Hole drilling tips/tails without filling them up under press

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K.Ström
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Hole drilling tips/tails without filling them up under press

Post by K.Ström »

Back again after one year off. Made 3pairs by vaccum the last time and now im back. But this time with a real press and a small cnc machine!
I will putt up a build log as soon as i have e everything up and running.

Started up this seasson with two skitest weekends here in sweden. Looked, sqeezed and tested a lot of skis and gave the skitips a special thought.
As it seems, lightening the ski tips and tails really are on the skiproducers mind right know. And why not? It really does semms to work if you ask me!
Most of the bigger ski producers also seems to want to go the same way when it comes to lighten the tips/tails. And thats by adding air with diffrent technics. Some drills or mills the materials, some adds diffrent "comb" materials, like aramid honeycomb, tube comb and so on...

Well, are they doing it... When I want to do it to! But how?
I do have a cnc machine so i thinking of doing something like the rossignol s7 series. Cnc mill the hell out of some abs spacer and make some cool pattern. So far no problems.
But how the heck am i going to succed to press the ski without filling all holes with epoxy?
Last edited by K.Ström on Wed Dec 11, 2013 12:35 pm, edited 1 time in total.
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chrismp
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Post by chrismp »

You could use some small sheets of fiberglass impregnated with epoxy to cover the holes in the core. Just wet out the first piece of glass, put it on a flat surface, place the core with the hole on top of it and clamp it down or apply some weight, flip the core and repeat for the other side.
K.Ström
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Post by K.Ström »

Hello Crismp
Well that would be one way to do it. But i dont know how much weight you will save by adding a extra layer of glass first. Even if it is a very thin one.
Do you think i will need one on the side below as well? Will the epoxy push up in the hole as well?


PS: Im not really sure but i think it was you i bought some paulonia cores from. I guess i forgott to thank you for those. So big thanks for them.
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MontuckyMadman
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Post by MontuckyMadman »

full sheets of rubber to kinda seal the holes from the wet glass and epoxy sqeeze?
sammer wrote: I'm still a tang on top guy.
twizzstyle
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Post by twizzstyle »

Anothing thing to consider is that the honeycomb won't be pressing on your main fiberglass layers the same as a solid core/tip spacer, and thus won't squeeze out as much epoxy from the fiberglass. If you didn't seal up both sides of the honeycomb first, you may end up with heavier skis!

Have you considered foam cores or tip spacers?

I had plans to do a honeycomb core ski, as I have a bunch of scrap honeycomb sitting around, but put that project to the side because of the same concerns you have about how to build it.

I've used pre-cured sheets of fiberglass on skis before, that might be the best option here (these sheets replace all of the fiberglass in the ski)
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chrismp
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Post by chrismp »

K.Ström wrote:Hello Crismp
Well that would be one way to do it. But i dont know how much weight you will save by adding a extra layer of glass first. Even if it is a very thin one.
Do you think i will need one on the side below as well? Will the epoxy push up in the hole as well?


PS: Im not really sure but i think it was you i bought some paulonia cores from. I guess i forgott to thank you for those. So big thanks for them.
Now that you mention it, I don't think the one below is absolutely necessary...however, I think you'll get a bit more epoxy in there which gets squeezed into the hole during pressing. It's quite easy to calculate how much weight the hole would save and how heavy a patch of thin glass would be...I guess running the numbers is the best way of telling.

Haha, thank you! I remember sending out a couple of planks to sweden :)
ben_mtl
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Post by ben_mtl »

Pre-preg composites are the way to go... I used to build stuff with Nomex honeycomb back in school and the only way you don't fill the honeycomb with resin is to use prepreg fibers. The resin contained in those fabrics will only squeeze enough to make a good bond between layers and "grab" the honeycomb... the fibre-to-resin ratio is very accurate and accounts only for that !
Problem is it's quite expensive, might be tough to source and shelf life is not that long.... + you have to store everything in the freezer.

Sure you might be able to do differently but will the advantages of using honeycomb to save weight be still there ? only a try will tell so go ahead .. and report back :)

Good luck !
A bad day skiing is always better than a good one at work...
twizzstyle
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Post by twizzstyle »

Not too hard to source!

SnowboardMaterials.com has it for $15/m

http://snowboardmaterials.com/pages/materials.htm

They used to have some that had carbon fiber stringers (that's what I used a few years back), but looks like he's only got plain fiberglass now. Good stuff though, worked out well for me. (I just did it over wood cores)
ben_mtl
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Post by ben_mtl »

Well the stuff sold by snowboardmaterial is preCURED fiberglass, not prepreg.

Prepreg is basically the fabric + just the right amount of resin but the curing reaction is thermically stopped by storing it @-20C. The impregnated fabric is then rolled and stored until it's used.
It's a process that requires a high temperature cure + vacuum or pressure (usually an autoclave is used) but produces very high quality parts, the fabri-to-resin ratio is perfectly controlled, the resin used is a specialty product and basically you can anticipate and control the resin "squeeze" while under pressure. Very low squeeze means the resin will not fill the honeycomb cells but will be just enought for a layer to bond to the next one.

Aircraft composite parts are almost exclusively made with this kind of products to ensure repeatabily and process stability.
A bad day skiing is always better than a good one at work...
K.Ström
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Post by K.Ström »

Well, you pretty much make "pre preg" everytime you wet out a weave... Only difference is that you do not use extremly slow curing epoxy and put in a freezer.
Im going to weigh pre cut weaves and then calculate how much epoxy thats suitable and roll it out wery carfully before i put it on the ski.
If prepreg works, this should work as well. But i need to do some test pieces first.
To pre fabricate glass layers on machined abs boards just seems to be a little bit to much work. But maybe its the way to go.
I dont want to use foam tips cause im pretty sure it would delam. I have made some test pieces with "foamed" pvc but it seemed to get fragile with the epoxy.
This hole drilling thing is something i came up with then i made my second ski with all paulownia cores i got from cris. That ski delamed in the wood itself and i had a hard time to walk 3 days after that wreck and my legg turn blue for a month! The same afternoon as the wreck i started to scetch on a ski whith a hollow drilled ash/oac core insted. To make it more sustainable. And now when i saw that they using this technic in the tips and tails a thoght i was going to give it a try.
ben_mtl
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Post by ben_mtl »

One key feature of the real prepreg (not the DIY you can make) is the control you have on how the resin flows while curing under pressure. Low flow with regular hardeners makes for poor fabric impregnation in our world.
DIY prepreg have its advantages though (more control on the resin-to-fabric ratio) but real prepreg have super slow setting hardeners...
If you can find some and you can validate that when in the heated press the resin won't go through a "low viscosity" phase (mine does that, and I'm glad it does since it's a pretty "thick" resin so it allows for a better fabric impregnation), then you're golden !
A bad day skiing is always better than a good one at work...
twizzstyle
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Post by twizzstyle »

ah, yes sorry, I was thinking something different.

I did some CF layups back in college with pre-preg, but as you said it requires a lot more control (and cost). The pre-cured sheets might still work though. Although when I used the precured sheets, I did an identical pair with a normal wet layup, and the skis with the precured sheets ended up a little heavier.
K.Ström
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Post by K.Ström »

I dont have a heated press and problably wont go for one either, Im only using a very low viscosity resins so heating it up wont help i guess.
Unfortunatly all of my resin sources around europe have got the idea that epoxy needs to be shipped as dangerous goods. And that means the shipping rates gone sky high!
Otherwise r-g had a 400min potlife epoxy.
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MontuckyMadman
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Post by MontuckyMadman »

Rubber. Best idea in here im doinit.
sammer wrote: I'm still a tang on top guy.
K.Ström
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Joined: Wed Aug 18, 2010 10:45 am
Location: Dalarna, Sweden

Post by K.Ström »

How thick rubber do you put in there? One piece to cover the complete tip or vds rubber stripes? Do you put it on both sides?
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