Bigger, better, tougher

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The use of glass fibre reinforcement along with integrative simulation in computer-aided design (CAD) have made possible the manufacture of large-scale oil pans in PA6 and PA66 materials. Even truck-sized parts, of 40l and more, can now be made lighter and with fewer seams, thanks to fibre-reinforced polyamide’s ability to be formed into complex shapes in a single piece. Frank Krause, application development high performance materials, Lanxess Deutschland, described the process that led to the development of a 12.6l truck engine sump made of 35% PA6, for the Mercedes Actros.

The 12.6l truck engine sump of the Mercedes Actros is made of 35% PA6. (photo: Daimler)

Simulation of the filling process: four gate points ensure an even filling of the mould. (photo: Lanxess).

The challenge

The oil pan, or sump, holds the oil that is essential for engine lubrication. While those in passenger cars are relatively small, with capacities of between 2-8l, truck sumps are much bigger; the larger engines need capacities of 40l or more. But the sump is not a simple tray; it is a fairly complex shape and is, as a result, ideally suited for production in plastic, rather than metal. Polyamides (PA) already are highly resistant to chemicals such as oils, lubricants, motor cleaners and corrosive substances resulting from oil decomposition and combustion, and some types have shown particularly high resistance to biofuels, including soy and rapeseed oil-based compounds. Tests have established that PAs maintain resistance throughout the life of truck engines – typically 20,000h. They also offer resistance to heat and wear, and have qualities of absorption of vibration and noise superior to metals’. Other properties, such as reversible absorption of environmental moisture and immediate readiness for use, with no installation holes to be drilled, further raise their attraction for underhood applications.

Adding glass fibre reinforcement to PAs improves mechanical strength and stiffness. However, glass fibres present particular challenges for injection moulding, because the anisotropic orientation of the fibres after injection means that many material properties are dependent on the orientation of the fibres themselves. This has made accurate simulation in CAD difficult but new and highly sophisticated calculation methods, such as integrative simulation, provide the basis for adequate cost-efficient calculations within a relatively short development time. These new methods can take localised anisotropic behaviour into account, along with their impact on the component’s properties. They also enable stresses to be calculated in advance and incorporated in the realisation process.

Simulation of the dynamic load when the entire engine block rests on the oil pan. The supporting feet are designed to crack under extreme stress (bottom right) while the main wall remains intact (top right). (photo: Lanxess).

Simulation results for filling, sealing, and stabilising

Distortion under stress through pressure or temperature has a considerable effect on the tightness of the flange, in large components especially. Simulation of the manufacturing process thus includes both the filling of the sump and its distortion behaviour and, as a result, special flow leaders have been incorporated into the pans. These channels ensure an even filling of the tool, especially in the flange area, which serves to minimise distortion of the finished component.

In order to evaluate system tightness during the component’s lifetime, the simulation also included distortion relative to the pressure on the gasket. Results of the simulation led to the increase of the sealing gap to be less than the maximum allowable value for the gasket itself, which removed any need for further reinforcement of the flange. The simulation also revealed that noise generated by the oil pan can be minimised by optimising the structure. The flat portion of the sump was designed with ribs, and the deep portion with a curved bottom.

The entire engine block and transmission – weighing 1.6t – is removed during maintenance or repairs and rested on the sump, so its behaviour under dynamic and static weight loads was also simulated. One of the outcomes was a modification to the ribs on the bottom, which now have areas designed as supporting feet. They absorb a significant amount of stress and prevent main wall failure. Simulation of resistance to stone chipping found that the front end of the deeper portion is at special risk, so its rib structure was duly optimised to withstand the stress.

Operating temperatures of 150°C present an ongoing challenge. PA has a much higher coefficient of heat expansion than aluminium, the material used in the flanges on the engine side. The simulation of the anisotropic heat expansion in the relevant area led to the development of walls and rib structures thick enough to ensure flange tightness over a wide range of temperatures and pressures.

This oil pan for a 12.8l engine holds 36l and is made of PA66 Durethan AKV 35 H2.0. It is used in the 12.8l Euro-6 engines of the Mercedes Actros. (photo: Lanxess)

Serial production begins

The first oil pan based on the outcome of the simulation was jointly developed by Lanxess and BBP Kunststoffwerk Marbach Baier  and initially made of heat-stabilised PA66 reinforced with 35% glass fibres, sold under the trade name Durethan AKV 35 H2.0. The model that entered series production consists of heat-stabilized PA66 reinforced with 35% glass fibres (Durethan AKV 35 H2.0). Measuring 125 x 40 x 35cm, it is used in the 12.8l Euro-6 engines of the Mercedes Actros. After further analysis, 35% glass fibre reinforced Durethan BKV 35 EF H2.0 (PA6) became be the material of choice. The glass fibres are less prominent on its surface, improving surface quality; the material is also tighter in groove areas; its demoulding properties are superior to PA66; and its good flowability reduces wear on the tool, which helps in the complex manufacturing process of large truck sumps. The shot weight amounts to 8kg/part. The tool is filled through four gate points, which requires a sophisticated heating process for each gate valve and uses a special temperature control system.

Lanxess says that high-quality PA/fibre composites offer major advantages, especially in technical applications. The trend towards PA sumps is expected to continue with plastic oil pans in passenger cars; this material is up to 50% lighter than die-cast aluminium, sheet steel, or sheet moulding compounds (SMC). PA also offers more flexibility in design and optimisation of underhood space. Their increased oil filling capacity translates into longer maintenance cycles. Lighter weight leads to improved fuel consumption and PA’s good sound dampening qualities also make a strong case for its increased use in oil pans.

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