I recently built my own rotomolder, a device that lets you make hollow parts like the hollow chocolate bunnies that they make every easter. It rotates a mold with resin inside 360º in two directions until the resin cures. Once hard you pop the mold open and there you have your hollow part!

I'm making this instructable to show what I have done with my rotomolder.

Step 1: Creating the Mold

You want to make your mold as light as possible so I used two plastic bowls from my local "one stop shop".

Take your bowl, flip it over and place it on the center of your board. Trace the outline of your bowl onto the board. Use a bandsaw or jigsaw to cut just inside the line you traced.

Remove the center and place your board on a flat surface with a sheet of wax paper underneath. Your bowl should fit in the hole you just cut out. You may have to sand the edges a bit to get it to fit.

Using a hot glue gun or two-part epoxy, squirt the glue into the space between the bowl and the hole making sure the bowl and the board stay flat, let the glue dry.

Cut your plexiglass to the same size of your boards.

Step 2: Assemble Your Mold

You should have two halves.

Drill a 3/4" hole in your bowl, this is where you will pour your resin.

Clamp both halves of together with the plexiglass in the center, then drill a 5/16" hole in each corner. These will be four bolt holes. This is how you will hold your mold together.

Spray your mold release on the inside of the bowl and BOTH sides of the plexiglass. You need to do this before you assemble the mold or you might ruin it and permanently bond your parts together.

Ask me how I know this....

Use your 1/8" sheet of plexiglass or acrylic and place it between the two bowls. This will act as a wall for the face of your speakers.

Bolt the mold together and you're ready! If the surfaces don't quite meet you can use tape along the outer edges to help seal the mold.

Step 3: MIX and ROTATE!

Place/secure the mold in the center of the rotomolder with the 3/4" holes facing up.

I mix up 300 grams of a two-part Urethane. You get about 5-7 minute work time with this and 15-20 minute cure time. Different resins will give you different results, but this particular resin can be de-molded in about an hour and sand-able in roughly 6hrs. It mixes clear but cures white, making it easy to be pigmented.

I pour about 150 grams of resin in each half, plug the holes with the two rubber stoppers and start rotating it. I turn it for about 20-25 minutes and then leave it alone for an hour. If I had a second mold I would swap it out and turn another set. MASS-PRODUCTION!!!!!

Right now the rotomolder is hand cranked, the trick is that the slower you rotate it, the more evenly you will coat your mold. If you spin it too fast all the resin will be slung up into the corners of the mold, making the side walls very thin.

I'm planning on adding a motor to it soon for using it with resins that require longer cure times. The handle is removable.

Step 4: How It Works

Here's the process.

Step 5: Demolding

Demolding and making the hole for the speaker.

Step 6: Finishing

You should now have two hollow bowl/speaker shapes, you're halfway there!

After the two parts are cleaned up, the next step is to make holes for the speaker, stands and speaker wire.

I used a handheld router but recommend a hole saw to cut the 5.5" hole in the front for the speaker. You could use a Dremmel as well, just trace the hole onto the shell and use it as a guide.

Three 1/4" holes for the stands and the speaker wire.

Insert the screw from the inside of the shell and thread it onto the stand on the outside.

Run the wire through the last hole and stuff the speaker shell with polyfill.

Lastly, screw the speaker in place.