DIY Autoclave

[ColinB]

Ok, First off what im about to describe to you i neither endorse nor recommend, this device is a pressurized vessel and as such if poorly constructed can cause death and mayhem so consider this article a work of fiction and do not ever attempt to recreate this yourself, all information here was conducted by a highly trained stunt man in a controlled environment. so DONT TRY IT AT HOME! and if you do dont blame me if it goes badly

 

 

ok so now i have abolished any blame for anyones stupidly let me explain to you the fun bits

 

The autoclave was constructed out of a 1 meter long propane cylinder with roughly a 370mm dia.

 

the flanges were sent to a water cutting company and came back as nice metal objects (i will include the dxf for the flanges incase they are of interest) the thickness is 12mm in mild steel with a grove machined into one flange for the seal to sit in, and then smeared with a high temp silicone grease to create a tight seal (superlube)

 

the heater element and fan are just domestic oven parts very cheap to get hold of (i got mine on fleabay) and are controlled using a PID temperature controller (also a fleabay item)

 

there are 2 air fittings as seen in the picture, 1 is for the vacuum line into the clave to the part and the other is for the airline to connect too, the normal valve on the tank is used as an air release

 

if you do decide to ignore what i said about not building one of these then please make sure on the pressure test you fit the cylinder with water as much as you can, this way the uncompressable water reduces the amount of compressed air inside the cylinder and if there is a burst the amount of energy is significantly reduced

 

 

 

 

 

 

ring.dxf

[TMann]

Nice post!

I would have loved to have had something alon those lines for the CS Spar and Canard. Just being able to get those parts up to the 150-160 range would be nice. Beyong that and I think you would be damaging the foam.

 

I used the DAU (Domestice Autoclave Unit) located in the kitchen to do some post curing as well as heating small parts to correct shape and the like.

 

Question though:

Why pressurize the vessel? If it was plumbed for a vacuum line, wouldn't that obtain the same objective?

[mak790]

I would have loved to have had something alon those lines for the CS Spar and Canard. Just being able to get those parts up to the 150-160 range would be nice. ...

TMann if you are looking for something like "homemade oven" try these links

 

Extremely simple and cheap

http://www.privatedata.com/byb/rocketry/composites/ovens/Airframe%20Composite%20Curing%20and%20Post%20Curing%20Oven.html

http://www.jrockdale.com/curing_oven.htm

 

This one is really nice and big but is also $$$.

http://www.dpcars.net/dp1v8/db.htm

 

Seb

[ColinB]

well dont forget the foams i use are capable of 350f so that isnt an issue for me

 

 

the reason for the extra pressure is consolidation

 

if you can imagine the force pressed onto the laminate at 14psi (atmospheric pressure with vacuum) then if you pressurize the cylinder to 90 psi, you end up with a huge amount of force pushing the lamination into the mold and compressing the layers or even if you have a 2 part closed mold, by time you take into consideration the surface area of the mold you have several tons of force compressing the mold shut (and squeezing the laminate onto the core)

 

this pressure is especially relevant when using prepreg materials to achieve good consolidation

[TMann]

this pressure is especially relevant when using prepreg materials to achieve good consolidation

Hmmmmm ....... well above my pay grade as far as projects go.

That king of pressure would result in damage to our foam core method of building but I can see where it would be applicable in prepreg applications.

 

I ran into some problems earlier in the year dealing with foam cores at the joints where they were glued together (3M 78) just using regular VB technique.

 

Cool stuff all around though.

[ColinB]

Hmmmmm ....... well above my pay grade as far as projects go.

That king of pressure would result in damage to our foam core method of building but I can see where it would be applicable in prepreg applications.

 

I ran into some problems earlier in the year dealing with foam cores at the joints where they were glued together (3M 78) just using regular VB technique.

 

Cool stuff all around though.

 

well the idea of this post is to show that its not actually above anyones paygrade

 

you can still use wetlay in a clave and use the extra pressure for consolidation, you just adjust the temperature accordingly (or use a higher temp curing resin)

[TMann]

TMann if you are looking for something like "homemade oven" try these links

 

I'm checking them out.

Looks good so far. Really wish I has seen this 6 months ago.

[TMann]

well the idea of this post is to show that its not actually above anyones paygrade

....... by that I meant I won't be going the prepreg route. Interesting stuff though.

I want to play a bit with vacuum infusion as well. Maybe after that I'll have an opportunity to look into some of the prepreg stuff.

 

This is good stuff ...... especially for winter building.

I work mainly out of my basement so I've been using electric blankets as my heat source. Even during the summer months, I've been adding this to bring the temp up a bit during the cure.

[ColinB]

yeah the great thing about working with pre-pregs is that you can do it in a nice cool clean room (air conditioned) and it doesn't make a huge mess everywhere

 

its the consolidation from the extra pressure that really gives the components the better quality and increased performance

 

so you can still get benefits using a pressure vessel even if the cure of the parts is only at room temperature (you can use an extra slow hardener then use a slightly elevated temp to send it off giving you a bit of extra time to lay up and bag)

[argoldman]

yeah the great thing about working with pre-pregs is that you can do it in a nice cool clean room (air conditioned) and it doesn't make a huge mess everywhere

 

its the consolidation from the extra pressure that really gives the components the better quality and increased performance

 

so you can still get benefits using a pressure vessel even if the cure of the parts is only at room temperature (you can use an extra slow hardener then use a slightly elevated temp to send it off giving you a bit of extra time to lay up and bag)

 

Colin, I am a little confused about the autoclave process.

 

Certainly the pressure increases the strength of the polymer. In using a simlar technique with methyl methacrilate we cure items under water pressure. This stops volatilization (outgassing) of the material as it polymerizes and gives a more dense and stronger product.

 

Is this the same as with the epoxy?

 

If you have a mold and lay prepreg on it and then subject the entire thing to pressure, the air pressure will be the same at the surface of the glass as it will be at the surface of the mold and thus there will be no pressure benefit of adapting the glass better to the mold. If the mold is porous and the pressure escapes through the mold, you will have a pressure differential possibly forcing the glass into the mold--- but you will probably only succeed in bleeding the epoxy from the glass through the mold.

 

The vacuum bagging technique (no heat) actually relies on the plastic film pressing the glass into the mold since the mold is non-porous, the plastic is non-porous and when vacuum is applied air pressure from the outside (greater than the vacuum produced) will try to make the space between the plastic and the mold as small as possible thus adapting the glass to the mold. In that process, there is many times a barrier material and an absorbent material used as excess resin is expressed.

 

It seems that without the differential pressure, there is no further adaptation other than that which existed when the glass was originally placed in the mold.

 

What am I missing???

[TMann]

What am I missing???

Rich, It's a combination of Pressure, vacuum and heat all rolled into one unit.

i.e.

Material goes in the mold.

then sealed with vacuum fim.

Vacuum applied, pressure applied to enhance efficiency of the vacuum.

Heat kicks off the prepreg.

 

Have I got this right

[ColinB]

Colin, I am a little confused about the autoclave process.

 

What am I missing???

 

Im not entirely sure what your asking, but ill try to explain best i can

 

inside the clave the part is put inside in a vacuum bag and is vacuum is applied

then pressure is applied to the clave

 

its the differential pressure between the part inside the vacuum bag and the outside pressure that creates a lot of force pushing the laminate into the mold

[argoldman]

Im not entirely sure what your asking, but ill try to explain best i can

 

inside the clave the part is put inside in a vacuum bag and is vacuum is applied

then pressure is applied to the clave

 

its the differential pressure between the part inside the vacuum bag and the outside pressure that creates a lot of force pushing the laminate into the mold

 

Got it,

 

I was missing the vacuum inside of the pressure.

 

Makes perfect sense now.

 

Thanks for the clarification:)

[trans4md]

I'm relatively green to composite construction and the term prepreg was completely new to me. This thread and some additional research have piqued my interest in prepreg construction. I'm interested in what ColinB is building. Are many others using prepreg materials and methods for canard-aircraft construction? Is this an acceptable alternative to the materials and methods used in the Cozy aircraft?

[TMann]

Are many others using prepreg materials and methods for canard-aircraft construction? Is this an acceptable alternative to the materials and methods used in the Cozy aircraft?

Our planes use the 'moldless' construction technique. Prepreg is used in molded, high pressure, high heat construction technique. Our foam would not stand the heat & pressure.

 

This is the difference between plans built and kit. A comparison might be Long-ez vs.Berkut.

 

To use this method would require a re-engineered approach.

There is a member of our EAA chapter that (and I use the term loosely) built an Epic Aircraft product. The wing skins were all prepreg pre-manufactured with CF ribs. The way it works up there is there are a team of guys who watch you squirt some epoxy on a part and then the team comes in and puts it together in their factory.

[Edge 513]

Its that kinda single splooge crap that is chit-canning our homebuilding amateur-built catagory with the EAA.

It really rankles me.

[TMann]

Its that kinda single splooge crap that is chit-canning our homebuilding amateur-built catagory with the EAA.

It really rankles me.

Yeah ...... feel the same way. It's hard to draw a comparison between my hard fought Long-EZ and his Epic. I mean really ...... an aircraft that is classified as experimental just like mine is ........ but it cones with stairs .

I kid you not. The door pulls dow and you climb the stairs and ride in that pessurized cabin.

 

He wasn't even there when they built it. He hired a retired SR-71 pilot in our chapter to do that and fly off the 40.

[Kraig]

Yeah ...... feel the same way. It's hard to draw a comparison between my hard fought Long-EZ and his Epic. I mean really ...... an aircraft that is classified as experimental just like mine is ........ but it cones with stairs .

I kid you not. The door pulls dow and you climb the stairs and ride in that pessurized cabin.

 

He wasn't even there when they built it. He hired a retired SR-71 pilot in our chapter to do that and fly off the 40.

 

agree, I mean whats the point? Why not just go out and buy a nice big expensive certified plane. You certainly won't be fooling anyone on the ramp at a fly-in.

 

There are builders and there are flyers. The difference is that some builders will become flyers of their craft and craftsmanship. I am under no illusion that it might be impossible to fly my completed Cozy the same number of hours as it will take to build. It does not deter me in the least. I really do enjoy building. And one day, when I am blissfully riding the sky's in it, I will feel even that much more pride in knowing that I had the ability and talents to have created it.

 

Kraig

[trans4md]

To use this method would require a re-engineered approach.

I assumed this would likely be the case, but in the interest of education I'm glad that I asked. Thanks for the response.

[ColinB]

I'm relatively green to composite construction and the term prepreg was completely new to me. This thread and some additional research have piqued my interest in prepreg construction. I'm interested in what ColinB is building. Are many others using prepreg materials and methods for canard-aircraft construction? Is this an acceptable alternative to the materials and methods used in the Cozy aircraft?

 

Pre-preg and autoclaves arnt really something that has been used in home building before

 

you have to understand that this is the latest technology in composites and is usually only used in high end motorsport and aerospace

 

the costs for this kinda thing are normally huge (autoclaves cost a LOT of money)

 

to make use of prepreg you need to have high quality molds that can withstand the temperature and pressures inside the clave

 

as well as this you need to start looking at the materials, in this process you need to use cores that can withstand the t&p's of the process, either honeycombs or high temp foams such as rohacell

 

im sure it will become more clear as my project progresses and you will see how these methods are utilised

[Edge 513]

Not just the autoclaves are hugely expensive. I was at Dave R's UAV-Berkut shop last year and he pointed to a 5 gallon of caulking adhesive used to mate all finished pieces of prepreged parts together and said. "That can of magic costs 5 grand, pretty cool huh?" I looked over at the all black carbon Berkut on sawhorses and whistled. Just a few minutes before we had forced the prepreg into a mold for one of the ancillary nose bulkheads and bagged it and vacced it to its pressure hold condition. Then walked it over to the 14 foot long heat box. No autoclave, just a highheat cure.

MY impression was, this was a mighty "state of the art"...read EXPENSIVE way to build an airplane. Just the TIME[i.e.manhours]to create the molds for everything was enormous Dave said.

 

So this/your thread transferring to an actual EZE animal of some sort will be interesting to see down the timeline. With your expertise, I hope you actually end up with something several years down the road and dont give up.

[ColinB]

oh yes the pattern and mold making can be hideously expensive if you paid full price for the pattern machining and making of the molds

 

 

i know what you mean about the expense of structural adhesives, they can certainly cost quite a lot, a smallish (i think 1L) tin of 3M 9323 is around 60-70 pounds, fortunately it goes quite a long way if you dont waste it

 

also you dont need to use autoclaves, i have made prepreg parts just using oven cure under vac, but you do get a far far better result if you can apply pressure to the bag as well in a clave

[airnico]

I was at Dave R's UAV-Berkut shop last year and he pointed to a 5 gallon of caulking adhesive used to mate all finished pieces of prepreged parts together and said. "That can of magic costs 5 grand, pretty cool huh?"

do you know the name of that adhesive?