Single screw extruder problems

Spalding, M. A. and Hyun, K. S., Troubleshooting Mixing Problems in Single-Screw Extruders, SPE ANTEC Tech. Papers, 49, 229 (2003)... 

The above equation can be simplified assuming a Newtonian isothermal problem. For such a case Pawlowski reduced the above equations to a set of characteristic functions that describe the conveying properties of a single screw extruder under isothermal and creeping flow (Re < 100) assumptions. These are written as... 

TPs are characterized by low thermal conductivity, high specific heat, and high melt viscosity. Preparation of a uniform homogeneous melt and its delivery at adequate pressure and a constant rate could pose considerable problems if not properly processed (Chapter 3). The principal extruder variants are the single-screw and the twin-screw types. Of these, the single-screw extruder is by far the most versatile and popular in use. 

In the past twin and other multi-screw extruders were developed to correct the problems that existed with the single-screw extruder. Later the single-screw designs with material developments practically eliminated all their original problems. 

The goal of this contribution about the operating behaviour of single-screw extruders in the extrusion of ceramic masses is, on the basis of an easily comprehensible presentation, to explain the scenarios emerging in practice so as to be in a position, in the case of any problems arising, to initiate purposefully the necessary steps. 

A single-screw extruder or a twin-screw extruder is used for the manufacturing of PVC rigid foam. This type of foam is widely used as the core of some sandwich and multilayer panels. The flexible one is used as the foam layer in coated fabric flooring. Its low vapour transmission is an advantage when condensation might be a problem. 

In the extrusion of medical devices, molecular degradation is often a critical issue. This degradation of the MW can be strongly influenced by the geometry of the extruder screw. Unfortunately, little information on this problem is available in the open literature. Paakinaho et al. [127] published results of a study on the MW reduction along the length of a single screw extruder for three polylactic acid (PLA) resins see Fig. 8.111. 

The simple extruder design analyzed here would not be implemented in practice because of obvious mechanical problems, but, as we shall see subsequently, it is sufficiently close to the description of a true single-screw extruder that the calculations done here are all relevant. There are three weaknesses in the analysis. First, we have considered only a Newtonian fluid, while most real polymers have highly shear-dependent viscosities. Second, our heat transfer analysis is inadequate, both because we have considered temperature- and pressure-independent physical properties and because we have been able to obtain explicit solutions only for certain limiting cases. Finally, we have not dealt with the flow in the neighborhood of the transition from the extruder channel to the die. All of these restrictions can be relaxed, as we shall see, but to do so for the latter two generally requires the use of numerical algorithms to solve the full equation set. We shall address this topic in Chapter 8. 

Additives required to give properties for specific applications such as pigments, flame retardants, fillers etc are almost always in the form of very fine powders as this is usually necessary for them to perform effectively. Although this can cause problems (with the single screw extruder normally being unable to satisfactorily incorporate these additives), they are more usually the concern of the compound and masterbatch supplier. 

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