grand plate flanges...

[Jim]

Joel and Priscilla:

I tho't I'd jump in and answer this one.

When a plate has been designed, a wooden pattern (often solid mahogony) is
prepared using a shrink rule (1/8 inch per foot larger than normal).
This pattern is mounted to a follow board (heavy platform). A rectangular
flask (like a heavy formed steel band) is placed on top of the board.  This
flask is about 12 inches high all the way around. There is a steel
reinforcing grid across the top of the flask which holds the sides rigidly.
A powdery releasing compound is lightly dusted over the entire follow
board and the pattern.

Sand is poured down thru the grid to completely cover the pattern.  Workers
standing on top of the grid begin ramming the sand tightly around the pattern
and solid upon the follow board. Jack hammers and ramming rods are used.

This sand is a special mixture of sand, bentonite, molasses and perhaps other
ingredients which make the sand stick together when compacted.

There a flask which holds the molded sand for the shape of the top side of
the plate pattern and then another flask which holds the molded sand for the
shape of the bottom side of the plate pattern (like the horn, bosses, etc.).

These two flasks are called the cope and the drag.  When they are lifted
off of the pattern, the draft angle is important to avoid any sand being
accidently dislodged and falling down from the flask which holds
the reverse shape of the pattern.  The cope side has little sprue holes
carved into the sand to allow pouring in the white hot metal mix when
the cope and drag are mated together.  When they are mated together, there
is a piano plate shaped space between the cope and the drag.  The
sprue hole are placed so that all parts of the cavity between the cope
and drag are completely filled and running over.  After the metal cools
sufficiently, the flasks are removed, or the sand is shaken out.  The sand
usually goes onto conveyor belts which takes it up to be renewed and
processed for use in some future casting.  The plate is then shot peened,
sent for curing, then ground down at rough edges, and then drilled.  If any
sand had fallen into the cavity before pouring, it would have ruined the
plate.  Hence, the need for good draft angle.  the foundry would
really like 4 to 7 degrees draft angle to help avoid cave-ins.

Jim Coleman, Sr.


On Wed, 27 Nov 1996, Joel Rappaport wrote:

> Danny,
> You are the perfect one to ask a question that occurred to me while
> reading the various responses in this thread.  How many times is a sand
> mold used to cast a plate?  Is the draft angle necessary for a clean
> release so as not to disturb the sand of the molding so it can be used
> again, or for a clean casting?
>
> ----Joel
>
> Danny Dover, RPT
> Dartmouth College, Hanover, NH wrote:
>
> > Patternmaking has its own set of rules, just as we have, to design
> > pieces that will release cleanly from the sand mold.  One of those rules is,
> > the deeper the impression in the sand (that is, the higher the vertical wall of
> > the piece coming out of the sand), the more draft angle required to get a clean
> > release.  So if the height of the flange changes, you can expect the
> > patternmaker to have changed the angle as well, to make sure they pour a clean
> > plate every time.
>