Melt pool

Maddock s and Street s famous solidification experiments revealed that melting in many situations takes place in a specific order. The experiments showed that after melting began, there was a continuous solid bed and a melt film over the inner barrel surface. Later the solid bed was completely surrounded by melt. Farther downstream, a melt pool developed between the pushing flight and the solid bed. The... 

Figure 6.12 Schematic for the zones of the new melting concept Zone A is the solid bed. Zone B is the melt pool, Zone C is the melt film located between the solid bed and the barrel wall, Zone D is the melt film between the solid bed and the screw root, and Zone E is the melt film between the solid bed and the trailing flight. The cream color represents molten resin...

As discussed in Section 6.2.1, the meting process has the potential to generate pressure in the downstream direction. The local pressure affects the melting rate and the compaction of the solid bed and provides a pressure at the entry to the metering section. Here, the dominating pressure is assumed to be the pressure in the melt pool. Cross-channel pressures have been measured as shown in Fig. 12.3, but it is not as important as the pressure in the z direction. This section will develop the equations for the pressure profde in the z direction based solely on the flow in the melt pool. 

The locally high pressure underneath the solid bed and the positive dP/dx in Film D causes some flow of resin from Film D to the melt pool. Thus, for a local Az increment for Film D, there is material entering the element from the melting process and from the drag motion of the screw core, and there is material leaving the increment from the motion of the screw core and from the flow of material into the melt pool due to a positive dP/dx. These complex flows are consistent with observations from Maddock solidification experiments. This flow is shown in detail for the Maddock experiment shown in Fig. 6.35. 

Recalling the discussion earlier in this chapter, in most cases melting in the channel typically occurs at all four edges of the solid bed, with the majority of the melting occurring at the solid bed-melt film interface located between the solid bed and the barrel wall, as shown in Eig. 6.2. The newly molten resin from this location is then conveyed by the motion of the screw to a melt pool located at the pushing side of the channel. Eor very special and sometimes unpredictable conditions, the melting process can occur by a different mechanism. In these cases, the... 

Figure 6.30 Photographs of the conventional melting of a commercial PP resin in an experimental extruder with a glass barrel. In both photographs a melt pool forms near the pushing side of the channel. The screw was rotated at 15 rpm and the barrel temperature was 90 °C. The ruler Is In Inches...

Figure 6.31 Photographs of one-dimensional melting of the same commercial PP resin as used in Fig. 6.30. Melt pools do not form under these conditions. The screw was rotated at 7 rpm and the barrel temperature was 100°C...

Flow from melt Film D to the melt pool... 

Figure 6.35 Photograph of resin solidified in the transition section after a Maddock solidification experiment for an ABS resin (a) the arrow is pointing at the flow lines created by the flow of material from Film D out to the melt pool, and (b) an enlargement of the flow area...

Figure 6.36 Schematic showing the recirculating flow in the melt pool and the flow entering the melt pool from Film D. A low-flow region exists where the flow streams merge...

The degradation ribbon at the merger of the flows occurs because of the crosschannel flow of material from the region between the solid bed and the screw root to the melt pool. As shown by Fig. 6.35, this flow is relatively large. As previously stated, the flow occurs because of pressure-induced flow and the dragging of fresh material under the solid bed by the backwards motion of the screw root. This process is consistent with the physics presented for screw rotation. The flow fields developed for a barrel rotation system would not create the low-flow region such as shown in Fig. 6.37. 

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