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foam build - rocker by heating foam

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merl
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foam build - rocker by heating foam

Postby merl » Wed Aug 11, 2021 12:41 pm

[changed thread name as it ended up being more about thermoforming ] I have a nice chunk of 25mm x 500 x 1080 divinycell style foam (75kg/m3). Last time I built a pocket board from a similar bit I kerfed it to get the rocker. I am pretty sure it is too thick to bend, at least without heat, and that idea seems pretty scary to me. Then I remember than some folks build by cutting out cross-sections with the rocker pre-cut, and gluing them together. So for a 45 wide board I'd need about 18 curved bits of foam from my sheet. But I realise that this is something that needs to be drawn up and maybe printed out in order to get it right. What software would one need to do this? Or maybe there is a simpler way...
Last edited by merl on Wed Aug 25, 2021 1:22 pm, edited 1 time in total.

tmcfarla
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Re: foam build with lengthwise slices - how to design?

Postby tmcfarla » Wed Aug 11, 2021 2:07 pm

If it is divinycell, it is easily thermoformed in an oven. I built a mini rocker template and screwed the front of the board down to it, then placed the nose of the board in an oven at 300 degrees F for about 30 seconds at a time until it took shape. The rocker template was just some pieces of wood stacked under the nose to get desired rocker. Whole thing took maybe 20 minutes and was surprisingly easy.

If you have a bit of spare, do a small test piece to work out time and temperature

merl
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Re: foam build with lengthwise slices - how to design?

Postby merl » Wed Aug 11, 2021 2:25 pm

Several people say it is easy but the info online is a bit vague. What does the "oven" look like and how does it work? Or do you literally mean a kitchen oven...!?? You do make it sound easy...

ps. I found this: http://www.fiberglasssupply.com/pdf/cor ... rmDiab.pdf which suggests a 25mm sheet would need 100C (212F) for 10 minutes (if I am reading the table correctly).

BWD
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Re: foam build with lengthwise slices - how to design?

Postby BWD » Wed Aug 11, 2021 3:17 pm

You can also thermoform foam with hot water. Corecell A for example bends from ~75-85 C.
You can do this with divynicel too, but only the lighter grades h45 and h60 will bend <100 C.
With corecell you can even thermoform it again at a slightly higher temperature if the result is not perfect.
I formed a board with ~12 cm rocker on a rocker table with hot water from 20mm corecell.
I cut the sides of the table from plywood and used the off cut to make a male form.
Boil lots of water.
Put a towel on rocker table, pour a few liters of hot water on it.
Place foam on top of hot wet rocker table.
Put another towel on top of foam.
Pour hot water over the top towel.
Put male form over all and press down.
Foam will bend easily.
Set some weight on top to hold the pile together.
After it cools enough to touch the rocker will be set.
This is easy, cheap and precise enough for good results.
Better with vacuum forming in an oven perhaps but I prefer building boards not ovens. I have also used a “poor man’s vacuum bag” by using a shop vac to suck polyethylene trash bag film down to hold foam on the mold. This works well also. Making a matching mold set is easier than you think though, with the towels in the stack the match need not be totally perfect.
Remember to wear gloves that are insulating and waterproof if you try this!
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thewindego
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Re: foam build with lengthwise slices - how to design?

Postby thewindego » Wed Aug 11, 2021 3:57 pm

I use corecell M80 20mm and it bends on a rocker table without heat just pressure.. I would think 25mm would as well but maybe not. Depends on how steep you do the rocker.
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tmcfarla
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Re: foam build with lengthwise slices - how to design?

Postby tmcfarla » Wed Aug 11, 2021 5:35 pm

I meant a kitchen oven. Just leave the oven door cracked, and probably open up some windows for ventilation.
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Re: foam build with lengthwise slices - how to design?

Postby fluidity » Thu Aug 12, 2021 9:08 am

You can draw up the forms in Inkscape and split the long pieces to fit on A0 document size, which is huge and good for printing commercially.
if you want to design in 3D first then take projections, slices through the model and save those to DXF then import to inkscape.
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merl
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Re: foam build with lengthwise slices - how to design?

Postby merl » Thu Aug 12, 2021 11:13 am

tmcfarla wrote:
Wed Aug 11, 2021 5:35 pm
I meant a kitchen oven. Just leave the oven door cracked, and probably open up some windows for ventilation.
Cool. I am not clear how you bent it. Did is just sag into the rocker form, or did you press it somehow. I am worried that the foam will damage easily when it is hot...

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Re: foam build with lengthwise slices - how to design?

Postby tmcfarla » Thu Aug 12, 2021 2:47 pm

merl wrote:
Thu Aug 12, 2021 11:13 am
tmcfarla wrote:
Wed Aug 11, 2021 5:35 pm
I meant a kitchen oven. Just leave the oven door cracked, and probably open up some windows for ventilation.
Cool. I am not clear how you bent it. Did is just sag into the rocker form, or did you press it somehow. I am worried that the foam will damage easily when it is hot...
I screwed the foam to the rocker form. Then held it in the oven until the foam had taken that rocker. In my case, the rocker was gentle enough that the foam could take that bend without breaking. For extreme rocker, maybe do this in stages. It left a few holes in the board where the screws where, but that is easy to fill afterwards.

merl
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Re: foam build with lengthwise slices - how to design?

Postby merl » Thu Aug 12, 2021 3:51 pm

tmcfarla wrote:
Thu Aug 12, 2021 2:47 pm
merl wrote:
Thu Aug 12, 2021 11:13 am
tmcfarla wrote:
Wed Aug 11, 2021 5:35 pm
I meant a kitchen oven. Just leave the oven door cracked, and probably open up some windows for ventilation.
Cool. I am not clear how you bent it. Did is just sag into the rocker form, or did you press it somehow. I am worried that the foam will damage easily when it is hot...
I screwed the foam to the rocker form. Then held it in the oven until the foam had taken that rocker. In my case, the rocker was gentle enough that the foam could take that bend without breaking. For extreme rocker, maybe do this in stages. It left a few holes in the board where the screws where, but that is easy to fill afterwards.
OK - I have a plan forming based on this idea. The thing I didn't mention was that my foam is in two pieces that need to be joined. So I can do this with the front half only before I joint them. That said, if you are using a US oven then it is much much bigger than a European oven...


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