UNDERCUTS

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GUIDELINES:

Keep the distance that the undercut projects into the part to a minimum. Typical undercut sizes are .375" with some localized tabs of up to 1". Because the undercut will require even more stretching of the sheet stock it is important to keep the draw ratio in mind when designing for undercuts.

OVERVIEW:

Undercuts are a feature that can be added to thermoformed parts very cost effectively. Undercuts offer increased part strength, a locating edge, a fastening point, and/or the ability to hide a trimmed edge. Tooling costs will be increased, but not nearly the amount it would be if it were an injection molding or structural foam molding tool.

SPECIFICS:

Typically most undercuts are an inward facing flange. However, other types of undercuts might include a reverse drafted wall, a molded in countersink or a design line that is not parallel to the direction of pull out of the mold. Because these features increase the surface area of the part they increase the draw ratio of the part. One of the most common requests is to carry an undercut flange into one or all four corners of a part. The problem this presents is that it causes the material to stretch even more in an area that is typically the thinnest on the part, if it is a female tool. By stepping the undercut back in the corners you allow for better material distribution.

Some undercuts do not require the mold to be collapsible or removable. The undercut may be small enough or the material flexible enough to allow the part to strip out of the mold. This is more likely to occur on a female mold because the part will shrink away from the sidewalls of the tool, as opposed to a male mold which finds the material shrinking tighter around it.

If the undercut requires a moving section in the mold you must allow for a parting or witness line on the part. This is not normally a problem on an undercut, which is an inward facing flange, since the parting line can be hidden at the point at which the part turns in. But on other undercuts there needs to be an allowance for the parting line. Many times the parting line is used as a point of demarcation between a textured and untextured surface.

The safest way to incorporate an undercut in the mold is with an articulating section. In the past removable loose pieces have been used and placed back in the mold once they were freed from the part prior to the next shot. Because of the risk of tool damage due to improper alignment or the marring of a textured surface, the best long term approach is to incorporate the articulation of the undercut section with the controls of the forming machine operating it automatically. A quality thermoformer will have this capability.