The same fabric that makes a packraft airtight is turning up in some surprising places: design studios, engineering labs, and university research projects building inflatable structures and soft robots. So what is it about heat sealable TPU fabric that makes it so useful, and how does a flat sheet of cloth become something that holds air? Here is a plain-English guide.
Two materials doing two jobs
Heat sealable fabric is really two materials working together.
The base fabric is woven nylon. It provides the strength: it resists tearing, stretching and abrasion, and it is what stops an inflated shape from bulging out of control. The coating is thermoplastic polyurethane, or TPU. It fills the gaps in the weave, making the fabric airtight and waterproof.
Neither would work alone. Nylon on its own leaks air straight through the weave. TPU film on its own is airtight but stretches and tears far too easily. Bonded together, you get a material that is light, strong and airtight.
What does “210D” actually mean?
The number before the D is the denier, a measure of how heavy the yarn is. Technically it is the weight in grams of 9,000 metres of a single fibre. In practice, a higher denier means a thicker, heavier, tougher yarn.
- 210D is a great all-rounder. It is what we use for the tubes of our packrafts.
- 420D is heavier duty, better for floors and high-wear areas.
The weave matters too. Our fabrics use an Oxford weave, which gives a strong, stable fabric that handles well when you are cutting and sealing it.
Why TPU can be “welded”
The secret is in the word thermoplastic. Some plastics, once set, can never be softened again. Thermoplastics are different: heat them and they soften and begin to flow; let them cool and they harden again. You can repeat that cycle.
Heat sealing takes advantage of this. Put two TPU coated surfaces face to face, apply heat and pressure, and the two coatings soften and flow into each other. When they cool, they have become one continuous layer of TPU. There is no glue line and no stitch holes, just a single fused seam. Done well, the seam can be stronger than the surrounding fabric.
One important detail: our fabrics are coated with TPU on one side and PU on the other. It is the TPU faces that bond, so plan your seams so the TPU sides meet.
Heat, pressure and time
Every good seal comes down to three things.
- Heat: enough to soften the coating fully. With a heat sealing iron on our fabric, that is roughly 220°C.
- Pressure: to push the softened coatings together so they fuse without air gaps.
- Time: long enough for the heat to get through, but not so long that you scorch the fabric.
Too little of any one and the seam may peel. Too much heat and you risk damaging the nylon. That is why the best advice is always to test on an offcut first.
Ways to seal it
You do not need a factory. Most people start with a handheld heat sealing iron, the kind used by model makers, and a sheet of baking paper to protect the fabric and stop the iron sticking. A heat-proof work surface and a firm roller help you apply even pressure.
Workshops and labs often add a hot air welder, which is useful for long straight seams, or a heat press for sealing flat shapes in one go. Industrial manufacturers go further with machines that weld seams at speed, but the principle is exactly the same.
Our top tips for airtight seams
- Practise on offcuts until you can make a seam you cannot peel apart.
- Keep surfaces clean and dry. Dust and grease stop the coatings fusing.
- Work in overlapping passes and keep the iron moving.
- Let seams cool under light pressure before you handle them.
- Test finished pieces by inflating them and brushing soapy water over the seams. Bubbles show you exactly where to reseal.
- Work somewhere well ventilated, and remember the iron and the fabric get very hot.
Why engineers and researchers love it
For university teams, heat sealable TPU fabric solves a real problem: how to prototype inflatable or airtight shapes quickly and cheaply, without expensive moulds or specialist suppliers.
- It starts flat. Designs can be drawn in CAD, printed or cut as flat patterns, then sealed into three-dimensional shapes.
- It is fast to iterate. A new version of a design can be cut and sealed in an afternoon.
- It is strong for its weight. The nylon base means inflated structures hold their shape under pressure.
- It is accessible. Students can learn to seal it with simple hand tools.
That makes it popular for projects such as soft robotics (flexible grippers and actuators that move when inflated), inflatable structures and deployable shapes, waterproof enclosures, and all kinds of student design and engineering work. Add a top-up valve or Boston valve for inflation and you have a working prototype.
Get started
We sell heat sealable TPU coated nylon by the metre from UK stock, in 210D and 420D, at a full 148cm usable width. If you are buying for a university department, school or research lab, or need a larger quantity, email info@iron-raft.co.uk and we will be happy to help.
And if you would rather skip the building and just get on the water, our ready-to-paddle Paddle Ready packrafts are made from the same family of fabrics.
