Capillary Rheometry Limits in Detecting Third Pass Regrind Fractions below Ten Percent
Capillary rheometry fails below 10% third-pass regrind because high shear rates mask molecular weight drop beneath equipment noise floors.
Structural Degradation
Meaning
Polymer chain scission and oxidative degradation reduce the average molecular weight of a plastic material during thermal processing. Structural degradation describes the permanent breakdown of long molecular chains into shorter fragments due to excessive residence time or temperature in the barrel of an injection machine. This loss of chain integrity compromises the physical properties required for the molded part to perform under load.
The boundary for this phenomenon occurs when the measured melt flow index deviates from the original resin specification provided by the supplier.
Polymer Stability
Heat history accumulates whenever plastic enters a molten state, driving the degradation process forward through multiple heating cycles. Virgin resin withstands a limited number of passes before the mechanical performance of the resulting part falls below the minimum design requirements. Regrind material accelerates this decline because each previous moulding cycle already reduced the polymer length.
Process engineers maintain stable output by monitoring barrel temperatures and minimizing the time molten material dwells within the heating bands.
Defect Frequency
Parts experiencing this decline often show brittleness, visible discolouration, or premature failure during mechanical testing. A molder detects the onset of the problem when tensile strength or impact resistance drops while the melt flow index spikes higher than the grade specifications. Excessive degradation changes the flow characteristics to the point where filling the cavity produces inconsistent dimensions or surface blemishes.
These chemical shifts within the resin prevent the material from meeting the physical criteria established for a specific part application.
Specification Control
Datasheet values represent the performance of high quality virgin resin tested under controlled laboratory conditions. Production batches often vary from these laboratory metrics because shop floor conditions alter the thermal experience of the material. A molder holds a range of acceptable process parameters rather than a single fixed point to account for this reality.
Precise control of the melt temperature protects the molecular structure from permanent harm during the conversion process.