Identifying Reprocessed Engineering Thermoplastic Additions through Visual Screening
Visual inspection of pellet geometry, color shift, specks, and cross-polar optics rapidly identifies reprocessed additions in engineering resin lots.
CIE L*a*b*
Meaning
Three-dimensional color spaces define colors numerically across three axes representing lightness, green-red opposition, and blue-yellow opposition. The coordinate system known as CIE Lab provides a device-independent method to measure and compare color differences in manufactured goods. It allows plastic compounders to specify raw material colors and match them against molded product samples under standardized lighting conditions.
Molders use this system to identify and correct thermal degradation or pigment dispersion issues during the injection molding process.
Color Calibration
Spectrophotometers measure the reflected or transmitted light from a polymer sample to generate coordinates in the color space. The L axis ranges from zero for total absorption to one hundred for perfect diffuse reflection. The a and b coordinates represent chromaticity, where positive values denote red and yellow, and negative values represent green and blue.
This measurement enables precise adjustment of colorant dosing equipment to maintain uniformity across different production batches.
Tolerance Limit
Part specifications usually dictate a maximum allowable color difference, designated as delta E, to ensure that assembled components match visually. The calculated difference combines the deviations of all three coordinates into a single vector value. For demanding applications such as automotive trim panels or consumer electronics, the acceptable tolerance is often set below one unit.
If the difference exceeds this threshold, the batch is rejected, requiring the processor to purge the cylinder or recalibrate the pigment feeder.
Process Optimization
Regrind usage alters the optical properties of the polymer and shifts the color coordinates. Tracking these coordinates across successive molding cycles identifies thermal damage. Adjusting the melt temperature or reducing the cycle time helps restore the color back to the target coordinates.