The Standard Extruder Screw

In many discussions on extrusion, reference is made to a so-called standard or conventional extruder screw. In order to define this term more quantitatively, the general characteristics of the standard extruder will be listed see also Fig. 8.30  

Channel depth in feed section 0.15-0.20 D Channel depth ratio 2-4 

These dimensions are approximate, but it is interesting that the majority of the 

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There are a large number of modifications of the standard extruder screw in use today. It will not be possible to mention all of them, but an effort will be made to discuss the more significant ones. Figure 8.31 shows the standard screw with an additional flight in the feed section. 

In the parallel portion of the barrier section, the width of the solids channel is constant. The highest melting performance will be reached if the width of the solid bed fills the entire width of the solids channel. If the solid bed velocity is assumed constant, the total melting length can be found by using the equations derived from the standard extruder screw see Eq. 7.116. The total melting length for the parallel barrier portion is simply ... 

In terms of processing there is no need for pre-drying PCHE granules, a standard extruder screw as used for polycarbonate may be used and discs are said to release well from the mould. Question marks remain on the oxidative stability of the polymer and on the quality of adhesion of the reflective layer but Dow claim that metallising is possible. 

Figure 8.27 The LL-extruder screw versus a standard extruder screw...

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... 

Standard production lines for obtaining granulated aminoplast by the use of screw mixer-extruders are shown in Fig. 14a, b. The production line illustrated in Fig. 14a is the Buss line for obtaining crushed material35 ... 

Extrusion of tubes, hose, and profiles is done on standard extruders for rubber. The usual temperature pattern is a gradual increase of temperature from the feed zone to the die. The die temperature is typically 100°C (212°F) and the screw temperature is approximately the same as the temperature of the feed zone.37 Processing aids are almost always required to improve the surface appearance and to increase the extrusion rate. Extrusion represents only a small proportion (about 10%) of the total consumption of fluorocarbon elastomers.37... 

Once the co-rotating extruder had become the standard element for plastics conditioning and processing, compounding in other words, and the patents referred to in Section 2.2.2 had expired (approximately 1970), the co-rotating principle was adopted by numerous mechanical engineering companies. Today, approx. 50 companies around the world can supply small or medium-sized (approx. < 120 mm) screws. However, just five are able to supply large screws of 120 mm to 380 mm (estimated values). 

We are also witnessing a considerable increase in new specialized literature. The most important of these is the standard work Twin Screw Extrusion by J. L. White [44]. It is a rich source of information, both about the historical development and the state-of-the-art technology, as well as about patent literature and specialized publications. A few examples [45,46] from the wide-ranging specialized literature on the subject of co-rotating extruders should be mentioned here. 

The cycle starts with the plastification of the core component in the injection unit. Then the extruder moves to the bottom position, the injection unit moves forward to the extruder nozzle to link the nozzles of the extruder and the injection unit. The extruder starts plastification of the skin component and extrudes the melted skin component into the screw antechamber of the injection unit. Thus the skin and core components are located one after the other in the screw antechamber. After the extruder moved back to the top position, the injection unit moves forward to the mold followed by a conventional filling phase. Due to the fountain flow effect the first injected material forms the skin layer followed by the second component forming the core. Compared to the standard sandwich process the injection phase of the monosandwich process is less complicated as it is identical to the conventional injection molding process. 

For extrusion blow moulding, the blow moulding machine is based on a standard extruder barrel and screw assembly to plasticise the polymer. The molten polymer is led through a right angle and through a die to emerge as a hollow (usually circular) pipe section called a parison. 

The form is typically pellets, flake, or powder. Most pellets and flake are easy to feed since they have a high bulk density, their individual dimensions are sufficiently small with respect to extruder channel depth, and they do not lubricate the screw and thus impede the drag flow mechanism. Standard profile screw elements with a pitch of 1-2D are appropriate. 

Paul Troester started manufacturing single-screw extruders (SSE) in 1892, but only in 1935 the first SSE for thermoplastics was introduced. Four years later, Paul Leistritz built electrically heated, air cooled SSE, having L/D = 10, automatic temperature control, variable screw speed, and nitrided barrel. The machine is considered prototype of the modem extmders. The SSE offered continuous processing capability, but it was notoriously poor as a mixer. Furthermore, under the standard processing conditions, a significant non uniformity in the shear history had resulted in large temperature differences of the melt, AT = 60°C. To alleviate the... 

Extruder screws and cylinders are made of standard tempered and high strength steels. For increasing the wear resistance, nitrating or heat hardening is used. Cooling of the entrance zone is... 

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