Proper Heat-Sealing Technique

Contents:

  • Introduction
  • Work Surface
  • Ironing Technique
  • Finding the Right Temperature
  • Troubleshooting
  • Curved Seams
  • Check Your Work

 

Introduction

This page describes the heat-sealing technique in detail and I recommend you familiarize yourself with the information before starting to construct your DIY Packraft so you can be sure you’re making good, strong welds. It will also help you troubleshoot any problems you run into.

If you’re getting ready to start your first heat-sealing project, practice using your iron on a few scraps of fabric before starting on your packraft (fabric scraps are included with every packraft kit). That way, the build will go smoothly and you’ll be confident in your welds.

The following tips apply only to the fabrics offered in the DIY Packraft Shop; fabrics from other suppliers may behave differently and I cannot vouch for their quality or properties!

Note: In the How-To pages I use the words “join,” “seal,” “weld,” “bond,” and “heat seal” interchangeably. The meaning is always to apply heat to the fabric with your iron so that the TPU coatings melt and fuse together.

 

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Work Surface

I generally heat seal on a smooth, unpainted wood or medium density fiberboard (MDF) surface, and I cover it with a sheet of parchment paper (baking paper). Cardboard, such as the inside of a cereal box, also works well as a smooth surface, and even a piece of paper over a table top works fine, but remember that heat will transfer through thin cardboard or paper, so the surface below must be heat-resistant.

Avoid heat-sealing over corrugated cardboard because the ribs inside cause an uneven pressure distribution under the iron. For the same reason, you should also avoid rough plywood, oriented strand board (OSB), and rough dimension lumber… basically anything without a smooth surface.

 

Ironing Technique

A strong weld between two layers of TPU coated fabric requires two things: heat and pressure. Heat is the obvious one, but don’t forget to press down firmly with the iron.

Orient the pieces of fabric with their heat-sealable TPU coatings facing each other, and then press down with the iron on the area you want to seal. If the area is larger than the tip of your iron, you can slide it back and forth.

Press with the iron and slide it back and forth over a short section of the seam. This sliding motion works well because, with most of the colors of fabric that I offer, the fabric darkens slightly as the TPU begins to fuse (it almost looks wet), and sliding the iron back and forth allows you to see exactly when this happens, so you won’t overheat the fabric. The darkening is temporary – as the TPU cools, the fabric returns to its original appearance.

The thicker your fabric is, the longer you will have to apply heat and pressure for the bond to form properly, because the heat has to transfer through the woven fabric to the TPU layers below. If your iron is set to the correct temperature, this should only take a few seconds.

For best results, use a rag (or something else like a piece of cardboard or a block of aluminum) to continue applying pressure to the weld for a few seconds after the iron is removed. 

When working on a seam, always iron out from a single starting point – do not try to seal two ends of a seam and then meet in the middle, or you could end up with a wrinkle.

If you’re finding that the fabric moves around too much as you work, you can use weights or masking tape to hold it in place.

Note: The generic masking tape I’ve used doesn’t leave a noticeable residue on the fabric, even after ironing. The tape doesn’t stick to the fabric particularly well, but it works well enough for a temporary hold. If you use a tape that leaves a residue, it’s not the end of the world because nylon and polyurethane have good resistance to most solvents, but do a quick search online to check compatibility before washing your packraft with a solvent.

 

Finding the Right Temperature

The fabrics offered in the DIY Packraft shop fuse at approximately 200 °C (400 °F). Temperature settings on mini irons and adjustable soldering irons are pretty inaccurate, so the best way to find the right heat-sealing temperature is by trial and error.

After plugging in the iron and turning it on, wait 5-10 minutes for it to heat up fully.

Note: For Clover irons and similar designs, before plugging in the iron, make sure the screw that holds the iron’s tip in place is tight to ensure maximum heat transfer from the heating element to the tip of the iron.

You’ll know if your iron is much too hot as soon as you touch it to the fabric, because the woven fabric will start to melt and stick to your iron. This is unlikely to happen with the DIY Packraft fabrics, as they are designed to have a melting point much higher than the TPU activation temperature, but beware of heat sealable fabrics from other suppliers.

If the TPU coating is melting to a liquid state and not sticking to itself, your iron may still be too hot even if the woven fabric isn’t melting. In this case, you should either use a lower temperature setting on your iron, or apply heat for a shorter period of time.

If the iron is too cold, the TPU will not bond properly no matter how long you press the the fabric. If your iron is too cold even on the hottest setting, you may be able to boost the temperature by wrapping the heating element and shaft in aluminum foil so more heat will be conducted to the tip, depending on the iron’s design. Be careful not to damage the iron’s internal circuit board by overheating!

When testing to see if you’re getting a good seal with your iron, you must wait for the sealed fabric to cool before trying to peel it apart. If the TPU is still hot, the layers will pull apart easily even if your iron is set to the right temperature and you are doing everything else correctly. Wait until the bond feels cool to the touch before testing it. You can blow on it or wave it in the air to speed up the cooling process, but if the fabric is curling/lifting or if you are working on a curved seam, you may need to hold pressure on the weld to keep it in place while it cools for several seconds. A clean rag works well for this – if you use your bare hand, you may burn yourself!

A good bond will be very difficult or impossible to peel apart when cool (depending on how wide it is and how strong you are). When I have a good bond between two pieces of fabric a few cm wide, I have difficulty pulling them apart even when I pull as hard as I can. When the pieces do come apart, the TPU peels away from the woven fabric in patches on both sides, indicating that the TPU layers have fused into a single layer – this is what you want to see:

Here the clear TPU coatings have fused into a single layer and when the two pieces of fabric are forced apart the TPU turns white and peels away from the blue and black woven nylon in patches.

A bond with insufficient heat will peel apart more easily and will look like this:

Here the iron was not hot enough or I did not apply heat long enough for the TPU layers to bond fully; the clear TPU coatings have not fused into a single layer and can be peeled apart without separating from the blue and black woven nylon.

If the TPU peels off in patches from both pieces of fabric when you force them apart (after allowing them to cool), your iron is the correct temperature and you have made a strong bond.

 

Troubleshooting:

If the TPU layers peel apart from each other instead of peeling away from the woven fabric in patches, either

  • Your iron is not hot enough to fuse the TPU properly
  • You’re not applying enough pressure
  • You’re not allowing enough time for heat to transfer through the fabric
  • Or some combination of the above factors

If the TPU layers fuse into a single layer that peels away from only one piece of fabric when forced apart (the one that you applied the iron to), either

  • Your iron is too hot
  • You’re not pressing hard enough
  • You’re not allowing enough time for heat to conduct all the way through both layers of TPU
  • Or some combination of the above factors

If the temperature seems right and you’re pressing firmly with the iron but some sections are wrinkling or lifting out of place, hold pressure on the seam for several seconds after the iron is removed so the weld cools in place (use a rag to avoid burning your fingers).

 

Curved Seams

Heat sealing curved seams (such as the tube-to-tube seams in a packraft) presents a challenge because the curves prevent the fabric from lying flat, so they must be draped over a rounded surface, such as an overturned bowl (see the picture at the top of this page and the video below) or a purpose-built form like the one pictured here (click pictures for descriptions):

As you work your way along the seam, keep the finished portion hanging as straight as possible (aligned with the part you are sealing) to prevent biasing the curve toward one side of the seam. If you allow the sealed part of the seam to pile up consistently to one side, the corners of the tube pieces will not line up properly when your reach the end of the seam even though the sides are the same length. Watch this video for a demonstration of how this happens:

 

Note: Some people prefer to tape the seam first before heat sealing it. This takes longer, but allows them to make sure everything is lined up correctly before a permanent seal is made, and may make possible to seal the seam on a flat surface.

 

Check Your Work

Always check your welds after heat sealing by examining both sides and by manipulating the fabric to check for lifting at the edges. Attempt to slide your fingernail between the fabric pieces, and re-seal any spots that aren’t fully welded.

After you have sealed a seam to your satisfaction, you may choose to add a layer of Seam Grip or Aquaseal on the inside as insurance against small leaks. While this may seem like an obviously good idea, avoid relying on glue in areas that should be airtight when properly heat sealed because, while applying glue over an incomplete weld will stop the air leak, you’ll never know if there’s a weak spot in your seam that wasn’t completely welded. For this reason, avoid adding glue to areas that should be airtight when heat sealed properly. Areas that are nearly impossible to fully heat seal to airtightness (and where glue will almost certainly be required to plug small leaks) include spots where multiple pieces of fabric come together at different angles, such as where two tubes sections meet at a packraft’s floor, or where you’ve accidentally welded a fold into a seam.

Tiny leaks can be hard to find, so use the soap bubble technique shown on the Punctures, Leaks & Repairs page.

If you have questions or tips, please leave a comment below.

6 thoughts on “Proper Heat-Sealing Technique

  1. It is a bit difficult to tell from the stop motion in your videos and I couldn’t tell from reading this section so I thought I’d ask here. Is the heat sealing a press-pause-move, press-pause move, type process or more of a continuously pressing and sliding the iron back and forth along a short section of the seam as you move along the seam? Thanks

    1. Hi Quincy, that’s a good question. I never really thought about it, but you’re right – speeded up in the videos it’s hard to tell exactly what I’m doing.

      I usually press and slide back and forth over a short section of the seam (especially on curved seams, where I need to re-position the fabric after sealing a short section). This works well for me because, with most of the colours of fabric that I offer, the fabric darkens slightly as the TPU begins to fuse (it almost looks wet), and sliding back and forth allows me to see exactly when this happens and to what extent, so I don’t overheat it. The darkening is temporary – as the TPU cools it goes back to its original appearance – unless your iron is so hot it melts the nylon!

      If your iron is only slightly too hot, you can slide it faster and for a shorter period of time, and if it’s on the cooler side you can slide it back and forth over a shorter distance to concentrate the heat output into a smaller area.

      In some of the videos, especially when working on curved seams, you’ll see me heat a section with the iron and then set the iron aside and press on it briefly while it cools to below the TPU fusing temperature. This keeps the fabric from moving while the TPU is hot and ensures a good strong bond.

  2. Hello!

    Curious if you have found that un-bonding the fabric is as easy as bonding the fabric? Did some research on this TPU coplymer in general and found that it is unique in that the TPU can be bonded and un-bonded apparently endlessly and still remain as original due to the unique and strong way the crosslinks form, deform on heat, and then reform just as strong, supposedly. Excellent website and community knowledge! this awesome!!!!!

    1. Thanks, Paul! Yep, I’ve found that I can reheat and unseal/reseal the TPU without much difficulty. The only issue is that when you separate the previously sealed pieces the TPU isn’t distributed evenly over the two pieces anymore – not a problem if you just need to reposition it slightly, but it wouldn’t look good and might not remain airtight if you wanted to totally remove something that had been sealed down without covering the spot again with a new piece of fabric. Cheers!

  3. Matt,
    Can you heat weld all/any TPU coated fabrics? Does it just take more time and pressure with the iron if you’re using thicker material from different projects, for instance 600 to 800 d TPU coated material?

    Thanks,
    Joseph

    1. Yep, that’s right. Some fabrics melt at a temperature close to the TPU activation temperature, however, which makes them unsuitable for heat welding. Always test samples first if you’re buying elsewhere. Cheers 🙂

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