I guess inadequate premixing was the problem. I made two good casts yesterday (of race car tires). I gently rocked each bottle completely upside down and back six times before pouring. That seems to have done the job.
Glad you got it to work, Don.
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Btw, I just read on the latest FSM issue the tip of a guy that to avoid air bubbles in resin castings puts the mold inside an ultrasonic cleaner. Seems it shakes bubbles out of the mold. Has anyone tryed it? I wonder if it's safe enough...
The last time I looked at an ultrasonic cleaner they were lots of money. Did FSM mention an inexpensive model?
The way most pro casters do it is with a vacuum chamber. Vacuum on the RTV and pressure on the resin casting. hth
The Keeper (of too much crap!)
When I was still working for a living, we used a type of ultrasonic cleaner for our LeRoy pen tips. I was a metal cup that sat on top of the ultrasound generator. The liquid in the cup would be aggitated and forced into to pen to loosen any dried ink. Sit too close to it while it was running and it felt as if your fillings were going to pop out. Could this be what he was talking about? Units like this aren't too expensive, being old technology. Maybe $25 or $30.
-- John The history of things that didn't happen has never been written. . - - - Henry Kissinger
Old technology isn't always cheap. I did a quick web search and found one ultrasonic cleaner for $50, but all the others range from $100 to $800. The expensive ones are intended for professional jewelers etc.
Here's a link to the cheap one:
The ones made for jewelers are too small for our uses. I was looking at one for model railroading; I think it had an oil bath. It was around $1000. I looked into vibrating tables for casting; that was a little too pricy for my tastes as well. I may buy a pressure chamber soon. Cheers,
The Keeper (of too much crap!)
I don't use either vacuum, pressure, or a vibrator. Bubbles are not a problem. The only problems I had in casting 11 tire halves were the resin not hardening in two pours (but that has been fixed, as noted elsewhere. There are no bubbles in any of the pours that hardened.
The secret is in the mold design. I keep at least a one inch head of the pouring sprue above the highest mold cavity point. Every high point needs a vent tube. Also, my molds always are sort of J-shaped, to bring resin into BOTTOM of mold cavity, not TOP. The resin pushes air out the top.
One also should stir, not 'whip' the resin when mixing. Stirring is enough. As long as you can see no air bubbles when you pour, fine. Most of the bubbles folks have are due to air trapped within mold cavity. It took me several years to learn to design molds to eliminate that.
I did an article in FSM about a year and a half ago about mold design.
Don
Really? You were a metal cup?
Sorry, I couldn't resist :)
Hee, hee. I'd like to find an ultrasonic generator like that. It would come in handy for a lot of things where the ones I've seen offered for sale all have a receptacle for fluid and aren't very large. Cheers,
The Keeper (of too much crap!)
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