|Resin Ear Posts
To make the master ear post, I started with a couple of basic
wooden shapes from the craft store: a 1.25" ball, a pencil pot and a 1" dowel.
I turned down the dowel on the drill press (no lathe, yet) to make the stem,
drilled a hole in the ball and glued the stem. I then sliced off the end of
the pencil pot and turned it down, and glued it to the top of the ball. Some
wood putty to fill in the gaps, round off the base to fit the radar section, a
smaller dowel on the base, sand it smooth and apply about 4 coats of spray
shellac. The dimensions match Dave Painter's plans except where the post goes
into the radar section, which I made .625" instead of .600" for a better fit.
Next is the mold box, made of Legos, of course. At this point, I sprayed a
couple light coats of mold release on the inside of the box.
Next, I laid down a bed of plasticine clay, then sprayed the
master with mold release, pushed it into the clay and built up to the midway
point with more clay. The holes are to align the two mold halves. Next, I
mixed up a batch of of
Por-A-Mold, from Synair. Por-A-Mold is a 2-part polyurethane mold making
material, which gels in about 35 minutes, and cures in 24 hours at room
temperature. The two parts are mixed 1:1 by volume, so no scales are
necessary. I used the medium variety. I had a little trouble with the first
pour, in that some unmixed curative stuck to the bottom of the mixing cup and
ended up in the pour. It never cured properly, so I threw it out and mixed
up another batch. My shop is on the cool side, so this time I made sure the
materials were warmed up before mixing, and took it upstairs to cure.
After letting the top cure for 24 hours, I removed the clay,
and cleaned off the top of the master. Mold release on the master, mold half
and the inside of the box, then another batch of Por-A-Mold.
After pouring the mix into the mold box, I took a tip from another hobbyist
and set the box on my scroll saw table and turned it on, to help vibrate any
bubbles up to the top. It seems to have worked, since there were no bubbles
present when I peeled the mold apart. Note - Legos make a quick mold box, but
they're a pain in the butt to clean up, since the polyurethane gets in the