Determining Masterbatch Letdown Ratios for Recycled Polyethylene Compounds
Determining masterbatch letdown ratios for recycled polyethylene requires MFR matching, antioxidant depletion testing, and gravimetric dosing adjustments.
Volumetric Feeders
Meaning
Material dosing systems in plastic extrusion and compounding dispense a constant volume of resin, color masterbatch, or additive per unit time using a rotating screw or auger. These volumetric feeders operate on the assumption that the bulk density of the material remains constant throughout the production run. While simple and cost-effective, their accuracy depends on the consistent flow and density of the material being fed.
Feeding Accuracy
Rotational speed of the auger determines the rate at which the material is delivered to the feed throat. Since the system does not measure the actual weight of the material being discharged, any change in the bulk density of the material alters the mass flow rate. For this reason, these units are best suited for free-flowing pellets of uniform size and density.
Quality Issues
Density variations in the feed material can cause under-dosing or over-dosing of pigments and additives during the run. This variation leads to color drift and inconsistent physical properties in the moulded parts, resulting in higher reject rates. In recycling operations where flake density varies continuously, these feed systems can struggle to maintain the required compound consistency.
Equipment Selection
Selecting between a single-screw and twin-screw configuration depends on the flow characteristics of the material. Twin-screw designs are preferred for cohesive powders or sticky additives that tend to bridge, while single-screw units are sufficient for standard pellets. The feeder must be matched to the throughput requirements of the extruder to prevent starving or flooding the screw.
Properly sized feed screws ensure that the volumetric dosing remains stable across the entire speed range of the production run, minimizing the need for manual adjustments.