Initial Forwarding and Compaction of Pellets
Once polymer pellets enter into the screw channel through the feed throat of an extruder, they drop to the bottom of the barrel because of gravity. The advancing flight pushes the pellets forward along the barrel as illustrated in Fig. 2.4. When the screw channel is not full under the hopper, the pellets do not make full contact with the screw surface and the screw cannot grab the pellets to rotate with it. The pellets are efficiently pushed forward by the advancing flight until the screw channel becomes full. The initial forwarding mechanism is the same as that of screw conveyors such as the grain feeders used by farmers.
The screw surface becomes hot because of the heat conducted from the melt, and the screw tip at the die end is heated to the same temperature as the melt. The screw surface under the hopper is cooled continuously by the incoming stream of cold feed pellets in a steady-state operation. Thus the screw surface in this section stays below the melting point of the pellets in a steady-state operation, and the rubbing force of the pellets on the screw surface is controlled by the external friction of the pellets. Low external friction coefficient of the pellets on the screw surface allows easy sliding of the pellets on the screw, resulting in fast forwarding and compaction. However, the barrel surface immediately after the feed throat is usually set well above the melting point of the pellets, and the rubbing force of the pellets on the barrel surface is controlled by the viscosity of the polymer. High polymer viscosity gives high rubbing force on the barrel, resulting in fast forwarding and compaction.
The ratio of the viscosity on the barrel surface to the external friction coefficient of the polymer, (η/μe), may be used as a parameter to indicate the initial forwarding and compaction characteristics of the pellets.
If the screw surface under the hopper becomes hot and pellets stick on the screw surface, the pellets stuck on the screw will rotate with the screw, reducing the screw channel area and the output rate. Then the output rate slowly decreases with time after startup. Such phenomenon is called “feed bridging”. Thefeed bridging problem often occursonrestart after an interrupted operation because the screw surface under the hopper becomes hot during screw stoppage. Sticking of polymer pellets on screw surface must be avoided in the first several L/D of a screw to avoid feed bridging. If the sticking problem occurs, the screw over the first several L/D should be bored out and cooled by water or other suitable cooling medium.
The screw channel quickly becomes full, usually after 3–5 L/D from the hopper, and the pellets start to be compacted into a solid bed, developing pressure. High internal friction between the pellets is desirable to transfer the screw torque to the pellets for compaction. Spherical pellets like ball bearings with a low internal friction slide past each other and are not compacted easily. Soft pellets are compacted easily along the screw. Harder pellets
are more difficult to compact, and full compaction is achieved farther away from the
hopper.
The air between the pellets also goes into the screw with the pellets. It is remarkable that all the air is squeezed out of the screw as the pellets are compacted. There must be continuous flow paths for the air to flow backward from the compacting solid bed to the hopper. If the flow paths are blocked by penetrating melt, the air becomes entrapped in the melt and the entrapped air mixed in the melt is extruded. The air entrapment problem is common for hard polymers and powder feeds.
The initial forwarding and compaction rate of a screw usually increases proportional to the screw speed. At present, there is no mathematical model that can be used to predict the forwarding and compaction rate.
Preferred conditions for a high rate of the initial forwarding and compaction are:
- High rubbing force on the barrel
– High viscosity of the polymer
– Barrel temperature near the melting point of the polymer
– Grooved barrel surface
- Low rubbing force on the screw
– Low external friction coefficient of the polymer
– Low screw surface temperature far below the melting point of the polymer
– Polished screw surface
– Low friction coating on the screw surface
- High melting point
- High bulk density
- Soft pellets for easy compaction
- Shape and size favorable for high internal friction
