Extrusion torque

PA-12 or PA-1212 or PA-612 or PA-610 or PA-69 or PA-46 /SEES (0-20) / SEBS-g-MA (1.8% MA) (0-20) properties / TEM / ductile-brittle transition temperatures / interfacial tension estimates / effects of PA amine end-group concentration on copolymer formation (titration before and after extrusion) / torque rheometry ... 

The availability of power and torque to the shank of the screw is extremely important to the success of an extrusion process, especially if modifications to the line are planned to increase the rate. That is, the proper level of power must be available at the proper screw speed. Thus, the motor size and speed, belt sheaving if used, and gearbox reduction must be specified to provide enough power to the screw to plasticate and pump the resin at a high rate. 

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. 

Rate Limited by Discharge Temperature and Torque for Starch Extrusion... 

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... 

Particle size and size distribution mixing, amperage, power consumption) Torque or pressure of extrusion... 

Among the formulation variables that control extrudability of a product, several studies (25,26,33-35) point to the fluid or moisture content of the wet feed material being more critical than others. The force or torque of extrusion and power consumption are often inversely proportional to moisture content of the extrudate as seen from Figures 11 and 12 (26,36-38), while the particle size of pellets increases linearly with water content when extruded with gravity feed basket and roll extruders (33). Using instrumented gravity feed and radial screw extruders, a three- to fourfold decrease in the force has been noted, with a 10% increase in water content of feed material (37). 

Figure 11 Torque of extrusion as a function of moisture content of wet mass of MCC at varying screw speeds a-19 rpm, -28 rpm, c-38 rpm, d-4S rpm. Source From Ref 26.

Extrusion accounts for about 30% of nylon produced and is used in various processes (24). Nylons can be extruded on conventional equipment having the following characteristics. The extruder drive should be capable of continuous variation over a range of screw speeds. Nylon often requires a high torque at low screw speeds typical power requirements would be a 7.5-kW motor for a 30-mm machine or 25-kW for 60-mm. A nylon screw is necessary and should not be cooled. Recommended compression ratios are between 3.5 1 and 4 1 for nylon-6,6 and nylon-6 between 3 1 and 3.5 1 for nylon-11 and nylon-12. The length-to-diameter ratio, L D should be greater than 15 1 at least 20 1 is recommended for nylon-6,6, and 25 1 for nylon-12. 

Simulation of the extrusion process in the laboratory is one of the most important applications of the torque rheometer in conjunction with single-screw extruders. Figure 13 illustrates simulations of widely used extrusion processes in the industries. 

Figure 13. Application of torque rheometry to studying extrusion operations.

High Density Polyethylene - [Both regular and high molecular weight (HMW)]. The time to increase in torque at 220°C. by the Brabender Plasticorder was determined as a measure of crosslinking. Yellowness Index after the Brabender test at 220°C. and after the first extrusion at 260°C. were also performed. 

Problem understanding In many cases, experiments can provide only reliable integral values. In the case of twin screw extruders, for example, these are the shaft torque and the pressure and the temperature at the extrusion nozzle. Computational fluid dynamics, however, provide local information about pressure, velocity, and temperature within the overall computational domain. The calculation of gradients provides additional information about the shear rate or the heat transfer coefficients. 

Typical applications for products and processes with high and low torque requirement are shown in Fig. 14.9. Typical uses of high torques are reinforcing and alloying of engineering plastics and compounding and pelletizing of polyolefin powders into polyolefin pellets, or direct extrusion of films. Low torques are used for products that are shear-sensitive or difficult to feed, such as those often found in the chemical/food/pharmaceutical industries. 

The relationships shown in Fig. 14.11 were experimentally determined for direct extrusion of non-dried PET with two vacuum zones at 5 mbar absolute pressure the 20% torque increase in the ZSK Me PLUS may be used to halve hydrolytic degradation at the same throughput or to increase throughput by 75% by additional speed increases while maintaining the same quality. The same process was successfully transferred to non-dried PLA. 

Figure 14.11 Use of a 20% torque increase to increase quality or throughput in direct extrusion of non-dried PET...

Some of the test methods being used to measure the processing stability of polypropylene include melt flow drift measurements at elevated temperatures using an extrusion plastometer (melt indexer), melt viscosity retention measurements using a torque rheometer, retention of melt flow after repeated extrusions, and injection molded spiral test measured by the flow in inches at various temperatures and the retention of melt flow of the injected spirals. The nine commercial resins were evaluated by these methods. 

In the past with the development of single-screw extrusion techniques for newer TP materials, it was found that some plastics with or without additives required higher pressures (torque) and needed higher tempera-... 

Soh JLP, Liew CV, Heng PWS. Torque rheological parameters to predict pellet quality on extrusion/spheronization. Int J Pharm 2006 315 99-109. 

High Density Polyethylene (HDPE). The preferred method of evaluating HDPE was found to be the time to cross link at 220 0 using the Brabender apparatus, as is made evident by the rapid increase in torque. Determining melt viscosity by multiple extrusion showed only small differences compared to the base polymer. 

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