An outer contour cannot be derived by tracing a top-view photograph of a formed part. The free component can flex, and flange width or corners may not be uniform. Measurement must describe how the part will seat in the thermoform trim cutting die.

Separate the finished edge from the process flange

Define the edge that must remain on the product and the excess area used during forming. The trim line separates them. Copying the existing sample edge without a finished dimension can repeat variation from the previous process.

The finished edge comes from the customer’s product drawing or from the place of assembly: if the part seats in a recess, the recess dimension sets the edge; if it sits on a shelf, appearance does. When that dimension is given in writing the trim line is derived from the target, not from the sample, and the previous process variation is not carried into the new tool.

  • The finished outer dimension and the flange width to remain are given in writing.
  • If the part seats in a recess, the recess dimension is the reference.
  • The sample’s existing edge is not copied; the line is drawn from the target dimension.

Use a reference network rather than one centre

Measure opposing edges, corner radii and functional holes or recesses at several points. Define the seating surface. Compare the free part with a fixture-like seated condition.

The reference network starts from the surface the part will seat on: the base, the inner wall or the underside of the flange? Once that surface is defined, opposing edges and corner radii are measured relative to it; a part measured free and a part seated against a fixture-like stop give different readings, and the second is what counts.

  • The seating surface (base, wall, underside of flange) is defined first.
  • Opposing edges and corner radii are measured relative to that surface.
  • Free and seated readings are recorded separately; the seated reading counts.

Check variation between samples

Overlay multiple thermoformed samples when available and observe perimeter differences. Do not mistake local deformation in one part for intentional tool geometry. Set acceptable flange using the real process spread.

With several samples overlaid, the perimeter difference shows at a glance; if a local deformation on one sample is taken for tool geometry and cut into the trim line, the whole run carries that error. The acceptable trim allowance is chosen so that the narrowest and widest parts of the real production spread both pass.

  • At least three samples are overlaid to observe the perimeter difference.
  • A local deformation on one sample is not cut into the trim line.
  • The trim allowance covers both the narrowest and widest parts of the spread.

Keep dimensions and orientation in the drawing

Add part orientation, top face, locating points and required remaining flange to the DXF or AI contour. The cutting path should be one clean closed vector with duplicate lines removed.

In the DXF or AI file the contour is one closed vector; part orientation, top face and locating points are added as a layer or a note. The flange width to remain is written as a dimension; a note such as “leave some flange” forces the die workshop to guess.

  • The contour is delivered as one closed vector; duplicate lines are removed.
  • Part orientation, top face and locating points are added to the file.
  • The flange to remain is written as a dimension, not in words.

Application note

Send the physical sample, finished outer dimensions and flange that must remain so the trim line can be built from all three.

  • Use the current material and dimensions.
  • Compare a clean sample with the defective sample.
  • Record the decision with photographs and drawings.