Vacuum V.S. Press

[ed]

I've done a fair ammount of work in composites at University. Whenever we do any work in carbon fibre we always use plastic sheeting and a vacuum pump. The guys who do a lot of work in the stuff have been able to make complete carbon fibre monocoque race car chassis using this method.

I was wondering why a press frame seems favourable to vacuum pumping. Is it just because when people refer to vacuum puming on here they usually refer to using a fridge pump rather than a high powered vacuum pump? The equipment I have access to could produce the same ammounts of pressure most people press at on here, plus we have a large oven for baking off composites.

[endre]

Unless you have an autoclave there is no way you will achieve as high pressure with a vacuume setup (which is limited by atmospheric pressure) as with a pneumatic or hydraulic system. (I have a 4hp vacuume pump, it still delivers just 1/5th the power I use when I press with the pneumatic.) The reason atmospheric pressure is plenty when you press car parts etc. Is because you don't have a wooden core, steel edges, rubber, base and all those other materials which should fit perfectly together, and the trapped epoxy ponds in between the parts should be squeezed out. Because of the stiffness of all these materials you really don't need the extremely evenly distributed pressure which is the main benefit of a vacuume setup. The stiffness of the base and topsheet gives the ski a smooth surface anyway.

Plain vacuume in short: Great for the soft stuff, not quite enough power and too precise for the stiff stuff.

[big_game]

I'd like to hear more on this as well. I really like the idea of having a safe vacuum press over a dangerous hydraulic press, and I also like the precision of vacuum pressing.

However, I know you will never get 50psi w/ a vacuum. Is there any two step method for vacuum pressing.

Also, forgive my ignorance on this one, but if you double or even tripple bagged somehow (and this is a hypothetical), wouldn't that give you say 15psi + 15psi + 15psi = 45psi. Three times the work and parts though, maybe not a great idea....

[shralpster]

Using more vacuums won't work. It's all about the pressure difference between the inner and outer surface of the bagging material. Once you've removed all atmosphere and there's a total vacuum, you can't reduce any further. It would be like trying to create a vacuum in space... it's already a total vacuum, so there's nothing left to suck out.

[big_game]

yes very true..... vacuum definitely is not viable the more I look at this. The firehose setup is very good, safety concerns me of course.

[Idris]

yes very true..... vacuum definitely is not viable the more I look at this. The firehose setup is very good, safety concerns me of course.

Vacuum is very viable for ski building, just you don't get high pressure.

[hafte]

I'll second that notion. My vac bagged skis work great! The only “advantage” I see in the pneumatically pressed ski is that the laminate is a bit thinner and lighter since a bit more epoxy is squeezed out. Which in my mind could lead to dry laminates and delamb, camber loss issues. I can only pull about 21.5” HG with my venturi system and usually run 19” HG so the system doesn’t have to work so hard.

So far I no one has quantified the advantages of a pneumatic press over a vacuum press showing a meaningful gain in quality of the finished product, or in easy of use.

It’s a simple system you can use in a limited space i.e. your kitchen, a hallway, or a spare room. Just cover the floor and work surfaces with plastic so you don’t make a mess.

For your first few attempts, or indefinitely in my case, the vacuum bag is a good way to go. If you’re interested, I have posted some links to articles in other threads on this board about folks using old vacuum cleaners to generate the vacuum with masking tape and sheet plastic to make the bag. And he is making things as big and high performance as surfboards.

My $.02 for the day

If anyone has some info that shows the advantages of a pneumatic press as it relates to the strength of the finished product I would love to learn more.

Thanks


Hafte

[MartinJern]

Unless you have an autoclave there is no way you will achieve as high pressure with a vacuume setup (which is limited by atmospheric pressure) as with a pneumatic or hydraulic system.

True, but I dont think u can compare the pressure u get when vacuum baging and pneumatic pressure.
I think the reason vacuum baging works with such little pressure is that u suck out all the air.
With the pneumatic setup u will only get the mecanical force and therefore u need more pressure to remove the air. what do u think, does it make sense? I´d like to think so, i just love the simplyness of vacuum baging!

[big_game]

I can borrow pretty heavy duty pumps from work and set up a system... shouldnt be a big deal for that. I just don't want to do anything too complicated for my first try and I also like that you can see what your doing with vacuuming.

What about sucking epoxy out, anyone had problems with this? Is there any way to pre-treat so that more epoxy is absorbed to offset the lower pressure?

thanks guys... oh by the way, this site is the shit

[King_of_the_Hill]

When you use a vacuumpot between the vacuumbag and the pump the epoxy will flow in the pot. The pot is an airtight container with two connections a the top and a small cup placed at the bottom, by sucking out the air a vacuum is created and the epoxy will flow into the cup.

[Bucky Mullet]

I built my skis using a vacuum pump, and am currently trying to build a combo Vac / pneumo press. I got my pump from ebay quite cheaply, it's a "roughing" pump used for backing high vac systems, or clearing out atmosphere's in chemical environments in order to backfill with inert gasses, but I digress. A pump like this will pull at least 50 mtorr if it's in good shape (keep your eyes out for these if you are wanting one, there's a generational change in the industry right now where everyone is getting rid of their oil based pumps to replace them with oil-less scroll pumps).
The thing is, whether your pump pulls 1 torr, or 50 mtorr, you are still pretty close to 14 psi (less if you live at altitude). So you can pay more and more for a pump that will achieve ultra-high vacum, but the forces remain the same.
I like the vacuum setup however because you get really good impregnation of the epoxy into the fiber, as well as gaps in your core glue-up, imperfections in the wood (knots) etc... . Combining this with a pneumatic press, one can get away with running 35 psi and true pressure on the ski would be 49 psi. Thus one can use lighter I-beams in the press frame.
I'm also inclined to believe that heat is a factor as important as pressure, so that a vacuum press with a heating blanket setup may perform pretty near the high pressure setups.
One thing though is that Venturi setups are quite inefficient and don't really work all that well. Unless you are recycling you water, invest in a pump from ebay, or go looking for a retired oil-based pump from an appropriate business (small chemical plant or whatever).

[davide]

... because you don't have a wooden core, steel edges, rubber, base and all those other materials which should fit perfectly together, ...
Plain vacuume in short: Great for the soft stuff, not quite enough power and too precise for the stiff stuff.

Vacuum is no problem for skis. Just pre-bend the core and the edges, and you will not need the extra pressure to push them together.
Industry use penumatic because they can afford it, and they have large production to be pressed as fast as possible.
For few, handcrafted skis, vacuum bagging is great.

Autoclave was very popular for airplane carbon parts, but they have started to shift to normal vacuum bagging, because is much cheaper. It is possible to have no air bubbles when pressing large pieces just by ramping the temperature up quicky and using infusion system.
I read a paper on the subject few months ago.

[doughboyshredder]

. Combining this with a pneumatic press, one can get away with running 35 psi and true pressure on the ski would be 49 psi. Thus one can use lighter I-beams in the press frame.
).

If you have your vacuum at 15 psi and then put it in a press at 35 psi, there will be 35 psi on your board.

[LifeisRiding]

anyone use a heat blanket on their vacum setup?
i was thinking 180f might soften up the bag too much..

[hafte]

LifeisRiding, poly bag material might get soft. They do make some bagging material that can deal with high temps. I get mine here.
http://www.fiberglasssupply.com/Product ... gging.html its the WL 7400 36" Diameter Nylon Bagging Tubing. You can use it as a tube or cut it into sizes you want to use and use sealant tape around the edges. Might cost all of $5.00 for a pair of skis. Less if you buy more.

Hafte