Cylinder extruders, design

Normal Stress (Weissenberg Effect). Many viscoelastic fluids flow in a direction normal (perpendicular) to the direction of shear stress in steady-state shear (21,90). Examples of the effect include flour dough climbing up a beater, polymer solutions climbing up the inner cylinder in a concentric cylinder viscometer, and paints forcing apart the cone and plate of a cone—plate viscometer. The normal stress effect has been put to practical use in certain screwless extruders designed in a cone—plate or plate—plate configuration, where the polymer enters at the periphery and exits at the axis. 

The selection of the extruder design is based on the principal requirements of the extrudate and the nature of further processing. For the production of uniform granules to be dried in a fluid-bed drier, a low-compaction system, such as that provided by the various types of screen extruders may be suitable. Cylinder or gear-type extruders may be more appropriate when aiming for a densified extrudate, such as that required for spheronization. Ram-extrusion systems, which allow precision control of extrudate density, size, and shape, are ideal for the extrusion and forming of pharmaceutical polymers of the type used for sub-dermal implants. 

A precision knife cutter whose cutting surface is made of stainless steel will be designated. The knife edge should be 24 in long. The product is to be Vs in cylinders. The product rate is 2,000 lb/hr through each extruder. This should require about a 60 hp motor.41... 

Pellet mills exemplify the use of extrusion. Tn some designs the charge material is forced out of cylindrical or other shaped holes located on the periphery of a cylinder within winch rollers and spreaders force the bulk materials through the openings. A knife cuts the extruded pellets to length as they are forced through the dies. 

To overcome weld line problems, the cross-head tubing die is often used. Here, the die design is similar to that of the coat-hanger die, but wrapped around a cylinder. This die is depicted in Fig. 3.17. Since the polymer melt must flow around the mandrel, the extruded... 

The drive unit usually is based on an electric motor and turns the screw at a suitable speed set in advance the electric elements in the cylinder are connected to temperature controllers that maintain the levels required. While screw speed and temperature are of critical importance the ability to extrude a given material continually to a satisfactory standard depends also on more fundamental considerations—like the suitability in design and construction of an entire line, including the operation of the die and of any haul-off or other technique applied after material passes the die. 

One advantage of sintering is the close control of size and shape of the abrasive particle. Extruded, cylindiically shaped, sintered abrasives of circular cross section were produced from bauxite (24) and from calcined alumina (25). The Ueltz sintered bauxite was also later produced in extruded cylinder form and designated as 76A. Extruded sintered abrasives of a wide variety of cross-sectional configurations, eg, square and triangular, were later patented (26). 

Basic types of extruders include both screw extruders such as axial endplate, radial screen, and basket designs, as well as pelletization equipment described above, such as rotary cylinder or gear and ram or piston extruders (Fig. 21-136). See Newton [Powder Technology ana Pharm. Processes, Chulia et al. (eds.), Elsevier, 1994, p. 391]> Pietsch (Size Enlargement by Ag omeraUon, Wiley, 1992), and Benbow and Bridgwater (Paste Flow and Extrusion, Oxford University Press, 1993). 

Containers with restricted apertures suitable for packing liquids can also be produced by first extruding a cylinder and then necking or spinning it in at the open end. Seamless containers in a range of sizes are used for the bulk shipment of expensive items such as perfumes, essential oils, antibiotics, vitamins and hormones. These containers can be designed with no sharp comers so that they can be easily cleaned and sterilised by autoclaving if required. 

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