Contract and Spot Naphtha Spread behind a Thirty Day Quote
Contract and spot naphtha spreads undermine fixed thirty-day resin quotes, requiring indexation formulas and dock MFR testing to prevent margin erosion.
Index Lag
Meaning
Polymer viscosity deviation describes a thermal delay between the external barrel temperature setpoint and the internal melt temperature of a resin during high speed injection moulding. This phenomenon represents the time required for heat transfer to penetrate the pellet core inside the screw flights. Operators define this duration by the difference between the programmed heater output and the actual thermocouple readout at the nozzle interface.
The condition stops applying when the screw recovery speed allows for complete thermal equilibrium across the resin charge.
Injection Delay
During the transition phase of plastication, index lag governs the uniformity of the melt cushion and the accuracy of the injection stroke. If the screw rotates faster than the thermal conductivities of the polymer allow, the material core remains solid while the outer layer reaches degradation heat levels. This mismatch forces a variance in the volume of the plastic shot delivered into the cavity.
Technicians manage the effect by adjusting the back pressure to increase residence time under shear, which encourages better heat distribution within the melt. Higher back pressure demands more torque from the motor but improves the stability of the viscosity across the entire mould cycle. When the barrel geometry limits the time available for thermal soak, the resultant part suffers from incomplete packing near the gate.
Moulding Variance
Variations in regrind concentrations alter the response of index lag because distinct polymer chain lengths possess specific melting profiles. Virgin resin exhibits a predictable heat transfer coefficient that stays consistent under set conditions. Regrind introduces thermal history into the melt that confuses the standard heater control algorithms.
This creates a divergence between the nominal material specification and the performance of the shot in the mould. Moulders compensate for this by extending the recovery time to allow the regrind portion to reach the set viscosity. Failure to account for the altered heat absorption results in fluctuating part dimensions and inconsistent surface finish quality.
Each material batch requires a revalidation of these parameters to ensure the part specification remains within tolerance.
Thermal Calibration
Process control engineers monitor the recovery torque and the nozzle pressure to detect when index lag exceeds the acceptable limits for complex geometry. A high torque value signals that the screw must work harder to displace the semi solid melt, which often indicates an insufficient soak period. Conversely, a low torque reading during rapid cycling indicates that the plastic lacks the required shear energy for uniform flow.
Production teams calculate the optimal screw speed by checking the recovery time against the overall cycle length. Stable moulding depends upon keeping the thermal input consistent relative to the mass of the part. Persistent deviation in the temperature gradient confirms that the heating system cannot maintain the target melt density.
Consistent heat transfer through the screw diameter dictates the final quality of every moulded part.