Random sampling in plastic moulding
Example: button for a seat belt buckle

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Inspecting samples for statistical process control

The button on a seat belt buckle in the car must meet high quality standards to ensure that this safety-relevant component works faultlessly. Even though it is subjected to frequent mechanical stress in daily use, the button's stability over the life time of the car must be 100% guaranteed.

Quality assurance requirements

Quality assurance for the button should ideally be as comprehensive as possible and should be performed immediately after its manufacture in the plastic injection mold. Otherwise, defects are only identified after the button has been installed, leading to high subsequent costs because the entire safety-relevant module must be exchanged. In a worst-case scenario, this defect can impair safety, e.g. because the seat belt can no longer be used.

Best fit of CT data (blue) and CAD data (gray). Measuring values outside tolerance are marked in red.

Best fit of CT data (blue) and CAD data (gray). Measuring values outside tolerance are marked in red.

Statistical process control

Regular random sampling guarantees the quality of the production process because, for financial and technical reasons, it is not possible to inspect every seat belt button. Defined component dimensions are captured regularly and deviations are monitored exactly overtime. Statistical analysis enables the process to be stopped at the ideal moment as well as the early identification of gradual deterioration in the process. The shape of the value progression also provides information about potential defect causes.

Detail analysis of a curve form on the component using ZEISS CALYPSO CT.

Detail analysis of a curve form on the component using ZEISS CALYPSO CT.

Characteristics

These features predominantly determine the quality of the button:

  • The dimensional stability of the shape, i.e. the correspondence to the defined nominal values in the CAD model.
  • Stability which could be potentially limited by cavities, if applicable.
Top: Color-coded display of dimensional stability.
Bottom: Inspection of pores

Top: Color-coded display of dimensional stability. Bottom: Inspection of pores

Conventional inspection with a coordinate measuring machine

The stability is typically inspected using a coordinate measuring machine with contact or optical sensors. The more dimensions to be captured, the more time is required for inspection. In some cases the component must also be reclamped. Moreover, it is difficult or even impossible to measure the concealed structures using a coordinate measuring machine without destroying the component. Pores in the interior cannot be inspected.The time required for regular inspections increases with the frequency of random inspections. However, more frequent measurements help stabilize the process and minimize tolerance deviations, reducing both the number of rejects and the subsequent costs of defective production.

CT inspection with ZEISS METROTOM

With a ZEISS METROTOM computertomograph, all dimensions to be captured within the scope of process control can be inspected with just a single scan. It is even possible to insert multiple components into the CT system at once and inspect them. Depending on the component size, a complete shot is also possible. Moreover, additional dimensions can be inspected at any time using the voxel data generated to perform supplementary analyses. If necessary, a color-coded display of the nominal/actual comparison provides an overview of the dimensional accuracy of the entire component. The particular strength of the ZEISS METROTOM: the measuring results are traceable and dependably meet the current metrology standards.

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