8.24

Extruder Controls G. A. PETTIT

(1972, 1985),

B. A. JENSEN M. L. BERNS

Extruder

(1995), (2005)

Flow sheet symbol

INTRODUCTION

Polymer Types and Characteristics

Integrated extrusion control requires more than controlling the extruder and associated equipment variables at appropriate set points. Extrusion control integrates all the functions an operator normally performs in the correct order at the proper time. This involves the complete integration of controlling, interlocking, sequencing, and scheduling of the extruder system with the other parts of the plant. An equally important consideration in the automation effort is the interfacing to allow the operator or the higher level plantwide control system to retrieve the necessary data and to provide the means to control the operations of the equipment up- and downstream of the extruder. Implementation of the various forms and levels of extruder automation is a continuing stepwise process. Achieving the benefits of extrusion automation is a matter of layering information functions and advanced supervisory strategies upon the basic regulatory and interlocking control strategies.

All lists of commercial polymers are incomplete owing to the rapid progress of polymer science and production. The common extruded products, such as polyethylene and polystyrene, extrude rather simply and are relatively stable, and their behavior is reasonably predictable. Polymers in the vinyl chloride group are not so stable at extrusion temperatures. If not promptly removed from the process, they decompose, emitting hydrogen chloride gas, which is very corrosive to metal. Consequently, dies for vinyl chloride are commonly chrome-plated and designed to eliminate pockets or crevices where the material may become entrained and decompose. An important characteristic of synthetic polymers that complicates their extrusion is sensitivity to shear rate, a phenomenon known as non-Newtonian behavior (Equation 8.24[1]). The relationship between apparent viscosity and shear rate is also dependent on temperature and must be considered when 4 extruder controls are considered. Certain polymers, such as polyvinyl chloride (PVC) and acrylonitrile-butadiene-styrene (ABS), are much more sensitive to shear rate than are more crystalline materials, such as nylon and acrylics. This is especially true if color pigment additives are used. A wide variety of additives or fillers are available to reduce or eliminate various types of degradation (thermal, mechanical, chemical, environmental). Additives such as UV stabilizers, antioxidant, processing aids, flame retardants, lubricant concentrates, and impact modifiers are normally used in very small quantities. Other additives include reinforcing materials such as fiberglass and, of course, pigment additives for color. Fillers include mineral fillers, such as talc and CaCO3 plus mica, which are added to improve physical properties and function as low-cost extenders.

Extruder Production Rate The extruder output flow is a function of the screw speed, plastic viscosity, screw leakage, pressure build-up, and so on. Assuming an uninterrupted resin supply and negligible screw overflight leakage, the relationship between the production rate of the extruder and the above listed factors is as follows: Q= where k1 = N= k2 = P= m=

k1N − k2 P µ

8.24(1)

constant, based on geometry of the last screw flight screw speed constant, based on geometry of melting zone pressure of the melt at the nose of the screw viscosity of plastic

Equation 8.24(1) shows that the output of the extruder is proportional to the speed of the screw and that an increase in output pressure reduces output flow rate. 1932 © 2006 by Béla Lipták

COMPONENTS OF THE EXTRUDER SYSTEM The extruder itself is the main component in an extrusion process. It is composed of a drive motor, gear box, screw, barrel zones, and die. On older systems, the screw drive motors are generally DC drives, while on newer systems

8.24 Extruder Controls

Hopper #1

Hopper #X

Feeder #1

Exhaust fan and vacuum pump

Feeder #X

Lube oil pump Main drive

1933

Gear box

Pelletizer/ dicer

Extruder Dryer

Air

Screener

Water bath Packaging bins

Main drive cooling fan

FIG. 8.24a Extrusion flow diagram.

variable-frequency AC drives or open-loop vector-controlled AC drives are more often used (see Section 7.10 for details on variable-speed drives). The most important component in the compounding process is the screw-type extruder. In addition, an extruder system consists of several other pieces of equipment, including: blenders and feeders on the inlet of the extruder and coolers (water baths), pelletizers, tube cutterpullers, shears, and stackers on its outlet side. In addition, the system includes such auxiliary equipment as fans and vacuum pumps. An example of an equipment layout is shown in Figure 8.24a. Blenders The resin feed usually is blended with different additives such as pigments and fillers before it is fed into the extruder. The blenders can be volumetric and gravimetric. Both types consist of a resin station, one or more dosing stations, the mixer, and a controller. The feed rate of a volumetric feeder typically ranges from 3 3 3 to 1500 ft /hr (0.085 to 42.5 m /h) and is determined by the gear ratio and the diameter of the metering screw. The associated microprocessor-based controller performs the following functions: 1) allows the operator to create, store, and recall recipes, 2) controls the running time of the dosing stations, 3) controls the running time of the agitator, 4) maintains the level of the material in each dosing station, 5) displays the status of the operation, and 6) warns the operator if abnormal conditions occur. Figure 8.24b provides a block diagram of the volumetric blender. A gravimetric feeder measures the actual weight of each additive before loading it into the mixer. The rate of a gravi-

© 2006 by Béla Lipták

metric feeder typically ranges from 0.55 lb/hr to 16,500 lb/hr (0.25 kg/h to 7500 kg/h). A microprocessor-based controller performs the same control functions on a gravimetric blender as was described for the volumetric blender. Blender manufacturers use the same controllers for both types of feeders. Figure 8.24c provides a block diagram of the gravimetric blender. Cooling System When the molten extrudate exits the die, cooling is required to solidify the product. Different types of cooling systems are used, including: 1) water baths with vacuum sizing, which are used in pipe and tube manufacturing, 2) cooling rollers, which are used in sheet manufacturing, and 3) water baths, which are used in coating processes, including wire and cable covering. The vacuum sizing tank system consists of the tank, which is under vacuum, a water reservoir, water, and vacuum pumps. The water pump circulates the water between the water reservoir and the vacuum tank through entrance and exit rings. The continuous circulation of the water ensures adequate cooling of the extrudate. The molten extrudate enters the vacuum sizing tank through an entrance ring. The diameter of the entrance ring determines the size of the pipe or tube. The difference between the air pressure inside the tube or pipe and the vacuum outside it determines the inside diameter of the tube or pipe. The vacuum pressure in the tank can be controlled manually or automatically. (For a description of vacuum gauges and vacuum transducers, refer to the first volume of this handbook.) When the extruded sheet leaves the die it passes through an air space of 3–10 ft (1–3 m). Here, additional cooling is

1934

Control and Optimization of Unit Operations

Resin Pigment Resin dosing control Resin level Pigment level Pigment dosing control Agitator control

Controller

Mixer level

Mixer

Extruder hopper level

Extruder

FIG. 8.24b Block diagram of a volumetric blender and its controls.

provided by a set of cooling fans, which are placed above and below the sheet. In some applications, water-cooled rollers are also used to provide adequate cooling of the material. When covered wires and cables are manufactured, they are cooled by passing through a cooling trough. To reduce the length of the trough and to maintain the high speed of production, it might be necessary to have several passages inside the trough.

Cutters Depending upon the end product of the extrusion process, a cutting machine can be used to cut the extrudate strands into shorter lengths. Pelletizers and Dicers Depending upon the type of the extrudate, a pelletizer, dicer, or some other kind of cutting machine is used to cut the strands into shorter lengths. Pelletizers

Resin Pigment

Mixer Load cell

Resin weight Resin dosing control Resin level Pigment level Pigment dosing control Pigment weight Agitator control Mixer level Extruder hopper level

Extruder

FIG. 8.24c Block diagram of a gravimetric blender and its controls.

© 2006 by Béla Lipták

Controller

8.24 Extruder Controls

generally cut spaghetti-like strands into small pellets suitable for packaging in bags or boxes. Dicers are used to cut sheets into strips suitable to be wound up into rolls, or they can also be left flat. Control of the pelletizers and dicers primarily involves the control of their speed so as to maintain the size of the pellets and the ratio of the pelletizer and feed speeds constant. The speed ratio is achieved by feedforward control of the pelletizer/dicer speed based upon the dynamically compensated master feed rate to the extruder. Pullers-Cutters Cutter-pullers are used in the production of pipes and thin-wall tubes. As their name implies, they pull the extrudate through a vacuum sizer/cooler and cut the tube or pipe to the desired length. The speed of the puller is set by a slave controller to correspond to the speed of the extruder. Reciprocal movement of the cutting head is synchronized with the linear speed of the sleeve or pipe, and at a preset length, a blade cuts the sleeve and retracts. Then the cutting head moves back by the distance equal to the required tube or pipe length. Auxiliary Equipment Exhaust fans and vacuum pumps are some of the auxiliary equipment required to remove fumes from the extruder. Because during compounding a variety of gaseous by-products can form, special vent sections are provided in the extruder to allow the removal of these gases. Venting is also required to remove the vapors generated by moisture removal and devolatilizing. A decompression chamber is incorporated at the point of venting so that the process material does not extrude out through the vent port. When the processing involves the sending of strands through a water bath, often these strands are sent through a vacuum system to dry them. Screeners are used to sift pellets, ensuring common diameters and lengths.

EXTRUDER TYPES AND SUBSYSTEMS Single-Screw Extruders Single-screw extruders usually convert granular resin feeds into sheets, films, pellets, and shapes such as pipe. These extruders are described by their screw diameters (in inches or millimeters) and by their L/D ratio, L being the screw length and D the screw diameter. Single-screw extruders are available in almost any size imaginable. Common sizes are 5/8, 3/4, 1, 11/2, 21/2, 31/2, 41/2, 51/2, 6, 8, 12, 15, and 20 in. (16, 19, 25, 37.5, 62.5, 87.5, 112.5, 132, 150, 200, 300, 380, and 500 mm). L/D ratios range from 5:1 to 48:1, with 20:1 to 30:1 being some of the common choices. The single-screw extruder is by far the most

© 2006 by Béla Lipták

1935

frequently used design; however, the twin-screw extruder is 1 becoming even more popular. Twin-Screw Extruders Machines using twin screws are generally large-volume production units used for resin pelletizing in petrochemical plants. They are equipped with various combinations of intermeshing and nonmeshing screws that can be either the corotating or the counter-rotating variety. The following types of processing can be performed in a single machine: 1) melting, 2) mixing and blending, 3) homogenizing, gelling, and dispersing, 4) reacting, 5) pumping, 6) compounding and formulation, 7) devolatilizing and degassing, and 8) drying. Twin-screw machines are often melt-fed directly from polymerization reactors and perform multiple functions on the polymer prior to pelletizing and packaging it as a finished product. The twin-screw extruder is normally selected as the solution to many compounding and reactive extrusion tasks. Twin-screw extruders can be either intermeshing or nonintermeshing. Nonintermeshing extruders behave like two singlescrew extruders with only minor interactions between the two screws. A further subdivision of twin-screw types is the direction of rotation. Co-rotating extruders have both screws rotating in the same direction, and therefore the material is exchanged between the screws, while counter-rotating screws transport the process material through the extruder in a figure eight channel. Compounding requires that the resin be melted and homogenized while incorporating additives or fillers at a given shear level. The key is the isolation of the high, medium, and low shear sections along the screw length, as well as the feeding of additives at the appropriate points. The advantages of twin screws include the capability for mixing, dispersion, and heat control in addition to efficient conveying. Extruder Dies and Barrels Die Types The shape and the ultimate use of the extruded product are defined by the die shape. Dies are broadly classified as follows: 1. Sheet dies for extruding flat sheets, up to 120 in. (3 m) wide and 1/2 in. (12.5 mm) thick. 2. Shape dies for making pipe, gaskets, tubular products, and many other designs. 3. Blown film dies, using an annular orifice to form a thin-walled envelope. The diameter of the envelope is expanded with low-pressure air to roughly three times the annular orifice diameter to form a thin film. The process is used for films up to 5 mils thick (0.005 in., or 0.125 mm) at the upper limit. 4. Spinnerette dies for extrusion of single or multiple strands of polymer for textile products, rope, tire cord, or webbing.

1936

Control and Optimization of Unit Operations

L

Gear box

Hopper

Die

Extruder barrel

D

Feed section

Compression section

Metering section

Drive Motor

FIG. 8.24d Typical extruder screw.

Control zone Zone temperature sensor

Band type heaters

Typical single screw extruder

FIG. 8.24e Single-screw extruder with band heaters.

5. Pelletizing dies for granular products in resin production, synthetic rubbers, and scrap reclaiming. These dies form multiple strands roughly 1/8 in. (3.125 mm) in diameter. Rotating knives continuously cut the strands in short lengths, after which the pellets drop into water for cooling. 6. Cross-head dies for wire coating, in which the bare wire or cable enters the die and emerges coated with semimolten polymer. The wire enters and leaves the die at an angle of 90º to the extruder axis. Many special configurations of extruder dies are used to produce composite films. In such designs, two polymers enter the die from two extruders and exit as a sheet. The top and bottom polymer layers are of different chemical composition so that one might obtain a film or sheet of two colors or to utilize the other desirable characteristics of both materials. Dies for rigid foam production are similar to blown film dies. Special dies with moving parts can continuously extrude netting. Most dies require a short connecting pipelike piece, commonly called the adaptor, which connects to the extruder head. Also, twin-screw extruders require an additional special adaptor piece called an “eight-to-oh” (8/O) zone to convert the twin configuration (which physically looks like a figure eight lying on its side) to an output configuration of the adaptor (zero or the letter “O”). A typical extruder screw is shown in Figure 8.24d.

is determined by the type of feed, the screw type, and the resin feed location. External heating of the barrels can be provided by electric resistance heaters and by hot oil systems. Band-type silicon control rectifier (SCR) resistance heaters are of two-piece construction to facilitate removal. Another type of electric heating is inductance heating, which is accomplished by coiling a copper wire about the barrel to induce an electromagnetic field. This technique is more responsive than resistance-type heating. The coil is energized with 60 cycle current and is usually controlled by contactors. After extrusion has begun, heat is internally generated from the friction and shear of the rotating screw. This heat is a function of the square of the screw speed. The amount of heat generated is also influenced by screw design, head pressure, and resin viscosity. In some zones, the melt temperature may rise above accepted maximum, and barrel cooling is required. Cooling Systems The barrels are commonly cooled by fans (Figure 8.24f) or by a water-cooled jacket. Many extruders use aluminum heating shells with casting heaters of encased nichrome coiled

TIC

Barrels and Heater Rings The barrel of an extruder is usually divided into roughly 15–18 in. (375–450 mm) long temperature control zones. A 41/2 in. (112.5 mm) extruder, for example, may have four to six barrel zones. The number of zones depends upon the L/D ratio, the number of feed points for additives, and on the type of material being extruded; 4–6 zones is most common, but it is not unheard of to have 12–15 zones (Figure 8.24e). Conventional temperature control loops include a power control device, temperature sensor, and heater for each zone. Adapter, 8/O, and die zones may all be provided with temperature controllers. Extruders require large heater ratings to decrease heating time. A typical barrel zone electrical heater can be rated anywhere from 1.2 to 11 kW and greater. The size generally

© 2006 by Béla Lipták

Electric power supply

Power supply

TSH

S

S

Extruder barrel

Automatic M

Off On

FIG. 8.24f The extruder barrel is provided with fins and with temperature controls using electric heating and cooling by air fans.

8.24 Extruder Controls

Temperature sensor

TIC

Electric power supply

TSH

S

1937

the water from the passages and to eliminate trapped fluid, which could cause erratic cooling. Cooling water is often injected in very short pulses, which are followed by the immediate removal of the water to reduce cooling. Running the exchanger sump at higher temperatures also reduces the severity of water-cooling.

Heater terminal

Solenoid valve

SENSORS, VARIABLES, AND THEIR CONTROL End view showing split heater with supply and drain connections Visible drain TCV

Drain Heat exchanger sump

Cooling water

FIG. 8.24g The extruder barrel is provided with fins and with temperature controls using electric heating and cooling by water.

throughout the aluminum jacket. The shells are constructed in two segments clamped together to surround the extruder barrel. Shells for fan cooling have fins cast to the outside surface to increase their surface area. Motor-driven fans or blowers are positioned directly below the zone, and when switched on by a high-temperature switch, they cool the barrel by forced convection. A zone fan on a 21/2 in. (62.5 mm) extruder can typically remove the equivalent of 5 kW per hour. Water-cooled extruders are provided with aluminum shells with cast-in tubing as well as heaters. Treated water is continuously circulated through the coils by a common pump and heat exchanger (Figure 8.24g). Solenoid valves, regulated by the zone temperature controller, allow circulation to each zone in order to maintain the proper barrel zone temperature. The solenoid valves can be operated by an auxiliary high-temperature switch, which is part of the zone temperature controller. The switch is designed to operate in a timeproportioning manner with extra slow cycle rate and capability for very short pulses. Water-cooling can be too effective compared with fan cooling. The cooling water is usually flashed into steam at most barrel temperatures used for processing thermoplastic materials. Some machine builders use compressed air to clear

SP

PID loop calculations

FIG. 8.24h Block diagram of a time-proportioning controller.

© 2006 by Béla Lipták

Temperature Measurement Accurate temperature measurement is very important for efficient extrusion and overall quality of the end product. The most common extruder temperature sensors are thermocouples (TCs) and resistance temperature detectors (RTDs). RTDs detect the change in their electrical resistance, which is proportional to their temperature, while thermocouples produce a millivoltage output, which is related to the temperature of the TC’s junction. A detailed discussion of all temperature sensors is provided in Chapter 4 of the first volume of this handbook. Temperature Control The zone temperature of extruders is usually controlled by time proportioning PID controllers. In such a control system, the on-time of the electric heater is modulated by the PID controller, while the total cycle period (usually 8–10 sec) remains fixed. In some systems each zone is provided with its own single-loop controller, while in others a programmable logic controller (PLC) executes the PID algorithms in sequence. Time-proportioning control is less expensive than continuous modulation, because it allows the use of electromechanical or solid-state contactors. A block diagram of the time-proportioning controller is shown on Figure 8.24h. Duration of on-time period of the final control element is a function of the continuous output of the PID controller. Therefore, if the output of the controller is 40% and the cycle period is 10 sec, then the on-time period of the solenoid or contactor is 4 sec. The output signal generated by the time-proportioning control system in response to a continuous PID controller output is illustrated in Figure 8.24i. Temperature control is complicated by several factors. These include heating caused by the shearing action of the screw, changes in the feed rate, conduction of the heat along

Time proportioning

Process

PV

1938

Control and Optimization of Unit Operations

Sampling time

Continuous

Output

in the 4th edition of Volume 1 of this handbook.) The major problem is to maintain an optimal temperature profile, which is a function of the dynamics of heating, cooling, shear heat, and conduction. Adaptive tuning and the use of dead time PID control can improve performance. Pressure Measurement

On

Off

FIG. 8.24i Illustration of the continuous output signal from a PID controller (top) and the corresponding output generated by a time-proportioning controller (bottom).

the barrel, and the difference between the process gains that exist during heating and during cooling. An increase in screw speed increases the shear in the screw channel and the mechanical work, which raises the resin temperature. At the same time an increase in screw speed and output flow rate reduces the amount of cooling, because the resin spends less time in contact with the same heat-transfer surface. If the operating pressure is low, due to a low-resistance die size, the amount of frictional heat developed may not be sufficient to compensate for the decrease in conducted heat, and therefore, the resin temperature will drop as the screw speed increases. At higher pressures, the temperature may show an initial increase before dropping off. At still higher pressures (heavy screen packs or small dies), the same screw and operating conditions can result in an increased temperature with increasing screw speed, because the mechanical 2 heating effect exceeds the decrease in conducted cooling. In some cases, as much as 80–100% of the heat produced throughout the extruder can be generated by screw shear alone. Effective temperature control requires accurate removal of heat as well as accurate application of the heat from the barrel heaters. Zone temperature control is designed to maintain a constant temperature in each barrel, adaptor, and die zone to achieve a desired profile and thus guarantee the consistent melting of the extrudate. Feeding and compression zones are most effectively controlled at relatively constant temperatures, because under such conditions they will provide the optimum wall friction for efficient feeding. Temperature can have a dramatic effect on both throughput and product quality. Good temperature control in the rear zones can increase the hourly output for some materials. On the other hand, some crystalline thermoplastics are relatively impervious to temperature variations of as much as 50°F. Zone temperature is usually controlled either by singleloop PID control loops or by PLC-executed PID controls. However, the installation and maintenance of the RTD-, thermocouple-, or thermistor-type sensors is critical. (See Chapter 4

© 2006 by Béla Lipták

The accurate knowledge of extruder pressure is very important for efficient extrusion. Chapter 5 of the first volume of this handbook provides an in-depth description of all the available pressure sensors. The commonly used extruder pressure sensors include grease-sealed gauges. These are direct-reading Bourdon tube gauges with a tube tip capillary for complete filling with hightemperature grease, usually silicone. The grease remains viscous at high temperatures and prevents the molten polymer from entering and solidifying in the gauge or piping. Disadvantages of this sensor include the need for periodic greasing and the occasional contamination of the product. Force-balance transmitters provide a linear output signal that can be sent to digital or analog pressure indicators, recorders, and controllers. Their force-balance operating principle makes them less sensitive to process temperature variations. Strain gauge-type pressure transducers are higher accuracy pressure sensors, and for that reason, they are widely used in extruder applications. Pressure Control Synthetic fiber processes frequently use an extruder to melt and transport nylon and polyesters to a bank of gear pumps feeding individual spinning die heads. The shear characteristics of these polymers are near-Newtonian and their viscosity is relatively constant at wide variations in shear rate. In such processes, the pressure is nearly proportional to screw speed, and therefore pressure can be controlled by the manipulation of the screw speed. In contrast, PVC can be pumped through a restriction orifice at increasing rates without an increase in the pressure drop across the orifice, because its apparent viscosity decreases with increased pumping shear rate. In this case, the pressure cannot be controlled by the manipulation of the screw speed. Figure 8.24j illustrates the control scheme used in fiber processes. Because some of the fiber spinning pumps frequently fail or are stopped intentionally, it is necessary that the pressure controller quickly respond to such events by immediately slowing the extruder screw to a new output rate determined by the number of constant volume pumps remaining in operation. In the past, screw drive motors were generally DC drives or eddy current clutches. Today, these drives are being substituted by variable-frequency AC drives. These drives can

8.24 Extruder Controls

Constant volume gear pumps

(Temperature control not shown)

Variable-speed motor

M

1939

DIC

NIP rolls

PT Motor speed control Panel

Inflated envelope (plastic tube)

HK PIC P10 Man/auto station

DT

FIG. 8.24j Extruder pressure control used in the production of synthetic fibers.

be modulated by either digital or analog control signals. The variable-frequency drives provide virtually instantaneous response, which is important to guarantee stable control. Depending upon shear characteristics, die head pressure can remain uncontrolled or can be controlled by either feed rate or screw speed. Synthetic fiber processes frequently use an extruder to melt and transport nylon and polyesters to a bank of gear pumps feeding individual spinning die heads. The shear characteristics of these polyesters are near-Newtonian and, thus, have relatively constant viscosity at wide variations in shear rate. This characteristic is responsible for the discharge pressure being nearly proportional to screw speed, which is necessary for pressure control by the regulation of the screw speed. In contrast, PVC can be pumped through a restricted orifice at increasing rates without an increase in back-pressure because its apparent viscosity drops with increased pumping shear rate. The pressure of such a process is nearly impossible to control by manipulating screw speed.

Extruder

Beta ray source

PI

Blow film die

PCV Air supply

FIG. 8.24k Film thickness control through nip roll speed manipulation.

Polymer viscosity variations of the feed constitute the most difficult aspect of extruder design, simulation, and control. The feed viscosity tends to have more influence on the extrusion process and on the dimensional quality of the pelletizing extruder output than all other variables. For the description of viscosity detectors, see Sections 8.62 to 8.64 in Chapter 8 in Volume 1 of this handbook. In tube and pipe production lines, the outer diameter, wall thickness, and concentricity are determining factors of produce quality. Outer diameter is set by the entrance ring of

Film Thickness Control Blown film, which is used in the production of tubular and flat film to 5 mils (0.125 mm) thickness, requires thickness measurement and control. Thickness can be adjusted by the speed of the take-up rolls if the thickness uniformly varies (in machine direction) across the film. Increasing the takeup speed reduces the film-gauge, while decreasing the speed increases its thickness (Figure 8.24k). Thickness variations in die direction (across the width) can be measured, but most attempts to automate sheet die nip adjustments have been unsuccessful. Sheet dies extrude materials up to 1/2 in. (12.5 mm) thick and 120 in. (3 m ) wide. Measurement of film thickness to 100 mils (2.5 mm) is made with radiation instruments (Figure 8.24l), using beta rays. Among design variations are scanning heads that measure and record thickness over the entire width of materials. Infrared, LVDT, laser, capacitance, ultrasonic, and mechanical devices have also been used. For details on these sensors, refer to Section 7.20 in Chapter 7 in the first volume of this handbook.

© 2006 by Béla Lipták

Wind up roll

Display meter, controls and amplifier ∆I Radiation detector

Material being gauged

Collimated radiation source

FIG. 8.24l Radiation absorption gauge.

1940

Control and Optimization of Unit Operations

the water tank and can be slightly adjusted by the speed of the puller. Concentricity can be adjusted by controlling the vacuum inside the vacuum tank. Wall thickness can be adjusted by controlling extrusion speed.

Advanced Control

EXTRUDER CONTROL SYSTEMS

Melt Temperature Control The output melt temperature is a function of the internal shear energy (converted to heat energy) plus or minus the conducted heat (barrel cooling), depending on the operation. The temperature of the melt is as important, because it influences the output rate for quality extrusion. The cascade control to reset the zone controller set point achieves continuous temperature control of the melt. 6 Figure 8.24m shows the arrangement for cascade feedback. One special feature is that the system allows only depression of the zone temperature controller set points, which is a safety consideration because of heat degradation and pressure build-up in polymer systems. A safety interlock with the extruder screw drive is usually incorporated in order to prevent both polymer freezing during shutdowns and drive damage at start-up. Zone controllers provide both heating and cooling, and their set points are regulated by the melt controller. Each

The overall extrusion control system serves more than controlling the extruder and associated equipment at appropriate set points. Extrusion control should integrate all the functions that an operator normally performs into a control system that keeps the steps of the extrusion process in the correct order at the proper time. This includes the complete integration of controlling, interlocking, sequencing, and scheduling the ex5 truder system with the other parts of the plant. An equally important consideration in the automation effort is the interfacing provided for the operator to retrieve data and to provide the means for control. Modern extruder control systems are based on programmable logic controllers or industrial computers to control temperatures and screw speeds. Using programmable logic controllers and networking offers systems that not only govern machine functions and parameters but also integrate them with upstream equipment, such as the feeders and mixers, and with downstream equipment, such as pelletizers, dryers, and conveyors. Basic Control Regulatory, discrete, and interlock controls are needed to operate an extrusion system. The typical basic control system consists of the several subsystems, including: 1) feeder mass flow rate control, 2) individual zone temperature control along the extruder barrel, 3) motor speed control, 4) die head pressure controls, and 5) auxiliary discrete devices for on/off operation of fans, pumps, pelletizers, and so on. Mass flow rate control is designed to allow the operator to set the overall mass flow rate to the extruder. Feed rate control for blending is implemented by setting mass flow rate set points for each feeder, based upon the individual percentages determined by the recipe, and multiplied by the overall mass flow rate. For a detailed discussion of mass flow meter designs, see Sections 2.11 to 2.13 in Chapter 2 in the first volume of this handbook. Main screw speed control is designed to maintain a reasonably consistent torque and specific energy of the extrudate at various feed rates. Implementation is based upon feedforward control from the master feed rate to the extruder screw speed, which is adjusted in a simple ratio. Extruders used in compounding tend to be run at top speed for extended periods and require only minor variations in RPM. Interlocks build upon regulatory and discrete control functions in order to provide additional equipment and personnel protection in the case of hardware failure. The most common interlocks control the main drive motor, lube oil pump, feeders, feed hoppers, and pelletizers/dicers.

© 2006 by Béla Lipták

Advanced controls include the cascade control of melt temperature, zone flux control, auto-tuning of temperature controllers, maximizing production, and extrudate quality control.

Tachometer generator on screw motor drive

ST

TY

SY

Drive system status interlocks

1

Proportional only zone controllers

Electric heater controls

2 or 3 mode melt temperature cascade master controller TIC

%

TY

Σ

SY

1

Set point

TIC

%

TY

Σ

SY

1

Set point

TIC

%

Σ

Set point

TIC

Control signal to electric heater Coolant

SCR

Coolant SCR

Feeding

Coolant

TIC

SCR

Compression Metering

Extruder barrel zones

FIG. 8.24m Melt temperature cascade system.

Adapter

Die

8.24 Extruder Controls

zone temperature set point can be adjusted individually to follow a certain percentage of the feedback cascade signal so that a preset program of zone depression will follow a definite barrel temperature zone profile curve. The extruder metering zones are capable of the greatest heat transfer and, therefore, receive the greatest percentage of feedback and most set point depression. Feeding and compression zones are most effective at a relatively constant temperature to provide optimum wall friction for most efficient feedings; therefore, they usually receive a small percentage of the feedback signal. The melt controller 7 must have proportional and integral control actions with rate action being beneficial. Some cascade systems also incorporate a tachometer feedforward signal to supply set point depression proportional to screw speed. This function reduces temperature departures from the control point caused by screw speed drift or by intentional speed changes. Flux Control Zone flux control is designed to maintain the desired temperature at all points throughout the extruder so that a consistent melt of the extrudate is achieved, which is the same objective as that of basic zone temperature control. Flux controllers are used to include feedforward control from feed rate and the drive motor screw. Flux is defined as the heat per unit area in the extruder zone. It is the energy due to the heating elements, plus the energy due to the screw shear, minus the energy removed by cooling, and minus the energy required to heat and melt the resin. These factors must be dynamically compensated and applied to the measurement of the flux controller. Both the motor and the feed energy terms provide feedforward responses to “load” disturbances. These disturbances are generally not significant during normal operation but become important during start-up and shutdown. This is because as the screw starts, the motor power begins to rise, which increases the flux controller’s measurement. In response to this, the flux controller calls for a decrease in heat, resulting in a decrease in output to the heating element or an increase in the rate of cooling. As feed is added to the extruder, the heat required to warm and melt the feed results in a decrease in the flux controller’s measurement. In response to this, the flux controller’s output increases, calling for more energy to be put into the zone. Therefore, the control algorithm must include dynamic terms, which have to be field-tuned. Figure 8.24n illustrates the control block diagram for zone flux control. Auto-Tuning Maintaining the extruder barrel temperature profile is the most important factor in controlling the quality of the final product. Therefore, the temperature control loops must be tuned to ensure their optimal performance. Loop tuning is performed by initiating a small change of the set point (usually 10%) and adjusting the gain of the control loop to achieve the fastest response (see Figure 8.24o). For an in-

© 2006 by Béla Lipták

1941

depth discussion of PID controller tuning, refer to Section 2.35 in Chapter 2. Most PLCs and microprocessor-based single-loop controllers have an auto-tuning feature that implement the ZieglerNichols method of tuning. Adaptive auto-tuning is designed to continually update the PID-type zone temperature controller’s tuning constants in response to process gain variations caused by load variations. There are three methods of adaptive auto-tuning: Programmed adaptive auto-tuning is based on automatic adjustment of the proportional gain as a function of changes in process dynamics. For example, the proportional gain of the controller can be affected by changes in the controlled variable, in the set point, and so on. The programmed adaptive algorithm can be set for independent gain adjustment using any combination of the process-related variables. Model-based auto-tuning uses an internal model of the process to determine the optimal PID settings. In this case, the controller introduces step changes above and below the set point and observes reaction to these changes. The PID settings are then calculated according to predetermined criteria. The pattern recognition method continuously examines the response of the process to naturally occurring disturbances such as set point or load changes. When the resulting error exceeds some threshold value (controller looks for peak values) and after several peaks (usually two or three), the controller determines the period of oscillation that is related to the dead time of the loop and, based on it, calculates the new PID tuning constants. See Chapter 2 for a general discussion of control theory and specifically for process modeling and controller tuning. Maximized Feed Rate Control Figure 8.24p illustrates a master feed control system that is designed to maximize the throughput of resin to the extruder by increasing the feed rate until a constraint is reached. The master feed rate controller set point is constrained by the outputs of a number of limit controllers, which are sent to a select network. The output of the selector is the lowest of any of the constraint controllers. The output of the low select then becomes one of two inputs into a high selector limit. The other input is the minimum feed rate setting of the extruder. The resulting output is the cascade master’s feed rate set point for the feed hopper flow controllers. A set point for minimum feed rate is included in order to have a low limit clamp. The constraint variables normally include the die head pressure, drive motor amps (or motor torque), extruder throat or chute level, maximum feed rate, and minimum feed rate. Many compounding extrusion processes can take advantage of a maximum feed rate control because of excess upstream storage capabilities. However, when feed to the extrusion system is from some other continuous process, this type of control may not be possible because of limited feedstock availability. The ultimate test of the quality of any control system is the quality of the extrudate. Quality is affected by many

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Control and Optimization of Unit Operations

Melt temperature set point

RTIC PIC A M + + Σ

Melt temperature

Dead time

Motor speed

Output to heater SCR power rating Motor energy (AMPS) K Cooling water valve position K Feed rate K

CALC

CALC

Dead time Dynamics

CALC

CALC

+ _ _

Σ

+ BTU/HR (KW) R

Dead time Dynamics

Heat flux set point L

Zone area

PID

BTU/HR/FT2 (KW/M2) Split range

CALC

CALC

AMPS out

H2O out

FIG. 8.24n Heat flux control.

parameters, including feed mixing, barrel temperatures, melt temperatures, screw speed (shear rate), enthalpy (from external sources), pressure, and rheological conditions. On-line rheometers, machine vision analyzers, or other specialty

instruments can be used to measure certain extrudate properties. They are becoming more numerous and more reliable. Quality indicators measured by these instruments include such properties as melt flow index, viscosity, gels, color,

Over-damped response

10%

SP

Critically damped response

PV Under-damped response

FIG. 8.24o The response of a PID loop as a function of narrowing the proportional band (increasing the controller gain). The cycling curve corresponds to a high gain setting, the critically damped curve to the correct setting, while the overdamped curve to a low gain setting.

© 2006 by Béla Lipták

8.24 Extruder Controls

Maximum die head pressure set point

Die head pressure

Maximum motor AMPS set point

PIC PID A M

Maximum feed rate set point

High chute level set point

FIC PID A M

LIC PID A M

JIC PID A M

1943

Master feed rate Motor AMPS

Low selector

Chute hopper level High selector

Limits L R

Feed hopper #1 percentage

Feed hopper #1 flow rate

Master feed rate set point

Feed hopper “N” percentage

X R

Minimum feed rate

L

FIC PID

Feed hopper #1 set point

X R

L

Feed hopper “N” set point

FIC PID

Feed hopper “N” flow rate

FIG. 8.24p Optimized control system, serving to maximize the feed rate to the extruder while obeying limit constraints.

agglomerates, and excess ingredients, which are all described in Chapter 8 of Volume 1 of this handbook. A combination mass flowmeter and viscosity detector is illustrated in Figure 8.24q.

Pendulum

FIG. 8.24q A pendulum mounted on a single Coriolis tube generates both torsional and lateral oscillations, which allow for the simultaneous measurements of mass flow and viscosity.

© 2006 by Béla Lipták

A measurable, repeatable value can be used as feedback to one of the above-listed quality-related parameters. Calculations determining extrudate properties can also be used. Choosing a manipulated variable from the above list is challenging because of the lack of sensitivity and the limited number of degrees of freedom available. One method of controlling the quality of the extrudate is the manipulation of the feed ratio, i.e., the mixing of a particular feed resin with another resin of different properties while all other parameters are held constant. Model-based control or multivariable control can also be used to control extrudate quality, but in most polymer processes the majority of quality parameters of the feed can only be corrected at the reactor and not at the extruder. The large dead time of the process can also render feedback control impractical. Engineering Calculations Increased capabilities of modern digital controls allow engineering calculations to be displayed on the human-machine interfaces (HMIs) for the operator. The displayed variable trends include specific energy, extruder motor speed, torque, and current. Additional parameters such

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Control and Optimization of Unit Operations

as actual zone temperatures and deviations, equipment run times, and other values are also displayed on auxiliary screens. One such calculation is the physical property known as melt factor or melt flow index (MFI) of the extrudate. Another calculation could include viscosity or other indexing properties. The quality calculation is normally computed as a continuous on-line value. Because the laboratory result of the sample is not available until some time after the sample was taken, recording the current conditions allows a timesynchronized comparison of the laboratory result with the calculated value. The process conditions are recorded at the time that the sample is taken. The conditions from which MFI is normally calculated are die pressure, melt temperature, and mass flow rate. However, feed mixing, screw speed, and rheological conditions are other nonmeasured parameters affecting extrudate properties. Another useful calculation is specific energy of the extruder at the die head. The power input, whose units are horsepower (watts), is multiplied by an efficiency constant and by a constant conversion factor. This value is divided by the total feed rate to give a result in the units of hp/lb (kw/kg). This property relates to the shear rate and, thus, to the viscosity of the resin. Torque and percent maximum torque are important operational parameters for the extruder. A linear relationship can be assumed to exist between the motor current (in amps) and screw speed (in RPM). A constant voltage is assumed, allowing power to be calculated from motor amps. Percent maximum torque is also calculated from the current and screw speed. The current torque measurement is divided by the maximum current rating and maximum drive speed, and is expressed in percent. Equipment run-time numbers are generally of interest to maintenance personnel. The number of times the main drive motor has been started, the percentage up-time since initialization, total run time, and percentage up-time can be calculated for each extruder. The cutting time and the number of times each pelletizer/dicer has been started gives maintenance personnel information on when cutting blades should be sharpened or when the equipment should be overhauled. Sequences Sequences are automated so that step-by-step procedures control the routine activities. Included in this category are heater/contactor check, heat-up, start-up, shutdown, and alarm disabling/enabling. Heater/contactor checks provide the ability to test proper operation of the heater/contactors and extruder thermocouples, for those heating systems that are using electrical heating elements. A heater/contactor is suspected to be “bad” whenever the maximum output is sent to the zone and the expected amperage read back is not within a certain percentage of the expected value. The problem is either a bad contactor or bad heater legs. Extruder heat-up provides the ability to heat the extruder to preset values in a series of steps and to display the duration of the heat-up. This results in a heat-up profile in accordance

© 2006 by Béla Lipták

with the manufacturer’s specifications in order to minimize thermal stress on the equipment. Extruder start-up provides the ability to bring the extruder equipment on-line from a heat-up condition to preset values in a series of steps. The procedure checks system interlocks, starts auxiliary equipment, and ramps the extruder speed and feed rate to their preset recipe values. In addition, the appropriate alarms are enabled as the sequence progresses. Once the feed material is ready and a heat-up of the extruder has been performed, the extruder can be started up in an orderly stepwise manner. Extruder shutdown sequencing provides the ability to bring the extruder equipment off-line from a running condition in a series of steps. The procedure turns off the feeders, decreases screw speed in a preset manner, ramps the pelletizer speed to a minimum value, and shuts down the main drive and auxiliary equipment. Appropriate alarms are inhibited as the sequence progresses. In addition, a “panic button” is normally provided for emergency shutdown. This button cuts all power to the line, resulting in immediate shutdown of the extrusion process in case of emergency. The enabling and disabling of alarms is automated to reduce the frequency of unnecessary signals. Normally this function can be performed by the start-up and shutdown procedures. Whenever a unit is being started up, alarms need to be enabled to warn of abnormal occurrences during normal operating conditions. Similarly, when a unit is being shut down, certain alarms need to be disabled or inhibited to prevent alarms from actuating, because it is normal that during shutdown a number of alarm thresholds will be violated as equipment is being shut down. Integrated Control Management information control is a higher level automation activity. Timely information provided to engineers and management allows them to better evaluate performance and maintain consistency. Integrated control includes those additional steps that serve to optimize the overall utilization and performance of the extruders. Included in this category are line scheduling, lot history, recipe management, and statistical process control and statistical quality control (SPC and SQC). Line scheduling provides the ability to optimize extruder availability. Optimizing extruder availability maximizes overall throughput. An individual extruder, chosen from a set of many extruders, may need to be made available to run a particular type of raw material to produce a certain grade of product. Factors affecting the scheduling of a single extruder line includes the amount of material to be run, availability of raw material, grade of material, color of material, size of extruder (screw diameter and number of barrel zones), extruder configuration (single or twin screw), and the availability of auxiliary equipment. Lot history is a function that captures all the information associated with the process of extruding a resin product. This is analogous to batch tracking in a chemical plant. The information associated with extrusion of a resin product is normally

8.24 Extruder Controls

categorized according to a lot number. Select process alarm information and process conditions are captured and stored as part of the production history on file. Quality standards within the batch industry mandate the reporting of all activity associated with a batch run. Recipe management maintains a database of master recipes for various products, formulas, and procedures. Specific information for the extrusion equipment and extrusion lines that can perform the required operations are contained within the recipe. The recipe is selected and accessed according to the relationship of the material to be processed with a specific extruder line. The recipe management function provides the ability to automate the start-up and normal operation of the extruder. Set points stored from previously run recipes and formulas are useful, because they store the relationship to the grade of materials that were processed in the past and can be inserted into the controllers for new runs on the same extruder and associated equipment. These set points include barrel zone temperatures, feed rates, and screw speed. Statistical quality control provides a history of quality data for the extrudate. This process flags statistically improbable events in real time, evaluates process performance, and validates newly installed operating conditions. Various calculations and statistical tests can be made for each run. Realtime and historical graphs can then display these data to the operators and engineers as required. CONCLUSIONS During the last ten years digital control systems including programmable logic controllers have become more powerful. Today, even mid-size PLCs can perform complex calculations, including some modeling and floating-point mathematics. This allows extruder manufacturers to implement control systems with complex algorithms that include auto-tuning of temperature control and pressure control loops. In addition, the human-machine interfaces allows storage of recipes and can display historical trends of many operating parameters. Because most mid-size PLCs have extensive networking capabilities, extruder manufacturers are able to offer integration with upstream equipment, such as feeders and mixers, and with downstream equipment, such as pelletizers, dryers, and conveyors. System control and data acquisition (SCADA) packages provide plant management with production information that allows them to improve quality control, planning, and allocation of resources. References 1.

2.

Collins, S. H., “Update on Continuous Compounding Equipment, Part 2: Twin-Screw Extruders and Specialized Compounders,” Plastics Compounding, November/December 1982, pp. 29–44. Maddock, B. H., “Factors Affecting Quality in Polyethylene Extrusion,” Thirteenth Annual National Conference of the Society of Plastic Engineers, 1967.

© 2006 by Béla Lipták

3. 4. 5.

6. 7. 8. 9. 10. 11.

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Mielcarek, D. F., “Twin-Screw Compounding,” Chemical Engineering Progress, June 1987, pp. 59–67. Bernhart, E. C., Processing of Thermoplastic, New York: Van Nostrand Reinhold, 1959. Jensen, B. A. and George, J., “Extruder Automation,” Proceedings of the ISA/90, Vol. 45, Part 2, New Orleans, LA: Instrument Society of America, October 1990. Pettit and Ahlers, “Extruder Controls,” SPE Journal, Vol. 24, November 1968. Meissner, S. C. and Meneges, E. P., Technical Papers, Vol. XVII, Society of Plastic Engineers, May 1971. Bateson, R. N., Introduction to Control System Technology, 5th edition, Engelwood Cliffs, NJ: Prentice Hall, 1996. Johnson, C. D., Process Control and Instrumentation Technology, 6th edition, Upper Saddle River, NJ: Prentice Hall, 1996. Petruzella, F. D., Programmable Logic Controllers, New York: McGraw Hill Companies, 1990. Hensen, F., Plastic Extrusion Technology, 2nd edition, Cincinnati, OH: Hanser-Gardner, 1997.

Bibliography Bouilloux, A., Macosko, C. W., and Kotnour, T., “Urethane Polymerization in a Counter-Rotating Twin-Screw Extruder,” Industrial and Engineering Chemistry Research, Vol. 30, pp. 2431–2436, November 1991. Chui, Sh.-H. and Pong, Sh.-H., “Fuzzy Supervisory Predictive PID Control of Plastic Extruder Barrel,” Journal of Applied Polymer Science, September 1999, pp. 541–555. Colbert, J. A., “Optimization of Compounding Extruders,” Plastics Compounding, December 1989, pp. 25–32. Collins, S. H., “Update on Continuous Compounding Equipment, Part 1: Single-Screw Extruders,” Plastics Compounding, September/October 1982, pp. 69–82. Fisher, H., “Electric Drives in Extrusion,” Kunststoffe Plast Europe, September 1998, pp. 26–29. Hopkins, B., “Temperature Control of Polymerization Reactors,” Instrumentation Technology, May 1973. Janssen, L. P., Twin-Screw Extrusion, New York: Elsevier, 1978. Kepper, D. and Haney, J., “An Overview of Modern Extrusion Technology,” Rubber World, June 1987. Kilham, L. B., “A Systems Approach to Quality Control in the Production of Polymer Products,” Tappi Journal, April 1989, pp. 113–116. Klein, I. and Klein, R. J., “The Effect of Resin Properties on the Extrusion Process,” Tappi Journal, August 1989, pp. 99–103. Kramer, W. A., “Extruder Temperature Control with Adaptive Reset,” Engineering Plastics, September 1996, pp. 341–350. Marschall, U. and Reyer, K.-H., “Automating Extruder Control,” Kunststoffe Plast Europe, February 1998, pp. 10–12. Martin, T. L., “Temperature Control in Plastic Processing,” Plastics, April 1967. Mercure, P. K. and Trainor, R., “Modeling and Optimization of Extruder Temperature Control,” Polymeric Materials Science and Engineering, Vol. 58, pp. 692–696, 1988. Mober, C., “Control Schemes for Plastic Molding Machines Conserve Energy,” Instrument and Control Systems, April 1982. Tsai, Ch.-Ch. and Lu, Ch-H., “Fuzzy Supervisory Predictive PID Control of Plastic Extruder Barrel,” Journal of the Chinese Institute of Engineers, September 1998, pp. 619–624. Yang, B. and Lee, L. J., “Process Control of Profile Extrusion Using Thermal Method, Part 1. Mathematical Modeling and System Analysis,” Polymer Engineering Science, November 1988, pp. 697–707. Yang, B. and Lee, L.J., “Process Control of Profile Extrusion Using Thermal Method, Part 2. Closed Loop Control,” Polymer Engineering Science, November 1988, pp. 708–717.