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Comfortable ride for automotive plastics

01 October 2008

Pressures to save weight and costs in the automotive sector are leading to innovative plastic replacements for more traditional metal components. In this recent application a moulded glass reinforced nylon was chosen in preference to die cast magnesium for the housing of an electronic air suspension system controller

Electronic Air Suspension Systems (EAS) have been used in the commercial vehicle sector for many years to keep a vehicle at a specified level regardless of load status. With regard to passenger vehicles, they are principally used today in high-end models (for example, the Mercedes S and E Class, Audi A8) as well as Sports Utility Vehicles, (for example, the Mercedes M-Class, Audi Q7, VW Touareg) for a more comfortable and quieter ride. Benefits include driver-adjusted suspension settings for a sportier or more comfortable drive, the automatic lowering of the bodywork for reduced consumption when travelling at high speed, as well as several different height settings, for more chassis clearance when driving off-road.

An air suspension system consists of four air spring struts and integrated dampers, each with continuously adjustable characteristics and switched, additional air volume. In addition there is a sensor system with travel and body acceleration sensors. A pressure supply and switchover unit with accumulator, plus valve block and pressure sensors, completes the system.

The control unit uses signals from the travel sensors to calculate the current height of the chassis, which it then manages by inflating or deflating the air springs. In addition, it prevents the car from bouncing up-and-down when travelling on uneven roads. The car body position can be held very accurately under these conditions, ensuring a very smooth ride for the occupants.

Normally, these units are housed in die cast magnesium enclosures, but for automotive component specialist, Woco Industrietechnik, DuPont’s Zytel HTN was the material of choice when it set out to design a pneumatic switchover unit for an air suspension system being developed by Continental Automotive Systems. The polyphthalamide (PPA) with 35% by weight glass fibres met all of the project’s requirements in terms of strength, dimensional stability (even at high temperatures), chemical and hydrolysis resistance as well as suitability for injection moulding.

In particular, the manufacturing costs are considerably lower than those associated with the die cast-magnesium housings traditionally used for such units. This is mainly due to the fact that no further refinishing work is required, such as de-burring, and the fact that complex-shaped parts can be produced in a single manufacturing stage.

The switchover unit, with the housing made of Zytel HTN, contains coils, electrically and pneumatically actuated valves, as well as several check valves. It is connected directly to the compressor unit and manages the air supply to the suspension system. Depending on the vehicle make, it is installed either under the front bumper, in the vicinity of the exhaust or in a part of the car’s interior, such as the spare wheel cavity. The housing is therefore exposed to various aggressive substances, including brake fluid, lubricants as well as road borne debris such as dust, sand and so on, during its entire lifetime. Working temperatures range from -40ºC to +80ºC, but short-term temperature peaks can reach 120ºC. Moreover, the housing is required to withstand a burst pressure of 35bar. Woco development engineer, Peter Henninger takes up the story:

“We chose Zytel HTN PPA 51G35 due to the positive experience we have had with the material to date. It meets all our requirements in terms of mechanical strength and is highly suited to this application. Moreover, the material maintains its very high dimensional stability and accuracy even at very high temperatures. This helps ensure that an air-tight seal is created between the switchover unit and the compressor unit for its entire lifetime, which is, in turn, critical for the reliable operation of the air suspension system.”

The complex-shaped housing (133mm long, 83mm wide and 70mm high) is produced via a precision injection moulding process – much aided by the good flow characteristics of Zytel HTN, and its very low warp tendency, which enabled tolerances of +/-0.05mm to be achieved.

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