Bubba Don't Forget Your Rubber

Making a Two Part Rubber Mold for the Streetlamp/Line Pole Project.

Even though this is a rather crazy project, the making of this rubber mold is pretty straight forward. I watched tons of YouTube videos on how to pour rubber molds until I was confident I knew exactly what I needed do. This video in particular comes closest to what I need to do, except that I'll be sandwiching a brass conduit and a couple of LEDs in the mold along with the casting resin (I told you this is a crazy project!).

I'm a beginner when it comes to casting, so I need everything, rubber, sealer, mold release, and casting resin. The guys on the Railroad Line Forums recommended using Oomoo Silicone Rubber from Smooth-On. Smooth-on offers a starter kit that has everything I need to make a mold and castings, so I went with that.

Last week I built this two-part mold box for the two-part mold (bottom part shown). Since the streetlamp master is suspended in the box I was tempted to fill the box with rubber just like that...

...but the rubber crept up the sides of the master and completely covered it in some parts. That's unacceptable for casting. I tried trimming it back when it set, but that didn't turn out too well so...

Take Two!

This time I'm going to do it the right way like all the instructions said to.

That means filling the bottom of the half of the mold box with clay, carefully working it up around all the details of the lamp right up to the half-way line. Packing the clay tightly against the master so there aren't any voids that will be filled with rubber. Some strips of clay protruding up will create a keying effect so that the two halves of the mold will snap together with some precision. 

The top of the mold box is attached and the whole thing is sprayed with sealer. After two coats and drying time, spray on mold release (clean the sprayers afterwards, mine clogged. Now I have to brush it all on!) and let dry. Now were ready for rubber!

To determine exactly how much rubber I need, I had filled the mold box with sand...

... and then poured that sand into the mixing cup. I marked its level with a Sharpie. Remove the sand, pour the liquid silicone Part 1 half way to the mark, and then part 2 the rest of the way. Mix well.

Oomoo is easy. Mix equal parts of A and B and mix it really well. A woman in one of the YouTubes suggested warming up the bottles a little first so that the compound is even thinner.

And then fill up the mold box and let set. So far so good!

Flipped the whole thing over and dug out the clay revealing the master and the first half of the mold. Success! Attached the second half of the mold box and then added the coats of sealer and release. Now were ready for the second pour.

Success! It turned out beautifully. The rubber captured the strata layers inherent from the 3D printing process perfectly!

Next week comes the crazy part: filling the two halves of the mold with resin and then placing the brass armature with LEDs into it and then sandwiching the whole thing together! That's the plan anyway. I told you this was a crazy project. Yes sir! Crazy!

Dandy