Make an exact copy of the core from Styrofoam.

Make a box that will accept the size of the core. Suspend the "core pattern" about 1" off the bottom of the box.

Attach using large nails pushed through the box and into the pattern.

Fill box with plaster of paris up to halfway (parting line) up the pattern.

 

Place 2 bolts into the plaster before it dries, at opposite ends of the mold. These will act as guide pins when making the cores later.

Place a straw over the bolts before pouring the second section and a piece of tape over the end.

 

After the first section has completely dried, spray with wax or paint to keep the sections separate.

Fill the last section with plaster and allow to harden.

After removing the foam and lightly sanding the core pattern, it is ready.

Open "core box". Notice the guide pins and corresponding holes.

  Cores love to stick in the mold, I use a spray wax as a mold release.