January 2010

January Meeting

Decoupled Molding & Cavity Pressure Sensing with eDART
from RJG

If you are a plastics parts processor, injection molding technician, mold or part designer, quality engineer or quality manager, RJG has the tools to help you succeed.

The RJG name is synonymous with advanced injection molding techniques and equipment: Scientific and/or DECOUPLED MOLDINGSM, the eDART System™, the Master MolderSM and Tryout Shop Certification programs and more

Bob Reese of RJG, Inc. will present cavity sensing methodology at the Manufacturing laboratory of Houston Community College. The seminar will include on-machine training with a mold instrumented for cavity pressure sensing and monitoring.

Speaker: With RJG for three years, Bob is Regional Implementation Manager for the South Central Region. In this position, he is responsible for Consulting, Training, and Equipment Implementation for the Region. Previously was with Perlos (major molder for Nokia), where he implemented and utilized RJG technology and scientific molding methodologies for twelve years. His last position with Perlos was Global Engineering Manager, with responsibility for tool design, tool manufacture, and process development.

President’s Message

Happy New Year!

The New Year gives us the chance to close the books and reflect on the prior year, make adjustments for the New Year, and start with a fresh hope for new opportunities and challenges to come. As I enter 2010 I have taken the pulse of my customers, suppliers, and friends to gain perspective of the prospects for 2010. I would summarize the collective sentiments as “Uncertain Optimism”. All share an optimistic confidence that 2010 will be better than 2009 and the economists tell us that we are finally climbing out of this recession. However, we are uncertain as to the availability of credit from banks that will allow for capital investment and business growth, the policy actions our government will take to manage growth and curb inflation, the timing of a recovery that adds jobs, and the pace and magnitude of the recovery.

2009 was challenging for all of us involved in the SPE, however, I am encouraged by the camaraderie among colleagues to take a genuine interest in helping one another with job searches, supporting technical challenges and networking among professionals to facilitate the prosperity of our businesses and careers. We also continued our efforts to support our local student sections at 10 colleges and universities. 2010 will surely bring more success with these types of professional relationships working together to expand the technical knowledge of plastics and build better friendships.

Engaging and including young professionals is critical to our on-going success as a professional section. To this effort, Ayush Bafna of Dow Chemical will be kicking off our Young Professionals network in January. Please look for announcements on the SPE South Texas website and on Linked In. http://www.linkedin.com/groups?gid=2073302&trk=myg_ugrp_ovr

We are fast approaching our International Polyolefins 2010 February 21 – 24 at the Hilton Houston North. Please visit http://www.spe-stx.org/PolyolefinsConference.htm and plan to be part of this outstanding conference. We have had great early response in receiving over 90 new technical papers for this conference to date. I look forward to seeing you there.

If you have not been active in the South Texas section of the SPE, please consider coming to one of our monthly social breakfast or lunch meetings or contact any of the board members about participating in one of the committees. There are many opportunities to get involved, meet others in the industry, support education and learn something new. More information can be found at our website.

The mission of the SPE is to promote scientific and engineering knowledge relating to plastics. In addition to our professional technical programs, we continue our commitment to support local SPE student chapters, scholarships and grants at 10 colleges and universities in our region.

Happy New Year! I look forward to working with all of you to make 2010 a great year in the SPE and in our businesses. On behalf of all of the directors and chairpersons, we look forward to serving you in a way that will grow our businesses, expand our technical knowledge and our relationships.

Many blessings to you and your family in 2010!

Jeff Applegate
SPE South Texas President 2009-2010

Polyolefins Conference and FlexPackCon – Mark your calendars!

The 2010 International Polyolefins Conference and FlexPackCon will be held at the Hilton Houston North (formerly known as the Wyndham), Feb 22-24, 2010. 

This year’s program will again include both the traditional Polyolefins focus as well as the Flexible Packaging program. One of the themes for FlexPackCon will be sustainability and will include talks by Chet Rutledge, Director of Packaging Procurement,WalMart; Larry Dull, Partner, Packing Knowledge Group (PKG) LLC; and Victor A. Bell, President, Environmental Packaging International. They will discuss the WalMart scorecard and it’s extensions, how to used the scorecard to evaluate new opportunities, and packaging Life Cycle Analysis. FlexPackCon will also provide updates on food law (e.g. FDA) and potential regulatory expansions.

If you know of particular developments which should be presented at the conference, please contact Robert Portnoy for Polyolefins rportnoy@portnoytechnicalservices.com) or Andy Christie (andy@optexprocesssolutions.com) for FlexPackCon.  If you’d like to sponsor a coffee break or exhibit, please contact Emery Jorgenson (emery@jorgensonmachinery.com).

We look forward to seeing you at the conference!

http://www.linkedin.com/groups?gid=2073302&trk=hb_side_g

Join the South Texas Section of the Society of Plastics Engineers on Linked-In. Click on the above link and you will be directed to group page. Linked-In will serve as another communication tool for SPE-STX.

Linked-In  is a free web site for professional networking with over 40 million members as of May 2009.

Linked-In enables discussions and networking between SPE-STX members and those related to the plastics industry. Linked-In compliments the current SPE-STX web and membership to Linked-In is free. 

News Highlight
Linked-In SPE-STX group currently has 116 member as of 1 October. To date, the tool has been used primarily to promotes and discuss SPE-STX meetings. 

Members are also notified of meetings from board members plus email blasts sent by SPE International.  

As always, both the monthly newsletter and STPE-STX web site are the leading source of information on SPE-STX activities.  

Lunch and the Economy

Please join the MIT Enterprise Forum of Texas as they kick off 2010 with an Economic Forecast for the coming year! Given the tough business climate many faced in 2009, you won't want to miss hearing what these experts believe the coming year has in store.

Speakers: The distinguished panel includes Dr. R.W. (Bill) Gilmer, VP in charge of the El Paso Branch of the Federal Reserve Bank of Dallas, Dr. David H. Papell – Joel W. Sailors Endowed Professor of Economics and Chair: Department of Economics, University of Houston, and Ryan Colburn, Houston City President for Regions Bank, who will serve as our Moderator.

Bulletin Board

Plastics Information: Check it Out

The Houston Public Library on McKinney has resources on plastics and polymers. Check out their catalog at www.hpl.lib.tx.us. If you are not near the McKinney location, you can arrange to pick up your books at your local branch.

The Fondren Library at Rice University has the most complete collection of books on plastics and polymers. This is also a prime resource for patent and trademark information, as well as other US Government documents. You cannot check out books there unless you join Fondren Library [$50], but you can arrange for books to be sent to your library by inter-library loan. Use their catalog at http://library.rice.edu/.

The next best place to browse is at the MD Anderson Library at the University of Houston central campus. South Texas Section has donated many plastics books to this library. If you plan ahead, you can get a TexShare library card from a library where you are a member, which will allow you to check out books from any U of H library. Their catalog is at www.library.uh.edu/.

Book Bag

	Additives for Polyolefins Michael Tolinski, 304 pages, Jul-2009 ISBN-13: 978-0-8155-2051-1 $150.00 This book focuses on the polyolefin additives that are currently important in the plastics industry, alongside new additives of increasing interest, such as nanofillers and environmentally sustainable materials. As much as possible, each chapter emphasizes the performance of the additives in the polymer, and the value each relevant additive brings to polypropylene or polyethylene. Where possible, similar additives are compared by capability and relative cost. With major sections for each additive function, this book provides a highly practical guide for engineers and scientists creating and using polyolefin compounds, who will find in this book a wealth of detail and practical guidance. This unique resource will enable them to make practical decisions about the use of the various additives, fillers, and reinforcements specific to this family of materials. Contents: SECTION I: OVERVIEW OF POLYOLEFINS AND ADDITIVES 1 Introduction 2 Trends in polyolefin & additive use SECTION II: ENVIRONMENTAL RESISTANCE 3 Antioxidants and heat stabilization 4 Ultraviolet light protection & stabilization 5 Flame-retarding additives 6 Additives for modifying electrical properties SECTION III: MECHANICAL PROPERTY ENHANCEMENT 7 Overview of fillers & fibers 8 Factors determining selection of fillers and fibers SECTION IV: APPEARANCE ENHANCEMENT 9 Colorants 10 Nucleation and clarity SECTION V: PROCESSING AIDS 11 Processing aids for molding 12 Processing aids for extrusion SECTION VI: OTHER MODIFICATIONS OF FORM AND FUNCTION 13 Reducing density: Polyolefin foams 14 Coupling, compatibilizing, recycling, and biodegradability 15 Cross-linking 16 Sterilization & radiation resistance 17 Aesthetics enhancement and surface modification SECTION VII: CONCLUSION: INCORPORATING ADDITIVES 18 Adding Additives to resin References Index
	Flame Retardants for Plastics and Textiles: Practical Applications Edward D. Weil, Sergei V. Levchik, 2009 ISBN: 9781569904541 $100.00 An overview of flame retardants that are either in commercial use or in advanced stages of market development, reviewed polymer-by-polymer, supplemented by a brief overview of mode of action and interaction. It is more of a “how-to-do-it” book than an academic study. As such, it names trademarked materials as well as products in active stages of development, gives suggestions for selecting among alternatives, provides suggested formulations, and offers a starting point for the compounder or plastics fabricator to pass commercial flammability requirements. Contents: Flame Retardants in Commercial Use or Development for Polyolefins Polystyrenes and Thermoplastic Styrene Copolymers Flame and Smoke Retardants in Vinyl Chloride Polymers – Commerical Usage and Current Developments Current Practice and Recent Commercial Developments in Flame Retardancy of Polyamides Flame Retardants for Thermoplastic Polyesters Flame Retardants in Polycarbonates and Polycarbonate Blends Commercial Flame Retardancy of Unsaturated Polyesters, Vinyl Resins, Phenolics and their Composites Flame Retardants in Commercial Use or Advanced Development in Polyurethanes Current Flame Retardant Systems for Epoxy Resins Flame Retardants in Commercial Use or Development for Textiles Comments on Flammability and Smoke Tests Overview of Modes of Action and Interaction of Flame Retardants Directory of Flame Retardant Manufacturers, Distributors, and Compounders
	Plastics Manufacturing Systems Engineering: A Practical Approach David O. Kazmer, 2009 ISBN: 9781569904626 $100.00 Plastics manufacturing is a highly interdisciplinary endeavor requiring knowledge related to materials science, physics, engineering, and management. Because of this diversity, the plastics process engineer interacts with many stakeholders, including customers, designers, materials suppliers, machine builders, mold/die suppliers, systems integrators, operators, quality engineers, and managers. With so many stakeholders involved, it isn’t surprising that many plastics manufacturing processes are not precisely engineered systems. The resulting processes can be poorly designed, requiring too much investment to achieve too little productivity. This book was written to educate and support plastics processing engineers, but is also highly useful to others involved with plastics manufacturing who are performing process development, research, and even machinery design. It uses a manufacturing systems engineering approach to provide guidance about plastics manufacturing as an integrated system with broadly applicable analysis of the underlying subsystems. Contents: Background, Plastics Manufacturing Systems, Heating and Cooling, Hydraulics and Pneumatics, Electric Drives, Process Sensors, Signal Conditioning, Data Acquisition Systems, Machine Controllers, Process Control, Process Characterization, Process Optimization, Quality Control, Automation, Statistical Labor Data, Material Properties, Unit Conversions, Matlab Primer
	Lawsuit!: Reducing the Risk of Product Liability for Manufacturers Randall L. Goodden, 2009, 359 pages ISBN: 9780470177976 $80.00 Reduce your exposure to civil lawsuits. Addressing product liability and laws in both the U.S. and internationally, this book helps product manufacturers and engineers develop and implement proactive processes that can reduce liability concerns and potential lawsuits. It discusses preventive measures in the engineering, development, and manufacturing of products and explains the procedures and processes manufacturers must have in place to reduce the likelihood of liability – as well as to provide the best defense in case of a lawsuit.
	Qualifications, Startups and Tryouts of Injection Molds W. J. Tobin, 2003, All inclusive CD $32.00 The truth is you don’t need fancy gizmos to do scientific molding. All you need is a machine where you can read the settings, an accurate weigh scale, and a computer. A universal setup guide only needs a little homework on your part to read each machine’s manual and the ability to read the settings. If you had a few days and an engineering textbook you could actually write these spreadsheets yourself! It is NOT MAGIC. This CD uses a practical, down to earth, easy to use approach to bringing a mold on-line and producing consistently good parts. CD contains both filled in tutorials showing what inputs are required and worksheets for you to fill in. CD Contents: Contents of the 90 page book. Spreadsheets in Microsoft Excel .xls files: Optimum fill calculation – the melt viscosity curve, Gate Freeze off, Cooling time, Cavity balance for multi cavity tools, Cp CpK calculator, Universal setup sheet – includes how to translate from one machine to another, a waterline map, Runner size optimization, and a calculator to determine your process window settings. Microsoft Word .doc Files: Mold Check List, Mold History / Maintenance forms. The text includes a complete explanation of the experiments to optimize your injection molding cycle. Other Topics are: Mold Maintenance – Inspection and General Maintenance, Mold hang and Mold Pull procedures, Mold Storage, Basic Chemistry, Quality Control/ Quality Assurance – Cp and CpK explained and how to use them, Sizing Sprues, Nozzles and Runners for your mold, Hard Copy of the Mold Check List, Mold History and a Data Form to fill out for the experiments, and how to calculate a process window that will allow you to adjust the process as it drifts and still make acceptable parts.
	Plastic Conversion Processes: A Concise and Applied Guide Eric Cybulski, 2009; 165 pages $70.00 Many books describe a single plastic-conversion process, like injection molding, but until now, none has described and compared several processes. This book provides a basic overview of seven conversion processes used in the industry. These processes account for more than 97% of all plastic products. Each chapter begins with a process attribute table to serve as a quick guide. The particular conversion process is then briefly described, along with a short history. To better explain each process, sections detailing equipment, tooling, and materials have been added. Also included are sections on design guidelines and on how to identify which process was used to manufacture a plastic part. Contents: Injection Molding Plastic Extrusion Blow Molding Thermoforming Reaction Injection Molding Rotational Molding Compression Molding
	Mixing and Compounding of Polymers: Theory and Practice, 2nd Edition Ica Manas-Zloczower, 2009; 850 pages ISBN: 978-1-56990-424-4 $249.00 Finally available in its second edition, this classic monograph covers everything from the basic principles to the various practical applications of state-of-the-art mixing and compounding. It discusses the basic mixing mechanisms encountered in polymer processing; the latest results in modeling, flow simulation and visualization, and scale-up rules for the most important batch and continuous mixers; the properties of various additives used in the plastics and rubber industry and their effects on the properties of the compound; working principles and practices for reactive polymer compounding; compatibilization mechanisms applicable to blends and composites; mixing practices in the current commercial mixing devices; key aspects of mixing at nanoscale; and scale-down of mixing equipment and fundamentals of microfluidics. Contents: Part I: Mechanisms and Theory Basic Concepts - Mixing of Miscible Fluids - Mixing of Immiscible Fluids - Dispersive Mixing of Solid Additives - Distributive Mixing - Distribution Functions and Measures of Mixing Part II: Mixing Equipment - Modeling, Simulation, Visualization Batch Equipment Simulation - Batch Equipment Visualization - Continuous Equipment Simulation - Dispersive Mixing Devices in Single Screw - Twin Rotor Mixers - Co-Kneader - Visualization - Scale-up of Mixing Equipment - Scale-down of Mixing Equipment Part III: Material Consideration, Properties and Characterization Solid additives (inorganic) - Solid additives (organic) - Compatibilizers (mechanisms, theory) - Material Consideration for Mixing at Nanoscale - Effect of Mixing on Properties of Compounds - Effect of Mixing on Rubber Properties Part IV: Mixing Practices Internal Mixers - Single Screw Extruders - Twin Screw Extruders - Intermeshing Twin Screw Extruders - Reciprocating Screws - Reactive Compounding - Farrel Continuous Mixer
	Injection Molding: Fundamentals and Applications Musa R. Kamal, Avram I. Isayev, S. Liu, 2009; 800 pages ISBN: 978-1-56990-434-3   $249.00 This book surveys the state of the science and technology of the injection molding process. It represents a comprehensive, balanced mix of practical and theoretical aspects for a wide range of injection molding applications. The authors of the 21 chapters are experts and leaders in their respective areas of specialization in the injection molding field. While it is not possible to cover all aspects of such a dynamic growing field, readers should find sufficient information and background to become acquainted, at various levels of depth, with key components of the science and technology of injection molding. Contents: Injection Molding: Introduction and General Background Injection Molding Machines, Tools, and Processes The Plasticating System for Injection Molding Machines Non-Conventional Injection Molds Gas Assisted Injection Molding Water Injection Techniques (WIT) Flow Induced Fiber Micro-Structure in Injection Molding of Fiber Reinforced Materials Injection Foam Molding Powder Metal Injection Molding Micro Injection Molding Internal Visualization of Mold Cavity and Heating Cylinder Injection Molding Control Optimal Design for Injection Molding Development of Injection Molding Simulation Three-Dimensional Injection Molding Simulation Viscoelastic Instabilities in Injection Molding Evolution of Structural Hierarchy in Injection Molded Semicrystalline Polymers Modeling Aspects of Post-Filling Steps in Injection Molding Volumetric and Anisotropic Shrinkage in Injection Moldings of Thermoplastics Three-Dimensional Simulation of Gas-Assisted and Co-Injection Molding Processes Co-Injection Molding of Polymers
	ANTEC™@NPE 2009 Conference Proceedings - Thumb Drive SPE, Chicago, Illinois USA, June 2009, Thumb Drive $250.00 In Chicago in 2009, ANTEC® (Annual Technical Conference), sponsored by the Society of Plastics Engineers, celebrated its 67th year of excellence. The largest peer-reviewed technical conference serving the plastics industry, ANTEC® is perfectly positioned to help the plastics specialist achieve new levels of professional development. Order the ANTEC® 2009 proceedings on thumb drive or CD-ROM. Includes 700+ papers detailing the latest developments in: Alloys and Blends, Applied Rheology, Automotive, Biopolymers, Blow Molding, Color and Appearance, Composites, Decorating and Assembly, Electrical and Electronic, Engineering Properties and Structure, Extrusion, Failure Analysis and Prevention, Flexible Packaging, Injection Molding, Joining of Plastics and Composites, Marketing and Management, Medical Plastics, Mold Making and Mold Design, Nano/Micro Molding, Plastic Pipe & Fittings, Plastics in Building and Construction, Plastics Environmental, Polymer Analysis, Polymer Modifiers and Additives, Process Monitoring and Control, Product Design and Development, Radiation Processing of Polymers, Rotational Molding, Thermoforming, Thermoplastic Elastomers, Thermoplastic Materials and Foams, Thermoset, and Vinyl Plastics.
	Injection Molding - Thumb Drive SPE, Chicago, Illinois USA, June 2009, Thumb Drive $50.00 Injection Molding Titles Include: Optimizing the Molding Process — Chapter from Plastics Technician’s TOOLBOX, by Jerry Golmanavich, Golmanavich Enterprises Shear Induced Imbalances and MeltFlipper® Technology, by John P. Beaumont, Penn State Erie & Beaumont Technologies, Inc. The Warpage Simulation with In-mold Constraint Effect in Injection Molding, by Dr. Venny Yang, CoreTech System Co., Ltd. The Investigation of Flow Behavior of Polymeric Melts in Water Assisted Injection Molding, by Dr. Chao-Tsai Huang, CoreTech System Co., Ltd. Injection Molding of Woodfiber Plastics Composites, by Michael Burgoyne State of the Art of Electric injection Molding, by M. Barr Klaus, Electric Injection Services, Inc. Mold Ejector Pin Melt Flow Volume Sensor, by Fred Buja, FjB PlasTechnology Water up to 400°F – The Best Way To Heat up Your Mold, by Anna Birkhofer, consultant Asia Tooling and Molding: How To Be Successful, D.B. "Dusty" Rhodes, Waypoint Bellwether, Inc. It Is as Easy To Be an Injection Molding Compounder as It Is To Be an Injection Molder, by Peter Lipp, Krauss-Maffei Corporation Effect of Injection Molding Process Parameters on the Morphology and Quality of Microcellular Foams, by Jingyi Xu Microcellular Injection Molding of Polylactide-Montmorillonite Nanocomposites, by Sarah Gong, Assistant Professor, University of Wisconsin-Milwaukee

SPE-STX Board Meeting

Location: Houston Engineering & Science Society, Galleria area, Houston
Date: December 14, 2009

A Review of In-Mold Pressure and Temperature Instrumentation

David O. Kazmer, Peter Knepper, Stephen Johnston
University of Massachusetts Lowell

Abstract

A survey of commercially available and broadly used pressure and temperature sensors for injection molding is presented. The various pressure and temperature sensing means are reviewed along with the geometry and performance of common transducers. Usage and trade-offs in sensor design and selection is discussed.

Introduction

The economic and technical requirements for injection molded components continue to increase, supported by corresponding improvements in process monitoring and control technologies. To obtain the desired critical quality attributes in molded products, the injection molding process must be consciously instrumented and designed such that the key process variables are observable and controllable. Failure to understand and control the linkage between process variables and quality attributes may result in undesirable levels of defects during production, unattainable levels of precision, as well as technical and economic infeasibility of the molding application.

A fundamental difficulty in monitoring and control of injection molding is that few of the final molded part properties can be ascertained from process instrumentation within the molding cycle. Instrumentation does not yet exist, and may never exist, to yield information about aesthetics, structural integrity, and other part properties prior to opening of the mold and inspection of the part. For instance, a pressure sensor may be placed at the end of flow to detect the arrival of the melt. Yet such a sensor may or may not be able to consistently predict the formation of flash during filling or sufficient part packing so that the molded component(s) meet the desired level of precision. As such, injection molding applications require integrated product and process design, in which the product development process provides not only the design of the component (geometry, material, specifications), but also the validated design of the molding process (process parameters, sensors, allowances).

The injection molding process consists of multiple coupled stages, in which the dynamics of each stage are determined through control of different but related machine elements such as motors, heaters, servo valves, etc. These machine elements are typically controlled via a closed feedback loop as shown by the inner most loop in Figure 1, in which the control signal is determined by real time comparison of the desired machine set-points with their corresponding observed state, such that the difference (or error) is used to correct the process.

The performance of a closed loop controlled machine is dependent upon a number of system properties, such as the inertia or dynamic behavior of the machine elements, availability and amount of control energy applied to the machine element, the time response and resolution of the sensors providing feedback, and the validity of the control laws which convert perceived errors into corrective actions. It should be realized that sustained advances in hardware devices and software algorithms have led to substantial gains in process control performance, such that the molding process control is no longer limited by the response time or algorithmic complexity of the controller. Control system response times (from input of feedback signal to output of control signal) of 2 milliseconds are quite common, with sub 10 microsecond response times widely available.

While machine control is important, it has become generally accepted that the polymer state (pressure, temperature, and morphology) ultimately determines the molded part quality. As such, recent technology developments have rightly focused on monitoring these state variables and (when possible) closing the loop between the machine parameters and the polymer state as shown by the intermediate control loop in Figure 1. When implemented, advanced control strategies can provide increased molded part quality and consistency, leading to lower labor content and substantial economic advantages in production. Towards this end, the two most dominant variables (pressure and temperature) that define the state of the melt are next discussed.*

Pressure Sensors

Pressure sensing of the polymer melt poses some significant challenges relative to pressure sensing of compressed air or hydraulic fluid. One significant issue is that melt pressures routinely exceed 150 MPa, with some molding applications requiring nearly 300 MPa. A second issue is that the surface of the sensing head is exposed to abrasive and/or corrosive materials at high and fast cycling temperatures, frequently above 300C. A third issue is that the body of the sensor is embedded in a mold at varying temperatures, often causing long term variation in output.

A brief listing of several available pressure sensing means is provided in Table 1 [1]. Of these, the most common cavity pressure sensor designs are based on 1) strain gage, 2) piezoelectric, and 3) indirect methods. Typical layouts are provided to relative scale in Figure 2; Table 2 lists several pressure sensors available from several prominent manufacturers. Strain gage sensors utilize a diaphragm that displaces when exposed to the melt pressure. In this design, the relatively large diaphragm displaces mercury fluid in a thin capillary, thereby gaining mechanical amplification of the diaphragm displacement. Furthermore, the mercury fluid and control electronics are removed from the melt temperature, which tends to improve the robustness and repeatability of the sensor. Unfortunately, this sensor contains mercury which may be undesirable in some production facilities, and also has a relatively slow dynamic response on the order of milliseconds.

Piezoelectric transducers measure the charge generated on a piezoelectric crystal under an applied stress or load. Piezoelectric pressure sensors measure dynamic and quasi-static pressures, and are not apt at measuring nearly static loads (e.g. extrusion) due to leakage of the charge over time. The common modes of operation are charge mode, which generates a high-impedance charge output; and voltage mode, which uses an amplifier to convert the high-impedance charge into a low-impedance output voltage. Continued improvements in piezoelectric sensors have yielded sensors that are robust with respect to structural design and signal conditioning. Given that piezoelectric sensors have very high stiffness and provide for direct charge generation under applied load, these sensors provide for very fast dynamic response, typically on the order of microseconds.

Surface mounting of pressure sensors may not always be feasible due to part surface requirements, mold actuation, physical obstructions, and other issues. In such cases, the melt pressure can be measured indirectly via a load pin and load cell combination as shown at right of Figure 2. Load cells in molding applications typically sense load via a strain gage or piezoelectric cell as discussed above. Often, the load sensor is placed underneath an ejector pin, which simplifies the pin, sensor, and wiring installation. However, care must be taken with this strategy to ensure that there is no interference between the pins and the mold plates. Also, the pin diameters must be closely sized with the load cell to ensure adequate signal to noise ratio.

Temperature Sensors

Temperature sensing of the polymer melt poses many of the same significant challenges as pressure sensing, though temperature sensors typically have no moving parts and may be more mechanically robust and less expensive then pressure sensors. However, temperature sensors are generally not as accurate recording melt temperature as pressure sensors are recording melt pressure. The reason is that melt temperature sensors are necessarily embedded in mold steel, so there tends to be significant heat transfer from the sensor head to the surrounding metal. Accordingly, temperature sensors may have a significant phase lag and steady state error in the measurement of melt temperature.

A brief listing of several available sensing means is provided in Table 3 [2]. Of these, virtually all melt temperature sensors used in industry are thermocouples that provide an output voltage based on the Seebeck effect. In thermocouples, metals with dissimilar conductivities conduct heat and electrons at dissimilar rates, thereby providing an output voltage. Common melt temperature transducers are shown in Figure 3; Table 3 lists several temperature sensors available from several prominent manufacturers. Immersion melt temperature sensors, shown at left, are commonly used in nozzles to measure the temperature of the flowing melt stream. This type of temperature sensor has limited response time and accuracy due to its reliance on heat conduction from the melt stream to the thermocouple, which occurs at the same time as heat conduction from the melt and the thermocouple to the nozzle. However, the melt stream thermocouples certainly provide more information about the process than thermocouples located in the steel that are commonly used for machine control.

Surface mounted temperature transducers, shown at center of Figure 3, are commonly used in molds to estimate the temperature of the melt:mold interface. This type of temperature sensor has limited response time and accuracy due to its reliance on heat conduction from the cooling melt to the thermocouple, which occurs at the same time as heat conduction from the melt and the thermocouple to the surrounding mold steel. However, surface mounted temperature transducers can be very small in size (1 mm diameter in this example) and have been shown to yield valuable information relating to the location of the melt front and consistency of the molding process.

Shown at the right of Figure 3 is an infrared melt pyrometer that was developed and sold by Sandretto around 1990. While this product is not commercially available, this melt pyrometer measures the emitted radiation through an emerald lens to estimate the temperature of the polymer melt. Compared to conduction-type temperature transducers, pyrometers have excellent dynamic response and sensitivity. Unfortunately, issues related to cost, calibration, size, and reliability prevented widespread commercial adoption. Recent work in fluoroscopy with doped polymers may prove to provide many of these benefits.

Discussion

There has been a definite trend towards increased instrumentation in support of automation on the shop floor. This trend is due, in part, to the increased capabilities and reduced costs of sensing systems that technological progress has enabled. This trend is also due, in part, to the consolidation of smaller molders into mega-molders who have greater technological sophistication and can provide the economic and educational investment required to leverage a company-wide implementation of process and quality control systems.

Selection: It is vital to consider both the control system specifications and the molding process characteristics when implementing sensing systems. Sensor selection should be driven first by the specifications that originate from the process. These specifications most commonly include: allowable size of sensor head, operating pressure, operating temperature, abrasion resistance, accuracy, and response time. In general, trade-offs abound between multiple specifications. For instance, smaller sensor heads are generally less accurate due to their lesser contact area and input energy than larger diameter heads. Similar trade-offs often exist between operating pressure/temperature and accuracy/response time.

When selecting sensors, the system builder should also consider inter-operability with other sensors, signal conditioning, cabling, data acquisition, and related software. Just as with mechanical components such as fasteners and hoses, support and cost issues abound with sensing systems. It is a good practice, where possible, to standardize on input and output specifications, connector types, etc. The alternative to standardization is the procurement and maintenance of multiple power supplies, signal conditioners, cables, and connectors. These “add-ons” are very necessary components in the sensing system that often exceed the cost of the sensors. For this reason, several consulting firms have arisen that provide turn-key process and quality control systems, which reduces the technical burden on end-users.

Process Development: There is the unfortunate tendency in the industry for molders to blindly use sensing systems that don’t work for their application. As of this date, there is no “magic bullet” for process instrumentation and control that is universally successful across the molding industry. The most fundamental issue preventing the successful development and deployment of sensing systems is the lack of observability of part quality during the molding process. Given that the diversity of application geometry and requirements across molding industry, it is quite impossible for a single sensing strategy to fulfill all of the industry’s needs.

A tour of many lights-out molding facilities indicates that successful sensing systems are typically developed inhouse, with the design based upon deep application expertise. This implies that end-users should reflect on the process to quality relationships that they have established in their applications, and use this knowledge to develop a protocol for specifying the number, type, and location of sensors in their molding process. A methodology for system development is provided in Figure 4. While generic turn-key systems may not fit exact end-user requirements, most providers will customize their sensing systems on a cost-plus basis to deliver validated capabilities to the end-user. Documented strategies have included the use of [3]:

• plastication time to ensure material consistency;
• cavity pressure transducers at end of flow to guarantee cavity filling;
• cavity pressure transducers near the gate to indicate repeatability of packing;
• use of tie-bar strain gauges to estimate pressure and tonnage across all cavities;
• ultrasonic transducers to estimate solidification;
• part weight to estimate dimensions;
• robots, measurement fixtures, and/or laser metrology to estimate dimensions;
• cameras to ensure ejection of parts from the cavities;
• and others.

Sensing systems should identify and avoid the production of a significant fraction of defective products. Successful lights out operations effectively use conservative process control approaches for round the clock, automated production of precision parts in which a small percentage of suspect molding are discarded without human inspection. On the other hand, improper use of sensing systems may prove worthless or negative in value by continuously accepting defective moldings and/or rejecting acceptable moldings. As such, automated on-line quality assurance must be validated continuously with off-line, higher fidelity metrology techniques.

Trends: Some significant trends in process instrumentation and control include improvements in sensor miniaturization, sensor fusion, and system integration. With respect to miniaturization, molders are seeking to incorporate a large number of very small and lower cost sensors into molding applications. This trend is being driven by standardization of protocols, for instance, to instrument every cavity in precision applications, or to detect different features within a molding process such as the arrival of the melt at the beginning and end of the cavity. Sensor suppliers have responded with increasingly small sensors with reasonable performance characteristics.

While further miniaturization of sensors is unlikely since the sensor diameter is approaching the cable diameter, the number of sensors per application will continue to increase as cost permits. The use of sensor arrays as proposed by Coulter [4] and Gao [5] will enable the estimation of pressure and temperature contributions throughout the cavity. When coupled with on-line process simulation, it is likely that many molded part quality attributes will be predicted to reasonable certainty on a cycle by cycle basis, thereby rendering a decision on the acceptance of the molded part(s) before the mold opens.

Advances in system integration are providing for easier installation and utilization of sensors. Specifically, the lower cost of integrated circuits is moving much of the signal processing from the controller cabinet to the sensing module. In these newer designs, the sensor itself includes analog to digital acquisition, internal logic for automatic calibration and error diagnostics and noise filtering, as well as serial communication protocols. While reliance on 0-10 volt and 420 milliamp analog signals will not disappear, new sensor communications such as Internet 0 (I0), Highway Addressable Remote Transmitter (HART®), universal serial bus, Profibus, Devicetnet, Fieldbus, Ethernet, and power over Ethernet will significantly improve sensor performance while lowering total system cost.

Conclusions

The technical and economic requirements of injection molded components can best be fulfilled through a strategic and rational use of process instrumentation and control systems. Since molded part quality attributes are rarely observable from the process, estimators should be developed based upon real time process data and validated with corresponding control techniques to guarantee the quality of the moldings. Continuous advances in sensor technology will enable highly competitive production of high quality parts for those molders who are able to understand, invest, and leverage the technology.

Acknowledgements

This work has been partially supported by the National Science Foundation under grant DMI-0428336 and DMI-0428669.

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