Howdy,
Just ran across this site about a week ago. Before finding it, I had thought about building my own skis from time to time (I build my own bike frames), but I didn't think I had the background in skiing to know how to design a ski layup when it came to # of layers of fiberglass, core thickenss, etc. However, it seems like it's a lot more straightforward than I'd thought.
Making a ski press doesn't seem to difficult. Perhaps a bit pricey, but mainly I'm concerned about where I'd keep it. Vacuum bagging, however, seems a lot easier to me. It seems the lower pressure of a vacuum bag is an issue. I wonder if the one+ fewer PSI I could achieve living near Denver would be a problem vs. sea level? Pretty sure I'm being stupid here, but gotta start somewhere...
And if you think I'm being _really_ dumb with bypassing the press, then please convince me.
Vacuum bagging at altitude?
Moderators: Head Monkey, kelvin, bigKam, skidesmond, chrismp
-
twizzstyle
- Posts: 2204
- Joined: Tue Mar 07, 2006 8:25 pm
- Location: Kenmore, Wa USA
You are absolutely correct with the higher altitude.
Vacuum bagging essentially is like using a press at atmospheric pressure (~14.7psi at sea level). The lower the barometric pressure, the less pressing force you have. Thats what autoclaves are used for.
According to Googlez, Denver on average is about 0.83 of sea level, so more like 12.2psi. So yes, you're losing about 2psi on the pressing force.
Now is it enough to make a difference? Probably not. Either way you are FAR better off with a pneumatic press where you can press at 50psi? 70psi? As high as you feel safe with your press really. 50 vs 14 is a big difference.
But, lots of people make skis with bagging, and they turn out great. You just might have a heavier ski compared to a pneumatically pressed one because you aren't squeezing as much resin out.
Vacuum bagging essentially is like using a press at atmospheric pressure (~14.7psi at sea level). The lower the barometric pressure, the less pressing force you have. Thats what autoclaves are used for.
According to Googlez, Denver on average is about 0.83 of sea level, so more like 12.2psi. So yes, you're losing about 2psi on the pressing force.
Now is it enough to make a difference? Probably not. Either way you are FAR better off with a pneumatic press where you can press at 50psi? 70psi? As high as you feel safe with your press really. 50 vs 14 is a big difference.
But, lots of people make skis with bagging, and they turn out great. You just might have a heavier ski compared to a pneumatically pressed one because you aren't squeezing as much resin out.
OK I saw this and felt I needed to point out a few things. I do not want to start a debate on what is best just point out a couple of things I have learned. Yes, at elevation you will see a decrease in achievable vacuum. I live at ~4500 above sea level. The pro’s where I work can pull 25 HG on a good day (~11 psi). My system, with a used vacuum pump, will pull ~21 HG (~10 PSI). That’s the best I have been able to do.
There are a lot of variables to consider. Vacuum bagging uses more consumable items i.e. sealant tape, bag film, release ply, perforated release ply, peal ply, and breather ply etc. The squeeze out of resin can be the same as a pneumatic press IF you chose the right resin. Do not use QCM epoxies in a vacuum press. They are too thick and will not flow properly for that use (at least the ones I read about here). I use http://resinresearch.net/_wsn/page3.html for my skis. The 2070 version with fast hardener. It’s water thin and is designed for RIM and infusion bagging setups. And high temp post cures work really well on this product. I experience very little camber loss from my skis, and actually notice that the camber will increase as the temp gets colder on a few pair.
As for weight I think the core material has more to do with the final weight of the ski than the ounce or so of difference you might get in resin squeeze out. I made two pairs of skis, my nihilists, one pair with an ash core (9.5 lb) and one with a poplar core(7.5 lb). The poplar cored skis are almost 2 lb lighter than the ash cored skis. And to be honest I can’t tell the difference in ski ability of the two pairs. I also made two pair of reverse camber skis… well the tips and tails anyway. One pair with a Cyprus core and one with an ash core. They have a 1 lb difference in weight. And the same for ski ability. Neither my friend or I could tell the difference.
The other thing to consider with vacuum setups is that you will need to fit/pre-bend your parts more carefully. I boil my core tips so that I can pre-shape them to the tool as well as bending the edges to fit perfectly in the tool.
I like the skis I have made. They ski well and are for the most part very durable for a wood side wall ski. I did have a compression fracture in the top sheet on the original pair of nihilists right behind the heel riser of my hammerheads. Most likely due to my fat crack skiing on them for 60 days. I also made a longitudinal crack in the core of the other ski last year and was able to repair that and ski it until we saw the fracture in the other ski. And due to the glass top sheet and wood side walls they need a touch up every year or so to keep the wood sealed that tops shiny.
FWIW, vacuum bagging works great and takes up less space to store tools/press (have a look at plywoods process for other pneumatic ideas). You will get some really good skis if you pay attention to the details and take your time to get everything right. Even at your altitude you can make some great skis with the vacuum process. And all you have to build is the tool to bag the skis to. ~1 sheet of MDF.
My $.02, YMMV, yada yada yada
Hafte
There are a lot of variables to consider. Vacuum bagging uses more consumable items i.e. sealant tape, bag film, release ply, perforated release ply, peal ply, and breather ply etc. The squeeze out of resin can be the same as a pneumatic press IF you chose the right resin. Do not use QCM epoxies in a vacuum press. They are too thick and will not flow properly for that use (at least the ones I read about here). I use http://resinresearch.net/_wsn/page3.html for my skis. The 2070 version with fast hardener. It’s water thin and is designed for RIM and infusion bagging setups. And high temp post cures work really well on this product. I experience very little camber loss from my skis, and actually notice that the camber will increase as the temp gets colder on a few pair.
As for weight I think the core material has more to do with the final weight of the ski than the ounce or so of difference you might get in resin squeeze out. I made two pairs of skis, my nihilists, one pair with an ash core (9.5 lb) and one with a poplar core(7.5 lb). The poplar cored skis are almost 2 lb lighter than the ash cored skis. And to be honest I can’t tell the difference in ski ability of the two pairs. I also made two pair of reverse camber skis… well the tips and tails anyway. One pair with a Cyprus core and one with an ash core. They have a 1 lb difference in weight. And the same for ski ability. Neither my friend or I could tell the difference.
The other thing to consider with vacuum setups is that you will need to fit/pre-bend your parts more carefully. I boil my core tips so that I can pre-shape them to the tool as well as bending the edges to fit perfectly in the tool.
I like the skis I have made. They ski well and are for the most part very durable for a wood side wall ski. I did have a compression fracture in the top sheet on the original pair of nihilists right behind the heel riser of my hammerheads. Most likely due to my fat crack skiing on them for 60 days. I also made a longitudinal crack in the core of the other ski last year and was able to repair that and ski it until we saw the fracture in the other ski. And due to the glass top sheet and wood side walls they need a touch up every year or so to keep the wood sealed that tops shiny.
FWIW, vacuum bagging works great and takes up less space to store tools/press (have a look at plywoods process for other pneumatic ideas). You will get some really good skis if you pay attention to the details and take your time to get everything right. Even at your altitude you can make some great skis with the vacuum process. And all you have to build is the tool to bag the skis to. ~1 sheet of MDF.
My $.02, YMMV, yada yada yada
Hafte