Melt pumping

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. 

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Ethylbenzene, generally in 6- 15% concentration (Z 26/ 27) is used as a diluent with the feed, most likely to reduce viscosity and thereby facilitate melt pumping and heat transfer. It also serves as a chain transfer agent (30 ). ... 

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.]...

Polymer feeding Additive feeding Liquid injection Venting Melt pumping... 

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. 

Figure 15.3 shows the influence of specific energy input SEI on the WSA for an extruder with melt pump and the new ZSK-NT system. It is clear that the new processing system has benefits in terms of quality and homogenization without requiring higher energy inputs. 

Downstream of the die, the extrudate (melt) is calibrated, cooled and packaged by an array of ancillary auxiliary devices including vacuum calibrators, water tanks, cooling rolls, haul-offs, cutters, and winders (Chapter 18). Upstream of the die, a melt pump may be interposed between the extruder and the die to produce a more uniform extrudate. The exact selection and arrangement of these component parts of an extrusion system will depend on the end product and tolerance requirements that have to be met.25 27> 33> 143 196>476... 

FIGURE 14-16 Schematic diagram illustrating melt pumping. 

The term QAP is the power required to pressurize the melt (pumping power) and represents less than 5% of the total power required for the process (9). Hence, as can be seen from Equation 3, the power requirement is essentially determined by the product CQAT. Typical processing temperatures are listed in Table III (10, 11. 12). Thus, it is found that high-density polyethylene requires the most power per pound of product while polystyrene requires the least. It should be noted that the average heat capacity values in Table II include the heat of fusion for the semicrystalline polymers such as high- and low-density polyethylene. 

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... 

The polymer was extruded into strands by the use of a melt pump on the bottom of the reactor. The strands were cooled in a water bath and... 

Monofilament Extruder-melt pump-spinneret-quench-stretch- heat-set -wind. For Newtonian fluids the critical draw-down ratio is about 20 (take up velocity/ die exit velocity). Extrusion speeds 1 to 8 m/s. Feed must be dry. 20 to 200 filaments per spinneret. Operating limit is linear speed filaments can be drawn. 

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. 

Malloy, A. M., Closed Loop Inlet Pressure Control of Melt Pump Extrusion, SPE ANTEC, May 1984. 

The polymer was extruded into strands by the use of a melt pump on the bottom of the reactor. The strands were cooled in a water bath and chopped into amorphous pellets. The pellets had a 0.60 dl g inherent viscosity in 60/40 phenol/tetracWoroethane at 30 °C. After pelletization, the molecular weight of the polymer can be increased by solid state polymerization. 

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... 

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