CompositesWorld
Issue link: https://cw.epubxp.com/i/546021
CompositesWorld.com 35 NEWS N E W S N S N E W S E N W S W In-Mold Monitoring for RTM Given Kistler's experience in manufacturing control sensors for injection molding, Lagonegro says the company's decision to branch into composites was a natural stepping stone. In view of the growing sophistication of composites manufacturing, the company displayed a prototype sensor for composites processing at the 2012 K Show, and is introducing its frst commercial mold- cavity pressure sensor, the 6161 AA, designed for both high- and low-pressure composite processing, this year. Te sensor is designed for mold cavity pressures as high as 200 bar, as well as vacuum infusion, and comprises a 4-mm diaphragm welded into a 9-mm cartridge. Te 6161 AA is a direct-contact sensor. Te resin acts directly on the front of the diaphragm, which generates a pressure curve via the use of a "charge amp" (transducer/amplifer that gauges the pressure, then converts it to a scalable output value). Cavity pressure measurement in RTM provides a number of distinct benefts, some similar to the injection molding of ther- moplastic, and others unique to both composites processing and composite materials. Like injection molding, the sensors in an RTM mold can be used to monitor and control sequencing of multiple gates, to monitor the fow front and to shut of injection. More distinctively, with respect to RTM, cavity pressure sensors can monitor, as suggested above, mold cavity vacuum pressure as well. Tis provides, in turn, better control over injection pressure and quality-related issues, such as air inclusions and voids, which could arise from an insufcient or inconsistent vacuum. Also, much like injection molding, the pressure curve gener- ated during RTM drives each part cycle and is a key tool that helps the fabricator establish process optimization (see the "Sensor- controlled mold cycle" graph, at top right). During the RTM cycle, pressure sensors inside the mold will detect if a vacuum is created and held. Assuming the vacuum is held, the process begins with infusion of the resin and catalyst. As infusion of the preform continues, the curve travels from a negative value, through zero, to the pressure threshold lower limit, at which time the pump is turned of, preventing overfll and potential uncoupling of the mold halves. Te use of a vacuum to infuse the part is, of course, one of the chief diferences between injection molding and a process such as RTM, and this poses a challenge to sensor design. "Tradi- tionally, our sensors have not been exposed to a vacuum," says Lagonegro, noting that the negative — as opposed to positive — pressure afects operating conditions and, therefore, posed a diferent set of sensor design considerations. Te 6161 AA, there- fore, is designed with an O-ring, which prevents low-viscosity resin from entering the gap in the mounting fange while the cavity is under vacuum. Te sensors are calibrated by Kistler before they are shipped to the customer. Te calibration data, and a NIST-traceable Calibration Certifcate, which is delivered with the sensor, confrms that the sensor performs with a linear output throughout the manufacturing pressure profle and, as a result, those output values will be absolutely reliable as a measure of pressure during the manufacturing process. Filling the void As a manufacturer of highly technical, structural composite parts for the automotive racing and aerospace industries, Calloway Carbon (Santa Ana, CA, US) decided to take the matter of process control in composites manufacturing, or lack thereof, into its own hands. Reeves Calloway, founder and owner, says the combina- tion of downward cost pressure and higher quality standards throughout the industry had been pushing the company in the direction of RTM and increasing amounts of automation for a number of years. Having reached the limit of what the company could do without process control technology, it opted to develop its own solution, in house. Sensors, it turns out, are merely part, albeit a key part, of a holistic process control system developed by the company, one that encompasses hardware and software to close the loop on RTM manufacturing. "In order to really automate RTM, or any closed molding process, we had to take it one step further than merely installing sensors in molds," Calloway says. Sensors, he reports, have to work hand- in-hand with software that will replicate all the sequences of part infusion, and monitor and control those sequences throughout each part cycle. "We created software, resulting in a recipe for a part that can control an automated injection system." In-mold sensors deliver continuous pressure readings to the process control software, which, in turn, monitors and controls resin metering, delivery, infusion and heating/cooling of the mold in a closed-loop confguration for each part cycle, a process the company refers to Sensors, as it turns out, are merely part, albeit it a key part, of a holistic control system developed by the company. Time Pressure 0 Sensor-controlled mold cycle Much like injection molding, a pressure curve generated during RTM drives each part cycle. Pressure sensors inside the mold detect when a vacuum is created and held. If the vacuum holds, the process begins with infusion of the resin and catalyst. As infusion of the preform continues, the sensor-recorded pressure, represented by the curve, travels from a negative value, through zero, to pressure threshold lower limit, at which time the pump is turned of, preventing overfll and potential uncoupling of the mold halves. Source | Kistler Instrument Corp.