Screw melt pumping

Figure 10.37 The effect of discharge pressure on screw speed and discharge temperature at constant rate. The analysis was for a 150 mm diameter extruder running an engineering resin. An extruder discharge pressure of 8 MPa would be typical for a process running a melt pump while a high pressure of 32 MPa would represent a process without a melt pump...

Fig. 10.7 Schematic view of the CM/single screw extruder combination used in polymer compounding operations. The chute, connecting the two processing machines, transfers the molten and mixed stream at low pressures to the throat of the melt pump SSE, which generates the pressure needed for pelletization. [Reprinted by permission from E. L. Canedo and L. N. Valsamis, Farrel Continuous Mixer Systems for Plastics Compounding in Plastics Compounding—Equipment and Processing, D. B. Todd, Ed., Hanser, Munich, 1998.]...

We now discuss the elementary steps of processing as they occur in the twin screw segment of the CRNI tangential extruders melt pumping in the single screw segment was covered in Chapter 9. 

Melting the Plastic Material. The plastic pellets are fed through a hopper into an extruder screw, typically with an L/D ratio of 20/1. The first half of the screw compresses the pellets and squeezes out air. The third quarter of the screw melts the pellets, 70 percent by friction, 30 percent by conduction from heater bands on the extruder barrel. This is called preplastica-tion. The fourth quarter of the screw pumps the melt forward to the front of the extruder, where it goes through a one-way valve to prevent backflow. As the molten plastic accumulates at... 

Direct observations of Tm (P) and AV may be made in a sapphire optical cell with simple screw-press pump by measuring the offset in the pressure versus volume curve. AH can be measured at room pressure using a simple differential calorimeter comprised of two paper nut cups outfitted with kitchen thermistors and containing water in one and a standard solid material in the other for which the heat capacity curve is known. Direct observations of pressure-release freezing of water (as compared to pressure-release melting in silicates) may be observed in such an optical pressure cell by sudden release of pressure. 

The change in flight depth along the screw (i.e., the ratio of the solids metering channel depth to the melt pump channel depth) is known as the... 

Melt pumps are most appropriate when the screw and die characteristics combine to give a relatively poor pumping performance by the total system. This can happen when die pressures are low but more often occurs when they are extremely high (5,000-8,000 psi), or when the melt viscosity is extremely low. When pumps are used to increase the production rate by reducing the extruder head pressure without a corresponding increase in the screw speed, the extrudate solids content often is increased. The result is an inferior product. This problem often necessitates additional filtration, which serves only to increase pressure and may counteract many of the benfits expected from the pump, as well as increasing the financial investment even further. 

To incorporate flow and mixing mechanisms in a conventional screw, the ideal screw length for operations of material transport, melting and melt pumping would be excessively long with regard to both... 

Near the end of the screw transition section, melting should be complete and the melt pumping function takes place. The purpose of this function is to convey melt toward the end of the extruder and generate pressure to overcome the flow resistance of the head and die. 

Polymer flow rate is kept constant when the screw processes occur stably. That is, the stability of solids conveying, melting, mixing, and melt pumping is necessary to maintain a uniform flow rate through the die. It is not uncommon, particularly with significant... 

Screw wear, most often resulting in increased flight clearance, reduces output in two ways. First, leakage of melt over the flight in the screw s pumping section decreases the amount of polymer conveyed forward with each rotation. Second, a reduction in shear rate over the solid bed decreases the amount of solids that are converted to melt with each screw rotation. As discussed above, it is important to monitor and document screw wear over time. One potential way to differentiate between a contaminated screen problem and a screw wear problem is to check for notable increases in head pressure, which is associated with clogged screens. 

These two sets of curves indicate that an increase in screw speed can be offset by a reduction in the throttie ratio. The throttie ratio is determined by the iniet and outiet pressure of the meit conveying zone. It is ciear, therefore, that reducing barrei pressure will reduce polymer melt temperature. This can be achieved by using a less restrictive screen pack, a less restrictive extrusion die, higher die temperatures, or by using a melt pump to generate most of the diehead pressure. 

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