Disk Extruders

Gear or Disk extruders, e.g., Maxwell Melt-elasticity Extruder, Tadmor s Disk Extruder,... 

There are several extruders that do not use an Archimedean screw for transportation of materials, but still belong to the category of continuous extruders. These machines are sometimes referred to as screw-less extruders. Usually they employ a disk or a drum. Most disk extruders are based on viscous drag transport principle. To this category belong stepped disk extruders [Westover, 1962 Raleigh, 1879], drum extruders, spiral extruders [Ingen Housz, 1975], diskpack extruder [Tadmor, 1979, 1980 Tadmor et al, 1979, 1983 Hold et al., 1979 Valsamis, 1983], and many others. 

Gear or disk extruders like MaxweU melt-elasticity extmder, Tadmor s disk extruder, etc. 

Multi screw extruders Gear pumps Planetary gear extruders Multi (>2) screw extruders Spiral disk extruder... 

Viscous drag extruders Drum extruder Diskpack extruder Stepped disk extruder... 

There are a number of extruders, which do not utilize an Archimedean screw for transport of the material, but still fall in the class of continuous extruders. Sometimes these machines are referred to as screwless extruders. These machines employ some kind of disk or drum to extrude the material. One can classify the disk extruders according to their conveying mechanism (see Table 2.1). Most of the disk extruders are based on viscous drag transport. One special disk extruder utilizes the elasticity of polymer melts to convey the material and to develop the necessary diehead pressure. 

Disk extruders have been around for a long time, at least since 1950. However, at this point in time the industrial significance of disk extruders is still relatively small compared to screw extruders. 

One of the first disk extruders was developed by Westover at Bell Telephone Laboratories it is often referred to as a stepped disk extruder or slider pad extruder [24]. A schematic picture of the extruder is shown in Fig. 2.13. 

A practical disadvantage of the stepped disk extruder is the fact that the machine is difficult to clean because of the intricate design of the flow channels in the stepped disk. 

Another development in disk extruders is the diskpack extruder. Tadmor originated the idea of the diskpack machine, which is covered under several patents [31-33]. The development of the machine was undertaken by the Parrel Machinery Group of Emgart Corporation in cooperation with Tadmor [34-39]. The basic concept of the machine is shown in Fig. 2.17. 

Several modifications have been proposed to improve the performance of the elastic melt extruder. Fritz [43] suggested incorporation of spiral grooves to improve the pressure generating capability, essentially combining the elastic melt extruder and the spiral disk extruder into one machine. In Russia [47], several modifications were made to the design of the elastic melt extruder. One of those combined a screw extruder with the elastic melt extruder to eliminate the feeding and plasticating problem. Despite all of these activities, the elastic melt extruder has not been able to acquire a position of importance in the extrusion industry. 

Single Screw Extruders. In the plastics industry, there are three main extruder types the screw extruder, which is the most common the ram extruder and the drum or disk extruder, which is the least common. In a screw extruder a screw rotates in a cylinder the rotation of the screw creates a pumping action (see Fig. 1). 

Disk extruders. There are several types of screwless extruders. These machines employ a disk or a drum to plasticate, mix, and extrude. Most designs are based on viscous drag flow. They include Maxwell s elastic melt extruder, Westover s stepped disk and drum extruders, and Diskpack [39]. The Diskpack extruder has the capability of performing all the elementary steps of plastics processing by combination of differently shaped rotating... 

In the co-rotating disk extruder, the material is transported by the action of two moving walls, the sides of the disks, which can form a parallel-faced or a wedge-shaped chamber as shown in Figure 5.23. An analysis of the velocity distribution in the parallel-faced chamber with a restriction to flow at e outlet shows two flow patterns [21]. A circular flow in planes perpendicular to the axis of rotation and another circular flow in cylindrical surfaces around the axis of rotation. The superposition of both flows is conducive to extensive mixing. 

Figure 5.23 Sectional view of co-rotating disk extruder without mixing blocks. (Reprinted from Ref. 21 with kind permission from John Wiley Sons, Inc., New York, USA.)...

Co-rotating disk extruders are useful in the preparation of color concentrates, polymer blending as well as in the compounding of fillers with polymers. 

The same system was tested by Baramboim and co-workers [119] at 20°C for 15-45 min. Maleic anhydride was 5-15% of the system. Related experiments have been performed on polyethylene in a disk extruder (see below). The properties of poly(vinyl chloride) have been improved (processability and toughness) by milling with styrene [120],... 

Protasov and Baramboim modified the properties and structure of polyethylene by reaction with maleic anhydride in a disk extruder [134, 135, 235, 236, 240]. The following conclusions were reached ... 

There are basically three classifications of extruders screw extruders, disk extruders, and ram extruders. The most common ones are the screw extruders, and for this reason they are emphasized in this book. 

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