Composites Oust Magnesium In Big Volume Engine
DaimlerChrysler shaves part cost, investment and time to market in replacing magnesium valve covers with vinyl ester BMC.
By Dale Brosius, Contributing Writer | December 2003
In the design and approval of a critical engine component, the automotive engineer concentrates first on performance and reliability, then balances the material and process decisions with economics. Engines today incorporate a host of improved technologies, designed to ensure high reliability and minimal maintenance costs. One material gaining acceptance in engine components is cast magnesium. With its 1.8 g/cc density (about two-thirds that of aluminum) and favorable property profile, magnesium was initially specified, over steel and plastic, for the left and right valve covers of DaimlerChrysler's (Auburn Hills, Mich., U.S.A.) redesigned 4.7L V-8 engine, introduced in 1999. Built in volumes of approximately 500,000 per year, the engine is common to several platforms, including the Dodge Durango, RAM and Dakota trucks, and the Jeep Grand Cherokee.
DaimlerChrysler employed advanced mold-fill simulation software to optimize part strength/weight values by dramatically reducing SMC part wall thickness — to 2.5 mm/0.100 inch, from the standard 3.5 to 4.0 mm/0.140 to 0.160 inch — within a short, 11-month turnaround.
In late 2001, DaimlerChrysler saw an opportunity to convert the valve covers to glass-reinforced BMC and, in so doing, save a lot of money in purchased part costs. Demand and other factors have driven up the cost of magnesium substantially, and the process used to make the covers involved a number of expensive secondary operations not required with vinyl ester BMC valve covers. The company already used BMC in valve covers for its 2.7L and 2.0L engines, so the performance was well-established. By some estimates, over 50 million BMC and SMC valve covers have been produced since 1988 without a single field failure, confirming the material's long-term reliability.
Key to gaining approval to proceed, emphasizes Steve Crawford, design release engineer for DaimlerChrysler, was that desired reduction in cure time and material costs could only be achieved if the wall thickness of the composite covers could be reduced to about 2.5 mm (0.100 inch), from the then-current standard of 3.5 to 4.0 mm (0.140 to 0.160 inch). In addition, the project needed to be completed quickly, which limited any prototype effort. Given these constraints, the conversion relied extensively on computer simulation and parallel analysis and validation efforts, resulting in project launch to full production in only eleven months — a dramatic reduction compared to the typical three-year development cycle.
Computer simulation provides confidence in design
The computer analysis involved simulation of the mold filling and resulting fiber orientation, followed by structural and vibration modeling. Due to the layout of the cylinders, the 4.7L engine develops considerable heat, and the valve cover temperature can reach as high as 204°C (400°F), and both thermoplastic nylon 6/6 and thermoset materials were explored, Crawford says. For the composite covers, DaimlerChrysler selected Premi-Glas 1265, a glass-reinforced vinyl ester BMC from Premix Inc. (North Kingsville, Ohio, U.S.A.), due to its high temperature properties and approved use on the 2.0L engine, which is molded via injection-compression by Dana Corporation (Paris, Tenn., U.S.A.). In the injection-compression process, the mold is held slightly open during the injection of the material, then closed to achieve final shape. This reduces fiber damage but achieves properties slightly short of those possible with pure compression molding. However, for the 4.7L engine, Dana and DaimlerChrysler made the decision to mold the covers using straight injection molding, which would shorten the cycle time and make the parts easier to deflash. However, with the thinner wall, issues such as shear heating and complete mold filling would become even more critical and demand greater control over the process.
The fully redesigned 2004 Dodge Durango comes standard with the 4.7L engine.
To gain initial confidence to go to injection molding, Dana modified the molding process for the 2.0L covers and produced a series of parts, which were quickly tested for performance and validated. Simultaneously, DaimlerChrysler turned to The Madison Group (Madison, Wis., U.S.A.) to simulate the mold filling of the 4.7L cover using its CADPRESS BMC software. While mold-filling software has existed for many years in thermoplastic injection, simulation of the thermoset injection process is further complicated by several factors, explains Dr. Paul Gramman, president of The Madison Group. "With thermosets, you have the combination of a very hot mold, a viscous skin layer of material, and a chemical reaction," he says. "In addition, the reinforcing fibers play an important role in determining how well the mold will fill, and whether knit lines will occur, creating potential weak points."
CADPRESS BMC was used to predict the filling of the part and the resulting fiber orientation. The injection simulation showed that slight "race tracking" would occur at the seal edge. Usually considered an undesirable effect in mold-filling operations, race tracking in this case would occur where the flow length was greatest, and therefore would actually assist in the mold filling process, enabling the BMC to flow from the injection gate up and over the top of the cover, without forming knit lines or trapping air. Dana and DaimlerChrysler conducted a series of "short shot" experiments performed on the actual mold, where less than the required quantity of material is injected and the resulting flow pattern observed empirically. A comparison of the predicted and observed results of the short shot experiments demonstrated a correlation to within approximately 2.5 mm (0.100 inch), notes Gramman.
