Rocket Man VW Heads... What alloy?

On Jan 13, 3:00*am, Stealth Pilot
wrote:
On Mon, 12 Jan 2009 22:38:54 -0600, "Maxwell" #$$9#@%%%.^^^ wrote:

There is also the possibility of molding this in plaster, but that's another
story.

unless you've tried it you would have no idea how dangerous that
suggestion actually is.

firstly the strength of plaster vanishes to nothing beyond a certain
temperature. that temperature is just 5 degrees celcius above the
melting point of aloominum.

That is because it is water molecules that bind the other molecules
together when the plaster cures. When you heat it, you break those
bonds and drive off the water.

You can test this out by weighing the water and plaster you mix,
then weigh the cured solid when it appears dry. It will still weight
a lot more than the dry plaster did. After baking it, weigh it
again.

As long as the weight is greater than the original dry plaster, you
know there is still water in there.


I have tried the technique to cast a mills 0.75cc diesel crankcase.
this is a tiny thing about an inch and half by an inch and a half in
overall dimensions. relatively tiny and easy to work with.
it was done lost wax.
a day after the plaster was set the mold went into the oven to melt
out the wax. it was then cooked up to about 180 degrees farenheit for
a while (2 hours afaik) to evaporate off the moisture. it stayed in
the oven overnight (I've never been allowed to use the oven again) it
was then given 15 minutes in the microwave to really dry it out.
it was wrapped in a cloth and raced out to the workshop hot.

it looked alabaster white and as dry as a piece of chalk.

when the aluminium was poured into the mold a guyser of molten
aluminium ejected to the ceiling of the workshop. *this seemed to
continue for some time. an examination of the mould later showed it to
be totally empty. I dont know how dry you need to get it but I gave it
a fair go at getting a bone dry mould.


I've no experience, but there is a nice newsgroup called
rec.crafts.metalworking or something like that where you
can get good advice from people who routinely have a
less exciting experience.

Plaster of Paris retains water of crystalization when it is
cured. It takes a lot of heat for a long time to get all of that
water out, like maybe 5 or 6 hours at 500 F and you should
keep it hot when you our. If I understand the chemistry, by
the time it is dry enough to cast aluminum you will have returned
it to much the same material that it was before you mixed
it. Since you didn't do that, it was the molten aluminum
that decomposed the plaster releasing the water with the
observed result.

Jewelers work with much higher melting point metals, platinum
even. They use investment, sometimes called investment plaster.
The difference between that and ordinary Plaster of Paris is that
it has materials added like silica that do not decompose when the
mold is burnt out, and other materials (maybe wood flour) that are
there to create microporosity when it is burnt out. A high quality
investment may be overkill for casting aluminum, but it will be
better than a molten aluminum geyser.

But please do check with people who, unlike myself, have
actually done this.

On to the subject of cooling fins:

I HAVE designed cooling fins back in a former life when I
was working in the nuclear industry. I believe you will
get the best heat transfer with pure aluminum. Alloys
will have a lower thermal conductivity, although the
more important parameter is heat capacity, the product
of thermal conductivity with density. But for an airplane
you probably don't want denser (and therefore heavier
materials).

You can improve the heat transfer and reducing the
weight by tapering the cooling fins from root to tip. A
good undergraduate engineering book on heat transfer
should address the optimization of cooling fins.

Remember also that convective heat transfer is
governed by Newton's law of cooling:

q = mdot * h * A * deltaT

DeltaT is the temperature difference, A is the surface
area (which you increase with fins), h is the convective
heat transfer coefficient (usually a function of Reynold's
number) and mdot is the mass transfer rate of the cooling
fluid.

Bumping up the flow increases both mdot (directly) and
h, by increasing the Reynold's number. It helps a lot.

IOW, concentrating on improving the airflow may be more
helpful than improving the fins.

Hope this helps

--

FF