Modern Plastics Handbook 2011 Part 6 pps

along the extruder. Since the pressure is greatest just before the die, this head pressure creates two other flows, pressure flow Q P and leak- age flow Q L . In pressure flow, the head pressure forces the melt to rotate in the channels of the extruder screw. Leakage flow occurs when the head pressure forces melt back over the flights of the screw. Since they both counter the forward motion of the melt, pressure and leak- age flow are often lumped together as back flow. As depicted in Fig. 5.25, during normal extruder operation, drag flow conveys the polymer along the barrel walls, whereas pressure flow forces the material near the screw back toward the hopper. A simple mathematical modeling of extrusion assumes that: (1) the extruder is at steady state, (2) the melt is newtonian, (3) the extrud- er is isothermal (at a constant temperature), and (4) the metering zone makes the only contribution to output. Thus, the net output Q, of the extruder can be expressed as the sum of the three flows: Q ϭ Q D Ϫ Q P Ϫ Q L (5.15) Drag flow is proportional to a screw constant (A) and the screw speed, expressed as Q D ϭ AN ϭ N (5.16) where D is the screw diameter, h is the channel depth in the metering zone, ␾ is the helix angle of the screw, and N is the screw speed in rev- olutions per second (r/s). Pressure flow is related to a screw geometry constant B, the head pressure ⌬P, and the apparent viscosity of the melt in the metering zone ␮. This is given by ␲ 2 D 2 h sin ␾ cos ␾ ᎏᎏᎏ 2 5.42 Chapter Five h To die Pressure flow Drag flow Barrel Screw Figure 5.25 Drag and pressure flow in the metering zone of a single- screw extruder. 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.42 Q P ϭϭ (5.17) where L m is the length of the metering zone. Leakage flow is a function of a constant C, the head pressure ⌬P, and the apparent viscosity in the flight clearance (␮). This is expressed by Q L ϭϭ (5.18) where ␦ is the flight clearance and e is the flight width. With similar assumptions, die output becomes: Q Die ϭ (5.19) where the die constant K varies with geometry. Selected constants are given in Table 5.8. 61 When drag flow dominates, extruder output increases linearly with screw speed, and larger screws and deeper channels carry more melt. However, head pressure also rises with screw speed. Although this increases pressure flow, the actual effect depends heavily on melt vis- cosity. With high-viscosity melts, pressure flow may be minimal and have little effect on extruder output. In contrast, low-viscosity melts produce less head pressure but greater pressure flow. Thus, pressure flow will reduce the expected output. Deeper channels, neutral screws (Fig. 5.26), and shorter metering zones enhance these effects. Leakage flow varies with the flight clearance. It is also enhanced by low-viscosity melts and high head pressures. With new screws and barrels, leakage flow is minor and has no apparent effect on extruder output. As the flight clearance increases, leakage flow rises, thereby reducing output. Consequently, the decrease in extruder output over time is used to monitor screw and barrel wear. In contrast to extruder output, die output increases with head pressure (Fig. 5.27). Die output is also enhanced by low-viscosity melts and larger die gaps. The match between extruder and die output shifts with operat- ing conditions. The simple die characteristic curve in Fig. 5.27 shows the optimized processing conditions. However, this curve does not consider extrudate quality. Other “lines” would be required to locate the onset of surface defects, such as melt fracture, and for incomplete melting. Head (melt) pressure is measured at the end of the extruder. One pressure transducer is typically mounted just before the breaker plate while others may be placed in the die adapter or die itself. Pressure is monitored for safety purposes, product quality, research and develop- ⌬P ᎏ K␮ ⌬P ᎏ ␮ ␲ 2 D 2 ␦ 3 tan ␾ ᎏᎏ 12eL m C ⌬P ᎏ ␮ ⌬P ᎏ ␮ ␲Dh 3 sin 2 ␾ ᎏᎏ 12L m B ⌬P ᎏ ␮ Processing of Thermoplastics 5.43 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.43 ment, screen pack or changer condition, process monitoring, and trou- bleshooting. Since head pressures can reach 69 MPa (10,000 lb/in 2 ), pressure is monitored during extruder start-up and operation to adjust operating conditions or halt operation before the pressure opens the rupture disk. Variations in head pressure (surging) produce variations 5.44 Chapter Five TABLE 5.8 Selected Die Constants 61 Die profile Die constant, K Circular 128(L ϩ 4D c )/␲D c 4 Slit 12L/WH 3 Annular 12L/␲D m H 3 Note: L ϭ die land length, D c ϭ diameter of a circular die, D m ϭ mean diameter of an annular die, W ϭ width of a slit die, and H ϭ die opening (gap). Coolant H > H' H H H' Cooled screw Neutral screw Barrel wall Figure 5.26 Channels in neutral and cooled screws. Large die Q T Small die High N Low N ∆P Figure 5.27 Die characteristic curve. 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.44 in output. These changes are used to track product quality and trou- bleshoot the extrusion process. Larger pressure increases also trigger the movement of screen changers or signal the need to replace screen packs and breaker plates. Melt temperature is also monitored during extrusion. Melt tem- perature varies with the placement of the measuring device, mate- rial, and processing conditions. Thermocouples measure temperature at one point in the melt stream. As shown in Fig. 5.28, 62 melt temperature measured with a flush-mounted thermo- couple is influenced by the barrel wall temperature. Protruding thermocouples interrupt flow and produce varying levels of shear heating. While straight protruding thermocouples measure more shear heating, they are more robust than upstream fixed or radial- ly adjustable thermocouples. A bridge with multiple thermocouples measures melt temperature at several points in the melt stream. However, the bridge produces a greater interruption of the melt flow. Infrared sensors measure the average melt temperature and are more sensitive to temperature variations; these sensors are expensive and have limited availability. A material’s sensitivity to shear and temperature produces varying levels of shear. While melt travels fastest in the center of the channel, shear is highest near the wall. Cooling effects are also greatest near the wall, producing a melt temperature differential as great as 50°C in the melt channel. Processing of Thermoplastics 5.45 (a) (b) (c) (d) (e) Figure 5.28 Various temperature sensor configurations: (a) flush-mounted, (b) straight protruding, (c) upstream fixed, (d) upstream radially adjustable, and (e) bridge with multiple probes. 62 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.45 5.2.3 Vented extruders Vented extruders (Fig. 5.29 63 ) contain a vent port and require two- stage screws. As described previously, the screw has five zones: feed, transition (or melting), first metering, vent, and second metering. Material is fed, melted, and conveyed in the first three zones of the extruder. Melt pressure increases gradually as the plastic moves down the barrel. However, the channel depth increases abruptly in the vent zone. Since the thin layer of melt from the metering zone cannot fill this channel, the melt is decompressed and volatiles escape through the vent. The melt is repressurized in the second metering zone and this pressure forces the melt through the die. While vented extruders are used for devolatilization, they can only handle materials with a volatiles content up to 5 percent. 64 They are also subject to vent flooding. If the die resistance is too high or the screen pack is clogged, the melt pressure will rise in the vent zone, causing vent flooding. In screws with high pump ratios or when the feeding rate is too high, the second zone cannot convey the material fed from the first metering zone and this floods the vent. Consequently, vented extruders are often starve fed and pressure is monitored care- fully during operation. 5.2.4 Twin-screw extrusion Single-screw extruders are relatively similar in design and function. All single-screw extruders convey the polymer to the die by means of viscous drag (drag flow). While some variations occur in screw and extruder design, single-screw extruders generally provide high head pressures, uncontrolled shear, and a degree of mixing that relies on the screw design. Output depends on material properties, particularly 5.46 Chapter Five Second melting zone l 2 (h 2 , ␾ 2 ) Vent zone First metering zone l 2 (h 1 , ␾ 1 ) Melting zone Feed zone Compression Pressure Die resistance High Intermediate Low Decompression Figure 5.29 Pressure profile associated with a vented barrel extruder. 63 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.46 the bulk properties (coefficient of friction, particle size, and particle- size distribution). In contrast, the design, principles of operation, and applications of twin-screw extruders vary widely. While the two screws are usually arranged side by side, the introduction of two screws pro- duces different conveyance mechanisms, varied degrees of mixing, and controllable shear. The low head pressure generated by twin-screw extruders initially limited their use to processing of shear-sensitive materials, such as polyvinyl chloride, and to compounding. Although changes in design have permitted higher speeds and pressures, 65 the primary use of twin-screw extruders is still compounding. Twin-screw extruders are used in 10 percent of all extrusion. The two screws are the key to understanding the conveyance mech- anisms and probable applications of different twin-screw extruders. The screws may rotate in the same direction (corotating) or in opposite directions (counterrotating). In addition, the flights of the two screws may be separated, just touch (tangential), or intermesh to various degrees. The flights of partially intermeshing screws interpenetrate the channels of the other screw, whereas the flights of fully inter- meshing screws completely fill (except for a mechanical clearance) the channels of the adjacent screw. While many configurations are possi- ble, in practice the major designs are: (1) nonintermeshing, (2) fully intermeshing counterrotating, and (3) fully intermeshing corotating twin-screw extruders (Fig. 5.30 66 ). Nonintermeshing (separated or tangential) twin screws do not inter- lock with each other. The polymer is conveyed, melted, and mixing by drag flow. Since two corotating nonintermeshing screws would provide uncontrollable shear at the nip between two screw and little distribu- tive mixing, they are not used commercially. 67 Counterrotating screws must rotate at the same rate to produce sufficient output. With matched flights, little plastic material is transferred between screws, however, substantial interscrew transfer occurs with staggered Processing of Thermoplastics 5.47 Figure 5.30 Twin-screw extruders: (a) counterrotating, fully intermeshing; (b) corotat- ing, fully intermeshing; and (c) counterrotating, nonintermeshing. 66 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.47 flights. 68 As a result, counterrotating nonintermeshing twin-screw extruders provide good distributive mixing but little shear. The screws of commercial counterrotating tangential (CRT) twin-screw extruders are either matched or one screw is longer than the other. With the lat- ter configuration, the single screw at the end of the extruder improves pressure generation. Thus, counterrotating nonintermeshing twin- screw extruders have been used for devolatilization, coagulation, reac- tive extrusion, and halogenation of polyolefins. With intermeshing twin-screw extruders, the flights of one screw fit into the channels of the other. Since the extruders are usually starve fed, the screw channels are not completely filled with polymer. By transferring some polymer from the channels of one screw to those of the other, the intermeshing divides the polymer in the channel into at least two flows. Thus, intermeshing twin-screw extruders provide pos- itive conveyance of the polymer and improved mixing. In counterrotating, intermeshing twin-screw extruders, a bank of material flows between the screws and the barrel wall. The remainder is forced between the two screws and undergoes substantial shear. With little intermeshing, drag flow between the screws is greater than that at the barrel walls. However, for the commercial fully intermesh- ing screws, most material flows along the screws in a narrow channel (C chamber) and is subject to relatively low shear. Consequently, the degree of mixing in counterrotating, intermeshing twin-screw extrud- ers depends on the degree of intermeshing and screw geometry. Increasing the distance between the screws increases flow between the screws and permits effective distributive mixing. However, increased screw separation only decreases the shear rate in the nip, and hence reduces dispersive mixing. Since screw length and geometry are also used to prevent excessive shearing, melting in these extruders is lim- ited, and most of the heat transferred to the polymer is conducted from the barrel. This mechanism provides very sensitive control over the melt temperature. With good temperature control and the low shear, these extruders are well suited for compounding and for extrusion of rigid poly(vinyl chloride). Typically, high-speed (200- to 500-r/min 69 ) extruders are employed for compounding, whereas low-speed (10- to 40-r/min 69 ) machines are used for profile extrusion. Conical twin-screw extruders with their tapering screws (Fig. 5.31 70 ) are utilized almost exclusively for chlorinated polyethylene and rigid poly(vinyl chloride). Corotating fully intermeshing twin screws are self-wiping. Thus, they tend to move the polymer in a figure-eight pattern around the two screws, as shown in Fig. 5.32. 71 Typically, a screw flight pushes the material toward the point of intersection between the two screws. Material is then forced to change its direction through a large angle, 5.48 Chapter Five 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.48 which mixes the material. Very little material is able to leak between the screws. Finally, the material is transferred from one screw to the next. The flow pattern provides a longer flow path for the material, and hence, the longer residence time of corotating extruders. Mixing elements, such as kneading blocks, are not fully self-wiping, but are usually incorporated to improve melting and mixing. However, unlike counterrotating screws, the shear between the corotating screws is rel- atively mild. Consequently, the combination of longer flow paths, more uniform shear, and self-wiping conveying elements make corotating intermeshing twin screws well suited to mixing and compounding applications. The design of intermeshing twin-screw extruders also differs from that of single-screw extruders. With the exception of conical screws, the Processing of Thermoplastics 5.49 Figure 5.31 Screws for a conical twin-screw extruder. 70 Figure 5.32 Flow pattern in a corotat- ing, fully intermeshing twin-screw extruder. 71 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.49 screws are usually not a single piece of metal, but two shafts onto which component screw elements are arranged (Fig. 5.33 72 ). Thus, screw pro- files may be “programmed” to impart specific levels of shear, mixing, and conveyance. Conveying elements (Figs. 5.34a 73 and b 73 ) do not mix the plastic, but merely convey the material down the screw. Single-flighted conveying elements provide rapid transport, whereas triple-flighted elements impart shear to the plastic; the performance of double-flighted elements is intermediate to the other two. Monolobal elements dominate in counterrotating extruders while corotating extruders use bilobal and trilobal elements. The latter divide the flow to enhance mixing. Kneading blocks (Figs. 5.34c 73 and d 74 ) impart shear to the melt. They have three critical dimensions: length, disk thickness, and degree of stag- ger. Although increasing length improves mixing, changing disk thick- ness and stagger angle alter the balance of dispersive and distributive mixing. Typically, increased thickness and angle increase dispersion at the expense of distributive mixing. For trilobal elements, 30° provides forward conveyance, 60° is neutral (no conveyance), and 90° forces melt backwards along the screws. With bilobal elements, 180° is backwards conveyance. Left-hand kneading blocks, a restrictive element used prior to vent ports, also induce back flow. Gear and slot mixing elements (Figs. 5.34e 73 and f 74 ) provide distributive mixing. When programming the screw, elements facilitate the required func- tion of the screw (Fig. 5.35 75 ). Since single-flighted conveying sections have a large volumetric capacity, they are used in the feed zone. Kneading blocks impart shear and facilitate melting of the material. Small-pitch, double-flighted conveying elements slow conveyance, while a left-handed element seals the vent and increases distributive mixing. Pressure increases in the kneading blocks and double-flighted elements and drops with the left-handed elements. The pattern is repeated for the second vent zone. However, single-flighted conveying elements increase the melt conveyance near the die, which facilitates the generation of pressure that forces the melt through the die. Small pitch is used to reduce conveyance and increase residence time in reac- tive extrusion. 76 With conventional twin-screw extruders, the time can be extended to 10 min, whereas special twin-screw extruders can pro- vide residence times up to 45 min. 77 Narrow kneading blocks are used after fiber addition to prevent fiber degradation. 76 The extruder barrels are also modular in design. Extruder L/D may be changed by adding or removing barrel segments. Features, such as multiple stages and venting sections, may also be added, subtracted, or moved. Finally, special barrel sections, such as those with expensive abrasion-resistant barrel liners, may be located after the appropriate feed ports. 78 Metered feeding of material is typically required to keep the channels of intermeshing twin-screw extruders from filling com- 5.50 Chapter Five 0267146_Ch05_Harper 2/24/00 4:55 PM Page 5.50 [...]... absence of clamshelling 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM (b) Figure 5.42 Manifold designs: (a) T manifold and (b) coat-hanger manifold.99 Page 5 .65 (a) 5 .65 0 267 1 46_ Ch05_Harper 5 .66 2/24/00 4:55 PM Page 5 .66 Chapter Five Three mechanisms produce the fine adjustment for flow through the die In a flex lip die, a metal bar with a flexible hinge is machined into the upper part of the die Bolts at an... Sizes92 Extruder diameter, mm Blown film die diameter, mm Cast film die width, mm Extrusion coating die width, mm 38 64 89 114 152 203 Ͻ100 75–200 150–380 Ն220 Ն220 — — Ͻ900 60 0–1520 900–1830 1520–3050 — — — 61 0–1220 915–2290 1370–3550 Ն4 065 0 267 1 46_ Ch05_Harper 5 .62 2/24/00 4:55 PM Page 5 .62 Chapter Five tion (TD) Stalk height depends on material properties and processing conditions Typically, low-density... 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM Page 5.59 Processing of Thermoplastics Bubble Bubble Air ring Q2 Q1 ;;;; ;;;; ;;; ; ;;;; ;;;; ;; Air ring Air 5.59 Air Die bushing Die mandrel (a) Die mandrel (b) Q2 Q1 Die bushing (c) Figure 5.40 Cooling methods for blown film lines: (a) single-lip air ring, (b) dual-lip air ring, and (c) internal bubble cooling.88 0 267 1 46_ Ch05_Harper 5 .60 2/24/00 4:55 PM Page 5 .60 ... bar.101 5 .67 0 267 1 46_ Ch05_Harper Nip (or pinch) rolls Extruder 2/24/00 4:55 PM Treater bar Rubber Stainless steel Trimmer (slitter) Idler rolls Rubber nip (or pinch) roll Die 2 (or more) water-cooled highly polished chill rolls Stainless-steel nip (or pinch) roll (driven) Windup (usually twin station) Figure 5.44 Chill-roll stack for film extrusion.103 Page 5 .68 Powered carrier rolls 5 .68 0 267 1 46_ Ch05_Harper... method is seldom practiced With the latter two systems, the film veers back and forth by 3.2 to 6. 4 mm (0.125 to 0.250 in).1 06 As illustrated in Fig 5.44,103 slitters (or trimmers) are also required to remove the edge bead in many film and sheet 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM Page 5 .67 Processing of Thermoplastics Melt thermocouple Breaker plate Heaters Thermocouple Holding bolts Cooling channel Sloped... film (a) (b) (c) (d) Figure 5.41 The effect of (a) BUR on tensile strength, (b) BUR on tear strength, (c) DDR on tensile strength, and (d) DDR on impact strength. 96 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM Page 5 .63 Processing of Thermoplastics 5 .63 suitable for shrink-wrap films Impact strength is also enhanced by the biaxial orientation while any blocking tendency decreases due to the rapid cooling Although... produce nonuniform flow Figure 5.38 Blown film line 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM Page 5.57 Processing of Thermoplastics Thermocouple pocket 5.57 Air Air hole Die centering bolt Spider leg Mandrel pin Die Breaker plate seat Thermocouple pocket Air inlet (a) Retaining ring (b) (c) Figure 5.39 Extrusion dies: (a) side-fed die, 86 (b) spider-arm die, 86 and (c) spiral-flow die.87 Thus, film tends to be... polymer, and other defects However, if the barrel temperature or melt temperature is too high, the viscosity becomes too low, and the bubble becomes unstable and may 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM Page 5 .61 Processing of Thermoplastics 5 .61 break Thus, the temperatures should be as high as the resin and the cooling equipment permit A lower die temperature may be used when starting up the film extruder,... surfaces adhere to each other, particularly in the film roll) and failure to slip (internal surface stick to each other) Blown film extrusion is a scrapless operation with high outputs The films are versatile; they can be used as tubes or slit to become flat film Finally, the process inherently produces biaxial orientation However, 0 267 1 46_ Ch05_Harper 5 .64 2/24/00 4:55 PM Page 5 .64 Chapter Five blown film... difficult to 0 267 1 46_ Ch05_Harper Heaters Extruder Adapter Trimmer Screw Resin Die To windup Screen pack Breaker plate Air gap Water outlet Quench tank Water inlet Guide shoe Figure 5.45 Water cooling of flat film.104 Roll – alternative to guide shoe Page 5.70 Nip (or pinch) rolls 2/24/00 4:55 PM Stock thermocouple 5.70 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM Page 5.71 Processing of Thermoplastics 5.71 Adjustable . associated with a vented barrel extruder. 63 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM Page 5. 46 the bulk properties (coefficient of friction, particle size, and particle- size distribution). In contrast,. Thermoplastics 5.47 Figure 5.30 Twin-screw extruders: (a) counterrotating, fully intermeshing; (b) corotat- ing, fully intermeshing; and (c) counterrotating, nonintermeshing. 66 0 267 1 46_ Ch05_Harper. of Thermoplastics 5.43 0 267 1 46_ Ch05_Harper 2/24/00 4:55 PM Page 5.43 ment, screen pack or changer condition, process monitoring, and trou- bleshooting. Since head pressures can reach 69 MPa (10,000