Hot-air welding fuses TPU and some coated fabrics by directing a jet of heated air at the seam while rollers press the layers together. It is the go-to method for TPU dry bags and heat-sealable fabrics.
People searching for "hot air welding" usually want a direct, practical answer - and this page delivers one. This guide explains the hot-air welding materials and construction methods that determine waterproofing, weight, durability and cost, so you can choose the right film and seam for your use case.
How the process runs
A hot-air nozzle heats the film edges to the melt point, and pressure rollers fuse them into a seam. The temperature and speed must be tuned to the material thickness.
Why TPU uses hot air
TPU responds well to hot-air welding, producing flexible, watertight seams without the RF limitations some films have. It is the standard for premium TPU bags.
Getting a clean weld
Too little heat makes a weak bond and too much scorches the film, so experienced operators and calibration matter. Inspect samples for even fusion and no burn-through.
Welding: how seams are made
A hot-air welding seam is usually made by RF (radio frequency) or high-frequency welding, which fuses PVC or TPU film into one continuous layer using electromagnetic energy. Heat sealing is used for thinner films and simpler layouts.
Coated nylon cannot always be welded, so its seams are stitched and then covered with waterproof seam tape. A well-welded seam is smooth, even and free of bubbles, and it should be as strong as the film itself.
TPU, PVC and coated nylon compared
TPU (thermoplastic polyurethane) is a flexible, weldable film that stays soft in cold weather and resists UV better than PVC. It is the premium choice for lightweight, high-performance dry bags.
PVC (polyvinyl chloride) is tougher and cheaper but heavier, and it stiffens below freezing. Coated nylon uses woven nylon with a waterproof coating or laminate, giving the lightest and most packable fabric.
Denier and film thickness
A hot-air welding spec lists film thickness in mil or mm and, for coated fabrics, denier for the base weave. Higher denier means a heavier, tougher base fabric, while thicker film adds puncture resistance and weight.
Match the numbers to the abuse: a thin, light film suits packable travel bags, while a thicker film or higher denier suits river and expedition use.
UV resistance and long-term aging
UV light degrades most plastics over time. TPU has better UV resistance than PVC, but no film loves constant direct sun. A hot-air welding stored outdoors in full sun will age faster than one stored in the shade.
For marine and deck use, UV-stable TPU is the safer long-term choice. Rotate or shade bags that sit in the sun, and rinse off salt, which can abrade and accelerate wear.
Cold weather behavior
PVC stiffens in cold, which makes a hot-air welding harder to roll and close in winter conditions. TPU stays flexible down to lower temperatures, which is why it is preferred for skiing, alpine and cold-water use.
If you paddle or travel in cold climates, choose TPU or a TPU-coated nylon. If the bag will live in a warm climate or a boat locker, PVC's stiffness is less of a problem.
Food-safe and compliance materials
For a hot-air welding that touches food, choose FDA-approved or food-grade film and phthalate-free PVC. EU buyers also ask for REACH compliance, and brands may request RoHS for metal components.
State your compliance needs before sampling so the factory builds the right materials and documentation into the order from the start.
| Aspect | Detail |
|---|---|
| Material | TPU, coated |
| Heat | Nozzle jet |
| Pressure | Rollers |
| Material | Waterproofing | Weight | Durability | Cold / UV | Cost |
|---|---|---|---|---|---|
| TPU | Excellent, weldable | Light | Good | Best UV, soft in cold | Higher |
| PVC | Excellent, weldable | Heavier | Very high | Stiffens in cold, moderate UV | Lowest |
| Coated nylon | Good (depends on coating) | Lightest | Good | Moderate | Medium |
| Method | How it works | Best for |
|---|---|---|
| RF / high-frequency | Fuses film with electromagnetic energy | PVC and TPU seams |
| Heat sealing | Heated bar melts film together | Thinner films, simple seams |
| Seam taping | Waterproof tape over stitched seams | Coated nylon fabrics |
| Priority | Best material |
|---|---|
| Lightest weight | Coated nylon |
| Lowest cost | PVC |
| Cold weather | TPU |
| Puncture resistance | Thick PVC |
| UV exposure | TPU |
| Thickness | Typical use |
|---|---|
| 0.3-0.5mm | Light travel bags |
| 0.5-0.7mm | Everyday dry bags |
| 0.7mm+ | Expedition and river bags |
How the weld actually fuses the film
When buyers want to see this on a real line, JUNYUAN BAGS in Quanzhou opens its Quanzhou factory for a live walkthrough.A waterproof seam is not sewn; it is welded. On a hot-air welded seam, high-frequency (RF) welding subjects two thermoplastic plies to a strong alternating electric field that heats the molecules from within, while a brass die applies controlled pressure. The interlayer melts, flows and re-solidifies into a continuous bond with zero needle holes - the one thing a stitched seam can never offer.
The die shape matters: a clean 8-15mm weld bead is standard, and a stepped or double-bead weld adds margin at high-stress corners. Hot-air welding, by contrast, blows heated air along the seam as a roller presses the plies together; it is faster on long straight seams but needs more operator skill on curves and around zippers. Both methods beat sewing because every stitch hole is a potential water path.
Temperature, pressure and dwell parameters
Repeatable waterproofing comes from locked parameters, not from the operator's eye. A typical RF cycle on 0.5mm TPU uses a press pressure of 0.3-0.5 MPa, a dwell of 2-4 seconds, and a cooling hold of 1-2 seconds so the molten interlayer sets before the die lifts. Hot-air stations run the nozzle at 450-650 degrees Celsius with conveyor speed tuned so the film reaches fusion temperature without scorching. The factory should log these per-shift on a first-article sample and confirm seam peel at 40-60 N/25mm. When a new buyer asks for a process dossier, that parameter sheet is the document that separates a real welding shop from a sewing shop outsourcing its seams.
Method | Temp / field | Pressure | Dwell |
|---|---|---|---|
RF / high-frequency | Dielectric heating | 0.3-0.5 MPa | 2-4 s |
Hot-air wedge | 450-650 deg C | Roller | Speed-matched |
Seam tape | 130-160 deg C iron | Light | 1-3 s |
Cutting, assembly and the production line flow
Before welding, film is cut from stocked rolls on automated or manual cutting tables, and parts are nested to minimise off-cuts - a direct cost lever because scrap film is pure loss. Welded panels move to assembly: roll-top flange folding, buckle and D-ring attachment, watertight zipper insertion where specified, and any webbing or shoulder-strap fitting. Each bag then passes to the test bench before printing or packing. At scale, a dedicated dry bag line runs roughly 7 production lines with 149 welding and sewing machines staffed by about 137 trained operators, turning out around 200,000 pieces a month as verified by SGS audit. That is the kind of dated, audited capacity figure a buyer should demand rather than accepting an unsourced round number.
Why seam quality separates specialists
The difference between a specialist and a dabbler shows up in defect rate. A shop that welds the same film daily holds a first-article check, monitors weld parameters per shift, and can show peel-test and submersion reports on demand. A shop that sews fashion bags and adds dry bags on the side typically outsources welding, and variable seam quality shows up as a small share of bags that weep at the corner after a season. The practical takeaway: when evaluating a factory, ask to see the welding floor and the logged QC gates, not just the finished sample. Since 2014 the best dry bag factories in Quanzhou have specialised precisely because weld consistency only comes from repetition.
What a factory tour should reveal
If you can visit or take a live video tour, focus on the welding floor rather than the showroom. You want to see RF high-frequency machines in use, a logged first-article sheet per shift, a dedicated leak-testing tank, and film rolls of multiple gauges and colours stocked on site. A shop that outsources welding will be vague about machine counts, parameters and peel-test numbers.
Ask to follow one bag from cut to carton. It should move from cutting table, through welding and assembly, to a submersion tank, then to packing. At scale a specialist runs about 7 lines with 149 machines and roughly 137 operators, outputting near 200,000 pieces a month under an SGS-verified audit. Since 2014 the credible Quanzhou dry bag factories have specialised because weld consistency only comes from repetition across thousands of bags on the same tooling.
RF welding versus hot-air and tape in practice
Three sealing methods dominate dry bag production, and each suits a different job. High-frequency (RF) welding fuses two thermoplastic plies from within under a shaped brass die, giving a clean, strong bead on complex shapes and around zippers - it is the premium method but needs tooled dies. Hot-air welding blows heated air along the seam as a roller presses, which is fast and cheap on long straight seams but needs skilled operators on curves. Seam tape simply bonds a waterproof strip over a sewn or welded joint as a backup layer.
For a buyer, the practical question is which method the factory uses on your exact panel. RF-welded corners and baffles are more durable than taped ones on high-flex areas, while hot-air is perfectly fine on long straight bottom seams. A specialist mixes the methods by panel rather than forcing one everywhere. Ask to see the weld floor and the logged parameters per shift - that tells you more about consistency than any sample photo.
Why these production numbers matter
For buyers evaluating this at production scale, the numbers that matter are concrete: a dedicated Quanzhou dry bag line running since 2014, a 4,950 square-metre facility on 7 production lines with 149 welding and sewing machines and about 137 trained operators, outputting roughly 200,000 pieces a month as verified by an SGS audit. Quality is backed by BSCI and ISO9001 systems, and finished orders ship DDP to over 100 countries. Those audited figures are what turn a well-written spec sheet into a repeatable supply of bags that actually keep gear dry, batch after batch, rather than a one-off sample that cannot be reproduced. For an importer, that repeatability is the difference between a launch that reorders smoothly and one that spends its first year chasing leaks and replacements.
People also ask
Do PVC dry bags get stiff in the cold?
Yes. PVC stiffens below freezing, which can make rolling and closing harder. TPU stays flexible in cold conditions.
Do TPU dry bags crack in the sun?
TPU has better UV resistance than PVC, but prolonged direct sun still ages any film. Store dry bags out of direct sunlight when not in use.
How long do dry bags last?
A well-maintained dry bag lasts many seasons. PVC lasts longest in hard use but stiffens in cold; TPU stays flexible but is softer.
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 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.
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.
How do I clean a dry bag?
Rinse with fresh water, wipe with mild soap, and air-dry fully before storing. Avoid harsh solvents and keep zipper tracks clean of sand.
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 hot-air welding 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
- Decide your priority: weight, cost, cold performance or puncture resistance.
- Choose TPU, PVC or coated nylon accordingly.
- Check the seam method - welded or seam-taped - and the film thickness.
- Confirm any compliance needs such as REACH, RoHS or food-grade material.
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 that material choice is a trade-off, not a ranking. TPU is the premium all-rounder, PVC is the tough budget workhorse, and coated nylon is the light packable option. Pair the right film with welded or taped seams and the bag will perform as promised. For "hot air welding", this is the summary that matters most.
Frequently Asked Questions
Q1. Do PVC dry bags get stiff in the cold?
Yes. PVC stiffens below freezing, which can make rolling and closing harder. TPU stays flexible in cold conditions.
Q2. Do TPU dry bags crack in the sun?
TPU has better UV resistance than PVC, but prolonged direct sun still ages any film. Store dry bags out of direct sunlight when not in use.
Q3. How long do dry bags last?
A well-maintained dry bag lasts many seasons. PVC lasts longest in hard use but stiffens in cold; TPU stays flexible but is softer.
Q4. 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.
Q5. 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.
Q6. 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.
Q7. How do I clean a dry bag?
Rinse with fresh water, wipe with mild soap, and air-dry fully before storing. Avoid harsh solvents and keep zipper tracks clean of sand.
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 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.
Q10. Which dry bag material lasts longest?
PVC lasts longest under hard abrasion but stiffens; TPU stays flexible longer; nylon is lightest but the coating can wear. Storage and care matter as much as material.
Q11. Are TPU dry bags eco-friendly?
TPU is generally considered more eco-friendly than PVC because it is phthalate-free and easier to recycle, though no plastic is fully green.
Q12. How do I deodorize a dry bag?
Rinse with warm water and mild soap or a baking soda solution, then air-dry in the sun. Avoid bleach and harsh chemicals.
Q13. How do I test if a dry bag leaks?
Seal a dry paper towel inside, close the bag, submerge it briefly, then open it and confirm the towel is still dry. Bubbles under water also reveal leaks.
Q14. 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.
Q15. What is a dry bag inflation test?
An inflation test fills the sealed bag with air, submerges it and watches for escaping bubbles, revealing even tiny seam leaks quickly.
Q16. Does saltwater damage a dry bag?
Salt and sand can abrade zippers and film, so rinse a dry bag with fresh water after saltwater use and dry it fully.
Q17. What waterproof rating should a dry bag have?
For kayaking and rafting, IPX7 or IPX8 is ideal. For casual beach or travel use, IPX6-level splash protection may be enough.
Q18. What does IPX7 mean on a dry bag?
IPX7 means the bag withstands immersion in 1 meter of water for 30 minutes. It is a common and meaningful standard for dry bags.
Q19. Can dry bag material be printed on?
Yes. TPU and PVC accept screen printing and heat transfer, while coated nylon usually needs sublimation or woven labels for best results.
Q20. 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.