Let's talk Kerfs (curving your foam)

Canvas covered foamies (Thrifty Alternatives...)

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Re: Let's talk Kerfs (curving your foam)

Postby GPW » Thu Oct 09, 2014 12:46 pm

Pike .... they do !!! :thumbsup: 8)
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Re: Let's talk Kerfs (curving your foam)

Postby jgraham698 » Thu Nov 20, 2014 11:25 am

I posted a short video on you tube titled " how to bend styrofoam with a compression strap" that people
might find interesting. The foam is 1" esp from Lowe's. My first test piece is 1 1/2 thick by 1 and the last
piece is part of a food tray. For straps I used aluminum flashing, luan , and vinyl sheet. Since making the
video (my teenage children filmed and uploaded it for me), I've bent 2" xps for the tear drop I'm building.
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Re: Let's talk Kerfs (curving your foam)

Postby OP827 » Wed Jun 10, 2015 2:23 pm

Here is also a discussion on doing foam kerfs and shapes to produce spherical shape - http://www.tnttt.com/viewtopic.php?f=50&t=62852&p=1132701#p1132701
My foldable foam trailer build: http://www.tnttt.com/viewtopic.php?f=50&t=61344
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Re: Let's talk Kerfs (curving your foam)

Postby Fred Trout » Wed Jun 10, 2015 11:48 pm

jgraham698 wrote:I posted a short video on you tube titled " how to bend styrofoam with a compression strap" that people
might find interesting. The foam is 1" esp from Lowe's. My first test piece is 1 1/2 thick by 1 and the last
piece is part of a food tray. For straps I used aluminum flashing, luan , and vinyl sheet. Since making the
video (my teenage children filmed and uploaded it for me), I've bent 2" xps for the tear drop I'm building.


A link would be nice. A quick search revealed lots of nothing :?
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Re: Let's talk Kerfs (curving your foam)

Postby jgraham698 » Sun Jun 21, 2015 2:51 pm

I made a new and longer (more boring) video to replace the first that you can find at https://m.YouTube.com/watch?v=wUb8patN6Mw
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Re: Let's talk Kerfs (curving your foam)

Postby Fred Trout » Sun Jun 21, 2015 4:11 pm

Thanks for posting the youtube link. Very interesting !

Have you tried bending a larger sheet using the compression strap to the sort of smaller angles used in foamie construction ?
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Re: Let's talk Kerfs (curving your foam)

Postby jgraham698 » Sun Jun 21, 2015 7:48 pm

I have my roof made for my trailer. It's 2" xps that I bent over a form using 1/4 Luan for a compression strap. Just as an experiment I have bent 2" eps and xps around a 6" drain pipe. I used aluminum flashing for that strap. Ratcheting cargo straps work well to do the bending and for clamping. For a radius more than 2 feet you would probably need a strap that can bend more than luan.
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Re: Let's talk Kerfs (curving your foam)

Postby OP827 » Sun Jun 21, 2015 11:56 pm

Thanks for posting, it is interesting method. How long ago did you build your trailer and was it in various temperature conditions? I think that the XPS being closed foam the little bubbles of gas inside that XPS foam are remaining compressed and I do not know if they and the XPS foam will remain stable shape in changing climate (temp and air pressure) conditions :thinking: or you feel that the new form shape is quite stable as the closed cells have conformed to the new shape? It is definitely a new approach :thumbsup: thanks!
My foldable foam trailer build: http://www.tnttt.com/viewtopic.php?f=50&t=61344
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Re: Let's talk Kerfs (curving your foam)

Postby jgraham698 » Mon Jun 22, 2015 4:17 pm

I started my build about 10 months ago. I'm using a 5' wide trailer, so I bent two halves that were 32" wide and 12' long (8' + 4' pieces epoxied together). Each half was bent over the same form with plywood laid on the form first, titebond glue spread evenly over the entire surface and the 2" xps foam nested in a plywood strap made of two pieces joined together with a scarf joint and end blocks 2 x 4 lumber with 1/2 plywood to make a full 2". The front end was bolted to the form and the other end was pulled down to the back end with cargo straps. Some clamping was needed to get good contact between foam and plywood. I need to clear up something about the plywood I'm using. The product is made by Patriot Timber Products called RevolutionPly and is 5 mm (about 3/16) thick. I bought it at a Lowe's store. Anyways, I left each piece clamped up for a week to let glue dry and epoxied them together and covered the outer surface with polyester fleece material and epoxy. That was as far as I got before running out of good weather. After one winter, spring and the beginning of this summer my roof, that's been sitting on the trailer with no other support than it's ends, hasn't changed except for some bubbling of the fiberglass. I think air escapes from the foam after it's been bent. I don't know how to prove that but it explains better my experiences so far than assuming that "closed cell" means airtight.
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Re: Let's talk Kerfs (curving your foam)

Postby OP827 » Mon Jun 22, 2015 7:43 pm

jgraham698 wrote:I started my build about 10 months ago. I'm using a 5' wide trailer, so I bent two halves that were 32" wide and 12' long (8' + 4' pieces epoxied together). Each half was bent over the same form with plywood laid on the form first, titebond glue spread evenly over the entire surface and the 2" xps foam nested in a plywood strap made of two pieces joined together with a scarf joint and end blocks 2 x 4 lumber with 1/2 plywood to make a full 2". The front end was bolted to the form and the other end was pulled down to the back end with cargo straps. Some clamping was needed to get good contact between foam and plywood. I need to clear up something about the plywood I'm using. The product is made by Patriot Timber Products called RevolutionPly and is 5 mm (about 3/16) thick. I bought it at a Lowe's store. Anyways, I left each piece clamped up for a week to let glue dry and epoxied them together and covered the outer surface with polyester fleece material and epoxy. That was as far as I got before running out of good weather. After one winter, spring and the beginning of this summer my roof, that's been sitting on the trailer with no other support than it's ends, hasn't changed except for some bubbling of the fiberglass. I think air escapes from the foam after it's been bent. I don't know how to prove that but it explains better my experiences so far than assuming that "closed cell" means airtight.


If I got it right from the above description, you have a compression bent foam sandwich of thin(how thick?) plywood interior, 2" foam and then polyester fleece and epoxy lamination extrior, right? And you have noticed that the exterior fleece plus epoxy lamination is bubling? If it is the exterior bubling I do not think the bubling is related to interior layer closed cells degassing and decompressing to form the outer surface bubles. It could be related to temperature expansion and softening affects to the exterior when the foam is heated by sun, it is quite common to have delamination issues, many homebuilders reported that. I think if some penetrations are done prior to lamination of outside foam surface and then finished exterior is painted (off-)white, the skin delamination should be mitigated. I personally have embeded some plywood strips every 12 inches and also did many small hole penetrations before I do lamination, but I did not start my panels glassing to report on any of my experience to be different or better.

I feel that XPS rigid foam surface by itself is way too weak to be able to resist the stress and still be attached to foam when under expansions and contractions of the epoxy laminate and foam weakening under the sun heat.

My question is:
Did you make penetrations in the foam or other type of measures to provide epoxy to have a good grip to the foam?
My foldable foam trailer build: http://www.tnttt.com/viewtopic.php?f=50&t=61344
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Re: Let's talk Kerfs (curving your foam)

Postby GPW » Tue Jun 23, 2015 7:28 am

"I feel that XPS rigid foam surface by itself is way too weak to be able to resist the stress and still be attached to foam when under expansions and contractions of the epoxy laminate and foam weakening under the sun heat. “ ....
Huh , what ??? :thinking:
JMHO, if you’re bending foam , HEAT is the way to do it !!!
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Re: Let's talk Kerfs (curving your foam)

Postby OP827 » Tue Jun 23, 2015 8:36 am

GPW wrote:"I feel that XPS rigid foam surface by itself is way too weak to be able to resist the stress and still be attached to foam when under expansions and contractions of the epoxy laminate and foam weakening under the sun heat. “ ....
Huh , what ??? :thinking:
JMHO, if you’re bending foam , HEAT is the way to do it !!!


Sorry GWP, I must have been not clear enough, the discussion here was not about heat bending the foam. What I meant was the weak strength of foam surface bonding when it is just a plain surface glued to canvas or fiberglass. When the lamination done without foam penetrations or other reinforcements to allow glue to grab to the foam more than just a surface, I just saw more than two builders on different forums which have reported exterior skin delamination issues with either canvas or fiberglass, and almost always when their trailer was painted darker colors. That wallpaper pricking roller or similar tool seems to be more of a requirement than an option. This is what I was talking about.
My foldable foam trailer build: http://www.tnttt.com/viewtopic.php?f=50&t=61344
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Re: Let's talk Kerfs (curving your foam)

Postby GPW » Tue Jun 23, 2015 4:50 pm

Oh , OK !!! Yes ,you’re Right !!! ... darker colors in summer sun could be problematic if the covering wasn’t properly applied . On the other hand , our Foamie #1 sat out in 4 summers worth of Sun and didn’t develop any de-laminations or bubbles, and it had some very dark spots in the camo ... If anything , it helped heal the foam itself after that big Hail storm we had . We had only rough sanded the foam finish before covering. As we‘ve learned, a good bit of the adhesion process is not only preparing the foam surface , but also thoroughly and completely wetting the canvas skin with glue . Fiberglass, I dunno ... we really haven’t tried fiberglass over foam other than gluing on some FG tape with T2.
I think , just maybe the Fiberglass doesn’t expand at the same rate as the foam and may (or may not ) lead to de-lamination ..... much as we’ve seen with folks trying to glue Aluminum skin to plywood .... eventually the difference tears the Al away from the ply and floats ... On a foamie , that would be a Bubble ... which BTW can be easily fixed . Not so easy with Aluminum , or fiberglass .
There are limitations and compromises to every building method ... Once we fully establish the full parameters of Foam camper building , hopefully the process will be Clear to all ...

Goodness knows , we’ve had our share of naysayers over the years , who absolutely believed a foamie trailer would blow apart at highway speeds. All that turned out to be wrong , with tens of thousands of miles now on Foamie trailers ,and almost FIVE years now in development ... and aside from the one that got blown over, the only mention is a few bubbles ... A Good record one might think, eh !!! :thumbsup: 8) :beer:
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Re: Let's talk Kerfs (curving your foam)

Postby Toby » Sat Jan 02, 2016 12:41 am

I'm going to be building my teardrop in pieces inside my heated basement this winter. When the weather warms up, put it together like a puzzle outside. Unless I've missed a few posts, it looks like kerfing is still the best way outside of heat to get those curves. If I'm piecemealing this build, I need to get those curves prior to attaching them to the TD. Looking over the calculations seemed simple enough but how do I get the inside and outside radiuses to plug in the formula? If it helps, I'm using the superleggera model. My plans have the radius for all the curves, but I'm not sure how to apply only one given radius. Does it apply to the inside or outside? I'll be using 2" foam.
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Re: Let's talk Kerfs (curving your foam)

Postby OP827 » Sat Jan 02, 2016 12:54 am

Toby wrote:I'm going to be building my teardrop in pieces inside my heated basement this winter. When the weather warms up, put it together like a puzzle outside. Unless I've missed a few posts, it looks like kerfing is still the best way outside of heat to get those curves. If I'm piecemealing this build, I need to get those curves prior to attaching them to the TD. Looking over the calculations seemed simple enough but how do I get the inside and outside radiuses to plug in the formula? If it helps, I'm using the superleggera model. My plans have the radius for all the curves, but I'm not sure how to apply only one given radius. Does it apply to the inside or outside? I'll be using 2" foam.


The internal radius could be determined by subtracting the foam thickness from external radius shown on the drawing.
My foldable foam trailer build: http://www.tnttt.com/viewtopic.php?f=50&t=61344
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