In-mold Sensors – A Great Help in Achieving Automated RTM

 

In-mold sensors have been used for decades in plastic injection molding as part of a diverse process control portfolio. But recently, similar technologies have begun to make inroads into composite manufacturing through resin transfer molding (RTM), which is the most similar to injection molding of all closed-mold molding methods.

 

As customers place a high premium on higher part quality and better part-to-part consistency, the need for sensing technology has arisen. In response, several suppliers and at least one composite part manufacturer are developing and marketing new sensor technologies specifically for RTM, resin infusion and other types of composite manufacturing processes.

 

RTM’s processing characteristics, in particular, are a perfect match for the capabilities of process control sensors, says Paul Lagonegro, applications manager at Kistler Instruments. Composite part manufacturers appreciate RTM’s manufacturing capabilities because it can produce lightweight parts with short cycle times and achieve close tolerances. However, RTM typically requires an additive manufacturing process of preforms, which adds processing steps and requires a higher level of automation to achieve cost-effectiveness.

 

Given Kistler’s experience making control sensors for injection molding, Lagonegro says the company’s decision to move into composites was a natural progression. The company showed a prototype sensor for composite processing at the 2013 K show and launched its first commercial cavity pressure sensor, the 6161 AA, designed to meet the needs of high- and low-pressure composite processing.

 

The sensor, which measures cavity pressure up to 200 bar and vacuum inside the mold, consists of a 4mm diameter sensitive element welded to a 9mm outer diameter metal sleeve.

 

The 6161 AA is a direct pressure sensor. The resin contacts the front face of the sensor directly and the resulting signal is converted into a pressure profile by a charge amplifier.

 

Measuring cavity pressure for RTM has many distinct benefits. As with injection molding, sensors in an RTM mold can monitor and control multiple gate sequences, monitor flow fronts and stop injection. More specifically for RTM, cavity pressure sensors can also monitor vacuum pressure within the mold cavity, thus providing better control over injection pressure and quality-related issues such as air entrainment and porosity caused by insufficient or inconsistent vacuum.

 

Even more similar to injection molding, pressure profiles are developed during the RTM processing cycle of each part and are an important tool to help manufacturers achieve process optimization. During the RTM cycle, if vacuum is established and maintained, the pressure sensor in the mold can detect it. If the vacuum is maintained, the process of infusing resin and catalyst can begin. As the preform continues to infuse, the curve goes from negative through zero to the lower pressure threshold, at which point the pump is shut off to prevent overfilling and potential separation of the mold halves.

 

Of course, the application of vacuum when infusing the part is one of the main differences between the RTM process and injection molding, and it also presents challenges in sensor design. “Traditionally, our sensors have not been exposed to vacuum,” Lagonegro says, and because pressure affects the sensor’s operating conditions, the company came up with a different sensor design.

 

The 6161 AA design uses an O-ring to prevent low-viscosity resin from entering the gap in the mounting flange when the cavity is under vacuum. Kistler calibrates these sensors before shipping them to customers. Calibration data, along with a calibration certificate with “test methods traceable to NIST (National Institute of Standards)”, is delivered with the sensor to confirm that the sensor performs linearly across the entire manufacturing pressure profile, making the output values ​​absolutely reliable as a measurement of pressure during the manufacturing process.

 

 

Sensor-controlled molding cycle: During each RTM molding cycle of a part, a pressure curve is generated that is more like the pressure curve of injection molding. When a vacuum is created and maintained, it is detected by the pressure sensor in the mold. If the vacuum is maintained, the process of infusion with resin and catalyst can begin.

 

As the preform continues to be infused, the pressure value recorded by the sensor is reflected by the curve, from negative through zero to the lower pressure threshold. During this time, the pump is turned off to prevent overflow and potential separation of the mold halves

 

Filling The Gap

Calloway Carbon, a manufacturer of high-tech structural composite parts for the automotive racing and aerospace industries, decided to take a process control approach to composite manufacturing. Reeves Calloway, the company’s founder and owner, said that for many years, the industry’s demand for reducing costs and improving quality standards has been driving the company in the direction of RTM and increasing the amount of automation.

 

The company has developed a process control system that includes hardware and software to form a closed-loop control in RTM manufacturing. “Sensors were originally just one part of the process control system (albeit a critical part), but to truly automate RTM or any other closed-mold molding process, we had to go further than just putting sensors in the mold.”

 

The sensors must work in conjunction with software to replicate all the processes of part infusion and monitor and control those processes throughout each part cycle, Calloway said. “We created software to bring a recipe to a part that controls an automated injection system.” In-mold sensors provide continuous pressure readings to the process control software, which in turn monitors and controls the metering, delivery, infusion of resin, and heating and cooling of the mold in a closed-loop layout for each part production cycle. The company calls this process “recipe.” Once the recipe is understood, Calloway noted, it’s just a matter of replication.

 

The company has been using sensor-based automated RTM manufacturing for five years. Calloway says the advancement of sensor technology has been critical to successfully developing its own automation technology. “Fortunately, the sensor technology available commercially is really good.”

 

Equally important, RTM is the most amenable composite manufacturing method to automation because it is primarily a continuous process. Calloway says process control development for RTM can be broken down into a series of programming problems, such as how to ensure that step B is actually executed after step A is actually completed.

 

Of course, the growth of process control systems in composites will require the involvement of moldmakers. Kistler’s Lagonegro says the company’s early efforts will include moldmakers and product manufacturers.

 

Calloway Carbon cuts its own molds for this purpose, and in addition to having a process control system, it also controls the process settings. Calloway says the number, type and location of sensors in the mold depends on the part. Almost all of the company’s molds are CNC-machined aluminum molds. Calloway says many molds use two or more piezoresistive pressure sensors located on the top of the mold.

 

He calls the process of “deciding where to place the sensors” “part science, part art and part luck,” based on years of experience. Before production, each mold goes through an automated calibration process until the sensors are set within a predetermined range (the recipe) to allow the cycle to begin and process control monitoring can begin. Calloway’s software also allows production to run manually.

 

 

In-situ detection in the mold: Calloway Carbon embeds off-the-shelf piezoresistive pressure sensors in its polished aluminum molds, which, combined with its own proprietary software, monitor the RTM production process of structural carbon fiber composite parts. It is worth noting that the infusion of the part can only be carried out when the sensor is within the predetermined pressure limit.