I am just getting started and have a question:
What is the drawback to vacuum bagging? If it were possible to create higher pressures than ~15psi would it be a more viable technique or superior to the hydraulic presses many are using?
vaccum bag
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I would say that yes, vacuum bagging has many advantages. Especially when you can increase the atmospheric pressure (i am guessing you are thinking of an autoclave). Cat-track presses provide vertical force, good for sandwhich construction but not much use for cap. With vacuum bagging you can do cap or sandwhich, you have a device literally removing air pockets, and you can even upgrade the process to resin transfer or vacuum infusion...so less messing with resins. The autoclave is a real possibility to do affordable with the right design. I have been planning to do this for a while and will as soon as the space is available! Clearly I recommend vacuum bagging (cheaper startup and tooling aswell).
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doughboyshredder
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Head Monkey
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Brewster wanted me to elaborate on my apparent dislike of vacuum bagging based on my statement in another thread: “IMO, a pneumatic press is so, so, so much better than bagging, but bagging is a great way to get started on the cheap.”
I got started with vacuum bagging, and built boards that way for many years. It is a fantastic way to get started since it is cheap and flexible, and way easier to get started with than building a pneumatic press. And you can make some nice boards vacuum bagging. Lots of small builders have started this way. However, of all the small builders that I know that started vacuum bagging, all of them are using a pneumatic press now, as am I. Don’t get me wrong below… vacuum bagging isn’t all that bad, but you asked why I believe it is inferior to a pneumatic press, so that’s what I’ll explain now…
On the process of building:
1. Vacuum bagging uses a lot of extra disposable materials over a press: bag material, breather fabric, sealant tape. Extra money, extra prep, extra fuss late in the layup cycle, extra trash at the end of it all. Sure, you can try to reuse the bag material, but it’s a pain to salvage and it’s easy to end up with a poor seal next time. To keep cost reasonable you can buy in bulk from excellent suppliers like Airtech International (http://www.airtechonline.com/), but now you’ve got even more big rolls of supplies in the shop.
2. Heating the laminate evenly is far easier with a press than with vacuum bagging. With a press you can place a heat blanket in close contact with the top and bottom of the laminate, which is what you honestly need to produce a good, consistent board. With vacuum bagging you’re more limited. You can try to place blankets inside of the bag with the board… more hassle, more likelihood of screwing up your blankets, etc. You can heat from below with one blanket, and heat from above with another blanket outside of the bag, but that’s not as good. You can heat the top with infrared heaters, but again, not as even and good.
For the quality of the board, the issues come down to: final top sheet quality, voids, and compression of the glass layers.
1. Most top sheet material is somewhat rigid, but not that rigid. Uneven distribution of resin under the top sheet and in the top layer of glass translates into an uneven surface finish on the top sheet. Compare the top sheets of a bagged board and a pressed board. One will be irregular, the other will be super-flat. You can mitigate this by precuring the top sheet with a light weave of glass and a careful spread of resin, but now your back into the extra hassle zone.
2. People assume that if you’re vacuum bagging that there will be no voids in your laminate, no air pockets. Wrong. It’s easy to have voids even when drawing “a vacuum”. You can draw a vacuum out of a small jar, right? The jar doesn’t collapse. It’s rigid enough to withstand the force. You can end up in the same situation vacuum bagging a board. If your core wont bend into the mold completely at ~12psi then drawing a vacuum isn’t gonna bend it in there. You’ll have a void. Maybe it will fill with resin and you won’t notice the weakness, or maybe it won’t and it will have a partial atmosphere in there (because, I assure you, you have not drawn a perfect vacuum.) But the void will be there. It’s really easy to end up with fine air pockets in your laminate vacuum bagging, too, even when your core does bend into the mold. I’ve got more than a few boards with clear top sheets to prove this.
3. The PSI you get on the board with vacuum bagging without an autoclave is quite low. Measure how many in/Hg you’re drawing from the opposite end of the layup. You’ll find you’re not getting a full atmosphere. I used to get 25.5 in/Hg max with a nice, continuous duty Gast vacuum pump. 1 atmosphere is 29.92. Do the math and you’ll find that translates to about 12.5psi, assuming I’m at sea level and the weather is nice. If I’d drawn a perfect vacuum, I’d still have only gotten, what? 14.7psi or so, again assuming nice weather. The effective PSI you can get on the board with a pneumatic press is significantly better. The net result is you don’t get as good squeeze-out with a vacuum bag, and you end up with a thicker, heavier laminate.
The way you correct these issues is with more pressure, and the easiest (and I believe safest) way to get this is with a pneumatic press. You also get a nice, smooth aluminum plate over the top of your board with a press, which helps impart that nice smooth surface finish. This is quite difficult with a bag.
Now, you can build an autoclave and increase your pressure when vacuum bagging. I’ve never done that, so I can’t give you first-hand experience like I have above with a normal bagging process. However, I’d imagine it adds to the hassle factor. Now you have the added step of getting the bagged board into the autoclave and pressurize the thing. Now you’ve got a pump and a compressor going. Great, two points of mechanical failure now, in addition to the extra materials and the more difficult heat transfer. This is getting better why?
The difference in my boards when I moved to a press was night and day. Thinner, lighter boards with significantly more consistent and improved physical properties.
So, that’s why I like a press better than vacuum bagging. Again, don’t get me wrong, vacuum bagging isn’t horrible, and you will produce some nice results with a bag. I’d would simply assert that you can produce better results with a press.
One final note: cap construction often gets tied up with vacuum bagging, since it’s darn hard to do cap on a small scale with a press, but it’s easy with vacuum bagging. A word of caution: it is very, very hard to get a good pinch of the top sheet material over a cap with the pressure you get from normal vacuum bagging. This means you end up with voids right over the edge, right where you don’t want that weakness. I highly recommend sidewall construction, especially if you are vacuum bagging.
Good luck!
I got started with vacuum bagging, and built boards that way for many years. It is a fantastic way to get started since it is cheap and flexible, and way easier to get started with than building a pneumatic press. And you can make some nice boards vacuum bagging. Lots of small builders have started this way. However, of all the small builders that I know that started vacuum bagging, all of them are using a pneumatic press now, as am I. Don’t get me wrong below… vacuum bagging isn’t all that bad, but you asked why I believe it is inferior to a pneumatic press, so that’s what I’ll explain now…
On the process of building:
1. Vacuum bagging uses a lot of extra disposable materials over a press: bag material, breather fabric, sealant tape. Extra money, extra prep, extra fuss late in the layup cycle, extra trash at the end of it all. Sure, you can try to reuse the bag material, but it’s a pain to salvage and it’s easy to end up with a poor seal next time. To keep cost reasonable you can buy in bulk from excellent suppliers like Airtech International (http://www.airtechonline.com/), but now you’ve got even more big rolls of supplies in the shop.
2. Heating the laminate evenly is far easier with a press than with vacuum bagging. With a press you can place a heat blanket in close contact with the top and bottom of the laminate, which is what you honestly need to produce a good, consistent board. With vacuum bagging you’re more limited. You can try to place blankets inside of the bag with the board… more hassle, more likelihood of screwing up your blankets, etc. You can heat from below with one blanket, and heat from above with another blanket outside of the bag, but that’s not as good. You can heat the top with infrared heaters, but again, not as even and good.
For the quality of the board, the issues come down to: final top sheet quality, voids, and compression of the glass layers.
1. Most top sheet material is somewhat rigid, but not that rigid. Uneven distribution of resin under the top sheet and in the top layer of glass translates into an uneven surface finish on the top sheet. Compare the top sheets of a bagged board and a pressed board. One will be irregular, the other will be super-flat. You can mitigate this by precuring the top sheet with a light weave of glass and a careful spread of resin, but now your back into the extra hassle zone.
2. People assume that if you’re vacuum bagging that there will be no voids in your laminate, no air pockets. Wrong. It’s easy to have voids even when drawing “a vacuum”. You can draw a vacuum out of a small jar, right? The jar doesn’t collapse. It’s rigid enough to withstand the force. You can end up in the same situation vacuum bagging a board. If your core wont bend into the mold completely at ~12psi then drawing a vacuum isn’t gonna bend it in there. You’ll have a void. Maybe it will fill with resin and you won’t notice the weakness, or maybe it won’t and it will have a partial atmosphere in there (because, I assure you, you have not drawn a perfect vacuum.) But the void will be there. It’s really easy to end up with fine air pockets in your laminate vacuum bagging, too, even when your core does bend into the mold. I’ve got more than a few boards with clear top sheets to prove this.
3. The PSI you get on the board with vacuum bagging without an autoclave is quite low. Measure how many in/Hg you’re drawing from the opposite end of the layup. You’ll find you’re not getting a full atmosphere. I used to get 25.5 in/Hg max with a nice, continuous duty Gast vacuum pump. 1 atmosphere is 29.92. Do the math and you’ll find that translates to about 12.5psi, assuming I’m at sea level and the weather is nice. If I’d drawn a perfect vacuum, I’d still have only gotten, what? 14.7psi or so, again assuming nice weather. The effective PSI you can get on the board with a pneumatic press is significantly better. The net result is you don’t get as good squeeze-out with a vacuum bag, and you end up with a thicker, heavier laminate.
The way you correct these issues is with more pressure, and the easiest (and I believe safest) way to get this is with a pneumatic press. You also get a nice, smooth aluminum plate over the top of your board with a press, which helps impart that nice smooth surface finish. This is quite difficult with a bag.
Now, you can build an autoclave and increase your pressure when vacuum bagging. I’ve never done that, so I can’t give you first-hand experience like I have above with a normal bagging process. However, I’d imagine it adds to the hassle factor. Now you have the added step of getting the bagged board into the autoclave and pressurize the thing. Now you’ve got a pump and a compressor going. Great, two points of mechanical failure now, in addition to the extra materials and the more difficult heat transfer. This is getting better why?
The difference in my boards when I moved to a press was night and day. Thinner, lighter boards with significantly more consistent and improved physical properties.
So, that’s why I like a press better than vacuum bagging. Again, don’t get me wrong, vacuum bagging isn’t horrible, and you will produce some nice results with a bag. I’d would simply assert that you can produce better results with a press.
One final note: cap construction often gets tied up with vacuum bagging, since it’s darn hard to do cap on a small scale with a press, but it’s easy with vacuum bagging. A word of caution: it is very, very hard to get a good pinch of the top sheet material over a cap with the pressure you get from normal vacuum bagging. This means you end up with voids right over the edge, right where you don’t want that weakness. I highly recommend sidewall construction, especially if you are vacuum bagging.
Good luck!
Everything I know about snowboard building, almost: MonkeyWiki, a guide to snowboard construction
Free open source ski and snowboard CADCAM: MonkeyCAM, snoCAD-X
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Vacuum with catrack
In reply to Mikes comments on vaccum pressure.
It's true that you can attain 12.5 psi, but what you have to remember is that it's 12.5 psi over the entire pressed area where as a bladder produces its pressure over maybe a 3" wide section of cat track. What I have been doing is using a cat track under a vacuum bag. I press 1 ski at a time and use a 7.5in wide cat track. This will produce the equivalent of a 12.5x7.5/3=31.25 psi on the ski. Although it's still not as high as a pressess it's still not that shabby. If you want to increase the pressure on the ski just make a wider cattrack.
I will have to agree that the hassels are there with vaccum bagging compared to a press but I've got a system now that I have an adjustable mold that can slide into a multi resued bag (20 presses so far) in 5 seconds by one person. No breather or tape /release flim required. This is basically a pneumatic press system (mold/top cover/cattrack) using a bag instead of a hose. This mold can be completley disassembled and put away along with the bag so I can have my garage back. I don't have any way to heat the mold other than a tarp with a space heater.
I don't think my setup will hold a candle at the production capacity of a press but for a home builder with little storage room it's pretty good.
Just my 2 cents
Bill
BRD Factory
It's true that you can attain 12.5 psi, but what you have to remember is that it's 12.5 psi over the entire pressed area where as a bladder produces its pressure over maybe a 3" wide section of cat track. What I have been doing is using a cat track under a vacuum bag. I press 1 ski at a time and use a 7.5in wide cat track. This will produce the equivalent of a 12.5x7.5/3=31.25 psi on the ski. Although it's still not as high as a pressess it's still not that shabby. If you want to increase the pressure on the ski just make a wider cattrack.
I will have to agree that the hassels are there with vaccum bagging compared to a press but I've got a system now that I have an adjustable mold that can slide into a multi resued bag (20 presses so far) in 5 seconds by one person. No breather or tape /release flim required. This is basically a pneumatic press system (mold/top cover/cattrack) using a bag instead of a hose. This mold can be completley disassembled and put away along with the bag so I can have my garage back. I don't have any way to heat the mold other than a tarp with a space heater.
I don't think my setup will hold a candle at the production capacity of a press but for a home builder with little storage room it's pretty good.
Just my 2 cents
Bill
BRD Factory
Thanks for your comments. For now, I will have to take your word on many of the comments particularly as it regards to additional hassles and mechanical failures.
However, the more I think about this the more I am wondering if a uniform pressure distribution is really what is required. The primary reason why aerospace engineers use autoclaves for composites is because these typically have complex geometries (such as a wing) and require high temperatures where a simple press won't work. In the case of a ski/board you are essentially constructing a 2D structure. The 1D pressure distribution forces the topsheet to be flat regardless of the underlying structures. Imagine epoxy as being a somewhat loose water balloon. If you want to make your water balloon as flat as possible, you smoosh it between your hands (1D pressure). If you were to put that water balloon under uniform pressure nothing would happen (water is incompressible)!
It would be cool to make an identical layup: one with 40psi in a press and the other with 40psi in an autoclave and to see the result.
Mike,
You mention seeing visible voids through a clear topsheet while vacuum bagging. Have you ever seen voids while using your press?
However, the more I think about this the more I am wondering if a uniform pressure distribution is really what is required. The primary reason why aerospace engineers use autoclaves for composites is because these typically have complex geometries (such as a wing) and require high temperatures where a simple press won't work. In the case of a ski/board you are essentially constructing a 2D structure. The 1D pressure distribution forces the topsheet to be flat regardless of the underlying structures. Imagine epoxy as being a somewhat loose water balloon. If you want to make your water balloon as flat as possible, you smoosh it between your hands (1D pressure). If you were to put that water balloon under uniform pressure nothing would happen (water is incompressible)!
It would be cool to make an identical layup: one with 40psi in a press and the other with 40psi in an autoclave and to see the result.
Mike,
You mention seeing visible voids through a clear topsheet while vacuum bagging. Have you ever seen voids while using your press?
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Head Monkey
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That’s correct, it is 12.5 psi over the entire pressed area. I calculate the psi I get via my press using the surface area of the snowboard, the contact patch of the bladders, etc. The psi comparisons are fair, and you will double or triple your effective psi with a pneumatic press immediately.breid19 wrote:
It's true that you can attain 12.5 psi, but what you have to remember is that it's 12.5 psi over the entire pressed area where as a bladder produces its pressure over maybe a 3" wide section of cat track.
Your vacuum setup sounds cool. Nice work! Again, don’t get me wrong, vacuum bagging isn’t bad. But I was asked to compare…
Correct! When the lamination is simple, like with a ski or a snowboard, then a pneumatic press is good. When it gets too geometrically complicated then you’ll see people switch to other methods. You’d be amazed where I see things just like our pneumatic press showing up… the ski/snowboard industry did not invent these things, that’s for sure! A friend built an absolutely amazing custom home in North Vancouver, BC, and he jointly developed a new glue lamination technique in conjunction with a company that makes large glue-lam beams for wood construction homes. He showed me pictures of them making the laminations that involved pneumatic presses, complete with yellow fire hose, that were on the order of 50ft long. Amazing stuff.brewster wrote: The primary reason why aerospace engineers use autoclaves for composites is because these typically have complex geometries (such as a wing) and require high temperatures where a simple press won't work. In the case of a ski/board you are essentially constructing a 2D structure. The 1D pressure distribution forces the topsheet to be flat regardless of the underlying structures.
I would also like to see the result of that!brewster wrote: It would be cool to make an identical layup: one with 40psi in a press and the other with 40psi in an autoclave and to see the result.
brewster wrote: You mention seeing visible voids through a clear topsheet while vacuum bagging. Have you ever seen voids while using your press?
No, no voids when pressing, and I have made the same board shape, same construction, and same clear top sheet with a bag and a press. There are voids in the bagged board, and none in the pressed board. The glass layers are about 40% thinner (measured from the cut edge, above and below the sidewall) in the pressed board. Part of this is the better pressure. Another part, I believe, is the heating. The heated epoxy flows like water, so you get more even distribution and better squeeze out, even at the same pressure.
Everything I know about snowboard building, almost: MonkeyWiki, a guide to snowboard construction
Free open source ski and snowboard CADCAM: MonkeyCAM, snoCAD-X
Free open source ski and snowboard CADCAM: MonkeyCAM, snoCAD-X