Screw left-handed elements

It is well known that screws may have a mirror image. This is equivalent to a screw having a negative pitch. It is possible to produce such a left-hand element in all the variations of design developed for screws (Figure 6.15). This includes the number of threads starts and axial diameter profile. 

Figure 7.14 Flow visualization for a highly viscous silicone oil along the mixing zone of a twin-screw extruder (a) orthogonal kneading block (90°) (b) kneading block with negative staggering (—30°C), plus left-handed element [146].

Right-handed or neutral elements Left-handed elements Screw elements... 

Screw rotation. The symmetry element is a screw axis. It can only occur if there is translational symmetry in the direction of the axis. The screw rotation results when a rotation of 360/1V degrees is coupled with a displacement parallel to the axis. The Hermann-Mauguin symbol is NM ( N sub M )-,N expresses the rotational component and the fraction M/N is the displacement component as a fraction of the translation vector. Some screw axes are right or left-handed. Screw axes that can occur in crystals are shown in Fig. 3.4. Single polymer molecules can also have non-crystallographic screw axes, e.g. 103 in polymeric sulfur. 

Three-flight conveying screw elements Left-handed kneading element Right-handed kneading element... 

Polyesters, polyetherimide, melamine-formaldehyde, polyurethanes, polyurethane-ureas and polyamides (nylons) are examples of condensation polymers prepared by REX. " Because a small molecule is produced in condensation reactions, vent ports are employed. Between the ports are melt sealing screw sections, to prevent back mixing of volatilizing melt. Sealing screw sections are constructed by a right handed-left handed sequence of screw elements. Another chemistry feature of step-growth reactions is their sensitivity to errors in stoichiometric feed proportions. This poses problems with solid feed materials, which are normally converted to liquid form for more... 

Twin-screw extruder Flow of Newtonian fluids in kneading disc elements and (right and left handed) screw elements. Szydlowski et al., 1987... 

Left-handed kneading elements Right-handed kneading elements Melt homogenizing screw elements... 

Left-handed screw elements are used to generate backpressure and therefore create a fully filled section. Left-handed screw elements are inserted behind melting or mixing sections where the polymer melt is fidly filled and mixing takes place. 

In a 1964 patent application, Robert Colombo [117] of LMP described a mixing section consisting of a left-handed screw element with a helical groove cut in it (Figure 6.19). This should have distributive mixing capability. 

A left-handed screw element used before vent port to create a melt seal... 

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