Design for reliability – Weld lines in structural components

How do we guarantee the reliability and lifetime of highly loaded structural components with weld lines?

Due to the integration of functions and lightweight design, weld lines are present in almost all technical components. In the design process, numerical simulation methods or empirical values are used in an attempt to relocate weld lines to less heavily loaded or invisible component areas. However, this is not always successful as desired, because external influences such as the rheology of the melt, process parameter or component geometry often cause the weld line to shift to non-predetermined, critical or failure-relevant component areas. If you only become aware of this during component sampling or during the release test due to premature damage, the start of production and product lauch can usually no longer be met. At such an advanced stage of component approval, changes to the gating system or component design are only possible at great expense in terms of time and money.

In particular, the increased use of recyclates due to the EU's ELV-Regulation makes the reliability and lifetime assessment of recyclates a particular challenge. Due to the material mix and previous history, recyclates exhibit a greater range of variation in rheological, morphological and process properties, which are a particular challenge for weld lines.

Research questions on the interactions of weld lines in highly loaded structural components

The following research questions are intended to investigate the fundamental interactions between rheological, morphological, process and long-term properties in highly loaded applications:

• What are the influencing parameters on ‘good’ and ‘poor’ weld line quality?
• What microscopic filler- or fibre orientation and distribution is present in the weld line, particularly in the area of the flow fronts?
• Which morphological differences (viscosity, molecular weight distribution, filler and reinforcing material) influence weld line properties?
• Which mechanical properties (static, cyclic, crack and crack propagation) result from weld lines for different thermoplastics depending on their filler and reinforcing material?
• What possibilities are there to improve the lifetime and reliability of components with weld lines?

By understanding the interactions between rheology, morphology and process, weld lines can be better evaluated in terms of their reliability and long-term properties. You can acquire this expertise in this joint industrial project ‘Design for Reliability - weld lines in structural components’. Take part and benefit from the expertise of Fraunhofer LBF.

Focus and approach

Basic literature research and evaluation

Based on the materials selected in the composite (e.g. PP, rPP, PA6, rPA6 or PPA) and taking into account their fillers and reinforcing materials, a research of available materials is carried out. For this purpose, the material property profiles and the expected weld line quality are compiled and initial resulting challenges are derived.

Determination of the rheological and morphological properties to assess the application-specific suitability and weld line quality

Analytical tests are used to determine the actual state with regard to the rheological and morphological material properties. These are used for evaluation and description, as well as the derivation of process parameters for good or poor weld line quality.

Investigating the long-term properties

Using an injection moulding tool with special inserts, test specimens with a weld line are produced at the Fraunhofer LBF. Process parameters are to be specifically influenced in such a way that ‘good’ and ‘poor’ weld line properties result. The interaction of the notch effect (shape of the moulded core) on the static and cyclic long-term properties will be specifically investigated.

Microscopic and macroscopic morphology in the weld line

Special attention will be paid to the microscopic and macroscopic morphology in the weld line. Microscopically, the area of the weld line will be examined using µ-CT and microscopy for the local fibre orientation and confluence of the melt flow fronts. Macroscopically, the crack initiation and crack propagation as well as the damage pattern under static and cyclic loading will be analysed.

Derivation of a catalogue of measures

The material properties determined from rheology, mechanics and microscopic and macroscopic morphology are incorporated into a catalogue of measures for reliability assessment of weld lines. With this catalogue of measures, the partners are able to better evaluate the process parameters, long-term properties and reliability and become aware of critical conditions and areas of the weld line in the early phase of product development.

The participating partners are thus able to address their requirements specifically to material suppliers, manufacturers and subcontractors and have the opportunity to adapt their material formulations, component design or process parameters in line with requirements.

Build up expertise and stay ahead in the market

Participating partners in this joint project will be able to

• evaluate the challenges and potential of weld lines
• derive the necessary material analyses for the qualification of weld lines
• adapt their own design methodology for component design and thus design technical components with weld lines to be operationally stable and reliable
• to be able to formulate very specific requirements for material manufacturers

This joint project is aimed at companies along the value chain from the granulate to the component:

• OEMs and manufacturers of plastic components from the automotive, trucks or white goods industries who will have to produce components with weld lines from virgin and recycled materials in the future and ensure their reliability and lifetime
• Recyclers, compounders, processors, additive manufacturers who want to further qualify their materials for demanding applications with weld lines in the future