Processing extrusion plants

Reactive extrusion plants are more compact than batch polymerization processes and require less floor space. This results in a smaller overhead fee for space, but this benefit strongly depends on the internal accounting system. 

Obviously, concern about safety exists in small as well as large plants. However, small plants often do not have the resources to devote to an extended safety assessment and design phase. This was the case in a 100-employee plastic extrusion plant with which we consulted. To meet this plant s needs, we worked with a six-member design team in the planning of its safety process and kicked it off within six weeks. The safety process was designed as discussed throughout this book. 

Finally, the onset of peroxide decomposition is of considerable interest during the manufacturing process. The temperature of the polyethylene in the extrusion plant and dies must remain below the decomposition onset of the peroxide or else cross-linking will begin to occur there. This process results in inhomogeneities in the extruded insulation, which present a risk of electrical failure in service and are therefore unacceptable. 

Continuous Solvent—Extrusion Process. A schematic for a typical continuous process, widely used for making solvent propellant for cannons, is shown in Figure 7. This continuous process produces ca 1100 metric tons of single-base propellant per month at the U.S. Army Ammunition Plant (Radford, Virginia). Continuous processes have also been developed for double- and triple-base propellants and for stick as well as granular geometries. A principal aspect of these processes has been the extensive use of single- and double-screw extmders instead of the presses used in the batch process. 

Other, more recently developed, uses include microwave oven parts, transparent pipelines, chemical plant pumps and coffee machine hot water dispensers. One exceptional use has been to produce, by an extrusion moulding process, very large rollers for textile finishing for use where cast nylons cannot meet the specification. Also of growing interest are medical equipment applications that may be repeatedly steam-sterilised at 134°C, filtration membranes and cartridges for ink-jet printers. 

Pyromellitic dianhydride (PMDA) is generally used in PET at concentrations ranging from 0.05 to 2%. Reactive extrusion of PET with PMDA has been reported by Incamato et al. [9], These authors used PMDA to increase the molecular weight of PET industrial scraps sourced from a PET processing plant. They found that concentrations of PMDA between 0.50 and 0.75 % promote chain extension reactions that lead to an increase of MW, a broadening of the MWD and branching phenomena which modify the PET scrap in such a way that makes... 

Extrusion processes are often rate limited by motor power or torque, discharge temperature, or the melting capacity of the screw. Other root causes associated with the design of the screw can limit rates as shown in previous sections. The problems, however, are typically associated with other defects such as flow surging or resin degradation. Chapters 11 and 12 discuss process defects associated with resin degradation and flow surging, respectively. Rate limitations due to inadequate motor power and torque are common problems for commercial plants. Two case studies are presented in the next sections that show rate limitations due to the lack of torque and motor power. 

For example, if a two-stage vented extruder has a screw that was designed for resins with an Mi range of 2 to 20 dg/min, and plant personnel are asked to manufacture a resin with an MI of 100 dg/min, the extruder may not process the resin acceptably. For this case and flood feeding, the extruder is likely to have resin flowing into the vent because the first stage can deliver more resin than the second stage can pump. One practical solution to this problem is to operate the slide valve in a partially open manner such that it controls the rate of the extrusion process. This will allow the extruder to operate at a lower specific rate and allow operation without the flow of resin into the vent. 

Fig. 15. Specific power consumed in the process of extrusion of polypropylene filled with 10% (by mass) of chalk (a) and 20% (by mass) of asbestos (b) versus capacity of the plant at core vibration at amplitudes, degr. 1 — 0 2 — 4.8 3 — 11.5 4 — 22.3...

Reactive extruders and extrusion dies of different designs can be easily included in standard technological scheme of polymer production plants, such as those for polycaproamide synthesis, as shown in Fig. 4.39. In this case, a reactive material premixed in a tank 1 is fed into a static device 2 for prepolymerization, where part of the polymerization process takes place. Then the reactive mixture enters the extruder-reactor 3. The necessary temperature distribution is maintained along the extruder. Transfer of the reactive mass proceeds by a system of two coaxial screws mounted in series in a common barrel. Controlling the relative rotation speed of both screws provides the necessary residence time for the reactive mass in the extrader, so that the material reaching the outlet section of the die is a finished polymer. 

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