RTV - degassing vs. bombs away vs. venturi

qc · Post by qc » Wed Nov 07, 2007 11:50 am

i'm wanting to up my casting game..but not sure i want to put down the big bucks yet for a nice vacuum pump.

i'm looking at two choices...

bombs away method
or
hooking up a venturi to my iwata sprint compressor.

1-am i loosing a lot of quality by using a prof. vacuum pump?
2-how MUCH am i loosing (not looking for a quantitative number...just a general idea/discussion)?
3-is one of the above (bombs vs. venturi) preferred...or better?
4-any idea how much Hg i can pull with a harbor freight venturi and my iwata?
5-or perhaps i should do both...the venturi then bombs away it?

many thanks

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Wed Nov 07, 2007 3:03 pm

I am guessing, but I think your air brush compressor would not move enough air...

The venturi piece needs 4.2 CFM to pull the vacuum.
Your airbrush compressor would have to have a pretty big tank resevoir.

I used to just pour thing layers to start out with, and get the masters coated, then slowly fill the mold. Once full I would jiggle the mold to get
any airbubbles to rise and pop them.

A friend still drops some old sneakers in his dryer, and sets his molds on
top of the dryer and lets the dryer "jiggle" the molds.

If you are just casting for yourself, then the jiggle or bombs away, or even the 'bucket on a rope centrifugal' method are reasonable methods
to de-air resin.

qc · Post by qc » Wed Nov 07, 2007 3:07 pm

umi,
i was doing a search last night..
and seem to remember seeing one post where you thought a shop vac could pull the needed vacuum to degas...or get close to it.

have you tried this...or just theorizing?
and how would one make an adapter from plastic hose to pressure pot?

thanx

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Wed Nov 07, 2007 3:13 pm

I did it in a one quart container...
Scared the heck out of me, as the first time I did it, the
RTV expanded to fill the container, and I almost sucked the
entire contents into the shop vac...

So for small volumes it could work...
I am skeptical whether it would work for a larger pot...

qc · Post by qc » Wed Nov 07, 2007 3:37 pm

umi,
the cup you had the RTV was in was one quart...or the whole pressure vessel was 1 quart?

and how did you rig the shop vac to the pressure container? i suppose i could get a thick piece of acrylic..and make a small fitting to capture the end of the nozzle of the shop vac?

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Wed Nov 07, 2007 10:00 pm

The cup was 1 quart, and I had a spindle end from a roll of blueprint paper that I wrapped door weather strip around. The vacuum hose fit the center of the spindle. This created a lid that when placed in the top of the quart container, would seal the top and allow the vacuum to remove air...

When I stuck the contraption into the top of the quart container, the 1" of RTV expanded and would have gotten sucked into the vac if I hadn't pulled the hose out of the "lid"

schweinhund227 · Post by **schweinhund227** » Fri Nov 09, 2007 10:41 pm

Ok guys you got my attention....

I was going to buy a venturi vacuum pump from ALUMILITE and they got Rid of that line of products...

Basically looking like this ONE !
http://www.hyvac.com/images/Other_Pumps ... e%2002.jpg

Driven by a Household Compressor... while this little marvel is pumping air out of my Tank...

BUT .. I can't Find any distributors .... around here...

Do you guy know how to get HOLD of a Venturi Pump ! for a reasonable price... those babies can go Pretty high depending on how many Inches of mercury per minute they can pump out !!!

SO...

Give me a hint !!! please... cuz I would really like to Learn how to Degas my RTV and get better moulds.... tired of Micro bubbles filling with resin and screwing up my parts....

thanks in advance for those that will leave the info I need !

see ya !

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Fri Nov 09, 2007 11:47 pm

These are the ones most people seem to run across,

http://search.harborfreight.com/cpisear ... rd=venturi

The one you posted looks interesting, like it has a one way valve, or something on it. However, I have never seen one of the harbor freight versions, they may have something similar inside...

schweinhund227 · Post by **schweinhund227** » Sat Nov 10, 2007 8:29 am

Thank you Umi_Ryuzuki

the Smaller one in my picture is 99$ runs off your Compressor.. air goes in and sucks from another inlet and has a gauge !

I had seen those in a ALUMILITE Resin and Mould Making Video, So I imagined those were the only ones I could get !

Your solution... is very viable... at 19$ it's a steal if it does the same thing.... all I have to do is get a Negative pressure gauge and a good valve !

I will never get close to 99$

Thanks for pointing the obvious !!!

qc · Post by qc » Sat Nov 10, 2007 11:08 am

watch what you're doing
i did some quick research
the venturi at HF took 4.2 cfm...to get up to 28" Hg....
my little iwata sprint compressor only pushes out something like 0.4 cfm...even tho it can get up to 58 psi...

if i hooked up my comp. to the HF venturi....i would only pull 1/10 of what i need. i would think you'd need a big compressor and storage tank to get that kinda volume needed for the HF venturi to work.

point:
do all the math before you buy!

Joseph Osborn · Post by **Joseph Osborn** » Sat Nov 10, 2007 3:15 pm

schweinhund227, the actual venturi inside the Harbor Freight units looks a lot like the little unit in your picture. Biggest difference is there is no gauge or muffler with the HF piece. I have one, and using a 20-gal compressor that puts out about 6cfm at 90psi (exceeding the venturi's minimum specs) I can only get to -27"Hg on my gauge, not the advertised -28.3"Hg. It is loud and obnoxious and only makes the RTV frothy. There are more efficient venturis available that will pull hard enough to degas RTV, but as you see they are more expensive than the HF units. Look on Ebay for "vacuum generators" and "venturi" and you'll find some nice ones. You'll still need a stout compressor to drive the venturi. For the price of a decent venturi and compressor you can buy a good vacuum pump. There's no inexpensive route to vacuum degassing.

schweinhund227 · Post by **schweinhund227** » Sat Nov 10, 2007 7:21 pm

qc wrote:watch what you're doing
i did some quick research
the venturi at HF took 4.2 cfm...to get up to 28" Hg....
my little iwata sprint compressor only pushes out something like 0.4 cfm...even tho it can get up to 58 psi... point: do all the math before you buy!

Don't worry QC I wasn't talking about my Paint compressor but My big BERTHA.... 100psi shop compressor....

and THANK you Joseph Osborn for the description of those HF venturi pumps... I went out today in Princess auto and other parts stores.. to no avail... they all looked at me like I had 4 heads... I guess its not common in town to look for a Vacuum pump...

So.. I'll try to order one... or look on Ebay like you said..

Thanks again all !!!

Degassing isn't cheap !!! you've got that right !!! LOL

see ya ! and thanks again for all your help !