Extruder axial screw

Medium Pressure Axial Screw Extruders Axial screw extruders normalty operate with low (see Section 8.4.1) or high (see Section 8.4.3) pressure. The basic principle of an axial screw extruder was shown in Fig. 6.4b.1 (Chapter 6) and Fig. 8.24 (Section 8.4.1). The pressure that is developed by the screw(s) depends on the power of the drive and the frictional resistance in the extrusion channel or other discharge device. Axial screw extruders that rely solely on the pressure developed by the rotating screw(s) employ hydrostatic pressure as the driving mechanism for extrusion. Such machines generally use high pressure. However, under certain conditions and for specific applications, some can be classified as medium-pressure agglomerators. 

Transport of energy in the screws was modeled previously for single-screw extruders [30-32] and for twin-screw extruders [33]. In order to predict the axial screw temperature in a single-screw extruder, heat conduction along the screw has to be modeled. The model developed by Derezinski [32] included heat conduction from the barrel through the screw flights to the screw surface, heat conduction from the polymer to the screw root, and heat conduction in the axial direction. The model showed that the screw does not behave adiabatically and that the steady-state heat conduction in the screw depends greatly on the size of the extruder. 

Target quantities power consumption P, axial screw force F, pumping pressure Ap, temperature of the extrudate expressed in temperature difference AT = T — To, and volumetric throughput Q... 

C. E. Wyman, Theoretical Model for Intermeshing Twin Screw Extruders Axial Velocity Prohle in Shallow Channels, Polym. Eng. Sci., 15, 606-611 (1975). 

Screw-fed extruders have screws that rotate along the horizontal axis and hence transport the material horizontally they may be axial or radial screw extruders (Fig. 5). Axial extruders, which have a die plate that is positioned axially, consist of a feeding zone, a compression zone, and an extrusion zone. The product temperature is controlled during extrusion by jacketed... 

While most of the axial screw extruders operate solely according to the hydrostatic pressure principle (Figure 311) several other types use an extrusion blade to additionally create a wiping effect at the die plate (Figure 312(a)). This blade looks like and performs in a fashion somewhat similar to a propeller. Nevertheless, the material discharges axially from the end of the extruder barrel. 

Hollow, perforated cylinder(s), feed from the outside Hollow, perforated cylinder, feed from the inside Flat, perforated die plate with press roller(s) Gear-shaped press rollers, feed from the outside Medium pressure axial screw extruders... 

Three different medium pressure axial screw extruders will be discussed in the following as examples. 

High Pressure Axial Screw and Ram Extruders Extrusion is possible by low, medium, and high pressure. In all three cases the same underlying basic principle is responsible for the agglomeration process. Tab. 8.8 lists the requirements. 

In high pressure axial screw extruders, dies and their orifice(s) can have different shapes and sizes. Their selection depends on the plasticity and flow properties of the materials to be extruded and, obviously, on the desired product shape and size. Two basic types of dies are used in extrusion. For the formation of small agglomerates, such as strands or pellets, extrusion plates are used. Such plates can be simple or more complex, the latter, so called sandwich types, consist of multiple plates that are made of different materials. Fig. 8.78 presents sketches of two extrusion plate designs. Parameters are based on manufacturer know-how as well as vendor experience from different applications and can not be discussed here. 

Fig. 8.81 Photographs of two high pressure axial screw extruders showing different die changers. (a) Extruder (model 90AD with horizontal mixer/pug sealer and dual hinged mouthpieces (= dies), (b) extruder (model 90AD with hydraulic horizontal die changer dies are not installed in the die holders (courtesy J.C. Steele, Statesville, NC, USA).

Injection equipment consists essentially of two parts the injection and clamping units. The injection unit melts the polymer and injects it in the mold. The vast majority of modern injection presses use a reciprocating screw. As in an extruder, the screw rotates, melts the polymer and pumps it. Furthermore, in most injection molding units, the whole screw can also axially move forward, acting like a plunger to inject the resin into the mold cavity through runners and gates. 

In the twin-screw extruder, the screw system is open in the axial direction. It is actually open in the longitudinal direction of the screw channel with passage from the inlet to the outlet of the machine. Figure 5.2 illustrates the flow pattern of material from the feed section in a co-rotating compounding extruder. 

Fluid film thrust bearings have been applied to extruders on a few occasions. Their load carrying capability at low speed is generally poor and a loss of fluid film would have disastrous results. If a hydraulic drive is used to turn the screw, application of hydraulic thrust bearings may deserve some consideration. Reference 28 describes an extruder with hydraulic drive that incorporates a patented thrust bearing assembly with hydraulic axial screw adjustment. By measuring the pressure of the hydrostatic chamber of the thrust bearing, the pressure in the polymer melt at the end of the screw can be determined. 

Ln-line screw system. The in-line screw system, or screw ram system as it is sometimes called, is the most important development in injection moulding. In this system the plastics is softened by passing it through a heated cylinder by the use of an extruder type screw, the softened plastics building-up in front of the screw. The build up of plastics in front of the screw causes the screw to move axially backwards. The shot size can therefore be easily controlled by stopping the screw rotation at a predetermined position. When the preplasticising operation is complete, the screw is moved forward under pressure pushing the plastics into the mould (Fig. 2). 

An exceptional type of single-screw extrader is the pin extruder. In this extruder the screw has interrupted flights and it rotates in a barrel with stationary pins. The effect is twofold because of the rotation of the screw a good mixing action can be achieved, and because of the increased resistance against slip the throughput is more stable than in ordinary single-screw extruders. However, the pressure built up in this type of machine is rather poor. In some special types the screw not only rotates but also oscillates in an axial direction. 

Roberts (1970) has identified the exact mathematical form for axial dispersion in a screw extruder by noting that the devolatilized film is remixed with material in the bulk at an upstream position which can be determined exactly once the geometry of the system is specified. According to Roberts, the Peclet number is given by... 

A number of issues relative to the prediction of solids conveying in smooth bore single-screw extruders are exposed from the theoretical fits to the data in Fig. 5.32. First, the data needed to carry out an effective simulation is difficult to take and is very time consuming, and only a few labs have the proper equipment that is, bulk density measurement, dynamic friction data, lateral stress, and solids conveying data. Moreover, care must be taken to develop an accurate representation of the surface temperature for the barrel and screw as a function of the axial position. This would be quite difficult in a traditional extruder with only a control thermocouple to measure the temperature at the midpoint of the barrel thickness. Second... 

Figure 6.7 Axial pressure profiles measured for a 63.5 mm diameter extruder running an ABS resin at 60 rpm for screws with a 8.89 mm deep feed channel, 6 diameters of feed section, and a metering channel depth of 3.18 mm (C = 2.8) for the photographs of Fig. 6.6...

Figure 6.38 Axial pressure profiles for a 63.5 mm diameter instrumented extruder running two different pellet geometries at a screw speed of 90 rpm...

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