i want to build a big 6'x6'x6' oven for powder coating and some vacuum bagging. it will need to seal and hold pressure for the vac bagging.

the idea i have at the moment is to use perforated angle iron to frame it, put sheet metal inside the frame weld all the seams and weld to the frame, then insulate it and sheet metal the outside of the box. and use the thermostat and elements from an electric range and oven to heat it. and some kind of sliding rail system for the inside top

the main thing im having trouble picturing is how to make the door and how to hinge and seal it, and i have to figure how to get the heating elements in and sealed.

if anyone has an oven like this they have built or bought can you please share pics and/or ideas. if i get a solid plan together i'll probably turn this into a build thread but i need more info at this point

 

Um i dont know about the vacuum part but if u go to a major aplliance parts supply im sure they have the ceramic insulated connectors for the elements to get power from and all the electrical parts needed to accomplish the oven part, hell find some old ovens to salvage the insulation and elements plus the outside sheeting

i just installed a new oven and the elements arent on the inside but beneath the interior sheeting so u could make the inside of the case out of thinner ss weld it up and then make provision in the outer shell for maintenance of the elements (make the panels removable and self insulated that should get u closer to ur goals

 

I think this is an interesting project. I always brainstorm about these sorts of things that I can build myself on my drive to work, haha! I, too, have been thinking about constructing a heaving system, but it's for a different purpose. I have a few points I would like to share:

I'm not familiar with making composites, but how much vacuum does the vacuum bagging process require? I ask this because I have a feeling that the pressure differential between the inside and outside of the oven would be enough that the perforated angle frame and the sheet metal skin wouldn't be strong enough.

Try to make the door as simple as possible. I would use barrel hinges in conjunction with a manual lever locking system (much like "ship doors"). You can purchase high-temperature seals from McMaster to seal in the pressure/vacuum and you can make the lock levers yourself.

How accurate do you want this oven to be? House ovens are very inaccurate and have hot/cold zones within them. If I were to build this, I would use several thermocouples, each monitoring a "zone" in the oven. This way, you get a more even temperature throughout the oven. They usually use J or K type thermocouples in big industrial ovens. I would also use one heating element per zone, or at the very least, use a low-velocity air circulation system in the oven to keep the temperature even.

Best of luck!

 

http://forum.caswellplating.com/oven-building-forum

 

motorhead: doing the vac bagging all the vac is inside the bag and as low as i can get it, and then placed in a pressurized oven, so if i could get the oven to just 30psi and the bag at 28" hg then that would be about 4x atmospheric pressure i think that should be enough for what im doing and shouldnt be too much press for the oven.

rotary: holy ****! awesome link, thank you. i have to go read now!

 

no problem, you should try the powder Caswell sells they get their powders from high quality company's like Tiger Drylac also every few months they have buy a pound get a pound for free.

 

Just FYI, a 6ft side at 30psi will be experiencing 155,520lb of force

 

he could reinforce the walls with 1 by 8 metal studs from a hardware store and space them like every 6 inch

 

its going to be very difficult to make a square oven with the capability to be pressurized to 30psi. The forces are extremely high . There is a reason why autoclaves are round with dished ends. Remember that 30 PSI means Pounds per Square Inch which means that every inch of the oven will have 30 pounds of force on it trying to make a ball out of your square oven. A door would have an enormous amount of energy if it were to fail.

 

yeah i guess i forgot to do the math on that, lol. i'll have to figure out what spacing i'll need on the studs. i'll probably also run horizontal studs with everything fully seam welded.

 

so iv'e been thinking about this whole **** ton of pressure thing pushing out on the inner walls, and thought of making the inner to outer wall cavity 1-2" and filling it with concrete. this should solve the insulation issue but do you guys think it will be strong enough? of course i will still have vertical studs and tie in some horizontal re-bar.

i also still want this to be somewhat portable does anyone know a good source for HEAVY-DUTY casters?

 

Give up on the pressurization thing. Not gonna happen.

 

Dude, what you are trying to construct is an autoclave. The easiest way to go is cylindrical steel tube, a lot like a submarine. The door is going to be the main concern for a pressure vessel. Don't expect to pay less than $7,500 for an effective 30 psi system, home brew style.

What type of parts are you trying to make?

 

There is a reason why autoclaves are $200k-million and arent sold to the general public...

 

there was a reason why computers used to cost $200k-million and not sold to the public also but here i am typing on one.

iv'e always believed that anything is possible or buildable within reason. and i dont think a 30 psi box is that unconcievable. also im pretty sure autoclaves are rated for a lot more than 30 psi, but i could be wrong, and for safety and liability reasons usually those ratings are only one third of what the device is actually capable of.

for a fab forum i figured there would be a little more forward thinking than this.

 

Find a decommissioned submarine and hook up your air compressor to it. ;-)

I would suggest a cylinder shape over a box so the sides don't bulge out. I used to run an autoclave at a composites company in WA. Nitrogen is used for the pressure inside because it's a pretty stable gas at the high temps.

If you are wanting to get high compaction pressure on composites a heated platen press would be way easier to make. Basically the concept is to use fire hose and inflate it up to 100 psi and use the pressurized bags to press down on your composite parts. A two sided mold would be used and then you would have two tool surfaces instead of one bag side.



A picture says a thousand words...That's a snowboard press.

 

thats definitely an idea i'll keep for smaller parts, but is there a larger version?

also, when you were running the autoclave what kind of pressure and heat were you using and for what kind of parts? also was vac bagging used along with the pressure of the autoclave, or is it kinda redundant to use both methods together

 

What do you need an autoclave for? What are you making?

Yes, you're right. Computers were very expensive and not available to the public. But they didnt produce the pressure of 3-4 atmospheres at 250+ degrees.

Are you trying to make dry carbon parts? Is this something you're trying to make a living off of? Im just trying to understand what you're trying to accomplish here. You can definitely make a straight up oven. But a autoclave could be out of the question. Im sure there are tons of regulations and licenses involved in owning one. Im pretty sure its not something you want to operate in the garage of your home.

 

I was making test panels for new materials made for the Joint Strike Fighter. The smaller autoclave was 6' long by 36" diameter went up to 140 psi. The larger autoclave that was 25' long by 10' diameter went up to 110 psi.

90 psi is the standard pressure for all the carbon prepreg I've worked with. If you have to much pressure on the part it can actually squeeze the resin back out and dry the fabric. Bagged parts go in with vacuum being pulled, when the autoclave completes ramping up the internal pressure with Nitrogen the bags are vented to open atmosphere.

Some of the more common prepreg cure temperatures are 250F and 350F. There are lower cure temp prepregs available though. The cure cycles are detailed in how many degrees rise per minute, the amount of dwell time at max temp, and the cool down ramp. All of which coordinate with the pressure rise. Each type of resin can have a different recipe.

What type stuff are you making?

 

i dont really have any exact plans on what ill be making, but i'd like to be able to make hoods and fenders, at this point more for personal projects than to sell. i've done a ton of fiberglass work and a little wet layup and vac bagging with carbon, but ive always wanted to try working with prepreg. i want to make the oven big so i can powder coat pretty much whatever i want and i figured if i could kill two birds with one stone i'd try to through some pressure into it.

will there be a problem making non structural parts with prepreg if i cant get the pressure that high?


edit: for a side note, i just did the math on the size and pressure of the autoclaves you were using. the smaller one at 140psi has over 10.254 MILLION pounds of pressure and the larger at 110psi has over 373 MILLION pounds of pressure trying to get out. if made to 6x6x6ft at 30psi would only be 933120 pounds of force, still a very scary number and i think if i do get this thing built i'll probably dig a big hole in the ground to put it in for its first couple test runs. lol.

 

Vacuum pressure is usually just fine.

 

good to hear! these last few post really have me questioning whether or not i want to risk blowing a wall or two out of my garage, although im still tempted to do it just to prove i can


oh, do you have any idea how thick the walls on those autoclaves were?

 

Seriously dude you have no idea the forces you are dealing with...Have you ever even taken basic statics? Or even have any sort of engineering background at all? I'm not trying to be an *** or anything, but building something like this without knowing what you are doing makes you a prime candidate for a darwin award.

 

Probably quite thick... Every autoclave ive ever seen pictures of looked like the internal space was a 1/3 of the actual outside dimension. They are usually cylindrical shaped too, but im sure cmerce would be able to correct me.

I do ALOT of vacuum bagging. I know you said you have done some, but did you do vacuum infusion? Thats the best way to keep the resin to fiber ratio extremely low almost reaching prepreg fabrics.

 

It's hard to tell how thick the autoclave walls are because I believe they are hollow like a thermos to help with insulation. I would estimate 2-3" worth of metal, but spaced apart to appear 8" thick.

It would be easy to calculate the forces inside there with a little bit of applied statics knowledge, pressure vessels are made for everyday things all the time. High pressure transportation tanker trucks, underwater equipment, etc. Although I wouldn't try making one myself with my 110v welder. I would recommend finding an industrial welding company with experience making pipelines. Larger diameter pipe can be sourced that is rated for high pressure or just the 30 psi you are talking about. The fabrication company would have certified welders that are experienced with heavy metal welding. Cap off one end, and find a submarine hatch door for the other end and call it a day.

But then there is the heating elements and computer controls that you'd have to integrate. For the autoclave I used to work with it would take $250 worth of Nitrogen to fill it and the gas heater was expensive to run. So consider all of that too. Making it is definitely possible, it's all about how much you have to spend.