Robert Barnard wrote:

Lost foam can be a fairly fast and inexpensive way to make aluminum parts in
small production batches.  The steps include: making the master pattern,
making tooling, making foam cores, packing foam cores in "green sand",
pouring the metel into the foam.

Step 1. The pattern
This is the product shape usually made from wood.  Mahogany is one of the
best to work with.

The pattern must be over sized to compensate for metal shrinkage.  We used
1.01 to 1 for aluminum castings.

All sharp acute or small concave shapes are filleted with modeling clay.
The (rounded) fillet shape is formed and blended into other fillets using a
ball of the desired radius on the end of a stick.

Lacquer the entire pattern.

Step 2. Tooling
Lots of materials (plastics) are usable for creating a foam core mold.

If the pattern has no draft angles (making removal of the cores difficult,
or damaging to the mold), urethane is the ideal maternal to create a mold
from.  If other plastics are used, consider a 2 - 4 degree draft to vertical
surfaces (assuming the parting line of the mold is horizontal).

Make one half of the mold first****
Make a (wood) box large enough for half the mold.
Draw a line around the pattern where the parting line of the mold should be
(taking care to orient the pattern so no product geometry will become
"hooked" in the mold).
Cover the pattern in mold release spray.
Screw or nail (small nails or screws) supports in to the part of the pattern
that will be above the plastic in this box.
Partially fill the box with plastic.
Lower the pattern into the plastic until the plastic "water-line" meets the
line you pencilled on to the pattern.
Support with flexible (goose neck) clamps so pattern does sink or role over.

Add locator pins*********
When the plastics curred, drill vertical holes to accept locator pins and
insert/thread them in.

Make the other half of the mold*********
Spray mold release on the lower box (still containing the pattern and
locator pins).

Take another box (actually 4 sides of a box, no top/bottom) the same size as
the first, place in on the first box.  This will be the upper box (half).

Fill it with plastic covering the pattern (still in the lower box), and
locator pins.

Let cure, then dissassemble and remove the pattern.

Add vents (screen or slotted).

Step 3. Make the core(s)
Steam is used to expand the foam granules and fuse them together into one
piece, the vents are to facilitate steam to and from the mold.

After filling the mold with foam granules, clamp the box together.  A long
of pressure is going to be created.  Plan for up to 20 psi over the entire
face of the mold (not just product geometry). So a 12' by 12' mold could
create 144X20 pounds of force (2880 pounds).  If the mold is over filled,
pressures can be much higher, the mold may end up looking like a large
popped corn.

Running short on time so lets to go "green sand".
This is fine sand mixed with clay and some grease.  It's able to be somewhat
plyable and provides a good surface finish while holding product shape once
the foam is gone.  Some hobbyist's use sand with grease only since clay has
water in it, getting to much water in the sand/clay can be a real disaster.

Good luck!

Rob Barnard

 
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