Conventional metering section
A particular difficulty of many prior designs is the fact that in the development of the flighting to create the solids compaction channel separate from the melt channel causes a sudden change in the pitch or the depth of a developing channel and results in a flow disruption, a discontinuity, and often the packing of the melt channel with solids. These disruptions can cause a variation in pressure, consistency and also, rate of output of the screw.
While prior designs have used a barrier flight to separate a melt channel from a solids compaction channel, the barrier flight has commonly had a constant clearance with the barrel, with the result that a much higher specific energy input is applied at the beginning of the barrier section when the melt viscosity is high than at the end of the melt channel when the viscosity of the melted phase is much lower. Such an arrangement inherently causes a non-uniform shear energy input to the melt over the length of the barrier flight. As a result, twin screw extruder that polymer initially traversing the melt barrier experiences a high shear energy input and a long residence time at the elevated melt temperature while that polymer passing over the melt barrier at the end of the solids channel experiences a lower shear energy input, and a reduced barrier at the end of the solids channel experiences a lower shear energy input and a reduced residence time at elevated melt temperatures creating an inconsistent melt quality. The present invention eliminates this inconsistency.
Further, in the transition between a barrier section and a conventional metering section, designs have been proposed in which there has been no thought of maintaining a balance of pressures between the compaction channel carrying the solids and the adjacent melt channel, nor has provision been made to accommodate polymers of varying melt rates. An abrupt transition at the end of the solids channel can cause a pressure fluctuation, adversely affecting the performance of the screw. This invention is directed to a extruder screw in which a melt flighting develops out of one of the flank of the flights of the primary flight, and does so gradually define a melt channel which maintains a volume, as it develops, which corresponds to the developing melting volume from the remaining unmelted plastic material. This channel is designed as to cause a minimum of disruption by eliminating abrupt changes in pitch or depth of the developing melt channel and is done in such a way as to enhance the compaction of the pellets into the solids channel with minimum disruptions.