A seam strength test pulls a welded or taped dry bag seam until it fails, measuring the force the seam can take. It confirms the weld is strong enough to survive packing, flexing and rough handling.
People searching for "seam strength test" usually want a direct, practical answer - and this page delivers one. This guide explains seam strength testing ratings and how waterproofing is actually tested, so you can read a spec sheet and a test report with confidence instead of trusting a label.
How it is measured
A sample is cut across the seam and pulled in a tensile tester until the weld or fabric fails. The force at failure is recorded and compared to a standard.
Why seam strength matters
A waterproof seam that tears easily is useless, because the bag fails at the joint under load. Strength and watertightness must both hold.
What to expect
Ask for the seam strength in newtons or kilograms and compare across factories. A higher, consistent value signals better welding control.
Test methods explained
A seam strength testing is tested in a few standard ways: submersion under pressure, inflation and submersion to reveal bubbles, and seam pull tests to confirm weld strength. Each targets a different failure point.
Ask which tests a supplier runs and at what sample rate. A supplier that tests every batch is more reliable than one that tested a single prototype years ago.
How the IPX scale works
The IP scale uses two digits: the first rates solids from 0 to 6 and the second rates liquids from 0 to 8, with X meaning the digit was not tested. A seam strength testing only concerns the liquid (second) digit.
The key levels for dry bags are IPX6 (powerful water jets), IPX7 (30 minutes at 1 meter) and IPX8 (continuous immersion beyond 1 meter under stated conditions). Higher is not automatically better without the test context.
Rating vs real-world performance
A seam strength testing rating is a controlled test, not a promise for every situation. A bag that passes IPX7 in the lab can still leak if it is overpacked, the closure is dirty or the seam is damaged.
Treat the rating as a baseline and protect against the gaps: close the bag correctly, inspect seams and closures, and re-test periodically. Real-world care extends what any rating delivers.
How factories test waterproofing
Factories test a seam strength testing with submersion jigs, inflation-and-submersion to reveal air bubbles, and seam pull tests. A paper-towel test inside the bag after immersion is a simple, reliable verification.
Third-party labs can certify IP ratings with controlled equipment. For production, ask for the batch testing plan - typically AQL sampling plus a leak test on every batch - rather than a single prototype certificate.
Reading a test report
A credible seam strength testing test report states the standard, the test conditions, the sample and the result. Check the date and that the report matches the product and material you are buying.
For OEM buyers, ask for the test report before bulk and keep a copy with your compliance documents. A dated, matching report protects both your brand and your customers.
What to ask a supplier about testing
Before you trust a seam strength testing, ask for the test report, the standard used, the depth and duration for any IPX8 claim, and the production batch sampling plan.
Written answers are better than verbal ones. A dated report that matches your product and material is the evidence your brand and customers need.
| Check | Detail |
|---|---|
| Method | Tensile pull |
| Metric | Force at failure |
| Goal | High + consistent |
| Rating | Protection | Typical dry bag use |
|---|---|---|
| IPX6 | Powerful water jets | Splash and rain protection |
| IPX7 | 30 min at 1 meter | Kayaking, brief submersion |
| IPX8 | Deeper / longer immersion | Expedition, sustained water |
| IP68 | Dust-tight + immersion | Beach, dust and water |
| Check | What to look for |
|---|---|
| Test standard | IPX7 / IPX8 with stated conditions |
| Depth & duration | Must be stated for IPX8 |
| Report | Dated, matching the product |
| Batch testing | AQL sampling plus leak test |
| Seam test | Seam strength test result |
| Test | What it checks | How |
|---|---|---|
| Submersion | Water ingress | Immerse under pressure |
| Inflation | Tiny leaks | Air bubbles in water |
| Seam pull | Weld strength | Pull seam to failure |
| Zipper | Closure seal | Water pressure on teeth |
| Claim | What it means |
|---|---|
| IPX6 | Resists powerful jets |
| IPX7 | 30 min at 1 meter |
| IPX8 | Deeper/longer, as specified |
The peel test, explained
Seam strength is measured by a 90-degree or 180-degree peel test on a universal tensile tester: a 25 mm wide strip of welded seam is pulled apart at 100 mm/minute, and the force at failure is recorded in N/25mm. A consumer-grade TPU roll-top should hit 80 N/25mm or higher; a commercial PVC bag should exceed 120 N/25mm.
The mode of failure matters as much as the number. A clean film tear (the film breaks before the seam) means the weld is stronger than the material - that is the goal. A peel-at-the-weld-line failure means the weld is under-built and will delaminate in service.
Sampling plan buyers should demand
For a production lot, ask for a sampling plan: pull 3 to 5 seam samples per 1,000 pieces, test peel strength, and require a minimum 80 N/25mm for TPU and 120 N/25mm for PVC. If any sample fails, the lot is 100 percent re-tested. This is a basic AQL 2.5 inspection level for seam integrity.
The cost of this test is about $15 to $30 per sample at an outside lab, or free in-house if the factory has a tensile tester. A factory without a tensile tester cannot really verify its own welds - ask for a photo of the tester and a sample test report before you place a deposit.
What weak seams look like in the field
A weak seam does not leak on day one. It leaks after 50 to 200 roll cycles when the edge begins to lift. The early warning sign is a slight ridge or bubble along the weld line after the bag is inflated and squeezed. Buyers who receive a sample should inflate it, submerge it, and flex the seam 50 times before approving - that simple cycling catches 90 percent of weak welds.
AQL sampling for bulk orders
For a 3,000-piece order, AQL 2.5 means inspecting about 125 bags, checking seams, closure, printing and dimensions. Critical defects (a leaking seam) result in lot rejection; major defects (a crooked logo) allow a small percentage; minor defects (a loose thread) are tolerated. Build this into the purchase order so the pre-shipment inspection is not a surprise.
Breaking force by material
A good rf-welded TPU seam hits 90 to 130 N/25mm peel. A welded PVC seam hits 120 to 180 N/25mm. A hot-air taped PU nylon seam hits 50 to 70 N/25mm. A stitched-only seam (not recommended for waterproof bags) holds 15 to 25 N/25mm. These numbers are on the test report - ask for them.
Reading a peel test report
A peel test report lists the sample ID, the material, the seam width, the test speed (100 mm/minute), the peak force in N/25mm, and the failure mode. The failure mode is the key: 'material tear' means the weld is stronger than the film - ideal. 'Adhesive failure at weld line' means the weld let go - bad. 'Cohesive failure' means the film split inside the weld - acceptable.
Require a minimum floor: 80 N/25mm for TPU, 120 N/25mm for PVC. If the report shows 60 N/25mm, the bag is under-built. Ask for three samples per lot and average them. A factory with no tensile tester on site cannot verify its own welds - that is a walk-away signal.
AQL sampling for seam defects
For a 3,000-piece order, AQL 2.5 means inspecting about 125 bags for critical and major defects. A leaking seam is a critical defect - one found and the lot is 100 percent inspected. A crooked logo is a major defect; a loose thread is minor. Build this into the PO so the pre-shipment inspection is contractual.
The buyer pays for the third-party inspection (about $200 to $300 per day at SGS/Bureau Veritas). It is the cheapest insurance in the process. Do not waive it to save $250 - a rejected container costs $5,000+
QC testing cost
An in-house tensile tester costs the factory $2,000 to $5,000. Per-sample peel test is free in-house. An outside SGS test costs $300 to $800 per report. For a buyer, the in-house test on 100 percent of seams matters more than the outside report.
Budget $200 to $300 for a third-party pre-shipment inspection at AQL 2.5. It catches critical defects before the container ships.
Sampling plan in the PO
Write the sampling plan into the purchase order: pull 3 seam samples per 1,000 pieces, test peel to 80 N/25mm TPU or 120 N/25mm PVC. Any failure means 100 percent retest. The third-party inspector enforces this. A written plan prevents post-shipment arguments about what was tested.
Contractual QC
Write the peel floor into the PO: 80 N/25mm TPU, 120 N/25mm PVC. Require 3 samples per 1,000 pieces. Reject the lot on failure. The third-party inspector enforces it. A written spec turns a waterproof claim into a measurable contract. For brands sourcing this spec at volume, our affiliated manufacturer keeps the MOQ at 500 and turns pre-production samples around in 6 to 10 days.
Seam strength test method
Seam strength is measured by pulling a welded or taped seam apart on a tensile tester at 200 mm per minute, reporting the force in newtons per 25 mm width. A good welded TPU seam holds 30 to 50 N per 25 mm; a taped stitched seam holds 15 to 25 N; a cold weld fails at under 15 N. The test uses a 25 mm wide specimen, clamped 25 mm on each side, and pulled until the seam separates. Record both the peak force and the mode of failure (weld peel, film tear or hardware pull-out).
For pre-shipment QC, AQL 2.5 on seam appearance and AQL 1.5 on function: pull-test 5 samples per 1,000 bags on a destructive basis, and visually inspect every bag for weld beads under 8 mm. At the factory, the weld temperature and speed are logged per batch, so a weak batch can be traced to a cold die. Reject any batch where the average peel strength is under 25 N per 25 mm, and ask for a dated test report signed by the QC manager.
People also ask
What is the difference between waterproof and water resistant?
Waterproof means it keeps water out under submersion or jets; water resistant means it resists light rain and splashes but will leak under pressure or immersion.
What is a D-ring on a dry bag?
A D-shaped metal or plastic ring used as an attachment point for straps, carabiners or lashing the bag to a boat.
Can you put a phone in a dry bag?
Yes, using a dedicated phone dry bag or a padded roll-top bag. Test the bag first and add padding for impact protection.
What is a purge valve?
A one-way valve that lets air escape after rolling, compressing the bag for packability.
What is the difference between TPU and PVC dry bags?
TPU is lighter, softer and more UV-stable; PVC is heavier, more puncture-resistant and cheaper. TPU suits premium packable bags; PVC suits rugged, high-volume bags.
What is the difference between a dry bag and a dry sack?
They are the same product. Dry sack is the more common British and outdoor-store term, while dry bag is used more in the US.
What are the parts of a dry bag?
Body fabric, roll-top or zipper closure, buckle, D-rings or attachment points, straps or handles, and optional purge valve or window.
How are dry bags made?
The body is cut from waterproof film or coated fabric, seams are welded or taped, hardware is attached, and each bag is leak-tested before shipping.
Key takeaways
- Choose seam strength testing by fixing the activity, size, material and closure before comparing options.
- Verify waterproofing with a stated IPX rating and a real test method, not a label.
- For bulk orders, confirm MOQ, lead time and a pre-production sample before committing.
- Match material to conditions: TPU for cold and UV, PVC for abrasion, coated nylon for weight.
- A physical pre-production sample is the only reliable way to lock material, color and printing.
- Store dry bags loose and shaded, and re-test the seal before trusting them with electronics.
Related guides
- What is a dry bag?
- Dry bag materials: TPU vs PVC vs nylon
- IPX / IP68 waterproof ratings
- Types of dry bags
- OEM dry bags
Step by step
- Find the stated IPX or IP rating and the test conditions.
- Confirm the depth and duration for any IPX8 claim.
- Look for a dated test report that matches the product.
- Run your own leak test before trusting expensive gear.
Specification checklist
- Fix the specification first - material, size, closure, printing and packaging - so quotes are comparable.
- Verify waterproofing with a stated IPX rating and a real test method, and confirm the batch sampling plan.
- Order a pre-production sample and approve it before bulk production begins.
- Confirm MOQ, lead time, payment terms and the correct HS code before you commit.
- Match the material to the conditions: TPU for cold and UV, PVC for abrasion, coated nylon for weight.
- Ask for a dated test report and a pre-shipment inspection (AQL) for waterproofing and seams.
- Keep the most valuable items in an inner pouch as a second layer of protection.
- Rinse, dry and store the bag correctly to extend its lifespan season after season.
Bottom line
The bottom line is to treat an IPX rating as a claim to verify, not a guarantee to trust. A stated IPX7 or a fully specified IPX8 with a dated test report is meaningful; an unsupported IPX8 is marketing. Confirm the conditions and run your own leak test before relying on any bag. For "seam strength test", this is the summary that matters most.
Frequently Asked Questions
Q1. What is the difference between waterproof and water resistant?
Waterproof means it keeps water out under submersion or jets; water resistant means it resists light rain and splashes but will leak under pressure or immersion.
Q2. What is a D-ring on a dry bag?
A D-shaped metal or plastic ring used as an attachment point for straps, carabiners or lashing the bag to a boat.
Q3. Can you put a phone in a dry bag?
Yes, using a dedicated phone dry bag or a padded roll-top bag. Test the bag first and add padding for impact protection.
Q4. What is a purge valve?
A one-way valve that lets air escape after rolling, compressing the bag for packability.
Q5. What is the difference between TPU and PVC dry bags?
TPU is lighter, softer and more UV-stable; PVC is heavier, more puncture-resistant and cheaper. TPU suits premium packable bags; PVC suits rugged, high-volume bags.
Q6. What is the difference between a dry bag and a dry sack?
They are the same product. Dry sack is the more common British and outdoor-store term, while dry bag is used more in the US.
Q7. What are the parts of a dry bag?
Body fabric, roll-top or zipper closure, buckle, D-rings or attachment points, straps or handles, and optional purge valve or window.
Q8. How are dry bags made?
The body is cut from waterproof film or coated fabric, seams are welded or taped, hardware is attached, and each bag is leak-tested before shipping.
Q9. What are the different types of dry bags?
Roll-top, zipper, backpack, tote, phone, floating, waist pack, duffel and pouch styles, each built for a different activity and access need.
Q10. Are dry bags safe for electronics?
Yes for water protection, but they do not protect against impact. Use a padded or dedicated electronics dry bag and test it first.
Q11. What is the difference between a dry bag and a backpack?
A dry bag is waterproof and seals shut; a backpack is structured for carry but is rarely fully waterproof. A dry bag backpack combines both.
Q12. What does IPX8 mean on a dry bag?
IPX8 means the bag survives continuous immersion beyond 1 meter under manufacturer-specified conditions. The depth and duration are only meaningful if stated.
Q13. Does dry bag material affect waterproofing?
The material supplies the waterproof barrier, but the closure and seams decide real-world waterproofing. A good film still leaks if seams or closures fail.
Q14. Are IPX ratings on dry bags trustworthy?
Only when test conditions are stated and backed by a report. Many listings misuse IPX8 without specifying depth or duration.
Q15. What is an IPX rating?
IPX ratings classify protection against water. IPX6 resists powerful jets, IPX7 survives 30 minutes at 1 meter, and IPX8 survives deeper or longer immersion as specified.
Q16. What is a nylon dry bag?
A nylon dry bag uses woven nylon coated or laminated with a waterproof layer such as TPU or silicone, giving a light, packable fabric.
Q17. Is TPU better than PVC for dry bags?
For most premium dry bags, yes: TPU is lighter, softer and more UV-stable. PVC still wins for the cheapest, most abrasion-resistant high-volume bags.
Q18. What is a dry bag leak test?
A leak test seals the bag, submerges it under pressure and looks for air bubbles, or places dry paper inside and checks for moisture after immersion.
Q19. Are transparent dry bags useful?
Yes. A transparent or windowed dry bag lets you see contents without opening it, which is convenient for travel, events and security checks.
Q20. What is a camera dry bag?
A camera dry bag is a padded, waterproof pouch or case that protects a camera from water, spray and sand during outdoor shoots.