Ski Builders Forum

I´ve made a mold and using water in my hose. So far so good...

Here´s the set up

Base
Rubberstripes
Glassfiber 22 oz triax
Maplecore laminated (2mm - 10mm - 2mm)
Glassfiber 22 oz triax
Graphicdesign on ricepaper
Top sheet

Now to the problems. I made a pair of skiis, one at a time. What happens is that the core cracks longside where the steeledge is. I´ve made the core around 10 mm wider then the base. Why is the core cracking? Too much pressure?

Next problem: My skiis are way to soft! What can I do? Put more fiberglass under the core? Make the core thicker? Is it good making the core thicker than 2 mm? Put some other wood in and laminate with the maple? What is good to make it stiffer? Did I use to much pressure so that the epoxi ran out and thats why the ski so soft? There was plenty of epoxi that ran out of the mold when pressing. The lamination seems good anyway.

I also found out now that my tip needs more curve. Is there any solution to that or do I need to make a new mold?? :-( Or can I do something with my mold to fix this??


I can´t really tell how much pressure i put on my skiis as I´m using a waterhose. The hose is hard as a rock when I have maximum pressure.

I never had the cores crack like that. Can you post some pics of your setup? How much pressure are you using in the hose, approximately? Are you using a cat track? Did you use wood glue when you laminated? Could be you squeezed too much glue when you clamped the core together and the joints failed? What are the ski dimensions?

May be double check your epoxy/hardener ratio when mixing.

The thickness of the cores sound ok to me. I made cores that were roughly 2.5mm, 13mm, 3mm and they were super stiff. I used solid ash, no glue up/lamination. Also had the same layup for the most part, ie 22oz triax under/over the core.

I think your core is cracking because your hose is pushing down on the overhanging core, and there's nothing to support it from underneath.

Ways to solve this problem:
1) cut your core to the right size
2) put some extra base underneath the overhang
3) use a cat track

Yes, the core probably cracked because it overhangs the edges too far.

You can achieve a much stiffer ski by increasing the core thickness just a little. For a quick calculation, you can use elementary beam theory to estimate increase in stiffness. Assuming a sandwich construction, and a relatively soft core (fiberglass carries the bulk of the stresses).

EI'/EI = (t'/t)^2

EI = section modulus (stiffness)
t = ski thickness

In order to double the stiffness (k'/k = 2) your new cores need to be 1.4 times as thick. If they were 2mm, 10mm, 2mm thick. You might try making them 2.8mm, 14mm, 2.8mm thick.

It looks like your cores are hardwood (maple), so you wouldn't have to increase the core thickness as much to double stiffness. If we assume your core material is just as stiff as the fiberglass, it would have to be 1.26 times thicker (2.5mm, 12.6mm, 2.5mm). EI'/EI=(t'/t)^3

Ok thanks guys! Will try your tips!

Anybody have a suggestion on my problem with too little curve on the tip? I don´t want to build a whole new mold.

Is there anything you can do to change the curve angel of the mold or is there any chance of afterbend the tip?? Cause my tip defenitly need a bigger curve..!

I think unless your mold is modular you're stuck making a new one.

you can try and make a jig with clamps and such and heat the tips with a heat gun and bend a new bend in there. You have to get it up around 180 F or so. Pretty damn hot

Be careful not to burn your top sheet and this process may dramatically weaken the resin bond in this area. I put a bunch of rocker in some skis but I jigged the bend over half more what it ended up being.

I just used some crap 2 x 4's and some rope and clamps. Might be really hard to get a tight radial bend. Rocker is much easier to induce.

nate wrote:I think unless your mold is modular you're stuck making a new one.

That's true. That's one reason to make a modular mold. One thing I encountered was when the ski (in between the cassette, ie 1/8 inch hardboard) was in the press and I inflated the hose to fast, the hose tended to push the ski and cassette downward so it slid back a little so the tip curve was not as great as it should have been. On the second pair I inflated the hose slowly and the ski/cassette didn't slide as much.

I'm working on a better system to keep the cassette in place once it's in the press....(thinking out load) Like having a groove/dado across the middle of the mold and a rod on the bottom of the cassette that fits the groove/dado. This would prevent the cassette from moving front to back. Another option would be to have some holes in the mold and pegs go through the cassette and into the mold.