Machinable molds for use in autoclave?

[quickbird]

Im looking for machinable mold material for use in an autoclave. The max temperature is 350 degrees F so it needs to be able to withstand that for three hours or so. Has anyone tried using wood as molding material? What i'm thinking is getting hardwood and just machining that to the desired size then sealing it with hi-temp paint. Any suggestions?

 

[patprimmer]

Wood will swell from the moisture.

What precision do you need to maintain.


Regards
Pat
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[MikeHalloran]

Arboron or Benelex is probably a better choice than hardwood.

Aluminum may be a still better choice.
I personally like to work with 2024-T3 or -T351 because it machines like butter and is hell for strong.

Be aware that it will move around a little as you machine away the locked-in stress, so you might want to use a rough/ finish machining process if your tolerances are tight.

I honestly don't know how it will behave in an autoclave; it's susceptible to stress corrosion cracking, and a lot of stress is already built in.



Mike Halloran
Pembroke Pines, FL, USA

 

[quickbird]

I am doing R&D work on some small UAVs used at work. We were donated 1600 pounds of prepreg CF which is more than we will ever need for these small planes but I want to experiment with making wing skins. The tolerances are not very tight, around 1/16 inch since this is still testing phase. Obviously the closer to design the better but we're talking low Reynolds numbers here. I don't have the budget to get some 2"x18"x48" pieces of aluminum or I would. I would be able to build a plug then do a simple wet-layup but I don't think it would withstand the heat.

 

[MikeHalloran]

Phenolic/glass wet layup would take the heat.

The cure is a little weird; phenolic emits water as a reaction product just before it gels, so you may be troubled with voids.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Wood impregnated with epoxy to stabilise it may work. West System makes low viscosity epoxies that penetrate well into the wood before curing. I never tried it for that but it may work. The system is typically used for wood/composite hydroplanes and tunnel hull high speed race boats.

Regards
Pat
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[quickbird]

What about using concrete? I know this was/is used in laminating wood, but I assume it is able to withstand the heat without cracking.

 

[MikeHalloran]

I used to buy a lot of custom ceramic parts. I once had a buyer shop around for two months trying to get better delivery times. After blowing my top, I patiently explained to said buyer that ceramics typically take 8 to 12 _weeks_ to dry, during which they shrink about 50 percent, and that no amount of magic or microwaves can accelerate the drying process without cracking the parts.

I don't think concrete shrinks anywhere near that much, but you do have to let it dry for a while.






Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

You can get limited shrinkage or even expanding concrete by adding a blowing agent (aluminium hydroxide maybe?) This is sold under the name hard block where they charge a fortune for a slightly modified sand cement blowing agent mix. It is used to fill water jackets on drag race engines to back up the bore under very high load, hence the need for very slight expansion.

It sets like a firm gell in a few hours, hard overnight and rock hard after a week or so.

If you can find the blowing agent and figure out exactly how much to use, it might just work.

Regards
Pat
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[unclesyd]

I would get in touch with CMT to what they have that may be suitable for your application. I don't think you are going to find anything cheap.

http://www.cmtmaterials.com/hytac.htm