Aluminum Casting Techniques
I’d like to kick off our manufacturing series with an invitation to learn some valuable pieces of information about manufacturing over the next few weeks. First, I’ll be sharing different manufacturing techniques for various components. Next, I’ll explain how to perform a cost analysis and finally, put together a business case to help you easily decide the best path for your business.
This week we’ll start with aluminum castings. Below are a few descriptions of how to manufacture these parts and the pros and cons of each.
Machining
Machining is the quickest and easiest way to produce a part from block aluminum that might normally be cast. Typical lead times for machining a part can range from a few days to 2 weeks. Whereas lead times can be significantly longer for other manufacturing types. Machining has some limitations though, in that you often can’t do intricate parts with internal cavities or bores. You can really only do convex shapes that can be done with a straight bit. Machining can be done on a mill or a lathe. It can be done manually by a trained machinist, or can be computer coded onto a CNC mill or lathe. Keep in mind that while 5 axis machines are more expensive to run, they often produce parts more quickly. Therefore, the part cost is lower. If you have a more complex part with machining on multiple surfaces in the x, y and z dimensions, you might want to specifically look for machine shops that have a 5 axis mill. The advantage of this is that the machine shop tech will fixture and set up the part only once, and then run the milling pattern and the end result is a completed part. When a 3 axis or 4 axis mill is being used, the part has to often be rotated or flipped to continue machining all the surfaces.
Machining can be quite expensive though. For instance, the part below can cost $1200 when done in Asia or up to $3000 when done in the US. Because of the high cost, machining is typically reserved for prototypes where only 1 or 2 pieces are needed quickly or high precision parts where budget is not an issue.
Sand casting
Sand casting is a great method for producing higher volume aluminum cast parts. It also has the advantage that it is an incredibly versatile process that can produce parts ranging from a few grams to several tons. Sand casting is done exactly as it sounds. A mold is made out of sand and molten aluminum is poured through a hole to fill a cavity. The link below is a good demonstration of a simple sand casting process.
However, sand casting can also be used to produce complex, seemingly impossible parts. The below example is of a motorcycle swing arm produced by TEI that won casting of the year. Its organic shape and hollow tube construction means that it is incredibly strong while also lightweight. This kind of part could never have been produced with any other method other that sand casting. The core that produced the hole tubes was 3D printed from sand as was the rest of the mold. Sand casting can be as low tech as in the video above or as high tech as a computer controlled 3D printer. The link below shows a video of sand casting an aluminum skillet.
Investment Casting
Investment casting, shell casting or lost wax casting is another versatile casting process that can be used to cast intricate highly detailed parts. While typically reserved for smaller parts, under 50lbs, it can produce thinner walls, finer details and higher tolerances as molded. Wax models of the end parts are produced. These are then coated in ceramic and the wax is melted out from the inside. These wax parts can be molded from a die for higher volumes or 3D printed for lower volumes or prototypes. The video below shows the typical Investment casting process.
Investment casting can also produce similarly complex and intricate parts with hollow cores like sand casting. However, the hot process where the ceramic mold is fired and then molten aluminum is poured in allows the metal to fill in smaller finer details (in sand casting the metal will cool before it can fill in the tiny details) and thus allows for the more delicate parts that are typically seen this process. The below image is a great example of a typical investment casting part with thin walls and an intricate pattern.
High Pressure Die Casting
High pressure die casting is typically what is used for high volume parts. While it is a large up front investment for the tooling, the piece prices can come out as little as 25% that of a sand casting. High Pressure die cast has similar limitations in form and shape as machining a part does. But with an added complexity – die cast parts must have a draft so they can easily be released from the mold. A perfectly straight walled part will not release from the mold and must have a least a 1-3% wall draft angle. This can cause some complications depending on the part.
One advantage that high pressure die casting does have is that it can do small hollows or other forms of pockets that a machined part can’t have due to highly technical collapsing cores and slides. The link below shows a video of how collapsing cores work.
Forging
Forging is a method where a block of aluminum is pressed under high pressure and forced into shape. Advantages are low porosity and better metal structure as the metal does not ever become molten again. Forgings are typically stronger than castings and are often used for parts that see high loads. Forgings can be produced as cold or hot forgings. Cold forgings are typically simpler shapes and are usually not heat treated after forming thus making them lower in cost. Hot forging can produce more complex parts because of the higher ductility, but often hot forged parts need secondary heat treatment.
Forgings have a big disadvantage though where the process can only be used to produce parts that have no undercuts or cavities.
The below video shows the general process involved in forging. It requires high tonnage machinery and hardened steel tooling to force the aluminum into such shapes.
The up front investment in forging tooling is also higher and therefore the process is better suited to medium and high volume production. Its quite common to see heat sinks forged as shown in the image below.
So which process do you pick for your part? Stay tuned for next weeks post where we run through different cost scenarios and a business case analysis.