Plastics pressure generation

Secondly, they showed that under the high hydrostatic pressures generated in contact of a crystalline material (rock-salt) and a steel slider the rock-salt ceases to be brittle, and can undergo marked plastic deformation. Such plastic deformation in... 

Eastern European farmers produce a variety of fruit juices, which they wish to dehydrate to prolong shelf-life and facilitate transportation. One very simple dehydration method is to put the juice in a plastic bag and drop the bag into brine at 283 K. If the bag is permeable to water but not to salt or juice components, then osmotic flow will concentrate the juice. Is the osmotic pressure generated in this way significant Assume that the juice contains a solids concentration equivalent to 1.0 wt% sucrose (342 g/mol molecular weight) and that the brine contains 35 g of NaCl per 100 g of water. For simplicity, assume that the juice and the brine are ideal solutions. 

Ottawa Sand-Silt Mix The data for clean sand and non-plastic silty sand up to 25% silt content (Figures 2a-c) follow the envelope for clean sand reported by Seed at al. (1976). They agree with the results reported by others (Guo and Prakash 1999, 2000). The data for Ottawa sand-silt mixtures at intermediate silt contents of 40 and 60% (Figures 2d-e) fall above the Seed et al. envelope for clean sand and indicate a faster pore pressure generation initially and a slower rate with further cyclic loading. The data for 100% silt (sil-co-sil 40) follows the upper boimd Seed et al. envelope (Figures 2f). 

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. 

For a Newtonian fluid (n = 1), the familiar linear output-pressure relationship is found. However, when the power law index is less than unity, substantial deviations from Newtonian characteristics occur. The deviations increase as the material becomes more pseudo-plastic (more strongly non-Newtonian). The result is that for a pseudo-plastic fluid, the pressure generating capability is drastically reduced compared to a Newtonian fluid. Or, at the same pressure gradient, the output is drastically reduced. For a fluid with a power law index less than 0.8, the use of the equations for Newtonian fluids will result in large errors ... 

This screw is more appropriate for a plasticating unit of an injection molding machine. The zero-meter screw is used to reduce the temperature build-up in the material by deepening the depth of the channel in the melt conveying zone of the extruder. The obvious drawback is that the pressure generating capability of the screw will be adversely affected, but this is not a major concern in injection molding applications. In other applications, however, the approach outlined in Section 8.3 is recommended. An extension of the zero-meter screw is the zero-feed zero-meter screw shown in Fig. 8.34. 

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include 1) non-uniform plastics flow in the hopper 2) troublesome bridging, with excessive barrel heat that melts the solidified plastic in the hopper and feed section and stops the plastic flow 3) variations in barrel heat, screw heat, screw speed, the screw power drive, die heat, die head pressure, and the take-up device 4) insufficient melting or mixing capacity 5) insufficient pressure-generating capacity 6) wear or damage of the screw or barrel 7) melt fracture/sharkskin (see Chapter 2), and so on. 

These conclusions are compatible with Radcliff s ( ) observation of what is thought to be a similar phenomenon with respect to isostatic pressure generation of dislocations which move on subsequent activation. He found that if iron-carbon alloys were isostatically pressed, discontinuous yielding was eliminated. Bullen et al. ( °) observed a similar effect in iron specimens containing carbon and were also able to obtain up to 60% plastic deformation... 

The injection pressure (p ) is the pressure generated by the screw or the ram in the heating cylinder and it should be sufficiently high to press the plasticized mass from the cylinder into the mold, to fUl the cooled mold completely before the material solidifies. The maximum injection pressure is determined on the basis of the diameter (D) of screw or ram, an external diameter (d) of the hydraulic piston in the shove and retract system of the screw or ram, the pressure in the hydraulic system (pt), and the friction force (F) at screw shove [3] ... 

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