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 Trailer Hitch Covers

In this section, I'm working on making rocketry logo trailer hitch covers. I'll

be making a "match plate" to help speed up production. A match plate is a

board that sits between the cope and the drag. Half of the pattern is on

each side of the board along with the runner and gates used to distribute

liquid metal in the mold to the patterns.

In this case I am using 3/8" plywood as the base for the match plate. This

photo shows the drag half of the mold. The initial layout shown here is on

a cardboard template.

I screwed the wooden runner to the center of the match plate. The metal

patterns were attached with hot melt glue. Florist's clay was used to form

the gates and cup on the runner.

This is the back (cope) side showing the square bosses that will be used

in the final casting to attach a 2" square aluminum tube that will slide into

the receiver on the vehicle. The dark wooden piece will form a hollow in the

mold to provide additional hot metal to the thin plates on each side and

hopefully will prevent shrink cavities from forming in the plates below the

square bosses. The red areas are where I used automotive spot putty to

fill and smooth holes in the plywood.I sprayed clear-coat paint on the

matchplate to protect the wood from moisture and also further smooth the

surface. Florist's clay is used here to cover wood screws used to

attach the patterns to the match plate.

Here are the first 2 castings. At first they look pretty good, but sadly the

extra piece described above did not prevent the shrink cavity. Look

closely at the "IP" in Tripoli.

The letters are distorted because of the large volume of metal in the boss

behind these letters shrank as the metal cooled.

Here is a view of the boss on the back side...see the shrink cavity in the

center? I'll have to re-design the pattern/match plate to fix this problem.

Larger/wider gates would help. These thin areas are probably solidifying

early and starving the pattern from receiving additional liquid metal as it sets.

At any rate, the Kloudbusters plate was ok to use. The next step is to paint

the plate as shown below. After the paint dries, the surface is sanded to

reveal the aluminum below.

Finally the finished plate is friction fitted to a 2" square aluminum tube. A

5/8" hole allows the hitch plate cover to be secured in the receiver with a pin.

August 6, 2009

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Updates:

Here are the changes I made to the match plate. The square boss was

replaced with small wedges.

Here is the resulting casting. The shrinkage is almost gone. I forgot to make

the clay gates larger which should eliminate any remaining shrink.

I'm thinking of replacing the wedges with aluminum angle. It will use less

will be easier to pull from the sand without tearing the edge of the mold.metal to make the casting,will be easier to align with the front plate, and

Update March 2012

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While updating the web site I decided much of the shrinkage problem here was caused
by the metal being too hot when it was poured. I can tell because of the rough, bubbly
texture of the metal in the sprue.


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