A properly designed screw can improve cycle time, melt quality, color mixing and profits. Knowledge of the resin to be processed and an understanding of screw geometry are crucial to proper screw design.
The standard metering screw has three zones: the feed zone, where the plastic first enters the screw and is conveyed along a constant root diameter; the transition zone, where the plastic is conveyed, compressed and melted along a root diameter that increases with a constant taper; and the meter zone, where the melting of the plastic is completed and the melt is conveyed forward along a constant root diameter, reaching a temperature and viscosity that is necessary to form parts. The length of each zone has an impact on how a resin reaches a melt condition. A longer feed zone creates a greater potential throughput. A longer transition zone results in less shear heat and more time to compress and melt the resin. A shorter meter zone allows less time to assure an isothermal (uniform) melt quality.