5 9 Extrusion 229 Table 5ol Example of thermoplastics that are extruded (courtesy of Spirex) Resin data" ~ c~ ~ ~ ~ ~ "~ ABS, extrusion 1.02 64.0 27.0 0.980 435 0.34 0.25 ABS, injection t.05 65.0 26.0 0.952 0.40 0.40 0.20 Acetal, injection 1.41 88.0 19.7 0.709 0.35 0.25 Acrylic, extrusion 1.19 74.3 23.3 0.839 375 0.35 0.30 Acrylic, injection 1.16 72.0 24.1 0.868 0.35 0.20 0.08 CAB 1.20 74.6 23.1 0.833 380 0.35 1.50 0.15 Cellulose acetate, extrusion 1.28 80.2 21.6 0.781 380 0.40 2.50 Cellulose acetate, injection 1.26 79.0 2t.9 0.794 0.36 2.40 0.20 Cellulose proprionate, extrusion 1 77 76.1 22.7 0.821 380 0.40 1.70 Cellulose proprionate, injection 1.22 75.5 22.9 0.828 0.40 2.00 0.25 CTFE 2.11 134.0 13.1 0.473 0.22 0.01 FEP 2.11 134.0 12.9 0.465 600 0.28 <0.01 lonomer, extrusion 0.95 59.6 29.0 1.050 500 0.54 0.07 Ionomer, injection 0.95 59.1 29.2 1.060 0.54 0.20 Nylon-6 1.13 70.5 24.5 0.886 520 0.40 1.60 0.15 Nylon-6,6 1.14 71.2 24.3 0.878 510 0.40 1.50 0.15 Nylon-6,10 1.08 67.4 25.6 0.927 0.40 0.40 0.15 Nylon-6,12 1.07 66.8 25.9 0.935 475 0.40 0.40 0.20 Nylon-ll 1.04 64.9 26.6 0.962 460 0.47 0.30 0.10 Nylon-12 1.02 63.7 27.1 0.980 450 0.25 0.10 Phenylene oxide based 1.08 67.5 25.6 0.926 480 0.32 0.07 Polyallomer 0.90 56.2 30.7 1.110 405 0.50 0.01 Polyarylene ether 1.06 66.2 30.7 0.940 460 0.10 Polycarbonate 1.20 74.9 23.1 0.832 550 0.30 0.20 0.02 Polyester PBT 1.34 83.6 20.7 0.746 0.08 0.04 Polyester PET 1.31 8.18 21.1 0.746 480 0.40 0.10 0.005 HD polyethylene, extrusion 0.96 59.9 28.8 1.040 410 <0.01 HD polyethylene, injection 0~95 59.3 29.1 1.050 480 <0.0l HD polyethylene, blow molding 0.95 56.9 28.8 1.040 410 <0.01 LD polyethylene, film 0.92 57.44 30.1 1.090 350 <0.01 LD polyethylene, injection 0.92 57.4 30.1 1.090 400 <0.01 LD polyethylene, wire 0.92 57.4 30.1 1.090 400 <0.01 LD polyethylene, ext. coating 0.92 57.1 30.0 1.090 600 <0.01 LLD polyethylene, extrusion 0.92 57.4 30.1 1.087 500 LLD polyethylene, injection 0.93 58.0 29.8 1.075 49_5 Polypropyiene, extrusion 0.91 56.8 30.4 1.100 450 0.03 Polypropylene, injection 0.90 56.2 30.7 1.110 490 <0.01 Polystyrene, impact sheet 1.04 64.9 26.6 0.963 450 0.10 Polystyrene, gp crystal 1.05 65.5 26.2 0.943 410 425 0.03 Polystyrene, injection impact 1.04 64.9 26.6 0.968 440 0.t0 Polysulfone 1.25 77.4 22.3 0.807 650 680 0.30 0.05 Polyurethane 1.20 74.9 23.t 0.834 400 400 0.10 0.03 PVC, rigid profiles 1.39 86.6 19.9 0.720 365 0.02 PVC, pipe 1.44 87.5 19.7 0.714 380 0.10 PVC, rigid iniection 1.29 83.6 21.0 0.756 380 0.t0 0.07 PVC, flexible wire 1.37 85.5 20.2 0.731 365 PVC, flexible extruded shapes 1.23 76.8 22.5 0.814 350 PVC, flexible injection 1.29 80.5 21.4 0.776 300 PTFE 2.16 134.8 12.9 0.464 <0.01 SAN 1.08 67.4 25.6 0.927 420 470 0.03 0.02 TFE t.70 106.1 16.3 0.589 610 0.01 Urethane elastomers 0.83 51.6 33.5 1.210 390 400 0.07 0.03 '~Specific information on all machine settings and plastic properties is initially acquired by using the resin supplier's data sheet on the oarticular compound or resin to be used. ~These are strictly typical average values for a resin class; consult your resin supplier for values and more accurate information. 230 Plastic Product Material and Process Selection Handbook discs or rotors arc used to generate shear. Howcvcr, TPs cxtrusion depends almost entirely on the rotating screw as a melt delivery device.143,476 TPs arc characterized by low thermal conductivity, high specific heat, and high melt viscosity. Preparation of a uniform homogeneous melt and its delivery at adequate pressure and a constant rate could pose considerable problems if not properly processed (Chapter 3). The principal extruder variants arc the single-screw and the twin-screw types. Of these, the single-scrcw cxtruder is by far the most versatile and popular in use. The single-screw extruder consists essentially of a screw that rotates in an axially fixed position within the close-fitting bore of a barrel. Extruder sizes are identified by the inside diameter of their barrel. Size range from 1/4 to 24 in. diameter with the usual from 1 to 6 in. (Europe and Asia sizes rangc from 20 to 600 mm with the usual from 25 to 159 mm.). The screw is electrically motor driven through different devices such as a gear reduction train or belt to meet different performance and cost requirements. These gear reducers arc rated in mechanical horse- power and thermal horsepower as defined by the American Gear Manufacturers (AGMA). The AGMA rating system is based on the understanding that not all gear reducers are used the same way. There are also gearlcss drive systems such as those using Siemens high-torque motor with an unusual low-inertia hollow shaft. 476 The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure dcvclopcd in the extruder system is largely a function of die resistance and dependent on die geometry and melt viscosity. Extrusion pressures are lower than those encountered in injection molding. They are typically 500 to 5000 psi (3.5 to 35 MPa). In extreme cases, extrusion pressures may rise as high as 10,000 psi (69 MPa). Variants on the single screw include the barrier or melt extraction screw and the vented screw (Chapter 3). The twin-screw extruder may have parallel or conical screws, and these screws may rotate in the same direction (co-rotating) or in opposite directions (contra-rotating). Extruders with more than two screws are known as the multiple-screw extruder. These extruders are normally used when mixing and homogenization of the melt is very important, in particular where additives, fillers, and/or reinforcements arc to be included in the plastic. They are extensively used for plastic compounding that includes heat- sensitive materials such as PVC, proccssing of difficult-to-feed materials (such as certain powders), reactive processing, 197 and for plastic devola- 5. Extrusion 231 tilization. Twin-screw extruders particularly offer a wide processing variability. They can be starve-fed so that residence time, amount of shear, and control of melt temperature can be controlled by means of their segmental modular designs. Component There are different components that make up the extruder each with their specific important function. M1 components have to operate efficiently otherwise the extruder's operation is inefficient. A very important and essential parameter in the extruder is the plasticator's pumping process. It is the interaction between the rotating flights of the screw and the stationary barrel wall. For the plastic material to be conveyed, its friction must be low at the screw surface but high at the barrel wall. If this basic criterion is not met, the plastic will usually rotate with the screw and not move in the axial output direction. In the plasticators output zone, both screw and barrel surfaces are usually covered with the melt, and external forces between the melt and the screw channel walls have no influence except when processing extremely high viscosity plastics such as rigid PVC and UHMWPE. The flow of the melt in the output section is affected by the coefficient of internal friction (viscosity) particularly when the die offers a high resistance to the flow of the melt (Chapter 3). Figure 5.2 shows the extruder's components where the following identifications are listed: 1 Drive motor from 20 to 2000 hp infinitely variable speed drives directly coupled to reducer for maximum efficiency deigned to save floor space. Gears and gearless to provide high efficiency capability to process plastics. 476 Efficient performance heat treated helical or herringbones (gears equipped with shaft-driven oil pumps and oil cooler). Thrust bearing with long life expectancy (of well in excess of 30 years' continuous operation). 5 Large rectangular standard feed opening (round with lining, optional, for use with crammer feeders). 6 Long lasting barrel heater/cooler elements that heat quicldy. Cooling tubes run parallel with heating elements. The cast-in stainless steel tubes closed-loop system provide non-ferrous distilled 232 Plastic Product Material and Process Selection Handbook water that is automatically adjusted via microprocessor-based temperature controllers providing uniform, efficient cooling. 8 High-performance screw with bimetallic lined cylinder designed for processing a specific plastic; can be cored for cooling. 9 Prepiped and prcwired power installation. 10 Safety heat conservation and heat protection guards that are one- piece, hinged, no loose parts insulated. 11 Heavy single unit steel base machine foundation prcassembled so all parts are in place ready to be used. 12. When required, patented two-stage vented plasticator is used (that can be plugged in minutes). 13 Screen changer for continuous operation without shut down using standard hinged swing-bolt gate. 14 Gear pump to ensure absolute volumetric output stability. 15 Static mixer to provide thermal and viscosity homogeneity. 16 Die designed to produce single or multi-layer sheet without modifi- cation; strand dies, etc. Figure 5~ Schematic identifies the different components in an extruder (courtesy of Welex Inc.) Purpose of the screens is primarily twofold: (1) to change the melt's spiraling motion, caused by the screw rotation; and (2) to filter contaminants out of the melt. Most plastics contain contaminants and these particles can be conveniently removed by means of a screen placed after the extruder barrel and before the melt flow reaches the extrusion die. The simplest means for filtering plastic melts are woven wire mesh disks of about the same diameter as that of the extruder barrels. Several 5. Extrusion 233 layers of different screens are usually made up into one screen pack. The innermost layer is the finest mesh screen that determines the particle size that will be caught by the screen pack. Against the forces exerted by the melt flow, the screen packs are backed by a thick, densely perforated steel disk called a breaker plate. The outer rims of the breaker plate and of the screen pack fit into a round recess in the end of the extruder barrel and are clamped in place by the adapter flange of the adjoining piece of equipment, usually that of the extrusion die. To change a clogged screen pack, the die adapter flange has to be removed, the old pack taken out and replaced with a new one, and the equipment reassembled. Screen changers arc mechanical devices that permit changing screens in a faster and more convenient way. Screen changers fall into three main categories: (1) manual, (2)intermittent (reciprocating), and (3) continuous screen changers. Other types of reciprocating screen changers employ valves by means of which the melt flow may bc diverted from one screen pack to the other, and back again. The ever- changing pressure conditions that are inherent in all intermittently operating machines can bc eliminated by the use of continuous screen changers. If it is at all possible to do without screen packs they should not be used. Various reasons exist. Complete and continuing displacement of melt from all points in the screen pack is rather difficult. Hundreds of small dead spots are filled with melt as soon as the pack is put into service, and the material in these spots is moved only very slowly, if at all, by the drag of neighboring melts. This action can cause contami- nating and degrading of the extrudate. The gear pump is a component that has been standard equipment since the 1930s in textile fiber production. During the 1980s they established themselves in all ldnds of extrusion lines. Gear pump is used to generate even melt pressure. Two counter-rotating gears transport a melt from the pump inlet (extruder output) to the pump discharge outlet. Gear rotation creates a suction that draws the melt into a gap between one tooth. This continuation action from tooth to tooth develops a surface drag that resists flow, so some inlet pressure is required to fill the cavity. 492 Static mixer, also called a motionless mixer, provides a homogeneous mix by flowing one or more plastic streams through geometric patterns formed by mechanical elements in a tubular tube or barrel. These elements cause the plastic compound to subdivide and recombine in order to increase the homogeneity and temperature uniformity of the 234 Plastic Product Material and Process Selection Handbook melt. There are no moving parts and only a small increase in the energy is needed to overcome the resistance of the mechanical baffles. These mixers are located at the end of the screw or before the screen changer or between the screw and gear pump. The temperature profile required along a barrel, adapter, and die depends largely on the specific extrusion process line with its screw design, plastic used, and available process control (Chapter 3). The thermal condition of the plastic is essentially determined for a given material by screw geometry with its rotational speed and the total restriction or pressure existing in the die. The electrical heaters are normally placed along the barrel grouped in separate and adjoining zones; each zone is controlled independently. Small machines usually have two to four zones. Larger machines have five to ten zones. Table 5.2 provides information on the different types of heater bands. TabJe 5.2 Selection guide for barrel heater bands (courtesy of Spirex) STYLE Mica Ceramic Mineral Insulated Tubular , Cast Aluminum , , Cast Water Cooled Cast Air Cooled Ceramic Air Cooled INSULATION Plate Mica Cordierite Steatite Silicon Carbide MGO MGO MGO MGO MGO Steatite MAX. TEMP. 900 F 1400 F 1400 F 1200 F 650 F 650 F 650 F ,, 1200 F MAX. ADVANTAGES WSl LOW cost, 50 Versatile High temperature, 50 Flexible, Energy efficient 230 High temperature, Response time 100 Durability 35 Uniform heat 35 Efficient cooling 35 Durability, Cost 1 50 Cost, High temperature DRAWBACKS , Low temperature Prone to contaminants , Cost, Versatility, Energy efficiency Energy efficiency Cost, Low temperature , Cost, Water leaks, Scaling ,, Cost Cooling, Efficiency MGO = magnesium oxide Information on dies and process control is in Chapter 3. Different control systems are used to process the different extruded products. Simplified examples of different controls are provided in Figures 5.3 and 5.4. 5 9 Extrusion 235 Figure 5,3 Blown film control Extruder type/performance The popularly used single-screw and multi-screw types have their differences. Each has its benefits, depending on the plastic being processed and the products to be fabricated. At times their benefits can overlap, so that either type could bc used. In this case, the type to be used would depend on cost factors, such as cost to produce a quality product, cost of equipment, life cycle of equipment, and cost of maintenance. In the past with the development of single-screw extrusion techniques for newer TP materials, it was found that some plastics with or without additives required higher pressures (torque) and needed higher tempera- 236 Plastic Product Material and Process Selection Handbook Figure 5~4 Sheet line control turcs. Thcrc was also the tendency for thc plastic to rotate with the screw. The result was degraded plastics. The peculiar consistency of some plastics interfered with the plasticators feeding and pumping process. The problem magnified with bull~ materials, also certain typcs of emulsion PVC and HDPE, as well as loosely chopped PE film or sticl~ pastes such as PVC plastisols. In the past twin and other multi-screw extruders were developcd to correct the problems that existed with the single-screw cxtrudcr. Later the single-screw designs with material dcvclopmcnts practically elimi- nated all their original problems. The conveyance and flow processes of multi-screw extruders are very different from those in the single-screw extruder. The main charac- teristic of multi-screw extruders include: 1 their high conveying capacity at low spccd; 2 positive and controlled pumping ratc over a wide range of temperatures and coefficients of frictions; 3 low frictional (if any) heat gcncration which permits low heat operation; low contact time in the extruder; relatively low motor-power requirements self-cleaning action with high degree of mixing; 6 very important, positivc pumping ability which is independent of the friction of the plastic against the screw and barrel which is not reduced by back flow. 5. Extrusion 237 Even though the back flow theoretically does not exist, their flow phenomena are more complicated and therefore far more difficult to treat theoretically than single-screw flow. Result has been that the machine designer has to rely mainly on experience. Although there are very few twin-screw (TS) extruders in comparison to the many more single-screw extruders, they are used also to produce products such as window and custom profile systems. Their major use is in compounding applications. The popular common twin-screw extruders (in the family of multi-screw extruders) include tapered screws or parallel cylindrical screws with at least one feed port through a hopper, a discharge port to which a die is attached, and process controls such as temperature, pressure, screw rotation (rpm), melt output rate, etc. ~43 Twin-screws with intermeshing counter-rotating screws are principally used for compounding. Different types have been designed that include co-rotating and counter-rotating intermeshing twin screws. The non- intermeshing twin screws are offered only with counter-rotation. There are fully intermeshing and partially intermeshing systems and open- and closed-chamber types. In the past major differences existed between co- rotating or counter-rotating; today they work equally well in about 70% of compounding applications, leaving about 30% where one machine may perform dramatically better than the other. Similar to the single-screw cxtrudcr, the twin-screw extruder, including multi-screw, has advantages and disadvantages. The type of design to be used will depend on performance requirements for a specific material to produce a specific product. With the multi-screws, very exact metered feeding is necessary for certain materials otherwise output performance will vary. With overfeeding, there is a possibility of overloading the drive or bearings of the machine, particularly with counter-rotating screw designs. For mixing and homogenizing plastics, the absence of pressure flow is usually a disadvantage. Disadvantages also include their increased initial cost due to their more complicated construction as well as their higher maintenance cost and potential difficulty in heating. The market for counter-rotating twin-screw (TS) extruders is basically dominated by two designs. One has cylindrical screws called parallel TS extruder and the other TS extruder is fitted with conical screws. Performancewise, the superiority of the conical principle to parallel does not only appear in the theoretical comparison, but in practice as confirmed by users. Flexibility of conical turns out an extrudate of consistent quality at both low and high output rates which are not sensitive to raw material fluctuations. It appears that the parallel have reached their efficiency limit unless a means of drastically increasing the 238 Plastic Product Material and Process Selection Handbook screw torsional strength is developed. Conical continue to offer what appears to be endless improved benefits through further development. An example of a conical extruder is Milacron's CM92 that is the world's largest of this design. It produces the highest output extruder for processing wood flour filled plastics. Depending on the flour-plastics ratio, output rate ranges from 1,000 to 1,800 lb/h. It uses a feed crammer to properly handle the low bulk density and fluffy wood flour. The tapered screw design that allows for a larger feed zone and applies a natural compression on the material during processing, results in the wood flour being more effectively "wetted out" by the plastic melt. The large diameter screws [184 tapering to 92mm (7.24 to 3.62in.)] with a 27:1 L/D ratio optimize feed zone surface area for faster, more uniform heat transmission from screws to material. Small exit diameter reduces rotational shear and screw thrust, while increasing pumping efficiency into the die. High torque at low speed of 34 rpm enables gentile plasticizing and a wide processing window. Critical to this extrusion process is maintaining consistent, controllable heating and cooling. It has five-barrel zones with a total heating capacity of 86 kW. Four of the barrel zones arc provided with cooling, using a heat-transfer fluid designed to dissipate heat. Six die zones (including entry adapter) are provided with maximum heating capacity of 4:5 kW. This extruder was designed with high output capacity in order to provide economic advantages in volume markets such as composite lumber, fencing, decking, windows, and doors. Operation Startup Machine operation can take place in three stages that go from startup to shutdown. The first stage requires operating the extruder for warm- up with operational settings of up-stream and down-stream equipment. The next stage involves setting the required processing conditions to meet product requirements at the lowest cost. The final stage is devoted to fine-tuning and problem solving the complete line. A successful operation requires close attention to many details, such as the melt quality, temperature profile adequate to melt but which does not degrade the plastic, production of a minimum of scrap, and procedures for startup and shutdown that will not degrade (or minimize) the plastic. Processors must also become familiar with troubleshooting guides. 143 [...]... pipe production consist essentially of a female die ring that shapes the pipe outside diameter, and a male mandrel that shapes the 253 2 5 4 Plastic Product Material and Process Selection Handbook inside diameter The difficulty is to support the mandrel in rigid and accurate alignment with the die ring without compromising the product The spider type uses three or four spider legs to support the mandrel... (Chapter 3) Checkup includes the careful handling of: (a) heater bands and electrical connections, (b) thermocouples, pressure transducers, and their connections, (c) inspect all machine heating, cooling, and ventilation systems to ensure adequate flow, (d) be sure flow path through the extruder is not blocked, 239 2 4 0 Plastic Product Material and Process Selection Handbook - - ... period when the plastic cools A metal cooling device (steel or brass sleeve, fiat sizing plates, etc.) can be used Cooling is provided by a water trough/tank and/ or a water cascade, except for those rigid plastics that can be cooled in air With certain plastics, water-cooling sets up internal stresses and gives a poor surface appearance Other 256 Plastic Product Material and Process Selection Handbook Figure... for use with certain plastics, such as nylon, to maximize their performance (toughness, stress relaxation, etc.) by using in-line annealing and moisture conditioning equipment Variations in Figure 5,t 6 Example of a wire coating extrusion line 262 Plastic Product Material and Process Selection Handbook extruder and plastic performances, such as melt index (MI), influence output and properties of the... melts, a two-roll stack for thinner sheet gauges, and others to meet diffcrent material requirements The function of a stack is to cool and polish the sheet Alternatively, an cmbossed roll may be used to impart a texture to the sheet surface Figure 5.12 Schematic of a three-roll sheet cooling stack 252 Plastic Product Material and Process Selection Handbook Stack rolls are usually of double shell design,... operations arc made at rates people can handle The process is very slow compared to standard operating speeds The puller starts its movement at just about the same speed the person has been pulling or therc may be a pile-up or tear-off of melt at the die That will usually mean threading up again 242 Plastic Product Material and Process Selection Handbook Skill on the part of the person will involve pulling... buttons and the line is set-up in 41/2 minutes 476 Flat Film Flat film identifies cast film Other names used include chill roll film, roll cast film, slot cast film, water quench, water chill film, etc These cast film lines require dies that yield a wide range of diverse products Widths may range from less than 6 in (15 cm) to more than 33 ft 2 48 Plastic Product Material and Process Selection Handbook. .. under a pair of idler rollers in the bath and, for any given rate of extrusion, it is the rate of downstream haul-off that regulates film draw-down and finished thickness Sheet The thickness range of extruded sheet is normally between 0.010 and 3 in Generally, 0.500 in is the upper limit in the conventional range, 250 Plastic Product Material and Process Selection Handbook Figure 5.9 Simplified water quenched... In turn sheet can be classified as: 244 Plastic Product Material and Process Selection Handbook (a) intermediate sheet in the range of 0.04:0 to 0.250 in (0.01 to 0.06 ram); (b) thin gauge sheet up to 0.060 in (0.015 mm); (c) heavy gauge sheet at 0. 080 to 0.500 in (0.02 to 0.13 mm) Most commercial plastic films are produced with a thickness of less than 0.005 in and most packaging films are less than... minutes Skill on the part of a good operator is very evident at startup Cooling the extrudate during hand pulling is important It gives strength and form stability to the extrudate Without cooling, the melt will string out and pull apart The steadiness of pulling and the evenness of cooling determine what the hand-pulled product will look like and how easily it can be threaded and fed into the take-off . order to increase the homogeneity and temperature uniformity of the 234 Plastic Product Material and Process Selection Handbook melt. There are no moving parts and only a small increase in the. 232 Plastic Product Material and Process Selection Handbook water that is automatically adjusted via microprocessor-based temperature controllers providing uniform, efficient cooling. 8 High-performance. was found that some plastics with or without additives required higher pressures (torque) and needed higher tempera- 236 Plastic Product Material and Process Selection Handbook Figure 5~4