Screw design performance

Table 21-5 indicates screw-conveyor performance on the basis of material classifications as listed in Table 21-4 and defined in Table 21-3. Table 21-6 gives a wide range of capacities and power requirements for various sizes of screws handling 801 kg/m (50 lb/ fU) of material of average conveyabihty. Within reasonable limits, values from Tables 21-5 and 21-6 can be interpolated for preliminary estimates and designs. 

Plasticator A very important component in a melting process is the plasticator with its usual specialty designed screw and barrel used that is used in different machines (extruders, injection molding, blow molding, etc.). If the proper screw design is not used products may not meet or maximize their performance and meet their cost requirements. The hard steel shaft screws have helical flights, which rotates within a barrel to mechanically process and advance (pump) the plastic. There are general purposes and dedicated screws used. The type of screw used is dependent on the plastic material to be processed. 

As mentioned in the introduction to this chapter this is a necessary condition when approximating the cylindrical screw in the Cartesian coordinate system. The screw rotation theory, New Theory line, predicts that the rate should constantly increase as the channel gets deeper. When a fixed positive pressure occurs for the screw rotation model, the New Theory with Pressure line, the predictions fits the data very well for all H/Ws. Thus for modern screw designs with deeper channels, reduced energy dissipation, and lower discharge temperatures, the screw rotation model would be expected to provide a good first estimation of the performance of the extruder regardless of the channel depth for Newtonian polymers. 

An extrusion trial was performed at the processor s plant using a 38.1 mm diameter production extruder, a proprietary screw design, and resin that had previously exhibited flow surging and reduced rate. The extruder was equipped with three barrel zone heaters with control thermocouples (labeled Tl, T2, and T3) and two pressure sensors. One pressure sensor was located in the midsection (zone 2) of the barrel (P2) and the other at the end of the barrel near the tip of the screw (P3). Both transducers were positioned over the top of the screw such that a pressure variation due to screw rotation would be observed. 

Many of the geometric features of a VBFT screw are similar to those for an FT screw. The VBFT screw, however, has three additional enhanced features that can be adjusted for optimal processing performance. The channel depths for a 114.3 mm diameter VBFT screw designed for use with a specific polyolefin resin is shown by... 

The same analysis was performed using the identical screw design and an LDPE resin with a 25 dg/min (190 °C, 2.16 kg) Mi, as shown in Fig. 15.11(b). For this case, the specific rate was 5 to 20% less than that for the 2 Mi resin shown in Fig. 15.11(a). Moreover, the highest specific rate possible before resin flows into the vent was calculated at 63 kg/(h-rpm), a value that was about 10 % less than that for the 2 Ml resin. For the 25 Ml resin, the optimal clearance for the second blister is about 3.0 to 3.8 mm, providing the highest specific rate while not forcing resin to flow into the vent dome. 

In the hardwood hydrolysis experiment with the screw reactor, NREL researchers found that overmixing and an uneven flow pattern existed in the reactor. These factors have a negative effect on biomass hydrolysis. To enhance the screw conveyor reactor s performance, it was necessary to redesign the reactor to achieve adequate mixing and an even flow pattern. In the present work, CFD was utilized to analyze the flow behavior in the screw conveyor reactor, and a new screw design was proposed based on CFD analysis. 

The major processing methods that process well over 80wt% of all plastics are extrusion, injection molding, and blow molding. These processes as well as a few others use a plasticator to melt plastics. It is a very important component in a melting process with its usual barrel and screw. If factors such as the proper screw design and/or barrel heat profile are not used correctly fabricated products may not meet or maximize their performance and very important not provide for low cost process. 

Figure 4 shows all the experimental Cq data for the short screws and the corresponding values calculated from Figure 2. Molecular diffusivities at the various temperatures were taken from Figure 3, and the equilibrium values from Figure 5. The scatter is not too bad, and indicates that the model fairly well predicts the effect of rotational speed, throughput rate, and molecular diffusivity on devolatilization performance. The data also indicate that the geometry efficiency, p (which were different for the two screws) has some merit, and provides a convenient means for comparing the relative efficiency of different screw designs.

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