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Venturi Vacuum Degassing Apparatus for use in Rapid Biomedical Part Prototyping

Picture of Venturi Vacuum Degassing Apparatus for use in Rapid Biomedical Part Prototyping
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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.
 
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Step 1: Parts List:

Picture of Parts List:
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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.

Step 2: Making the Vacuum Chamber: "Dollar Store Style"

Picture of Making the Vacuum Chamber: "Dollar Store Style"
The container that we'll use has to be sturdy enough so that it won't collapse when you pull a vacuum on it (I learned that the hard way) and it needs to be deep enough to hold the mold for the model that you are working with. The container in this image has strong corners, a strong rim and is deep enough to hold the small molds that we want to the pull bubbles out of (de-gas).

The short segment of hose has to be rigid enough so that it won't collapse under the vacuum. The quick connect for the other end is not necessary but you can get it at any hardware store. The threaded end is only threaded on one end, the other end of the connector has a bayonet tip that goes into the tube.

Step 3: Attaching the Vacuum hose: Drill a hole in the side of the container

Picture of Attaching the Vacuum hose: Drill a hole in the side of the container
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Make a hole in the side of the container.

Instead of using a drill, you might consider heating the connector and melting your way through the plastic container. You might also heat the base end of a drill and melt it into the side of the plastic container. This will assure that the nozzle fits snugly.

When you have a hole in the side of the container, screw in the connector and then hot-glue or rubber cement the nozzle in place from the INSIDE.

This fitting has to be air-tight

Step 4: Two part plastic resin material:

Picture of Two part plastic resin material:
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Image 1: Example of two part plastic resin available through Micro-Mark or Room Temperature Vulcanization (RTV) silicone rubber two part mold mixture available from Polytek

Image 2: Example of a flawless, bubble free, plastic blank made by pouring the liquid two part plastic resin into a PVC pipe that the inner surfaces have been coated with a layer of Vaseline for release agent.

Image 3: Here is an example of a two part plastic resin "blank" that was degassed. The blank was then placed on a CNC milling machine to make models of various human hand bones. Most importantly there are no void spaces in the material.

Step 5: Another good vacuum source

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This small "Hand-vac" will pull enough of a vacuum to pull the bubbles out of most resins and RTV silicone molds and still costs under 20 dollars. Its also available from Harbor Freight.

Step 6: Disclaimer

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Follow all safety guidelines, including but not limited to:

1. Using vacuum systems or compressors.
2. Power-tools that you never read the instructions for.
3. All tools that you never learned to use properly.
4. Anything that heats up, pinches, cuts, squeezes or causes traumatic, caustic or thermal injury.

We are sharing our experience, not telling you to do it.

If you choose to try this, then it is at your own risk!

No really, we're not kidding about this.

Step 7: Project Description

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Facility: RWJUHH Exp Division and Center for Parabiotics Research

Section: Special Projects

Application: Part Manufacture for Scientific Apparatus

Technique: Rapid Prototyping in laboratory development

Title: "Making a Venturi Vacuum Degassing System"

Step 8: The End

Picture of The End
Good Luck and Be Safe!

is there any way you could post the names of the connectors used (especially the one connected to the r134a on the pump)? i am unfamiliar with what is standard for this type of thing. i have had several unsuccessful trips to the hardware store! thanks for the great tutorial!

quantafoto2 years ago
Could you use a standard household vacuum cleaner or shop vac as a vacuum source?
siderits (author)  quantafoto2 years ago
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 (author) 2 years ago
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_daniel2 years ago
awesome, thanks for the great Instructable
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.
Tomdf2 years ago
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!
Fabulous! Thanks so much, this is EXACTLY what I was hoping to find today.
castothomas2 years ago
I have an old motorized breast pump, I was going to use for a portable vacuum former table. cool instructable
w2sqr5 years ago
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 (author)  w2sqr5 years ago
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
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 (author)  brainstorm325 years ago
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.
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 (author)  brainstorm325 years ago
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
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 (author)  brainstorm325 years ago
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
Hah, nice. I remember that episode too, DIY for the win!
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 (author)  The Ideanator5 years ago
Its the simple things in life. Thanks for the comment. Cheers; RS