Thermoforming

To see pictures of thermoformed parts, click here.

To watch a video about thermoforming, click here.

Process Overview

Thermoforming makes parts out of plastic sheet. The sheet is heated until it is pliable, and then it is clamped around one side of a mold. The plastic sheet is drawn towards the mold, most commonly by vacuum, but also by air pressure. The mold is cooled so the sheet rapidly solidifies, and then it is ejected. Because of the low pressures and the nature of the process, great detail in the part cannot be achieved. Thermoforming is commonly used to make cheap cups (Taco Bell makes their cups this way), disposable plastic plates, blister packs, and other products.

Fundamental Vacuum Forming

This is the simplest kind of thermoforming. As in all of the thermoforming processes, a plastic sheet is heated until it becomes pliable. The soft sheet is clamped against the top of the mold. A vacuum pump then evacuates the air beneath the sheet, causing atmospheric pressure to push the sheet against the mold walls. Because vacuum forming actually uses atmospheric pressure to form the part, pressures of only about 14.5 psi are possible. Vacuum forming is used when details are needed on the outside of the part.

Pressure Forming

This process is similar to vacuum forming, except a compressed air source is used instead of atmospheric pressure. A plastic sheet is heated until it becomes pliable. The sheet is then clamped against the mold. Air pressure, usually between 15 and 300 psi, is applied to the upper sheet surface, forcing it against the mold walls.

There are some advantages of pressure forming over vacuum forming. Because higher pressures are used, greater part detail is possible, the part can be formed faster, and lower sheet temperatures can be used. These last two advantages reduce cycle time.

Plug Assist Forming

Plug assist forming is used for parts with deep draws, or parts where a fairly uniform wall thickness is required. Basically, the softened sheet is initially pushed down by a plug. Then, either vacuum or air pressure is used to finish forming the part.

If a deep draw part is formed without plug assist, there will be areas where the wall thickness will be extremely thin. In plug assist, the plug carries the material closer to the wall, stretching it more uniformly so thin spots do not occur.

Drape Forming

Drape forming is essentially the same as vacuum forming, except the sheet is formed over a protrusion instead of a cavity (it uses a male mold instead of a female mold.) The process is exactly the same.

Matched Die Forming

In this process, both halves of the part are formed by molds. The sheet is heated until it is soft, and then both mold halves clamp together to form the part.

Summary

Thermoforming produces parts from a flat plastic sheet by heating it until it is pliable, and then forcing it against a mold with vacuum or air pressure.

Some characteristics of thermoformed parts are:

· Constant wall thickness is difficult / impossible to achieve

· Only very simple geometry can be made

· Detailed surfaces (texturing, text) cannot be achieved