New processes for lightweight components

Leichtbau Innovation Träger Konstruktion
© Fraunhofer IMWS

Front cross members made of UD tapes enable significant weight savings.

Lightweight, crashproof and cost-effective in series production: Prof. Dr. Peter Michel of Fraunhofer IMWS and his team of around 25 people are currently developing a manufacturing process for new continuous fiber-reinforced thermoplastic components for the automotive industry. At the end they are to be transferred into industrial production. In the interview, Prof. Michel explains the prospects for success.

Lightweight thermoplastic components for cars – why?

Well, our clients are primarily interested in the lower weight and the costs naturally play a role, too. For example, we have developed a prototypical car front crossmember made of so-called UD tapes, i.e. reinforced thermoplastics: Depending on the vehicle, the weight of this component is 4 to 6 kilograms, ours is 2 kilograms lighter.

What exactly does continuous fiber-reinforced thermoplastic mean and what is its advantage?

It is a polymer matrix, i.e. plastic, in which a continuous mineral fiber is embedded. Unlike thermosetting components, thermoplastics have the enormous advantage that they can be processed very quickly. The heating, the handling above the melting temperature and the cooling each takes far less than a minute. This means that components can be made in around one minute. Thermosetting systems need almost 10 minutes, smaller components around 3 minutes. The investment costs of both processes are comparable. Thermoplastic processes become much more attractive due to the higher quantities.

How large is the industry’s interest?

We are receiving significant interest. Hybridization, i.e. the combination of different materials, continues to progress further in the automotive industry. We work in public projects and also directly with car manufacturers and suppliers.

What is necessary to establish the manufacturing process?

In the aerospace industry the process steps, at least for dry woven and unidirectional (UD or UNI) fibers, i.e. thermosetting components, are already state-of-the-art. We now have to transfer this to the thermoplastic systems. Because with the established techniques in aircraft construction we would be a too expensive by a factor of 5. There each kilogram of weight saved is paid for immediately by the reduced fuel consumption. Depending on the assembling position, the kilogram saved in car manufacturing is »only« worth 5 to 10 euros.

You have to become more cost effective?

Yes. Our objective is therefore a high degree of automation. It’s an enormous challenge, but the large quantities in the automotive industry also offer a large opportunity. By way of comparison: In aircraft construction the lot size is 1 to 10, in car manufacturing we frequently have lot sizes of over 100,000. Even a Mercedes S Class reaches 50,000 vehicles a year. The costs can be lowered through large production series – without any quality losses.

How precisely do you develop the process?

Initially, we reproduce the process chain virtually, step-by-step, to see which steps interact. In this way we identify the respective critical parameters for the component properties. At the end we bring the steps together. At the start the requirements profile for the component is decisive; it determines the design. The component, a car door say, must be able to withstand both the normal operating loads and the misuse loads. We calculate this and select the fiber and matrix based on the results. This then determines how the individual layers of the continuous fiber-reinforced UD tapes have to be assembled and how many layers of tape we need to withstand the calculated loads. The fibers showcase their potential best under tensile loading. In the case of the car door we cross the fiber layers. Each layer is given its own optimum orientation. The second simulation step is the manufacturing process simulation: The tapes are two-dimensional and have to be draped in the third dimension in the pressing mould so that at the end the car door planned in the design is produced.

How do you determine the necessary parameters?

We characterize individual layers and performed compression, tensile and shear tests for different fiber orientations. We also have to take into account that polymer is always viscoelastic, viscoplastic, temperature-dependent and strain rate-dependent. All these phenomena are described in the model mathematically, because in car manufacturing we have to be able to precisely predict the damage profile in the case of a crash.

What exactly is the production process?

We produce in the Fraunhofer PAZ, the Pilot Plant Center, which is operated from the Fraunhofer IAP and IMWS institute. The synthesis know-how for polymers, coupled with the know-how for fibers, comes from IAP. The processing plants are operated by the Fraunhofer IMWS: a UD tape line is being set up, we use the injection moulding compounder (IMC)  to carry out initial technology tests, and we use the injection moulding technology for the final pressing of the components. We will soon be extending the PAZ in order to set up the intermediate steps too – the cutting to size and heating of the UD tapes. The UD tapes produced are cut to size. The cutting is followed by the stacking, the technical jargon for laying the layers on top of each other. In the first pressing step the tapes are consolidated in the transfer mould. Air inclusions would otherwise reduce the stiffness, incidentally just like deviation from the ideal fiber position by only a few degrees does. In the second phase the injection mould is closed and injection moulding compound is added. This is how a homogeneous component is produced: Continuous fibers reproduce the load paths, the remaining volume is produced in a one-shot process. The component screw mounting and hold points are also added in the mould.

What is your time perspective?

The Fraunhofer IMWS wants to be ready to develop simple tools in the next one to two years. If we take into account the product development cycle of the automotive industry, this means: Simple parts would be seen in vehicles in four to five years. This fits in well with the new EU targets for CO2 savings: maximum 95 gram CO2 emission per kilometre driven in 2021. If initial experience with the thermoplastic components is gained soon, one could be well-placed with the vehicles in 2021 and really reduce CO2 emissions.