Differences Between 3D Printing and CNC Machining of Plastic Parts

Plastic parts manufacturing is on the rise. Globally, there is a massive e demand of plastic machined parts but new technology is also echoing. It is 3D printing (additive manufacturing). Both methods boost unique strength and properties. Let’s have a look on some latest research on both methods and try to figure out which one is the best.

In a study held by Ömer Bayraktar and Gültekin Uzun on “3D-printed plastic parts and mechanical properties”, they used FDM technology to check the mechanical strength of 3D-printed parts made from PLA plastic.

They figured out some amazing facts:

Three factors affecting tensile strength were examined:

  1. Melt temperature (190°C, 205°C, 220°C)
  2. Layer thickness (0.06mm, 0.10mm, 0.19mm, 0.35mm)
  3. Raster pattern orientation (+45°/-45° crisscross, horizontal, vertical)

In this experiment, they found that the raster pattern and tensile strength achieved high accuracy (R² values of 0.999+).

In another research held in February 2024 on 3D Printing and CNC Machining, it states the following findings:

  1.       Traditional manufacturing methods struggle with complex shapes. However, 3D printing can produce them layer-by-layer.
  2.       CNC machining offers a post-processing solution for 3D-printed parts
  3.       Possibilities to combine both methods for higher precision parts

You might be wondering when both methods carry significant advantages, which is the best method to machine plastic parts.

This is the crux of our article today.

Key Takeaways of the Content:

  1.       3D Printing & CNC Machining Process Breakdown
  2.       Design Features & Surface Finishing
  3.       Applications

O no matter if your focus is on rapid prototyping for complex designs or looking for affordable high-volume production, understanding these factors will help you to take the best decision.

Process Breakdown of 3D Printing & CNC Machining

As mentioned above, both 3D printing and CNC machining boost excellent features. However, they manufacture parts in different ways. Read a brief comparison below:

1. 3D Printing

3D printing, also known as “Additive manufacturing,” is a revolutionary concept. To understand that concept easily, let’s take an example. You have a digital model of some object. With 3D printing technology, you develop the shape layer by layer by adding the material. So ultimately, you bring a digital model into real life.

This technology incorporates a digital blueprint. It is called an STL file.  This file is sliced into incredibly thin horizontal layers. Just like the above example, a printed builds the layers and convert that digital file into a plastic object. We shall read about the different plastics after reading about more printing technologies.  

3D printing process

1) Fused Filament Fabrication (FFF):

This method has another popular name; FDM (Fused Deposition Modeling). This is a complex process; a thin filament of plastic, ranging from 1.75mm to 2.85mm in diameter, works in the following pattern:

  1.       It is unwound from a spool
  2.       Heated to a molten condition
  3.       Finally, extruded through a tiny nozzle

A computer precisely controls this tiny nozzle. The nozzle deposits the molten plastic layer by layer. This process continues until the final part is completed. FDM offers a suitable range of filament options. For example:

 

Comparison Table of ABS, PLA, and Nylon

Property

ABS

PLA

Nylon

Material Type

Thermoplastic (Amorphous)

Thermoplastic (Semi-crystalline)

Thermoplastic (Semi-crystalline)

Printing Temperature

210-250°C

190-220°C

220-260°C

Heat Resistance

High (up to 105°C)

Low (deforms around 60°C)

High (up to 150°C)

Chemical Resistance

Moderate

Low

High

Warping

More prone to warping

Minimal warping

Less prone to warping than ABS

Cost

Moderate

Lower cost option

Higher cost

2) Stereolithography Apparatus (SLA)

SLA adopts a laser-focused approach. What does it mean? A laser beam cures liquid resin. It solidifies the resin layer by layer following the digital model. For your information, SLA possesses significant resolution, which can produce parts with layer thicknesses as low as 0.025mm to 0.1mm. This thickness is ideal for complex designs with in-depth details.

3) Selective Laser Sintering (SLS):

SLS incorporates a bed of powdered plastic material. A high-powered laser is used for this method. The laser selectively fuses the plastic particles together. This process is done layer by layer to build the final part. Compared to SLA, SLS has wide options of materials such as engineering-grade nylons.

Layer thickness in SLS ranges from 0.07mm to 0.1mm.

2. CNC Machining

CNC machining is a highly satisfying process that involves several machines to complete a custom part. How exactly is the machine able to cut such complex geometric patterns? If you want to produce a custom part, you must feed the CNC machine a code called G-code and M-code.  

What happens when you feed this code to a CNC machine? The tool’s movement is beautiful, but it’s slightly difficult to understand. 

Suppose you want to engrave this logo on a steel plate. The tool motion needed to make this engraving seems simple, requiring only two dimensions. Just by moving the tool in the X and Y directions, we can perfectly achieve this engraving. Such CNC machines are known as two-axis machines.

What are G and M codes? First, G stands for geometry. This means the G-code transfers all the information required to complete a shape. M, on the other hand, stands for machine.

 This means all the machine-related information, such as;

 

Now, we shall discuss two main CNC machining processes:

  1. CNC Milling
  2. CNC Turning

CNC Milling 

CNC milling is a machining method that removes material from a workpiece by rotating a cutting tool. During machining, the tool moves along multiple axes, such as 3-axis and 5-axis. Each axis configuration has its own unique advantages and areas of application.

Comparison of 3-axis and 5-axis machines

 

 

Material Selection in CNC Milling

CNC milling technology is widely used for machining a variety of plastic materials, with common materials including ABS, PC (polycarbonate) and Delrin (homopolymer acetal). Each material has its own unique properties and is suitable for different application scenarios:

 

 

Advantages of CNC Milling

CNC milling technology is popular for its excellent accuracy and speed in plastics processing. Key advantages include:

CNC Turning

CNC turning primarily focuses on rotating the workpiece. During this process, a stationary cutting tool removes material. This method is used to create custom metal and plastic parts with cylindrical shapes and features. CNC turning in machining can accelerate mass production, ensuring precision and consistency while maintaining efficient production speeds.

Common CNC Turning Parts

CNC turning technology is widely used to manufacture various precision parts that play crucial roles in multiple industries:

 

Material Selection in CNC Turning

CNC turning parts can be made from various metal and plastic materials, each with unique characteristics suitable for different applications:

Key Comparison Between CNC Machining and 3D Printing

Previously, you have read about the basic understanding of CNC machining and 3D printing, including some pros and cons. So it is crucial to understand the technical details of each process. Therefore, in the following section, you will read about factors like design complexity, part strength, production volume, surface finish, and lead time.

1. Design Complexity

3D Printing

3D printing has an advantage over CNC machining in terms of design freedom. Its additive feature can produce parts with complex geometries. 

FDM printers, for example, can reach minimum feature sizes of around 0.2 – 0.4 mm. Subsequently, SLA delivers finer resolutions down to 0.025mm. So ultimately, 3D printing is the best process for prototypes with complex geometries. Building such shapes will be nearly impossible for CNC subtractive methods. That is the reason not every company provides CNC machining services for plastic parts; it is difficult.  

CNC Machining

CNC machining cannot be underestimated. If your focus is tight tolerances, then CNC machines deliver you the optimum results. For example, CNC milling machines handle moderately complex geometries. Milling machines can effortlessly manufacture features like:

Minimum feature size in CNC milling depends on two factors:

  1. The tool size
  2. Material

However, it falls within the range of 0.1mm to 0.5mm. So again, if your focus is on precision, CNC machining has a significant advantage for parts requiring tight tolerances.

2. Part Strength and Function

CNC machining takes the lead in part strength. The subtractive process incorporates solid plastic workpieces, resulting in parts with isotropic properties. This means the strength is consistent throughout the material. CNC-machined parts are ideal for functional applications.

3D-printed parts show some degree of anisotropy. Why does this happen? This is due to the layered nature of the printing process. The strength between layers (often referred to as the “Z-axis”) is weaker compared to the strength within the layer (the “XY-plane”). This feature can make parts vulnerable under significant stress. 

3. Production Volume and Cost

For low-volume production or prototypes, 3D printing is an affordable and efficient option. This affordability stems from the minimal initial setup costs associated with 3D printing. Unlike traditional manufacturing methods, 3D printing does not require complex tooling, molds, or jigs, which can be expensive and time-consuming to produce.

Advantages of 3D Printing for Low-Volume Production

  1. Minimal Initial Setup Cost: One of the primary reasons 3D printing is cost-effective for low-volume production is the lack of need for specialized tooling. This significantly reduces the upfront investment required to start production.
  2. Lower Cost Per Part: In low-volume production runs, the cost per part in 3D printing is generally lower compared to traditional manufacturing methods. This is because the expenses associated with setup and tooling are spread over fewer units, making 3D printing a more economical choice.
  3. Flexibility and Speed: 3D printing allows for rapid prototyping and quick iterations. Design changes can be implemented easily without the need to modify or create new tooling, saving both time and money.
  4. Complex Geometries: 3D printing can produce complex geometries and intricate designs that might be difficult or impossible to achieve with traditional manufacturing methods. This capability is especially beneficial for creating prototypes that require detailed features.

 

Cost Reduction with CNC Machining for Large-Batch Orders

While 3D printing excels in low-volume production, CNC machining offers significant cost advantages for large-batch orders, particularly with the use of advanced technologies such as 5-axis machining and high-speed tooling.

  1. 5-Axis Machining: This advanced machining technology allows for the simultaneous movement of the cutting tool along five different axes. This capability reduces the need for multiple setups and fixtures, significantly cutting down on production time and costs. It also enhances precision and allows for the production of more complex parts in a single setup.
  2. High-Speed Tooling: High-speed CNC machines can operate at faster speeds than traditional machines, reducing cycle times and increasing production efficiency. This leads to lower labor costs and higher throughput, making it more cost-effective for large-batch production.
  3. Economies of Scale: In large-batch production, the cost per part decreases as the total volume increases. The initial setup and tooling costs are amortized over a larger number of units, reducing the overall cost per part.
  4. Consistent Quality: CNC machining ensures high precision and consistency across all parts in a production run. This reduces the likelihood of defects and rework, further lowering production costs.

4. Surface Finishing

The appearance of a part is as crucial as its functionality. In this regard, CNC machining and 3D printing offer a range of surface finishing options to achieve the desired aesthetic and functional qualities.

CNC Machining Surface Finishing Options

CNC machining provides numerous surface finishing techniques to enhance the appearance and performance of the final product. Some of the common CNC machining surface finishes include:

3D Printing Surface Finishing Options

3D printed parts typically require post-processing to achieve a smooth and aesthetically pleasing finish. Common post-processing techniques include:

  1. Sanding: Manually or mechanically sanding the part to remove layer lines and achieve a smoother surface. This is labor-intensive and adds to the overall production time and cost.
  2. Vapor Smoothing: Exposing the part to solvent vapors that melt the outer surface slightly, resulting in a glossy, smooth finish. This is particularly effective for ABS and other solvent-reactive materials.
  3. Infiltration Techniques: Applying infiltrants such as resins or epoxies to fill in gaps and layer lines, enhancing the part’s strength and surface finish. This method is beneficial for porous materials or parts requiring additional structural integrity.

Surface Finishing Considerations for Different 3D Printing Technologies

The targeted surface finish for 3D printed parts also depends on the materials and printing process technology used. For example:

5. Speed and Lead Time

Turnaround time is a crucial factor in making any business successful. It directly impacts project schedules, customer satisfaction, and overall competitiveness in the market. For any project, the first thing that determines the project duration is part design. Depending on the complexity and volume of the order, different manufacturing methods can be chosen to optimize turnaround time.

Choosing the Right Method Based on Part Design

  1. Simple Designs:

    • 3D Printing: For simple designs, 3D printing is an excellent choice. It allows for rapid prototyping and production without the need for complex tooling or setup. This makes it possible to quickly iterate on designs and make necessary adjustments, significantly shortening the overall project duration. Additionally, the ability to produce parts on-demand reduces lead times, making 3D printing ideal for projects with tight deadlines.

  2. Complex Designs and Large Batch Orders:

    • CNC Machining: For large batch orders and complex designs, CNC machining can outperform 3D printing in terms of speed and efficiency. CNC machines are capable of handling high volumes with consistent quality and precision. Advanced CNC technologies, such as 5-axis machining and high-speed tooling, enable faster production cycles and reduced turnaround times. This is particularly important for complex parts that require intricate geometries and tight tolerances.

Advantages of CNC Machining for Fast Turnaround

  1. High-Speed Production: Modern CNC machines are equipped with high-speed spindles and advanced control systems, allowing for rapid material removal and efficient machining processes. This reduces the time needed to produce each part, enabling faster completion of large orders.

  2. Simultaneous Multi-Axis Machining: 5-axis CNC machining can perform multiple operations simultaneously, reducing the need for multiple setups and fixtures. This capability not only speeds up the machining process but also improves accuracy and consistency, further enhancing turnaround time.

  3. Automation and Repeatability: CNC machining benefits from automation, which minimizes manual intervention and allows for continuous production runs. Automated tool changers, robotic arms, and integrated quality control systems ensure that parts are produced quickly and with minimal downtime.

  4. Scalability: CNC machining is highly scalable, making it suitable for both small and large batch productions. Once the initial setup and programming are complete, CNC machines can operate continuously to produce large quantities of parts without compromising on quality or speed.

Applications

1. Applications of 3D Printing

3D printing is a versatile technology that finds applications in various industries due to its ability to produce complex geometries, customize designs, and reduce lead times. Here are some key applications of 3D printing:

Prototypes

3D printing is widely used in the prototyping phase of product development. It allows for the rapid creation of multiple iterations, enabling designers and engineers to test and refine their concepts quickly and cost-effectively.

Low-Volume Functional Parts

3D printing is also ideal for producing low-volume functional parts. It allows for on-demand manufacturing, customization, and the creation of parts with complex geometries that would be difficult or impossible to achieve with traditional manufacturing methods.

Custom Tooling and Fixtures

In addition to final parts, 3D printing is useful for creating custom tooling and fixtures that streamline the manufacturing process and improve efficiency.

Medical and Dental Applications

The healthcare industry benefits significantly from 3D printing due to its ability to produce patient-specific solutions, such as prosthetics, implants, and surgical guides.

Educational and Research Applications

3D printing is a valuable tool in educational institutions and research laboratories for creating models, conducting experiments, and fostering innovation.

Consumer Goods and Customization

The ability to produce customized products on demand makes 3D printing attractive for the consumer goods market, where personalization is a growing trend.

2. Applications of CNC Machining

CNC machining is a highly versatile and precise manufacturing process that is essential for producing high-quality parts in a wide range of industries. Here are some key applications of CNC machining:

High-Volume Production

CNC machining is particularly well-suited for high-volume production runs due to its ability to produce consistent and precise parts with minimal variation.

Parts with High Strength and Precision

CNC machining excels in producing parts that require high strength, precision, and intricate geometries. This makes it ideal for industries where the performance and reliability of components are critical.

Hybrid Approaches Combining 3D Printing and CNC Machining

In some cases, a hybrid approach that combines the strengths of both 3D printing and CNC machining can be employed to optimize manufacturing processes and achieve the best of both worlds.

Conclusion

In summary, both CNC machining and 3D printing are essential for manufacturing plastic parts. 3D printing excels in customization, rapid prototyping, and small-volume production, making it ideal for unique and complex designs. Conversely, CNC machining is perfect for high-precision and large-volume production needs. To determine the best method for your specific project, contact RALLY Machining today. Our experts will help you choose the optimal manufacturing solution to meet your requirements. Reach out now and ensure your parts are produced with the highest quality and efficiency.

FAQs

1. What is 3D printing?

Three-dimensional printing (3D Printing) is an additive manufacturing technology that creates three-dimensional objects by adding material layer by layer. Common 3D printing methods include fused deposition modeling (FDM), stereolithography (SLA), and selective laser sintering (SLS).

2. What is CNC machining?

CNC Machining (CNC Machining) is a subtractive manufacturing technique used to create parts by removing excess from a block of raw material through cutting, drilling, and milling. It is commonly used in the manufacture of high-precision and high-complexity parts.

3. Which method is better for prototyping?

3D printing is usually better suited for rapid prototyping because it does not require complex programming and setup and can produce a prototype of a design in a shorter period of time.

4. Which method is better suited for high volume production?

CNC machining is usually better suited for high-volume production because it ensures high consistency and accuracy on a per-part basis and is faster.

5. What is the difference in material selection?

Plastic materials commonly used for 3D printing include PLA, ABS, and Nylon. CNC machining can use a wider variety of plastic materials such as PP, ABS, Acetal, Nylon, PTFE, Acrylic, PEEK, HDPE and PVC.

6. Which method is less expensive?

For low-volume or single-piece manufacturing, 3-D printing is usually less expensive because expensive molds and fixtures are not required. For high volume manufacturing, CNC machining may be more cost effective.

7. Which method is better suited for high precision applications?

CNC machining typically provides higher accuracy and better surface finish, making it better suited for applications that require high precision and tight tolerances.

Work with RALLY for CNC Machining Parts

Request a quote for new project today!  No minimum order quantity and free samples available!


Quick Quote