Author Topic: Making RTV molds with pressure  (Read 1556 times)


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Making RTV molds with pressure
« on: May 09, 2008, 12:16:44 PM »
After playing around with RTV & resin for the last 16 years, I have managed some spectacular successes and a decent amount of stunning failures. One thing for sure, I never new what the first casting would have in store. What new and exciting air bubble or void was there? How long before the void in the mold tears out from repeated pulls?

I recently switched RTV from Platsil 71-20 to 71-11. The main difference is 71-11 is much softer and as a result, ANY air bubbles in the RTV causes trouble. As the resin heats while curing, it expands against the mold and presses into the bubbles leaving bumps on he finished part. I called Platsil, and they suggested I get a pressure pot. By putting the liquid RTV under ~60 psi, any air bubbles in the RTV will essentially be compressed out of existance. The RTV will cure in the compressed state, holding that shape. Additionally the pressure will push the RTV tighter against the master, helping to prevent voids in the mold. By casting with pressure, the sam effect should occur--any air bubbles in the resin will be compressed too, which should result in a cleaner casting.

Well, I finally deflowered my new pressure pot from Harbor Freight. Of course, the first thing was to cast those damned elbows for the Sazabi.

Here is the resin shortly after mixing, and after pouring.  The airbubbles are stunning.  I have already decided I will not use this product again--Platsil 71-20 is the only way I'll go in the future.

Here's the chamber, and the gauge--the gauge is off--I think it is about 20 psi low judging from my compressor.

After curing, I pulled the mold. Here it is, held up to a light.  Not a bubble in sight!  The blip on the lower right is not a bubble, but some sort of surface defect.

And here was a disastrous old mold that was not pressurized.

Pretty big difference.

The proof is in the pudding, though.  So far, I have done about 11 pulls, and the castings are coming out great.  The older nasty mold with all the air bubbles resulted in dimpled castings.  The new ones are smooth as glass...

I did try the RTV under pressure, but with a superthin resin like Easyflo 60, it really was way more effort than it was worth.
« Last Edit: May 09, 2008, 04:37:20 PM by MarkW »

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