Extrusion chamber

Fig. 8.32 Photograph of a small, low pressure flat die extruder in operation. The front half of the extrusion chamber is transparent to make the press rollers visible (courtesy LCI Corp., Charlotte, NC, USA).

If we analyse the cross-section of an extrusion mouth at the aperture, the clayey body is subject to a pressure q exerted by that portion of the body contained inside the extrusion chamber and the attrition t exerted on the walls of the die itself bear in mind that the attrition is uniform over all points of the edge between die and body. Under stationary conditions the mass is in equilibrium as are, therefore, the forces in play. It thus follows that ... 

The porosity expressed as the wt% water uptake for the pilot material is in the order of only 2%. An explanation for this lies in the forming process. Indeed, during the extrusion of the material, the extrusion chamber is vacuum sucked and, moreover, the material is compressed when it leaves via the mouth of the extruder. 

Extrusion viscometer operated under high shear rates is used to determine viscosity of organosols and plastisols. A sample conditioned to proper temperature is placed in an extrusion chamber, and the extrasion pressure and the rate of flow through the orifice are measured and used for result calculations. The orifice has the inside diameter of 3.17 nun... 

Vacuum Pug. A pug (q.v.) with a vacuum chamber in which the clay is de-aired before it passes into the extrusion chamber. 

The press is equipped with a top- and bottom hydraulic cylinder a and h being positively connected to each other by columns c. The doubleacting main pistons d and e move the crossheads / and g. The container h is stationary and mounted in the container-holder i which in turn is fixed to the columns c by nuts j. The top movable crosshead / carries the ram k. The die-block I with hohow ram m rests on the bottom crosshead. The hollow ram m moves into a filling chamber n being connected to the extrusion chamber p through a ball check valve. 

A common indicator in an extruder console is the power, E, consumed by the motor drive in transporting material from feed to product exit. A higher power consumption could mean greater friction during product movement or an overloaded chamber. The torque of extrusion (F) is the energy expended by the motor drive in rotating the screw(s) and is expressed as... 

The formation of moisture gradients within the material during extrusion, resulting in migration of liquid from within the interparticulate spaces to surface of the extrudate, has been reported (40,25). At low initial moisture content, the friction within the chamber can result in higher temperatures leading to loss of moisture of up to 12% from the extrudate surface. This effect is more pronounced with A-A extruders. 

Using a melt extruder with only feed screw elements (similar to wet extruder), a dense extrudate was obtained at 20 rpm that became soft and powdery at 50 and 100 rpm (42). At a lower speed, the material resides longer within the chamber and that could lead to greater homogeneity of binding fluid (water) in the material. In addition, the slower rate of extrusion provides a more uniformly dense extrudate. 

An interesting study of the extrusion of lubricated polytetrafluoro-ethylene powders has been published by Lewis and Winchester (LI), who have concluded that much of the shearing takes place in the taper of the die before the fluid enters the tube, or hole, of the die itself— i.e., that the end affects are of primary importance. Furthermore the data were interpreted to indicate an outstanding importance of the flow of the polymer in the material chamber well ahead of the die itself, an importance which is difficult to accept. Much further work is needed to... 

One can also recognize that application of sufficient pressure (above the equilibrium osmotic pressure n) to the right-hand chamber in (7.67) must cause the solvent flow to reverse, resulting in extrusion of pure solvent from solution. This is the phenomenon of reverse osmosis, an important industrial process for water desalination. Reverse osmosis is also used for other purification processes, such as removal of H20 from ethanol beyond the azeotropic limit of distillation (Section 7.3.4). Reverse osmosis also finds numerous applications in wastewater treatment, solvent recovery, and pollution control processes. 

Figure 4.38. Extrusion die with automatic pumping of a lubricant. Layout of regulating system 1 - high pressure cylinder 2 - air pressure regulator 3 - joining tubes 4 - lubricating chamber 5 - porous element 6 - room for the liquid lubricant 7 - transducer for lubricant pressure 8 - transducer for polymer material pressure 9 - electronic governing unit 10 - recording unit.

In addition, a machine much like a meat grinder can force clay through a chamber to remove air pockets. To make this more effective, the clay extrusion can occur in a vacuum. This produces a smooth, dense clay impossible to produce by hand. 

Fig. 10.12 Schematic representation of two melting mechanisms observed by Janssen (24) with PP processed in a double-flighted 47-mm counterrotating TSE operating at low rotational speeds, (a) Melting when the die pressure was set at the low value of 50 psi, where chamber-to-chamber leak pressure backflows are negligible, (b) Melting under the high die pressure of 2700 psi, which enables leak backflows, which result in chamber solid bed compaction and introduce the possibility of dissipative mix melting. [Reprinted by permission from L. P. B. M. Janssen, Twin Screw Extrusion, Elsevier Scientific, Amsterdam, 1978.]...

Gasket materials are chosen primarily on the basis of their internal friction or extrusion resistance under pressure, compressibilities, thermal stabilities, chemical inertness, and ease of fabrication. Often several materials comprise the gaskets of a particular apparatus. Suitable materials include certain kinds of slightly porous natural stone (e.g., pyrophyllite, or various ceramic materials, and some metals such as copper and steel). Usually slippery or organic materials are unsatisfactory because they tend to fail catastrophically in shear and thereby allow violent extrusion of the gasket and part of the chamber contents. 

Wolinski4 proposed a method by which polyethylene film could be treated as it emerged hot from the extruder in this process, the film passed after extrusion through a chamber in which it was exposed to the action of ozone and of an accelerator, preferably in the presence of ultra-violet light. 

The simplest,technique for converting plastic powder, pellets, or flakes into useful shapes is by melt extrusion. An extruder is a machine that operates somewhat like an old-fashioned crank meat grinder (see Figure 8-1). Polymer is fed into a chamber where it is melted and mixed with other polymers... 

Different methods are used. An online ordinary common blown or cast film line uses a machine direction orienter (MDO) on the front end of biaxially oriented film heated chamber extrusion line. If only the machine direction is to be stretched, a series of precision controlled heated rolls can be used. Film is fed through a series of rolls where it is sequentially heated, drawn around rolls that increase in rotational speed providing the stretching action, annealed around larger diameter roll(s), and cooled on a final roll(s). 

Owens Corning manufactures a similar extruded polystyrene foam under the trade name FoamulaR. A single-screw tandem extrusion line is used to produce foam boardstock in a vacuum chamber connected to a barometric leg that acts as a vacuum seal [78]. Owens Corning also licenses their process to a number of international parties. 

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