Design screw

This chapter offers an overview of screw modeling, and focuses on process technology, with reference to the two main areas of process technology and machine technology . 

As explained, the ideal situation for a model that would enable a purely mathematical screw design would be a straight path to the right, as in Fig. 6.1. In fact, however, trials are usually needed in practice. If for instance, another type of product is to be run on an existing production machine, this can be tried out experimentally on a large scale. If the trial is successful, everyone will be happy but if not, this kind of trial can be very expensive. Modeling may not be able to simulate the whole process but it is useful preparation for production trials. It can serve to define certain details or screw sections in order to prevent the worst outcome. 

In many cases it is not possible to predict exactly how the end product will behave in a specific screw machine for a specific a product manufacture/modification operation. Examples  

Tests are usually carried out on a smaller scale before an extruder is built or configured on a production scale. Here again, modeling helps to support the process. A scale-down is used first in order to decide at what scale the miniature trials should be carried out. If only a small amount of resin is available, it might be practical to carry out initial tests with a small laboratory extruder to produce a sample quantity. These tests therefore tend to be used for product development. Once this is done the question is What will the production screw for the manufacture of the product look like For this purpose it may be necessary to carry out 

With practically all machines, only the cylinder temperature is directly controlled (see Chapter 1). The actual heat of the melt, within the screw and as it is ejected from the nozzle, can vary considerably, depending on the efficiency of the screw design and the method of operation. Factors affecting melt heat include the time plastic remains in the cylinder the internal surface heating area of the cylinder and the screw, per volume of material being heated the thermal conductivity of cylinder, screw, and plastic (Table 1-6) the heat differential between the cylinder and the melt and the amount of melt turbulence in the cylinder. In designing the screw, a balance must be maintained between the need to provide adequate time for heat exposure and the need to maximize output most economically. 

In general, heat transfer problems have led screw designers to concentrate on making screws more efficient heat transfer devices. As a result, the internal design and performance of screws vary considerably to accommodate the different plastics that are used (1-4, 85, 100-102). Most machines are single, constant-pitch, metering-type screws to handle the majority of plastics (Fig. 2-3). 

DIMENSIONS, IN. RIGID PVC IMPACT POLYSTYRENE LOW-DENSITY POLYETHYLENE HIGH-DENSITY POLYETHYLENE NYLON CELLULOSE ACET/ BUTYRATE 

The output of a metering screw is fairly predictable, provided that the melt is under control. With a square pitch screw (conventional screw where distance from flight to flight is equal to the diameter), a simplified formula for output is  

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Considerable work has been done on mathematic models of the extmsion process, with particular emphasis on screw design. Good results are claimed for extmsion of styrene-based resins using these mathematical methods (229,232). With the advent of low cost computers, closed-loop control of... 

Attention also has to be paid to screw design. The screw metering zone should be long and shallow to ensure melt homogenisation but not so shallow that... 

There are two types of cold feed extruders, the standard screw design type (Fig. 15) and one with a screw feed using a ram action to feed the extruder. In both... 

Plasticator A very important component in a melting process is the plasticator with its usual specialty designed screw and barrel used that is used in different machines (extruders, injection molding, blow molding, etc.). If the proper screw design is not used products may not meet or maximize their performance and meet their cost requirements. The hard steel shaft screws have helical flights, which rotates within a barrel to mechanically process and advance (pump) the plastic. There are general purposes and dedicated screws used. The type of screw used is dependent on the plastic material to be processed. 

The multiplex extmsion screws have to cover a variety of applications and depending on the task and the type of mbber compound the following principal screw designs are available ... 

A Uniroyal patent originated the principle of using pins protruding into the extruder barrel to disrupt the laminar flow of compound, and this principle was further developed by Troester in their QSM (cross-flow mixing) machines. The protrusion of pins into the barrel necessitated a modification to the screw design to accommodate them. Many papers can be found in the literature dealing with this type of pin extruder. 

An extruder with a heating device for resin plasticizing or melting. The screw design and the temperatures depend on the injected material. 

The advantages of the LFRT products are offset by requirements to optimize the whole process chain including extruder screw design, processing parameters and mould design, thus needing higher investment and production costs. 

As shown in Fig. 4.1, resin feedstocks have a considerable level of interparticle space that is occupied by air. This level of space and thus the bulk density of the feedstock depend on the temperature, pressure, pellet (or powder) shape, resin type, and the level and shape of the recycle material. For a specific resin feedstock, the bulk density Increases with both temperature and the applied pressure. Understanding the compaction behavior of a resin feedstock is essential for both screw design and numerical simulation of the solids-conveying and melting processes. Screw channels must be able to accommodate the change in the bulk density to mitigate the entrainment of air and the decomposition of resin at the root of the screw. Typically, screw channels are set by using an acceptable compression ratio and compression rate for the resin. These parameters will be discussed in Section 6.1. 

The model developed here uses a fitting parameter to obtain melting lengths that are consistent with those observed experimentally. This fitted model is utilized to adjust for some of the non-linearities in the model. The model is not meant as a screw design tool. An improved model could be written based on the original Lindt and Elbirli model [27], The improved model would set the boundary conditions for screw rotation rather than barrel rotation as used by Lindt and Elbirli. 

Meijer, H. E. H., Melting in Single-Screw Extruders Models, Calculations, and Screw Design, Ph. D. Thesis, Technische Hogeschool Twente, Netherlands (1980)... 

Dekker, ]., Improved Screw Design for the Extrusion of Polypropylene , Kunststojfe, 66, 5 (1976)... 

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