How to Mold A Plastic Ball

Cut a square piece of ½ inch MDF, 1 inch larger than the diameter of the plastic ball. Cut a hole in the center of the MDF squar, which will allow the ball to just fit through.

Cut a second MDF square which is 1 inch larger than the first square. Remove the center of this square to allow the ball to fall through.

Place a bead of hot glue on the bottom of the smaller square of MDF and quickly center and place it on top of the larger square. Press down to get a tight fit.

Place the plastic ball in the center hole. The goal is to have exactly half of the ball above the surface of the MDF square. It may be necessary to put small pieces of MDF under the bottom of the larger square to raise the squares up and allow it to sit deep enough.

Using modeling clay, close any gaps between the ball and the edges of the hole in the MDF. Take care to smooth the modeling clay to the surface of the MDF. A better job here means less cleaning of casting flaws on the finished cast plastic ball.

Cut 2 pieces, 2 inches long from the plastic drinking straw. Glue these pieces vertically on the top of the ball, leaving about ¼ inch between them. Use a minimum amount of glue, as any excess glue here will result in a larger area to smooth on the finished ball.

Cut 4 pieces of MDF to form vertical walls around the outside of the larger MDF square. Attach to the larger square with hot glue placed on the edges of the MDF square. The walls must be at least ½ inch taller than the height of the center of the ball.

Coat the ball and all interior surfaces of your MDF box with mold release agent.

Mix silicone mold making rubber and pour in the box to a depth ½ inch greater than the height of the ball. The straws will protrude from the rubber.

Allow the mold making silicone rubber to cure for 24 hours. After this time, remove the MDF box and the MDF squares. Do not allow the ball to move from the silicone rubber. The straws can be removed from the rubber at this time.

Place the half-mold on the table of the bench with the ball side up. Remove any traces of modeling clay from the ball and mold. Install the MDF pieces back around the mold and glue in place. Make sure the walls are higher than the center of the ball.

Coat the ball and MDF pieces liberally with mold release agent.

Pour mold making silicone rubber into the box, covering the ball completely. Pour at least ¼ inch deep over the center of the ball.

Allow to cure for 24 hours. Remove the MDF box. Separate the mold halves by pulling them apart at the seam. The ball should be removed at this time.

Cast balls from the mold by closing the mold and placing on a bench or table with the 2 small holes up. Pour or inject the casting material in one of the holes. When the material is extruded from the second hole, stop pouring, the mold is full. Allow to cure and remove from the mold.