Messages In This Digest (1 Message)
- 1a.
- Re: Is roto-casting like a salad spinner? From: Clark Cone
Message
- 1a.
-
Re: Is roto-casting like a salad spinner?
Posted by: "Clark Cone" archive.clark@gmail.com clark_cone
Mon Feb 25, 2008 3:28 pm (PST)
good stuff, Tom... Would you mind taking some pictures of the tanks?
Clark Cone
On 2/21/08, Tom or Gail Madden <tgmadden@worldnet.att.net > wrote:
>
> > i would like to say that pressure casting is safe as long as you know
> > how it works and respect the fact that it can go wrong if not done
> > proper and can cause injury.
>
> If you're going to do pressure casting, you'll likely have a tank-type air
>
> compressor in the house. If you're comfortable with that being at 100 to
> 120
> PSI, you should have no problem dealing with pressure pots running at 40
> to
> 60 PSI with safety valves set slightly above those pressures. I have two
> tanks in regular use - 2.5 and 5 gallon ones from McMaster-Carr - and run
> them at 35 to 40 PSI. If you're unfamiliar with pressure casting, the
> primary purpose is _not_ to force resin into all the nooks and crannies in
>
> the mold, or to replace deairing the mix in vacuum, but to prevent
> reaction
> product gasses from forming bubbles in the material as it heats up and
> cures. It doesn't take much pressure to do that. Trapped air in the mold
> is
> bad news, and your mold design, including gating, venting and fill angle,
> should be such as to eliminate the possibility of trapping air. You have
> to
> provide escape paths for the air as it's displaced by the incoming resin.
> If
> you don't, it'll be trapped in the mold. Boyle's Law says that gas volume
> is
> inversely proportional to pressure. Going from atmosphere to 60 PSI will
> reduce the volume of a trapped bubble by a factor of four, but volume is
> related to the cube of the radius, so the diameter of that bubble will
> still
> be two thirds of what it was. That might be enough of a size reduction to
> jar the bubble loose so it moves somewhere else, but not enough to make it
>
> disappear. If it floats up to an unimportant surface, fine. If it's in an
> unvented, inverted cavity, not fine.
>
> For what it's worth, I have my tanks mounted in saddles, lying on their
> sides. All my larger molds sit on bases cut from laminated shelving, and I
>
> just slide them into the tanks on their bases. For smaller molds, I have a
>
> shelf cut from the same laminate to fit each tank. (A "large" mold, for
> me,
> would be two parts, 12" x 6" x 3" when assembled.)
>
> Tom Madden
>
>
>
[Non-text portions of this message have been removed]
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