Commercially manufactured kettle corn machines can cost hundreds or even thousands of dollars. If you are fluent in the fundamentals of how they work and are comfortable working with metal, you can build your own machine for a fraction of the cost. Be sure to invest in quality parts, and do not skimp on safety either, as the high temperatures involved in preparing kettle corn pose a fire hazard, and are capable of causing severe burns.
Purchase a kettle with a rounded, wok shape. This kettle will need to be sturdy enough to withstand heat of several hundred degrees. If you have the option, choose a NSF-certified kettle. For a commercial popcorn operation, your kettle should be as large as 40 gallons, although you can buy a much smaller kettle as your needs dictate.
Purchase an iron or steel plate, square-shaped, with dimensions at least six inches larger than your kettle diameter. Cut a circular hole in the centre of the plate with a diameter large enough that most of the kettle will pass through, leaving only about the top 3 inches sticking out above the plate. Lay the kettle into the plate, and weld it into place on both the top side and underside of the plate along the outside of the kettle.
Build a steel box with the same cross-sectional square area as the metal plate. Make it tall enough so that there will be room to put a gas burner beneath the kettle, with about three inches of clearance between burner and kettle for the flame to burn. Mount the box on wheels to aid movement, effectively transforming it into a cart.
Connect the top metal plate to the metal box by means of a heavy-duty hinge. This way, you will be able to swivel the kettle into almost an upside-down position by means of an external handle. Test the apparatus in an open position to make sure that the cart will not become imbalanced. If this is a concern, add iron ballast on the far side of the cart. Mount an insulated handle to one side of the plate. Optionally, install a spring latch mechanism to aid in lifting the plate and kettle.
Install a gas burner beneath the kettle, and cut small ventilation holes into the bottom of the cart to facilitate the flow of oxygen and to prevent the temperature inside the cart from getting too high. Make sure you buy a heavy-duty, UL listed burner capable of withstanding the high temperatures. Mount the burner ignition switch on the outside of the cart in a convenient location.
Connect the burner, by way of a shutoff valve, to a 100-pound propane cylinder. Mount the flow regulator for the propane gas tubing next to the ignition switch on the outside of the cart.
Purchase a large metal sifting bin, equivalent or greater in volume to the kettle, with grating on the bottom large enough to permit unpopped kernels to slip through, but not popped ones. You will dump the freshly popped corn into this bin immediately after it pops.
Purchase a stirrer that will safely stir the popcorn at the high temperatures you will be cooking with.
Visit a kettle corn machine vendor and inspect their commercially manufactured units to get construction ideas for your machine.
Always use safety goggles and gloves when preparing kettle corn.