Are you under pressure????

[SuperDave]

I have my Pressure/Vacuum Chamber (pot) almost completed and hope to order my supplies to be in casting test mode in a few days.

My questions for those of you that are casting currently or have cast in the past:

1. What pressure do/did you find most successful...
2. What duration do/did you require for the cast...
3. And what material(s) were you casting under those criterion...

I can easily maintain 70 PSI and the tank is rated for 100 PSI. I still have a couple of small pressure leak issues to resolve but I get 20-30 minutes of 70 PSI before the compressor cycles back on. I am trying to see if I have an acceptable pressure system currently, just in case I cannot get the remaining leaks closed in a timely/affordable fashion.

Next is the Vacuum Leak Test.

Thanks!

 

[MesquiteMan]

Dave,

I cast my prickly pear cactus blanks at 60 psi. I seriously doubt I need that much but what they heck! I can hold 60 psi for 24 hours if I want. I use Alumilite clear urethane resin which sets in 10 minutes or so. I leave my blanks under pressure for 15 minutes. Hope that helps.

 

[gketell]

To help find the pressure leaks use a squirt bottle fully of slightly soapy water.

I had several of my cast bronze elbows/Ts/etc leak THROUGH THE CAST. Soaking with thin CA solved that problem.

My two casts have both been done at 60psi. Don (aka its_virgil) does all his at 30psi.
I cast a printed paper label wrapped around the tube (no vacuum) and then a copper scrubber pad (vacuum followed by pressure).

GK

 

[SuperDave]

Curtis,

That's exactly the info I am looking for. Thanks!

It looks like I can pressurize and set a load, during the 20-30 minute period before the compressor cycles again, with the system "as is."

GK,

I did the soap bubble search and the main leak(s) are in the lid o-ring. I pulled the lid and the o-ring has noticeably "flattened out" some in a couple of spots and the prospects of removing and replacing are not something I look forward to. The small leak in the regulator knob is minor and can be cured with a replacement.

So, I have to decide if I want to remove and replace the lid o-ring, which, I fear, can cause more problems than I have right now, or use it as is for a while and postpone the o-ring.

Anyone replaced a 9" diameter recessed o-ring????

Dave

 

[Glass Scratcher]

For your lid seal you could always take some vaseline/pam/liquid soap (test'em first) and give a light coat inside and out around the lid seal, top and bottom, get some auto gasket silicone and make a "New" one.

 

[SuperDave]

Scratcher,

Not a bad idea... I can get some sillycone material from Artstuf and turn a mould from MDF on my bowl lathe and then cast a whole new gasket that the lid (o-ring and all) can compress into and then use the gasket and lid as a combination...

Nice idea... I like it!

 

[SuperDave]

Well... like I feared, the Vacuum Leak Test yielded significant issues.

I can pull a full vacuum and hold it indefinitely (which means the vacuum reservoirs and lines leading to the chamber are working 100%), however, once the vacuum line ball valve to the canister is opened, I lose vacuum rapidly (about 2 Hg within a couple of minutes), and the vacuum pump cycles back on.

Now that is not a big issue if I only need the vacuum for a few minutes and then switch off the vacuum ball valve and open the pressure ball valve and pressurize the casting. If I need the vacuum for a much longer time-frame, then I will have to isolate the leaks and make repairs.

Stay tuned.

Dave

 

[gketell]

Dave,

The way Alumilite recommends and many of us do is to pull a full vacuum for a few minutes to remove the majority of the air and then gently switch to pressure and hold it until the acrylic is fully set to compress any remaining air to invisibility.

GK

 

[SuperDave]

Greg,

Thanks... if I can't reduce/eliminate the vacuum leaks in a timely fashion, then I feel good about using it as is for the time being.

Pulling the vacuum and having maybe one additional cycle before switching to pressure is not bad at all.

Off to make a new gasket for the lid... then ordering some casting supplies!

Thanks!

 

[Glass Scratcher]

I mentioned this to Greg? in an email recently that over the years working with pressure for my glass biz, the places that the air system leaked was where I used teflon tape. One of the problems with T-tape is that it lubricates the fittings so you can over tighten and swell a fitting, or that there may be an imperfection in the fittings, T-tape doesn't seal a fitting, tightening down may. I use and have used Permatex gasket seal, availble at the auto parts store and I am sure at Walmart. The GE Gasket sealants will probably work as well. You apply sealant, turn the fitting snug and to the desired position, then walk away, Just Walk Away! Let it completely cure for the time specified, and don't fiddle with it. Just read the list of compatabilities. The Permatex 1 & 2, Form-A-Gasket will disolve with alchohol, so don't use them. The Red High Temp RTV, gives good perfomance but you must let it cure for 24 hrs. If you are going to use your setup for Vac Stabilizing with Plexi-Acetone (I'm calling it A-Plex from now on) be aware that silicone seals will deteriorate with time and exposure.

 

[woodman928]

I had to replace the big o-ring on the pots I had in my woodworking business a couple of times. I would just take them to a spray equiptment supplier and let them replace them and the most I ever paid was $10.00 []
Jay

 

[SuperDave]

Scratcher,

Permatex... good idea. Used that in my drag racing days many moons ago. Didn't know that about the tape... go figure. I did try to minimize the number of fittings just to reduce the possible leak locations.

Woodman,

I will look for a local spray equipment shop. The flat spots on the o-ring are the #1 major leak locations right now and I would like to bring it back to original specs.
_________________________

I see where Alumilite is selling their Vacuum chamber with two 1/2" Lexan sheets for $156. I would like to see the vacuum process to gauge when the air has dissipated, so I am getting a 12" x 12" Lexan sheet off Ebay for $18 and some silicone gasket material to make a vacuum "lid" for the canister... Hopefully, the Permatex and the silicone gasket will make the vacuum more effective.

More to come... thanks for all the advice.

Dave

 

[MesquiteMan]

I had the same problem with my new HF pot. No matter how tight I tightened the bolts, it would still leak at the lid seal. I was about to take it back and decided to try something first. I sprayed the entire rubber seal with WD-40 and put the lid back on and tightened it down. I let it sit overnight and it has held perfect every since! Don't know why since I would not think that WD-40 would soften the rubber but it did something. I pressure my pot up and then I have a valve that I cut off and remove the hose. It will hold 60 psi for at least 24 hours (longest I have tested it) without loosing any.

I also have a second pot that the seal kept blowing out on. I would pressure the pot up and the o-ring would actually bulge out and blow all the pressure. I never could get it to work correct. I ended up removing the seal and filling the space where it was with some butyl rubber sealant (caulk) found at the local BORG. I let it skin over until it was no longer sticky but still somewhat soft. I then put some grease on the top edge of the pot and gently set the lid on the pot. I let it sit there overnight and the next day I had a custom fitted seal that will also hold 60 psi for 24 hrs!!

If you do this, I would suggest putting some alignment marks on the pot and the lid so you get the same orientation each time. I actually did that with both of my pots.

 

[Tubby]

I had a HF pressure pot that would not hold a seal also. I just gunked Vaseline all over the seal......tightened it down and tested it overnight. It works like a champ now. It may have conditioned the seal.....or it was pure magic.....but it worked for me.[] []

Ed Jordan
Wichita Falls, TX

 

[SuperDave]

Curtis...

I have to give the WD 40 a tryout!

As for the butyl rubber sealant, that is along the line I was thinking by using the black-gasket-in-a-tube I use for my oil and transmission pans... let it sit over night and then lock the lid down and create a "positive-negative" impression type seal... but WD 40 first, to give the o-ring a chance at making a "good impression."

Thanks for the heads-up on the registration marks!

Do you use the pots for vacuum as well?

Dave

 

[MainePenMan]

Originally posted by MesquiteMan
I sprayed the entire rubber seal with WD-40 and put the lid back on and tightened it down. I let it sit overnight and it has held perfect every since! Don't know why since I would not think that WD-40 would soften the rubber but it did something.

A refrigerator repairman once told me that the hand cleaner that mechanics use (not the kind with grit) is excellent for softening rubber gaskets . I suspect your WD-40 trick is temporary because it may eventually dry out the material.

 

[Fred]

If you know a printer, then ask them where they get their roller cleaners and solvents. Printing presses have to keep their rubber rollers soft and they have a special "softening" agent just for that purpose. It does not ruin the rubber as I believe the WD-40 might do in time. Also, vasaline jelly has been known to eventually cause damage to rubber seals, but it takes some time for th damage to develop. []

 

[RONB]

One thing that I would like to add is to apply the pressure very slowly. The first time I cast in my new pot I just blew half of the resin out of the mold all over the inside of the pot.I use the vacuum first and pressure next method. I have cast snake skins and the paper labels.The results were good.

 

[Glass Scratcher]

Did you leave the inlet apperature open into the pot or did you use a elbow like some of us? You can use a black iron, or galvanized 90deg street elbow on the inner fitting and it will vent the air to the side of the pot instead of down onto your resin.