Do it Yourself Thermoplastic Molding

By Charles Clay
Thermoplastic molding allows you to create thin plastic models from a mold or form.

Thermoplastic molding is a method of creating objects using a mold and heated plastic. When the plastic is heated, it becomes soft and is capable of being formed into a wide variety of shapes. It can be used as an inexpensive way to create replacement parts for plastic components or durable and attractive art projects. You can build a thermoplastic molding rig at home designed around your kitchen stove and shop vac. This design allows you to tightly mold heated sheets of thermoplastic material to a premade form, allowing you to create multiple precise copies of thin plastic.

Building the Molding Frame

Measure the inside of your oven. Cut four lengths of 2-inch by 4-inch timber long enough to make a rectangular frame that will fit inside your oven with a few inches of clearance on each side. Fasten these boards together using a pair of 3-inch wood screws in each corner.

Cut a square of plywood slightly larger than the bottom of your timber frame. Cut four lengths of plywood long enough to form the sides of a shallow box with the square of plywood as the bottom. The side pieces of the plywood box should be slightly taller than the nozzle of the angled attachment on your shop vac. The angled attachment needs to be fastened to the outside of the plywood box and maintain a good seal against the wood.

Drill a large hole in one of the plywood side pieces. Screw the plywood boards together into a shallow box using the wood screws. Attach a 2-inch by 2-inch cube of wood to the inside of the plywood box at the center. This will help to support the top of your box during vacuum forming. Cut a square of pegboard large enough to overlap the frame of the box on all sides. Attach it to the top of the box using wood screws.

Use small screws to fasten the angled attachment of the vacuum cleaner over the hole drilled in the side of the plywood box . Drive the screws through the lip of the angled attachment on all four sides to ensure a good seal. Seal all the seams of the plywood box and the seam between the angled attachment and the side of the box using duct tape.

Molding the Plastic

Place your form in the center of the pegboard on top of the plywood box. Put some coins under the edges of the form to allow the air to flow freely between the form and the pegboard. Attach your vacuum cleaner hose to the attachment on the side of the plywood box.

Cut a sheet of thermoplastic material to the size of your timber frame using a sharp hobby knife. Place the sheet over your timber frame and attach it to all sides of the frame using a stapler, placing the staples no more than an inch apart.

Place the timber frame in the oven with the plastic side up. Set the racks in the oven so that the plastic is about five inches away from the broiler. Set the broiler on high without closing the oven door. As soon as the plastic begins to sag in the center, press down on one corner with a pencil eraser. If the plastic is soft, remove it using a pair of oven mitts.

Place the frame on top of your form, plastic side down. Press gently until the sides of the frame are flush with the top of the box. Turn on the vacuum. Continue pressing down for 20 seconds until the plastic has hardened. Turn off the vacuum and lift the frame.

Remove the form by tapping one edge of the frame against the side of a counter until it falls out. Remove the plastic from the frame and trim away the edges using a sharp hobby knife. Sand the cut edges smooth using high-grit sandpaper.

Tip

You can use almost any three-dimensional object as a form, as long as it cannot be damaged by contact with the hot plastic. You can also make your own object out of plaster, wood or urethane foam.

Warning

When the plastic comes out of the oven, it is hot, soft and sticky. Keep it away from your skin and clothes or you could be badly burned.

Have plenty of ventilation in your work area. The buildup of fumes from thermoplastic can be harmful in a confined area.

Don't leave the plastic under the broiler for too long. If it is overheated, the plastic melts completely and then burns.