Parts thermoformed from an extruded thermoplastic film will exhibit tapering in the wall thickness when thermoformed. This makes sense when you consider that you start with a flat film that has a uniform thickness, heat it, and then form it into the mold geometry using pressure, typically supplied by vacuum. The degree of tapering and gauge loss will be different with each mold geometry.
There are two types of mold design – female and male. In female molds, all surfaces are recessed below the upper flat plane of the mold block. The heated film is drawn into the female geometry and a larger surface area is created. This larger area is drawn directly from the film thickness. The deeper the female cavity, the thinner the formed film will be.
Parts that are tall and/or thin may require changing to a male mold geometry, which presents different challenges.
In both cases, male and female mold geometries, the thinnest wall thickness will be along the vertical side walls and at the bottom of “inside” corners, where the film is stretched the furthest. Conversely, film that touches a more horizontal mold surface first will be more like the starting gauge of the film as it is stretched the least. The engineer, when designing a part for thermoforming, must consider tapering and gauge loss when considering thermoforming as a manufacturing process. As a rule of thumb, the taller the vertical surfaces, the greater the taper in the side walls and the thinner the film will become.