In extrusion operations, a solid thermoplastic material is melted, forced through an orifice (die) of the desired cross section, and cooled. This method was adapted from metallurgists who use a similar form of extrusion to process molten aluminum and was first adapted in 1845 by Bewley and Brooman to extrude rubber around cable as a coating.16 Extrusion processes are used to continuously produce film and sheet; shapes with uniform cross-sections, such as PVC pipe, tubes, and garden hose; profile with nonuniform cross-sections, such as PVC window moldings and gutters; synthetic fibers; polymer coatings for insulating wire and sealing paper, plastic, and metal packaging. Although there are many types of extruders, the most common is the single-screw extruder (shown in Fig. 1.13).
This extruder consists of a screw in a metal cylinder or barrel. Electrical heater bands and fans that surround the barrel help bring the extruder to operating temperature during start-up and maintain barrel temperature during operation. One end of the screw is connected through a thrust bearing and gear box to a drive motor that rotates the screw in the barrel. The other end is free floating in the barrel. The barrel is connected to the feed throat, a separate “barrel section,” with an opening called a feed port, and is connected to the feed hopper. A die adaptor is usually connected to the opposite end of the extruder. A breaker plate and a screen pack are sandwiched between the extruder and die adaptor. The breaker plate provides a seal between the extruder and die, converts the rotational motion of the melt (in the extruder) to linear motion (for the die), and supports the screen pack. The screen pack filters the melt, thereby prevent unmelted resin, degraded polymer, or other contaminants from producing defects in the extruded products and/or damaging the die.
During extrusion, solid resin in the form of pellets or powder is fed from the hopper, through the feed port, and into the feed throat of the extruder. The solid resin falls onto the rotating screw and is packed into a solid bed in the first section of the screw (called the feed zone). The solid bed is melted as it travels through the middle section (transition zone) of the screw. The melt is mixed, and pressure is generated in the final section (metering zone) of the screw. Although the heater bands and cooling fans maintain the barrel at a set temperature profile, conduction from the barrel walls provides only 10 to 30 percent of the energy required to melt the resin. The remainder of the energy is generated from the frictional heat generated by the mechanical motion of the screw; this mechanism is called viscous dissipation. Extruder screws are design to accommodate this pattern of packing, melting, and pressure generation. As illustrated in Fig. 1.14,
the outside diameter of the screw, which is measured at the tops of the screw flights, remains constant. The root diameter of the screw, however, changes. In the feed zone, the root diameter is small so that the large channel depth (i.e., distance between the outside and root diameters) can accommodate the packed solid resin particles. The root diameter of the transition or compression zone increases with the distance from the feed zone. This change in channel depth forces the solid
into better contact with barrel wall, thereby promoting better melting. It also compresses the molten polymer in the screw channels. The root diameter becomes constant again in the metering zone, but the channel depth is very small. This geometry facilitates pressure generation and helps maintain the temperature of the polymer melt (i.e., polymers are poor conductors of thermal energy, and so thin melt layers have more uniform temperatures). The compression ratio (i.e., ratio of the channel depths in the feed and metering zones) and length of the transition zone significantly affect the melting in the single-screw extruders. Typically, extruder screws have length to diameter (L/D) ratios of about 30:1, with each zone requiring about one-third of the screw length. Barrier screws are used to improve melting performance while an assortment of mixing elements incorporated into the metering zone enhance mixing and the melt temperature uniformity of the melt. These include the addition of mixing pins on the barrel of the screw, ring barriers, and modified designs that involve very large screw diameters so as to force molten polymer through a small clearance between the mixing head and the inside of the barrel wall. Two stage-screws permit devolatilization of polymer melts, thereby eliminating entrapped moisture, air, and other volatiles from the melt. Typical extruders have diameters of 25 to 150 mm, but this can vary from 20 to 600 mm (6 to 24 in). They typically operate at 1 to 2 rev/s (60 to 120 rpm) for large extruders and 1 to 5 rev/s (60 to 300 rpm) for small extruders. Output varies as a function of processing parameters (particularly screw speed and pressure), the thermal and mechanical properties of the polymer, and the design and geometry of the screw. A 600-mm dia single-screw extruder is capable of delivering 29 metric tons of product an hour, whereas the smallest 20-mm dia single-screw extruders have a throughput capacity of 5 kg/h.20 Operating pressures are typically 1 to 35 MPa (200 to 5000 psi). Single-screw extruders account for 90 percent of all extruders, with the three types of twin-screw extruders making up the bulk of the remaining 10 percent. In nonintermeshing (tangential) extruders, the counter-rotating screws do not interlock with each other and convey the polymer using drag flow (i.e., like a single-screw extruder). These extruders permit tight control of heating and shear and so have been used for devolatilization, coagulation, reactive extrusion, and halogenation of polyolefins. With intermeshing twinscrew extruders (Fig. 1.15),
the flights of one screw fit into the channels of the other, and polymer is transferred from the channels of one screw to those of the other, thereby providing positive conveyance of the polymer and increased mixing. In counter-rotating, intermeshing twin-screw extruders, some material flows between the screws and the barrel wall, and the remainder is forced between the two screws. Polymer in co-rotating twin screws moves in a figure-eight pattern around the two screws, with little material flowing between the screws. The longer flow path produces longer extruder residence times than observed with counter-rotating, intermeshing twin-screw extruders but increases the degree of elongational flow and enhances mixing. Intermeshing twin-screw extruders are typically used in applications where mixing and compounding need to be accomplished, because the screws’ elements can be rearranged (programmed) to suit a specific application. They are highly capable of dispersing small agglomerates such as carbon black and can be used, for example, to blend the components of duct tape adhesive as well as coat the finished adhesive onto the tape backing. Counter-rotating, intermeshing twin-screw extruders, which permit tight control of shear and residence time, are also employed for the extrusion of PVC pipes and profiles. Although twin-screw extruders have relatively low pressure-generating capabilities, some materials can be compounded and formed directly if a gear pump is added to the end of the extruder. Die designs depend on the product that will be formed. Typically, spiral flow and spider arm dies are used for blown film, tubing, and pipes. Crosshead dies are employed for tubing and wire coating. Wide dies with tee, coat hanger, and exponential are employed for film, sheet, and extrusion coating. In die design, it is critical to avoid “dead spots” where the polymer melt can become stagnant and risk thermal degradation. It is also important that the polymer molecules be allowed to return to an equilibrium position to the greatest extent possible to minimize the orientation as a result of flow. Laminar flow is desired, and finite element analysis is used to design dies that enable laminar flow to the greatest extent. Multimanifold dies, such as plate dies, and feedblocks (along with film, sheet, and extrusion dies) combine melt streams from multiple single-screw extruders to produce co-extruded multilayer products. This common technique is used for producing multilayer packaging films, where each layer provides a particular feature. For example, garbage bags are often multilaminate constructions, as are packaging films where a PVDC layer may be incorporated for moisture or oxygen barrier properties, and HDPE may be used as a less-expensive, relatively strong, layer. EVA is a common “bonding layer” between different plastic layers. As many as eight or more extruders may be used to form highly specialized, multilayer films. Common defects encountered with extrusion include effects associated with the viscoelastic nature of plastic melts. As the melt is extruded from the die for example, it may exhibit sharkskin melt fracture and extrudate (die) swell. Diagrams of these defects are shown in Fig. 1.16.
Sharkskin melt fracture occurs when the stresses being applied to the plastic melt exceed its tensile strength. Extrudate swell occurs due to the elastic component of the polymer melt’s response to stress and is the result of the elastic rebound of the polymer as it leaves the constraints of the die channel prior to cooling. Pressure generated in the extruder forces the melt through the breaker plate, die adaptor, and die. The die forms the melt into the desired shape. Downstream equipment, such as a water bath cools the melt, and a puller draws the extrudate away from the die and through the water bath away from the die. Figure 1.17
illustrates the downstream equipment for tube extrusion. The annular tube exiting the die is pulled though a calibration unit, which maintains the outside diameter of the tube, while being cooled by a water bath. The puller stretches the molten tube, and a cutter slices the tube into preset lengths. In blown film extrusion (Fig. 1.18),
the melt forced though an annular die is expanded into a bubble using air blown through a hole in the die mandrel, stretched axially by take-up rolls, and cooled by forced convention. This biaxial orientation, thinning of the tube of film through the internal pressurization of the bubble, combined with the thinning of the film as it is stretched upwards, results in a strong, biaxially oriented film. Stretching continues until the freezing line is reached, at which point the film has cooled off to such an extent as to provide a high enough modulus to resist further deformation. Crystallization also enables the orientation to be maintained. A pair of collapsing rolls is used to flatten the bubble and allow the film to then be wound into a master roll for later converting processes such as slitting.
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