Extruder designs

Figure 15.2 Schematic showing the channel depths for a 304.8 mm diameter melt-fed extruder designed to pump an ABS resin...

Normal Stress (Weissenberg Effect). Many viscoelastic fluids flow in a direction normal (perpendicular) to the direction of shear stress in steady-state shear (21,90). Examples of the effect include flour dough climbing up a beater, polymer solutions climbing up the inner cylinder in a concentric cylinder viscometer, and paints forcing apart the cone and plate of a cone—plate viscometer. The normal stress effect has been put to practical use in certain screwless extruders designed in a cone—plate or plate—plate configuration, where the polymer enters at the periphery and exits at the axis. 

Extrusion and extruder design are apparently almost entirely empirical at present. This field might well defy rigorous theoretical analyses until the simpler problems of flow in annular spaces and rectangular ducts and of nonisothermal fluid flow are understood. 

The shear viscosity can be used for relating the polymer flow properties to the processing behavior, extruder design, and many other high shear rate applications. Elongational viscosity, die swell measurements as well as residence time effects can be estimated. Typical data are shown in Figure 6. 

The extruders used for rubber do not require the length of those used for plastics, because they do not need as long a melting section as plastics do. When the plastics industry adopted the rubber machinery, it imitated the rubber extruder design, but as demands for output and quality mounted, the length-to-diameter ratio of the extruder grew over time, until it leveled off at current values. 

Example 9.2 The Design of a HDPE Pelletizing Extruder Design an 18,000-lb/hr pelletizing extruder for high-density polyethylene (HDPE) melt at 450°F to generate 2500 psi head pressure. Assume a constant channel depth extruder with an axial length of 60 in. The melt density is 54 lb/ft3, the viscosity is 0.15 lbfs/in2, and the specific heat is 0.717 Btu/lb°F. 

Numerical methods are a working tool which we encounter almost daily in the fields of engineering and science. The complexity of numerical methods ranges from simple spreadsheets to the solution of complex, non-linear differential equation systems that occur in flow dynamics. The aim of computational fluid dynamics, or CFD, is to obtain a deeper understanding of the flow processes that take place within the extruder and to combine the findings with experiments to produce reliable and economic extruder designs. 

The selection of the extruder design is based on the principal requirements of the extrudate and the nature of further processing. For the production of uniform granules to be dried in a fluid-bed drier, a low-compaction system, such as that provided by the various types of screen extruders may be suitable. Cylinder or gear-type extruders may be more appropriate when aiming for a densified extrudate, such as that required for spheronization. Ram-extrusion systems, which allow precision control of extrudate density, size, and shape, are ideal for the extrusion and forming of pharmaceutical polymers of the type used for sub-dermal implants. 

The main degrees of freedom in twin-screw extruder design are geometry (cross-section profile of screw pair), power (torque capacity) and speed (rpm). The essential geometry of any co-rotating twin-screw extruder is defined by two key parameters 1) centerline distance [a] between the shafts, and 2) outer diameter to inner (root) diameter ratio [OD/ID], see Fig. 3. For a fixed centerline, the OD/ID ratio defines the free volume of the extruder. 

Al Processes for Model-based Design (Marquardt/ Schneider) A2 Information Models for Conceptual Design (Marquardt) A3 Extruder Design Processes (Haberstroh)... 

As part of the polymerization scenario described in Subsect. 1.2.2, the compounding extruder is to be designed by a company specializing on extruder design and construction, supported by simulations. As the necessary knowledge is often not present in chemical companies, aspects of cross-organizational cooperation come into play, which will be treated later in Subsect. 4.1.7. 

A more detailed description of the simulation and design tasks can be found in Subsect. 5.4.2 (subproject A3). For the possibilities of experience-based support for extruder design, refer also to the FZExplorer tool in Subsect. 3.1.5. Here, we will concentrate on structuring the domain model for annotation of the videos resulting from simulation. 

A3 the models for extruder design, and for the annotation of simulation results ... 

The scenario ends with the discussion of the information returned from the contractor. Video conferencing tools, as developed by the subproject B3, can well be apphed here to allow direct communication between the experts of chemical and plastics engineering (cf. Subsect. 3.3.2). After entering the final decision into the PDW, earlier design steps might need to be revisited if a central assumption of the extruder design had to be changed, e.g., the rotational speed. In cooperation with the AHEAD administration system, PRIME and the PDW can follow the traces and determine the necessary steps (see Subsect. 3.2.6). 

The Core Ontology has been introduced and exemplarily extended with the domain model(s) of extruder design. The current prototype of the PDW, as realized on top of this Core Ontology, has been demonstrated in an example scenario from plastics engineering. It has been shown how the process steps and product artifacts are captured, structured, and later on offered for reuse in the scenario. Extensions for cross-organizational engineering have been introduced and described. 

Nowadays, the extruder design itself and the specification of process parameters are usually done on an empirical basis. This is due to the fact that a high number of different screw and barrel elements exist which allow for a huge number of combinations. Some special features of the compounding process, for example the degassing capability, can not be assessed adequately by empirical methods. 

Dispersion-type screw mixers are in an entirely separate class from the extruder designs described above. They are applicable to processing of either dry materials or moderately viscous pastes, creams, or lotions. These machines normally consist of a conical vessel equipped with a conical or inclined screw. There are either single- or twin-screw models that provide a gentle mixing action, and thus... 

---