Engineering Insights: BMC Creates New Dimensions in Underhood Apps
Specially formulated material enables production of high-tolerance automotive engine air intake component.
By Dale Brosius, Contributing Writer | August 2007
Source: Karl Reque
The past 20 years have seen the introduction of numerous metal-to-composite conversions in automotive engine-compartment applications. Although weight reduction can be a factor in such conversions, the focus increasingly is on improving performance at the same cost, or reduced costs vs. metallic components through parts integration, materials and processing innovations or both.
Most of the easy substitution of plastics for metals in engine compartments was done in the 1980s and 1990s. Today, composite-for-metal replacement is complicated because the opportunities for part conversion involve more stringent mechanical and thermal demands and, therefore, require the use of high-strength, usually glass-reinforced polymers with excellent mechanical performance at operating temperatures of 140°C/284°F and higher. Fortunately, modern design tools and improved molding technologies are enabling replacement of metals with high-temperature thermoplastics and thermosets in applications that only a few years ago were thought to be too difficult for polymer-based materials.
A case in point is the throttle body used in fuel-injected engines. While some simple mechanical air-control valves have converted to composites, more sophisticated electronic throttle control (ETC) valves steadfastly remained in die-cast aluminum until last year, when 1.4L and 1.6L engines built by a joint venture between automakers BMW (Munich, Germany) and PSA Peugeot Citroën (Paris, France) debuted with ETCs manufactured from injection molded bulk molding compound ( BMC). Used in vehicles such as BMW’s Mini and the Peugeot 207, the ETCs take advantage of dimensional stability advancements offered by a specially-formulated bulk molding compound (BMC) and precision molding technology. Tier 1 air and fuel management systems supplier Siemens VDO Automotive AG (Regensburg, Germany) designed, developed and supplies composite ETCs to BMW and PSA, working with molder Helvoet Rubber & Plastic Technologies NV (Lommel, Belgium) and BMC supplier TetraDUR GmbH (Hamburg, Germany), a subsidiary of Bulk Molding Compounds Inc. (BMCI, West Chicago, Ill.).
Source: BMCI
The Siemens VDO electronic throttle control.
Multifunctional device keys engine efficiency
The replacement of conventional carburetors with fuel injection was a major leap in gasoline engine fuel efficiency and emissions control. At first, the flow of air to the engine in injection systems was directly controlled by the movement of the gas pedal via a mechanical linkage between the pedal and the throttle body. But engines developed in the last decade, especially those in Europe, have incorporated the ETC. The ETC uses a more sophisticated system: the gas pedal sends a signal to a sensor that activates a motor housed in the control body. The motor adjusts a valve that controls the amount of air entering the engine. Additionally, oxygen sensors adjust the amount of fuel that is injected to maximize fuel efficiency and reduce emissions. Other electronics optimize engine torque, idle speed control and transmission control to improve the smoothness of the engine and the vehicle ride.
The key to achieving optimum ETC performance is accurate and smooth valve movement and repeatable valve seating — characteristics controlled by the throttle body’s dimensional precision and stability. The traditional method used to manufacture the valve plate and throttle housing is to die cast the parts in aluminum and machine them to the final dimension, a costly extra step. Also, because the parts operate in the engine compartment, they must withstand temperatures as high 150°C/302°F and resist damage from fuels and other engine fluids.
In 2001, faced with rising costs for aluminum and a need to reduce ETC weight and cost, Siemens VDO turned to Helvoet to explore production of ETC bodies in reinforced plastics. Already a significant supplier of high-tolerance, precision-molded components, such as phenolic fuel pump impellers, which require dimensional control at the micron level, Helvoet quickly determined that phenolic and high-temperature thermoplastics, such as polyphenylene sulfide (PPS), would be too expensive. The decision was made to investigate polyester BMCs, in part because they are 40 percent less expensive than aluminum on a cost per cubic centimeter basis. The primary question was whether polyester BMC could meet the demanding tolerance and temperature requirements.
Modified material prevents in-mold shrinkage
The first step in evaluating substitute materials, says Herman Koks, sales/marketing manager for Helvoet, was the fabrication of a test mold, consisting of a cylindrical bore and a flanged landing area around the cylinder. With this tool, Helvoet and Siemens VDO could quickly screen materials for mold shrinkage, concentricity, dimensional tolerances and other characteristics. In mid-2002, TetraDUR was invited to submit a range of compounds for evaluation. “We had to make some modifications to obtain a material that met the requirements,” notes Brett Weber, currently VP of European and Asian operations for TetraDUR’s parent, BMCI. At the time of development, Weber was stationed in Europe and active in the material’s development. He explains that the conventional method for formulating low-shrink molding compounds involves the use of thermoplastic additives in the base polyester resin. The additive, however, often reduces the compound’s mechanical strength and thermal performance, so a delicate balance between part dimensional stability and performance must be achieved.
In the end, the selected material, BMC L4220, was a mixture containing 15 percent chopped glass and 60 percent specially blended mineral fillers in a modified polyester resin. It provides thermal performance above the required 150°C/302°F, and the coefficient of thermal expansion (CTE) is 18µ/m/°C (18 microinches/inch/°C), very close to that of aluminum and less than half that of glass-reinforced thermoplastics. The compound easily passes all the requirements for strength and chemical resistance, including resistance to FAM-B, a high-methanol proxy fuel mixture used for testing, which Weber says is more severe than 90 percent of the fuels sold in Europe.
Significantly, the BMC L4220 exhibits near-zero in-mold shrinkage such that the molded part dimensions are an accurate replication of the mold cavity. As a result, it produces parts with exceptional dimensional repeatability. The measured variability of the BMC parts is typically one-fourth to one-half the allowed diametrical tolerance, which for the throttle body’s bore size of 60.05 mm/2.36 inches, is only ±0.05 mm (±0.002 inch), permitting proper operation of the valve plate, which is now produced in PPS (polyphenylene sulfide) with a steel shaft. This means the parts need no postmold machining. After demolding, they are deflashed and ready to ship, emphasizes Koks. “There’s no machining or postcuring,” he notes, “and no threaded inserts to install.” Instead, bolts pass through the housing’s mounting holes and into threads in the engine’s intake manifold.
