Application: Defect Detection in Nylon Granules

The problem

NIGHTSEA was approached by a manufacturer of plastic granules, specifically nylon 6,6. These are small granules, about 2 x 2 x 2.5 mm, that constitute the raw material for injection molding. In-process faults in polymerization can result in the formation of a defect called ‘gel’. This may occur in only a small fraction of the particles and even in those particles the gel may make up only a very small fraction of the granule volume. But if there is too much gel in a batch it can result in problems in downstream production.

The manufacturer already knew that both the good polymer and the gel fluoresce when illuminated by longwave ultraviolet light, with the gel fluorescing more brightly. They were using a UV light for inspection but they couldn’t detect the gel well enough to meet their stringent quality standards. The contrast between the gel and the granule background was just not strong enough for efficient detection.

The empirical study

The engineer tasked with solving the problem wondered whether there might be an alternative to ultraviolet that would do a better job of revealing the defects. He sent a representative sample of granules and we tried all 5 of the fluorescence excitation/emission wavelength combinations available with our Stereo Microscope Fluorescence Adapter. It took just a few minutes after they were received to put the particles under the stereo microscope and cycle through the options. It was quickly obvious that the Royal Blue excitation/emission set worked best, providing high fluorescence brightness along with good contrast between the defective areas and the bulk of the material. You can see the results in the images below. (Note that while the contrast looks good with several other excitation wavelengths, the images do not capture the actual brightness apparent to an observer.)

(Click any image for larger view)

This empirical investigation was a quick and effective way to test available technology to find a practical solution to a real-world problem. The manufacturer purchased the Stereo Microscope Fluorescence Adapter to help improve their process control, and also purchased fluorescence excitation flashlights and filter glasses so that they could quickly scan larger batches of particles. NIGHTSEA fluorescence equipment is now a standard part of their quality control and process improvement operations.


For more information or to get answers to your fluorescence questions contact NIGHTSEA.