Twin-screw Machine Research Articles

Optimisation-based design of extruders

An optimisation-based extruder screw design methodology using available numerical modelling routines of the plasticating process, able to deal with various process criteria simultaneously, is applied to both single and twin screw machines. The general specifications of the method and its main features are presented. The results obtained for several examples are discussed. Finally, an attempt is reported to validate the method experimentally. Generally, the results are encouraging, although the success of the method is strongly dependent on the capabilities of the modelling routine used.

Influence of Sb 2O 3 particles as filler on the thermal stability and flammability properties of poly(methyl methacrylate) (PMMA)

The thermal stability behaviour and the flammability properties of poly(methyl methacrylate) (PMMA) filled with antimony oxide (Sb 2O 3), at different Sb 2O 3 amounts: 5, 10 and 20 wt%, were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), cone calorimetry and limiting oxygen index (LOI) measurements. Samples were prepared using a twin-screw machine and subsequently injection molded. The experimental results obtained show that Sb 2O 3 particles retard the degradation and flammability of PMMA. The LOI increases to 24 for PMMA/Sb 2O 3 with just 20 wt% Sb 2O 3. An increase of the thermal stability between about 25 and 40 °C for 5–20 wt% of Sb 2O 3 was demonstrated by TGA in air. Cone calorimeter measurements showed that the heat release rate (HRR) significantly decreased in the presence of Sb 2O 3. Epiradiateur tests showed an increase of the ignition time between 10 and 20%. In a tentative way to establish how Sb 2O 3 acts as a flame retardant for PMMA, the activation energy of the thermal degradation was determined by the model-free isoconversional method of Vyazovkin.

Extruder manufacturers meet varied market needs

Although the major trend in twin screw extruder development is towards increasing speeds to achieve high throughput rates in commodity materials, manufacturers have also developed a number of special types for other products which, although not in the mainstream, have existing or potentially important markets. Chief among these are materials filled with wood or glass fibre. Alan Smith reports on the latest trends in twin screw and other machines.

Polypropylene/polyperoxide interaction in the melt phase

This paper deals with the effect of polyperoxide on the properties and characteristics of PP. Interaction of PP with polyperoxide was carried out by their mixing in melt phase applying co-rotating twin screw machine. The analysis of melting/crystallisation processes was performed using DSC. Influence of polyperoxide concentration and mixing condition (temperature, residence time) on PP properties was investigated via determination of PP molecular weight. Mechanism of the PP degradation and decomposition of peroxide groups was judged by thermogravimetric analysis, FT-IR-, and NMR-spectroscopy. It was observed that decomposition of peroxide groups brings about degradation of polypropylene. However, simultaneously some part of short, chains of PP are grafted onto macrochain of the polyperoxide decomposed.

Mechanical, microscopical and fire retardant studies of ABS polymers

Antimony trioxide (Sb 2 O 3 ) is a common additive in ABS flame retardant formulations and the effects of adding it, with four commercial brominated materials, to ABS are reported here. The results focus upon mechanical, rheological, microscopical and flame retardant properties, and the effects of four Sb 2 O 3 materials with average particle sizes ranging from 0.5 to 11.8 μm. Compounds were produced using a twin-screw co-rotating machine and subsequently injection moulded into flexural and impact specimens, the latter being tested with instrumented falling weight equipment. The effects on rheological properties were studied using a capillary rheometer. Dynamic mechanical thermal analysis (DMTA) and LOI flame testing were also carried out. The brominated materials had more influence than Sb 2 O 3 on the ABS rheology, although a trend of increased melt viscosity with increasing Sb 2 O 3 particle size and loading was noted. The brominated materials generally increased the flexural modulus and reduced the deflection at peak force values, whereas the modulus and strength were relatively unaltered with Sb 2 O 3 at the loadings studied. The impact strength of the ABS was detrimentally affected by both additives as loading level and average particle size increased. The presence of 5 wt% sub-micron Sb 2 O 3 caused a 20% reduction in impact strength whilst the reduction due to the brominated materials depended upon the specific halogenated compound used. Scanning Electron Microscopy (SEM) showed good dispersion of Sb 2 O 3 regardless of particle size and Transmission Electron Microscopy (TEM) revealed that Sb 2 O 3 resided in the SAN phase of the polymer. The DMTA response of ABS was not altered by Sb 2 O 3 .

Fracture resistance of short-glass-fiber-reinforced and short-carbon-fiber-reinforced polypropylene under Charpy impact load and its dependence on processing

Abstract In this paper, short-glass-fiber-reinforced and short-carbon-fiber-reinforced polypropylene composites were investigated with respect to the work of fracture (WOF), here viz the notched Charpy impact energy. Short glass fibers and short carbon fibers were incorporated into polypropylene with a twin-screw extruder and all of the specimens were injection molded into dumbbell-shaped tensile bars in a twin-screw injection-molding machine. Rectangular impact bars were obtained from tensile bars by removing the two clamping parts. Charpy impact tests were performed with a Charpy impact tester on specimens with V-notches. The values of the notched Charpy impact energy were obtained from each group of eight specimens. The effects of glass and carbon fiber volume fractions on their lengths in single and hybrid short fiber composites were investigated. The composite WOF was studied qualitatively by taking into account the effects of fiber volume fraction and fiber length distributions using the total WOF theory. Fiber length distributions have been previously shown to depend on the processing conditions. On the other hand, the composite impact resistance was shown to depend on fiber length and hence on processing. The dependence of the composite impact energy on processing is discussed.

Modular Tangential Counter-Rotating Twin Screw Extrusion: Non-Newtonian and Non-Isothermal Simulation

Abstract A non-Newtonian flow model for individual elements of a modular tangential counter-rotating twin screw extruder is developed. A non-Newtonian non-isothermal flow model for a composite modular machine is then described. Fill factor, pressure and temperature profiles along the axis of the twin screw machines were predicted for various modular screw configurations. Finally, experiments were carried out on the modular machine to verify the predictions. Generally, good agreement with the flow analysis was found.

Development and Optimization of Screw Machines With a Simulation Model—Part II: Thermodynamic Performance Simulation and Design Optimization

This paper presents a method for the design of twin screw compressors and expanders, which is based on a differential algorithm for defining the rotor profile and an analytical model of the fluid flow and thermodynamic processes within the machine. Part I of the paper presents a method for screw rotor profile generation which simplifies and improves design procedures. An example is given of its use in the development of a new “N” rotor profile, which is shown to be superior to other well-known types. Part II describes a numerical model of the thermodynamic and fluid flow processes within screw machines, which is valid for both the compressor and expander modes of operation. It includes the use of the equations of conservation of mass and energy applied to an instantaneous control volume of trapped fluid within the machine with allowance for fluid leakage, oil or other fluid injection, heat transfer, and the assumption of real fluid properties. By simultaneous solution of these equations, pressure-volume diagrams may be derived of the entire compression or expansions, process within the machine. The procedure has been developed over a period of fifteen years and validated with experimental results obtained from both reciprocating and screw compressors and screw expanders, some of which are included. The rotor profile generation processor, thermofluid solver and optimizer, together with preprocessing facilities for the input data and graphical post-processing and CAD interface, have been incorporated into a design package which provided a suitable tool for analysis and optimization of twin screw machine design. An example of its use is given in the optimization of the gate tip radius of a selected compressor design.

The effect of pH on the volatiles formed in an extruded starch-glucose-lysine model system

Starch-glucose-lysine extrudates were produced using a twin-screw co-rotating machine. The final barrel zone and die temperatures were 150 ± 3 °C and 150 ± 7 °C, respectively. The residence time was 32 s, the moisture content of the feed was 18% (m m −1) and the feed pH was 4.0, 5.0 or 7.7. Isolates of the volatile components were prepared using continuous steam distillation-solvent extraction with pentane:ether (9:1, v/v) as the solvent. Analysis by gas chromatography-mass spectrometry resulted in the identification of 32, 20 and 16 compounds, respectively, from each isolate. At pH 7.7, nitrogen compounds dominated the isolate and pyrazines accounted for over 80% (m m −1) of the volatiles. At pH values of 5.0 and 4.0, the yield of pyrazines was less than 10% of that at pH 7.7 and furans comprised more than 80% (m m −1) of these aroma isolates. The yield of total volatiles was more than 60% greater at pH 7.7 than at the other pH values.

Reactive processing of LLDPEs in counterrotating nonintermeshing twin-screw extruder. III. Methods of peroxide addition

An ethylene–octene linear low-density polyethylene (LLDPE) was treated with peroxide in a reactive extrusion system. A counterrotating nonintermeshing twin-screw extruder (System 2) was contrasted with a corotating intermeshing twin-screw machine (System 1). In System 2, the peroxide solution was pumped into the melted polymer, while it entered with the polymer pellets in the feed section of System 1. Molecular structure changes and the rheological behavior of peroxide-modified resins are similar in both operations but System 2 is much more effective. Much lower peroxide levels were needed in System 2. However, reactions in this setup were also more difficult to control. The presence of microgel was clearly evident in System 2 products but not in those made in System 1. The results of such reactive extrusion processes depend critically on the method of the peroxide feed and mixing conditions. Reaction conditions that favor optimum economy and peroxide efficiency are those which may compromise product homogeneity. © 1996 John Wiley & Sons, Inc.

Development of the Trilateral Flash Cycle System: Part 3: The Design of High-Efficiency Two-Phase Screw Expanders

An extensive research and development programme carried out at City University, London, has led to an improved level of understanding of how Lysholm twin screw machines may be used to recover power from two-phase flash expansion processes. The mode of operation of such machines is described together with the various types of rotor shapes used. Details are given of a computer simulation of the expansion process which was used to analyse 636 test results. These were obtained from earlier investigations as well as those of the authors and include three different working fluids, varying rotor profiles and sizes and power outputs of 5–850 kW. Good agreement was obtained between predicted and measured performance parameters and statistical analyses of the results indicate that this is unlikely to be improved without the development of more refined methods of two-phase flow analysis than are currently in use. Included in the tests are a set of measurements of pressure-volume changes within the expander carried out by the authors which confirmed a hitherto unappreciated feature of the expansion process. This is the relatively large pressure drop associated with the initial filling of the volume trapped between the rotors and the casing. The analytical technique thus developed was used both to explain the poor results of earlier studies with water expanders and to estimate optimum design performance. It is shown that, when expanding wet organic fluids, adiabatic efficiencies of over 70 per cent may be obtained at outputs of only 25 kW while multi-megawatt outputs are possible from machines no bigger than large compressors with efficiencies of more than 80 per cent. Two-phase screw expanders may be used not only for large-scale power generation in trilateral flash cycle (TFC) systems, but also in place of throttle valves in vapour compression systems to drive screw compressors in sealed ‘expressor’ units. The coefficient of performance of large refrigeration, air conditioning and heat pump systems may thereby be raised by up to approximately 8 per cent.

A Unified View of Modeling Flow in Counter-rotating Twin Screw Extruders

Abstract Intermeshing counter-rotating twin screw extruders have played an important role in polymer processing technology in the post World War II period. They operate on basically the same principle as low viscosity multiple screw pumps, i.e. they involve forward pumping of fluid contained in C-chambers, which are regions between the screw root and barrel between the interlocked intermeshing regions. Tangential counter-rotating twin screw extruders with open screw channels have also been manufactured in the same period. A new more general model of flow of a Newtonian fluid in a counter-rotating twin screw extruder with partially intermeshing screws is presented. The model allows transition from fully intermeshing to tangential screw machine systems. Calculations are made to show the effect of level of intermesh on pumping capability. Modular counter-rotating twin screw machines are analyzed. The problem of starvation is discussed.

Optimizing the processing of rigid PVC compounds

AbstractThe optimization of the extrusion of a simple rigid PVC compound in both single and twin screw machines is considered. Mass throughput, short term mechanical properties (tensile and impact). degree of mixing, and energy efficiency were selected as important parameters of the process. Using a statistically based experimental design, regression polynomials of the above variables were defined. The corresponding responses were superimposed and adequate processing windows were defined. Generally, processing rigid PVC at high screw speeds, small die resistances, and medium to high temperature profiles led to the most adequate values of the process parameters.

Fluid Mechanics of Distributive Mixing in a Modular Intermeshing Corotating Twin Screw Extruder

Abstract A theory of distributive mixing in a modular intermeshing corotating twin screw extruder is described. This is based on the Spencer – Wiley premise of representing mixing through interfacial area increase and its relationship to shear strain. The increase of interfacial area in both screw elements and kneading disc elements is analyzed. The flow in screw elements has a figure eight configuration but basically is analogous to flow in a single screw extruder. The primary mixing mode in these machines is within the kneading disc elements. A model for increase of interfacial area, which includes backflows through kneading disc tips, is developed. The kneading disc and screw element models are combined to compute interfacial area development in modular twin screw machine. We consider that modular twin screw extruders often have regions of starvation. To apply this model it is necessary to know flow configurations such as may be determined by a composite model of the total flow such as has been developed by the authors and their coworkers in earlier papers.

Flow in a Modular Intermeshing Co-rotating Twin Screw Extruder

Abstract The flow of viscous power law fluids in the various modules of a modular intermeshing co-rotating twin screw extruder. This includes right-handed and left-handed screw elements and kneading disc assemblies with various staggering angles. The calculations are based on hydrodynamic lubrication theory used a new calculation method based on combining methods by Tadmor, Broyer and Gutfinger, and by Griffith. The behavior of a composite twin screw extruder consisting of assemblies of screw and kneading disc elements is investigated and the potential of having a sequence of fully filled and starved regions is considered. The existence of such distribution of polymer and starvation behavior in a twin screw machine was found in experiments with a polyethylene. The material distribution is reasonably well predicted by the non-Newtonian flow model of this paper.

A new model of flow in a tangential counter‐rotating twin screw extruder

AbstractThe flow of Newtonian fluids in a tangential counter‐rotating twin screw extruder with matched and staggered screw flights is analyzed using hydrodynamic lubrication theory. The flow analysis network (FAN) method of Tadmor et al. as modified by Szydlowski et al. [Int. Polym. Proc., 1, 207 (1987)] has been applied to determine pressure fields and velocity fields. The results show lower pressure development and greater back flows for staggered flight as opposed to matched flight tangential counter‐rotating twin screw extruders. The predictions agree well with experimental studies. Earlier theories of Kaplan and Tadmor, and Nichols overpredict the performance of matched flight twin screw machines.

Twin screw extruders; development of technology and analysis of flow

AbstractA critical evaluation of the technological development and studies of mechanisms of flow in twin screw extruders is presented. We include (i) intermeshing counter‐rotating twin screw machines, (ii) tangential counter‐rotating twin screw extruders, and (iii) intermeshing corotating twin screw machines.

Designing of extrusion lines

AbstractThe design of extrusion lines deals with the extruder and the die as well as with the down stream equipment, especially the cooling section. This paper aids in the design of these three units, regarding different aspects. The scale‐up of extruders by means of the model theory is demonstrated in its practical use with examples. Emphasis is laid on the design of series of single screw extruders with mixing‐pins and of twin‐screw extruders (scale‐up). There the adequate selection of the necessary restrictions for the compilation of model laws is discussed in detail. This is an approach to the safe machine design on the basis of model theory.The present standard of the calculation of threedimensional flow patterns in extrusion dies by means of the finite‐element‐method (FEM) is shown in order to point out how design‐aids for the die‐designer are derived from these calculations. After a discussion of application‐criteria for the FEM, examples will show possible ways of using FEM‐results.For the shaping of Y‐branches in twin‐screw machines the FEM‐results point to possible plate‐out areas and to their avoidance by geometrical modifications. The calculation of a die for a T‐profile helps to design the passages between entry and exit sections. Finally, flow coefficients for the calculation of pressure losses in different cross sections are given.The design and optimization of cooling sections, as it can be done with the aid of the actual cooling models, is shown for some important extrusion processes. Heat transfer coefficients for practical cases (film‐sheet‐, pipe‐ and parison extrusion) are presented. Examples show, how diagrams for the design and optimization of cooling sections (for example the reduction of internal stresses and warping) can be made, using a microcomputer.