Characterization Study On Screw Function Of Twin Screw Extruder

Screw is the principal component of an extruder. Twin screw extruder has expanded operational capabilities and extended range of application due to interchangeable screw configuration that are created by slipping individual screw sections onto parallel shafts. Screw structure is described by screw configuration parameters, which are composed by screw element, element length, and element location. Screw element is composed by element type and element geometry and element location is composed by element-die distance and element space. Screw with different configuration parameters leads to different system parameters or extrudates parameters due to different functions of conveying, shearing, mixing and building pressure. Studying on screw functions of twin-screws extruder is helpful to improve the theory and technology of high moisture extrusion texturization of soybean protein and to serve for designing the screw or developing the new products.The objective of this study was to find the relationship among screw configuration parameters, system parameters and extrudates parameters and to find index that was suitable for characterizing the screw function. Defatted soybean flour was extruded with 45% moisture using a co-rotating, intermeshing twin screw extruder (DSE-25, Brabender, Germany). The effect of 21 types of screw configuration, which included six kinds of screw elements, 25mm~75mm element length, 0~150mm element-die distance and 0~150mm element space of reverse kneading element, on system parameters, such as screw fill degree, specific mechanical energy, pressure and residence time distribution during extrusion, and on extrudates properties, such as sensory properties, texture properties, and molecular properties were investigated. Statistic methods of descriptive analysis, ANOVA, correlation, principle components, canonical correlation and regression were used.Conclusion showed that it was more directly and clearly to characterize the screw function using system parameters than extrudates properties. (1) Screw element was the major factor in influencing residence time distribution, screw fill degree during extrusion, and tensile distance of extrudates. Variance proportion of screw element was 29%,27%,46%,16% and 27% separately, in influencing torque, specific mechanical energy during extrusion, tensile strength, a*, and tensile force of extrudates. The influence of screw element on intrinsic viscosity, chewiness, and ratio of non-covalent binding protein of extrudates was insignificant. (2) There were regression functions between specific mechanical energy during extrusion, a*, intrinsic viscosity, tensile force of extrudates and element length, element-die distance, element space, disc width and angle. R2 of those functions were more than 60%. (3) Screw fill degree or mean residence time was suitable for characterizing conveying capability of screw. Conveying capability of screw was poor when screw fill degree or mean residence time was high. Conveying capability of screw depended on screw elements and element length. Conveying capability of screw became better when reverse screw element was changed into forward screw element, or thick kneading disc was changed into thin kneading disc, or long reverse screw element was changed into short reverse screw element. (4) Specific mechanical energy was suitable for characterizing shearing capability of screw. Shearing capability of screw was strong when specific mechanical energy was high. Shearing capability of screw depended on angle of screw elements and element length. Shearing capability of reverse elements was stronger than forward screw element. Shearing capability became stronger when kneading disc was changed from thin to thick, or reverse screw element was changed from short to long. (5) Mean residence time and Pe number were suitable for characterizing mixing capability of screw. Mixing capability of screw was good when mean residence time was long and Pe number was small. Mixing capability of screw depended on screw elements. Range of screw element type was kneading elements, gearing elements, and conveying elements when mixing capability of screw became good gradually. Mixing capability of screw became better when forward elements were changed into reverse screw elements, or thin kneading disc was changed into thick kneading disc.