Heat and concave bases
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Heat and concave bases
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?
concave base
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
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
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- Posts: 1354
- Joined: Mon Sep 17, 2007 7:37 pm
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.
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.
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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...
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...
A bad day skiing is always better than a good one at work...
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.
I like top and bottom heat for best results.
- MontuckyMadman
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- MontuckyMadman
- Posts: 2395
- Joined: Fri Jun 20, 2008 9:41 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?
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?
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:...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.
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.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?
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...
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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…
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
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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.
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.
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- Joined: Mon Sep 17, 2007 7:37 pm
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.