Home / Blog / Waterproof Technology

What Happens When a Seam Fails

The four ways a welded seam fails

A radio-frequency or hot-air welded seam joins two film surfaces by melting them into each other. It fails in four recognisable ways: peel, where the two faces separate starting from an edge; pinhole, where a gap is left inside the weld track; delamination, where the film lets go of the face fabric it is laminated to; and creep, where a stressed seam slowly elongates until the weld track thins and lets go.

Each mode has a signature, and reading the signature is faster than any test. Peel shows a lip you can catch with a fingernail at a seam end. A pinhole shows nothing at all until the bag is pressurised, and then it shows as a stream of fine bubbles. Delamination shows as a cloudy, milky patch beside the weld with a slightly raised edge. Creep shows as a seam that has visibly stretched by 2 to 3 mm and gone glossy.

Two of these are production defects and two are service-life issues. Pinhole and most peel failures originate in the welding window on the line or in contamination at the time of welding. Delamination and creep develop over seasons of load, heat and UV, which is why a bag that passes every test in year one can still fail in year six. The progression is tracked in our seam service life guide.

The first sixty seconds on the water

A paddler rarely sees the failure itself. What they notice is a bag that sits differently: a floating bag that has lost its buoyancy and is riding lower, or a hatch where a small pool has formed under one corner. Those two observations cover most real-world detections, which is why a quick visual sweep of the bilge and the hatch every couple of hours is worth more than any gadget.

Once the water is in, it spreads fast inside a sealed bag because there is no cavity and no drain. Two hundred millilitres entering at the top of a 20 litre bag wicks through clothing in 10 to 20 minutes as the bag is moved around. The practical consequence is that the moment you suspect a leak, the priority is to isolate the contents, not to find the hole.

Sort the contents by consequence before anything else. Insulation, communications, medication and electronics come out first and go into a secondary pouch. Food and kitchen items can wait. If you are on a boat with a hatch, get the suspect bag out of the hull and onto a deck or a dry surface so it stops acting as a reservoir sitting against everything else you own.

Peel failure: the most common and the most repairable

Peel starts where the weld track ends. The last 10 to 15 mm of a seam carries the transition from welded to unwelded material, and that is where stress concentrates when the bag is overstuffed or when a corner is folded back on itself repeatedly. Once the faces begin to separate, the crack propagates along the track at a rate set by how much tension the bag is under, typically 1 to 3 mm per day of hard use.

The good news is that peel is slow, visible and repairable. Caught in the first few centimetres, a peel can be stopped with a TPU patch welded or bonded across the seam end, and the repair is stronger than the surrounding film because the patch spreads the load over a wider area. Caught at 100 mm, the same repair becomes a full seam rebuild that most people will not attempt in the field.

Prevention is mostly packing discipline. A bag rolled three times that is also stuffed to 100% of volume puts continuous tension on the top seam every time it is lifted. Leave 15% headroom, and never use the closure as a handle; carry by the webbing or the body. Our seam tear repair guide covers the patch technique in detail.

Pinholes: the failure you cannot see

A pinhole is a gap left inside the weld track, usually 0.2 to 1 mm across. It comes from a weld window that was slightly off, from a fold or a contaminant in the film at the moment of welding, or from a film thickness variation between two rolls. Nothing is visible on the bench. Under air pressure, it produces a fine, steady stream of bubbles that is unmistakable.

What makes pinholes dangerous is the flow rate. A 1 mm round aperture at one metre of head passes roughly 120 to 200 millilitres per minute of clean water at 20 °C, and the same aperture at 30 cm of head still passes 60 to 120 millilitres per minute. That is a litre in five to ten minutes, which is why a pinhole bag is a retire-now bag for anything critical, whatever the patch kit says.

Pinholes are also the cheapest failure to prevent on the line. An air inflation check at the end of welding catches essentially all of them, and it takes under 20 seconds per bag. That single station is the reason a bag from a controlled line is far less likely to surprise you in year one than a bag with no documented pressure test. The method is described in our air inflation leak test guide.

Delamination: when the film lets go of the fabric

On a laminated construction there are two bonds: the weld between two film faces, and the lamination bond between the film and the woven face fabric. Delamination is the second one failing. It shows as a milky or cloudy patch running alongside the weld, with a slightly raised edge where the film has lifted, and it grows outward from the weld rather than along it.

The causes are heat, hydrolysis and plasticiser or additive migration, and the onset is slow. A bag stored damp and warm for a season can show the first patches at two to three years; the same bag rinsed, dried in shade and stored loose can go eight years without a sign. Delamination is the failure mode that most reliably correlates with how a bag has been stored rather than how hard it has been used.

Delamination is barely repairable, because the bond has failed over an area rather than along a line. A patch can bridge a small patch of delamination if it extends 25 mm beyond the affected area on all sides, but once the cloudy area exceeds roughly 80 mm across, the film will keep lifting beyond the patch edge and the repair will fail within weeks. Retire the bag and move it to non-critical duty.

How much water actually gets in

Numbers help here because the intuition is usually wrong in both directions. People expect a breach to be catastrophic, and it is not; they also assume a few drops an hour is harmless, and over a long trip it is not. The table below is based on clean water at 20 °C flowing through a round aperture, measured as millilitres per minute, which is the unit you should use when deciding whether a bag can finish a trip.

Translate the rate into trip terms. Forty millilitres per minute is 2.4 litres per hour and roughly 29 litres over a twelve-hour day, which is more than the volume of most bags in the first place. Ten millilitres per minute is 0.6 litres per hour, enough to soak a sleeping bag overnight and still leave the bag looking only damp when you open it.

Depth changes the rate far less than people expect, because the driving force is the square root of the head. Doubling the depth from 0.5 m to 1 m raises the flow by about 40%, not by a factor of two. Aperture size matters much more: doubling the diameter quadruples the area and therefore the flow, which is why a weep that widens from 0.5 mm to 1 mm is the moment to act.

Detecting a failure before it matters

Four tests cover almost everything, in ascending order of effort. The air inflation check is the fastest: roll the bag closed, insert a nozzle or a drinking straw through the closure, inflate until the bag is firm but not drum-tight, and listen. A hiss you can hear across a room is a failure you cannot ignore; a slow bubble in a soapy solution is a failure you can plan around.

The submersion test is the second: roll the bag closed with a towel inside, hold it under 20 to 30 cm of water for 30 seconds and check the towel. It catches pinholes that the air test misses because the water pressure opens a marginal track that air at low pressure does not. The third is a soap solution painted along every seam with the bag inflated, which localises the hole rather than just detecting it.

The fourth is a weighted test and it is the one that most closely matches a real capsize: inflate the bag, lay it under a 5 kg weight, and check pressure retention over 30 minutes. Our immersion testing guide sets out the full protocol, and the at-home test sequence covers the version a paddler can run in a bathroom.

Field triage: the twenty-minute repair

1. Get the contents out, sorted by consequence, and put the critical items in a secondary pouch.
2. Dry the area around the failure completely with a cloth; no patch bonds to a wet film.
3. Round the corners of the patch and cut it 25 mm larger than the damage on every side.
4. Bond or weld from the centre outward, pressing out air as you go, then weight it for ten minutes.
5. Re-test with the air inflation check before the bag goes back into service.
6. Log the repair: date, location, size and what the bag is allowed to carry from now on.

Temperature and cleanliness decide whether a field repair holds. Most TPU patches need a surface above 10 °C and a film wiped with alcohol or fresh water and dried, and a patch applied to a gritty, sunscreen-contaminated surface will lift within hours. If the repair is on a corner, extend the patch around the corner rather than folding it, because a folded patch creates its own peel initiation point.

Cure time is the part people skip. A bonded patch reaches maybe 60% of its final strength in ten minutes and full strength in 24 hours, so a bag repaired at camp in the evening should carry non-critical kit the next morning and go back to full duty the day after. Our patch kit guide lists what to carry: two 100 mm patches, a 200 mm tape strip, alcohol wipes and a small roller.

Repair or retire: the decision rule

Three questions decide it. Is the failure on a seam or on the panel? Is the aperture under or over 1 mm? Is the bag carrying insulation, electronics or communications? Two yes answers and the bag is retired from critical duty; three yes answers and it is recycled rather than repaired. That rule is deliberately conservative because the cost of a new bag is far below the cost of a failed one mid-trip.

Retirement is not the same as disposal. A bag with a corner peel and no other damage makes an excellent wet-gear pouch, a wash bag or a bucket liner, and a bag with a panel pinhole works perfectly as a dry bag for items that never leave a tent. The honest approach is to downgrade the bag, mark it visibly so nobody re-issues it as a critical bag, and keep using it until the film itself goes brittle.

Why a welded seam behaves differently from a taped one

A stitched and taped seam carries three weaknesses that a welded seam does not: the needle holes, the thread, and the tape adhesive that holds the tape over the holes. Under pressure, water only needs a path, and needle holes give it thousands. A tape that has aged, been folded or been contaminated by sunscreen lifts at the edge and the whole row becomes a leak path in minutes.

A welded seam has one bond instead of three, and it fails in the modes described earlier rather than gradually degrading along its whole length. That is why the failure profile is different in kind: welded seams tend to be binary, either passing or failing at a point, while taped seams tend to degrade diffusely. The comparison is drawn out in our welded versus stitched guide.

How seam strength is measured

Two numbers define a seam: peel strength and shear strength. Peel strength is the force needed to separate the two welded faces, expressed in newtons per 25 mm of width, and it is the number that predicts the edge failures described earlier. Shear strength is the force needed to pull the joint apart in the plane of the film, expressed in newtons per 50 mm, and it predicts whether a load-bearing seam will creep.

Test methods for coated and laminated fabrics are published by standards bodies, and a serious production line quotes the method alongside the value. ASTM publishes the coated-fabric test methods that cover grab tensile, tongue tear and seam strength, and ISO publishes the textile and hydrostatic test series used alongside them. A spec sheet that gives a number without a method is not comparable to anything.

Sampling is the other half of the story. A single seam pulled once a week says little; a controlled line pulls samples per machine per shift and records the value, so a drift in the weld window shows up as a trend before it shows up as a failed bag. The inspection framework for that is set out in our AQL inspection guide and the quality control guide.

Designing the failure out of the specification

• Require a weld width and a peel value in the specification, not just a waterproof claim.
• Require 100% air inflation checking at the end of the welding line, not spot checks.
• Specify weld starts and stops away from corners wherever geometry allows.
• Ask for peel and shear values per machine per shift, recorded as a trend.
• Match film thickness to the load: 0.5 mm on load-bearing seams and attachment points.
• Require a repair patch and instructions to ship with any bag rated IPX7 or higher.
• Compare the seam construction across the roll-top range and the product line before standardising.

The weld window is the single most important process variable: energy, pressure and dwell time have to sit inside a band that is wide enough for normal film variation and narrow enough to be controlled. A line that records those three parameters per cycle can trace any field failure back to a shift, a roll of film and a machine, which is what turns a warranty claim into a process correction rather than an argument.

Programmes that need that level of traceability are run with documented parameters and retained samples. Waterproof bags have been produced on our line since 2014 in an SGS-verified 4,950 m2 facility with seven production lines, 149 machines and AQL 2.5 inspection; sampling runs 6 to 10 working days, bulk runs 35 to 50 days, terms are T/T 30/70 FOB Xiamen and the minimum is 500 pieces. The process is documented and the rating language that goes with it is explained in our IPX ratings guide. For the standard behind the ingress grades themselves, see the IEC.