Optimizing Extruder Controls, Part Two Lesson 6 I. The Compression Zone A. Most of the melting of raw material from solid plastic particles to molten plastic melt occurs in the compression zone. B. Goals of the compression zone: 1. Completely melt the solid bed of plastic. 2. Help mix the melt. C. Goals are accomplished through: 1. Heat 2. Compression 3. Shear NOTE: The plastic pressure is increased in the compression zone. D. The melting process Layer of molten plastic In the compression zone, the plastic is in contact with the heated inner wall of the extruder barrel. A thin layer of molten plastic adheres to the inner barrel wall. The rotating screw shears the plastic at the barrel wall which generates heat and accelerates the melting process. Page 67
Layer of molten plastic thickens Further along the compression zone, the layer of molten plastic adhering to the barrel wall thickens, until it exceeds the clearance between the screw flight and the barrel wall. Melt pool The screw flight scrapes off some of the melt, which accumulates at the front or pushing face of the flight. As more melt accumulates in front of each screw flight, a melt pool begins to fill the screw channel, pushing the solid or semi-solid bed of plastic granules forward. The unmelted plastic continues to be forced into the region of high shear at the barrel surface. The solid bed of plastic continues to melt and join the melt pool. Page 68
Completely melted To complete the melting process, all unmelted plastic in the solid bed should be displaced by the molten pool and forced against the barrel surface. 1. Unmelted plastic a. Sometimes as the plastic melts, the solid bed may break up, leaving unmelted plastic floating in melted plastic. This unmelted plastic may not be forced against the hot barrel surface, so some may remain unmelted. b. Mixing section 1. If the screw has a mixing section at the end, this section should complete the melting and mixing. c. Screen pack 1. A screen pack will block unmelted plastic and force it to melt. However, the trapped plastic particles could remain trapped in the screens long enough to degrade. Page 69
2. Barrier screw Barrier Barrier flight Solid bed Melt pool The barrier screw segregates the solid bed in its own channel and allows the melted plastic to flow over a barrier flight into a melt channel. The solids channel gets smaller and the melt channel larger as the plastic moves through the extruder. The flight clearance between the barrier flight and the barrel is about 1 percent of the screw diameter. II. Operating Variables in the Compression Zone A. Barrel temperature 1. Raising barrel temperature in compression zone: a. Causes earlier melting. b. Raises temperature in melt film region. 1. Decreases plastic s viscosity. 2. Reduces shear heating. c. Can either increase or decrease the melting rate, depending on how much the viscosity changes. Page 70
1. A decrease in overall melting rate is more likely with amorphous plastics because the viscosity of amorphous plastics drops quickly when the temperature is raised. Shear heating is reduced. 2. The viscosity of semi-crystalline plastics is less affected by an increase in compression zone barrel temperature. Shear heating continues at a high rate. 2. Cooling the barrel temperature in the compression zone. a. Used to reduce melt temperature. 1. Shear heating is increased in the melt film region. The increased shear heating is usually not enough to increase the overall melt temperature. III. The Metering Zone A. Plastic travel in the metering zone. Plastic Travel in the Metering Zone In the metering zone, the plastic melt moves in a spiraling motion down the channel. The plastic near the surface of the barrel is turned downward by the pushing edge of the screw flight. The plastic then crosses the screw channel, where the trailing edge of the flight pushes the plastic back up to the barrel surface. This constant motion of the plastic as the screw rotates, shears and mixes the melt. Page 71