Forming a Thermoplastic Bowl

Build the Thermoformer

• 1

  Use a tape measure, framing square and carpenter's pencil to lay out a 24-by-24-inch dimension, for the thermoforming platen, a flat plate made of a piece of 3/4-inch plywood. Use a circular saw to cut the square. Draw diagonal lines from corner to corner, locating the center of the piece. Nail four 2-by-2-inch pieces of lumber 6 inches in length under the four corners of the plywood platen for its legs.

• 2

  Measure and mark a 6-inch square on a piece of heavy rubber or tire inner-tube. Hold a straightedge against the lines, and cut the rubber with a sharp utility knife. Apply rubber cement to one side of the rubber piece, and glue it firmly to the center of the 24-inch piece of plywood. Allow the cement to harden for 24 hours.

• 3

  Use a 1 1/2-inch hole-saw bit on a power drill to drill a 1 1/2-inch centered hole through the rubber gasket and the plywood platen. Start the cut gradually, allowing the bit to cut out a rubber circle before plunging it into the wood. Apply rubber cement to the inside surface of the cylindrical hole in the platen.

• 4

  Measure, mark and use a hacksaw to cut a 2-inch piece of 1 1/2-inch PVC pipe. Apply more rubber cement to the exterior surface of the piece of pipe, and insert it into the hole in the platen. Make the fit so the upper end of the pipe is flush with the rubber gasket. Attach the other end to a standard hose fitting on a high-powered shop vacuum.

• 5

  Select two 20-by-20-inch aluminum screens, and cut out the screening material with a utility knife. Smooth down the edges with a heavy rag or gloved hand. Use one of the frames to draw its outline on the platen, centered on the vacuum hole. Measure and use scissors to cut four 1-inch-wide strips of sponge rubber weather stripping. Pull off the adhesive-covering strips and attach them, centered and over the screen dimension lines on the platen.

Thermoform the Plastic Bowl

• 6

  Fold a piece of aluminum screening material twice, into a square larger than the diameter of the target bowl. Lay the folded screen on the platen, centered over the vacuum hole. Place the model bowl, sprayed with plastic molding release agent, centered over the vacuum hole in the platen. The air will pass under the model, and out through the vacuum hole in the platen, as the heated plastic is drawn over the bowl model.

• 7

  Use one of the 20-inch frames as a cutting template, and cut a 20-inch-square piece of thermoplastic material with the reciprocating saw. Insert it between the two 20-inch frames, and attach document binding clips, every 4 inches around the screens, to hold the plastic sheet tightly inside the frames.

• 8

  Place the screens into a 250-degree conventional oven, resting on glass or metal objects that elevate them, but do not touch the plastic. Check repeatedly, starting at 15 minutes, until a 2-inch or greater sag develops in the plastic material.

• 9

  Turn on the shop-vac and, wearing heavy gloves, remove the frames from the oven. Place the frames/plastic over and onto the upper surface of the model bowl, and press them down and onto a perfect fit on the sponge rubber weather stripping. A vacuum will form instantly under the plastic sheet and, as the air is drawn under the bowl and out of the vacuum hole in the platen, the heated plastic will conform to the outer surface of the bowl.

• 10

  Turn off the vacuum in 30 seconds and use a sharp utility knife to cut the plastic around the upper edge of the bowl model. Allow it to cool for 20 minutes, then clean up the upper surface with the knife and emery cloth, producing either a rounded or flat surface.