RF Welding

RF Welding: How Radio-Frequency Welded Seams Are Made and Tested

Radio-frequency (RF) welding, also called high-frequency (HF) welding or dielectric welding, joins thermoplastic materials such as PVC and TPU by heating them from the inside with a rapidly alternating electromagnetic field, usually at 27.12 MHz, while a shaped metal die presses the layers together. The two layers melt into one and cool under pressure, leaving a seam with no needle holes, no thread and no adhesive. That is why high-frequency welded seams are used wherever a product must keep water, air or liquid in or out: dry bags, waterproof backpacks, soft coolers, inflatables, medical fluid bags and many military and industrial containers.

This guide explains how the process works, which materials can and cannot be welded, how a welded seam compares with stitched, taped and glued seams, what makes a weld fail, and how welded seams are tested before a product ships. It is written from the factory floor: we have been making high-frequency welded bags for over 20 years at our factories in Dongguan, China and Ho Chi Minh City, Vietnam.

Explore This Section: RF Welding, Testing and Materials

This page is the starting point for our engineering articles on high-frequency welding. Each article below goes deeper into one part of the topic.

How welding works: 27.12 MHz high-frequency welding explained  |  How RF welding prevents leaks at the manufacturing level  |  A backpack is thirty welded parts before it is a backpack

Welded vs sewn: Welded vs sewn: how waterproof bags are really made  |  RF welding vs stitching: which creates truly waterproof gear

Weld quality and defects: The number that decides whether a weld is actually a weld  |  Common RF welding defects: an OEM audit guide  |  QA testing and audits for waterproof manufacturing

Lab tests on welded bags: The strap didn't tear, it came off the bag  |  3,000 pulls under load: testing the part that fails first  |  The machine that rubs our fabric until it fails  |  The buckle outlives the bag, or it doesn't  |  Will the colour come off on your shirt?

Weldable materials: TPU waterproof material guide  |  TPU vs PVC dry bags in freezing cold  |  PFAS-free TPU

What Is RF Welding?

RF welding is a way of fusing plastics without an external heat source touching the surface. Instead of a hot plate or hot air, the machine applies a high-frequency electric field across the material. Certain plastics contain polar molecules, which try to line up with the field. Because the field reverses millions of times per second, those molecules oscillate constantly, and the friction between them generates heat inside the material itself. When the plastic reaches its melting point at the joint, the pressure of the die forces the two layers to flow into each other.

The names RF welding, HF welding, high-frequency welding and dielectric sealing all describe the same process. Industrial machines operate on frequencies reserved for this purpose, most commonly 27.12 MHz, which is why the term "27.12 MHz welding" also appears in specifications. The process has been used industrially since the mid-twentieth century and is now standard for any product where a stitched seam would leak.

How the Process Works, Step by Step

On a bag production line, high-frequency welding is one stage in a longer sequence. Panels are first cut from TPU- or PVC-coated fabric, and any parts that need sewing, such as webbing, strap anchors and organiser pockets, are sewn onto flat panels before welding begins. This is why most waterproof bags are made on two lines, sewing and welding, working together rather than one replacing the other.

For each weld, the operator places the panels on the machine bed, often in a jig that holds them in exact alignment. A brass die shaped like the seam is lowered onto the material, and an insulating buffer sheet sits underneath to prevent arcing. The machine then applies RF energy for a few seconds while the die presses down. Once the energy stops, the die stays closed for a cooling period so that the molten material solidifies under pressure; releasing it too early produces a weak, rippled seam. Finally, excess material is trimmed and the seam is inspected.

Three settings control the result: power, weld time and cooling time, together with the die pressure. They must be set for each combination of material, thickness and die. A setting that produces a perfect seam on 500D PVC tarpaulin can burn through a thin TPU film or under-fuse a heavy TPU laminate, so every new material or die is validated with test welds before production starts.

Which Materials Can Be RF Welded?

Only plastics with polar molecules heat up in a high-frequency field, which limits the process to certain materials. PVC and polyurethanes, including TPU, are the most widely welded, and they are the main materials used for waterproof bags. Some other materials can be welded under special conditions, while common non-polar plastics cannot be welded this way at all.

For coated fabrics, the detail that matters most is the coating. A TPU-laminated fabric with a substantial film welds reliably, while a fabric with a very thin coating may not have enough material at the joint to form a strong seam. This is one reason two fabrics described in the same words can behave very differently on the welding machine. Our TPU waterproof material guide covers the material side in more detail.

RF Welded Seams vs Stitched, Taped and Glued Seams

None of this makes stitching inferior in general. Stitching is the right choice for organiser pockets, straps, padding and anything outside the waterproof shell, and our sewing lines are as experienced as our welding lines. The best waterproof bags use both: sewn sub-assemblies for structure and comfort, and welded seams wherever water must be kept out. Our article on how welded and sewn bags are really made shows how the two are combined in one product.

RF Welding vs Ultrasonic and Hot-Air Welding

Ultrasonic welding uses high-frequency mechanical vibration rather than an electric field. It is fast and works on some materials that do not respond to RF, but it is best suited to small, spot or continuous narrow seams. Hot-air and hot-wedge welding melt the surface from outside and can join non-polar materials such as polyethylene, which makes them common for large geomembranes and some tarpaulins. For bags made from PVC or TPU, high-frequency welding is usually preferred because it heats the joint evenly through its thickness, produces wide, consistent seams in complex shapes in a single press, and leaves the surface of the material largely unmarked.

What Makes a Weld Fail

A welded seam that leaks almost always comes back to one of a few causes. Under-fusion happens when power or time is too low: the seam looks closed, but the layers have only stuck together at the surface and peel apart under load. Burn-through or arcing happens when power is too high or the buffer is missing, leaving a thin, scorched or perforated line. Contamination, such as dust, release agent or plasticiser on the surface, stops the layers fusing properly. And a change in material, even a different batch or colour of the same fabric, can shift the correct settings enough to weaken the seam if the machine is not re-validated.

Most of these defects are invisible to a casual look, which is why welded products need destructive testing on samples and pressure or immersion testing on finished goods. Our guide to common RF welding defects shows what each one looks like when a seam is cut open.

How RF Welded Seams Are Tested

Testing starts before production. When a new material or die is set up, sample welds are made and pulled apart on a tensile tester to measure bonding strength, the force needed to peel the seam open. A good weld tears the material beside the seam rather than separating along it. Our lab article The Number That Decides Whether a Weld Is Actually a Weld explains this test in detail.

During production, every order passes through IQC on incoming fabric, IPQC on the welding line with regular sample checks, and OQC on finished products, with SGS or QIMA third-party inspection available on request. The finished-product tests match the rating the product carries: a heavy rain and water-jet test for products rated IPX5 or IPX6, and a full water-immersion test of 2+ hours for products rated IPX7 or IPX8. Tensile strength, abrasion, salt spray on hardware, a load test of 1,500+ cycles and zipper fatigue testing to 3,000 cycles complete the standard programme, along with colour fastness and colour difference checks against the approved sample.

For sealed and air-tight products, the most revealing test is also the simplest. Our standard customer inspection procedure is unboxing, vacuum extraction, a 24-hour static rest and air-leak determination. If the product still holds its shape after 24 hours, none of its welds, closures or valves is leaking, including slow leaks that a short immersion test can miss.

Where RF Welding Is Used

Outdoor and marine gear is the most familiar use: dry bags, waterproof backpacks and duffels, kayak and boat bags, motorcycle and bike bags, waterproof phone pouches and compression bags. Soft coolers use welded shells and liners because a welded liner does not leak meltwater and has no stitch lines to trap dirt and odour. Beyond bags, the same process makes medical fluid and blood bags, inflatable boats and toys, swim buoys, tarpaulins and truck covers, and a wide range of military and government products, including fuel and water bladders, inflatable shelters, weatherproof equipment covers and containment bags, where airtight seams and resistance to heat, cold and pressure are essential.

RF Welded Products From Our Range

Image	Product	Welded construction
	SL-D069 20L Ocean Pack Dry Bag	500D PVC tarpaulin, high-frequency welded seams, roll-top
	SL-E090 60L Airtight Dry Backpack	TPU body, welded seams and strap anchors
	SL-K060B Handlebar Bag, 7L	TPU-coated ripstop, welded seams and strap ports, IPX6
	SL-P1083 Compression Bag with Built-In Pump	TPU laminate, welded body, welded window and pump mounting

How to Evaluate an RF Welding Supplier

Ask to see the welding machines and the dies for your product, and ask how parameters are recorded and re-validated when the material changes. Ask for bonding strength results on sample welds, not only on finished products. Check whether the factory has its own sewing capability, because almost every welded bag also has sewn parts, and a supplier that subcontracts one or the other has less control over the result. Finally, ask how finished products are tested against the rating on the label, and whether you can follow the same test yourself on arrival. Our breakdown of what drives the cost of a waterproof bag explains how die and tooling costs fit into a quotation.

Frequently Asked Questions

Is RF welding the same as HF welding?

Yes. RF (radio-frequency) welding, HF (high-frequency) welding and dielectric welding are different names for the same process.

Are RF welded seams waterproof?

Yes. A properly made weld fuses the two layers into one piece of material, so there are no holes for water to pass through. Whether the finished product is waterproof also depends on its closure and any other openings, which is why products are tested as a whole.

Is RF welding stronger than stitching?

For keeping water and air out, yes. In a good weld, the material beside the seam usually tears before the seam itself separates. Stitching remains the better choice for load-bearing straps and padded parts outside the waterproof area, which is why the two are often combined.

Can nylon or polyester be RF welded?

Not on their own. Woven nylon and polyester become weldable when they are coated or laminated with TPU or PVC, and it is the coating that forms the weld.

What is the difference between RF welding and ultrasonic welding?

RF welding heats polar plastics from within using an electric field and produces wide seams in one press. Ultrasonic welding uses mechanical vibration and is suited to smaller or narrower seams and to some materials RF cannot weld.

Why do some welded seams leak?

The usual causes are under-fusion from too little power or time, burn-through from too much power, contamination on the surface, or a change of material without re-validating the machine settings. All of them are caught by bonding strength tests on samples and leak testing on finished products.

What is the MOQ for a custom RF welded bag?

Most of our welded bags start at 300 pieces per style, with some at 500. A new shape needs its own welding dies, which is a one-time cost included in the quotation. Samples typically take 7–15 days and bulk production 30–45 days.

Can you add our logo to a welded bag?

Yes. Screen printing, heat transfer, welded patches and high-frequency embossing, where the logo is pressed into the material with a die, all work on welded PVC and TPU products.

Work With a High-Frequency Welding Factory

If you are developing a welded product, send us your drawings, samples or target specification. We will recommend a material and seam design, make the dies and prepare samples for testing. Contact us at info@sealock.com.hk or call +86-769-82009361.