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Dickens & Cai - ASME 1999


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Publication in the Proceedings of the 1999 ASME NDE Engineering Topical Conference, San Antonio, TX

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Dickens & Cai - ASME 1999

  1. 1. NDE OPPORTUNITIES FOR QUALITY ASSURANCE AND PROCESS CONTROL IN THE FOREST PRODUCTS INDUSTRY James R. Dickens, Ph.D. Zhiyong Cai, Ph.D. Temple-Inland Forest Products Corporation Panel Products Division Applied Research Center P.O. Drawer N Diboll, Texas 75941 ABSTRACT manufacturing facilities still rely on off-line destructive testing methods for quality assurance. In April 1998, Temple-Inland Forest Products Corporation entered the medium-density fiberboard (MDF) market with This paper will highlight some of the existing NDE the startup of a state-of-the-art mill in southern Arkansas. applications in our Company and to examine how NDE Following the acquisition of two additional plants in October technologies may be applied to quality assurance and process (Clarion, Pennsylvania, & Pembroke, Ontario), Temple control in the near future. became the leading producer of MDF in North America – virtually overnight. In recent years, Temple has strategically INTRODUCTION AND BACKGROUND expanded its long-standing role in the particleboard and gypsum wallboard markets. This rapid expansion includes yet Temple-Inland Forest Products Corporation operates 20 mills another new product entry for the Company – cementitious which manufacture a variety of primary wood and building fiberboard – which is expected to become a dominant product products, including lumber, plywood, particleboard, medium- in the exterior siding market. density fiberboard (MDF), gypsum wallboard, cement fiberboard, softboard sheathing, and hardboard siding & trim. As the Company builds new mills and modernizes older mills, To support these operations, the Company owns and manages computer control technologies are being integrated into the approximately 2.2 million acres of forestland, the majority of production processes. PLC systems, which run from client- which is located in East Texas. The Company also owns and server platforms, rely on accurate feedback of process data for operates a gypsum quarry in Fredricksburg, Texas. Annual precise control. This is a relatively well-defined task with sales of the Building Products Group exceed $600 million. respect to temperature, pressure, mass flow, position, velocity, and the like. As yet, accurate on-line measurement of finished Within the Building Products Group, production facilities are product characteristics is unavailable, and would lead to divided between two divisions: Solid Wood and Panel potential breakthroughs in process control and optimization. Products. The Solid Wood Division operates five sawmills and one plywood mill; the Panel Products Division operates NDE has been in use for many years in the forest products the remaining 14 mills which include four particleboard mills, industry, primarily as a means for process quality assurance. three MDF mills, four gypsum mills, one cement fiberboard Examples of commercial applications include: ultrasonic blow mill, one fiber products mill, and one laminating operation. detection systems, laser-based thickness measurement, With the exception of the laminating mill, all operations run infrared moisture measurement, and x-ray density 24 hours/day, 7 days/week. Total annual production capacity profilometry. While laboratory methods have shown promise of these 14 mills exceeds 3.5 billion square feet of product – for quantitative measurement of mechanical properties, the enough to circle the globe 7 times!
  2. 2. Of the 14 mills in our division, fully half are less than five years old; five mills are less than three years old; one is still X-ray Methods under construction. The remaining 7 mills represent older X-ray techniques are used for density profilometry and may technology, but have been upgraded during the last few years either be on-line, real-time systems or desktop versions used to replace analog controls with state-of-the-art PLC and in the quality control lab. In both cases the goal is to client-server control systems. The newer mills are being determine density profile through the thickness of the panel designed from the ground up with state-of-the-art technology. (this is not an imaging process, per se). Control of the density profile is critical for achieving good mechanical properties Computerization opens up new possibilities with respect to and other measures of board performance such as process control. Dozens of individual process settings can be machinability, surface quality, and warp. continuously and simultaneously monitored and adjusted with respect to established process targets. Even so, feedback Density profilometry is the only NDE technique we use which control systems rely on accurate process measures in order to provides quantitative, real-time feedback on the product – but be effective. For variables such as temperature, pressure, mass this technology is only being used in one MDF plant. On-line flow, position, velocity, etc., accurate sensors are already (real-time) density profilometry is particularly useful for available. The remaining challenge is to quantitatively adjusting press variables (e.g., temperature, time, closing measure the output – i.e., the final product – of the process, in speed) which directly affect density profile. Depending on the order to judge the effectiveness of process targets and control end use of the product, it may be more desirable to have schemes. At present, we continue to rely on offline higher density on the surface and lower density in the core destructive testing of a small sample of finished product as a (i.e., a U-shaped profile), or it might be more desirable to means for representing the population of all material have a relatively flat density profile. produced. This is a costly, time-consuming, and relatively impractical task. Small sample sizes lead to high probabilities While average density is highly correlated to the internal bond for both Type I (missed defect) and Type II (false call) errors, strength of wood products, it is by no means the only resulting in a potentially high risk to producer and consumer determinant. Resin content and press variables also strongly alike. influence internal strength, thus, measurement of density alone is not a sufficient indicator of internal board strength. NDE TECHNIQUES CURRENTLY IN USE For this reason, density profile is not the most robust method for determining internal bond strength. While the forest products industry has not widely embraced the state-of-the-art with respect to nondestructive evaluation Optical & Visual Methods (NDE) for process control, there exists already a number of Laser thickness gauges have been in use for 20 years in our sensors that employ some of the basic nondestructive three older particleboard plants. These are used to provide techniques, e.g., acoustic/ultrasonic, radiographic, & feedback on out-of-press thickness. When indicated by optical/visual methods. This section will briefly describe how changes in board thickness, adjustments may be made to the each method is employed in the various manufacturing press closing variables. There is a delicate balance to strike processes. with respect to out-of-press thickness – if the board is too thin, the sanded board will have low spots and must be Ultrasonic Methods downgraded. If the board is thicker than necessary to prevent Ultrasonic “blow” detectors have been in use for 30 years in low spots, excess energy and material is lost in the sanding wood panel manufacturing facilities [1]. The blow detector is process and surface defects may result. Even under optimal simply an array of stationary ultrasonic transducers conditions, sanding removes 5-10% of the material from the configured for through-transmission. The blow detector is board. located on the finishing end of the production line, usually right after the hot press. Each board passes through the sensor Moisture content is one of the most critical process variables array on its way down the production line. in wood manufacturing. The offline method of oven-drying a small sample of wood is not practical from a process control The function of the blow detector is to assess the integrity of standpoint, as fluctuations in moisture content can be rapid the panel, i.e., to ensure that no internal delaminations exist. and significant. Moisture content depends on many factors, Delaminations may be caused either from resin precure, including the initial moisture content of the wood, the manner incomplete cure, or excess steam pressure inside the panel. in which wet and dry wood is mixed, and processing variables Current systems in use today rely on pass/fail criterion for such as dryer temperature, mass flow rate, and resin variables. rejecting a defective product. As a result, it is possible to pass Excessive moisture leads to blows during the press cycle; board which has internal continuity, but lacks sufficient insufficient moisture leads to delaminations and generally mechanical strength to meet performance specifications. requires longer curing times. Thus a careful balance must be Because the blow detector uses a pass/fail decision criterion, maintained. the potential remains high for Type I and Type II errors.
  3. 3. The most common in-process method for measuring moisture finished board properties. This helps the operator make utilizes near-infrared (NIR) technology. By analyzing the adjustments to the blending & pressing variables in order to spectroscopic characteristics of reflected NIR radiation, the compensate for changes in the raw material stream. As an moisture content can be estimated. While there is some loss of indication of the potential significance of this technique, the accuracy compared to oven-dry testing, the need for authors note that six papers related to NIR are scheduled to be continuous feedback from the process is of primary presented at the annual meeting of the Forest Products Society importance and outweighs any loss of accuracy. on June 27-30, 1999 – enough papers to justify a full-day technical session devoted to emerging measurement Visual inspection is still relied upon for final product quality technologies. assurance. Typically, the sander operator is responsible for grading the boards as they feed out of the sander. At Numerous controlled laboratory studies have shown some production speeds, the operator has about 2-4 seconds to promise with respect to quantitative NDE of wood materials make a final visual check of the board as it passes by the [5-8]. The most significant hurdle appears to be transferring inspection station on its way to the unitizer (boards are the technology from a controlled lab environment to the plant typically 4’x8’, but in our largest particleboard plant the site where heat, humidity, dust, and vibration are the norm. operator must inspect a 9’x25’ panel!). The inspection task is Another key obstacle with respect to plant deployment is the assisted somewhat by the use of mirrors and bright lights; fact that the freshly-pressed board is undergoing gradual nevertheless, heavy demands are placed upon the operator for changes in temperature, moisture content, degree of resin maintaining accuracy and consistency over long periods of cure, and – consequently – strength. Thus, time-temperature- time. property relations complicate deployment into the plant environment. LITERATURE REVIEW DISCUSSION AND CONCLUSIONS While the techniques described in the previous paragraphs give an indication of the utility of NDE for process control, There are a number of potential benefits that make it the ability to accurately predict mechanical properties of the worthwhile to pursue an on-line measurement system. For finished product in real time remains elusive. Two examples example: of developments in real-time NDE are cited here, each with caveats. • reduced process variability, leading to lower process targets, and reduced material and energy Back in the mid-80’s, one of Temple’s major competitors usage developed an on-line ultrasonic internal bond measurement • reduced downfall and reject: system which was subsequently deployed into four • less frequent destructive tests particleboard plants between 1988 and 1990. An overview of • more timely operator response to changes in the system is reported in [2], where the system utilized a the process output through-transmission arrangement with rolling wheel • lower risk of Type I errors transducers. While the paper indicates that deployment was • improved utilization of press time, leading to initially successful, we understand from one of the system increased production capacity developers that no further installations were made, and the • reduced labor costs four systems originally deployed have since fallen into disuse. • reduced risk of Type II errors, leading to improved The reasons why are not known to us at this time. customer satisfaction and fewer product claims About two years ago a commercial vendor advertised an In addition to improving quality control, an on-line board “add-on” to their standard ultrasonic blow detector, which measurement system would provide a valuable tool to guide claimed to predict internal bond strength by means of a neural process improvement efforts. Feedback from the process network [3]. At that time we concluded that the device was would be available in minutes rather than hours, so that the still at an early stage of development and deployment. No net effect of process changes would be more quickly other commercially-available quantitative systems are known understood. to be available at this time. Operationally, quantitative NDE methods would prove most NIR techniques are stepping up as a legitimate means for beneficial to our primary wood products – particleboard and improving product quality. One application that we are aware MDF – which we produce at a rate of 1.1 billion square feet of utilizes NIR technology to characterize the wood raw per year, with relatively high quality costs. Gypsum products material [4]. This principle appears to be similar to that used represent about 2.1 billion feet of production, but traditionally with NIR for moisture detection, but the sensors are calibrated have much lower quality costs. Thus, NDE technology might to detect specific chemical differences in the constituent wood yield a higher return by focusing on improved efficiency of which are then empirically correlated to variations in the
  4. 4. the gypsum manufacturing process. In products like cement fiberboard and hardboard siding, internal bond and flexural strength are important to ensure good long-term performance. As with the other panel products, NDE might prove beneficial either as a process control tool or a quality assurance tool for exterior products. Based on our experience with building materials, we feel that the method with the greatest likelihood of success for QNDE is air-coupled ultrasound. Air-coupled ultrasound has been studied for many years, but only recently have advances in transducer design and electronic instrumentation brought air- coupled ultrasound to a high level of technical feasibility [9]. Our goal during the next few months is to prove the viability of air-coupled ultrasound under laboratory conditions, then move into one of the production operations with a prototype system for continuous on-line testing. REFERENCES 1. Birks, A.S., 1972, “Particleboard blow detector,” Forest Products Journal, Vol. 22 No. 6, pp. 23-26. 2. Rodgers, J.M., et al., 1991, “Acousto-ultrasonic measurement of internal bond strength in composite wood products,” Materials Evaluation, pp. 566-571. 3. Electronic Wood Systems GmbH, Germany. 4. Johnsson, B., et al., 1998, “Process modeling system for particleboard manufacturing, incorporating near-infrared spectroscopy on dried-wood particles,” Proceedings of the 32nd International Particleboard/Composite Materials Symposium, Washington State University, p. 164. 5. Dickens, J.R., 1996, “Evaluation of finger-jointed lumber strength using critically-refracted longitudinal waves and constituent wood properties,” Ph.D. dissertation, Texas A&M University, College Station, TX. 6. Tucker, J., 1996, “Angled-beam ultrasonic inspection of particleboard quality,” M.A. Record of Study, Texas A&M University, College Station, TX. 7. Han, M-B., 1993, “Nondestructive evaluation of end- jointed lumber using ultrasonic techniques,” Ph.D. dissertation, Texas A&M University, College Station, TX. 8. Bethi, R., 1994, “Grading of lumber using stress waves,” M.S. thesis, Texas A&M University, College Station, TX. 9. Grandia, W.A., and Fortunko, C.M., 1995, “NDE applications of air-coupled ultrasonic transducers,” Proceedings of the IEEE UFFC Symposium, Seattle, WA.