Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. A plastic sheet is heated in an oven then stretched into or onto a mold and cooled to a finished form. This article will look at the types of thermoforming and the applications of plastic thermoforming.
What is plastic thermoforming?
In its simplest form, a small machine can be used to heat small sections of plastic sheets and stretch them over a mold using a vacuum. This process is often used for sample and prototype parts. However, in complex and high-volume applications, very large production machines heat and form the plastic sheet and trim the formed parts from the sheet in a continuous high-speed process. They can produce thousands of finished parts per hour depending on the machine and mold size, and the parts being formed.
Plastic material is supplied in rolls or resin pellets. Rolls of plastic sheet are either created in an extrusion facility or purchased. Resin pellets are used for in-line thermoforming. The extruded plastic sheet is fed into the thermoforming machine and carried into the heating area on chains. The chains advance the sheet through the thermoforming machine’s heating oven, form station, and trim station.
What are the types of plastic thermoforming?
The two main types of thermoforming are vacuum forming and pressure forming.
Vacuum forming uses heat and pressure to shape plastic sheets. First, A sheet is heated and placed over a mold, where a vacuum manipulates it into the desired shape. When the material is detached from the mold, the final result is a precise shape. This type of thermoforming produces dimensionally stable parts on one side with high-quality esthetics on the other material side.
It may seem simple, but a few problems can arise when vacuum forming isn’t completed correctly. If the plastic absorbs moisture, it can create bubbles within the inner layers. These bubbles significantly weaken the plastic. Therefore, the plastic is dried for an extended time before thermoforming when necessary.
Webs are also a risk in vacuum forming. Webs can form from overheating, oversized molds, or parts of the molds being too close together. However, careful temperature monitoring and attention to detail easily prevents this issue.
Pressure forming is similar to vacuum forming but benefits from added pressure. The process also involves heating a sheet of plastic and also adds a pressure box to the non-mold side of the sheet. The pressure box creates up to 60 pounds per square inch (PSI) of air pressure, in addition to the vacuum on the mold side. The extra pressure causes sharp detail.
Pressure forming creates parts similar to an injection-molded finish but has lower tooling costs. In addition, because of the higher pressure, this technique can provide parts with sharp detail and multiple textures on the same part. These details make it perfect for forming multi-part assemblies, housings, or even medical equipment. This operation also takes less time to design, which means less time tooling.