Trim Fixtures for Thermoformed Parts: Where the Accuracy Comes From

The fixture decides the accuracy

A thermoformed part is flexible and its dimensions vary from part to part, because the wall thickness varies and the shrinkage varies with it. Trimming it accurately therefore depends on holding it in a known position rather than on the cutter following a path precisely.

That is why the accuracy of a trimmed part is quoted by trimming method. The chart on the left gives positional accuracy of 0.80 mm for hand marking, 0.40 for a simple jig, 0.15 for a steel rule die, 0.08 for a CNC fixture and 0.05 for a five axis machine on a dedicated fixture.

The step from hand marking to a jig is the largest single improvement, and it costs almost nothing: a plywood or resin jig with a locating edge and two clamps. The step from a jig to a CNC fixture buys another factor of five, at the cost of a machined fixture and a program.

The cutter matters less than the fixture. A sharp router in a poor fixture produces a part with a clean edge in the wrong place, which is scrap. A modest cutter in a good fixture produces a part that fits, which is what the customer buys.

Locating strategy: 3-2-1 and its limits

The classical approach is 3-2-1: three points define a plane, two define a line, one defines a position. On a formed part the plane is the flange, which is the most dimensionally stable feature because it was not drawn.

The flange is not flat, though. It has a slight curvature from the forming and it varies from part to part. A fixture that locates on three points of the flange will rock, and the part will be trimmed differently depending on which way it rocked. The answer is more points, or a compliant support that follows the flange.

For a part with no stable flange, the locating features are usually the side walls or a formed feature such as a rib or a recess. Those are less stable than a flange but they are what the part has, and the fixture has to be designed around them.

The locating points should be adjustable. A formed part varies, and a fixture that is machined to the nominal dimension will hold some parts loosely and others not at all. Adjustable stops let the fixture be set to the actual parts, and that is worth more than precision machining.

Clamping without distorting

A clamp that holds the part also deforms it. A pneumatic clamp applying 200 N to a thin wall will deflect that wall, and the part will be trimmed in its deflected shape and spring back afterwards. The trim line then does not match the drawing, even though the fixture was correct.

The solution is to clamp on a stiff feature and to support the part underneath wherever it is clamped. A clamp with a matching support pad opposite it compresses the material between two surfaces rather than bending it, and the deflection becomes negligible.

The number of clamps matters as much as their force. A part held by two clamps will lift at the far end, and the trim at that end will be wrong. A clamp every 150 to 200 mm along the trim line is a reasonable starting point, with more where the part is thin.

Vacuum is an alternative to clamps and it is often better. A vacuum fixture pulls the part onto a shaped nest, which supports it everywhere and holds it without local force. It needs a smooth surface and a vacuum source, and it is the standard for thin wall parts.

Deflection under cutting load

The cutting force pushes the part sideways, and the fixture has to resist it. A fixture with a thin base or long unsupported spans will deflect under the cut and the trim line will wander, especially at the end of a long cut where the tool is furthest from a support.

The chart on the right puts fixture location error at 30 percent and fixture deflection at 24. Together they are more than half of all trim defects, and both are fixture design issues rather than cutting issues.

Deflection is controlled by stiffness, and stiffness comes from depth rather than thickness. A fixture base with a 50 mm upstand is many times stiffer than a flat plate of the same weight, and it costs no more to make.

The cutting direction matters too. A climb cut pulls the part towards the fixture; a conventional cut pushes it away. On a flexible part, the cutting direction and the clamp positions have to be designed together, or the part will move as the tool passes.

Choosing the method

A steel rule die is the fastest method for a flat part at volume, and it is accurate to about 0.15 mm. It needs a press and a die, and it only works if the part can be presented flat, which means the trim line has to be in one plane.

A CNC router with a fixture is the flexible method. It handles three dimensional trim lines, it is accurate to 0.08 mm, and the program can be changed in minutes. It is slower per part than a die and it needs a fixture per part.

A five axis machine handles trim lines that are not in any plane, such as a part with a curved flange. It is the most expensive per part and the most accurate, and it is used where the geometry demands it.

Hand trimming still has a place for prototypes and for very low volumes, and its accuracy of 0.8 mm is adequate for a part whose tolerance is plus or minus 1 mm. Using a CNC fixture for that part is a waste of money.

Table 1 — Trimming methods and what each is for

Keeping the fixture honest

A fixture wears at the points where the part is loaded and unloaded. The locating edges round over, the clamp pads compress, and the part starts to sit differently. The trim line moves by a few tenths of a millimetre and nobody notices until a customer does.

The check is a first article against the fixture every month, or after any maintenance. Trim a part, measure the trim line to the locating feature, and compare with the record. A drift of 0.1 mm is a warning; 0.3 mm is a fixture that needs reworking.

The fixture should also be marked with its part number, its revision and its last verification date. A fixture without identification ends up being used for the wrong part, and that is a scrap event rather than a drift.

Table 2 — A fixture maintenance schedule