Perfected at the Group’s experimental casting plant in Neckersulm, the process is now ready for full-scale production. The specialists in the team led by development chief Dr. Christian Klimesch and process engineer Ludger Urhahn have come up with a solution that combines the benefits of several different casting processes. The result? High-quality, highly resilient cylinder crankcases that offer the added advantage of being relatively quick to produce. The new high-tech sand-casting solution is now being tested in a 4-cylinder in-line engine with a grey cast iron liner. The liners used here are over-molded at the cylinder head by several millimeters, the idea being to keep the milling cutter from having to move through different materials during subsequent processing. This structural shape cannot be achieved using conventional low-pressure chill casting.
The new high-tech sand-casting solution is now being tested in a 4-cylinder in-line engine with a grey cast iron liner. The liners used here are over-molded at the cylinder head by several millimeters, the idea being to keep the milling cutter from having to move through different materials during subsequent processing. This structural shape cannot be achieved using conventional low-pressure chill casting. For Klimesch, however, the process no longer represents virgin territory, “because the same process is being used at our plant in Guangde to produce electric motor housings for a battery-power car for the Chinese market”. This development originated in Neckersulm as well, before being transferred to the plant in Guangde, which is located around 300 kilometers west of Shanghai.
High demands on the process
Among other things, the exacting requirements for positioning the liners in the sand proved to be quite challenging during work to perfect the process for crankcases. They had to be heated in the fully mounted core package, after being exactly centered in cold condition between 22 sand cores. In the process, the thermal expansion of the liners also had to be taken into account.
Moreover, chill castings had to be mounted in the area around the bearing block, which would subsequently have to bear the main load during operation of the engine. Since active cooling isn’t used in chill casting, the necessary heat withdrawal has to be controlled via the mass of the chill castings. This way, very high rigidity is achieved with simultaneous large extension of the finished block. And this is exactly what engine makers want for today’s highly stressed engine generations.
After low-pressure filling, the block sits for a couple of seconds before being turned 180 degrees by a robot. This pressure is necessary in order to avoid movement of the molten metal during the rotation movement and the quality problems this can cause. “The great thing about this process is that it gives design engineers maximum scope for creativity”, explains Urhahn, noting that it enables “undercutting geometries, for example, and ducts of every shape.” Moreover, it requires less material, leading to a reduction in weight of between 3 and 5 %.
The result? High quality, highly resilient cylinder crank cases
In addition, on account of the extremely low heat conductivity of sand, it’s possible to produce thinner walls than with chill casting, even at the same filling speed and molten metal temperature.
Another advantage: Low-pressure sand casting turns out to be a very stable process with comparatively few variables once issues such as connecting the core package to the form filling are mastered. As Klimesch points out, “This is where our longstanding experience and leading role in low-pressure casting comes in. Once you’ve got the basic parameters down pat, it’s like baking pretzels.”
The low-pressure sand-casting process is very stable and takes a quarter less time than the conventional method
And it’s at least as quick: Instead of the roughly eight minute-cycle time required in low-pressure chill casting, the new process can be completed at least four times faster.