Solid conveying section

Figure 5.1 A typical plasticating extruder with a smooth barrel solids conveying section...

Prior to these devices, Zhu and Chen [28] built a 45 mm diameter extruder with glass windows positioned In the barrel that allowed observation of the flows of a fully functioning machine. The extruder had a length-to-dlameter ratio of 25 and thus it allowed the observation of all sections of the process. In the solids conveying section, they observed that the pellets had relative movements between them and that the pellets did not behave as a ridged solid plug. This result is consistent with Moysey and Thompson [25-27]. 

Total helical length of the solids conveying section (4 diameters) at the barrel, Zj 0.99 m 1.11... 

A simulation was carried out with the melting model using the simulation conditions described earlier for the PE resin with a Newtonian shear viscosity of 220 Pa-s. For this simulation, the pressure at the end of the solids-conveying section... 

Flow surging can occur if the temperature of the screw becomes too high in the solids-conveying section. In general, the temperature of the screw in this section needs to be less than the Tg for amorphous resins or less than the melting temperature for semicrystalline resins. Small-diameter screws will typically operate at feed... 

Figure 12.27 The solids-conveying section of a single-screw, plasticating extruder with a worn feed casing. The schematic is not to scale, and it shows a ledge formed between the oversized (worn) feed casing and the flange of a new barrel...

The unit can be fed polymer in the particulate solids form or as strips, as in the case of rubber extrusion. The solids (usually in pellet or powder form) in the hopper flow by gravity into the screw channel, where they are conveyed through the solids conveying section. They are compressed by a drag-induced mechanism in the transition section. In other words, melting is accomplished by heat transfer from the heated barrel surface and by mechanical shear heating. 

The pressure rise ratio P2/P1 over one turn of the solids conveying section (Zb 0.0635/ sin 17.6° = 0.209 m) from downstream the hopper to the location where the barrel heating starts, is obtained from Eq. 9.3-8 ... 

Thus, the exit pressure from the solids conveying zone is 3.776 x 103 x 60.9 = 2.3 x 105N/m2 (33 psi). This result indicates that the solids conveying section functions properly and that higher outputs could be obtained at this screw speed before solids conveying limitations (e.g., starving ) were encountered. We should note, however, that the analysis of the solids conveying zone is very sensitive to the values of the coefficient of friction. 

Plasticating First we observe that, in a CDP, there is no solids conveying section or chamber, because the disks are heated and the pellets that come in contact with the rotating disks quickly melt. The solids are compressed into a solid bed as in the SSE, occupying 2ne of the circumference, as shown in Fig. 9.46. 

In the solids-conveying and melting sections in single-screw extruders, the solid particulate assembly is normally treated as a solids plug . This assumption can be nsed to predict the pressurisation of polymer solids in the solids-conveying section of single-screw extruders with different screw geometries and material properties. 

For a flood-fed system, the solids conveying section of the screw must be able to provide resin at a rate fast enough and under pressure to keep the melting and metering sections full of resin. 

---