Meaning
Process duration metrics quantify the average period that a material remains within a specific zone of a production system. In injection moulding and extrusion, residence time refers to the duration that resin spends in either the drying hopper or the heated barrel of the plasticising unit. Maintaining this duration within a defined window ensures that the polymer is fully dried without suffering from thermal degradation or oxidation.
Thermal Degradation
Extended exposure to high temperatures in the injection barrel breaks the polymer chains, leading to a loss of mechanical properties and cosmetic defects such as yellowing or black specks. If the material stays too long in the barrel during a machine stoppage, the melt can become highly fluid and drool from the nozzle, causing subsequent moulding cycles to fail. Moulders must purge the barrel if production is interrupted to prevent the resin from degrading.
Dryer Sizing
Insufficient duration in the drying hopper prevents the polymer from reaching the necessary moisture level, while excessive duration can consume excess energy or cause the pellets to stick together. The capacity of the drying hopper must be matched to the hourly consumption of the moulding machine to guarantee that every pellet receives the minimum drying time. This balance is critical when transitioning from virgin to regrind resin, which may dry at different rates.
Moulding Control
Calculating the screw throughput and shot size allows the operator to adjust the process to keep the material moving steadily. This calculation prevents both under-drying and thermal degradation, helping to maintain a stable viscosity throughout the production run.
