Three Ways to Join Coated Fabric
There are only three mechanical options for joining coated fabric into a watertight seam:
- Sewing, followed by seam tape. Panels are stitched together and the stitch line is covered with waterproof tape. Familiar, flexible, and dependent on the tape's adhesive.
- Gluing. Used for coatings that cannot be welded — certain rubber-based coatings can only be glued or sewn. It is labour-intensive and involves volatile solvents, which is why it has largely moved away from regulated Western production.
- Welding (heat sealing). The coatings themselves are melted and fused. Four methods are in use — radio frequency, hot air, hot wedge and ultrasonic — each with different geometry and tooling implications.
What Welding Does That Sewing Cannot
High-frequency welding at 27.12 MHz puts the coated fabric in an alternating electric field. Polar molecules in the coating vibrate, generating heat inside the material, and the mating surfaces melt and fuse under pressure into a single mass. The result is a seam that is part of the fabric rather than attached to it.
The performance difference is measurable rather than rhetorical: RF welded seams can handle up to twice as much stress as sewn-and-taped seams.
And there's a failure mechanism worth understanding, documented in pressure-vessel applications: needle holes expand in dimension under pressure. A stitched seam that is watertight at rest opens up when the bag is submerged, compressed or loaded. That is precisely why a dry bag pushed under water behaves differently from the same bag in the rain, and why welded construction is the only honest basis for a submersion claim.
What Welding Cannot Do
This is the part most factory marketing leaves out, and it's why a welding-only workshop struggles with complex products.
- Welding wants flat. RF welding uses a flat base plate, so it is best at flat seams. Three-dimensional dies, contoured base plates and vacuum fixturing all exist, but they are expensive — which constrains what shapes are economic.
- Welding is less forgiving in design. As Filson put it after moving into welded construction: when sewing, panel sizes can be adjusted easily and seams started and stopped anywhere; welding comes with equipment limitations, and designing for the strongest welded seam took real learning. Herschel described the same experience getting shapes right.
- Not every coating can be welded. Welding needs polar thermoplastics — PVC, TPU, polyurethanes, polyamides. Some rubber-based coatings can only be glued or sewn, and non-polar plastics can't be RF welded at all.
- Welding is poor at load-bearing joints. A welded strap anchor can peel under sustained shear. Sewing spreads the same force across a stitched footprint.
- The equipment is a real barrier. Modern RF welding machines cost upward of $80,000 each, before dies. That's why the capability is concentrated in specialist factories — and why nine lines is a meaningful number rather than a marketing one.
What Sewing Cannot Do — and the Rule That Follows
Sewing perforates. That's the whole limitation, and it has one absolute consequence that governs how a hybrid product must be built:
Sewing must never take place after welding, because stitching perforates the finished seam and renders it pervious to water. This is not a preference; it's a rule established in the seam-construction literature. It dictates the entire production sequence of a waterproof bag: every stitched load point — harness anchors, D-ring patches, haul handles, external pockets — must be applied to flat panels first, so the perforated footprint ends up beneath the welded skin. Where structural stitching has to remain exposed on a heavy product, the inside perimeter is sealed with hot-air backing tape as a separate controlled process.
A factory that treats sewing as a finishing step rather than a first step will produce a bag that leaks at its strongest points.
The Industry Already Uses Both — and More Than One Weld Method
The strongest evidence that this isn't an either/or is what serious brands actually do. Patagonia and SealLine use both RF and thermal welding in the same product lines, chosen by geometry: RF is simple but carries design constraints, so it gets used for patch welds on large flat areas, with other methods handling what RF can't reach.
The four welding methods each solve a different problem:
| Method | Strength | Constraint |
|---|---|---|
| Radio frequency (HF) | Molecular bond, highly repeatable, excellent on flat seams and patches | Needs dies; flat-plate geometry; high equipment cost |
| Hot air | Fast, handles three-dimensional patterns, needs no dies or tooling | Consistency is operator- and parameter-dependent |
| Hot wedge | Continuous long seams | Heat drop-off at seam start; contamination sensitivity |
| Ultrasonic | Same principle as RF with a different energy source | Material and thickness limits |
The Honest Part: Not Everything Needs Welding
A supplier that tells you welding is always the answer is selling, not advising. There is a genuine hierarchy of seam construction, and the right level depends on exposure:
- Critically seamed — only high-exposure seams sealed. Fine in light rain, fails in sustained downpour or heavy spray.
- Fully taped — every seam sealed with tape over the stitching. This is the serious standard for rain-exposure products, and for a great many uses it reaches the practical ceiling of what conditions actually demand.
- Welded — stitching eliminated from the barrier. The highest protection, at a cost premium, and the only construction that survives repeated submersion and pressure.
The distinction that matters commercially: for products that face rain and spray, fully taped construction is often sufficient and cheaper. For products that will be submerged — dry bags, kayak packs, phone pouches, anything sold on an IPX rating — welding isn't an upgrade, it's the requirement. We build both, and we'll tell you which one your product actually needs.
How to Read a Construction Claim
Seam construction is where marketing copy is vaguest, so a few precise checks are worth making:
- Look for the exact phrase. "Fully taped seams" or "fully seam sealed" means every seam. "Seam sealed" or "seam taped" without "fully" may mean critically seamed only — and "fully seam sealed at critical points" is critically seamed with careful wording.
- Ask which parts are welded and which are sewn. A factory that genuinely does both can answer instantly and specifically. One that can't is describing someone else's production.
- Ask about sequence. If load points are stitched after the shell is welded, the barrier is compromised. The answer should be that structural stitching precedes welding.
- Ask what happens to exposed structural stitching. Hot-air backing tape on the inside perimeter, run as a controlled process with its own parameters.
- Ask for peel tests. Weld strength should be verified with peel tests on sample welds at the start of each production run, not assumed from appearance.
How Sealock Runs Both
Our welding capability is the part buyers know about: nine high-frequency welding lines at 27.12 MHz in a 12,000 m² Dongguan plant with 400+ staff, around 100,000 units a month, plus two plants in Ho Chi Minh City. Welding cycles run a fixed sequence — position, clamp under pneumatic pressure, energise for a set dwell, fuse, cool under pressure, then release and inspect — with peel tests at each run start. The cooling step is the one that gets rushed elsewhere, and rushing it produces a cold weld that looks finished and peels later.
The part buyers often don't know is that our industrial sewing operation runs at the same standard, and on many products it accounts for more operations than welding does. That includes box-X and bar-tack reinforcement at load points, bonded nylon or polyester thread at Tex 40 or heavier, wedge-point needles that part the yarns of coated fabric instead of cutting oversized holes, digital thread-tension control, and hot-air seam sealing where structural stitching stays exposed.
Both disciplines in one building is what makes a hybrid product possible — and every backpack, sling, waist pack and fly fishing pack we build is a hybrid product.
Products That Show the Mix
| Image | Model & specs | Welded / sewn balance | MOQ |
|---|---|---|---|
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SL-D002 Tarpaulin Tube Dry Bag — 5/10/20/30L; 500D PVC tarpaulin; eight colours; adjustable shoulder strap + D-ring; floats. | Almost entirely welded — the simplest case, where a flat-seam tube plays to RF's strengths | 500 |
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SL-J033 Waterproof Expedition Pack — 45/55/65L; TPU on 420D nylon; IPX8 rubber sealing strip, fold-over flap and compression strap; reinforced abrasion base; reflective piping. | Welded shell, fully sewn suspension — padded harness, sternum strap, padded hip belt, haul handle, patch panel | 300–500 |
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SL-E008-1 Fly Fishing Pack 24L — 840D TPU double-sided lamination; seamless airtight construction; waterproof zipper; front tool suspension system; detachable extended vest with its own airtight pouch; side bottle storage. | The most hybrid product we build — welded body and pouches, sewn vest, harness and tool stations | 300–500 |
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SL-E103 Waterproof Backpack with Airtight Zipper — 45L (30×22×73 cm); TPU coating; internal computer layer; sample 15 days. | Welded body and zipper interface; sewn internal sleeve and harness | 500 |
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SL-F115 Waterproof Sling Waist Pack — approx. 3L (24×8×15.5 cm), 0.56 kg; 500D PVC; IPX7; airtight main zipper plus a water-resistant front zip pocket; breathable padded back panel; dual D-rings, lash webbing, side loops. | Welded sealed compartment; sewn padded back panel, strap system and hardware | 300 |
Verifying Both Constructions
Quality is gated in three tiers. IQC checks incoming coated fabric for coating thickness, adhesion and weldability, alongside thread, webbing, foam and hardware against the signed standard. IPQC runs on both lines: peel tests on sample welds at the start of each run, weld width and corner integrity on the welding side; stitch density, needle condition, thread tension and bar-tack or box-X execution on the sewing side. OQC covers ISO 2859 AQL sampling, real water-submersion batch testing and gold-sample comparison, with SGS or QIMA inspection available.
Sealock also strictly executes the complete customer inspection procedure on finished goods: unboxing, vacuum extraction, a 24-hour static rest, and air-leak determination — a pinhole that passes a brief dunk still bleeds vacuum over a full day. The lab programme covers both disciplines: weld bond and peel strength on the sealed side, seam strength and a 1,500+ cycle load test with jerk-loading on anchors on the sewn side, plus abrasion, tear, tensile, salt spray, UV, colour fastness and colour difference.
FAQ: Welded and Sewn Construction
Q: Is a welded bag always better than a sewn one?
A: For submersion, yes — welded is the only construction that supports the claim, and welded seams handle up to twice the stress of sewn-and-taped ones. For rain and spray exposure, fully taped construction often reaches the practical ceiling of what conditions demand, at lower cost. The right answer depends on the product's exposure.
Q: Why can't you just weld everything?
A: Because welding wants flat geometry and doesn't hold load-bearing joints well. RF welding uses a flat base plate; three-dimensional dies exist but are expensive. And a welded strap anchor can peel under sustained shear where a stitched footprint spreads the force.
Q: Doesn't sewing ruin the waterproofing?
A: Only if it's done in the wrong order. Stitching must never follow welding, because it perforates the finished seam. Structural stitching goes onto flat panels first and ends up under the welded skin; where it stays exposed, the inside perimeter is sealed with hot-air backing tape.
Q: Why do needle holes matter more on a submerged bag than in the rain?
A: Because needle holes expand under pressure. A stitched seam can be watertight at rest and open up once the bag is pushed under water or loaded.
Q: How do I tell whether a supplier really has both capabilities?
A: Ask which specific parts are welded and which are sewn, what the production sequence is, and what happens to exposed structural stitching. A factory doing both answers immediately and in detail; a trading office cannot.
Q: Which does Sealock do?
A: Both, in-house, at the same standard — nine high-frequency welding lines and a full industrial sewing operation. On most of our products the sewn operations outnumber the welded ones, and both are inspected on every run.
Talk to the Factory
To discuss which construction your product actually needs — welded, sewn-and-taped, or the hybrid most products require — contact Sealock at info@sealock.com.hk or +86-769-82009361. Over twenty years of high-frequency welding and industrial sewing under one roof, nine welding lines, dual China–Vietnam production, and a full customer inspection procedure on every shipment.