Heat and concave bases

bobbyrobie · Post by **bobbyrobie** » Sun Mar 14, 2010 7:47 am

Is anyone else having any type of issues with concave bases and heat? i have a heated press with heat on both sides. Iv been getting concave bases i have ruled out the mold. I pressed a board with only top heat and i ended up with a flat base with just less camber. Has anyone had an experience with concave bases with just heat on the bottom?

wingworks · Post by **wingworks** » Sun Mar 14, 2010 8:53 am

bottom heated press no cat track---- foam/wood core cap skis
air at 80 psi ramped heat up to 230 f (max temp for our epoxy) onfirst set of skis
-pulled out of press after 1.5 hrs---poplar wood core cap ski--130 mm base concave but sandable to flat edge cleats showing slight bumps on bottom of base

next tried 200 f 80psi pulled out of press after 1.5 hrs wood/foam core cap ski still a little concave but better slight bumps from cleats

next 185 f @ 65psi for 1.5 hrs 120mm wood core cap ski
turned heat off left air on for 6 hrs
dead flat no bumps

doughboyshredder · Post by **doughboyshredder** » Sun Mar 14, 2010 9:11 am

I have determined conclusively that MDF gets weird under heat. I am 100% positive that if the base of your mold is mdf ribs and you have no plate of some sort sitting on the ribs that if you are using pressure and heat you will have problems.

When I remove pressure from my mdf tip and tail ribs I can see the individual ribs swell and contract while cooling.

I am currently working on an adjustable tip and tail block so I can eliminate mdf entirely.

bobbyrobie · Post by **bobbyrobie** » Sun Mar 14, 2010 9:52 am

Thats a good point about maybe the wood expanding. my mold is cut from 3/4" thick plywood though so it may act a little differently under heat. I have been pulling the board out at around 140 degrees F. I think i will try leaving it under pressure till it cools down fully and see what happens.

ben_mtl · Post by **ben_mtl** » Sun Mar 14, 2010 12:29 pm

I have slightly convex bases with my setup (nothing that a sand belt wouldn't make flat)
My mold is MDF ribs for the camber + a 5/8 MDF plate over it and my tip and tail molds are solid MDF.
I'm thinking of switching the MDF top plate for a 1/4" steel.. or maybe have my camber mold machined in a solid bloc of MDF...

Dr. Delam · Post by **Dr. Delam** » Sun Mar 14, 2010 6:45 pm

When doing bottom heat only I have had concave bases, top heat only convex bases, and fairly flat with top and bottom heat. I was recently tuning a pair of skis(not mine) where the thermocouple fell out of place for quite some time doing bottom heat and they were super concave.

I like top and bottom heat for best results.

davide · Post by **davide** » Mon Mar 15, 2010 12:50 am

I guess curing at room temperature would make life easier...

MontuckyMadman · Post by **MontuckyMadman** » Mon Mar 15, 2010 10:06 am

davide wrote:I guess curing at room temperature would make life easier...

But You have to wait like 14 hours!

davide · Post by **davide** » Mon Mar 15, 2010 4:11 pm

MontuckyMadman wrote:But You have to wait like 14 hours!

Why are you in a hurry? Do you press 20 pairs per day? If not you may wait for a while.
I suppose most of the home skibuilders press a pair per month, or so. Then what about pressing during the night?

MontuckyMadman · Post by **MontuckyMadman** » Mon Mar 15, 2010 5:48 pm

I like to make a pair in a day so I can concentrate on the finishing as a pair.
If you do one at a time it takes to long and running my compressor for that long is annoying.
I cook slower than most on here and max out my bottom blanket at 180 as to prevent this type of problem.
I think people are experiencing a number of problems with this type of method. One being to hot and the other being to hot to fast.
You have allot of different materials all expanding and contracting at different rates.

Also isn't the epoxy like 7 times stronger as a heat cure and less of a chance to break down later on if exposed to heat again?

davide · Post by **davide** » Tue Mar 16, 2010 5:47 pm

MontuckyMadman wrote:...I think people are experiencing a number of problems with this type of method. One being to hot and the other being to hot to fast.
You have allot of different materials all expanding and contracting at different rates.

I know, that is why I would suggest to press at room temperature, but I understand that it may be a problem with a compressor; under vacuum it is easier to press for long time.

MontuckyMadman wrote:Also isn't the epoxy like 7 times stronger as a heat cure and less of a chance to break down later on if exposed to heat again?

Seven times seem too much, maybe only for certain epoxy. The room temperature curing epoxy do not improve that much if cured at high temperature.
R&G (www.r-g.de) is selling an epoxy for aereonautical use that must be cured at room temperature, and requires a post curing (out of the press) at 50 degree centigrades to achive aereonautical performaces.
I imagine is a bit difficult and expensive to cure a fusalage or wings at high temperature...

King_of_the_Hill · Post by **King_of_the_Hill** » Tue Mar 16, 2010 11:24 pm

@Davide
In aerospace, certainly in commercial and militair aircraft, it is standard to cure the parts @120C. The new Boeing 787's fuselage constists of three 20m long parts which are all cured @ 140C

davide · Post by **davide** » Mon Mar 29, 2010 1:21 am

Well, the 787 is going to be very expensive and is a more than a year in late...

I do not want to convince to use room temperature curing epoxy, if you like hot pressing just do it, but I think you understood my point: if room temperature curing is good enough for gliders, then it is good for skis too. If somebody find difficult to do hot curing, keep in mind that cold curing is a valid alternative.
Just one more thing: it is nice to look at what ski industry is doing, but remember that industry and home ski builders have different goals: the goal of home ski builders is to build durable and fun skis with the minimum amount of time, money and effort, while the goal of the industry is to make money by selling skis: what is good for one, may not be good for the other…

MontuckyMadman · Post by **MontuckyMadman** » Mon Mar 29, 2010 10:29 am

I have built the same exact ski both ways. With and without heat with the same epoxy.

The feeling of the skis are completely different.

The room temp skis are dead and the flex is different.

The heat cured feels correct, the room temp feels wrong when flexed side by side.

They make different sounds even when pinged.

This convinced me that heat was definitely what I wanted to work with.

doughboyshredder · Post by **doughboyshredder** » Mon Mar 29, 2010 11:25 am

The oft repeated statement that slow cure or cold cured epoxy is stronger, is not technically correct, or is technically correct, but not really. The longer epoxy takes to cure, the less brittle it is, so it will flex much further before breaking. This makes it stronger in terms of how far you can flex it till it breaks, but that's not necessarily what we want. Kind of like saying that rubber is stronger than carbon fiber.

Ski Builders Forum

Ski Builders Forum

Heat and concave bases

Heat and concave bases

concave base