Casting vs CNC Machining: Which One is Better for Your Parts

CNC Machining, product development

Using molten metal to pour into a mold, casting has been practiced for more than 6,000 years and is still commonly employed today. Similar to CNC machining, a more recent production technique that is also commonly utilized, advancements in casting technology allow for the casting of high-quality items with precise tolerances.
However, have you ever thought of CNC machining a casting that you typically use? Or do you think about the best way to produce your design? The advantages and drawbacks of the two manufacturing techniques will be covered in this essay. You’ll learn the answers to some queries about casting vs CNC machining.
casting machining - casting vs CNC machining

What is Casting?

Casting is a formative manufacturing process that utilizes molds to create parts. This article focuses exclusively on metal casting and does not address other materials like epoxy that can be used in multi-component casting.

In the realm of metal casting, the procedure involves the pouring or insertion of molten metal into pre-constructed molds, often equipped with cores or inserts for intricate internal features and cavities. Once the material cools and solidifies, the resulting part undergoes cleaning and processing to achieve the desired surface finish. In some cases, this secondary operation may entail CNC machining to achieve even tighter tolerances or to add features that are not conducive to the casting process.

Common examples of cast parts encompass a wide range, including train wheels, bus pedals, turbine blades, and engine blocks. Such cast components find application across diverse industries, such as automotive, aerospace, consumer products, and medical devices.

What is CNC Machining?

CNC machining is a subtractive manufacturing technique in which a cutting tool is used to remove material and attain the desired shape. A computer program, typically employing G-code instructions, guides the machine’s movements and cutting actions.

CNC-machined components are ubiquitous, finding applications in various industries. They are integral in crafting metal laptop enclosures, tiny components within mobile phones, as well as essential elements in aerospace and automotive parts. Moreover, CNC machining is also pivotal in the production of fixtures, jigs, and even casting molds.

Casting vs CNC Machining: Selecting Your Manufacturing Approach

When it comes to designing a part, there are typically two frequently employed approaches for determining the manufacturing method to produce the part, specifically CNC machining and casting. But how do we choose them: casting vs CNC machining?

Design engineers often begin with a preferred manufacturing method in mind to ensure that the design is practical, manufacturable, and incorporates features that align seamlessly with that specific process.

Conversely, the functional requirements of a part may necessitate particular dimensions, shapes, or geometries that inherently dictate the most suitable manufacturing process. CNC machining and casting, for instance, each excel in specific geometrical contexts.

Casting vs CNC Machining: Selection Based on Part Geometry

The choice of the most appropriate manufacturing method hinges on a careful evaluation of the part’s geometry. Consider, for instance, designs that necessitate the removal of a substantial amount of material, a process that can be time-consuming and generate significant waste. Such designs often find favor in casting. Take, for example, a hollow part with slender walls and extensive cavities. Casting is an excellent fit for such scenarios. Moreover, intricate and complex geometries that demand lengthy machining time are also well-suited for casting.

Internal cavities pose another interesting dimension for consideration. Casting is frequently preferred for these, especially when they are tightly sealed and require additional assembly or metal joining procedures alongside CNC machining. In contrast, casting simplifies the design of a part or assembly. Rather than machining numerous small components and assembling them, casting the entire part eliminates the need for multiple assembly steps.

However, it’s crucial to acknowledge that certain features pose challenges when using casting. Achieving undercuts or internal features can be intricate and may necessitate complex molds incorporating cores and inserts. Casting is also less favorable for sharp edges or corners, and it typically demands a draft angle on all features. Large or irregularly shaped parts with thick walls tend to be better suited to CNC machining because these geometries can be difficult to fill and cool uniformly during the casting process.

Casting vs CNC Machining: Selection Based on Materials

Casting is renowned for its efficiency in material utilization when compared to CNC machining. The casting process predominantly consumes only the necessary material for the final part, with minimal excess flowing through the gate, runner, and sprue. This surplus material can be readily removed, remelted, and repurposed in the creation of another part.

However, it’s important to note that the options for materials are more limited when it comes to casting in contrast to CNC machining. Virtually all alloys can be used in CNC machining, whereas the roster of alloys suitable for casting is considerably shorter.

Casting vs CNC Machining: Selection Based on Production Speed

The lead time for the casting process typically spans a few weeks. Once the mold is prepared, the casting process itself is relatively rapid, making it an excellent choice for producing high volumes of parts. Nonetheless, the design and machining of the mold necessitate some lead time. Additionally, the mold must undergo quality inspections and maintenance, although some casting methods employ expendable molds. In contrast, die casting employs a permanent mold, resulting in reduced time and cost per part as production volume increases.

CNC machining offers a swifter production speed, resulting in a quicker turnaround, particularly for low volumes of parts. No waiting is required for mold manufacturing, as the essentials for initiating part production are limited to the CAD model, CAM program, and access to a CNC machine.

Casting vs CNC Machining: Selection Based on Reproducibility and Precision

Reproducibility is a vital aspect in manufacturing, and the casting process excels at generating consistent parts. Once the mold and process parameters are finely tuned, each part can be replicated with exceptional uniformity. For instance, in die casting aluminum, tolerances can be maintained within the range of +/- 0.25mm for dimensions up to 25.4mm, and precision casting can achieve tolerances of 0.01″ for standard dimensions and as fine as 0.002″ per inch.

It’s worth noting that these tolerances can vary depending on the material used. For instance, when casting aluminum, which has a higher melting point than zinc, a more vigorous process is necessary, leading to somewhat looser tolerances compared to cast zinc. Tighter tolerances can be attained when casting zinc.

On the other hand, CNC machining of metal is an exceptionally reproducible and precise process, yielding significantly tighter tolerances than casting. In standard metal machining, a reasonable expectation is a tolerance of 0.025mm (0.001″). Precision machining, in particular, can produce parts with tolerances as tight as +/- 0.0002″.

Casting vs CNC Machining: Selection for Prototyping

During the prototype development phase of your design process, opting for casting is not advisable. This is primarily because each modification to your design necessitates the costly creation or adjustment of molds, leading to extended lead times and increased expenses. Prototype die casting, for instance, can incur costs ranging from thousands to hundreds of thousands of dollars. Even though sand casting tends to be more economical, the advantages of die casting over sand casting aren’t particularly relevant for prototype manufacturing.

CNC machining proves to be the superior choice for prototyping, primarily due to the flexibility it offers in adapting to evolving designs. When your design is still evolving, the only requirement is to update the model and revise the CAM program, eliminating the need to recreate molds or attempt modifications to existing ones.

Casting vs CNC Machining: Selection Based on Quantity and Demand

The quantity of parts required is a crucial factor to consider. In the early stages of prototyping, when the need for parts is limited, CNC machining emerges as the more practical choice. Another significant aspect to contemplate is the variability in your demand. How frequently must you ramp up production? If demand fluctuates, CNC machining may be the more prudent option.

CNC machining offers the advantage of swift, on-demand production, particularly with the advent of digital transformation in the manufacturing sector. This makes CNC machining an economically sound choice since you can order parts as needed, eliminating the need to predict demand and subsequently deal with surplus inventory.

Conversely, if you are transitioning into full-scale production with the expectation of high demand, casting becomes an appealing choice. Casting is well-suited for efficiently producing thousands of parts or more, making it a suitable solution for such scenarios.

Sungplastic’s Tips: Casting vs CNC Machining

So, when should you opt for casting over CNC machining, or the other way around? The decision of casting vs CNC machining depends on several key factors: your current stage in the development process, the quantity of parts you’ll require, the variability in demand, and the specifics of the part’s geometry.

Ultimately, the ideal manufacturing process hinges on your unique circumstances. You may encounter a distinct combination of requirements that necessitate innovative problem-solving to find the best solution.

Despite the growing popularity of CNC machining, casting still holds its place in manufacturing. As demonstrated in this article, each manufacturing method presents its own set of advantages and disadvantages. Armed with the knowledge you’ve acquired, you can confidently make the right choice for your project based on considerations such as part geometry, quantity, quality requirements, and your project timeline. In fact, your project might even benefit from a blend of casting, CNC machining, and finishing processes.

If you find yourself unsure about the pertinent process details or are struggling to reach a decision, we encourage you to reach out to our team of expert engineers. Discuss your project with us, and we’ll provide you with effective guidance to make an informed choice.

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