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 Post subject: Glass over urethane rails
PostPosted: Tue Jun 12, 2012 6:56 am 
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Joined: Mon Nov 28, 2005 11:44 pm
Posts: 168
Location: Great Falls, Montana, USA
I'm going to try urethane rails on my next board. I decided on Polytek 15-6. I did a test pour and it still seems a little hard, so I may add some silica. I've beat it up with hammers and center punches, and it's not brittle, but it feels very stiff and I'd like to add a bit of flexibility so it doesn't crack under repeated stress. Other than that, it's a very nice material to work with.

My question is... Do I need to do anything to the urethane rails to get epoxy and glass to stick? I plan to at least give them a good 80 grit sanding. Do I need to flame them like other plastics?

I also ordered one of the 220v adjustable vacuum switched rated for 21 amps from omega.com, and a Robinair vacuum pump. Got tired of borrowing air compressors for the vacuum generator. I will post results when they arrive.

Thanks,
Chris


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 Post subject: Re: Glass over urethane rails
PostPosted: Wed Jun 13, 2012 10:45 am 
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Joined: Thu Nov 02, 2006 5:12 pm
Posts: 76
I always scrub mine with 40 grit to make I remove all the PU curing residue.


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 Post subject: Re: Glass over urethane rails
PostPosted: Wed Jun 13, 2012 11:59 am 
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Joined: Mon Sep 19, 2005 12:49 am
Posts: 1594
Location: scott klandl burlington, VT
40 grit, x2 no delams yet, way better than heat treating ABS. no wonder snow boards ar switcdhing


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 Post subject: Re: Glass over urethane rails
PostPosted: Thu Jun 14, 2012 4:33 pm 
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Joined: Mon Oct 08, 2007 4:39 am
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I found that it can take quite a bit of flex without any problems. Also don't mix it on a day that is 98 deg.


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 Post subject: Re: Glass over urethane rails
PostPosted: Thu Jun 14, 2012 5:08 pm 
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Joined: Mon Apr 23, 2007 1:15 pm
Posts: 44
what kind of urethane is the best for rails, any brands for suggestions?


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 Post subject: Re: Glass over urethane rails
PostPosted: Thu Jun 14, 2012 6:29 pm 
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Joined: Thu Nov 02, 2006 5:12 pm
Posts: 76
I found with the stuff I use and which will remain nameless, sorry guys.

As the PU cures it leaves a slightly damp residue, its really important to remove that and the 40 grit then provides a mechanical key for the epoxy, glass to bond to.

Done 10 + boards with this PU and its amazin, not one delam or cracks.

I use a fast curing PU (60 seconds), it has a shore hardness of 90.
Its really hard to the touch but flexes more than enough for any board.

I tested the PU.

This stuff will shatter if hit hard with a hammer, it can be lethal.
However once encased in glass, it simply scuffs and does not seem to crack, chip or shatter despite the hammer test results ;-)

PU Noobs - Tip time ;-)

So you have your core upside down on a perfectly flat table.
The core has had a 4 or 6mm deep x 12,5mm wide Rabbit as the guys in the US call it or channel to the rest of the world ready to take the PU.

Tape the core to the table.

Mask around the inside of the channel leaving 2mm gap to the inner channel edge.
Pour the PU and remember to allow a little extra for shrinkage as it goes off.

Once you see it starting to harden, to the point where your finger nail wont leave a mark, grab a Stanley knife blade, hold it sharp edge onto the PU at an angle of almost 90 degrees.

Now use it as a hand scraper and shave off the excess PU until level to the core surface.
The 2mm gap will give a little extra bonding of PU to Core for the shaping process later.

Once the PU is fully cured, cut near to the outer edge of the poured rail and remove the excess corecell.
Sand back to the PU rail edge and make as smooth as a babies bottom.

Using some "bright" but thin masking tape, electrical tape or something like car pin strip tape, put this around the entire parameter of the finished "visible" side of the rail, then Lam up the top and bottom on your rocker table.

The idea of the tape is that it helps you when cutting the lam flash off and in the final sanding down as you know when you hit the rail as you hit that tape first indicating STOP, it also keeps the epoxy off the rail edge.

My finished rail edged are profiled later on to remove the sharp edges which avoids chipping even more.

I PU all the fixtures, from foot strap inserts to fin blocks.

I have made my fin block by drilling 12mm holes through the finished and shaped core, one hole for each fin screw and locator.

Using tape, block the holes from the shaped core side of the core.
Turn the core over and level the tips flat with packing.
Pour the PU and do the knife blade trick to level.

I have found Blocks like this allow the tips to flex better than one long block and the tape trick means no additional sanding.

Hope this helps.
Regards XS


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 Post subject: Re: Glass over urethane rails
PostPosted: Sat Jul 07, 2012 5:43 am 
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Joined: Mon Nov 28, 2005 11:44 pm
Posts: 168
Location: Great Falls, Montana, USA
criznach wrote:
I also ordered one of the 220v adjustable vacuum switched rated for 21 amps from omega.com, and a Robinair vacuum pump. Got tired of borrowing air compressors for the vacuum generator. I will post results when they arrive.


Wanted to follow up on this...

The Robinair pump works great. I got the 1.5cfm 2 stage. It pulls 25hg+ easily. I'm using a 7 gal air tank as a vacuum reservoir but it only takes a few minutes with a good seal. The bottle of oil that came with the pump was enough to fill it and not much more. Plan on getting another bottle of oil. You want a long enough hose to run this thing outside. Or an exhaust hose.

The omega.com adjustable vacuum switch would probably work great but I ordered the wrong one. I stupidly assumed that inHG and inWC were the same. Not even close... The switch I got was very sensitive and shut off almost immediately. I did play with the switch I received, and wondered what the range is between turn-off and turn-on. I ended up using the 21 amp rated micro switch from inside the omega unit and building my own adjustable switch using a hypodermic type "flavor injector" from walmart. Using rubber bands, which I'll eventually replace with a spring, I can adjust the turn-on and turn-off points (through trial and error mostly). For it's trial run it turned off at around 25hg and back on at around 10hg. I think i'll increase the turn-on limit to 15-20 next time by using more rubber bands of the same length.

For Urethane rails and inserts I ordered the polytek 15-6 4lb kit. This is probably enough for two boards. I've used the board around 3 times and it's holding up great. It dents rather than cracks on impact. Somewhat off-white in color, but it can be tinted. It sands nicely. It has bubbles, but I poured it in my shed on a hot day. It sets up FAST.

Other lessons learned...
Use mold release on your rocker table.
Leave the board in the bag for 24 hours or your rocker will flatten.
Make your rocker table oversized - by more than you think.


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