Take good care of your extrusion pressure transducers: eight tips on installing and maintaining melt-pressure sensors in your extruder will help them last longer and give you fewer problems.Melt-pressure transducers are used to improve extruder output and melt quality, improve production safety, and protect your machinery. Meeting product quality specifications such as part dimensions and surface finish with minimal material waste requires maintaining optimum processing pressure during production. Transducers are sensitive components that are easily damaged if not installed and maintained correctly. Following a few simple guidelines will help ensure the longevity of your sensors and enable you to get accurate, reliable measurements. Why sense melt pressure? Melt-pressure instrumentation on an extruder can range from a single transducer transducer, device that accepts an input of energy in one form and produces an output of energy in some other form, with a known, fixed relationship between the input and output. measuring one pressure point to a series of transducers measuring the entire process and tied to instrumentation that will record data, sound an alarm, give warnings to take corrective actions A corrective action is a change implemented to address a weakness identified in a management system. Normally corrective actions are instigated in response to a customer complaint, abnormal levels if internal nonconformity, nonconformities identified during an internal audit or , and relay information to a process-control system. Stable output with reduced scrap and material waste is maintained by pressure transducers Pressure transducer An instrument component which detects a fluid pressure and produces an electrical, mechanical, or pneumatic signal related to the pressure. at the die entrance in conjunction with a pressure control device. Pressure measurements across a screen pack and melt pump are also important for safety and optimum performance. A melt transducer downstream of the screen pack will alert operators if flow to the die is restricted, while one upstream of the screen pack will warn of a high-pressure situation that may produce excessive wear to the screw's thrust bearing. For processors using melt pumps, measuring both inlet inlet /in·let/ (-let) a means or route of entrance. pelvic inlet the upper limit of the pelvic cavity. thoracic inlet the elliptical opening at the summit of the thorax. and outlet pressures ensures constant melt flow at the die and signals any restriction in melt flow that could damage the pump. Tip # 1: Mount them correctly Damage to a pressure transducer is commonly caused by its installation in an improperly machined hole. In forcing a transducer into a too-small or eccentric hole, the transducer diaphragm diaphragm (dī`əfrăm'), term used to describe any of several large muscles, found in humans and other mammals, which separate two adjacent regions of the body. The most commonly known muscle of this class is the thoraco-abdominal diaphragm. may be crushed and the instrument will not function. Tool kits available for machining the mounting hole will help to make sure the holes are properly sized. A mounting torque of 100 to 200 in.-lb (for 1/2-20 UNF UNF University of North Florida UNF Unfinished UNF Ungdommens Naturvidenskabelige Forening (Denmark) UNF United Nations Foundation UNF Ungdomens Nykterhetsförbund UNF United Nations Fund UNF Unified National Fine threads) is essential to form an adequate seal, but excessive mounting torque will cause seizing. To prevent seizing, apply a high-temperature anti-seize compound to the transducer threads prior to installation. Transducers installed at a mounting torque above 500 in.-lb will be difficult to remove even when anti-seize compound is applied. Tip #2: Check hole thread size Abrasion abrasion /abra·sion/ (ah-bra´zhun) 1. a rubbing or scraping off through unusual or abnormal action; see also planing. 2. a rubbed or scraped area on skin or mucous membrane. from screwing a transducer into a mounting hole with an incorrect thread size will damage the instrument's threads. This may prevent a tight seal, resulting in melt leakage LEAKAGE. The waste which has taken place in liquids, by their escaping out of the casks or vessels in which they were kept. By the act of March 2, 1799, s. 59, 1 Story's L. U. S, 625, it is provided that there be an allowance of two per cent for leakage, on the quantity which shall appear , and the instrument will not function properly or safely. Correct dimensions for the mounting hole must be used to avoid thread galling. (The threads are generally the industry standard of 1/2-20 UNF 2B.) A mounting-well gauge plug should be used to verify that the mounting hole is correctly machined and cleaned. Tip #3: Keep mounting holes clean It is important to keep transducer mounting holes clean and free of any plastic build-up build·up also build-up n. 1. The act or process of amassing or increasing: a military buildup; a buildup of tension during the strike. 2. . Before an extruder is cleaned, all of the transducers should be removed from the barrel to avoid their being damaged. When they are removed, plastic is likely to flow into the mounting holes and harden hard·en v. hard·ened, hard·en·ing, hard·ens v.tr. 1. To make hard or harder. 2. To enable to withstand physical or mental hardship. 3. . If this plastic residue is not removed, extensive sensor tip damage will result when the transducers are reinserted. A cleaning tool kit can be used to remove the plastic contamination. But note that repeated cleaning may produce "too deep" holes and result in damage to the transducer tip. If this occurs, spacers should be used to raise the transducer. Tip #4: Select a good location Transducers may be located in the barrel, before a screen changer Changer The name given to a clearing member that is willing to assume the opposite position of a futures contract within a larger alternative exchange, of which it also is a clearing member. , before and after a melt pump, or in the die. When a transducer is positioned too far upstream in the barrel, unmelted pellets may abrade the transducer tip, resulting in damage. If a transducer is positioned too far back in the mounting hole, a stagnant stagnant /stag·nant/ (stag´nant) 1. motionless; not flowing or moving. 2. inactive; not developing or progressing. pool of melted plastic will build up between the transducer tip and the screw flights. Over time, this plastic will degrade TO DEGRADE, DEGRADING. To, sink or lower a person in the estimation of the public. 2. As a man's character is of great importance to him, and it is his interest to retain the good opinion of all mankind, when he is a witness, he cannot be compelled to disclose to carbon, which will prevent transmission of an accurate pressure signal. On the other hand, if the transducer extends too far into the barrel the screw flights can shear off the sensor tip. Tip #5: Clean with care All the transducers should be removed before cleaning an extruder barrel with either a wire brush wire brush n → brosse f métallique wire brush wire n → Drahtbürste f wire brush n → or special cleaning compounds. Either one of these can ruin the transducer's diaphragm. The transducer should be removed while the barrel is hot and its tip wiped clean with a nonabrasive cloth. The transducer hole should be cleaned at the same time with a cleaning drill and guide sleeve. Tip #6: Keep transducers dry Although a standard transducer's electronic circuitry is designed to withstand the rigors of the extrusion process, most transducers are not watertight and will not operate when wet. Make sure there are no leaks in the water-cooling jackets on the extruder barrel, which could damage the transducer. If you know that the transducer will be unavoidably exposed to water or moisture, specify watertight transducers. Tip #7: Avoid cold starts Both the transducer and extruder can be damaged if the extruder is not brought up to operating temperature before production begins. A sufficient "soak time" must be provided for the plastic to go from solid to molten state. In addition, if a transducer is removed from a cold extruder, material might adhere to adhere to verb 1. follow, keep, maintain, respect, observe, be true, fulfil, obey, heed, keep to, abide by, be loyal, mind, be constant, be faithful 2. the transducer tip, causing the diaphragm to tear off to pull off by violence; to strip. See also: Tear . Before removing the transducer, make sure the barrel is warm enough that any plastic present will be soft. Tip #8: Don't overpressure overpressure, n excessive pressure applied at the end of a physiologic joint range to confirm the severity of pain, thus helping determine the manual treatments. it Even though transducers are designed to withstand 50% overpressure (in excess of the rated maximum), avoid risk by making sure to use a model designed for your range of extrusion operating pressures. A good rule of thumb is to use transducers that are built to withstand twice the rated pressure in your process. If so, the extruder would have to run at an extremely high (and unsafe) pressure level for the transducer to fail. A Few Simple Rules * Threads must be machined to correct tolerances. * A metal-to-metal seal on the 45[degrees] seating surface is necessary. * The recommended recess for mounting holes is flush to 0.010 in. * Apply a high temperature anti-seize compound to the threads before installation. * A minimum 100 to 200 in.-lb of torque are recommended to achieve a firm seal. * Do not torque above 500 in-lb. NEED TO KNOW MORE? For more information, enter PT Direct code at www.ptonline.com. Dynisco Instruments, Franklin, Mass. (800) 396-4726 * PT Direct: 684EZ Douglas Joy is v.p. of marketing for Dynisco Instruments. He can be reached by e-mail at djoy@dynisco.com. |
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