Some interesting design variations are coming to the fore in the production of automotive airbag assemblies. Traditionally, such components have been mainly of steel, aluminium, or injection moulded thermoplastics. Here, we look at metal/plastic hybrids, and highly reinforced nylon composite sheet materials used in over-moulded hybrids.
Belgian Tier 1 supplier Quadrant CMS employs plastics/metal hybrid technology to save weight and money in production of automotive airbag assemblies with integrated lead frames. Production is carried out fully automatically and the assembly is being fitted on vehicles produced around the world.
The hybrid integrated lead frame fits in with the continuing trend in the automobile industry to reduce not only weight for savings in fuel, but also space—to allow room for the growing range of increasingly sophisticated safety- and entertainment-related devices now fitted to vehicles, especially in and around the steering wheel.
The plastic/metal hybrid moulding has replaced an assembly of cables on a full steel housing that was more complicated and costly to produce, heavier, and which took up more packing space. It was first designed for the Ford Transit van, which is assembled in Turkey. Production began in 2012, but now Ford is introducing it on four other vehicle models in Europe, USA, Brazil and India.
The integrated lead frame activates the horn when the driver presses the airbag cover in the centre of the steering wheel, completing an electric circuit. As such, the module is a safety-critical component, so design and manufacturing requirements are very demanding. It is produced by injection moulding a 40% glass fibre reinforced nylon (polyamide) over a pre-coated stamped steel plate.
The airbag housing contains the airbag itself as well as the gas generator, with its main function being to direct the airbag during deployment, by maintaining the form and integrity of the housing. The hybrid structure is created by combining the plastics housing with the metal lead-frame. During development, the metal part within the plastics part was shown to induce additional stresses during the deployment, so these had to be eliminated using FEM analysis and design changes.
The over-moulding process is carried out fully automatically in a purpose-built manufacturing cell at the main QCMS plant in Tielt, Belgium. A six-axis robot positions the lead-frame precisely into the mould. The reinforced polyamide is then injected around it. After moulding, the finished product is removed from the mould and placed in a neighbouring station that carries out a 100% quality check. Once the part’s quality is assured, it is positioned on a conveyer belt that takes it to downstream assembly operations.
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Continuous Glass Fibre Reinforced Nylon Composite
A different approach for airbag housing design is being deployed in Germany in a joint advanced engineering project between LANXESS, Takata AG, KraussMaffei Technologies, BondLaminates and Christian Karl Siebenwurst.
The long side walls are made of moulded TEPEX Dynalite 102 RG600 from BondLaminates. This nylon composite sheet of polyamide 6, reinforced with 47% continuous glass fibres by volume, is over-moulded and reinforced with Durethan, an impact-modified polyamide 6 copolymer from LANXESS. The design enables the wall thickness of the side walls to be reduced from 4mm to less than 1mm, resulting in considerable cost savings. This metal-replacement technology is a successor to plastic-metal “hybrid” composites, which involved preforming sheet metal and over-moulding it to add ribs, attachment points, and other features.
Glass in a unidirectional form is much stronger than other forms of glass or ferrous and non-ferrous metals. Such a sheet composite over-moulded with nylon 6 has tensile strength as much as five times that of metals. Strength-to-weight ratio is dramatically improved over metals — twice that of steel and three-to-four times higher than a standard injection moulded glass-filled thermoplastic. Stiffness-to-weight ratio with the continuous-glass composite laminate over-moulded with nylon is eight times as much as steel.
Substantial weight savings can be achieved when metal stampings are replaced with nylon 6 (polyamide 6) composite sheet. This material can also be used to make all-plastic structures significantly lighter. The use of PA 6 composite sheet cuts the weight of the housing by over 30 percent compared with a mass produced, injection-moulded version (also made of polyamide 6).
When the airbag is triggered in an accident, the base and walls of the housing must be able to withstand the explosion and the pressure during inflation of the airbag. Although the side walls are so thin, they can withstand the sudden pressure because of the high strength and stiffness of the composite sheet. LANXESS is able to accurately simulate all process steps in the manufacture of PA 6 composite sheet parts – including the highly complex processes involved in forming the material and its anisotropic behaviour.
This technology is also being used by ZF Friedrichshafen to make brake pedals, said to be the first to be made of reinforced nylon 6. It weighs half as much as a steel brake pedal. Other possible applications include automotive interiors, including seating area components, door side-impact beams, cross-car beams, steering-column holders and front ends. Other applications are foreseen in in aeronautics, trains, trucks, agricultural equipment, machinery manufacturing and renewable-energy systems.
Other thermoplastics considered as candidates for this technology include PP, PBT, PES, PEEK, nylon 612, and possibly nylon 66, depending in large part on how well they meet strict flammability requirements.
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