Venturi Vacuum Degassing Apparatus for use in Rapid Biomedical Part Prototyping

Project Team:

Richard H. Siderits MD
Christoper Sereni MSIII
Varun Singh - Investigational Pathology Team


Removing bubbles, when you're making blanks to be used for machining plastic parts or casting models from Room Temperature Vulcanization (RTV) silicone rubber is an important step. The bubbles leave behind void spaces that can spoil an otherwise flawless model or part.

A "degassing apparatus" or Vacuum "Desgasser" takes bubbles out of plastic resin or silicone rubber, before it sets. Commercial degassers can be expensive.

This project shows you how to make a small vacuum degassing apparatus for under 20 dollars.

The only thing that you need to use the apparatus is a standard air compressor.

Step 1: Parts List:

1. Plastic container for the vacuum chamber - from the "Dollar Store" (1 dollar).
2. Semi-rigid plastic hose and air-hose connectors - from the local hardware store (4 dollars).
3. Venturi-type vacuum pump* - from Harbor Freight (12 dollars).
4. Hot glue gun.

*Vacuum Pump specifications: A Venturi-type vacuum pump is connected to a compressor to supply the airflow that creates the vacuum.

Note: Air-Vacuum pump: Includes 1/2" ACME (R134a) connector. Vacuum level: 28.3" of mercury at sea level; Air consumption: 4.2 CFM; Air inlet: 1/4 NPT; Overall dimensions: 8-3/8" L x 5" W x 6-1/2" H
Weight: 1.75 lbs.

Comments

[quantafoto]

Could you use a standard household vacuum cleaner or shop vac as a vacuum source?

[siderits]

I don't see why not.
If you just want long duration, low vacuum then try using the old Water column trick first. Its allot quieter.

We used to call is a Wangensteen suction apparatus and it works pretty well for low viscosity fluid degassing.

http://en.wikipedia.org/wiki/Wangensteen_suction

Best of luck!

[siderits]

Good points!
The effectiveness depends on viscosity, area and thickness.
I tend to use this on shallow molds with larger surface areas. You can also hand pump and clamp the line for longer durations as well as pulse the vacuum. Pulsing changes the shape of a bubble in the medium and moves it to the surface faster.

[danm_daniel]

awesome, thanks for the great Instructable

[clubagreenie]

In regards to vacuum, you can't pull more than -1atm/-14.7psi, thats an absolute.

Venturi systems or the hand pumps are never going to get anywhere near that level. Hose size will not change anything, but a tip to keep a soft hose from collapsing is to stick a length of spring inside. Alternately use a hard poly tube such as that you'd find on a drink dispensing machine.

At 30mmHg you're seeing about -0.6psi, not a lot at all but should be sufficient to pull bubbles from a sufficiently thin mix. Another option wound be to combine the vacuum method with pressurisation to approx 50psi, using a different container of course.

With the check valves, check what the cracking pressure (effort required to open the valve) is. If it's greater than, or at the limit of what you can pull than the pump will be working very hard just to overcome the spring tension on the valve and the valve will only open momentarily until it builds sufficient suction again.

A Norgren T51 check valve would be perfect. Tube/Tube push fit, 4-12mm tube, 0.01 bar cracking pressure, cheap.

[Tomdf]

Say what? I've wanted one of these things for years and it turns out I had the stuff all along?! And this instructable has been around for that whole time?! Awesome idea, Siderits, thank you!

[Spyridoula Nemesis]

Fabulous! Thanks so much, this is EXACTLY what I was hoping to find today.

[castothomas]

I have an old motorized breast pump, I was going to use for a portable vacuum former table. cool instructable

[w2sqr]

A cheaper source of vacuum for this type of project is a "Water Aspirator". They are available for about $15. They fit on the end of your sink faucet and as the water rushes through it a port on the side of it pulls a vacuum, about 28" at sea level. I used these to de-gass jewelry molds for years. Nice Instructable!

[siderits]

NICE!
Never even thought of it.
You can also pull a low steady vacuum with a Wangensteen or gravity water suction apparatus. It was used in medicine a long time ago to re-inflate a collapsed lung (See episode of M.A.S.H.). All you need is two 1 gallon jugs, tape, latex tubing and some water. You can pull a pretty good vacuum based on the height of the column. See below:

http://forum.motorcycle-usa.com/attach.aspx/11156/wangenstein.JPG

[brainstorm32]

I made one exactly like this, but I used a stainless steel can with the glass see through lid that is sealed. I couldn't get it to degass anything. Do you have any ideas what could be causing the problem? 4.4 hp air compressor 25 gallon. I can,t get it to work for anything. My venturi is rated for 29.7 Mercury.

[siderits]

Thanks for your comment. One problem I had was a single hair was across the rim of the container and it prevented me from pulling and "maintaining" a high vacuum. An interlocking lid (like the one I used) and a fine layer of Vaseline around the rim may help. If you can pull and maintain a vacuum (test with a hose clamp) then try "ANY" available source of continuous vibration while pulling the vacuum, this can literally walk the bubbles up. Also be patient, under a vacuum the bubbles may be almost impossible to see (not like the big ones outside a vacuum. Also thinning your mixture to increase the de-mold time or Pot-life can give bubbles more time to get out. I use water thin mixes these days. Of course try not to make the bubbles in the first place when stirring the mixture. Hope this helps. Cheers.

[brainstorm32]

What size check valve and hosing are you using? Is that a 1/2 inch. Does the size of the hose make a difference in the amount of pressure it is going to pull. I am using a 1/4 inch hose. I am wondering if that is causing the pull to be weak. I bought a check valve and it is a half inch. It is huge.

[siderits]

I used 3/8 tubing and standard fittings. Remember to wet the fittings with soap water to get a good and easy fit. To close the system I used a hose clamp instead of a check valve. Guess I never had a check valve that worked reliably in a vacuum system. The smaller the diameter tubing, the longer it takes to pull the vacuum. Once you pull a vacuum, the diameter of the hose should not make any difference, the vacuum is the same everywhere in a closed system. Hope that helps and thanks for the comments! R

[brainstorm32]

What kind of air compressor are you using. Do you have a big heavy duty one. I am using a 4.5 hp 25 gallon. It has a 6 rating for cfm.

[siderits]

Greetings; I have an old Sears 3/4 that will push about 3 CFM and a harbor freight pancake that will push higher. The way the venturi pump works is that it should pull a higher vacuum based on faster CFM rather than higher PSI. So you might want to use a lower psi and get a higher CFM to have the venturi pump pull a higher vacuum. Most have upper psi limits of about 90 but thats for safety not performance. Hope this is helpful. Best regards; R

[0xCyrusx0]

Hah, nice. I remember that episode too, DIY for the win!

[The Ideanator]

I see you have a tube of E6000 glue in the first pic. I couldn't live without that stuff, it is the best.

[siderits]

Its the simple things in life. Thanks for the comment. Cheers; RS