Polyethylene Melt Flow Rate Test Load Selection Basics
Polyethylene melt flow rate test load selection matches resin density and molecular weight to standard ASTM D1238 masses of 2.16 kg, 5 kg, or 21.6 kg at 190 °C.
Extrudate Strand Cut
Meaning
Continuous polymer filaments exiting a multi-hole strand die are cooled and subsequently severed into uniform cylindrical pellets. This extrudate strand cut method represents the most common pelletizing technology used in compound compounding and masterbatch production. The technique accommodates a wide range of thermoplastic viscosities and filler loadings.
Reliable operation depends on the precise coordination between the pull rolls and the rotating cutter head.
Pelleting Process
Polymer strands must be drawn through a water bath to solidify the melt before reaching the cutter. The extrudate strand cut process pulls these solidified strands through a set of feed rolls that guide them into the path of a high-speed helical rotor. This rotor presses the strands against a stationary bed knife, creating a clean shear cut that minimizes fines and irregular pellet lengths.
Achieving a consistent pellet size requires a constant tension on the strands between the die face and the feed rolls. Any variation in strand tension leads to fluctuating pellet diameters and causes feed issues in subsequent injection moulding machinery.
Blade Wear
Abrasive fillers like glass fibres or mineral powders accelerate the erosion of the cutting edges. As the blade edges round over, the extrudate strand cut yields more irregular pellets and elongated stringers. This wear increases the percentage of off-spec material that must be screened out.
Regular sharpening or replacement of the rotor blades is necessary to maintain a clean cut. Tooling inserts made of tungsten carbide can extend the interval between maintenance cycles.
Moisture Control
Residual water on the surface of the pellets must be removed to prevent hydrolytic degradation during moulding. In the extrudate strand cut sequence, a vacuum suction unit dries the strands before they reach the cutter. This drying step ensures that the final packaged pellets have a moisture content below the critical threshold.