- May 24, 2017
By Marius M√ºtzel, Kistler Group
In the past, fiber-reinforced plastics (FRPs) were mainly used in the aerospace sector ‚àíbut in response to the trend towards boosting efficiency and optimizing resources, their use in the automotive industry is now increasing as well. It is essential to use lightweight construction materials so as to achieve the increased efficiency levels required by law, and also to reduce CO2 emissions.
By Marius Mützel, Product Manager, Kistler Group
Composite components offer outstanding mechanical characteristics as well as low weight –that's why their importance is constantly growing. In the past, fiber-reinforced plastics (FRPs) were mainly used in the aerospace sector −but in response to the trend towards boosting efficiency and optimizing resources, their use in the automotive industry is now increasing as well. It is essential to use lightweight construction materials so as to achieve the increased efficiency levels required by law, and also to reduce CO2 emissions. The reason is clear: lighter cars consume less energy – which means less fuel. To attain the fleet-wide targets by 2030, automobile manufacturers are obliged to increase the quota of lightweight components in their vehicles from 30% to 70% in order to compensate the increased vehicle weight due to electric powertrains or batteries. This is why a study by management consultants McKinsey & Company predicts that the automotive supply and plant engineering sectors will see a sharp increase in annual sales revenue from lightweight construction components made of high-strength steel, aluminum and carbon fiber reinforced plastics (CFRPs). Depending on the price trend for raw materials, this revenue is likely to grow from about EUR 70 billion to more than EUR 300 billion by 2030. Growth in excess of 5% is forecast for the use of fiber composites alone.
Because of their high strength-to-weight ratios and their rigidity, fiber composites are ideal for applications in the automotive industry. By using CFRPs instead of steel, the weights of individual components can be reduced by as much as 30%. Production volumes are on the increase due to the growing use of these materials in the automotive sector, so there is a need to step up the automation of manufacturing processes such as RTM or wet molding. More and more often, cavity pressure sensors are used to monitor these processes so as to guarantee reproducibility and quality. Such sensors assist manufacturers of CFRP components with process optimization and early detection (or even total avoidance) of rejects.
ComoNeo: the process monitoring system
Quality Assurance in the Automotive Industry
The production of an engine hood using the HP-RTM (High Pressure Resin Transfer Molding) process clearly shows the importance of quality monitoring. To minimize cycle times, very fast-curing resins are used in the process. However, this means that the mold has to be injected more quickly, so this is usually done under high pressure. Pressures in the mold may be as high as 150 bar.
An engine hood is a relatively large and heavy component in a car, so it harbors good potential for reducing weight by using lightweight construction methods. Engine hoods have to meet demanding requirements: their surfaces are categorized as class A, so the surface structure must be perfect −no imperfections or indentations are permitted. At the same time, the mechanical characteristics of the hood must be guaranteed. Process monitoring with cavity pressure sensors makes it possible to identify errors during production so that no further processing is carried out on defective parts. The high production and material costs for engine hoods also make it essential to identify errors at an early stage. This goal can be attained thanks to process monitoring systems. For this purpose, Kistler offers the entire measuring chain from the sensor and the connection technology through to the monitoring system (ComoNeo).
Cavity Pressure Measurements Ensure Quality Control in the HP-RTM Process
The pressure curve – which the ComoNeo process monitoring system can visualize and evaluate – is a critical factor in process optimization and production monitoring. Characteristic process phases such as evacuation, injection and curing are clearly visible in the pressure curve (see Figure 1). Anomalies in the pressure curve indicate production errors in the engine hood (Figure 2). Capture and recording of the pressure signal also ensures that the manufactured components are traceable. For all these reasons, the pressure curve is an indispensable tool for optimizing production and ensuring quality.
Evacuation of the mold is the first phase of the process. The vacuum level and consistency are key factors in ensuring that the engine hood is manufactured without air bubbles. An indication that the vacuum is too weak or inconsistent suggests that the sealing function of the mold is inadequate. This would result in a high proportion of air in the mold, possibly leading to air inclusions in the finished hood. Inadequate evacuation of the mold can be detected from the pressure curve. A monitoring window placed over the pressure curve allows fully automated detection of this effect so that the start of injection can be halted – before a part of insufficient quality is produced. The benefit: wastage of high material costs for reject parts can be prevented (see Figure 2, curve 1).
The second phase of the process consists of injecting the resin-hardener mix and impregnating the preform. As soon as the resin reaches the sensor position, the cavity pressure starts to increase continuously. This maps the constant increase in flow resistance which the resin must overcome during impregnation of the preform. The rise in the pressure signal during the injection phase depends on the permeability of the preform. Deviations from the normal pressure gradient therefore indicate issues such as differences in the composition of the preform or in the orientation of individual layers, or foreign bodies in the mold. Here too, special monitoring windows ensure compliance with the reference pressure rise (see Figure 2, curves 2 and 3). If the evaluation results are traced back to the mixing unit, the injection pressure can be reset provided that the deviations from the reference rise are small enough to exclude a faulty preform.
On complete injection of the mold, at the start of phase 3, the pressure gradient rises dramatically after injection is finished. This may cause the mold halves to open, because most of the components are large and the resins are incompressible. The result will be a deviation in the wall thickness of the component or −in the worst-case scenario −resin may escape. A pressure threshold on the cavity pressure signal can quickly detect the rise in pressure and send a signal for the mixing pump to switch from high injection pressure to a lower holding pressure (see Figure 1).
The fourth and final phase of the HP-RTM process consists mainly of curing the resin-hardener system. At this point, the cavity pressure curve is subject to cyclical fluctuations (which are large in some cases). These are influenced mainly by the resin used and its curing characteristics. In general, however, the sharp and sometimes abrupt pressure drop in the mold indicates that the volume of the resin is starting to shrink. The timing of this 'kink' depends largely on the resin used, making it possible to draw conclusions about the correct mixing ratio for the resin system. It is therefore a key factor in the reproducibility and quality of the component. This 'kink' can also be tracked with the help of a monitoring window (see Figure 2, curve 4).
Process-integrated production monitoring based on cavity pressure measurements and a process monitoring system enables rejects to be detected at the earliest possible stage, paving the way for lean production and automation. This type of monitoring also helps to optimize production at a very early stage. Examples of its use include the validation of simulation results, mold design and the determination of optimal process settings. Furthermore, quality testing is carried out on every part, and quality data is documentedautomatically. All in all, the advantages of process-integrated monitoring play their part in improving the cost-efficiency and quality of production – and manufacturers of lightweight components for the automotive sector stand to benefit from all these features.
About the Kistler Group
The Kistler Group is an independent, owner-managed Swiss corporation. Some 1,500 employees at 56 facilities worldwide are dedicated to the development of new measurement solutions, backed by individual application-specific support at the local level. Ever since Kistler was founded in 1959, the company has grown hand-in-hand with its customers. In 2015, it posted revenue of USD 341 million, about 10% of which is reinvested in innovation and research – with the aim of delivering better results for every customer.
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