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Casting details - more than you asked for...

To: ardunbill@webtv.net
Subject: Casting details - more than you asked for...
From: karl.payne@gm.com
Date: Sat, 9 Jun 2001 11:48:42 -0500
Bill,
A few comments of clarification on casting processes.

>From this it appears that the 355 T6 that Zora used for his Ardun
castings is indeed a stronger casting aluminum than the 356 T6
universally used today, in permanent molds at least.  I'm pretty sure
our original Arduns were all sand-castings, however, which I imagine
have inherently slightly less strength.

Aluminum blocks and heads are cast in a process called Semi-Permanent Mold
(SPM)
 in almost all cases.  Permanent mold processes are commonly used for pistons
and open-
deck blocks, like Honda uses.  (Permanent molds, by definition, use no sand
cores.)  There
are gravity-fed perm. molds, and pressure fed PMs, like die castings.
SPM refers to the bottom and sides of the mold being permanent (hardened, ground
and polished
tool steel) and the cores being made of sand (with a resin binder for
durability).
The way it works is the "floor" of the mold is fixed, the sand cores are
assembled and located
on that platform, then  the four side plates move in on hydraulic slides to
close the box.  At that
point molten AL is poured into the mold.

*COSWORTH NOTE:  In the Cosworth process the AL is pumped up from a reservoir
UNDER
the mold, filling from the bottom.  This does two things, both good.  First, it
displaces the air in a
natural, upward venting action.  Second, it never exposes the molten AL to air.
(Entrained air
produces porosity, oxides, and all kinds of bad things.)
 This is one reason Cosworth castings are so reliable, dense and free from
porosity.

BTW, 356 T6 is gaining popularity, but NOT universally used across the auto
industry.  It is a
higher strength, slightly more expensive, and harder to machine than the
pedestrian 319
alloy which IS the most common automotive alloy, for now.  But 319 sucks for
severe-duty racing
use.  As new generations of production engines roll out and make more HP per
liter, the
evolution from the cheap 319 to the better 356 T6 is a natural one.
And 355 T6 is certainly better for racing, as discussed.
Regarding your comment on all-sand castings, I'm not at all sure they would have
inherently
less strength.  If 356 all-sand castings are "cooked" to the same T6 heat treat
spec. as 356
SPM castings I see no reason why they would have less strength.

Sorry kinda long-winded, but I tried to cast some light on the discussion.
Hehehe...

Karl Payne

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