CPT 2010/2011

[Eirik Hanes]

Custom Powder Tools

Background:
Tought it was time to make a small thread about our skibulding project. We are five students/friends studying at the Norwegian university of life Sciences (4 engineers and one life Science masters). So this fall we figured let's build skis! And then I figured why not do my semester paper on something fun; Skibuilding! So this is how it started November 2010

Build:
So we where quite lucky, we where able to use one of the Engineering Shop's for our project, so we had space and some tools from the beginning

Skis:
We did not want to just use the "iteration method" for designing good skis, we wanted to be able to calculate strength and stiffness for our designs and make a quick and easy to use "software". So while building the press, we worked on this as well. To be able to know if our calculations where right, we made a bunch of different samples and tested them, for stiffness, strength as well as fiber content.

Pictures:
Instead of posting one picture at the time here, i made a (public) Facebook album, which should be view able even if your not "on facebook". Let me know if it does not work.
http://www.facebook.com/album.php?aid=5 ... aa7d47962d

Thanks to everybody that has and will answer questions and/or participate in discussions. And thanks to skibuilders.com for hosting this amassing resource

[Eirik Hanes]

Finished the first skimold yesterday, and got the wood to the cores ready for the router.

[skidesmond]

Looking forward to seeing the data from your tests.

[Eirik Hanes]

Here is some results from my tests
First my skidesign spread sheet with comparison of current design to other designs

Load to failiure

Force vs Deflection

Stiffness EI


All samples had a 10.2 mm aspen core (except the ones which say "bjørk" which have a brich core) DB 450 is really DB 830.

and L means longitudal
DB means biaxial crossing the l direction (+-45 degrees unless otherwise stated)
DBL is L pluss DB
, the number is the fabric dry weight. fiber content for the laminates where 60-75 % by weight for glass and around 60 % for carbon.

Most of the samples failed by compression in the wood, except for DB 830 which failed from inter-laminar shear. there where also some samples where it was difficult to determine the cause of failure.

[Eirik Hanes]

DB320H is a hybrid fabric with 125g glass and 25 g carbon (torray t700s) in each dir.

[Richuk]

Cross referencing the Force v Deflection diagram is mucho difficult.

[Eirik Hanes]

yeah, just for illustration, you should get about the same info from the two other histograms.

[OAC]

Lots of scientific stuff here. I hope it match for you IRL.

[Eirik Hanes]

@ OAC, I know there can be a difference between theory and RL. Have some experience with that :P

First pair out of the press!





seems like they turned out all right

[Eirik Hanes]

So I tought it was time for an update.

We have been making 4 pairs since last time i posted, gradually getting a little bit better every time (except for one pair).

The first few pairs we had some problems with delamination between the sidwall and fiberglass/carbon layer, because we forgot to heat treat the sidewall again after routering the groove for the steel edge.

This problem made us very aware of the bonding problems with UHMW PE, so we decided to change to Poly urethane glue (better bond, cheaper and easier to use) to glue sidewall and steel edge. we also bought a special ink to measure the surface tension of the surface. http://www.lindberg-lund.no/files/PDF% ... TEC_UK.pdf

Before and after flame treating the base material, picture taken about the same time after aplying the droplet of ink. Huge difference!




Sample of sidewall sanded the same way, one heat treated one not treated:


This ink gave us a lot more confidence in our heat treatment process and we where able to adjust it. Even dough the topsheet and base was soposed to be treated, we decided to test. And it turned out to have a lower surface energy than out test ink (56 mN/m). This does not mean that it's too low, but we decided to treat it -> 3 pases on the base to get 56 mN/m, 1 pas on the topsheet, and 5-10 pases on the sidewall. so to be sure we did 5 pases on the base, 3 pases on the topsheet and around 10 on the sidewall.

Other than that, this is new for the latest layup:

We used sprayglue to and a steel edge to hold the bases in place. Then we glued a few pins to the base with matching holes in the core.




New quadaxial carbon fiber fabric, DBLT i.e 0, +-45 and 90. 590 g/sqm



New method for routering sidewall, inspiered by riuck (?). Works great.


Last pair finished:



Spec:
sidecut: 173-145-163(?)
fiber: quadaxial CF 590g/sqm
Epoxy: araldite esr3/esh3
core: 2-12-2, ash and aspen. Uhmw pe sidewall and tip and tail

But got to go, will write more about what to do better next time.
Hope the post is not to long.

congrats with new skis Torbjørn!

[Eirik Hanes]

Core and stiffnes profile (compared to average 190 cm ski)




a little untraditional stiffness profile, gonna be exciting to see how it works out.

What to do better next time:

Calculate necessary amount of epoxy and mix it all at once before starting the layup.

Work faster, we videotaped ourself, looked slow...

Avoid bubbles under the topsheet, anyone got a tip how to avoid this?

[Brazen]

If you ramp the temperature up slower in the press, the bubbles will have more time to squeeze out. Stir the epoxy by hand to avoid bubbles. Use a small hand roller during layup to "press" the bubbles out.

[doughboyshredder]

If you ramp the temperature up slower in the press, the bubbles will have more time to squeeze out. Stir the epoxy by hand to avoid bubbles. Use a small hand roller during layup to "press" the bubbles out.

besides the roller, I disagree completely.

[Brazen]

DBS, you make an inordinate amount of insubstantiable comments on this board. So...you think ADDING bubbles to the epoxy on mixing will help or, wait for it...heating it quickly so the bubbles don't have time to work their way out is good. Hmmm...I see...

Something about limiting your comments here to an occasional seafood recipe and a series of commas might apply, but I would never say that.

[doughboyshredder]

I've gone into it in detail before.

I will tell you this. I have personally built thousands of boards mixing the epoxy with a drill and have not ever had a single air bubble. This includes boards with clear bases and clear topsheets. If you have air bubble problems there is something wrong with your process.

As far as the heat, that's not really an issue, either way. Most of us are not using powerful enough heat blankets to bring the temp up super fast anyway, so we are already slowly ramping up the heat. However, if you preheat your press and put your board in to a hot press you will find out that the heat will turn the epoxy so viscous that unless you have a serious flaw in your layup process any and all air bubbles will immediately disappear.

Here is a fact for you to ponder.
The guys that are building tens of thousands of boards do not mix their epoxy by hand, and they do not slowly ramp up their heat, and they do not have air bubbles in their boards.

Now, go screw yourself brazen.