Recycled Polyethylene Terephthalate Moisture Testing Methods
Accurate moisture testing below 50 ppm via Karl Fischer oven titration prevents hydrolytic intrinsic viscosity degradation in recycled PET during melt processing.
Pellet Morphology
Meaning
Physical attributes of polymer raw materials describe the shape, size, and surface characteristics of resin pellets before they are fed into processing machinery. Variations in pellet morphology affect the bulk density, feeding behavior, and melting rate of the resin in extrusion and injection moulding. These parameters are set during the compounding and pelletizing stages, and they are distinct from the final part dimensions.
Feeding Stability
Uniform pellet size and shape are necessary to maintain a constant bulk density in the hopper and feed throat of the extruder. Irregularly shaped pellets or those with a high proportion of fines can cause bridging in the hopper or erratic feeding along the screw, leading to pressure fluctuations in the melt. These fluctuations result in shot-to-shot weight variations and dimensional instability in molded parts.
Processing post-industrial regrind often introduces these feeding challenges due to its non-uniform morphology.
Melting Dynamics
The surface-area-to-volume ratio of the pellets determines how quickly they absorb heat from the heated barrel and screw friction. Small or thin pellets melt more rapidly than large, thick pellets, which can lead to uneven melting if the pellet size distribution is wide. Unmelted pellet cores can pass through the screw and appear as cosmetic defects or weak spots in the molded parts, which reduces the mechanical integrity of the component.
Quality Control
Measuring pellet size distribution using sieve analysis or optical sorting systems helps molders verify incoming material quality. Consistent morphology reduces the need for frequent process adjustments on the molding machine, ensuring a stable manufacturing window. Molders can set purchasing specifications for pellet uniformity to minimize these feed-throat issues and optimize cycle times.