Solid Conveying Function

Solid Conveying Function

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

2.2.2          Solid Bed Conveying

Polymer pellets inside a screw channel are compacted into a solid bed (or a solid plug) after 3–5 L/D from the hopper by the pushing force of the screw, as discussed in the previous section. For most polymers which are rigid at the feed temperature, the solid bed moves down the screw channel as a rigid body. Once the solid bed is fully compacted after 5–7 L/D, it is very strong under compression, like a rock, and it cannot be easily compressed or sheared. But, it can be easily split or broken up by tensile force because the pellets in the solid bed are not fused together. It will be important to remember various solid bed characteristics when the screw mechanisms are studied later, in more detail.

This case occurs if the barrel and screw surfaces are kept below the melting point of the polymer. However, the entire barrel, starting from the first zone next to the hopper, is usually heated well above the melting point of the polymer. The screw also becomes hot because of the heat conducted from the melt. The tip of the screw reaches the melt temperature unless the screw is bored to the tip and cooled. The screw temperature increases quickly along the screw and reaches the melting point after 5–7 L/D from the hopper. Thus the screw temperature, where the solid bed is formed, is usually well above the melting point. Because a polymer melts quickly upon touching a hot metal surface above its melting point, the solid bed melts on all barrel and screw surfaces. The solid bed becomes surrounded by the melt,

The rotating screw grabs the solid bed and makes the solid bed rotate with it. As the rotating solid bed rubs on the stationary barrel, the barrel exerts a breaking force on the solid bed and makes the solid bed slide slightly on the screw surface. Therefore, the solid bed rotates at a slightly lower speed than the screw. If the barrel is removed or lubricated, the solid bed rotates with the screw at the same speed. The difference between the rotational speeds of the screw and the solid bed results in the solid conveying rate according to the helical geometry of the screw channel.

The slippage of the solid bed on the screw, that is, the solid bed conveying rate down the screw channel is controlled by the difference between two forces exerted on the solid bed by

the rotating screw and the stationary barrel. The pressure inside the screw channel usually increases along the screw because the forwarding force accumulates along the screw. The increased pressure along the screw channel pushes the solid bed backward toward the hopper. The only driving force for solid bed conveying is the rubbing force exerted on the solid bed by the stationary barrel, resisting the solid bed rotation. The opposing forces are the rubbing force exerted on the solid bed by the rotating screw and the increased pressure along the screw channel. A high rubbing force on the barrel and a low rubbing force on the screw are desirable for a high solid conveying rate. It is common practice to highly polish the screw surface in order to minimize the rubbing force on the screw. The barrel surface near the hopper can be grooved and/or cooled by water to increase the rubbing force on the barrel.

The rubbing force on the barrel or screw surface may be frictional or viscous in nature, depending on the temperature condition of the metal surface. If the metal surface is at a temperature above the melting point of the polymer, the polymer melts as shown in Fig. 2.6, and the rubbing force is viscous in nature. Because the first barrel zone temperature next to the hopper is usually set well above the melting point of the polymer in most cases, the rubbing force on the barrel is viscous in nature and the pressure builds up linearly along the screw channel, as discussed in Chapter 4. A polymer with a high viscosity gives a high solid conveying rate in this case.

If the metal surface is at a temperature below the melting point of the polymer, the solid bed does not melt, as shown in Fig. 2.5, and the rubbing force is frictional in nature. The barrel is readily heated above the melting point of the polymer in operation by the heat generated from the frictional force of the solid bed unless it is cooled efficiently. The barrel section next to the hopper may be grooved and intensely water-cooled, in order to keep the barrel surface below the melting point of the polymer. Then, the rubbing force on the barrel is frictional in nature and the pressure increases exponentially along the screw channel, as discussed in Chapter 4. Extremely high internal pressures over 69 MPa (10,000 psi) can be developed in this case. However, a grooved barrel without intense water-cooling does not keep the barrel surface below the melting point of the polymer, and such high pressures are not developed. Even if water-cooling is not applied, a grooved barrel increases the solid conveying rate by increasing the rubbing force on the barrel [3].

Elastomeric polymers with a low melting or fusion temperature, such as thermoplastic elastomers, present a unique solid conveying problem. The pellets of these polymers can fuse together upon compression, forming an elastic band in the feeding section. The elastic band stretches by screw rotation and the stretched elastic band wraps around the screw, tightly holding onto the screw and thus stopping solid conveying. If such a “feed binding” problem occurs, the output rate is very low and increases only slightly with increasing screw speed.

The mass solid conveying rate of a screw is equal to [(the sliding velocity of the solid bed on the screw surface) × (the screw channel cross-sectional area perpendicular to the screw flight) × (the bulk density of the solid bed)]. The mathematical solid conveying

models presented in Chapter 4 are used to calculate the solid conveying rate. The solid conveying rate usually increases nearly proportional to the screw speed. The mass output rate of an extruder is equal to the mass solid conveying rate, because an extruder is a continuous pump.

Preferred conditions for a high conveying rate of the solid bed are:

•        Large screw channel area

•        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

–       Screw temperature significantly higher than the melting point of the polymer

–       Highly polished screw surface

–       Low friction coating on the screw surface

•        Small screw surface area in comparison to the barrel area

–       Low screw channel depth to width ratio

–       Large flight radius on the screw root

•        Low pressure increase along the screw channel

–       Long feeding section

–       No or low reduction of the channel area along the screw