IX Международная студенческая научная конференция Студенческий научный форум - 2017

Extrusion is a polymer conversion operation in which a solid thermoplastic material is melted or softened, forced through an orifice (die) of the desired cross section, and cooled. The process is used for compounding plastics and for the production of tubes, pipes, sheet, film, wire coating, and profiles. All extrusion lines include a melt pump called an extruder, but other equipment is specific to the particular process. Although there are many types of extruders, the most common types are single-screw extruders, intermeshing twin-screw extruders, and ram extruders for special processes.

Single-screw extruders

A single-screw extruder consists of a screw in a metal cylinder or barrel. One end of the barrel is attached to the feed throat while the other end is open. A hopper is located above the feed throat and the barrel is surrounded by heating and cooling elements. The screw itself is coupled through a thrust bearing and gear box, or reducer, to a drive motor that rotates the screw in the barrel. A die is connected to the “open” end of the extruder with a breaker plate and screen pack (or a screen changer) forming a seal between the extruder and die [1, p. 175].

During extrusion, resin particles are fed from the hopper, through the feed throat of the extruder, and into the extruder barrel. The resin falls onto the rotating screw and is packed in the first section or feed zone of the screw. The packed particles are melted as they travel through the middle section (transition or compression zone) of the screw, and the melt is mixed in the final section or metering zone. Pressure generated in the extruder forces the molten polymer through the die.

Extruder drive motors must turn the screw, minimize the variation in screw speed, permit variable speed control (typically 50 to 150 r/min), and maintain constant torque. In selecting drive motors, the three major factors are: base speed variation, the presence or absence of brushes, and cost. The speed variation of a drive motor is based on the maximum speed available for the motor. Since this variation does not change when the speed is reduced, screw speed, which is generally 5 to 10 percent of the motor speed, varies more than the motor speed. Brushes in the drive motor are subject to corrosion and may not be used with plastics like poly(vinyl chloride) [2, p. 340].

The three basic types of drives are alternating current (ac), direct current (dc), and hydraulic. While a number of drives have been used in extruders, the most common are dc silicon control rectified (SCR) and ac adjustable frequency drives. A dc SCR drive is a solid-state dc rectifier connected to a dc motor. The base speed is about 1 percent, but reduces to 0.1 percent when a tachometer is added to the drive. These drives are very reliable, can handle high starting torques, can maintain a constant torque through a speed range of 20:1, and are relatively easy to maintain (that is, replace brushes). However, since the drives have brushes, they are limited to noncorrosive polymers [5, p. 361].

In contrast, an ac adjustable frequency drive consists of a solid-state power supply connected to an ac “high-efficiency” or “vector” motor. The power supply converts three-phase ac line voltage to variable voltage dc power and then back to controlled ac frequency. Since the voltage-to-frequency ratio is adjusted to provide constant torque from the ac motor, speed-torque characteristics can be optimized by varying the voltage-to-frequency ratio. These motors provide constant torque up to base speed. While they are usually more expensive than dc SCR drives, price reductions and the improved base-speed variation have made them competitive.

The high-speed drive motor is coupled to the low-speed screw using a reducer or gear box. Typical reduction ratios are 15:1 or 20:1.28. While helical gears are most common, worm gears are used on older or very small machines. A forced lubrication system allows oil to cool the bearings and gears; this oil is water-cooled by a heat exchanger in high-load machines.

For small- and medium-sized extruders, the drive coupling is a belt. Switching to larger gears increases the torque but reduces the screw speed. However, to increase available power, the drive motor, and usually the gear box, must be replaced. For large drives, the drive motor is coupled directly to the screw.

A thrust bearing supports the screw and couples the gear box to the screw.

The bearing sees high temperature, high pressure, and possible contamination. Although there are various bearing designs, the thrust bearing should last the life of the extruder (10 or more years). If the operating head pressure and/or screw speed is greater than the “B-10 standard,” then the bearing life is reduced [3, p. 49].

The feed throat fits around the first few flights of the screw and is usually separate from the barrel of the extruder. It is insulated from the barrel and cooled with water to prevent bridging and premature melting of the resin particles. The feed port is the opening in the feed throat. Standard feed ports are round or square and should match the geometry and size of the hopper opening. Although these ports are suitable for plastics pellets and some granules, specialized designs are employed with other materials. An undercut feed port, which exposes the bottom of the screw, is used for rolls and strips of film or fiber, for film scrap, and for polymer melts. A sloped feed port is better suited to irregularly shaped particles, whereas a tangential feed port can be used for powders and regrind.

The feed hopper feeds material to the extruder. Single-screw extruders are usually fed gravimetrically through standard conical or rectangular hoppers. Although pellets and some granules flow smoothly in these hoppers, powders and other particles often require modifications for proper feeding. A spiral hopper improves dry flow, while vibrating pads or hammers are sometimes attached to hoppers to break up bridges (blockages at the base of the hopper). Vacuum feed hoppers reduce the trapped air that hinders proper feeding. In crammer feeders, an auger forces material into a barrel, whereas metered (starve) feeding uses an auger to

feed a set amount of material to the barrel.

The barrel is a metal cylinder that surrounds the screw. One end fastens to the feed throat and the opposite end connects directly to the die adapter. Since extruder barrels must withstand pressures up to 70 MPa, they are usually made from standard tool steels, with special tool steels required for corrosive polymers.

Extruder barrels typically have length-to-diameter (L/D) ratios of 24:1 to

36:1, but they can be larger. Since melting occurs over a longer transition zone, longer barrels provide increased output. However, the longer screws require larger drive systems and produce greater screw deflection [4, p. 100].

The clearance between the barrel and screw flights is typically 0.08 to 0.13 mm (0.003 to 0.005 in). To reduce barrel wear, barrels are nitrided or bimetallic liners are inserted into the barrel. Nitriding is the surface hardening of the barrel. Nitriding also provides poor abrasion and only moderate corrosion resistance. Although barrel and barrel liners typically have smooth surfaces, a liner or barrel with axial grooves can be installed in the feed section of the extruder. Groove depth is greatest at the feed throat and gradually decreases with axial distance. Since the grooves increase shear, friction, and pressure, they improve extruder output. However, the feed section requires additional cooling, thermal insulation must be installed between the grooved section and the rest of the barrel, the feed zone must also be able to withstand higher pressures (typically 100 to 300 MPa), and significant wear occurs with abrasive materials. Grooved barrel liners were developed for low-bulk-density materials, but have increased output for regular pellets.

The breaker plate acts as a seal between the extruder barrel and the die adapter, thus preventing leakage of the melt. The breaker plate also supports the screen pack, develops head pressure (restricts flow), and converts the rotational motion of the melt to axial motion. The screen pack filters melt for contamination and gel particles, generate head pressure, and minimize surging (pulsing of the melt). Five or more screens are used in a typical screen pack; screens are rated by the number of holes per millimeter (or inch). The screens become finer as they approach the breaker plate. A coarse screen next to the breaker plate supports the finer screens and prevents the melt pressure from forcing them through the breaker plate. Although the selection of screen sizes depends on the material and extrusion process, increasing the number of screens or the mesh size increases the pressure developed during extrusion [2, p. 358].

It should be mentioned that compared to other molding processes, plastic extrusion molding has a low cost and it is more efficient. Extrusion molding will also provide considerable flexibility in the products being manufactured with a consistent cross section. And plastics remain hot when they are removed from the extruder and this allows for post-extrusion manipulations. Many manufacturers will take advantages of this and use a variety of roller, shoes and dies to change the shape of the plastic as needed [6].

But plastic extrusion has also some disadvantages. Once the hot plastic is removed from the extruder it will many times expand. This is called die swell. And extrusion plastic molding does place limits of the types of products that can be manufactured.

References

1. Rauwendaal C. Polymer Extrusion, 2d ed. – New York: Hanser Publishers, 1990. – 934 p.

2. Harper Charles A. Modern Plastics Handbook / Modern Plastics, Charles A. Harper (editor in chief). – New York: McGrow-Hill, 1999. – 1232 p.

3. Bikales N. M. Extrusion and Other Plastics Operations. – New York: Wiley-Interscience, 1971. – 498 p.

4. Fisher, E. G., Extrusion of Plastics. – New York: John Wiley, 1976. – 271p.

5. Harper Charles A. Handbook of Plastic Processes. – New York: Wiley, 2006. – 760 p.

6. Plastic Extrusion: Advantages and Disadvantages of Plastic Extrusion. Available at: http://civilengineersforum.com/plastic-extrusion-advantages-

disadvantages/. (accessed 22.10.2016).

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