questions about clamping pressure, heat, expoxy

[oldmanriver]

Hey all,
Hope you can forgive some oddball questions. I'm almost done building a wooden kayak using fiberglass/epoxy for bonding. It's going great, and has inspired me to build myself some skis next. So here's my questions.

First- clamping pressure and presses: I'm using MAS slow cure epoxy, which takes several hours to set up, and many days to cure entirely. It cures at normal room temperatures, no heat required. Once cured it's solid like a rock and stronger than the wood it's bonded to. MAS specifically recommends light clamping pressure between pieces. No compressors or presses at all- just light pressure. Theory being that heavy pressure drives the epoxy out from between pieces, thus reducing the strength of the bond. Wouldn't this same idea apply to skis? Why are such high clamping pressures used in ski building? Maybe it takes all that pressure to get the tips and tails to bend correctly? One thing I like about the slow cure epoxy is that I've got a good 30 minutes after mixing to line things up before the epoxy hardens at all. A big advantage when you're not a professional, or are trying something for the first time.

Next queston: heat. I understand that some epoxies (like the ones I see recommended on this site) require heat to cure rapidly (or at all). Benefit to these epoxies is that the epoxy cures fast and your ski is ready to hit the slopes the day after it comes out of the press. I could understand heat being worth the trouble if you've got a press that's trying to crank out 3 pairs of cured skis per day, forever. For us homebuilders though, waiting a week wouldn't seem like a big deal. So why not use a regular low temp epoxy and some patience? Is there any disadvantage besides the wait? Are those fast cure epoxies stronger, or required?

Basically, I'd like to build some skis in as easy a fashion as possible, and since I probably won't ski 'em till next winter I've got plenty of time. I'd like to use my regular MAS epoxy, and a simple bottom mold without a press or adding heat to the equation. Just apply enough pressure to bend the core and hold things together, step back and wait a week, then voila! Can it be this simple?

paul

[tonyt]

Why are such high clamping pressures used in ski building? Maybe it takes all that pressure to get the tips and tails to bend correctly?
paul

By the way I'm with you on this one, the more research I did on epoxy and clamping pressures the more I was convinced that the high pressures are not required. I use a vaccum press and can only press to a max of about 12 psi. I made test pieces up using Mas epoxy and then tried to chisel them apart I could not break the bond. Next I did a dry layup of the ski to make sure that I could bend it at the tip and tail, I was quite prepared to build a separate top mold for the tip and tail but the vacuum molded the ski perfectly no top mold required. So I made the skis. I used medium Mas epoxy partly because my basement is cool and I always fear the vacuum pump will quit on long cure times. I vacuum for about 8 hours with a radiant heater over the mold so that it feels warm to the touch, I have no springback of the tip or tail and the camber stayed within 1-2mm . We have no snow here so I sent the skiis out to my daughter in Whistler to test, she works at the warming hut at the top of the Harmony chair if anyone wants to see them.
I think that the high pressures and temperatures used by manufacturers are for pre preg fabrics that need the pressure and temperature to cure. We get good squeeze out of the epoxy and good shape so why risk pressing too hard and creating a dry laminate that will delaminate with use. I notice lots of others use vacuum presses but the same results can be achieved with an air press at lower pressure.

Good luck, one thing I liked about the MAS epoxies was how clear they are making for a nice finish. I used texalium in the top layer so the top skin looks nice even though it is hard to see through the "clear" top skin.

[oldmanriver]

thanks tony, that sounds like the same thing i'm thinking. yeah, that MAS is really clear. i'd never heard of texalium before. sounds like a precured composite? maybe i'm reading the website wrong. where did you order it from?
paul

[Bambi]

In researching epoxies the one thing that a lot of manufacturers seem to reccomend for expoxy used an adhesive is not to apply too much pressure to the joint because you can make it 'dry' and consequently weak. This would seem to reinforce the idea that really high pressures are not necessarily a good thing.

Are people using higher pressures to ensure flatness or do the epoxy manufacturers reccomend it?

I would be interested to know if people who are using vacuum presses experiencing delaminations or if that is mor common among the positive pressure manufactured skis?

B.