Bubbles and Ultrasonics - Update

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BRobbins629

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11/4 - Update: Made another bubble free clear PR cast today, this time with ultrasonics only. Still ran ultrasonics for about 6 minutes without catalyst. Added catalyst and stirred very carefully. May have to find a new use for the pressure pot. This is definitely a simpler and less expensive way to get into clear casting. Also a better chance of not getting resin in tube (not guaranteed, just a better chance). Would still like to hear if anyone else has tried this.

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For a while, I have been considering the use of ultrasonics to get the bubbles out of clear resin casts. I got the idea from my days using this technique to get entrapped air out of solvents for liquid chromotography. Well, I finally got a small ultrasonic cleaning unit on ebay (the kind used to clean jewelry) for about $25 and have run several tests. Just looked at the cast I made last night and it is bubble free (will be my PITH pen). I did also use the pressure pot for insurance, but know that the ultrasonics did help.

Procedure for polyester resin is to pour desired amount in a mixing container and set this in about 1/2 inch of water in the ultrasonic cleaner before adding any catalyst. I ran the ultrasonics for about 6 minutes.

I was discussing this with a professor at one of the local universities. Our best theory is that the bubbles come from air disolved in the resin. As the resin heats up to cure or there are sharp edges or minute irregularities on the surface, the air comes out of solution in the form of bubbles. Vacuum degassing likely does the same thing as ultrasonics - it forces the air out of solution. Pressure, on the other hand probably does not shrink the bubbles, it merely forces them to go into solution. For those scientists or anyone else is interested in the theory, the mere shrinking of the bubbles by pressure cannot be explained by physics. The gas law says that pressure and volume are directly proportional, so an 2X increase in pressure can only descrease volume by 1/2. The pressure pot threads suggest running at 25 to 30 psi, or about 2 to 3 times atmospheric pressure. IMO, this is not enough to shrink bubbles to invisible, so I'm thinking any left after degassing are going back into solution.

For those who don't care how it works, I offer this as an alternative to vacuum. Ultrasonics degassing by itself may also work well if you don't want to make the investment in a pressure pot and compressor. Its a little lower in cost by itself.

Would love to hear from others if they decide to try this and can further develop the techinque.
 
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JimGo

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Wow, that's an AWESOME idea Bruce! Relatively low cost (cheaper than the paint pot). I have always had a fundamental problem with the idea of pressure casting. Can't argue with the results, but the theories being postulated just didn't make sense to me. The ultrasonics should help drive those pesky bubbles out once and for all!!
 

DCBluesman

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For those scientists or anyone else is interested in the theory, the mere shrinking of the bubbles by pressure cannot be explained by physics. The gas law says that pressure and volume are directly proportional, so an 2X increase in pressure can only descrease volume by 1/2.
Bruce - I applaud your experimentation with ultrasound in releasing bubbles from resin, but the science you quote regarding pressure and volume are inaccurate. There is a third element, Temperature, which must be dealt with (according to the <u>first law of thermodynamics</u>). These laws must be further adjusted based on the composition of the air due to the fact that certain gases are more naturally attracted to each other.

As depicted in the graphics below, the relativity of pressure, temperature and volume is a geometric, not a linear relationship.

<br />

(Details are available here.)

<b>I still may have to try the jewelry clearner though.</b> [8D]
 

dfurlano

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But if you add the catalyst and mix and pour after the ultrasonics you reintroducing air into the mixture. Then you still have to deal with the trapped air that causes bubbles in the cast.

I would mix and pour into the mold then but in on the ultrasonics for 6 minutes and then into the pressure pot.
 

BRobbins629

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Lou - you are correct - the gas equation is actually PV=nRT and there are intermolecular attractions. I was going to go into that, but the temperature in the equation is in Kelvin or absolute temperature. I realize that the temperature does increase during curing, but in absolute temperature a relatively small amount compared to the pressure increase. I was just using the more simple example to show that the bubble phenomenom cannot alone be explained by pressure shrinking the bubble. A increase in temperature only makes the argument more convincing.

Let me know how you make out.

dfurlano - correct also. The reason I didn't use ultrasonics before adding catalyst was I didn't know how long I wanted to use the ultrasonics, and I was unsure as to what contribution, if any, the ultrasonices would speed up the cure before I got a chance to put it in the pressure pot. More experiments to follow.
 

btboone

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It should be interesting experiments Bruce. I would imagine it would cure much faster. Water alone in an ultrasonic cleaner will heat up if left for a while. It really does a number on bubbles too. Putting an open bottle of coke in an ultrasonic cleaner will definitely unleash a geyser of cola. It would also be interesting to see if the resulting plastic is stronger than usual.
 
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Originally posted by dfurlano
<br />Let us know. I though about a shaker table but never tried one.
I don't know who it was but someone here tried putting the mold filled with resin on his scroll saw table to vibrate the resin. I don't recall whether it worked.
 
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