Lightweight Design worldwide

, Volume 11, Issue 3, pp 18–21 | Cite as

Mass production of composite leaf springs

  • Robert Ernst-Siebert
  • Sebastian Grasser
Cover Story Large-Scale Production

SGL produces glass fiber compound material-based leaf springs in large quantities for Volvo and Daimler. The specially developed component design and a fully automated production line are decisive for large-scale production. In 2019, production is to be increased to 500,000 leaf springs per year.

RTM for Series Production

The idea to develop a leaf spring for serial use in the automotive industry originated in 2009 as part of the then-young Benteler-SGL joint venture at the plant in Paderborn, Germany. The company had been formed one year prior as an initiative by Benteler Automotive and SGL Group. Benteler-SGL had already supplied customers including Audi, BMW, Lamborghini and Porsche with structural components such as car roofs, rear spoilers, B-pillars, and doors, made using Resin Transfer Molding (RTM) or hot pressing. While engineers started designing the composite leaf springs in 2009 to ready themselves for mass market production, the plant at Ried im Innkreis, Austria, was already starting on optimizing the RTM process for series production. By 2013, a front axle leaf spring design based on Glass Fiber-Reinforced Plastic (GFRP), Figure 1, had been developed for Daimler’s Mercedes-Benz Sprinter vans. Compared with steel leaf springs, the weight of these new springs was cut from 15 to 6 kg, a reduction of nearly 65 %.
Figure 1

A glass fiber material tailored to the product and production creates stable preforms and enables trouble-free automated processing (© SGL Group)

Mass Series Production

This type of component concept was finally used by the Swedish car manufacturer Volvo. With its new global vehicle platform, the Scalable Product Architecture (SPA), SGL Group supplies the rear axle leaf springs. Volvo’s first vehicle brought to market based on SPA was its XC90 crossover SUV in 2017. As an integral component of SPA, the leaf springs are also installed in the S60, S90, V60, V90, and XC60 models, as well as in all larger models that can be developed with SPA. To be able to supply the required quantities, SGL had to increase its production capacities, and there is no end in sight. 2019 is expected to bring an annual volume of 500,000 parts. This represents the biggest composite components program in the automotive industry worldwide. In detail, their performance depends on technical measures explained below.

This represents the biggest composite components program in the automotive industry worldwide.

Fully Automated Production

Production for the Volvo leaf springs has been expanded at the SGL plant in Ried, Austria. This plant was built in 2012 with the aim of further developing the manufacturing of composite components, from just being sophisticated factory work to serial production. With unit volume requirements in mind, a fully automated production line was introduced, among other measures. From material equipment on one end to output of the finished leaf springs on the other, all process steps are completely automated.

In the first production stage, the fiber textile, Figure 1, is cut and preformed. Here, a glass fiber material tailored to the product and production is used which creates stable preforms and enables trouble-free automated processing. Next, the workpiece is pre-stabilized, the preform is coated with a high-performance powder binding agent and then stabilized under heat and pressure. Once the workpiece cools, it is stable enough for a robot to place it in the RTM mold. A special robot gripper was developed to make this process foolproof.

The number of cavities in the mold used ultimately depends on the order volume. This number can even reach the two-digit range. Next, the resin system is prepared by a metering unit and injected into the evacuated mold. After the resin system has cured completely, the leaf spring workpieces are brought to the milling cell by a robot, where they are trimmed and bored on both ends. Not a single step is manual.

Self-stabilizing Process

Several quality controls monitor the production process with both visual and mechanical checks. This control ensures that each and every leaf spring leaving the unit meets stiffness and functionality requirements. Crucial to process stability and reliability is an inline parameter control for all materials. If, for example, when creating a preform, a layer should shift out of place, the system will report a deviation. This enables the teams to immediately identify where the problem is, remove the defective workpieces and adjust the part in the system.

Despite the high reliability of the process, it still isn’t fully developed. Gradually, the system’s self-regulation function will be optimized while boosting process robustness, all steps towards an integrated system based on the concept of Industry 4.0. With this system, SGL has reached a degree of automation unparalleled in the market when it comes to production with composites.

Distinct Orders and Changing Batch Sizes

The Volvo leaf springs presented another challenge: to supply springs tailored to model all SPA-based models, from the luxury XC90 SUV to the compact S60. To achieve the load-dependent capacity and rigidity required, the model-specific component design was modified. The system and production concept is designed to be able to produce variable quantities based on the variant. This wide variety is ensured by production logistics using different press tools depending on the variant, Figure 2 and Figure 3. As a result, different spring variants can be produced in different lot sizes with efficacy.
Figure 2

Two 1600 t-presses are used for the production of the Volvo leaf springs (© SGL Group)

Figure 3

This tool was used to develop the prototype (© Henkel)

The production concept is designed to be able to produce variable quantities based on the variant.

Material Selection

Putting together the materials not only determines the component’s properties, it is also crucial for a stable, efficient production process with fast throughput times. This applies to each and every constituent, from fiber mesh to the resin system to the binding agent. Especially the textiles used play a decisive role. In recent years, SGL has laid the foundations for optimizing the textile for use in automated serial production while meeting requirements resulting from the production process and the necessary component performance.

Shorter Development Cycles

With their leaf springs for Mercedes-Benz vans and Volvo cars, SGL in collaboration with former partner Benteler demonstrated how RTM components can be mass produced. While components such as leaf springs are normally developed in a standard model development engineering process over two to three years, this cycle can be cut shorter if necessary. With the resources of CFM’s business unit, SGL can work with customers to implement automotive serial production projects in significantly shorter times.

Following Daimler and Volvo, other automobile manufacturers have indicated plans to use lightweight composite springs in future models. Leaf spring production is experiencing major growth momentum. After cars and vans, development is moving towards light commercial vehicles, such as trucks and pickups, which are especially popular in the US. In the long term, leaf springs for large commercial vehicles are also conceivable.

Presenting Evidence

With successful leaf spring projects for Daimler and Volvo, SGL has demonstrated to the automotive industry that RTM component design is a useful and reliable option for the automotive industry. Scalable high-volume production no longer presents an issue. These leaf springs have shown that the complex relation between material mixes and process technology can be mastered and that the technology is ideal for robust, fully automated production processes.

Acquisition of the Joint Venture Shares

In 2017, SGL acquired the shares of Benteler-SGL held by Benteler Automotive. The entity now operates under Austrian subsidiary SGL Composites and is part of the Composites — Fibers and Materials (CFM) unit of SGL Group, reinforcing the company’s capacity in the serial production of fiber composite components while focusing on its core competences.

Within the CFM business unit, SGL Group already has development and production expertise in the area of fibers, in particular acrylic fibers and carbon fibers, as well as in fiber compound materials. The group now covers the entire value chain, from carbon fibers to materials to components.

Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Robert Ernst-Siebert
    • 1
  • Sebastian Grasser
    • 2
  1. 1.SGL’s two production facilitiesRied and OrtAustria
  2. 2.SGL GroupMeitingenGermany

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