Twin Screw Extruders Functions

Matthey, F. P. and Hanna, M. A. (1997). Physical and functional properties of twin-screw extruded whey protein concentrate-corn starch blends. LWT Food Sci. Technol. 30,359-366. 

Fig. 17. Experimentally measured values of the exit concentration as a function of pressure for a polymeric solution consisting of methyl methacrylate-polyfmethyl methacrylate). Data were obtained by Werner using a twin-screw extruder. (Reproduced with permission from Werner, 1980.)...

In addition to single screw extruders, there are twin and multiscrew extruders that perform essentially the same functions, but with additional benefits. Among these, the intermeshing twin screw extruders are the most important ones. They are used primarily for heat-sensitive resins (such as PVC), which are difficult to process. The intermeshing screws create a relative motion of one flight in another, such that it acts... 

Fig. 11.15 RTD cumulative functions of a single screw tangential counter-rotating twin-screw extruder (TCTSE) under matched and staggered conditions, and the back-mixed extruder reactor. [Reprinted by permission from Y. Lu, Ph.D Dissertation, Department of Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ, 1993.]...

Use of unmodified starches in combination with mild acids and carbonates has been claimed.214 Starch moisture content should be no greater than 25%. Acids such as tartaric, citric or malic acids are added at levels between 0.2% and 7%, while the carbonate level is 0.1% to 2%, both based on total starch composition. The acid is stated to serve a two-fold function it depolymerizes the starch during extrusion via hydrolysis, improving expansion at the same time, it liberates carbon dioxide, which acts as a blowing agent, from the carbonate. A typical formula extruded in a twin-screw extruder at 170-195°C yielded a product with a density of approximately 1 lb/ft3 (16kg/m3) and resilience of 60-85%. The foam had a continuous skin with a closed cell structure. 

If a twin-screw extruder is stopped and opened, several zones can be clearly distinguished [9]. Depending on whether the extruder is fed with a solid or a liquid material, two different situations occur. In the case of a solid feed (a polymer or solid monomer), the channels near the feed hopper are more or less filled with solids. This material melts, resulting in a zone where the channels are only partly filled. At the end of the screw, close to the die, a zone exists where the channels are completely filled with polymer. If the extruder is fed with a liquid monomer, the first part does not necessarily need to be partly empty but, as will be explained later, for reasons of stability it is advisable to create a zone where the channels are not fully filled. In particular, the fully filled zone is very important for correct functioning of the extruder. In this zone the pressure is built up, the mixing and kneading mainly... 

The enormous flexibility and variability of the system was possible thanks to the constant development of the process related components, such as the barrels and screw elements and new geometry variants being developed continuously. The initial aim of the design, to maintain the self-wiping function of the screw elements, is sometimes abandoned, in part or in whole, to permit innovative geometries. This enabled the co-rotating, twin-screw extruder to carry out the most diverse processes and handle the most diverse products with this single machine system. 

There are systems, which provide a dedicated mixing system. The use of mixing within a co-rotating twin-screw extruder system is described by Kleinebudde et al. (13). This system also claims the ability to be able to adjust the water content to the appropriate level and because it acts as both mixer and extruder, reduces the number of items of equipment required. This system is not widely employed in screen extruder systems, which without careful formulation can generate considerable heat that could be detrimental to heat sensitive formulations. Hellen et al. (14) describe the use of a granulator (Nica M6L, Nica systems AB, Molndat, Sweden) that functions with a high speed turbine to mix the powder with the water prior to extrusion. 

Even though in principal, the visualization of ID simulations in a three-dimensional space is not too meaningful, the visualization of a number of ID-functions with exact geometrical placement can be an interesting alternative. In addition to that, the simple and natural interaction metaphors that are used in VEs can ease the task of configuration construction. Figure 3.53 depicts the prototype we developed for the interactive configuration of twin screw extruders that uses fast ID simulations to prepare functional zones for expensive FEM simulations. 

From the mathematical point of view the complexity is reduced because the system of equations which has to be solved is a function defined on the two-dimensional manifold of the control volumes boundary and leads to a dimension reduction. Practically the discretisation of the boundary usually is more simple than the meshing of complex three dimensional volumes. Especially this pertains to the transient flow channel geometry in co-rotating twin screw extruders. The surface meshes for the screws can independently be rotated inside the screw and barrel mesh analogous to the batchwise working internal mixer (Banbury Mixer) shown in the bottom part of Fig. 5.26. 

The functional sections in co-rotating twin screw extruders are sequentially arranged and usually have no back couphng upstream due to the partially filled sections. Thus material modifications or flow effects have no influence and are decoupled one another unlike in single-screw extruders, for example. 

More expensive but easier to control is a twin screw extruder, TSE. Owing to the modular design with many types of elements fulfilling different functions, TSE can be optimized for specific tasks. The ratio of the dispersive to distributive mixing can be adjusted, and the width of the residence time can be controlled. TSE is excellent chemical reactor for polymerization, modification... 

Numerous compatibilizers can be produced by grafting monomers (containing some kind of functional groups) in melt onto homopolymers and copolymers of olefins or their blends (25). Reactive extrusion (26) is a basic process for this when the twin-screw extruder is used as a reactor of continuous action (27). 

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