By: Thomas W. Fisher, III.

Transcription

1 By: Thomas W. Fisher, III

2 1.0 Introduction Extrusion Control Primer and Reference Guide Table of Contents 2.0 Factory Information Systems Architecture 3.0 Extrusion Control Introduction 4.0 Extrusion Parameters Typically Controlled and Monitored 5.0 General Background on Control System Approach/Architectures 6.0 Extrusion Temperature Control 7.0 Pressure Monitoring & Control 8.0 Speed Control System: issues, drive types 9.0 Feed Systems 10.0 Melt Pumps (metering pumps, gear pumps) 11.0 Advanced Control Concepts 12.0 Other Extrusion Control Issues 13.0 Things to Consider in Selecting a Controller or Control System 14.0 Transducers: principals of operation & application considerations 15.0 Measurement Systems - Coming soon 16.0 Communications 17.0 Installation Guidelines 18.0 Common Electrical Formulas & Conversion Factors 19.0 Trouble Shooting Guide Revision History Rev. O Unformatted initial draft Rev. 1 Fix Formatting & Typos N:\Sales\Promotional Materials\Articles\Extrusion Control Primer.doc 2

3 1.0 Introduction This Extrusion Primer and Reference Guide is intended to introduce and discuss the basics of extrusion control, provide an overview of the overall factory floor information structure, and serve as a technical reference of electrical and control formulas, guidelines, and conversion factors. The Primer has general application to any process where temperatures, speeds, pressures, etc. are controlled but it is specifically oriented towards the plastic and rubber extrusion process. A discussion of the most common types of transducers and measurement systems is included. If you have suggestions for additional content, clarification, expansion, or corrections your input is welcome. Please your comments to facts@facts-inc.com This paper is intended as a living document that will be continuously expanded and updated. The latest version may be requested in hard copy or directly downloaded from the FACTS, Inc. web site. Visit our web site at 3

4 2.0 Factory Information Systems Architecture Historically management information systems have been driven from the top down, with suppliers of MRP and ERP systems adding terminals and forms to capture manufacturing data from the factory floor. This approach is expensive, inefficient, and the data accuracy is generally poor. The additional work load for the operators is also generally unpopular with the factory floor personnel. FACTS recognizes that manufacturers sell extruded products, not information. The FACTS approach derives the management information as a by-product of the automation of the extrusion line equipment. FACTS view of the future: It is our belief that this decade will see a significant expansion in the number of companies that will automate their processing systems and include information systems to enable them to: ¾ Co-ordinate and control the complete production line ¾ Analyze and optimize the process ¾ Track in Real Time all production status ¾ Report on good product produced, scrap, and raw material usage ¾ Manage the overall production process ¾ Down load job schedules directly to the floor system ¾ Provide the operator with quality information ¾ Minimize inventory ¾ Maximize productivity ¾ Provide end user documentation to assure they are receiving quality products made to their unique specification 4

5 We believe that manufacturers will be addressing the following two key issues during this decade: 1. Applying advanced process control and transducer technology to achieve improved quality and productivity while reducing material consumption, scrap, and labor content. 2. Fully integrate their management information systems with the factory floor control systems to better manage their factory operations. Some of the benefits realized from this integration include: reduced delivery times, labor content, and inventory, while improving machine utilization. FACTS customers are already exemplifying this trend. A major producer of telephone and data cables recently expanded their manufacturing operations by adding 17 additional singles and jacketing extrusion lines. They purchased completely integrated advanced process control systems for all 17 lines from FACTS based on the results achieved from the previous 7 FACTS systems. The FACTS TLC 2001FS system integrated the temperature controls, drive systems, transducers for measurement and control of hot and cold O.D., eccentricity and concentricity, and capacitance, and provided a complete Real Time & Historical process analysis system. American Superconductor, another customer, is building the world's first full production plant for the manufacture of High Temperature Superconducting wire. This new 355,000 square foot facility is being built in Devens, MA, 35 miles north west of Boston. American Superconductor contracted FACTS, Inc. to develop and implement a plant wide integrated information system that encompasses all the floor level process control systems through the top level enterprise systems including MRP, Scheduling, Quality, and Maintenance systems. This system will enable American Superconductor to: ¾ Track each individual wire component throughout the entire manufacturing process ¾ Provide comprehensive quality reporting ¾ Optimize the process with comprehensive analysis tools ¾ Provide accurate efficient data to the Enterprise level management systems, and ¾ Adapt to future manufacturing and information requirements FACTS views the overall information architecture for a manufacturing company as 4 levels. These levels are depicted in the attached block diagram and explained in the following description of the Integrated Process Control and Information Management Network Architecture. 5

6 Integrated Process Control and Information Management Network Architecture The architecture described in the following provides a comprehensive information network that will enhance your manufacturing operations. It will provide cost effective, accurate, timely information from the shop floor to help manage the manufacturing operation more effectively, improve quality, provide complete product genealogy, and help to evaluate and optimize the individual manufacturing processes. This architecture is based on implementing an integrated process control and information management system. Level Overview Level 1: Process Control and Operator Interface (HMI) This level directly controls the process and interacts with the operator so it is the key to the entire network scheme. The integration of the control of the overall process provides increased productivity and improved quality while reducing both scrap and labor. These systems will not only provide for normal operator interface functions, but will also serve as a data conduit to the upper level business systems. The information for the upper level systems is a byproduct of automating the process. The key functions of this level are: - Integrated control of the complete processing line ¾ Local trend plots for all key process parameters ¾ SQC charts for all key process parameters ¾ Schedule download through the HMI ¾ All specified process data is passed to the Level 2 system for storage ¾ Barcode support through local HMI ¾ MES reporting to upper levels from the HMI ¾ CTQ (Critical to Quality) data to Level 4 Quality System ¾ Controlled Document Interface and Display The potential users of these systems are: ¾ Machine operators ¾ Production Supervisors ¾ Maintenance Personnel ¾ Engineering Support 6

7 Level 2: Process and Product Analysis & Reporting This level provides centralized logging and storing of data from the factory processes for Real Time or future analysis. Continuous time sampled Product, Process, and Quality Data from the factory process systems would be stored on this database server. This data would be available throughout the plant via the Office/Business network for real time and/or historical Product and Process analysis. Configurable standard reports can be used to demonstrate and improve quality, reduce costs, diagnose machine or process problems, and to streamline and optimize the entire process. This system provides historical and real time analysis and reporting. The key functions of this level are: ¾ Product and process analysis ¾ Long term storage of process and product parameters ¾ Trend plots of any live or historical data ¾ SQC charts of any live or historical data ¾ Logging of any defined event that affects the process or product ¾ XY plots of any logged parameters ¾ Network Access of data available enterprise wide ¾ Alarm logging ¾ Real Time user interface The potential users of these systems are: ¾ Process Engineers ¾ Maintenance Engineers ¾ Quality Assurance Department 7

8 Level 3: MES/WIP (Manufacturing Execution System/Work In Process) Database Server This level uses the Level 1 information to automatically provide the lot or process step information to the upper Level 4 ERP systems. This system can be used to track all product throughout its production cycle. This system has a definition/model of the complete manufacturing process. It is therefore able to track the progress of each part through each process and assure that the proper components are used and that the correct process steps followed. The WIP, Work in Process, server will maintain complete product traceablity on the factory floor. It will provide for real-time manufacturing resource tracking, work-in-progress tracking, and inventory control. All necessary data required by the upper level ERP/MRP systems will be supplied on a batch summary basis. This may be hourly, by shift, or as required. The material is tracked from raw materials inventory through work in process finally to finished goods inventory. This tracking produces a description of what has been completed, status of a lot or batch; remaining time for the operation, hold time for materials, and operation yields for specific operations. The status of all jobs or lots may be viewed in Real Time. The key functions of this level are: ¾ Real time tracking and reporting ¾ Batch or process step summary information reporting to upper level MRP/ERP systems ¾ Real Time Work In Process Inventory ¾ Downtime reporting ¾ Production reporting ¾ Item/Product Status Reporting ¾ Scrap Reports ¾ Costing Data Capture ¾ Support for serialized and non-serialized identification numbers The potential users of these systems are: ¾ Production Management and Supervision ¾ Scheduling Department ¾ Maintenance Management and Supervision ¾ Enterprise Resource Planning System or MRP System ¾ Finite Capacity Scheduling and Optimization System The Real Time Manufacturing Resource Tracking functions for Level III will include: ¾ Tracking Lots, Batches, Individual Parts ¾ Tracking Raw Material ¾ Tracking Operators ¾ Tracking Production - summaries, by shift, by product ID, by work cell, etc. ¾ Tracking Machine Dependability, Availability, Utilization ¾ Ability to explode backwards and forwards at any step in the process 8

9 Level 4: Factory Management Information Systems (MIS) This level consists of the commercial packages that provide ERP, Quality System services, Maintenance System services, and Document Control system services. The scope of supply by FACTS includes exchanging appropriate data and documents with these systems but does not include supplying them. The key functions of this level are: ¾ Production Planning ¾ Scheduling ¾ Quality Tracking and Reporting ¾ Document control ¾ Labor reporting ¾ Costing Analysis ¾ Maintenance scheduling and reporting The potential users of these systems are Enterprise wide, and would include: ¾ Purchasing Department ¾ Scheduling Department ¾ Engineering/Maintenance Department ¾ Quality Assurance Department ¾ Production Management ¾ Plant Management ¾ Cost Accounting ¾ Profit/Loss Business Managers 9

10 Level 4 Factory Information Systems Overall Factory Information MRP/ERP System Quality System Maintenance System Level 3 Plant Network MES System Factory Network TCP/IP Level 2 Process & Product Analysis & Reporting QA Manager Testing Laboratory Testing Lab Stations Plant Manager Supervisor Stations TIM 3001NT Expert System Level 1 Process Control Line #1 Line #2 Line #3 10

11 3.0 Extrusion Control The control system is an integral part of the extrusion machinery and extrusion process. It is also the means through which the operator interacts with the process. The control system will have a very large impact on the productivity, quality, uniformity, and cost per production unit for the extruder and overall extrusion process. Control of the extrusion process involves monitoring and control of a wide variety of process and product parameters. To put the control of the overall process in perspective, a brief review of all common process parameters will be provided. This is followed by a detailed discussion of control of the principal parameters of temperature, pressure, speed, feed systems, and melt pumps. The principal function of a control system is to control all the individual process and product parameters. Barrel temperatures and extruder speed are minimal control requirements. All other measurable parameters that are not controlled should be monitored and displayed for the operator. Monitored parameters should at a minimum include melt temperature, melt pressure, and drive load. Additionally, the process control system should co-ordinate all aspects of the extrusion process. This might include tying in the feeders and down stream drives. A centralized operator interface facilitates the operator's job by providing a consolidated view of the overall process. 11

12 4.0 Extrusion Parameters Parameters typically controlled in extrusion: ¾ Feed System blending ratios of multiple ingredients, extruder throughput in lbs/hour or lbs/ft ¾ Extruder barrel temperature zones These are usually have heating & cooling, except for low temperature materials such as silicone which are often cool only ¾ Tooling temperature zones These are usually heat only ¾ Die temperature zones These are usually heat only ¾ Speeds - extruder screw speed, roll stands, pullers, conveyors, capstans, and wind-ups ¾ Tension - product tension is controlled by varying the appropriate speed(s) based on load cell or dancer feedback ¾ Melt Pump Inlet Pressure ¾ Product Dimensions such as OD (outside diameter), ID (inside diameter), Width, Height, Wall Thickness ¾ Roll Length ¾ Cut-to-Length Parameters typically monitored in extrusion: ¾ Drive Loads ¾ Melt Temperature ¾ Melt Pressure ¾ Tension ¾ Cut Counting ¾ Product Dimensions Recommended additional parameters for monitoring to detect problems before any damage occurs, track for maintenance purposes, or assure proper process conditions: ¾ Gear Box Temperature, Pressure, and Lube Flow ¾ Motor Temperature ¾ Drive Faults ¾ Cooling Tank Temperature ¾ Relative Humidity and/or Dew Point controlling cooling roll temperatures to just above the dew point can prevent corrosion and contamination of the product with moisture ¾ % Moisture of Raw Material 12

13 5.0 General Background on Control System Approach/Architecture Categories of controllers: 1. Discrete controllers 2. Operator Interface plus discrete controllers 3. PLC control with operator interface 4. Real Time Integrated line control 5. PC based control with Operator Interface 6. DCS Distributed Control System 1. Discrete Instruments Discrete Instruments are the traditional method of extrusion control. This approach consists of Multiple Single Loop Controllers. Speed control and co-ordination is typically accomplished with by potentiometers. Advantages include: ¾ Lowest Initial Cost and ¾ Familiarity due to being Traditional Approach Disadvantages include: ¾ Set Points must be entered at each instrument individually. This can lead to errors. ¾ No Data Capture for historical analysis is provided ¾ No Process history for Operator, that is, no trend charts are available ¾ There is No Integration of other systems nor ¾ Advanced Control Loops. And finally, ¾ No Interface to Higher Level Business Systems is available 2. Discrete Instruments with consolidated HMI or Human Machine Interface This scheme adds an Operator Interface but retains the same basic control equipment setup as the above. Advantages include: ¾ Provides the operator with the ability to preset all set points from a central location ¾ Access to process history is provided in the form of trend charts ¾ Data capture may also be provided in this approach. Disadvantages include: ¾ The addition of the operator interface adds moderate cost ¾ No integration of other systems ¾ No advanced control strategies and ¾ No interface to higher level factory information systems 13

14 3. PLC with HMI PLCs have achieved wide spread use throughout many factories. They offer a robust hardware platform with world wide support. PLCs or Programmable Logic Controllers, as the name implies, were originally developed as relay replacement devices but have evolved to include analog capability. When combined with advanced operator interfaces, a PLC based system offers significant advantages over the discrete instrument approach. Advantages include: ¾ Standardized hardware platform ¾ Availability of personnel familiar with PLCs ¾ Good industrial interface and ¾ Networking of piers and remote I/O is usually a standard feature of the PLC Disadvantages Include: ¾ PLCs are optimized for sequential logic (machine sequence control), not continuous process control ¾ There is no effective support, other than parts, available from the PLC manufacturer since they are not familiar with the extrusion process ¾ Any changes must be made in 2 separate software systems - the PLC & the HMI ¾ It is difficult to tie in other process subsystems such as feeders and gauges since communications drivers for PLCs are difficult and often require separate communications processors ¾ All advanced control beyond simple PID loops requires software development ¾ Most maintenance diagnostic support is hardware not process oriented and requires the connection of a separate programming terminal ¾ Networking support for Factory Information Systems is only available via the operator interface 4. Real Time Integrated Extrusion Control Systems Another class of control systems have evolved that are specifically designed to control the extrusion process and integrate the overall process along with all of the major subsystems. These are Real Time Integrated Extrusion Control Systems. Because these systems are designed specifically for the extrusion process, the operator interface, the advanced control strategies, process diagnostics, and other extrusion specific features and functions are already built in. Advantages Include ¾ Vendor support that is very effective since the vendor is already familiar with the extrusion process ¾ Process, Product, & Hardware Diagnostics can be built in ¾ Remote diagnostics and ¾ Advanced control strategies are built in ¾ Lower support costs ¾ Network support for Factory Information Systems can be easily provided Disadvantages include: ¾ The Initial cost is higher than the discrete instrument approach 14

15 5. PC Based Control The most recent general purpose entry in the control field are PC Based Control Systems. These systems are an attempt to achieve the best of both worlds. The traditional logic and sequential control functions of the PLC are implemented in a Windows based PC platform. PC Based Control Systems typically make use of standard PLC I/O as the process interface. Additional features and functions can be added that make use of the Windows operating system. The use of Windows facilitates the incorporation of Business Information Systems into the factory floor control systems. It also permits more sophisticated operator interface functionality to be implemented. Advantages include: ¾ Standardized hardware platform with a ¾ Good industrial interface ¾ Networking of piers and remote I/O is usually a standard ¾ Network support for Factory Information Systems is available through the Windows operating system Disadvantages Include: ¾ No effective support, other than parts, available from the manufacturer since they are not familiar with the extrusion process ¾ All advanced control beyond simple PID loops requires software development ¾ Most maintenance diagnostic support is hardware, rather than process oriented ¾ And finally, you should keep in mind that Windows is optimized for the office environment and it's robustness and reliability on the factory floor remain questionable 6. DCS Distributed Control System A distributed control system consists of a group of control nodes, each with its own local processor, that are all networked together. Each node usually completely controls the associated local machine or process while communicating and co-coordinating with the other nodes that make up the over all machine or process. DCS systems typically allow multiple operator graphical interfaces that can each access any portion of the distributed system. These systems are capable of handling large complex processes but are also usually quite costly. DCS systems are typically found on large chemical processes that are complex and cover large areas. True DCS systems are not often used for extrusion line control because their complexity and high price are not justified. 15

16 6.0 Extrusion Temperature Control Temperature control of the extrusion process typically involves controlling the temperature of the following: ¾ Extruder barrel zones with both heat and cooling control ¾ Tooling and die zones as heat only or as heat and cool zones ¾ Roll temperature zones are usually heat and cool control of circulating water loops ¾ Downstream equipment such as water baths and vacuum tank water temperature may be cool only, heat only, or both heat and cool. ¾ Curing Systems such as steam tubes, hot air ovens, microwave ovens, or salt tanks ¾ Inductive heating of carrier or core conductive materials Temperature control is implemented by measuring the temperature, typically with a J type thermocouple, comparing the measured actual temperature with the target temperature set point, and computing the appropriate output required to achieve the desired target temperature. (See the Section on Transducers for details on thermocouples) Temperature control is usually based on PID loop control. The PID algorithm is a 3 part calculation that is composed of the following: Output = Proportional + Integral + Differential where output is in % Proportional = Error x Gain where Gain is the tuning factor used to adjust the Proportional Band Where: Error = Set Point - Actual Reading Integral New = Integral Previous + (Error x Integral Factor) where Integral Factor is the tuning factor used to adjust the Integral Differential = ( New Error - Previous Error ) x Differential Factor The Proportional component generates an output that is directly proportional to the error within it's proportional band. Outside this band the controller functions as an 'ON-OFF' controller. Proportional Band is the temperature range over which the Proportional term is active and not saturated. For example a gain of 5 gives a proportional band of 20, since an error of 20 degrees or greater generates an output of 100%. 5 x 20 = 100. The Gain term is useful for fast response, but is not capable of achieving a zero error in most control applications since when the error is zero, the Proportional term is also zero. Most processes, including extruder barrel and tooling temperature zones, require some output to maintain the target set point. Integral is used to force the error to zero by continuing to increase the output until the error is zero. Integral is sometimes referred to as Automatic Reset. 16

17 The Differential term, also referred to as the rate term, compensates for the rate of change in the measured variable, temperature in our case, to minimize overshoot. The rate term also resists upsets by reacting to any shift or change in the process. It should be noted that since the Differential term resists or opposes change, it actually reduces the output and works in opposition to the Gain term when the process is approaching set point. The output for extrusion temperature control loops increases the heating or cooling to the appropriate zone. Temperature control may be direct acting or indirect acting. Direct acting systems directly contact the barrel or tolling zones. Electric heating elements, coolant tubes, or blowers directly mounted on the extruder barrel are direct acting systems. Indirect acting systems use heat exchangers to vary the heat or cooling input to a liquid loop, which in turn transfers the energy to the controlled machine zone. For indirect acting systems, a liquid loop circulates around or through each machine temperature zone. Heating is accomplished by increasing electrical resistive heat or steam heat directly to the controlled machine parts or to an associated circulating liquid loop. Liquid loops may be oil, water, or glycol. Cooling may be accomplished by varying cooling air or water. Air systems vary the duty cycle of electric fans or the position of vents. Water cooling control gates water directly around the barrel or tooling zone, or through a heat exchanger. Water loops may use chilled water in a closed loop system or use city water, which then goes to the drain. Cooling and heating are rarely equally effective. This means that the temperature change will be different, in some cases significantly, when comparing 100% cooling for a fixed time period with 100% heating for the same amount of time. For this reason most systems have a tuning factor called cooling effectiveness that establishes the relative difference in the effectiveness between heating and cooling. A cooling effectiveness of 1 means they are equally effective. The effectiveness of direct acting water cooling is often very non linear, with the initial few percent being disproportionately effective. This is because when the temperatures being controlled are above the boiling point, the water flashes to steam. The change of state from liquid to gas pulls a great deal of heat away from the process until the surfaces drop below the boiling point. Additional cooling will then be less effective. This characteristic makes tuning water cooled loops very difficult where the cooling water may flash to steam.. The response of a controlled temperature zone is dependent on many things. These will include the thermal capacity of the heating and cooling systems (BTU/hour capacity of each), the speed of response of the temperature sensor, sensor placement, the thermal mass of the equipment, and the amount of heat being put into the process by the drive, and chemical reactions. The tuning factors needed for proper control will be dependent upon all of these parameters. Any change in any of these parameters may necessite retuning the overall control loop. For example if water cooling is utilized on an extruder barrel zone and there are hand valves in the water line, any change in the valve position will obviously effect the cooling effectiveness or capacity, and therefore require a change in the tuning factors. For this reason all hand valves should be tagged with a warning and their positions noted so that they can be returned to their original position after maintenance. Seasonal changes in the cooling water temperature may also require changes in loop tuning. Processes that are slow with long lag times and/or that are capable of large rates of change will require wide proportional bands, that is smaller Gain terms to avoid oscillations. Increased integral and Derivative terms may also be required. 17

18 Auto-tuning is a feature provided to help determine the tuning factors automatically. This helps reduce initial setup. In processes that have product dependent tuning requirements, groups of preset tuning factors may be automatically loaded when the product code or setup is selected. This approach provides the optimal tuning setup for each product. Fuzzy Logic control is an alternative or supplement to conventional PID control. Fuzzy Logic based control is not fuzzy or uncertain, rather it is a unique application of artificial intelligence. Where conventional logic has only true and false states, fuzzy logic permits degrees of true and false. A set of rules is used in conjunction with tuning parameters to evaluate the process status and determine the appropriate response. Fuzzy Logic attempts to partially emulate the human thought process. Fuzzy Logic is best suited for applications where the process response is very non-linear or the process response changes based on various conditions. Fuzzy logic is often used for departure or upset recovery with the classic PID algorithm used near set point and under steady state conditions. Types of Output Output of the control loop may be either time proportional, usually called duty cycle, or analog proportional. Duty cycle is based on a predetermined time period in which the output is ON a percentage of the time and OFF the remainder. For example if 30% heat is required and the time period is 5 seconds, then the output will be ON for 1.5 seconds (.3 x 5 = 1.5) and OFF for 3.5 seconds (5-1.5 = 3.5) and then repeat. Duty cycle control normally is utilized with solid state, mechanical, or mercury displacement (mercury displacement relays are not recommended for new applications due to environmental considerations) relays for resistive heating, ON/OFF water solenoids, or cooling fans. The thermal mass of the equipment integrates or averages the ON/OFF heating or cooling to produce a smooth response as long as the time period is not excessive. For extruder applications a period of 5 to 10 seconds will produce a smooth response without excessive cycling of the equipment. An analog proportional output is based on 0 to 10 volts dc or 4 to 20 ma dc. For a 30% heat requirement the output would be 3 volts for a voltage output or 8.8 ma (16 x = 8.8 )for a current loop output. A 4 to 20 ma loop has a span of 16 ma and an offset of 4 ma. Analog proportional outputs are normally utilized with phase angle controllers for electrical resistive heating or proportional steam, gas, or water valves. In some cases the 4 to 20 ma may be converted to a 3 to15 psi pneumatic signal to operate the valve. A phase angle controller is an electrical controller that turns on electrical power to the electrical load at the appropriate time during the 50 or 60 Hz sign wave. Conduction stops when the current passes through zero at the end of the current half cycle. The turn on time is determined by the input signal and may be varied to provide 0 to 100% power. This type of power controller is more expensive and tends to be electrically noisy since it turns on under load. Its advantage is that it is fast and is used where rapid response is needed, such as with quartz heating elements. Solid State Relays, or SSRs, are available with zero voltage turn on and zero current turn off. This means regardless of when during the AC sine wave cycle the control command becomes active, the SSR will not begin to conduct until the AC voltage passes through zero. Conduction turns off when the AC current passes through zero after the control signal goes inactive. This type of SSR generates no electrical noise and is generally low cost. SSRs are available with either AC or DC input control. Note that AC control of SSRs from other solid state devices may require the use of a parallel loading resistor to prevent unintended turn on of the SSR by the normal leakage currents from the controlling solid state device. 18

19 7.0 Pressure Monitoring and Control Extruder Output Pressure Extrudate output pressure is an important process parameter that should be monitored. If a screen pack is present, then both before and after the screen should be monitored. Control of extruder pressure is accomplished by varying extruder speed to maintain a set extruder pressure. Extruder output pressure is normally controlled only when used in conjunction with a melt pump, which is covered in detail in a following section. However, extruder output pressure may be successfully controlled when the flow is sufficiently restricted and/or the viscosity is high. This is normally done as a means to control and maintain extrudate dimensions, although dimensional control is usually more reliably accomplished by maintaining a fixed extruder speed and varying the takeaway speed. Down Stream Pressures Pressure control of internal air support for such products as hose, tubing, and other hollow extrudates is accomplished by varying the air pressure. This may be accomplished by the use of proportional air pressure regulators, current to pressuretransducers, or voltage to pressure transducers (I/P and V/P). Proportional air regulators are actually mechanical air regulators that control pressure based on a bellows or diaphram working against a spring. An electric motor positioner is used to adjust the pressure regulators spring pressure. A cascade control loop may be used to vary the internal pressure to maintain ID or OD based on dimensional feedback from Laser or Ultrasonic gauges. Vacuum control is a variation on pressure control. Vacuum may be adjusted by vacuum regulators similar in design to pressure regulators, adjusting the speed of the vacuum pump, or by using a fixed speed vacuum pump and varying a bleed valve manually or with an electric positioner. Steam pressure is controlled in the same manner as air or gas pressure. Steam pressure may be used to control the curing or drying temperature of down stream equipment. Pressure transducers commonly used to measure extrudate pressures in extruder applications are of the strain gauge type. A precision power supply, typically 10.0 vdc, powers the transducer, which is usually in a Wheatstone Bridge configuration. The typical output with 10.0 volt excitation is 33.3 mv (.033v) full scale. This low level signal must be amplified and properly calibrated. Many transducers have a built in calibration resistor that causes the transducer to output 80% of full scale when the calibration terminals of the probe are shorted. The usual cautions for low level signals must be followed. This includes use of shielded cable and not running the signal wiring near noise sources such as power lines, motor leads, and other AC signals. Do not press with a finger nail or sharp edge on the strain gauge diaphragm as permanent damage can result. Most pressure transducers are available with a built in signal conditioner and amplifier to provide a high level signal where required. Calibration of both zero and span must be done when the extruder is stopped and there is no pressure on the probe. Avoid hot extrudate coming in contact with the probe wires, as this will cause damage. 19

20 Temperature can induce errors or even damage pressure transducers. Where practical, such as when measuring steam pressure, placing the transducer on a stand-off that reduces the temperature the pressure probe experiences can alleviate this problem. Extruder pressure probes are temperature compensated, but must be selected for the specific temperature range to which the probe will be subjected. It is best to calibrate the probe at the temperature at which it will be used. Many probes commonly used have several pin out options. The appropriate cable, connector, and wiring must be used. Use caution that the proper connector and cable is utilized. Pressure transducers that are used to measure vacuum, internal gas support pressure, steam pressure, and hydraulic pressure may be of the strain gauge, semiconductor, or displacement (measures physical displacement proportional to pressure) type. These may have low level or high level output. Use snubbers whenever possible in fluid pressure monitoring applications to prevent damage from pressure spikes like water hammer. 20

21 8.0 Speed Speed control in extrusion is a critical parameter that has a great impact on the extrudate dimensions and often on other physical properties. Two properties of extruders related to speed are worth noting. First extruders are not linear in their output. Doubling the screw speed does not exactly double the output. This is due to the variable blow by or slippage of the melt past the screw flights. Faster screw speeds typically generate more heat and this tends to lower viscosity. Remember the barrel temperature is measured at the outside of the barrel liner, not in the melt stream within the barrel. Therefore maintaining the barrel temperatures at a constant temperature does not necessarily maintain a constant melt stream temperature. In severe cases, increasing screw speed can actually result in a reduction in extruder output. Most downstream equipment such as pullers and conveyors are linear with respect to speed unless there is slippage. It is recommended that whenever possible extruders should be run at a fixed speed and the downstream speeds be varied to control dimensions. Extruder and dies are amplifiers of speed errors. This results from the fact that extruder output (volume) is a cubic function whereas the product sectional dimensions (area) are a squared function. This assumes that the takeaway speed of the extrudate is held constant. Extruders also exhibit a characteristic, partially as a result of the above phenomenon, that results in actual output requiring a good deal of time, up to several minutes, to stabilize after a speed change. To avoid these problems, careful speed co-ordination of extruder(s) and down stream equipment is important. It is recommended that whenever possible extruders should be run at a fixed speed and the downstream speeds be varied to control dimensions. Speed control of all extrusion line equipment should be to +/-0.1% or better. Modern drive systems with encoder feedback can easily provide +/-0.01% regulation and accuracy. Older drive systems can be upgraded where necessary by the addition of digital speed supervisors as part of an overall control system or as separate dedicated devices. Typical speed control accuracy: ¾ Modern digital AC Flux Vector or DC drive with encoder feedback +/- 0.01% ¾ DC drive without feedback +/- 1% to 3% ¾ AC V/Hz without feedback +/- 1% to 5% ¾ DC drive with AC tachometer feedback +/- 0.5% ¾ DC drive with DC tachometer feedback +/- 0.1% to 0.5% 21

22 DC verses AC Drives Older drives were almost exclusively DC. The current trend is towards AC drives, principally due to their lower motor maintenance since the commutator of the DC motor is eliminated. AC drives plus motors tend to have lower purchase cost in the sizes below 100 hp, while DC drives plus motors are lower in cost in the larger sizes. Within the AC drive category, AC Vector drives are used where precision speed control is required (most extruder and down stream applications), AC sensorless drives are used where speed accuracy is less critical, simple inverter drives without feedback are used where speed accuracy is not at all critical, for example for blower speed control. DC Drive Advantages over AC Drives ¾ Simpler design, therefore easier to repair ¾ Higher efficiency - typically 98%+ ¾ Easily retrofitted to existing DC motors ¾ Most cost effective for larger sizes, above 100 hp ¾ High reliability ¾ Less electrical noise and interference - less than half that of AC drive ¾ Simpler installation wiring ¾ Lower motor acoustic noise than AC motor ¾ Smaller size for same hp rating AC Drive Advantages over DC Drives ¾ Simple low maintenance motor - no commutator ¾ Improved dynamic response where required ¾ Motors have low purchase and rewind cost ¾ Motors more suitable for harsh and rugged environments ¾ Better open loop regulation ¾ Most cost effective for smaller sizes, below 100 hp ¾ Possible to share single controller with multiple motors, inherent load sharing ¾ Wider speed range available - up to 6000 rpm motors available ¾ Smaller motor frame sizes ¾ Good power factor - near unity When the load can be over driven by the product or other processing equipment, or fast stopping under load is required, then regeneration for DC drives or dynamic braking for AC drives must be specified. Extruders normally do not require dynamic braking or regenerative operation, but pullers, chill roll stands, etc. may. The drive must be carefully sized for the application with consideration of the frequency and duration of the over hauling or over driving load. It may be necessary to provide external braking resistors suitably sized for the load conditions. 22

23 Application Issues: ¾ Loss of feedback can result in a runaway drive condition ¾ Feedback signal reversal due to wiring errors will result in a drive fault or the drive running in the wrong direction ¾ The use of isolation transformers and/or line reactors to isolate the drive from the power line is necessary to prevent electrical noise from interfering with other factory equipment such as computers and control systems ¾ AC drives are particularly electrically noisy. It is therefore important that they be installed with shielded motor leads as recommended by the manufacturer. 23

24 9.0 Feed Systems Feed systems fall into 2 methods and 3 categories: Feed System Methods ¾ Flood feed ¾ Starve feeding of extruder throat In general twin screw extruders are most often starve fed with either volumetric or gravimetric systems whereas single screw extruders are usually flood fed. Feed System Categories: ¾ Simple Hopper feeder - flood feeding only ¾ Volumetric feeding and/or blending ¾ Gravimetric (Loss in Weight) feeding and/or blending Volumetric feeders are based on augers that are optimized for linear material transfer, but are still subject to nonlinearity due to the variations in bulk density of the feed stock. Volumetric feeders are controlled by varying the auger speed to maintain a constant ratio with respect to other feeders if in a blending operation or with respect to extruder speed or line speed if in a feeding application. Volumetric systems are straight forward to control since they involve simple speed ratio control. Variations in feed stock bulk density introduce errors since the auger speed is being controlled based on the assumption that auger output is linear and directly proportional to speed. This is a valid assumption only if the feed stock bulk density is constant and there is no bridging of material in the feed hopper(s). Gravimetric feeders incorporate load cells to measure the amount of material being transferred by calculating the change in hopper weight or loss in weight per unit of time. This permits calculation of the weight per hour and weight per meter of the extrudate. To control single extruder output rates the line speed should be varied to maintain a constant weight per unit length for the extrudate. In coextrusion operations the individual extruder speed must be varied to maintain a constant output rate. Gravimetric or Loss in Weight control is well suited for layer weight control or to maintain product dimensions for complex profiles where it isn't practical to measure these dimensions directly. Loss in weight feeders have the added benefit of being able to accurately report raw material consumption. In blending operations the individual auger speed ratio is varied to maintain the target component % weight blending ratio. 24

25 10.0 Melt Pumps Melt Pumps, also referred to as Gear or Metering Pumps, are used to provide a constant, smooth, linear output from an extruder. Melt Pumps are normally fed by an extruder operating in a constant pressure loop. The melt pump is placed between the extruder and the die. Melt Pump control consists of 4 parts: ¾ Start-up sequence ¾ Maintaining a constant pressure on the melt pump inlet by varying the extruder speed ¾ Controlling the Melt Pump speed ¾ Interlocks In steady state operation the extruder melt pressure, after the screen if any, is measured by a conventional melt pressure transducer and the extruder speed varied by the control to maintain a constant preset pressure at the inlet to the melt pump. In the event that the melt pump inlet pressure drops below a preset value for more than a preset time the extruder and pump must be shut down. This is because the melt pump uses the melt flow to lubricate the pump. Severe damage will result if the pump is run dry. The start-up sequence must assure that the melt pump is not run dry nor over pressured. To start up a melt pump system, the extruder is started and run at a slow priming speed with the melt pump off until a preset priming threshold pressure is achieved. Once the priming pressure is achieved, the melt pump is started and run at a slow speed until the normal inlet (controlled) pressure is reached at which time the pressure control loop is turned on or enabled. The pressure loop control then controls the extruder speed until the system is turned off. The start-up control sequence must work when the extruder is empty as well as when it is already fully primed. The melt is essentially incompressible. This means that pressure builds very quickly once the extruder barrel is full, since the melt pump completely blocks off the barrel exit. The melt pump speed is normally the line speed master in a single extruder line since the melt pump output is directly proportional to melt pump speed. The melt pump speed is varied to control the extrudate output rate. Interlocks consist of: ¾ Over pressure shut down at a preset level ¾ Shutdown after preset time when pressure falls below the preset threshold ¾ No melt pump startup without minimum inlet pressure Tuning of melt pump control is dependent on the viscosity of the melt flow, screw design, and the die configuration. Changing of the extruder temperature profile which affects viscosity, screw design or condition, polymer or blend ratios, or die may necessitate retuning. Varying the amount of virgin or regrind in the fed stream may affect performance. 25

26 11.0 Advanced Control Concepts Speed Ratio Control: Maintaining the relationship or ratio between various speeds, such as extruder and a puller, is normally required in an extrusion system. In some lines, such as co-extrusion sheet or film lines, there can be a large number of speeds to co-ordinate. Establishing the correct ratios between various pieces of equipment and maintaining the precise current ratios as the overall line changes speed will greatly improve quality and product uniformity. In some cases it is necessary to vary the ratio between various pieces of line equipment to maintain product dimensions or other process parameters at the specified target. For example, the extruder verses melt pump speed ratio may be varied to maintain melt pump inlet pressure. Another example would be varying line speed verses extruder speed to maintain a specified product OD. Cascade Control: Cascade control refers to adding an outer control loop that controls or adjusts the set point of an inner loop to maintain a target set point. A typical example would be a cascade control loop that controls the OD of a tube by varying the puller speed based on the readings from a Laser OD gauge. The inner loop is the speed control loop, the outer loop or cascade loop is the diameter control loop where the Laser OD gauge provides the input and the cascade loop output is a revised set point for the inner speed loop. Cascade loops can themselves be cascaded to multiple levels. In the previous example, an additional cascade loop could be added by incorporating an additional OD gauge. The inner extruder speed loop would be adjusted by the first cascade loop based on the hot gauge at the extruder output. This loop would provide fast response to process upsets. The outer cascade loop would be based on a cold gauge at the end of the line that measured the final product. This outer loop would have the cold gauge readings as its input and its output would adjust the hot gauge inner cascade loop target set point. This cold gauge outer loop would handle longer term variations such as shrinkage that occur after the hot gauge. Other examples of cascade loop control are: ¾ Varying vacuum pump speed to maintain a present target vacuum level ¾ Varying a bleed or regulator valve position to maintain a preset target vacuum level. The inner loop would be valve position control ¾ Varying oven temperature to maintain a preset surface temperature read by an IR (Infrared) surface temperature transducer ¾ Adjusting 1 or more barrel zone temperatures to maintain a preset melt temperature The distinguishing characteristic of a cascade loop is that its output is a revised set point of an inner loop. The inner loop is assumed to be a closed loop which may be another cascade loop as in the hot gauge and cold gauge example, or a PID loop such as a temperature control loop. Cascade loops update based on time, distance, or a combination of time and distance. The OD cascade examples would be based on distance while the above vacuum cascade control examples would typically be based on time. 26

27 12.0 Other Extrusion Control Issues Inductive heating works by inducing a large localized current in a conductive material with an oscillating field. The extrudate, with a conductive carrier or core passes through a coil in which a large high frequency current produces localized eddy current heating of the conductive material. The amount of power applied to the coil and the dwell time of the extrudate within the coil's field determines the temperature rise of the conductive carrier or core. Non-contact Infrared sensors may be used to measure the surface temperature. As speed increases the dwell time decreases. This means that to maintain the same temperature increase, it is necessary to increase the power input to the inductive heater in direct proportion to the speed increase. It is also necessary to turn inductive heaters off when the line stops or damage to the extrudate or carrier will result from excessive heat buildup. 27