Single screw extrusion control - A comprehensive review and directions for improvements

Kinetics of recovery of biaxially oriented styrene-acrylonitrile copolymer films

Various scale-up rules and theories have been presented for extrusion, including both single- and twin-screw extruders. Until now, however, most of these theories have concerned fully-filled channels, not only for twin screw extruders of the co-rotating fully intermeshing type (COTSE) or non-intermeshing counter-rotating type (NITSE), but single screw extruders as well. As the demand for distributive mixing and devolatilization devices increases, more and more nonintermeshing twin screw extruders with regions of partially-filled channels are being used. Therefore, developing scale-up rules for such screw extruders is imperative. In this paper, scale-up rules are developed, theoretically and experimentally, by examining the relationship between distributive mixing and important flow parameters. Two partially-filled NITSE's, with screw diameters of 0.8 and 2 inches, have been studied by using a flow visualization technique entailing a dye tracer to study the effects of distributive mixing by varying such parameters as : percentage of drag flow, screw stagger, and screw velocity. Qualitative evaluation of the spread of the dye with the number of screw revolutions was obtained from videotape of the experiments. Factorial experimental design method has been applied for evaluating these results. Finally, new scale-up rules were developed and compared with rules in the literature.

Scale‐up rules for mixing in a non‐intermeshing twin‐screw extruder

When a biaxially oriented polystyrene film was stretched along one direction and subsequently stretched along the perpendicular direction, the film showed enhanced ductility with pronounced yield softening and extended strain hardening. In the forward deformation, at least two types of shear bands were observed. The bands at the early stages of yielding did not seem to contribute to the reduction of thickness. They were approximately 200 μm thick and had an intersection angle of approximately 120°. The bands developed in the later stages contributed to the thickness reduction. These bands were smaller and possessed an intersection of approximately 90°. In the cross deformation, new shear bands developed that were likely related to the reverse shearing of the existing bands. Stress relaxation showed a power-law relationship between the stress rate and relaxation time. The internal stress of the cross deformation was significantly (ca. 3 times) lower than that of the forward deformation at the same strain. The enhancement in ductility may be attributed to the lowering of internal stress during the cross deformation. The internal stress increased with the applied stress and strain. Fracture occurred when the internal stress reached a certain level, about 57–68 MPa for deformation along both directions and approximately 44–47% of the final applied stress. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 687–700, 2003

Cross deformation and stress relaxation of biaxially oriented polystyrene films

The fracture behavior of oriented poly (methyl methacrylate) (PMMA) has been investigated. The stress intensity factor K IC , characteristic distance α and angle θ of switching propagation direction decrease with increasing draw ratio in the range of 1.25 and 3.25. The microstructures such as the plastic zone and the features of morphology are in good agreement with the variations of these three variables, i.e., K IC , α, and θ

The fracture behavior of oriented poly(methyl methacrylate)

The shear yielding processes in the deformation of biaxially oriented styrene-acrylonitrile (SAN) copolymer films (machine oriented with draw ratio = 6.9 in the machine direction and 2.9 in the transverse direction) were studied. In the transverse direction, two sets of shear bands with an intersection angle of about 123.6 degrees (61.8 degrees with the tensile axis) were developed. When necking occurred, the bands in the necked region were thin and discontinuous with an intersection angle of about 81 degrees. In the machine direction, shear bands appeared to be short and diffused with an intersection angle of about 119.2 degrees. Only a slight necking effect was observed. The Luders strain in the transverse direction was about 0.56 and in the machine direction, about 0.16. The shearstrain-volume of activation obtained from the strain rate dependence of shear stress was about 3440 ± 400 A3 in the machine direction and 2700 ± 500 A3 in the transverse direction. The work hardening behavior in both directions seemed to follow a linear relationship between 1n (tensile stress) and tensile strain at large strains. From a consideration of localized shearing toward tensile axis, the fraction of deformed materials was calculated. These observations indicated that (1) a strong orientation hardening effect existed in unbalanced biaxially oriented films, resulting from the difference of the amount of deformable materials between individual directions; and (2) deformation proceeded by shear banding with the high orientation direction involved more correlated molecular segments than the low orientation direction during the activation process.

L. C. Rubens

Papers

Shear yielding of biaxially oriented styrene-acrylonitrile copolymer films