Cast crankshafts, though obsolete in most modern car engines, still exist in older and production vehicles and smaller machines. Manufacturers opt for forging the crankshaft in high-performance builds today, but the cast crankshafts that are still around are made through a process that takes real effort. To give you an idea, we’ll let you know all about the process of crankshaft casting, along with the materials used.
What Is a Cast Crankshaft?
So here’s the thing about cast crankshafts. There are three ways you can manufacture a crankshaft: casting, forging, and CNC machining. Out of these, casting is the one that costs the least, so top crankshaft manufacturers usually go for it when they want to save money.
But the problem with cast crankshafts is that they are brittle, because their tensile strength and ductility are way lower compared to the ones that are machined out from a billet.
Cast crankshafts have a typical tensile strength of around 60,000 to 100,000 psi and an elongation rating of around 2% to 3%, which is the measure of ductility or how much a part can be deformed before it breaks apart.
The elongation rating of forged crankshafts is 20% to 22% before failure, and they are more durable than the cast crankshafts. When the percentage is low, you already know the part is not going to take much abuse.
Cast crankshafts are made by casting molten metal into a mold and letting it set. This usually results in a pretty random grain structure. The grain structure refers to the way the tiny metal particles inside the crankshaft are arranged, and it plays a huge role in how strong or weak the part turns out.
Back in the old days, you’d see cast crankshafts in cars, motorcycles, and pretty much anything that had an engine. But now, things have changed.
Although these crankshafts are still used in production vehicles or in any scenario where there is a need to reduce the cost, manufacturers today mostly stick to forging because it produces much stronger and more reliable crankshafts.
What are the Materials Used in Crankshaft Casting?
While casting crankshafts, the material used makes a big difference in how well the engine performs. There are three materials: gray iron, ductile iron, and steel.
Now talking specifically about gray iron, it’s actually an alloy. That means it’s a mix of iron, carbon, silicon, and manganese. The carbon in gray iron usually makes up around 2.5% to 4% of the mix, while silicon is usually between 1% and 3%.
What makes gray iron such a good choice is mostly that it’s cheap, easy to machine to tight tolerances, which means you can cut and shape it to accurate size, and it resists wear really well.
Then, you’ve got ductile iron, which is also called nodular iron. Instead of regular graphite flakes, it has a nodular graphite structure. The way that structure is formed is by adding either magnesium or cerium to the molten iron.
Because of that nodular structure, ductile iron ends up with a higher tensile strength than gray iron, so it’s a better choice for high-performance engines.
Finally, there’s steel, an alloy of carbon and iron, which is used in top-end engines because it has the highest strength and stiffness out of the three.
The carbon content in steel is up to 2%. However, it is more expensive and difficult to machine than gray iron and ductile iron.
What Is the Process of Casting a Crankshaft?
Let’s talk about the process of crankshaft casting. In this method, the crankshaft comes out more accurate and sharp.
Step 1. Mold Preparation
The process starts with creating a mold, and this is often done using sand casting techniques. That means a cavity is created in a sand mold, and the shape of that cavity matches exactly what the crankshaft is supposed to look like. To make that happen, a pattern is used, and it is pressed into the sand so the mold cavity forms around it.
At the same time, a gating system is designed during this stage. This includes parts like gates, risers, and vents. That system is there to make sure the metal flows properly into the mold during the next steps.
The mold is usually made in two parts, which are called the cope and the drag. That two-part setup is important because it lets the finished crankshaft be removed after the casting process is done.
Step 2. Melting and Pouring
Once the mold is ready, a strong round piece of metal port is placed right on top of the gating system.
Then, the selected metal alloy, which could be something like cast iron, steel, etc., is melted in a furnace. The metal is heated until it reaches the correct temperature and consistency for crankshaft casting.
After that, the molten metal is carefully poured into the mold cavity using the gating system.
While this is happening, the liquid metal needs to flow smoothly to avoid turbulence during the flow, and not to get involved in gas. Otherwise, the metal could become oxidized or develop pores.
The tangential velocity also needs to be kept under control, because if it becomes too high, it could splash and flush against the inner wall of the cavity and sand core.
Step 3. Cooling and Demolding
After pouring, the mold is removed from the securing bracket with care. It is then taken outside to cool at ambient temperature, and this cooling continues until the molten metal solidifies and takes the full shape of the crankshaft.
Once the part has cooled, the sand mold is broken away with a hammer or some other tool to reveal the crankshaft.
Step 4. Finishing the Cast Crankshaft
When the crankshaft casting is out, it goes through a number of machining processes so it can reach the final shape and accuracy required.
These processes include grinding, drilling, and balancing.
Once all the machining is finished, the crankshaft is polished and cleaned so it is fully ready to be installed in an engine.
Conclusion
Crankshaft casting is hard manual labor up till the very end to produce a vital engine part that is affordable, reliable, and requires less machining. Forging, on the other hand, is the norm where the additional strength and durability of forged steel are necessary to withstand the stresses of high-performance in an engine.
If you’re in the business to offer better products to your clients, Nanjing Woda Auto Technology Co., Ltd is where you start.
We have been serving worldwide since 1999 to provide all types of precision crankshafts and complete engine support—from ignition systems and electrical setups to exhaust, suspension, and steering components.
Every part is manufactured to help you deliver more reliable, high-performance builds. Reach out to Nanjing Woda today and let’s talk about a strong partnership with mutual benefit.
