Autoclaves

[G-man]

From another current thread, Easy wrote:

I think it's wise to open a new topic for auto claves.

Okay, done.

We've had some discussion on the forum in the past regarding autoclaves, but it doesn't hurt to update the earlier thoughts.

I live in a rural area where we don't have an infrastructure that supplies our water, so we have individual water wells (great taste, no chemicals ). The system that moves the water out of the ground and into the faucets in the house is pretty simple and includes a 'pressure tank' that stores and pressurizes the water that is pumped from the ground. Most pressure tanks are 24 inches in diameter and vary in height (length) depending on their storage size rating. Mine is about 56 inches tall. The tanks have a rubber bladder inside that is intended to keep the water and the pressurized air separated. Over time, the bladders develop leaks. The bladders are replaceable, but most people just buy a new tank and take the old ones to the dump. I've replaced a number of bladders over the years and have noted that the tanks themselves are usually in very good shape if the leaky bladder was replaced fairly soon after the leak developed (because an intact bladder isolates the water from the steel tank... so, no rust).

Anyway (I'm getting to my point now), The tanks are rated for well over 100 psi. Two of them welded together would make a really nice looking auto clave. Or, weld a mating flange to the open ends of two tanks and use a boatload of grade 8 bolts to bolt the two halves together after the mold has been placed inside. Plumbing businesses in rural areas often have a few discarded tanks out back that they'd gladly give away so they don't have to go to the trouble of getting rid of them. Our local dump also has a pile of them, so that could be a source if you live near a rural dump.

Okay, thread started.

G-man

[Easy]

Make sure not to kill yourself. Welding reduces metal strength by 50% which is a rough measure. It could be reduced less, but you can also reduce it a lot more.

I think an auto clave won't help much in skibuilding because you don't need that much pressure to build a good pair of ski's. Build a pheumatic press and buy a heatblanket, this should give you more than enough build quality.

However I'm not saying auto claving would not work, so if anyone wants to try I wiss them the best of luck.

[G-man]

Hey Easy, I'm confused. I thought you wanted to have a discussion about autoclaves .

I agree that a pneumatic press is more than adequate at providing sufficient pressure to press a ski. I press at 50 psi and really don't see any need to go higher. However, some builders choose to use vacuum systems... some only have vacuum systems available to them. A discussion about various methods of increasing pressing pressure could be quite helpful to those folks. I've thought about playing around with my vacuum system myself, but up here in the mountains, I can only pull about 21inches Hg (I believe about 11psi)... good for some projects, but not so good for pressing a ski. Placing a vacuum bagging system into an autoclave that is itself pressurized to 20 or 30 psi could really improve the efficiency of the vacuum system.

I'm sure that there are numerous benefits to using a vacuum system, such as being able to visualize the lay-up alignment during pressing. Also, the bag itself would help to better contain the resin residue, resulting in less mess... oh, and vacuuming requires no top old. A safe autoclave device could be a bit labor intensive to build, but it still might compare favorably to building a pneumatic press, especially if a builder had a pre-existing affinity for vacuum pressing. There are many options for heating an autoclave.

Hmmm... the more I think about it, the better vacuum pressing (with an autoclave) is beginning to sound. I'll probably not get round to really trying it because I have so many other projects in the works. But I really do think that vacuuming is a viable option for pressing a ski and, therefore, worthy of some discussion.

Good heads-up, Easy, regarding the reduction in metal strength caused by welding. Because the water pressure tanks are welded during their construction, I'm wondering if their psi rating is already calculated/reduced to compensate for the welding.

G-man

[bigKam]

hey G-man,

i'll talk about autoclaves with you. it's an interesting idea. some time ago Kelvin came up with a similar idea to the one you mentioned to put a vacuum bag inside a high-pressure chamber, but heat wasn't considered if i remembered correctly. at the time, we were interested in making cap skis using this method. i think there's a place for the autoclave for ski building: vacuum and heat is a great combination. i've used a vacuum press (to make the GK's) and i found that it was good option -- only pulled down to 11 psi though because of leakage issues. quality of ski was good. i liked the fact that there was no top mold and alignment of materials was easy to control; however, i've yet to try the latest method which borrows your aluminum cassette idea -- hope it all works out.

one thing i noticed about the vacuum press was it seemed like it 'sucked' out more air bubbles beneath the topsheet...

maybe next season i'll drop by for a visit and we can have some root beer then go over to the dump and i'll roll away with one of those cans for a try...

[G-man]

Hi bigKam,

I guess I've managed to confuse folks. I'm not actually thinking (at this time anyway) about building an autoclave myself. I started this thread because another forum user expressed an interest (via another concurrent thread) in having an autoclave discussion. Even though I do find vacuum pressing systems intriguing and would like to someday mess around with them some, I was mostly trying to facilitate a discussion by throwing out some ideas. Its always interesting and fun to watch all of the experimenting that goes on at the skibuilders forum... I'll encourage whenever I can.

Anyway, thanks for jumping in. I do find your observation regarding less air bubbles interesting. Some two-part resin based polymers require a 'vacuum degassing' process prior to use (after mixing the two parts) in order to remove trapped air bubbles. It seems that pressing a ski in a vacuum press probably performs a similar function in helping to remove trapped gases in the epoxy. And yes, if someone is trying out capped construction, vacuum is a good option.

I always have a cupboard full of root beer (good memory by the way). I even have an old pressure tank out behind the shed. I'm planning a trip up your way sometime this summer. Maybe we can hook up then. Of course, I'll bring skis.

G-man

[mattman]

I personally prefer the vacuum system but want more pressure...so i have always wanted to build an autoclave (tried a couple soft autoclaves with large bladders, but temporarily closing off the ends proves to be difficult). a while back someone on here mentioned that HDPE pipe was the way to go for cheap and safe autoclaves. after looking into it i agree, polyethylene is about the cheapest plastic you can find, so its not too pricey, and the failure is much more controlled than PVC's "explosive personality". but if you have metal for free...cant beat that! (if you in fact wanted to build one ). now if you were to bolt the two halves together, what would be the gasket between the two flanges? and is there a faster way to install all those bolts?
i used to go with my father to the tire retreading plant he ran, and they had large autoclaves for curing tread. with their door they had a simple lever to close it off, then a tubular gasket inflated between the flange and the door...i always liked that method, but seems tough to build for a homebuilder.

[G-man]

Autoclaves are used in hospitals for sterilizing surgical instruments and I've worked with a bunch of them. The door structure is pretty simple. It consists of a couple of 'strap-and-pin' hinges that basically support the door and allow it to swing into the closed position. In the center of the door is a round spin wheel, similar to what is used on a ship's hatch/door. The spin wheel drives movable pins that tighten and secure the door to the gasketed door mating surface. So, if I were actually going to build an autoclave, I'd probably just weld a couple of pressure tanks together to get the length I wanted, then cut off one of the ends and fashion it into a door. I'd install nice rollers in the autoclave so that I could easily roll the vacuum bagged mold assembly in and out. I'd roll the mold in, close down the hatch, pressurize to no more than 40psi,... and run ... well, at least I'd I'd stand clear of the end with the door.

I definitely think it's doable, but mattman is right, it would be a bit tough for a hobby builder.... but, doable.

G-man

[mattman]

I am not totally following the pins method...so are the pins on the outside? because you couldnt drill through the door for the wheel, to put pins on the inside (wouldnt be air tight). so if the pins are on the outside, they push into holes along the furthest hoop of the tank? how would this apply pressure against the gasket...unless the pins are tapered?

i always pictured a few handles around the edge of the door, then pneumatic fixturing clamps (like those used as machining hold-downs) along the outer surface of the chamber that would latch/buckle down the handles. a couple of the heavy duty mechanical hold-downs might work too. the problem would of course be buying all these hold downs (pricey)...and then attaching them to the plastic pipe without making holes in the cylinder (with the metal pipe you could just weld them on).

also, how would you run wiring through the wall of the clave to get power to your heat blanket (or to your webcam if you want to keep an eye on your press ). are there air-pressure-tight electical sockets that you could install into the door or back wall of the chamber?

[G-man]

Okay, I've got it. Let's say that you take two old tanks and cut one end off of each (could even be old water heater tanks) and re-weld them together at the center, leaving you with about an 8 foot long tank. Now, let's say that you cut a 14" wide x 16" high rectangular hole in one end of the tank. Next, take the rectangle piece that you cut out and some 1/4" x 2" steel strap iron and weld a border to the back side of the rectangle so that the border extends about 1/2 to 3/4" beyond the perimeter of the rectangle. Place some rubber gasket material on the welded-on flange and bolt or weld some sort of simple handle to the center of the rectangle. Slip the rectangle inside of the tank opening (angle it a bit to get it through the opening in the tank) and then recenter it over the rectangular opening. Hold it against the opening while you apply a bit of air pressure to the tank. The pressure will hold the 'door' in place without any need for bolts or pins or anything else. The 14" x 16" opening should be large enough to get a mold through... if not, just make the opening an little larger. The door opening doesn't have to be rectangular. It could even be round, but the flange would be more difficult to create.

Regarding the electrical connection, you could use an extension cord that has a molded female end on it. The ends usually taper a bit, so cut a hole in the tank that is shaped like the smaller end of the female plug. Cut the male end off of the cord and feed the cut end of the cord through the hole in the tank, from the inside. Push the female end into the shaped hole so that it's fairly tight and apply silicone seal to the inside and outside. The taper on the receptacle will prevent the pressure in the tank from pushing the plug through the hole in the tank. Attach the ends of the wires (where you cut off the male plug) to your SSR output (from the PID controller). Attach a male plug to the leads on your heat blanket and plug it into the receptacle in the tank (now an autoclave).

Well, how's all of that sound? The same approach be used with other tank materials, even plastics. There's also a lot of old fiberglass water pressure tanks out there. They are also rated for over 100 psi... oooh, that sounds even better. They are not too expensive to even buy new... just cut the ends off and re-glass them back together. Not sure how heat would effect them though.

G-man

[mattman]

ya, we have no welding equipment...but i sure can do fiberglass work!! plus, with the fiberglass tank, you could use the current end of the tank as a mold to make your slightly larger door. i like your door idea alot though...think it would really work? are there any applications where this concept is used currently? vacuum clamping tape (a one side adhesive foam strip) could be used for the gasket really easily...and no bolting needed is a very attractive concept. ok ya, you just got me hung up on the autoclave idea again...when you say fiberglass tanks can be bought cheap...how cheap? i have alot of extra 34oz triax that could be used for some really sturdy doors, so that would make this project far more feasable than it ever was before!

think this would work: http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... &rd=1&rd=1
its 16" diameter and 57" long, so it seems just large enough, but the unknown wall thickness makes me worried that the ID will be too small...guess i could just round the bottom edges of the mold!

[G-man]

mattman wrote:

i like your door idea alot though...think it would really work?

It does seem too good to be true doesn't it. It just suddenly came to me.. but I really don't see why it wouldn't work. Think about it this way... relative to the pressure on the inside of the tank (say 30 psi), there is a vacuum on the outside trying to suck to door out through the hole in the end of the tank. The flange on the door piece prevents this, and the gasket prevents air leaks. And yes, the vacuum clamping tape would work great, but you might need to double up the width if the tape is just 3/8" like mine is.

Yes, the ebay tank is exactly like the one's that I've installed for a few friends. I didn't remember them being that expensive, though. We have a neighbor in the valley who's a well system parts salesman, so he often gets our parts at a reduced price, so I've been a little out of touch with retail prices. Anyway, the ebay route may be a good option. Those tanks have a wall thickness of about 1/4 inch, so the I.D. isn't much smaller than the O.D.

Again, do some thinking/research regarding possible strength reduction of the fiberglass at elevated temps if you're going to be using a heat cure. Wouldn't want the thing to go boom.

G-man

[mattman]

for a heated cure we put a heat blanket in the bag, so i wouldnt be too worried about the entire tank heating up. i would be worried about the strength lost by cutting a hole in the end of the tank...but between running at a lower psi, and being able to add some layers of glass if necessary it shouldnt be a problem. so the 1/4" wall thickness...is that including this PE liner that they mentioned? is the PE liner pressurized also, or is it loose and open ended like in a paint can?

[G-man]

mattman,

I've never really seen the PE liner because I've never had the need to take one of these tanks apart. I'm betting that one of the major reasons for the liner is that it functions as a 'mold plug' for the tank shape during the fiberglass wrapping. They just pressurize the liner, then wrap it in glass. I really don't think the liner would be any thicker that 1/8". The liner wouldn't be pressurized in a regular functioning tank... just the rubber bladder. I don't really see much use for the PE liner other than being used as a plug and maybe it allows the bladder to slip around inside the tank a little more freely.

Have you considered laying up your own glass tank/autoclave? You could even throw some metal strapping into the lay-up for added strength. Might be a bit cheaper than the pre-manufactured pressure tank route.

G-man

[mattman]

I had thought of making my own tank also...but wasnt sure what to use as a mandrel. maybe a foam block that could be dissolved with a solvent after? then its just a question of how many layers (i figure a long reel of relatively wide fiberglass uni tape wrapped around at 45. -45, 90...repeatedly).

about the PE liner, i think it IS the bladder...I just am not really familiar with how these things work, so I figured it was a liner. a bladder would make sense because it think it said they are removable. would a bladder be removable?

[G-man]

I don' think that the liner is the same thing as the bladder, or 'air cell' as the ebay description states. In order for a pressure tank to work, the water that is pumped into the tank must compress a volume of air in the top portion of the tank (air compresses, water doesn't) in order to create a pressurized system. The air and water are separated by the bladder or air cell, which must be made of an expandable material like rubber. I don't know what the fiberglass tanks air cell is made from, but the description does indicate that it is removable/replaceable. PE doesn't have the ability to expand well enough to function as the water containing element of the tank.

I've been giving some thought to what might be used as a plug to form a home-made tank around, but haven't come with anything really creative yet. I'll keep trying.

G-man