Accurate sorting is essential to produce high-quality recycled material. An optical recycling sorting machine is a good choice for automatic sorting because it provides specificity without any surface pre-treatment. NIR spectroscopy reflects light from an incandescent lamp onto the material during the identification process.
During the identification, light from an incandescent bulb is reflected on the material. A certain type of plastic can be identified by the diffuse reflectance of the light back to a receiver, which is sensitive to the near-infrared part of the spectrum. Different types of plastics are then distinguished by their unique reflectance curve “fingerprint”, by using state-of-the-art machine learning algorithms. This makes the technique ideal for sorting unknown plastic waste with high accuracy, regardless of its color. (Black plastics still pose a challenge due to their poor reflectance of infra-red light, but many researchers are trying to overcome this limitation.)
The separation and sorting of wastes are a big step towards the circular economy of plastic. The wide range of plastic resins poses a unique challenge for the recycling process. Identification and sorting have traditionally been manual. This has made it slow, ineffective, and costly even with labeling systems and the help of optical sorting methods such as hyperspectral cameras, which are also expensive.
The spectral resolution in NIR optical sorting
The spectral resolution is like the size of the pixels in a pixelized image: Zooming in doesn't increase the detail in the image, it only makes the pixels bigger. A spectrometer with low resolution loses information that a higher resolution instrument would be able to reproduce.
The upper bound of the resolution is determined by the quality of the Fabry-Perot interferometer and its mirrors in particular. The lower bound of resolution is determined by the aperture. The aperture makes sure that the light reaching the interferometer arrives perpendicular to the mirrors. If the light can come in at an angle, the interferometer cannot perform its job.
Both the mirrors and the aperture hole have manufacturing tolerances. The quality of the mirrors is affected by doping and layer thickness. While the aperture may just be a hole, if it’s made too big or positioned off-axis, the resolution suffers.
The raw absorption spectrum coming out of the spectrometer should not be counted on for any sort of analysis. The actual absorption in the spectra is mixed with the unique spectral response of the spectrometer, which is affected by the measurement conditions. Consequently, it becomes impossible to tell how much light was absorbed by the target.
Why do you need NIR optical sorting system?
NIR optical recycling sorting machine is both faster and more accurate than manual sorters. Automatic sorters interface NIR sensing with air nozzles to sort plastic items of various resin compositions, for example, separating PET bottles from HDPE bottles. To obtain the highest material value, the equipment must sort the items effectively and accurately. Otherwise, the article is either directed to the waste stream or becomes a contaminant in another recyclable stream where it is likely to not be recycled. It is also common for some plastic reclaimers to use NIR equipment near the end of the processing line for final contaminant removal as well.