Initial development of the melt channel
Following the initial development of the melt channel, a barrier wall is provided which rises out of a base surface of the melt channel, and is formed with a clearance with the barrel which begins at a maximum and, at the conclusion of the barrier section, reduces to a minimum. The rate at which the clearance changes from maximum to minimum is such as to provide a substantially uniform energy input into the melt as the melt moves from the packing or solids channel across the melt barrier flight, between this flight and barrel wall, and into the melt conveying channel. In other words, the clearance between the barrier flight face and the barrel wall decreases as the melt temperature increases and melt viscosity decreases. As a result, the energy input to the melt is more uniform throughout the axial length of the barrel single screw extruder, creating a melt of improved consistency.
A further feature of the invention is that the melt barrier flight continues through the barrier section of the screw, forming part of the screw's melt section and closes gradually with the primary flighting while maintaining the depth of the solids channel substantially constant during the termination portion. In this manner, a balance of pressures between the solids and melt channels is maintained while the solids channel is closed off against the primary flighting leaving only the melt channel at the beginning of the metering section. This gradual balancing and closure of the barrier section maintains pressure stability and avoids a solid bed breakup due to instabilities of pressures which could otherwise occur. It is accordingly an important object of this invention to provide an extruder screw in which a melt section has a melt channel formed at an adjacent flank of the primary flighting and tapering an axial width from a minimum at the beginning of a compaction portion to a maximum where a barrier flight begins, while maintaining a compaction channel of substantially constant depth.
It is a further object of the invention, as outlined above, to provide a barrier flight throughout a major portion of the melt section, which barrier flight has a varying clearance with the barrel wall beginning at a maximum and narrowing to a minimum as the temperature increases and viscosity decreases of the melted plastic material. A still further object of the invention is the provision of a compaction and melt section in an extruder screw for plastics materials in which the respective channels for compaction of unmelted plastic material and for receiving melted plastic material are free of discontinuities or interruptions which can cause surges or pressure instabilities at axial portions of the screw.