Base flatness thread

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Dtrain
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Base flatness thread

Post by Dtrain »

K . It's been discussed lots, most recently on skiman20 journal.
I'm starting a thread as the facts are out there but they are spread out

I'm getting flat skis, flat splitboards, but snowboards are not coming out flat.

I've just done 3 test to try and figure it out, and it's driving me nuts.

Badass press. Full width bladder and a catrack

All tests done with 22oz vector ply glass. Symmetrical layup.

40,50,and 55 psi.

At 40psi the tips are conforming perfectly

Urethane sidewalls. Rabbetted and unrabbeted

Pulled at 180f hot
Pulled at 100f
Left to cool under pressure

Heats to 180 in 20 min with a 5 deg variance.
Cools down with no variance in temp

Molds are perfectly flat.

All boards came out flat in the middle. Flat in the tips.
But with probably 3mm base high between the insert packs and the start of the tip/tail radius.

I like the way it rides, but without an autofeed grinder applying even pressure grinding is a real bitch. Any snowboard without a auto feed on the grinder is a real bitch actually.

Point is I want the ability to get snowboards flat. Ski scientists please help. Snowboard builders.... Are you getting flat bases? The only thing I have not tryed is messing with how fast my heat ramps up.

Derek
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chrismp
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Post by chrismp »

3mm sounds like an awful lot! I get reasonably flat bases that don't need much grinding to get flat. I have a Sigma Sbi with an autofeed though, so that makes my life a lot easier.

The only good explanation I can think of is different heat on top/bottom. Maybe your thermocouples are giving bad readings or are placed under parts of your blankets that heat up differently.
Richuk
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Post by Richuk »

DT,

Skimann mentioned that his top mold could be a little tighter, so the first step is to iron that out.

A few things might help,

Have you tried put a blank snowboard core in the press and running a normal heat cycle? Does it come out flat? This is a good way to understand the potential effects of the heat cycle on the materials within the press. You should get a good sense of what is happening, as wood is orthotropic in nature.

Have you checked the resin ratio you achieve once the board is made?

Twenty minutes to ramp is fast in IMHO, creating a lot of internal stress and stain. That is to say - anisotropy of thermal expansion properties, residual stresses, chemical shrinkage and thickness change of plies affect the final shape of fiber reinforced laminate structures cured at elevated temperature [1,2,3,4,5]. Barnes et al.[1] showed that the difference of coefficients of thermal expansion and residual stress were the main sources of process induced distortion of curved laminates ; )

Changes in pressure, 40 - 55psi, this will affect the thickness and mechanical qualities of the laminate.
Last edited by Richuk on Mon Feb 16, 2015 5:01 pm, edited 1 time in total.
Dtrain
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Post by Dtrain »

Chrismp,

How do you cycle your heat. How long to get to temp? You still using the quick adjustable style style mold? And what kind of pressure you pressing at?

Thanks for any info.
Dtrain
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Post by Dtrain »

Richuk,

I have not measured post press resin ratio! Though the tests were done with clear base and top. I could see the glass had no dry spots.
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skimann20
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Post by skimann20 »

Way to call me out... ;-) Good idea Dtrain. This would go back to the idea of doing stickies on this web site.

The only other thing that I have in the layup that was not mentioned is I put CF top and bottom. So i'm sure that is taking away a little of the rabbet height.

Dtrain: 3mm sounds really high! That's more than twice the the base material?
Richuk
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Post by Richuk »

DT,

Most people haven't ) It means cutting up a board. Its a good thing to do. Let's you see what's going on and what needs adjusting.

The second calculator is a good one - it will give you are sense of expected thickness of the laminate. Dry would be at the very extreme end of the issue.

http://www.netcomposites.com/calculator ... -fractions
http://www.saertex.com/en/product_techn ... alculator/

I'm assuming you don't heat treat your boards before pressing?
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MontuckyMadman
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Post by MontuckyMadman »

How deep is your rabbet? .8mm? Moisture in the core? Def press a core by itself. Lots to learn there. Could you be pressing at a much higher psi at the laminate surface? Do your fibers conform to the edge shape and base In the layup or span and contract in the thin areas?
Ive never pressed a snowboard.
Only time its concave in skis the rabbet is not deep enough. What urethane? That stuff has a different co of expansion than allot of the other materials sometimes.
Dtrain
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Post by Dtrain »

I think I just may have figured it out. My bottom 2 ibeams in my press are not touching. They are 4" apart. Total width 18inches. We used to have a 1"inch thick Mdf sheet glued ontop of them, molds sat on that.

As my partner was having issues with cavity height while loading skis, he swapped the 1" sheet for 1/4" one to create more room.

Because this board is so thin, and the molds are ribs,I think the 4" inch gap in the beams is causing the mold to distort under pressure. I'm going to swapp it out for a 1/4" or 1/2" of steel and see if that fixes it.

I think I overlooked it as the problem occurred after we got a new bladder, but it was at that same time we swapped out the Mdf sheet.

Yes MM. .8mm rabbet. Core at 8percent moisture . Smooth on pmc-790.
Dtrain
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Post by Dtrain »

What pressure are others pressing at?
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chrismp
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Post by chrismp »

Yes, still using wood ribs as an adjustable mold. We're pressing at 4 bar (~60 psi) and it takes about 20 min to get to 100°C. At first we apply pressure and after we're at 4 bar we switch on the heat.
gav wa
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Post by gav wa »

Even 40 pound pressure is huge for that full width bladder. Probably doesn't hurt the final product reliability wise bit isn't needed. What did you press at with your old hoses? You could put one hose in and measure side to side to see how wide the contact area is then work out the pressure per inch of press area. Remember it then gets spread/divided over the cat track.
I thought my 1mm was a pain but 3mm is crazy, bet its fun in good conditions. Have you tried letting your resin gel in the press for 40 minutes before hitting heat? Remember the resin companies do call it post cure. I now do this and it has resolved issues. I can now even do asymetric layups with no issues.
During the layup the bottom is getting to gel a little further than the top and if the heat is hit straight away then the top is curing different from the bottom.

Not giving a guarantee but it has helped me and is easy to test. I've found some resins worse than others so maybe your resin is like mine. Plus as was discussed in the interview on this site with the resin industry rep, it isn't important to heat it straight away.
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MontuckyMadman
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Post by MontuckyMadman »

Well right theres ur issue
Image
gav wa
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Post by gav wa »

looks like a nice hybrid rocker eyebrow MM
Dtrain
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Post by Dtrain »

And I'm all like.......
Image

The test boards. 155 directional twins.
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