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Update on BASF’s collar-joint fastening technology |
Application research in detail
Update on BASF’s collar-joint fastening technology
Presented by Presented by Martin Völker, Engineering Plastics Application Development
Trade Press Conference K 2004, June 22, 2004, Ludwigshafen, Germany
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"Simple, reliable and economic" - that was how application researchers at BASF described their new plastic-to-metal fastening technology at its launch two years ago. Since then two significant advances have been made in what is now termed "collar joining":- The mechanical properties of complex components made from dissimilar materials can now be predicted accurately with the help of finite element models.
- A prototype vehicle front-end has been developed jointly with an automotive supplier. The part meets all engineering requirements and proves the feasibility of collar joining for complex assemblies (figure 1).
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What is collar joining?
Collar joining is a method of joining plastic and metal components, in which collars – circular protrusions formed on a metal part – are mechanically pressed into the plastic part. Collar joining has a number of advantages over the rival in-mould assembly technique: it provides more freedom when it comes to the design details; simpler, cheaper mould tools can be employed and cycle times shortened, and parts are less prone to warpage than those assembled in the mould (figure 2).
Collar joining has been improved continuously since it first appeared in 2000. Initially, efforts were concentrated on improving the design and dimensions of the fastening point. Next, a more complex assembly – what has become known as the “LU-carrier” – was developed to investigate experimentally the mechanical behaviour of a realistic component, as well as verify the results and accuracy of computer prediction (figure 3).
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Composite proves inseparable
Basic mechanical tests prove the performance of collar-joined metal/ plastic composites at high temperatures and under dynamic loads, including crash conditions.
Astonishingly, heat aging the test piece at 120°C for over a thousand hours resulted in an 18% increase in the force – to almost 800 N – necessary to separate the collar joint. Post-crystallization of the nylon seems to be responsible for the added strength of the joint (figure 4).
Cyclic temperatures ranging from -40°C to +150°C caused a similar increase in strength: 6% higher after 100 four-hourly cycles (figure 5).
Fatigue tests showed that even high dynamic loads fail to separate the joints. For example, in a flexural test with a peak load of 200 N – well beyond that likely to be experienced by the part in reality – the test piece was still good after one million load cycles. Any weakening of the fastening point merely corresponds to normal material fatigue in the plastic, not the metal/plastic interface.
The behaviour of the LU-carrier during a crash test is also quite remarkable (figure 6).
By placing the plastic component inside a metal U-shaped beam it was possible to control how the structure failed under the buckling forces. Having sustained a high initial load, the carrier exhibits excellent energy absorption immediately after the start of buckling. The additional lateral support provided by the diagonal plastic ribs significantly increases the strength of the structure (figure 7).
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Experiment proves theory
BASF's development engineers are now able to employ structural analysis methods when designing hybrid structures. Finite element methods tailored to the special requirements of collar joining enable the structural behaviour to be assessed, including when the fastening point is likely to fail. In particular, BASF was able to draw on considerable know-how when modelling the contact surfaces; for instance, the failure parameter for the so-called connector elements – features used to model the joins between plastic and metal – was based on precise measurement data. The computational and experimental results in both the buckling and torsional tests were found to agree well. The LU-carrier's buckling load can be predicted with an error of less than 15%, while the energy absorption during buckling was also calculated to a high degree of accuracy. Experimental and computed torsional stiffness values matched closely right up to the point of failure (figure 8).
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Front-end prototype fulfils engineering requirements
The feasibility of collar joining as an alternative to in-mould assembly has been investigated in a joint project between BASF and Visteon, a leading supplier of hybrid front-end assemblies. Using a modified mould tool, a plastic beam was produced and subsequently attached to the metal section by collar joints, which were optimized by computer analysis in terms of number and location. The resulting assembly fully met the demands, possessing the required stiffness and exhibiting the predicted strength (figure 9).
In the meanwhile, further components are already being developed by other BASF customers. BASF is confident that the near future will see the appearance of the first production parts employing collar-joining technology.
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