Not every bag can be sewn. A leak-proof cooler bag, a waterproof dry bag and a medical pouch with a film window all need seams that thread cannot deliver, because a needle makes holes. The alternatives are RF welding and ultrasonic sealing, and they behave differently enough that the choice affects the material, the tooling and the price.

This guide explains how both processes work, where each is the right answer, and how to specify a sealed bag so the seam performs in service.

How RF Welding Works

Radio-frequency welding applies an alternating electric field across two layers of a polar material. The molecules vibrate, generating heat internally at the interface, and pressure consolidates the softened layers into one. Because the heat is generated inside the material rather than conducted from a tool, the bond can be strong without scorching the surface.

The process works on polar thermoplastics, most commonly PVC and polyurethane-coated fabrics. Materials that are not polar, such as plain polyethylene or polypropylene, do not respond directly and need a weldable layer or a different process. This single compatibility rule decides whether RF is available for a given design.

Tooling is a welding die shaped like the seam. A new seam pattern means a new die, which is a modest cost and a lead time that belongs in the sampling plan rather than in the production schedule.

How Ultrasonic Sealing Works

Ultrasonic sealing uses high-frequency mechanical vibration applied through a horn. The energy melts the material at the contact point, and the seam forms under pressure as the tool travels. It suits thermoplastic films and non-wovens, including polyethylene and polypropylene constructions that RF cannot weld directly.

Because the energy is applied from outside, the process is sensitive to material thickness and to the consistency of the stack. Two layers weld readily; six layers with an insert between them need a different setup and sometimes a different joint design.

The advantage is speed and automation: ultrasonic sealing integrates with cutting in the same machine, which suits high-volume production of pouches and liners. The limitation is joint strength distribution, because the seam is a series of bonded points rather than a continuous molecular weld.

Choosing Between Them

Four questions decide it. Is the material polar? If yes, RF is available; if it is a polyolefin, ultrasonic or a weldable layer is needed. Does the seam need to hold pressure or water? If yes, a continuous weld is preferable. What is the volume and shape? Complex three-dimensional seams suit RF with shaped dies; flat, high-volume seams suit ultrasonic with travelling tools.

The fourth question is what the bag has to do afterwards. A cooler bag that will be folded and stored repeatedly needs a seam that tolerates flexing; a pouch that holds a product for years needs a seam that resists creep. Both can be tested, and the tests belong in the specification.

Seam performance can be measured: peel strength, tensile strength and leak or pressure tests are established methods, catalogued in the ASTM standards library. A supplier that reports results from these tests gives a buyer something to compare rather than an assurance.

Specifying a Sealed Bag

The specification should name the material and its weldability, the process, the seam width, the target strength or leak performance with the test method, and the appearance requirement. Seam appearance matters more than buyers expect, because a weld can look flat or rippled depending on the tool and the temperature.

It should also name the failure mode that is acceptable. A seam that fails by peeling gradually is easier to detect and safer in service than one that tears suddenly, and the joint design can be biased towards the first.

Finally, the first article should include a seam test, not only a visual check. A sealed bag that passes inspection and fails in the field is a design problem, and the test data from the first article is what makes that visible before bulk. Where a design mixes sealed and sewn closures, the closure comparison in bag closures compared is a useful companion, and the duty differences between zipper types are set out in zipper types on bags.

Where Sealing Fits a Bag Programme

Sealed construction is usually a small part of a broader bag range, sitting alongside sewn products. Keeping the sealed items in the same programme means the sampling, the materials and the inspections travel together, and the buyer deals with one specification set.

It also means the dies and tooling are managed in one place, which matters when a reorder arrives two years later and the seam pattern must be reproduced exactly.

If your range includes welded or sealed items, send the design intent to our materials team. We will confirm which process fits the construction and propose the seam specification and test method with the quotation.

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If your range includes welded, sealed or leak-proof bags, our materials team can confirm the process, propose the seam specification and quote the tooling with the order.