BC-720- Pic du Jour

[DaveC]

Regarding Acrylic resins.... polymethylmethacrylate (PMMA) - I used to mold dielectric samples using the small beads. In this form, it takes a bit of finesse to get clear, bubble free castings. We had to make SS plug molds, and use a hydraulic press with heated platens capable of reaching about 180C at 3000 to 10000 psi. The material is a thermoset resin so once cured, it retains its shape.

I think is is castable from a solution of dissolved, cured resin, but my efforts there were not very good at getting a clear preduct. But the sheet acrylic claims to be "cast" , also the rod material.

The resins I see in the local shops is either polyester, or epoxy resin( amine or amide cured)

Maybe someone here knows the details.

Dave Cooper

[guest]

I am sure that you can get what ever type of acrylic resin you might need from a professional model making supply house. This is different from the local hobby shop. I know a guy who is a dealer for a couple of companies. I will ask him what the scoop is. As far a getting clear bubble free work, you need to use a pressure pot and low humidity. You can make a pressure pot from an old pressure cooker. use an air compressor to apply pressure

[Tom Dressel]

I worked in the materials lab at Honeywell in the 60's. The plastics guys would mix up the two part plastics then put the mixture in a vacuum chamber, and pull a vacuum until the mixture stopped bubbling. This usually took only a few minuites. They could then pour the mixture into the mold and get bubble free castings.

Tom Dressel

[DaveC]

That would be great if it only takes a little pressure pot. I had to have very high pressures and temps, since the catalyst was activated at about 180 C, Obviously, you need also to keep air away at these temps or rapid oxidation happens.

Dave Cooper

[guest]

The best way to do this is to first place the plastic mixture in a vacuum chamber to de-gas. Then cast it in a pressure pot. But the vacuum is optional.
Monty

[DaveC]

Right... if it IS a two component, room temperature activated resin... then you do just light pull out the bubbles from each component, the mix them, then quick degas again.. and let it cure at atm pressure.

If the compound is cured at high temp, like PMMA (arylic resin usually is) then you can only vauum first to clear out the air and dry the pellets.

To cure it you first raise the temp to about 110 to 120 C and compress the pellets to fuse them, then rapidly raise the temp to about 180 C and maintain pressure up to about 3000 psi or so for about 5 mins and then turn off the heaters...and maintain pressure all the way to room temp.

Usually that will give a clear void free molded piece.

But the process is a bit of a black art.\\

Dave Cooper