Carbon composite manufacturing in automotive volume production
Lightweight constructions are a continuously increasing trend in the automotive industry. Main drivers for that trend are the challenging emission reduction targets regarding combustion engines and increasing ranges in electric mobility. This article presents different composite production methods and discusses their ability within mass production giving also an example within the automotive production.
Applications of Industrial Composites
Carbon composite structures are used in various industrial sectors. Different decision criteria regarding design, material, and manufacturing methods apply depending on the related sector. For instance, in mechanical engineering, we find self-supporting composite structures in accelerated and moved masses used to gain productivity and performance increases of machines and to reduce energy consumption in operation conditions. Moreover, specific characteristics of carbon fibers are used, for example, a negative thermal expansion coefficient or the high young’s modulus in order to produce components without thermal expansion or stiff carbon composite structures. Due to the wide spectrum of applications, it is difficult to make generalised statements regarding the driving factors of the usage of carbon composites in mechanical engineering. Therefore, it depends on the application case if costs, performance, high complexity, or functional integration of composites have priority.
Regarding sport and recreational products, carbon composites have a wide range of applications — from customised bicycle frames and hiking sticks to exchangeable pressure cartridges for avalanche backpacks. High selling prices allow the usage of various materials and production technologies.
In aerospace applications, decision criteria are mainly driven by the performance of lightweight components and quality aspects. Extensive and expensive licensing and approval procedures can highly restrict the selection of materials and production methods.
Composite fabrication in automotive serial production needs to be smoothly integrated in well-established manufacturing processes and synchronised in order to achieve minimum overall costs within production which are the main drivers in the automotive industry. This focus on costs as well as required cycle times further restricts the number of materials and production technologies. On the other side, operational costs of the car can significantly be reduced by integrating lightweight composites as they reduce the overall weight of, for example, electric cars, and thus enhance their range.
Design and Production Method
Design and production methods are closely linked regarding carbon composite components which require the design team to feature knowledge regarding the options of production methods including advantages as well as disadvantages. Parallel calculations of recurring forces, load paths and their application to components design complete the design process.
In colloquial language, we speak of carbon composites meaning a material mixture of carbon fibers and matrix material. Matrix materials within automotive composites are mainly duroplastic resin systems and thermoplastic materials. Carbon fibers contain a tensile strength up to 5800 MPa with a specific weight of 1,8 g/cm3 . This results in a weight-specific strength which lies clearly above those of steel or aluminum alloy. The material costs of carbon composites are mainly driven by the fibers due their energy intensive manufacturing process.
The manufacturing processes described in the following mostly work with duroplastic materials where viscous and chemical reactive resins are mainly used. They mostly start their chemical reaction within the production curing of the composite. A special type of material is called prepreg. Here, the preprocessed fibers composed of mostly woven and nonwoven fabrics get pre-impregnated with resins or thermoplastics without reacting or curing out chemically. The prepregs can be stored at minus degrees. Subsequently, they can be tailored and draped in the desired shape. The chemical curing reaction starts when increasing the temperature, so that the prepregs are curing out to the finished composite product within the mold.
Flexible fibers and matrix materials get mixed together and cure into a dimensionality-stable shaped composite component. The shape of the component is mainly achieved by using a mold which is usually a female mold in the shape of the desired component. Production including such mold-based processes is complex and cost intensive especially with respect to the production of the mold.
The number of parts produced from one mold can vary according to size, type, and material of the mold (for example, foam, artificial resign, composite, aluminum, or steel) between one or several thousand components. Moreover, the manufacturing costs are highly dependent on the type and size of the applied mold. Another type of form-giving method is the use of male molds that can be winded around with fibers or braided. After the composite component has cured into its stable phase, the male mold can remain within the component or be removed.
Hand Layup Process
The method of hand layup is one of the most cost-efficient methods to build single composite prototypes as well as for the production of low quantities of the product. Within this method, fibers and matrix are manually brought in shape where the placing of the fibers and fiber mats (woven and nonwovens) is mainly done with the help of female molds. For instance, fibers can be impregnated with resin by using a handheld roll. Nevertheless, this labor intensive production method has restrictions regarding its reproducibility and its fiber volume content. Therefore, this process is less suitable for serial production. One possibility to enhance the quality of composite components and its fiber volume fraction is the infusion of the dried semi-manufactured fiber product by using vacuum infusion.
A vacuum-tight installation is set on the dried fibers that have been draped in a female mold that can be composed of different functional material layers and is airtight-closed with a vacuum film. Additionally, valves are installed in order to evacuate the installation and to infuse the fibers with the liquid matrix (for example, consisting of duroplastic resin). The infusion of the fibers with the liquid matrix material is starting from the resin inlet valve and extends to the overall component if applied correctly. An additional textile layer can be applied between vacuum-film and the fibers in order to improve the flow properties. Thereby, a very good component quality can be reached with fiber volume contents of more than 50 %. The usage of this kind of production method is useful for the production of batches from lot size one up to low volume series. For serial production with a high number of production units, this method is too labor intensive. Besides, it is challenging to assure and maintain equal production conditions.
Resin Ttransfer Molding
Production characteristics (© Raphael Geiger)
Investment costs for molding
Quality of components
Cycle time per component
Regarding production costs of mold-based production methods (hand layup, vacuum infusion, RTM, press-based methods), we can state the following assessments: the more process pressure is applied within the method, the more complex and pressure-resistant forming tools are necessary which leads to increased investment costs. On the other hand, higher process pressures allow for shorter cycle times, due to a faster infusion of the component, a faster form giving within the pressing is possible as well as a faster hardening of the matrix material. Thereby, the productivity of high-volume series can be enhanced while also allowing for reduced production costs.
Pultruded composite structures allow for large scale production with lower costs than discontinuous manufacturing methods.
Fiber and matrix are attached to the component with the help of a press and molds. Press-based methods have a good reproducibility, so that the production process can be automated to a great extent. Involved materials are mostly semi-manufactured fibers such as woven and nonwoven textile fabrics. According to the applied press-based method, these can be pre-impregnated (prepreg) or infused already during the press-process. Different press-based methods are for example, hot pressing, cold pressing, wet pressing, and prepreg-pressing methods. Hot pressing uses a heated metal tool to press the composite. This type of method is applicable for large volume series of small and medium components. Instead, cold pressing uses an un-heated pressing tool.
Within wet pressing, dry semi-manufactured fibers are placed in the mold where, subsequently, the resin is inserted, the tool locked and put into the press. The closing speed of the press can be varied in order to control the infiltration whereas the tool temperature can be varied depending on the resin system. Usually, temperatures vary between 90 and 140 °C with a pressure on the component of 5 to 25 bar. 
Continuous Manufacturing Technologies as Pultrusion
Continuous manufacturing techniques allow for an increased productivity compared to part or batch production. In composite production, there are continuous manufacturing techniques such as pultrusion where fiber strings are pulled through a mold that is open on both sides. The resin is injected in the molding tool, so that the fibers are infiltrated within the mold and then cured to the composite. The continuous pull-off of the pultruded profile allows for a continuous production. The profiles are cut to the needed length at the end of the pultrusion line. Nevertheless, the two-sided open mold tool restricts the freedom of design. 2-D-profile cross sections can be produced, for example, tube profiles, rectangle profiles as well as T or I shaped profiles. Pultruded composites have high fiber volume contents and great shape accuracy. With the help of an upstream textile process (for example, braiding), different fiber angles can be realised, for example, for torsion-stressed structures. Moreover, thermoplastic profiles can be manufactured with pultrusion of a blended twin yarn. 
Pultruded composite structures allow for large scale production with lower costs than discontinuous composite manufacturing methods. This is due to the continuous manufacturing method on the one hand and due to lower process times in comparison to discontinuous production methods on the other hand. Besides, used materials such as fiber roving and resin systems are less costly than preprocessed textiles as woven or pre-impregnated prepregs. As long as geometry of the structural composite component allows the usage of pultrusion, a serial production can be realised with significant economic savings.
This method winds the fibers around a rotating core. Compared to the previously presented methods using female molds to shape the outer contour of the component, this method uses a male mold core. Depending on the design and shape of the produced part, the core can be removed and reused (with lightly conic shaped designs) or the core can remain in the component (lost core) and, if made out of removable material, for example, sand or foam, can be dissolved .
An advantage of this method is the high composite material output per time allowing the production of large components in a feasible time. The considerably cheaper cores gain economic potential within small product series and large components where cost-efficient fiber-rovings and resins are used as materials. Nevertheless, a disadvantage involves the restrictions in shape and design. For instance, only structures able to rotate such as tubes or rotation-symmetric profiles can be wrapped. Moreover, the method provides a lower finish quality compared to methods that use female molds. Of course, the surface can be reworked and finished in an additional subsequent work step.
Composite components need to withstand high loads which lead to increased requirements regarding the quality of components. There are various possibilities regarding the quality assurance of composites. Depending on the branch and requirements regarding the component, a continuous quality control along all production steps may be required. Regular control can be reasonable also during the operation of the component especially concerning cyclical-loaded components as well as supporting structures as in aviation and aeronautics.
Inspection can be executed employing two fundamentally different methods: Nondestructive material testing and destructive material testing. With the help of nondestructive methods, quality assurance within the production can be realised. 
Key to economically attractive lightweight products lies within the production process.
Automotive Application Example
Lightweight design is a continuously increasing trend in automotive engineering. Driving factors are the challenging emission reduction targets regarding combustion engines and increased ranges within electric mobility. For both cases, lightweight construction is a key technology .
Hereby, composite materials offer the most significant potentials. In electric mobility, it provides a great improvement of the battery range with a single charge and therefore constitutes an essential sales argument. This increase in range extends also the field of usage of the vehicle and is a decisive success factor. Besides, the heavyweight battery needs to be compensated.
SPIRI develops an electric car specifically for urban car sharing application. The optimised usage of the car allows a considerable higher utilisation and decreases parking and idle times. Moreover, the increased range allows for increased usage durations and reduced operational costs.
The objective of the example presented here is to develop a carbon fiber composite car body with a total weight of maximal 100 kg that can be produced in serial production while achieving marketable production costs. The weight-wise optimised car body design is aligned with the production methods of carbon fiber composites. The number of vehicle body elements within the body in white is reduced to a minimum. Such a design reduces the cars body weight and thus investment and production costs also with respect to the molding.
Internal cost comparisons of different composite manufacturing methods based on the production of several thousand units per year have been conducted. The underlying data is specific to the car body development within SPIRI and highly dependent on the manufacturing suitable car design. The production costs encompass the following single positions: material costs, labor costs, manufacturing process costs, and the costs for molding as well as quality assurance. A generalisation of the results regarding the production costs is not possible, but calculation needs to be performed according to the application case.
In this specific case, the application of the winding methods that is restricted due to geometry reasons is not possible with respect to the car body design. Press molding is the most cost efficient carbon composite manufacturing method for this case including serial production of a carbon fiber composite car body. The production costs are higher than the production of a comparable steel car body by the factor of approximately 1.35.
A mass production of carbon composite car bodies and supporting structures in the automotive industry is possible and economically feasible. The usage of carbon composite materials enables significant weight reductions in car body construction compared to metal materials such as aluminum and steel alloy. The weight reductions further reduce operational costs and extend the range of the presented vehicle.
Key to economically attractive lightweight products lies within the production process. With an intelligent design aligned with the chosen manufacturing method, significant potentials with respect to weight reduction as well as production costs can be achieved. This is not only valid for the automotive industry, but for every branch where weight and performance are relevant. |
- Milwich, M.: Pultrusion of braids 14. In: Kyosev, Y.: Advances in Braiding Technology: Specialized Techniques and Applications. United Kingdom: Elsevier Science Technology, 2016Google Scholar
- Rohde-Brandenburger, Dr.-Ing. K.: Was bringen 100 kg Gewichtsreduzierung? - eine physikalische Berechnung. In: Siebenpfeiffer, W.: Leichtbau-Technologien im Automobilbau. Wiesbaden (Germany): Springer Fachmedien, 2014Google Scholar
- Straß, B.; Conrad, C.; Wolter, B: Production integrated nondestructive testing of compositematerials and material compounds — an overview. Conference 19th Chemnitz Seminar on Materials Engineering, Chemnitz (Germany), 2017, pp. 1–10Google Scholar
- Suter Kunststoffe AG: Fertigungsverfahren, 2017. Online: https://www.swiss-composite.ch/pdf/i-Fertigungsverfahren.pdf, called: 21 June 2017
- Wiedemann, J.: Leichtbau — Elemente und Konstruktion. Berlin/Heidelberg (Germany): Springer Verlag, 2007Google Scholar