CNC Machining Metal vs. Plastic: The Key Differences

In CNC machined parts there is always a choice of material selection and it directly depends on the project and design requirements. Since we are going to discuss machining metal vs plastics, our focus will be the characteristics both materials come with and what are challenges to machine plastics and metals.

When creating parts, it is necessary to select the appropriate material. Replacing parts that were previously made with metal with plastic may lead to higher quality products and increased productivity. Is metal good? Or is it better to choose resin? In order to make the right choice, you must understand the characteristics of each material.

By understanding the properties of materials, you can select materials that meet the requirements of the part and are cost effective also.

Even though metals and plastics are commonly referred to their properties differ greatly depending on the material.

Metal vs. Plastic Machining

In recent decades, technology in the plastics industry has advanced, and many plastic materials with excellent heat resistance and durability have been developed. Highly heat-resistant engineering plastics and super engineering plastics can now be used as substitutes for high-temperature parts such as those around engines, which were previously only possible with metal.

While the core concept of CNC machining remains the same, there are some key considerations when working with different materials:

• Tooling: Different cutting tools are needed for metals and plastics due to their varying properties. Metals require stronger and more durable tools to handle the higher cutting forces involved.
• Machining Speeds and Feeds: The speed and feed rate at which the CNC machine operates will also be adjusted based on the material hardness. The harder the material is, the slower speed and feed will be. For example, steel needs slower speed and feed compared to Aluminum when machining. Generally speaking, metal materials need slower speed and feed rate compared to the plastics. Except some super engineering plastics such as FR4 or engineering plastics plus fiber are even harder than normal metal so it needs slower speed and feed when this kind of plastics material do machining process .
• Cooling and Lubrication: Machining can generate heat, and depending on the material, cooling techniques like fluids or mists might be employed to prevent warping or melting.

Materials: Metals vs. Plastics

The fundamental properties of plastic and metal are very different. If you have a rough understanding of the properties of plastic and metal, it will be easier to select the material. We will explain the characteristics of plastic and metals item by item, as well as the differences in processing methods.

Proportion

If common plastic is 1, aluminum is 2 to 3 times heavier, and iron and copper are 8 to 9 times heavier.

The properties of plastic materials, which are lighter than metals, are useful in ways such as reducing the weight of automobiles and improving fuel efficiency.

Electrical conductivity

Steel conducts electricity, but plastic basically does not, making it suitable for manufacturing insulating parts. On the other hand, plastic is easily charged with electricity and therefore attracts dust.

In addition, some plastics are called “conductive plastics” that conduct electricity.

Heat Resistance

General-purpose resins have lower heat resistance than metals. Therefore, generally speaking, it is not suitable for use as parts used in high-temperature environments such as around automobile engines. However, as the developing of engineering plastics, more and more engineering plastics such as PEEK, PBS plus fiber can be used as high-temperature parts around engines to replace with metal.

Impact Resistance

Generally, metal has higher impact resistance than plastic. However, there are also plastic materials whose strength has been increased by adding things such as glass fiber. Highly impact-resistant plastics have also been used as materials for automobile bumpers.

Chemical Resistance

With the exception of some types, metals are susceptible to oxidation and rust. On the other hand, plastics have the characteristic of being sensitive to organic solvents, but there are types that have high chemical resistance. Even if metal parts require rust-proofing, plastics do not need rust-proofing as they do not rust.

High degree of freedom in shape

General-purpose resins have a low melting point, making them easier to mold than metals.

In addition, it has characteristics suitable for mass production, and can reduce product production costs.

However, the suitable shape and productivity differ depending on the molding method, so it is important to select a molding method that suits the part you are manufacturing.

Ease of coloring

Both resin and metal can be plated or painted, but plastic can be molded in a colored state by mixing dyes with the material. Suitable for mass producing parts of the same color.

Dimensional stability

Compared to metals, plastics are more sensitive to temperature changes, so the higher the temperature, the more their properties such as rigidity change.

Cost

General-purpose resins are often available at lower prices than metals. On the other hand, high-performance plastics such as engineering plastics are often more expensive than metals.

Machining Process of Metal & Plastics

CNC machining process involves three main techniques to machine both metals and plastics:

1. Milling
2. Drilling
3. Turning

Milling

CNC milling is based on the rotational movement of the cutter, which moves along the surface of the processed material and cuts it. In this process, the workpiece remains stationary. Milling machines, unlike lathes, operate mainly on flat surfaces. The work is carried out using a multi-point milling cutter, which, by separating unnecessary layers of the workpiece, gives it the desired dimensions, shapes or structure.  

Drilling

CNC drilling is possible thanks to the use of special types of drills, but not only that. CNC drilling sometimes uses milling machines or lathes for this purpose. It all depends on the type of working material and the type of hole we want, or rather need, to obtain.

Thanks to the possibility of extremely precise positioning of the drill in any plane, it is possible to make virtually any type of hole with a precisely set depth and diameter.

Turning

CNC turning is a method of machining that involves reducing the diameter of an object and giving it a cylindrical or conical shape. This process is carries incorporating a cutting tool. The workpiece is clamped on the fixture and then subjected to high speed, constant rotation. The cutting tool is moved towards it to provide the necessary feed force. It moves in a straight line, cutting subsequent layers.

Can the same machine be used for metal and plastic machining?

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In most scenarios, yes we can use the same machines but there are some important factors to consider like:

Material-Specific Considerations

Along with milling, drilling, and turning there are some additional factors to consider machining both metals and plastics. There are:

1. Cutting Parameters
2. Cooling Strategies
3. Tooling Selection

Cutting Parameters

Cutting parameters define the feed and speed rate to operate a CNC machine. Due to the hard features, generally speaking, metal materials need slower speed and feed rate compared to the plastics. It will help to minimize the tool wear and excessive heat generation during the machining process.

Cooling Strategies

To maintain tool life and part integrity, it is crucial to control the temperature during the machining process. Therefore, the coolant selection plays a decisive role here.

Flood Cooling: It is a common practice where high volume of coolant is used at the cutting zone. It helps to dissipate the heat significantly.

Mist Cooling: To machine plastics, mist cooling technique is preferred. In this method, we use a smaller amount of coolant to control the temperature.

Tooling Selection

What type of cutting tool is best to cut metals and plastics? This is another vital point to consider. Since metals are strong, they need more wear resistant tools like carbide. On the other hand to machine plastics, we can use steel or diamond-coated tools.

Difficulty Levels: Metals vs. Plastics

There’s a general perception that metal machining is more challenging compared to plastics but plastic is more difficult to machine. It is due to the precision tolerances which are quite difficult to control. The reason is there is internal stress from plastic itself; as a result, deformation and cracking are prone to occur, which eventually leads to out-of -tolerance or discarded product.

To solve this problem, engineers need more experience in processing plastic so that they could manage every operational detail. Boyi Technology engineers  has over 10 years working experience processing on plastics .They deep know how to control the process well and make out unique and high quality  plastic parts for our customers . 

Greater Difficulty:

While metals are generally harder, but machining plastics can be very challenging due to their properties. Here’s why:

• Heat Sensitivity: Plastics have lower melting points compared to metals. A minor increase in the temperature can deform the parts.
• Varied Material Properties: To get the best results, it is paramount to choose the best plastic material. It will help you to make a perfect part without leaving any issue.

Precision Challenges in Metals and Plastics:

It is very hard to maintain the tight tolerance in plastics compared to metals in CNC machining process. This is due to two major reasons:

1. Material Deformation
2. Machining effects

Limited Availability:

Not all CNC machining companies provide CNC plastic machining due to lack of technical expertise .

Post-Processing Needs:

Plastic parts need manual finishing which requires skills and technology to deliver optimum results. On the other hand, metals are easy materials for surface finishing. In the cutting process, there are chances of small burrs on the pars. These burrs are removed using manual finishing technique.

For more information on what deburring is, please refer to this resource.

Applications CNC Machining with Metal and Plastic

Feature	Metal Machining	Plastic Machining
Typical Applications	Engine components (pistons, valves) Aerospace parts (wing components, landing gear) Medical implants	Prototypes and low-volume parts Consumer (phones, laptops) Medical devices
Material Properties Emphasized	Strong and heat resistant	Lightweight
Core Machining Techniques	Milling, Drilling, Turning	Milling, Drilling
Cutting Parameters	Slower speeds, lower feed rates	Higher speeds, higher feed rates
Cooling Strategies	Flood cooling (high coolant flow) common For high hardness steel ,use air cooling due to effect and cost consideration	Mist cooling Flood cooling (high coolant flow)
Tooling Selection	Carbide, High-speed steel	High-quality steel, Diamond-coated
Difficulty	More challenging	Difficult due to heat sensitivity
Precision Tolerances	Easier to get tight tolerances	Difficulty in tight tolerances
Post-Processing	Less manual finishing	Deburring, and cleaning often needed

Conclusion

However, recently, resin materials such as engineering plastics and super engineering plastics, which have excellent heat resistance and durability, have appeared. Resin is also used for parts that are subject to high temperatures, and metal substitution is progressing.

It is now possible to use metal substitutes for parts in engine systems, such as car power trains, transmissions, and thermal management systems, which were previously thought to be impossible to replace.

Boyi Technology Precision Machining

Boyi Technology is a leading company providing one-stop solutions for both metals and plastics CNC machining. We have state of the art CNC machines for milling and turning operations. In addition, by incorporation 5-Axis machines, we can effectively product complex parts with higher precision.

Simultaneously, we have seasoned staff with decades of experience in injection molding. There is no need to outsource anything; from design to surface finishing we provide you everything with quick turnaround time.

FAQs

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Tagged: CNC Machining Guide