Trim Lines

With rare exceptions, Thermo Pressure Formed parts must be cut out of the original sheet to separate the finished part from that portion of the sheet that was used to clamp it into the forming machine. This procedure can often be done as a part of the routing step used to provide openings through the side wall. However, it may be done as a separate step.

There are many ways to trim the final part out of the original sheet. However, the two most common locations for the trim lines are as shown at locations J and K on Figure 16. A trim line on the flange as shown at J is the most common and the preferred location.

The trim line can be located at position K when it is desirable to remove the flange to produce a trim such as that shown in I. This location has some disadvantages however. The flange acts as a reinforcing rib and its removal will significantly weaken long straight walls. This type of trimming normally requires more complex and therefore more costly holding fixtures.

Independent of whether the trim line is at position J or K, it must be located back from the side wall or the flange a minimum distance equal to the size of the outside corner radius plus .030 inches.

In cases where inward projecting undercuts are being provided at the open end of the part, the trim line can be completely hidden from view as shown in Figure 17 E. This trim location, however, is more difficult to accomplish. A trim line located as shown at 17 F will be more economical to achieve.

The most economical trimming of the finished part out of the original sheet will be accomplished if the trim lines are straight and outside corners are left square as trimmed. Irregular shaped trim lines can be provided. However, they complicate the trimming process.

The whole subject of numerically controlled machining of molded and formed parts is relatively new and many designers are not familiar with the capabilities of the process. In cases such as this, the project should be reviewed with a knowledgeable supplier before the design is finalized. The tolerances that are possible with various types of machining operations are reviewed on Page 19.