Posi-Melt™ Multipurpose Screw
When a screw design is considered for a particular polymeric family, the rule-of-thumb has been; Amorphous screws process better with low compression designs, while Semi-Crystalline materials prefer high compression designs. Since Semi-Crystalline materials have a sharp melting point and are either in the solid state or melt state, higher compression designs with shallower metering sections have done a better job of assuring that each pellet is melted. Whereas high compression designs with shallow metering sections have the tendency to over-shear and heat Amorphous materials. The Industry Standard GP screw, is a square pitch design that has a compression between 2.3-2.7:1 and a 50/25/25 feed, transition, metering length ratio. The GP geometry does an adequate job at best with both polymer groups within a narrow process window. Since Injection applications entail a broad range of polymer groups, residence times and swept volumes, a more advanced design is needed to assure a more uniform melt homogeneity.
The Posi-Melt TM geometry is a core change to the geometry of a compression screw. We have separated the coordinate systems of the root and the flight pitch to force the pellets through a channel that changes pressure and velocity while exposing 55-60% more pellets to the heated barrel wall for increased conductive heat transfer. The “stepped” root geometry exposes the polymer to a change in velocity and extensional strain that enables us to lower the volumetric compression for low shear heating while establishing a high “linear depth ratio” between the feed depth and the metering depth. The lower volumetric compression works well with Amorphous polymers by lowering the shear between the pellets in the transition zone and the high “linear depth ratio” assures a shallow meter depth for efficient melting of the Semi-crystalline polymers. The Posi-Melt TM screw has won acclaim from resin suppliers, OEM’s and our customers/fabricators on both ends of the crystalline spectrum from LCP to Rigid PVC and the improved melt quality broadens the process window for custom molding shops that process a variety of materials. The Multi-Purpose Posi-Melt TM geometry delivers a more thoroughly melted polymer with a higher level of melt homogeneity that is able to produce higher quality parts over a broader spectrum of residence time and swept volumes.