How to finish edges of cuben tarp

[Colin Krusor]

I have made two small cuben tarps before, but I left the edges unfinished, thinking they didn't need any kind of a hem because the material can't unravel.

However, testing of cuben samples by posters to this forum (in particular Chris Lucas) suggests that having folded edges significantly increases the resistance to tearing.

Kevin Beeden concluded, in light of Chris Lucas's results, that, when making cuben articles, it is important to ensure that "all edges are folded and bonded to ensure no micro-tears are exposed to propagate catastrophic larger tears."

Roger Caffin concurred, noting that this sensitivity to micro-tears is one of the properties of cuben that differ from silnylon.

So, it seems that my tarp needs hems. Do commercially available cuben tarps have hemmed edges? Has anyone folded over the edges of their MYOG cuben tarp to finish them? Any ideas about how to do this along a slightly curved (cat) edge (will I have to nick the folded-over edge to make a series of small straight edges)?

[Steven Evans]

Colin,
All my tarps/shelters have the cuben hemmed and sewn to hide/clean up the edges. I haven't bonded any of my edges. My MLD Cuben Duomid, MLD Cuben Poncho Tarp, and Refuge X all have hemmed and sewn edges. I have no idea if a bonded edge is stronger.

I am also curious how a catenery edge is done. Anyone have one that they can take a look at? I also don't understand how a cat ridgeline is done without putting the material in peel.

[Dan @ Durston Gear]

For my cuben tent fly I folded the edges over 1" and bonded that with Hysol. On hind sight, a 1" overlap was overkill and 1/4 – 1/2" would be plenty and it would be lighter. The bonded isn't that stressed in this application, so if you don't want to buy an expensive adhesive just pick up a tube of SeamGrip at any outdoors shop and use that. It works well enough for this application, it's cheap and there's no mixing required. Drying time is nice….you've got a bit of time to adjust it before it sets. If you are going to do a lot of bonding in more crucial areas then you may want to look into a higher end adhesive.

[Chris Lucas]

Sorry I've dropped off the face of the earth I've just been insanely busy at work lately. When I'm next in Miami I'll toss some cuben and cuben + 3M tape in the tensometer to see if bonded edges tear at a higher load than do non-bonded edges.

[Kevin Beeden]

> Kevin Beeden concluded, in light of Chris Lucas's results, that, when making cuben articles, it is important to ensure that "all edges are folded and bonded to ensure no micro-tears are exposed to propagate catastrophic larger tears."

I should point out that I've never worked with Cuben, but I am reasonably familiar with a range of materials, including films. It was this experience, and the result of the practical testing that made me make the suggestion about hemming the edges to prevent propagation of microtears.

I've not looked at a cut edge of Cuben, so my microtears comment is, again, based on previous experience with thin films.

Having said all that, I'm still fairly confident that this hypothesis is the explanation for the apparently premature failures that Chris originally found.

My gut feeling is that a small hem should be adequate (say 1cm). A larger hem may even cause trouble on a curved edge, as the cut edge would have to stretch, or be cut to stop the whole thing puckering. And putting deliberate cuts in would seem to go against the whole idea of why you're hemming…

[Colin Krusor]

Kevin, I think the remark you posted in Chris Lucas's tensometer thread, which I quoted here, seemed apt and I didn't mean to attach any implication of a claim to authority by you. I just thought it neatly summed up a reasonable interpretation of Chris's findings, and one which probably occurred to many other readers of that thread.

Nicking the raw edge of a hemmed catenary curve would defeat the purpose of the hem unless the nicks were reenforced. I thought about making a catenary edge as a series of short straight edges, with a bit of tape over each nick to prevent tearing. This seems a bit sloppy, though.

I posted a question to Lawson about the catenary edges on his cuben tarps but got no response. From the photos on the Mountainfitter site it looks like the edges of his tarps are unhemmed, but I can't be sure.

It occurred to me that a slightly flexible tape (maybe urethane tape) could be folded over a raw curved cuben edge. I wonder if this would offer good resistance to tear propagation without too much weight penalty…

Also, I wonder if cuben eges made by cutting the material with a hot knife (so some melting occurs at the edge) are as prone to tear propagation as edges made by cutting with a utility blade or rotary cutter…

[Kevin Beeden]

> I didn't mean to attach any implication of a claim to authority by you.

No worries. I just didn't want people getting the idea I was some sort of expert on Cuben, and taking my idle speculation as a pronouncement on 'This Is The Way You Must Finish Cuben Edges'…

[Kevin Beeden]

> Also, I wonder if cuben eges made by cutting the material with a hot knife

I was wondering that last night, too. It struck me that the structure of Cuben, with outer reinforcing fibres around a film core, might encourage microtears when cutting with a knife, as the blade 'judders' over the fibres during the cut. Okay, so it may not judder very much, but there must be an interesting effect on forces acting on the blade tip as you cut the fabric, which could result in lots of micro-impacts on the film.

Scissors might not be so bad, as the cutting action is different (perhaps more controlled) than a knife.

A hot knife ought to cut cleanly, but it might also leave little lumps of melted plastic that might act as stress concentrators too, leading to premature tearing…

Maybe I ought to get some Cuben and experiment…

[Colin Krusor]

"…outer reinforcing fibres around a film core…"

The plies of film are on the outside, but I think you are probably right that a utility knife might produce a microscopically jagged edge, perhaps due to bumping into the fibers.

I had in mind that I might like to run a bit of cordage (maybe some Spectra/Dyneema fishing line) along the edges of the tarp. A hem of some kind, or folded-over tape, would provide a convenient pocket for this purpose. I'm interested in knowing how hot vs. cold cutting affects resistance to tearing, but a hot-cut raw edge might not be ideal for my design anyway.

I plan to make a 9' x 9' tarp with a flat ridgeline and (hopefully) catenary edges out of CT0.6K.08 (which is 0.478 oz/yard on my scale). I think material of this weight might prove to be durable for me if I can adequately protect the edges.

[Colin Krusor]

I found a urethane tape used to protect the leading edges of helicopter rotors that I may try on the edges of my tarp. It's clear, tough but slightly stretchy, and allegedly has a very strong acrylic adhesive.

I plan to hot-cut catenary edges and fold the tape over the edges, trapping a continuous length of dyneema fishing line under the tape along the edges. The dyneema line would be sewn into the grosgrain at each tie-out point. Hopefully this would reduce the chance of a tear beginning at the edge if the cuben is whipped by a violent gust of wind.

Does anyone have any experience attempting something similar? Any tips or ideas?

[Dan @ Durston Gear]

That sounds like plenty strong to me. The spectra fishing line might be overkill, but it's also pretty darn light stuff so if it's not too much hassle then go for it.

Is the whole idea with tape and fishing line just to avoid having to fold the edge of the cuben over and thus maybe get wrinkles? From my experiences with cuben, you'll be able to just fold it over and create a caternay (sp?) curve shape with no troubles. Some of the bottom edges of my cuben fly (see avatar) came out a bit curved unintentionally and it was clear when I was bonding the bottom hem that I could easily make it curved and still have a nice hem. Pressing it while it dries would eliminate any wrinkles or you could do as I did and just examine the cuben every 10 min or so while it's drying and ensure any wrinkles are pressed down until it's dry enough that they are going to stick.

The wrinkles just kinda go away as you press the fabric together. Alternatively you could just create your own tape using strips of cuben with adhesive on them. That seems lighter than using anything else.

Have you considered using CT1K.08 over CT0.6K.08? The former has a lot more spectra and it's hardly heavier. It's 0.51oz/yd instead of 0.465oz/yd. I've got another cuben project idea (basically a single wall, 1 man tent) that I'm leaning towards CT1K.08 for. If you do end up buying this fabric I'd be happy to go in for a joint order.

[Colin Krusor]

I'm glad to hear that you found that the cuben can simply be folded over smoothly along a curved edge without too much wrinkling, Dan. That seems to me like a much better option than my tape idea. I just assumed that the cuben wouldn't accept that kind of fold.

[Dan @ Durston Gear]

That's sweet you got a bit of a discount since cuben is such darn pricey stuff. The black should be cool too. Hopefully you post pics when your tarp is done.

Regarding the edges, I haven't tried radical curves, but it can definately accept a mild curve that I imagine would be sufficient for the edges. I think this is how Lawson (MountainFitter) does it. If you have a look at the bottom edges of his 9 x 9 tarp it looks like he has done this. I would essentially try to replicate this:

[Scott Van Doeselaar]

I left my cuben tent edges unfinished and I am having trouble with tearing at the edges of my tieouts. I would definitely fold over my edges on all future projects and spread the load from the tieouts with a double layer that also folds over the edge of the material.

[Colin Krusor]

Thanks for the note about your first-hand experience, Scott. Do you plan to do anything to modify your design now to prevent further tearing? Maybe adding tape to those edges or folding over and bonding a ribbon of cuben would help.

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