Vacuum Degassing in Casting of Concrete Analogues
Hello,

I am searching for any technical papers or resources regarding degassing of materials that are roughly analogous to concrete/ceramics/refractories. We currently use variable frequency vibration to remove entrained air from many different cast shapes, but due to certain surface requirements, we have some molds with elastomeric mold surfaces, which under high amplitude or frequency can impart stratification into the cast part. 

Anyone have resources or reading to recommend on the topic of degassing, either prior to pouring into the mold or in the mold itself. 

Thanks in advance for any insight. 

-Miguel  
12 Replies
Thanks Paul and David, Degassing during pouring would be tough since there has to be a way to get material into the mold, unless I am missing something here. As far as smaller batches, we are working with relatively short pot life once the material is mixed, so we dont have the luxury of batching the material as it begins to develop transition lines between the layers of older and newer material leading to weakness and cracking during heat cycles (these are used at about 500 degrees F working temp). David, to your point about aggressive mixing, we use a enormous ribbon blender, which fits the description pretty well. 
David Sulli
20 Posts
Good thoughts, Paul.
David Sulli
20 Posts
Hello, Miguel.
I think your theory is a good one to verify. It certainly looks plausible that shaking can entrap air. Very viscous 2-part silicones have such high void volumes because of the required aggressive mixing. It wouldn't surprise me if shaking produced similar, if even to a lesser extent results, with concrete.
Dave
Not sure what your set-up looks like but have you tried to:
1. degas/vibrate as you pour?
0r 
2. degas/vibrate in smaller batches
Hi David, 

Thanks again for your feedback and links. I certainly understand the need for correctly sized chamber and sufficient flow. One of our struggles is that the material is non-flowable without vibration (think really thick concrete), in order to fill our mold, which is relatively thin walled at about 2"in cross-section and is more or less a tapered tube about 21" high and 31" in diameter tapering to 26" we have have the whole thing on a table with railcar vibrators bolted to the bottom with VFD control. This allows the material to flow, but in the process entrains quite a bit of air. My challenge is figuring out how to degas it after or during vibration. I am wondering if you have any experience with the method of pulling vacuum and holding and allowing the material to settle, rather than pulling full vacuum and requiring a large expansion volume? I have seen some people having success with this on resin degassing but I think I will see issues due to the high viscosity. I think if I could vibrate the mold under vacuum, even if it wasnt a lot that I could prevent new bubbles from forming, while pulling the existing bubbles to the top.

I think I may need to do some trial and error testing. 

Forgot to mention I attached a short video clip showing the difference of material consistency under vibration versus static.

Thanks again for your time,

-Miguel 

 
David Sulli
20 Posts
Hello, Miguel.
When I was actively creating purchase requisitions for vacuum pumps, I relied on a sales rep who specialized in this type of product. The volume of vacuum chamber is important, as the larger the chamber, the higher the vacuum pump flow requirement. Most will reach 29 inches of mercury or better but you don't want to wait 5 minutes for the pump to reach that level. 
I've used Welch, Alcatel, and one other manufacturer's vacuum pumps.
Here's an example of options for high-flow pumps:
New & Rebuilt Rotary Vane Pumps for Sale (provac.com)
I also have a low-flow (Welch) pump in my garage. It was used primarily for making polyurethane shifter knobs.
Best regards,
Dave
 
Thanks Carol, I will give them a read!
Great, thanks for the response. I will see if I can give this a shot. Any recommendations on reliable vacuum pumps? 
Carol Tower
32 Posts
Hi Miguel,

Here are 2 articles that may be helpful!

Best regards,
Carol

Carol Tower
SME e-Librarian
David Sulli
20 Posts
Yes, it is important to size the container so that its volume is at least double the material volume. When the degassing material gets to within a few inches of the top of the container, the valve needs to be closed until the material level drops down to its original level. Reopen the valve and repeat if necessary until the material ceases to rise more than a small amount that is well short of the top of the container.
This does not compromise the material properties but curing and pouring time should be monitored.
Hi Dave, 

Thanks for the reply. I have seen the process that you are talking about. But due to the non-homogeneous nature of our material, didnt know whether this would be viable. 

Can I ask how you approximate the volume increase as the material degasses? Our parts are quite large in some cases being 300lbs or more on the small end with a density of around 110lbs/cubic foot. 

I have seen some people doing pull and hold methods, where they pull vacuum at some fraction of the ultimate and hold repeating after the volume shrinks to avoid needing such a large chamber, but I dont know if this is detrimental to the final result. 

thanks again for your time.

-Miguel
David Sulli
20 Posts
Hello, Miguel.
I have experience with degassing low viscosity polyurethane and high viscosity silicone. I've found that a minimum vacuum pressure required to remove all entrapped air is 29 inches of mercury. My vacuum pump typically achieves 29.5" Hg or greater. 
 the polyurethane is mixed and then degassed for 1-3 minutes before being slowly poured into the silicone mold.
 Depending upon the complexity of the desired molded part geometry, some companies need to then pressurize the filled mold to effectively shrink any entrapped air. 
 I am not familiar with using concrete with this process but most likely, it can be done in a similar manner..
Dave

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