Time to read: 6 min
Casting, a process whereby molten metal is poured into a mold, has been around for over 6,000 years and is still widely used today. Advances in casting technology enable the casting of high quality parts with tight tolerances, not unlike CNC machining, a newer manufacturing method that’s also widely used.
But have you ever considered CNC machining a part that you usually cast? Or are you considering which method is best to manufacture your design? This article will discuss the two manufacturing processes and the advantages and disadvantages of each.
What is Casting?
Casting is considered a formative manufacturing process that uses molds to make parts. There are many types of casting and many materials that you can cast. In this article, we will only discuss metal casting, not other time-setting multi-component materials that can be cast, like epoxy.
In metal casting, molten metal is poured or inserted into a pre-made mold with cores or inserts if intricate internal geometries and cavities are needed. After the part cools and solidifies, it’s cleaned and processed to give it the desired surface finish. Sometimes this secondary operation includes CNC machining to produce even tighter tolerances or features that are not ideal for casting.
Some commonly cast parts include train wheels, bus pedals, turbine blades, and engine blocks. Cast parts are used in almost every industry, including automotive, aerospace, consumer products, and medical devices.
What is CNC machining?
CNC machining is a subtractive manufacturing method whereby a cutting tool removes material to achieve the desired geometry. A computer program using G-code tells the machine where to move and how to cut.
CNC machined parts are everywhere. Metal laptop enclosures, small components inside cell phones, aerospace and automotive parts, as well as fixtures and jigs — even casting molds are manufactured using CNC machining.
Choosing Your Manufacturing Method
When designing a part, there are two commonly utilized paths to determine a manufacturing method to make the part (CNC vs casting):
- Design engineers often start with a manufacturing method in mind so that the design can be realistic, manufacturable, and have features that are well-suited to that process.
- On the other hand, the function of a part may require a certain size, shape, or geometry that dictates the manufacturing process. CNC machining and casting are each best-suited to certain geometries.
Casting vs CNC Machining: Choosing Based On Part Geometry
Choosing the correct manufacturing technique starts with considering the part’s geometry. For example, certain designs require a large amount of material removal, which can be time consuming and create excessive waste. Such designs are often good candidates for casting — for example, a hollow part with thin walls and large cavities. Complex geometries are also good candidates for casting, as they take a long time to machine.
Internal cavities are often ideal for casting as forming them with CNC machining can be incredibly complex. This is especially true for sealed internal cavities that require assembly or metal joining processes in addition to CNC machining. In contrast, casting lets you simplify the design of a part or assembly. Instead of machining many small parts and assembling them, you can cast the entire part and eliminate assembly steps.
However, certain features are more difficult to achieve with casting. Undercuts or internal features are more difficult and require complex molds with cores and inserts. Sharp edges or corners are not ideal for casting, either, and cast parts require a draft angle on all features. Large or oddly-shaped parts with thick walls are also better suited to CNC machining because those geometries are difficult to fill and cool evenly during the casting process.
Casting vs CNC Machining: Choosing Based On Materials
Casting results in less wasted material than CNC machining. The casting process only uses the material needed for the final part, aside from a small amount of metal that flows through the gate, runner, and sprue. It is easy to remove this excess material, remelt and reuse it in another part.
Casting vs CNC Machining: Choosing Based off Production Speed
The casting process lead time is typically a few weeks. The casting process itself is pretty fast once the mold is ready, which is why it’s good for producing high volumes of parts. However, mold design and machining requires some lead time. In addition, the mold must go through quality checks and maintenance, though some types of casting use expendable molds. However, die casting uses a permanent mold, so the time and cost per part decreases as the volume of parts increases.
CNC machining provides a faster production speed that results in a quicker turnaround for a low volume of parts. There’s no need to wait for the mold to be manufactured, since all you need is the CAD model, CAM program, and a CNC machine to start making parts.
Casting vs CNC Machining: Choosing Based off Repeatability and Precision
Repeatability is important in manufacturing, and the casting process produces repeatable parts. Once the mold and process are dialed in, every part can be made exactly the same. When die casting aluminum, tolerances can be held to +/- 0.25mm up to 25.4mm, and for precision casting 0.01” up to 1” for standard and .002” per 1”.
Those tolerances vary depending on the material. Aluminum has a higher melting point than zinc, for instance, so it requires a higher injection pressure and a more aggressive process overall. That results in looser tolerances with cast vs machined aluminum. Tighter tolerances can be held when casting zinc.
CNC machining metal is also an extremely repeatable, accurate process that produces much tighter tolerances than casting. For standard machining of metal, 0.025mm (0.001”) is a reasonable expectation. Fictiv precision machining can produce parts with tolerances as tight as +/- 0.0002”.
Casting vs CNC Machining: Choosing for Prototyping
If you’re in the prototyping phase of your design process, casting is not a good idea. You’ll have to pay for expensive molds to be re-created or adjusted every time you change your design, which increases lead times and cost. A prototype die cast can cost thousands, if not hundreds of thousands of dollars. And, while sand casting is typically cheaper, there isn’t much benefit to choosing die casting vs sand casting for prototype manufacturing.
CNC machining is the better choice for prototyping, due to the relative agility of changing a CNC machining process. When the design is still in flux, all you need to do is change the model and redo the CAM program, rather than remaking a mold or trying to modify an existing mold.
Casting vs CNC Machining: Choosing based off Quantity and Demand
The quantity of parts you need is also relevant. Earlier in the prototyping process you may not need many parts, so CNC machining makes more sense. The variability of your demand is another consideration How often do you actually need to produce more parts? If demand fluctuates, CNC machining might be a better choice.
You can get CNC machined parts much faster and on demand, especially with digital transformation in the manufacturing industry. So, CNC machining is a good financial choice since you can order parts as you need them instead of trying to predict demand and ending up with excess parts in inventory.
However, if you’re moving into production with the expectation of high demand, casting is a good choice, because it’s better suited to producing thousands of parts or more quickly.
So, when should you choose casting over machining or vice versa? Consider where you are in the development process, the quantity of parts you’ll need, demand variability, and part geometry.
The right manufacturing process will always come down to your specific opportunity. You may find yourself with a unique combination of requirements that forces you to be creative and find an innovative solution.
And while CNC machining has soared in popularity, there’s still a place for casting! As we set forth in this article, each manufacturing method has its own advantages and disadvantages. But with the knowledge you’ve gained you can make the right choice for your project based on the part geometry, quantity, quality needs, and your timeline. Your project may even require a mixture of casting, CNC machining, and finishing!
Sourcing Simplified – Start Your Next Project With Fictiv
Now that you know the differences between casting and CNC machining and have our tips for choosing the right method, you may need to source a supplier for CNC machining. Check out Fictiv’s CNC Machining capabilities, then create an account and upload your part to find out how our instant quote process, design for manufacturability feedback, and intelligent platform can help you.
Fictiv is your operating system for custom manufacturing that makes part procurement faster, easier, and more efficient. In other words, Fictiv lets engineers, like you, engineer.