Need a pressure casting primer

Dr. Snuts, M.D. · Post by **Dr. Snuts, M.D.** » Tue Jan 01, 2008 5:06 pm

I am interested in getting into pressure casting as I want to create bubble-free pieces and clear pieces for my scratchbuilds. Can anyone fill me in on the basics of the process and materials required? I see Alumilite has one available for around $200. Can I build my own from a pressure cooker? Does the mold have to be poured inside the pot or does the process just involve pressurizing the resin and pouring it out in the open? Am i limited in the size and amounts of my molds? I am pretty much completely in the dark on the process and if anyone an help me out, I'd be very appreciative.
Thanks in advance,

Paul

Joseph Osborn · Post by **Joseph Osborn** » Tue Jan 01, 2008 10:28 pm

Quick answers: Pressure cooker = not good. The minimum pressure needed is about 30 to 35psi and pressure cookers can't handle that; Harbor Freight's paint pot is a much better alternative and can be kitted up with the proper hardware for around $50. You pour the resin into the molds, place the molds into the pot/chamber, then clamp on the lid and pump 'er up. Proper mold design is still very important! You must have a decent compressor to supply the pressure, too. By decent I mean something bigger than an airbrush compressor. If you use a slow-setting resin you can pour a whole bunch and stack them up inside the pot on little shelves. Your RTV molds must be bubble-free around the pattern or else you'll get warts and defects in the cast parts. There's probably something I've glossed over but those are the high points! Once you pressure cast a part you'll never go back.

Stu Pidasso · Post by **Stu Pidasso** » Wed Jan 02, 2008 1:54 pm

What about this? Would it be okay to use?

Joseph Osborn · Post by **Joseph Osborn** » Wed Jan 02, 2008 2:08 pm

Stu Pidasso wrote:What about this? Would it be okay to use?

The ad text mentions 20psi, but that is much lower than the pressure I normally use (35 to 45psi). I'm not saying that there's no pressure cooker anywhere that could be adapted for pressure casting, but typical consumer cookers are not suitable. Without knowing the rating for that particular pressure cooker, I wouldn't consider it.

modelnutz · Post by **modelnutz** » Wed Jan 02, 2008 3:26 pm

The small pressure pots from Harbor Freight are the ideal starter pots.
If you have a Harbor Freight Store near you, they sometimes offer a discount ( not too sure 'bout the deal..may be if you buy online and pick it up ) certainly worth the price.
Everything you'll need 'cept the moisture trap is included.

I picked up 4 when they had their last sale...think I paid about $60.00 ea.

Note...I also cast with a minimum pressure of 45 Lb.
(sometimes higher if details dictate)

Dr. Snuts, M.D. · Post by **Dr. Snuts, M.D.** » Thu Jan 03, 2008 6:48 pm

Thanks for the info, guys. I have a Harbor Freight not too far from me and I think I'll go check it out this weekend. I'm looking forward to getting this underway.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Thu Jan 03, 2008 8:19 pm

It seems like this link keeps disappearing, but it has been posted here several times...

Cobywan used to use a pressure cooker.
Please note how the entire locking ring has been sheared away from the lid.
The lock may have stayed put, but the lid hit the ceiling.
Cobywan was lucky

http://home.comcast.net/~cobywan/Templates/Whoopsie.htm

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It has also been suggested that a glass fishbowl makes a good vacuum chamber...

Please observe the collecton of shards recovered from a 1/2" thick piece
of Acrylic from my vacuum chamber lid.
http://img178.imageshack.us/my.php?imag ... ass4sf.jpg

I was standing to one side operating the valves when this occured.
(I always stand to one side of the pot)

The shards imploded and shot back out of the pot and across the shop,...
bounced off the work bench and landed 5-6 feet to either side...

I was cautious and lucky...

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Please pay attention to recommendations... There are good reasons for them.

Safety first please...

Dr. Snuts, M.D. · Post by **Dr. Snuts, M.D.** » Sat Jan 05, 2008 11:23 am

So when using a homemade vacuum chamber the lesson is to have someone else test it out first...Done and done.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Sat Jan 05, 2008 1:04 pm

Dr. Snuts, M.D. wrote:So when using a homemade vacuum chamber the lesson is to have someone else test it out first...Done and done.

Same for any cheap pressure cookers as pressure pots..

I knew that 1/2" thick was too thin, it was a cheaper replacement for the 3/4"
thick piece that I had originally been using. When a vacuum was pulled on the 1/2" cover, I could see it deform about 3/16' - 1/4" across the center.

That piece took over two years to fail.. Your testor will approve your vacuum chamber at your own risk...