Advanced adhesives foster hybrid structures

Nov. 3, 2005
Low-energy-substrate adhesives pave the way for metal-plastic structures.

Dave Recktenwald
Market Development Manager
Dow Automotive
Auburn Hills, Mich.

The front-end carrier on Volkswagen's new Polo features steel reinforcement bonded to a plastic framework with Betamate lowenergy-substrate adhesive from Dow Automotive. The resulting structure reduces weight by 25% compared with previous versions, lowers costs by 10%, and meets structural requirements.

Adhesive bonding of hybrid structures lets designers use molded closed-box sections, which maintain structural integrity and permit more cost-effective materials and thinner wall sections.

An emerging trend in the automotive industry is the increasing use of metal/plastic hybrid structures. They incorporate metal-reinforcing members strategically attached to a plastic framework, combining the structural integrity of metal with the net shape and weight-to-stiffness advantages of fiberfilled plastics. The resulting structures help meet increasing mechanical demands while addressing disassembly and recycling needs at the end of the vehicle's useful life.

Injection overmolding metal and plastic-components is one way to make hybrids. But adhesive-bonded systems are gaining favor. Closed-box sections can be created that have greater stiffness than the open sections required with overmolding. Adhesive bonding also offers more flexibility for design engineers.

Advances in low-energy-substrate adhesive (LESA) technology are helping spur interest in hybrid systems. The materials were developed to bond hybrids and all-plastic structures together without mechanical fasteners.

The two-component structural adhesives graft to low-energy surfaces and create a sturdy durable bond. And they are suited for materials that can be tough to bond with other types of adhesives. For instance, LESA bonds thermoplastic and thermoset resins, including polypropylene, polyethylene, polystyrene, polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), acrylonitrilebutadiene-styrene (ABS), and others without any surface pretreatment. It also bonds many thermoplastic-olefin elastomers (TPOs), as well as coated metallic substrates and glass.

Overall adhesive properties and longterm durability make LESA a good candidate for structural bonding of metal/plastic hybrids. For instance, Dow Automotive's Betamate adhesive performs well at typical automobile temperatures (40 to 120°C). It has a tensile strength of 3,250 psi and, when subjected to extreme loads, the plastic substrate typically fails before the adhesive interface. Environmental weathering does not adversely affect performance.

As with other adhesives, LESA can replace or supplement welds and mechanical fasteners. It applies and cures at room temperature and requires no extra pressure during curing — the only need is to maintain contact between parts.

The benefits of adhesive-bonded hybrid structures are seen in the automobile front-end carrier recently developed by Volkswagen and Dow Automotive. A front-end carrier (FEC) supports most of the cooling system, headlights, hood latch, and various other components. It connects the upper and lower longitudinal body rails and plays a role in vehicle structural stiffness.

Traditional steel FECs are bolted or welded to the body. Adding subcomponents, however, requires welding attachment brackets which increases weight and cost.

A recent trend is mounting the complete front-end system, as a module, onto a body with an open structure. This allows greater access to the engine compartment during assembly. The FEC module reduces cost, frees OEMs from most of the assembly work, and often improves overall quality while decreasing the time to market. However, the FEC must carry all front-end components and also make up for the front-end stiffness that was lost by opening the body structure.

The move to modular systems opened the door to plastics. Molded-plastic FECs consolidate parts as well as reduce weight and cost. However, in most cases, it still takes a hybrid structure of metal and plastic to resist loads on the carrier, particularly in a crash.

In traditional hybrid front-end carriers, metal reinforcements attach to the plastic using rivets or heat stakes, or through overmolding. This new solution uses Betamate LESA technology to join the plastic molding and metal reinforcement.

The adhesive produces a continuous load-bearing joint between plastic and metal parts, improving stiffness and reducing the high-stress-point loads associated with typical mechanical fasteners. It also enables bonding without pretreatment.

The assembly technique lets designers use molded box sections, which maintain structural integrity while permitting more cost-effective thermoplastics, such as long-glass-fiber-reinforced polypropylene, and thinner wall sections.

Metal reinforcements must be precisely positioned on the plastic frame. Yet adjustments can be made in the late stages of program development, as the design for bonding is less restrictive than for overmolding, resulting in lower tooling costs.

The FEC bonded with Betamate LESA entered production this past April with the launch of the new Volkswagen Polo in Europe. The Polo's new front end reduces weight by 25%, compared with previous solutions, and meets rigidity demands at reduced cost.

The FEC also meets or exceeds requirements for vibration, hood slam, and latch pull under various temperature and environmental conditions. It offers opportunities to integrate components such as air-intake ducting into the carrier, and facilitates a design that improves pedestrian safety in terms of leg impact.

Dow Automotive,
(248) 391-6300, [email protected]

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