uggg concave bottoms

jellyfish · Post by **jellyfish** » Wed Jan 19, 2011 2:25 pm

Looking for a little advise I did 3 snowboards all with different cores 2 ended up with 1 and 2mm concaves and one is perfect, and to add more confusion I did the heat a little different on all 3. the worst one was the last one, heat for 10 mins at 100*f 10 mins @150*f and then final 20 mins at 180*f 30 psi pressure its a mix of ash and red cedar alterating in the core ash cedar ash cedar ash cedar and so on , best or perfect one is bamboo core and bamboo veneer top sheet and the mid one is 2 ash stringers in a popular wood core pressed at 150*f for 20 mins then 180*f for 20 mins,

anyone have some isight into whats going on More pressure? different heat cycle? can it be the cores ?

twizzstyle · Post by **twizzstyle** » Wed Jan 19, 2011 2:43 pm

What are your layers of composite above and below the core, and how are they oriented?

If you're using triax, do you have the longitudinal fibers facing towards the core or away from it? Is it the same top and bottom?

Are you heating from the top and bottom, or just one side?

There are a few things that can cause base concavity or camber changes, but those two things always stick out in my mind. Uneven heating (I only heat from the top currently and I get camber changes, but since they're skis they don't show any base concavity) and asymmetric composite layups can cause big changes. Asymmetric layup could either mean more layers on one side than the other, or when using a woven triax material you aren't flipping the layer over on one side. Does that make sense?

jellyfish · Post by **jellyfish** » Wed Jan 19, 2011 3:16 pm

thanks twizz

22 oz triax top and bottom with opposite orientation longitudinal strands always down on the bottom and up on the top. I precured 6 oz carbon fiber top sheets on both boards that concaves happened, 2 new blankets 5000Watts each inbetween 20 guage galv sheets heats up fast you can count the degrees like a timer thats why I dialed it down, qcm resin, hardeners. the core thickness were 8mm , and 7.7 mm, very simular. had edge bleed through on the first one even though they were rabetted into the sidewall so i turned down the pressure from 40 to 30 psi use double cat track top and bottom. cassett is aluminum sheets.

the one with no concave is 6.4 mm bamboo core and same glass layup its has a bamboo veneer top sheet..

I think I need more pressure?

COsurfer · Post by **COsurfer** » Wed Jan 19, 2011 4:45 pm

What size are your inserts?

Richuk · Post by **Richuk** » Wed Jan 19, 2011 5:39 pm

Are you pressing with a cat-track?

jellyfish · Post by **jellyfish** » Wed Jan 19, 2011 7:52 pm

Inserts ? ok 6mm metric from camax 6 & 7 mm to fit the taper

cat track? yes top at bladder and below for the adjustable bottom mold.

I guess there is no easy answer i'll keep plugging away, concave in powder is probably not a bad thing on ice would be where it could be noticed ?

cheers
Jim

doughboyshredder · Post by **doughboyshredder** » Wed Jan 19, 2011 8:40 pm

base thickness? Edge thickness?
Personally, I would like to see more pressure.
There is nothing wrong with heat ramping up fast, as long as it's ramping up evenly. No need to dial it down.
Is the concavity noticeable immediately upon removing from press, or does it develop during cool down?
Have you checked your cores to ensure uniformity there?
Cat track material?
Are bladders extending past cat track, or does the cat track extend past the bladders?

Brazen · Post by **Brazen** » Wed Jan 19, 2011 9:35 pm

I wood never use popular wood.

hahahahahaha

Brazen · Post by **Brazen** » Wed Jan 19, 2011 9:36 pm

come on, that's a pearl ))

Richuk · Post by **Richuk** » Thu Jan 20, 2011 2:46 am

I can't find the tread I wanted, so it worth a search - I think shopvac lead the discussion and posted a few diagrams. This is worth a read:

viewtopic.php?t=2304&postdays=0&postord ... ve&start=0

MontuckyMadman · Post by **MontuckyMadman** » Thu Jan 20, 2011 9:39 am

We took out a pair we pressed at 70psi and the concavity was noticable rigt away and a few days later it was gone. Skis were flat. Maybe the rabbit isnt deep enough?
Boards just come out fuct intially?

jellyfish · Post by **jellyfish** » Thu Jan 20, 2011 10:04 am

Did cold pressing last year with the same setup rechecked those cores and they are fine the only thing I changed is I did my own cores by glueing up blocks and then having the cores shaved off with a portable mill. I plane the core to final thickness then cut outline attach edges then profile the tips only from the inserts out to 1.7 mm inthe tips I do this with a router and bridge after trying unsuccessfully to profile with a planner I admitted defeat for now I will try again when there is more time to fiddle with it. And there is the heat thing I will press with more pressure and allow it To cool
With the pressure still on.

Thanks for everyones help
Cheers
Jim

jellyfish · Post by **jellyfish** » Thu Jan 20, 2011 2:20 pm

We use abs for sidewalls, I think the rabbet was deep enough i will google up and see what the melting point of abs is.. I did check with a meat thermometer to see what kind of temps were in my cat track top and bottom after it was posted here that the built ins from MEi could fail, they were within 5 degrees of each other and 10 degrees less than the controllers showed.

I Initially though they were fine never even foresaw the problem or even checked to see it was flat i just expected them to end up flat total blindsided it was noticed when i took them to a shop for base grind, to my embarassment.

cheers
Jim

lex · Post by **lex** » Thu Jan 20, 2011 4:20 pm

Sorry, just to be clear by concave you mean edge high, so if you lay the board down the center area does not touch while the edges do.

On the rocker boards I have done I put a strip of carbon horizontally under the core to get a convex base in that area. So I assume the topsheet is doing the same thing, but since it's on top you're getting a concave base. Doesn't matter how much pressure you use. Letting it cool down in the press will help for sure.

Since you are using 6oz precured, is it equal weight in both axis? You might go with a lighter top glass as well....like a 19oz

Also, how soon after pressing are you starting your finishing work/bandsawing/grinding/sidewall shaping? On my personal stuff, I wait a day, otherwise things change for me.

windego · Post by **windego** » Thu Jan 20, 2011 4:26 pm

Hi Jim
It is for sure the carbon topsheets. The coefficient of expansion with temperature is much more with glass than carbon. So when the board cools the carbon top stays put and the bottom glass shrinks.
I had a similar dded up doing a kiteboard with carbon....potato chip! Of course the hotter you cure the more exaggerated this becomes. For a room temp cure like many have done with carbon tops it is not noticeable. But crank it up to 180F an bam WTF!
If you grind the carbon off it may settle back flat...maybe.

Good snow and windof late!

Stan

Ski Builders Forum

Ski Builders Forum

uggg concave bottoms

uggg concave bottoms