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Vacuum Molding with RTV

A major stumbling block encountered when restoring the once- beautiful bakelite radio is the fact that it was that way a long time ago.... What started out as a major investment for its time, after many years in the kitchen, found itself out of style and favor. Then came a stint in the kids' room, followed by the workshop, garage and summer cottage.

With all this moving, use and abuse...radios got broken and parts got lost. Parts as in KNOBS.

The best way to replace missing radio knobs is also the least likely to happen - find what you need at a flea market. Doesn't happen often.....

So I make them....

RTV rubber (Room Temperature Vulcanizing) is a readily available compound used for sealing, as an adhesive and for mold making within the hobby field. It is available in numerous viscosities, shear strengths temperature tolerance ranges.

In its most basic terms, molding a three dimensional object such as a radio knob with RTV rubber involves embedding a "master" (such as a matching knob) in the rubber while it is in its viscous state, allowing the RTV to cure into a solid, flexible mass, and then removing the master from the mass. This results in a void within the cured RTV of the exact configuration as the "master"

There are many acrylic or epoxy based compounds that can be used to fill the void, which, once cured, can be removed as a twin to the "master".

Easily stated....Not so easily done.

Since the uncured RTV is a two part compound of a somewhat gooey cosistancy, mixing the components prior to use will always introduce air into the liquid. Unless removed from the mix, the air, in the form of bubbles, small and large, can be trapped on the surface of the "master". These bubbles will prevent contact between the master and the RTV...

If used to duplicate knobs, a "gassy" mold will result in the appearance of the same bubbles on the twins...that were trapped on the "master" at the time the RTV cured and solidified,

To prevent this "bubble thing" from occurring on molds I produce, the RTV compound, once mixed, is subjected to high vacuum in a stainless steel chamber. The absence of air pressure within the chamber draws the air trapped within the RTV mixture, out of the mixture and into the surrounding vacuum. The amount of trapped air is demonstrated during the degassing process by the huge increase in the RTV volume just prior to the trapped air's release into the vacuum chamber. The RTV practically doubles in volume, followed by a rapid collapse as the trapped air escapes.

Once degassed, the "master" is embedded in the RTV. A second degassing is performed at this stage to remove any air introduced during the embedding stage.

The resulting mold will faithfully duplicate the master with no additional details having been added by unwanted trapped air.


acrylic lump