Industries which produce items such as packaging, automobile and aircraft parts, toys, and electronic components often employ a manufacturing process that consists of heating thin sheets of plastic and applying them to molds to form numerous shapes. This method is called thermoforming, and it can be done in several ways which all produce molded plastic objects.
Different approaches may be used, depending the the size and shape of the items being molded, as well as the kind of plastic used, but the process follows the same basic principles in all of its forms. A plastic sheet is secured down and placed inside an industrial oven, where it is heated to a pre-determined temperature. When the plastic is soft, it is then stretched over the mold, cooled down using fans, and left to become rigid. There are several types of plastic that may be used, but the most common is HDPE, or \"high density polyethylene\".
When a vacuum thermoform method is applied, the sheet plastic and mold are put into a temperature-controlled vacuum unit, the former over top of the latter. Heat softens the plastic, then the waiting mold is raised up until it presses into it, as air is drawn out from beneath, producing a vacuum and airtight seal between the two surfaces which molds the shape.
Pressure forming implements compressed air which is released out of the top of the thermoform machine and exerts a force anywhere from 15 to 300 pounds per square inch onto the softened plastic. The amount of force required depends on the size of the form. The plastic is pushed into or around the molded shape from the air pressure.
Mechanical forming is that which uses a double-mold that has a negative bottom half and positive top half. The plastic sheet is placed over the bottom of the mold, heated until it is pliable, and then covered with the top half of the form which squeezes the softened plastic in the particular shape being made. Air vents in the bottom piece provide ventilation.
Other types include pressure diaphragm forming which fabricates plastic parts using a vacuum and pressurized fluid. Also, twin sheet forming is used particularly for joining two parts that are separated by a hollow space, and it involves first melting two sheets of plastic then joining them together.
Many manufacturing companies prefer the thermoform approach over injection molding because it offers several advantages. The biggest benefit is that it costs much less for the equipment than it does for injection molding machines, in particular for larger sized objects. Developing a suitable prototype is also more easily facilitated and affordable for thermoform processes.
