Specific extruder throughput

By careful process monitoring we can often detect the effects of wear. The process parameters that are affected by wear are meit temperature, output, and motor ioad. As wear progresses, the quality of the extruded product tends to deteriorate. This may manifest itself as discoloration, streaks, discolored specks, holes, etc. ft is very important to monitor the specific energy consumption (SEC) and specific extruder throughput (SET) because changes in these parameters often correiate with wear. SEC is the ratio of motor power divided by the throughput, ft is the mechanical power consumed per unit mass of plastic. The SEC tends to correiate with the melt temperature. The SEC is normally expressed in kWh/kg. A typicai vaiue of the SEC is 0.25 kWh/hr for extrusion of polyoiefins. 

Output rate is a measure of the throughput rate or pounds/h that can be delivered by a specific extruder size or diameter. 

A constant total shear S suggests that the profiles of the material in the two different size screw extruders are identical or that the material distribution (homogeneity) in the extruder is independent of screw speed. In a similar manner, scale-up factors for power consumption, specific energy consumption, throughput rate, etc. have been compiled in the literature (21,22). 

Figure 8 The effect of screw speed on specific energy consumption at varying throughput rates for extruders of two different scales. Source From Ref. 29.

Knowledge of the operating data such as screw speed, throughput, and discharge pressure, as well as extruder-specific data such as screw plays, steel temperatures in the product chamber, heat transfer coefficients, or melt temperatures. 

Even without software support, the process engineer can still obtain certain predictions by a precise analysis of the processes involved. In this case, process-specific diagrams are very helpful. These illustrate, for example, the specific energy input (Fig. 11.9) or other quality-related characteristics as a function of viscosity, throughput, speed, or discharge pressure. With the aid of enthalpy (Fig. 11.10) as a physical, process-independent value, initial forecasts can be obtained as to the energy that will be required to melt a resin and to extrude at a specified end temperature. 

Today, large polyolefin extruders are built as ZSK MEGAcompounders, mainly as 320, 350, and 380 Me sizes. The reason for this is clear from Fig. 14.8 which shows the same relationship as Figure 14.7, but specifically for polyolefin applications, here with the ZSK Me limit curve for maximum throughput. Normally, the torque limit is only reached with LLDPE and FfDPE if pressure is also built up by the ZSK and the powder from the polymerization reactor is coarse enough to prevent intake problems. With PP or melt-charged LDPE or two-stage machines for bimodal polyethylene, the torque required is well below the Me limit curve. 

Twin-screw extruders (with the exception of most counterrotating profile extruders) are designed to be starve fed. Therefore, throughput is independent of screw speed. This permits the processor to control residence time, degree of fill, and specific energy input (kw/kg). 

Note that in the simulator, the extruder has a specific throughput of 1 Ib/hr/RPM. 

The problem of low output generally refers to a reduction in specific throughput (throughput per RPM). In most cases, it is possible to increase the output simply by increasing the extruder screw speed. This may solve the problem at hand however, increasing screw speed may have other detrimental consequences associated with it. So, the real problem here is when a process has deteriorated as measured by a reduction of material extruded per hour at a specific screw speed. 

This equation is used to calculate specific throughput of the extruder, which is often used as a measure of screw performance. 

Twin-screw extruders are usually operated at a specific throughput therefore, portions of the extruder are completely filled, whereas other locations are partially filled. Filled regions are formed behind restrictive and reverse pumping elements in extrusion screws. 

Screw configuration has a pronounced effect on the RTD. The effects of different screw configurations on the broadness of mixing compared their subtle differences. Specific throughput is defined as the ratio between throughput and screw speed. The residence time of a material element is the time it spends in the extruder. All material elements do not necessarily spend the same period of time in the extruder. Residence times have a distribution called RTD. 

Therefore, the target for the further development of this extruder system is to solve these problems as far as possible. Moreova, it should be tried to increase the specific throughput m /n significantly, in order to keep the screw speed and with it the melt tempo-ature at given throughput as low as possible. The necessary changes in the screw and/or barrel design should be kept at a minimum... 

Figure 6. Average melt pressure courses in grooved feed zone extruders with diffo ent grooves in the plasticizing barrels specific throughput 4,30 kg-min/h.

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