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 Post subject: The benefit of PU over epoxy for rails?
PostPosted: Tue Jul 23, 2013 2:58 am 
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Done my first PU rails. Not easy job.

To be fair I would go any time with epoxy over PU. But before I do that, what do you guys think?

Me thinks about epoxy:

- more brittle.
- easier to work with.
- not so healthy.
- sanding well.
- bonding 100%.

Any takers? Thx.


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 Post subject: Re: The benefit of PU over epoxy for rails?
PostPosted: Tue Jul 23, 2013 3:28 am 
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Into my first urethane railed board and I am more thinking of the positives, although I put off trying it for a few years, being somewhat happy with epoxy rails.

Good:
-flexible to get a good rocker more easily
-cure in 15 minutes (although could get a slow one if desired)
-theoretically should absorb some shock and resist chipping

Bad:
-by no means more "healthy" to work with than epoxy
-3 minute pot life is a minus at times
-limited shelf life compared to epoxy, so no economizing for small volume/hobby builder

So far I think I like it, but if you have an abundance of epoxy, it works...


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 Post subject: Re: The benefit of PU over epoxy for rails?
PostPosted: Tue Jul 23, 2013 4:11 am 
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Thanks BWD.

I think the problem is to find which PU "Big boys" are using, like Cab, LF, etc...

For example, this PU (PMC 790) I've got is somewhat soft but you can't dig your nail in. However, on production boards the PU in not soft by any means. And impossible to cut with a knife. This one you can.

When I look at the Cab production video (posted on forum I follow), I see they pour PU with hand and the rail is not sealed. They also use CNC to get rid of excess:

http://boardbuilders-forum.1077691.n5.nabble.com/Alternative-to-ABS-rails-A-weed-trimmer-line-td685.html

With so many PU products (and epoxies), might be pricey to test more than a few. If only we can find something to stick with...

Cheers

D.


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 Post subject: Re: The benefit of PU over epoxy for rails?
PostPosted: Tue Jul 23, 2013 4:48 am 
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Was wondering this myself,

I took a look at the material sheets for the PUs at my composites store and pretty much all of them had lower compressive strength, tensile strength and elongation to failure (flex till breakage)than a basic system 3 general purpose epoxy

Curious what brands you guys are using successfully.


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 Post subject: Re: The benefit of PU over epoxy for rails?
PostPosted: Tue Jul 23, 2013 5:19 am 
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Epoxy changes color over time. More or less they all do.
PU shouldn't (they say).


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 Post subject: Re: The benefit of PU over epoxy for rails?
PostPosted: Wed Jul 24, 2013 2:19 am 
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Relevant properties

Epoxy (system 3 general purpose)
Tensile Strength, psi 7,500
Tensile Elongation 11%
Tensile Modulus, psi 325,000
viscosity 300-1100 cps
hardness not listed
working time 15-60min

PMC 790
Tensile Strength, psi >2000 psi
Tensile Elongation 550%
Tensile Modulus, psi 325,000
viscosity 300-1100 cps
hardness 90A
working time 20min

I like the properties of the PMC I'd give this a go on my rails.
why?
High elongation at break means it can stetch alot before breaking and is unlikely to chip or snap when bent or impacted
Low modulus means it is not stiff so it will not be carrying any load when compared to the stiff glass/epoxy of the board.

The lower tensile strength is not important as it will not be a significant load bearing part of the structure due to the low modulus.

from my recollection 90A hardness was relatively hard when it came to skateboard wheels but if you're finding it soft you want a PU that has a higher shore hardness number.

The Modulus (stiffness) will never be in the neighborhood of fiberglass but if it gets too high it may end up taking more load and the tensile strength may become important.

the viscosity is pretty thick and may be a bitch to pour/mix without bubbles. How'd this go?

Vacuum degassing will likely not be very helpful at that viscosity although the 20min pot life gives you a chance to try.

Don't have experience with PU but hope this "paper analysis" was useful.


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 Post subject: Re: The benefit of PU over epoxy for rails?
PostPosted: Wed Jul 24, 2013 6:38 am 
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I did not find it too thick to work with.

It actually poured nicely and it can be spread, no probs with that. 20 min pot life is good enough.
Well, air bubbles is a problem in everything, and your suggestion about sealing rails before is a good one.

"From one of the world authorities on the topic.

Moisture. 0.2% moisture content in the materials will cause the polyurethane to foam. By foam I mean like house insulation foam. It takes a great deal of care and heat to drive the moisture from the materials. In the case of the wood core, it is next to impossible to do;"

I would say this is comparable with not vacuumed epoxy (and not flamed as well):

Image

However, I think that because PU is quite soft one won't be able to hit the ramp without some damage.
And to find something with a higher shore might be difficult.

I do think, and I will try this: it might be possible to mix PU resins with epoxy and achieve something.

http://boardbuilders-forum.1077691.n5.nabble.com/Shinai-131x41-bamboo-FG-carbon-kevlar-td897i20.html


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 Post subject: Well, this is no good...
PostPosted: Wed Aug 07, 2013 5:17 am 
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Learning from experience...



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 Post subject: Re: The benefit of PU over epoxy for rails?
PostPosted: Wed Aug 07, 2013 2:27 pm 
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This combination may work for your rails to make them less brittle ---

Go here for a glance at the full list :

http://www.jamestowndistributors.com/us ... &page=GRID

Mix the epoxy with some WEST System 404 High Density Filler.

Then add a bit of ---Milled Glass Filler--- ; this keeps the matrix stable.

Then add 5 to 10% WEST SYSTEM 423 Graphite Powder
(i make my own by sweeping up the black stuff any time i sand on anything
made with carbon & epoxy)

Play around with your own proportions by mixing several combinations
THEN
smash each with a hammer.

This is what we used to bond the spars on my Rigid-wing HG to
the carbon skins ; it Never broke and the engineer said it was good for
10G's.

Bille

Ps : You now have a Really NASTY item to sand on ; i "Always" use
wet/dry sandpaper or steel files AND water so the dust can't go airborne !!!!!
A dust mask simply is NOT good enough (.)
PLEASE don't go Macho without the water ,
your lungs will HATE you


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 Post subject: Re: The benefit of PU over epoxy for rails?
PostPosted: Wed Aug 07, 2013 5:49 pm 
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Joined: Fri May 09, 2003 6:44 pm
Posts: 16
Location: haifa
I have been using this product :
http://www.smooth-on.com/index.php?cPath=5_1120_1156
with great success on my boards ,it it easy to work with and dries to a real hard plasick like,
no problems with epoxy adhesion , just sanding with #60 , I could only get the crystal clear 202 in my country but it is long enough work time to do a full rail.

Image

Image

Image


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