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Adding Rocker to a PVC foam core

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faklord
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Adding Rocker to a PVC foam core

Postby faklord » Thu Dec 01, 2022 7:00 pm

A question that periodically crops up is 'how to add rocker to high density PVC foam'. In this case for a Kitefoil board.
There are a number of options:
1) Build up the thickness by bonding layers together, then carve the desired shape from the thick block. - weight penalty, wasteful, labour intensive.
2) Bond several thin layers together on former (rocker table?) - weight penalty, guesswork due to spring back, can't easily modify once it's done.
3) Kerfing (google it if you don't know), fill slots with thickened epoxy, sand back. - labour intensive, needs a lot of cuts to get a smooth curve.
4) Spring core to desired shape and use laminates to hold rocker. - difficult to do with with thicker cores, additional complexity during laminating process.
5) Set rocker using heat.
I have used methods 2,3,4 and now 5.

The Heat method is the subject of this post.
I wanted to use a 25mm core (https://www.easycomposites.co.uk/easyce ... l-pvc-foam)
Experimentation with heat guns on test samples was a bit awkward and gave hit and miss results, invariably ending up with a scorched surface. I don't know how that would affect the mechanical properties?
Some method of applying uniform heat was required & my cooker oven isn't large enough!
I had previously heard that boiling water would do the trick but the heat needs to be applied for some time (~20mins as it turns out) to penetrate into 25mm foam (which is by nature quite a good insulator).

So time to make steam box, in this case powered by a domestic wallpaper stripper.
The steam box was just knocked up from some old pallet wood.
I made it pretty small so I could introduce curvature as required along the length of the blank, rather than trying to do it all in 1 go.
Prior to trialling the steam box, I buried a thermocouple into the middle a sample of core material and stuck it in the freezer to determine how long the temperature took to settle (20 mins for this 25mm thick foam)

It worked perfectly. I found the simplest way to use it was to overhang the blank off the edge of a table and let the weight of the steamer box bend the core when the desired temperature was achieved (and as expected this took 15-20 mins). This was done in 3 steps and it was easy to go back and make changes to increase/decrease the curvature.

Steamer 1.jpg
Steam box inside

Steamer 2.jpg
Steam box outside. Hinged for ease of use. Steam inlet and outlet positioned to try to ensure steam covers all enclosed core surface.

Steamer 3.jpg
Steam box closed. Elastic 'catch'

Steaming 1.jpg
Steam box in action, power by wallpaper stripper. I found helped to stuff bit of paper towel into any leaking gaps between core and box.
Steaming 2.jpg

Rocker.jpg
Rocker applied

alpaia
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Re: Adding Rocker to a PVC foam core

Postby alpaia » Thu Dec 01, 2022 10:20 pm

Thanks really clever, in many ways solves the limitations of the existing techniques
known by the man skilled in the art of bending pvc foam as you mentionned.

As you said no removal of material with a grinder, no waste, no excess weight,
no brute force, no spring back, no house burnt down by a heat gun...

I really like the fact that your box only has to cover a section of the blank,
and you can form the rocker slowy I guess moving it where you want it to bend.

I had done sandwich (plywood xps plywood ) with rocker formed while vac bag glueing.
That was ok but I wanted to use a proper pvc core and did not know how to bend it.

Thanks for sharing that is a brilliant trick.

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Re: Adding Rocker to a PVC foam core

Postby merl » Fri Dec 02, 2022 9:12 am

Brilliant - and very useful data about the length of time needed to heat it through. A rather extreme rocker line though...

I think the oven approach is still a bit easier. Buying easycomposite foam in smaller bits is a bit cheaper, and it almost fits in a regular european oven.

viewtopic.php?p=1147629#p1147629

alpaia
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Re: Adding Rocker to a PVC foam core

Postby alpaia » Fri Dec 09, 2022 1:02 am

The ultimate PVC foam cookbook : baked or steamed ?
Thanks both of you for sharing ideas :D


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