Re: ATM Concrete moulds could be DANGEROUS

[Dennis Rech <dennisrech@home.com>]

At 07:55 AM 10/6/00 -0400, Brad Davy wrote:
>
>Thomas Janstrom wrote:
>> 
>> The correct amount of cement powder is 10% and the correct amount of water
>> is 5%. These are all by weight with the remainder being 80 mesh sand.
>> 
>> > >>>
>> >
>> > Pouring molten Al in a concrete mould can be VERY DANGEROUS.
>> >
>> > If the concrete is of normal mix the mould has a very high chance of
>> > exploding violently showering you with molten Al.

It is a normal practice when using cement and plaster molds with molten metals
to bake the mold in an oven to a temperature AT LEAST as high as the
temperature of the metal that you are pouring. You can then pour the metal
into
the mold (while it is still hot or after it has cooled) without fear of
generating lots of steam which will blow molten metal out of the spew and
vents.
All calcium based products chemically lock in hydroscopic water and no length
of time will cause this water to escape. As soon as molten metal hits the
plaster, the water will chemically unbind from the calcium and cause massive
steaming which will only break the mold if you are lucky. Or give you a molten
metal shower if you are not.
Green sand molds are safe because the sand is porous enough to allow the steam
pressure to vent safely. So are molds that have only a very small amout of
cement as a binder.




Dennis Rech
Mirror-o-Matic

http://www.oblivion.net/~astro/mirror/