Applying the edge

Advanced engineering resins are continuing to evolve to meet the requirements of designers intent on adding value to new applications in maturing markets.

An example of this trend is the hybrid plastic-metal automotive front end recently developed by Bayer Corporation’s parent, Bayer AG, Leverkusen, Germany, and several European suppliers for Ford’s new Focus model. The hybrid front end, which fits beneath the hood and serves as a structural body component, is manufactured from Bayer’s glass-fibre reinforced polyamide Durethan BKV 30 (nylon 6) and profiled steel plate (51). The composite part achieves about a 40 percent weight reduction in comparison to a part made entirely of metal.

Rob Cunningham, Bayer Corporation senior plastics engineer, Innovative Technology, describes the front end as essentially a hollow metal structure reinforced with plastic. Cunningham notes that the outside surface of a structural component is the primary load bearing area. This principle is applied in the case of this hybrid front end, where the outside metal bears the load, while the inside plastic reinforces the metal shape.

The sheet metal used in the Focus hybrid front end is only 0.5 mm, compared with 0.7 to 1 mm thick sheet metal that would typically be used for a load-bearing application in this area of the automobile, Cunningham reports. “A lot of structures in automotive applications such as these are stiffness limited, rather than strength limited. The plastic allows us to spread the load over a wider area, and use a thinner sheet metal to achieve the same level of stiffness performance.”

Another benefit to the design is part reduction. The Focus hybrid front end integrates brackets for additional components such as the radiator, condenser, headlamps and other parts, as well as anchoring elements such as clips. Part reduction, in turn, improves overall fit and dimensional accuracy of the part.

Cunningham notes that Durethan BKV 30 was chosen for the hybrid front end because of its excellent high temperature stability and superior chemical resistance. The entire structure passes through the e-coat stage and drying ovens, where the material must remain dimensionally stable during exposure to dipping chemicals and temperatures up to 180C.

SCORING PERFORMANCE POINTS

While advanced engineering resins allow designers the freedom to make complex designs while integrating overall part and weight reduction, a decision to use a resin in a particular application is ultimately based on performance.

Lexmark chose Shell Chemicals’ CARILON aliphatic polyketones polymers for two 30 mm gears in its new Optra S laser printer because of the resin’s ability to meet demanding performance criteria in this high wear application (52). The gears are an integral part of the toner cartridge assembly unit

“We specified CARILON Polymers initially because of its wear resistance and natural lubricity,” says Lexmark engineer Frank Carroll. “We have found CARILON Polymers to be ideal for precision gear applications,” says Carroll.

CARILON Polymers provide good tribiological performance in a neat form, at a lower cost than lubricated polymers. The material’s strong suits include excellent toughness, low noise, high wear and creep resistance and dimensional stability.

“This issue of friction and wear is basic to the function of engineering thermoplastics in a wide variety of machine applications,” says John Kelly, research engineer with Shell Chemicals. “One plus with CARILON Polymers is that compared to polyamides and polyacetals, they resist wear better against themselves.

CARILON polymers are also said to have a broad processing window, implying they can be used on conventional injection molding equipment without expensive re-tooling.

AlliedSignal Plastics’ Capron UltraTough polyamide 6 resin has successfully met the stringent impact and aesthetic criteria for the Thug (Thermoplastic Urban Gear) BMX bike wheel (53).

The Thug wheel is a hollow, one piece, five spoke wheel that utilizes lost-core injection molding. Key material performance requirements included light weight, maximum lateral stiffness and exceptional strength. Capron UltraTough BU50G1, a resin grade with a low level of glass-fibre fill, offered the required toughness combined with the superior surface aesthetics.

Capron UltraTough resin registers a Notched Izod Impact test figure of 1,060 J/m without break at -40 C. Under the same conditions, AlliedSignal claims, most other low temperature impact materials exhibit a clean break below 300 J/m. The low temperature performance of Capron UltraTough resins makes them ideal for applications in which weld line impact strength at extremely low temperatures is critical. These applications include automotive components, power tools, lawn and garden equipment, as well as cold-weather and high-impact sports gear.

Voted one of the top 50 most innovative products for truck and trailer fleets, the patented Cycloid ACS air compressor system utilizes a number of tailor-made composite resins supplied by LNP Engineering Plastics. For the unit’s cam, Cycloid Company chose Lubricomp SAL, an aramid fiber reinforced lubricated nylon 12, and Lubricomp KL, a lubricated acetal composite, for the follower (54). The materials were chosen because they offer low wear rate between the cam and the follower.

“Lubricomp has fibre reinforcement for strength and the lubricity to provide low friction and wear characteristics,” says Merv Carse, president of Cycloid Company.

Ems-Chemie AG has extended its range of high performance special polyamides with the introduction of four new injection molding materials in its Grilamid product group (55). These materials have been developed for the manufacture of technical parts which require a high level of stiffness, toughness, dimensional stability and hydrolysis resistance. Typical applications include electrical and electronic equipment as well as mechanical parts in direct fluid contact such as assemblies for pumps, liquid flow meters or connectors.

Three of the new materials are offered in un-reinforced form. Grilamid LV-3AH is reinforced with 30% glass fibres and exhibits a high hydrolysis stability, and a combination of high impact resistance with stability at elevated temperatures. Un-reinforced Grilamid LB 25 HX has tensile modulus of 1700 MPa. By comparison, conventional PA 12 range of materials have tensile modulii in the range of 1100 MPa. Grilamid DS 20 offers high dimensional stability, high stiffness and excellent chemical resistance. Further, Ems-Chemie AG claims it can be processed in 45% shorter cycle times in comparison to conventional materials.

GETTING THE METAL OUT

Car designers have made metal substitution as high a priority as part reduction in the development of new automotive platforms. A newly designed pedal-bracket plate made of DuPont’s glass-reinforced Zytel nylon accomplishes both objectives (58).

The Zytel pedal-bracket plate, used in all Mercedes-Benz A-class compact models, replaces a previous part made of die-cast aluminum. The Zytel plate measures about 30 x 30 cm and weighs 1.2 kg, or one-third less than a comparable aluminum part. An additional benefit provided by the nylon material is enhanced vibration-absorption characteristics in comparison to aluminum. The NVH (Noise Vibration and Harshness) characteristics of an aluminum plate fail to meet Daimler-Benz’s requirements.

Daimler-Benz and its supplier, WOCO, chose Zytel 70G35, a 35 percent glass-fibre reinforced nylon 66, for its dimensional stability, low warpage and the mechanical, thermal and chemical resistance the part requires. The resin’s flow properties allow short molding cycle times in spite of the plate’s size and weight. The part needs no post-molding finishing or surface-coating operations.

Part reduction is accomplished by integrating all three pedal-mounting brackets into the module. The mounting bolts for all three pedals are similarly made of glass-reinforced nylon. The dimensional stability of the nylon allows the bolts to pass smoothly, yet without excessive play,
through the appropriate openings in the brackets and pedals.

Several new Ultramid RS (reduced shrinkage) grades of glass-fibre reinforced nylon from BASF are claimed to provide reduced warpage and shrinkage of up to 50 percent in comparison to conventional grades of nylon (59).

According to BASF business director Raj Mehta, the enhanced dimensional stability of the RS grades are due to a complex, proprietary additive package which reduces moisture regain to two to three percent.

Says Mehta: “The simplest way to control dimensional stability is to keep adding mineral, but then you begin losing performance properties, such as stiffness or modulus. What we’ve done with our proprietary technology is achieve dimensional stability without compromising any of the mechanical properties.”

Mehta says that in particular the technology used in the RS grades reduces pronounced multi-directional (anisotropic) shrinkage, and replaces it with a more controlled and limited single-direction or isotropic shrinkage. Such dimensional stability, Mehta notes, makes the Ultramid RS grades ideal for parts with precise mating requirements, such as power drill housings, engine covers and plug connectors. CPL