Nondestructive Computed Tomography Volumetric Void Acceptance Criteria for Heavy Polymer Sections
Computed tomography volumetric void acceptance requires zoning heavy polymer walls by stress field, setting pore limits below 0.1% in tensile skins.
Sphericity Index
Meaning
Geometric roundness evaluation governs the dimensional uniformity of pellet feedstocks and finished particulate polymers entering a thermal screw or reciprocating barrel. The sphericity index quantifies how closely a particle approaches a perfect sphere by comparing particle surface area to volume ratios against a theoretical spherical baseline of identical mass. Variations in this ratio alter bulk density profiles and screw feeding rates across hopper throat boundaries.
Standard polymer pellet feeds maintain indices near the upper limit of particle symmetry to promote predictable volumetric metering. Regrind fractions produced from scrap runners exhibit irregular geometry that depresses the index significantly below virgin resin baselines.
Geometric Variance
Irregular morphological distribution creates packing voids that change the bulk density of the resin charge entering the feed zone. Feed throat bridging occurs when angular particles interlock and halt the gravitational descent of material toward the rotating flights. A low sphericity index forces processors to adjust barrel temperature profiles and screw rotational speeds to compensate for unstable solid conveying rates.
Virgin pellets maintain consistent geometry through strand pelletizing operations, whereas post-consumer flakes display high aspect ratio variance that disrupts steady state throughput. Thermal degradation often compounds these feeding irregularities because distorted particles soften unevenly under barrel heaters.
Moulding Tolerance
Part specifications demand strict dimensional stability that unstable feedstock geometry compromises during high pressure injection cycles. Melt front velocity fluctuates inside the cavity when irregular pellets melt at variable rates along the plasticising screw channels. Higher sphericity indices ensure uniform frictional heat generation and homogenous melt viscosity before the material enters the gate.
Tooling designers account for baseline particle uniformity when establishing runner dimensions and gate sizing parameters for crystalline thermoplastics. Processors face severe sink marks and internal stress gradients in moulded components if feedstock morphology departs from established limits.
Economic Penalty
Material cost differentials between virgin resins and recycled alternatives depend heavily on the morphological consistency of the delivered batch. Lower purchase prices for regrind offset the operational losses caused by erratic hopper feeding and frequent production line stoppages. Parts produced from geometrically irregular feedstock frequently fail mechanical testing standards due to localized fiber orientation shifts and void formation.
Quality control laboratories reject incoming polymer lots whenever particle roundness measurements fall below the minimum threshold required for automated processing equipment. Final product performance relies entirely on maintaining strict morphological thresholds from the initial compounding stage through final part ejection.