sensors tracking "marker lines" to ensure consistent pattern placement.

Folding: The film passes through a triangular folding plate, precisely overlapping the edges to form the front and back of the bag.

Bottom Sealing: The folded film passes under a bottom heat-sealing bar, where preset temperature, pressure, and time parameters fuse the inner layers to form the bag’s bottom edge.

Side Sealing and Cutting:

L-Shaped Seal/Cut: The most common method. An L-shaped heat-sealing bar simultaneously seals the side of one bag and cuts the top opening of the previous bag.

Continuous Seal/Cut: Side sealing is performed first, followed by transverse cutting.

Cooling and Counting: After sealing, the edges are immediately cooled (e.g., via water-cooled steel bars) to set and strengthen. Finished bags are conveyed out and automatically counted and stacked.

III. Quality Control Points

Strict monitoring at each stage ensures the quality of three-side seal bags.

Seal Strength: A critical metric tested with a tensile machine to ensure the seal can withstand handling without breaking.

Seal Integrity: Tested via negative pressure (e.g., underwater leak detection) or positive pressure methods to identify micro-leaks.

Appearance: Inspect for even, smooth seals without burns or weak spots; accurate color registration; and absence of scratches or stains.

Dimensional Accuracy: Length, width, and seal width must meet specified tolerances.

IV. Conclusion

The production of three-side seal bags is a systematic engineering feat combining materials science, mechanical engineering, and process technology. From precise material formulation and stable gravure printing to efficient dry or solvent-free lamination, and finally, high-precision folding, sealing, and cutting on automated bag-making machines, each step’s parameter control and coordination determine the final product’s performance and quality. With continuous advancements in materials and processes, three-side seal bag production is evolving toward greater efficiency, sustainability, and multifunctionality.