Quality control in plastics manufacturing presents a real challenge as out-of-specification components represent a financial burden to the manufacturer, not only because of materials and machine time involved with re-running the batch, but also because of disposal and recycling charges.
With injection-moulding batches of plastics components stretching into the millions, identifying when key dimensions are threatening to drift out of tolerance is crucial. However, in addition to plastic conversion processes being subject to natural fluctuations, the sheer speed of the injection moulding process poses a real challenge in terms of effective quality control. Equally challenging is the range of colors, textures, sizes and complexity of components that need to be measured. Manufacturers need to be able to check statistically valid numbers of samples yet avoid compromizing demanding production schedules.
In addition to edge-to-edge dimensions, the position, diameter, depth and profile of apertures may also need to be checked. Also, since plastics form the outer cover of many high quality items, their surfaces need to be completely blemish-free.
Stereomicroscopy plays a key role in allowing staff to examine surface finishes and spot minute imperfections.
Manual metrology provides an accurate means to assess prototypes, check the performance of injection moulding dyes and perform lower volume quality control checks. Non-contact z-height measurement can also be a useful asset on manual measuring microscopes used for lower volume QC work.
Automated, non-contact video-based measuring not only allows multiple edge-to-edge measurements to be reliably made on large numbers of simple plastic components, but also the position, diameter, depth and profile of apertures on lower volume, complex work pieces.
With the correct illumination settings, repeatable and reproducible edge detection, even the edges on dark and clear parts can be correctly refracted, detected and reproducibly measured. Non-contact video measurement can also be used to compare CAD vs actual data and perform real-time SPC.
Key techniques for QC in plastics manufacturing include:stereomicroscopy; extended depth of field; polarizing microscopy; phase contrast; non-contact z-height measurement; non-contact video measuring systems; twin-ring LED illumination; through-the-lens laser autofocusing (TTL AF); laser scanning; and automated edge detection.