Plastic CNC Machining – Material Selection and Machining

CNC machining is a common rapid prototyping process because of its accuracy, precision, compatibility with complex designs, and more. It is a subtractive process that involves computers controlling CNC tools through milling, turning, and other techniques to remove a portion of the workpiece used to form the desired product.

CNC machines are compatible with a wide range of materials, such as metals, plastics, and wood. However, with the rapid development of polymer plastics, many plastic product manufacturers now use plastic CNC machining, which involves using different CNC machine technologies to manufacture plastic products.

There are many aspects to plastic CNC machining. Therefore, this article will cover seven basic things you need to know about the process.

1.Can plastic be CNC machined?

Of course, plastics are perfectly suited to CNC machining and play an important role in numerous industries. Plastics are lightweight, durable, and relatively flexible. CNC milling, turning, drilling, and other machining processes allow for the rapid, high-precision production of complex parts while maintaining good dimensional stability. This machining method is relatively simple, more efficient than machining some metals, and less prone to tool damage.

Plastics’ chemical and corrosion resistance make them safe for use in a variety of applications, including medical, food, industrial, and electronics. Furthermore, different types of plastics can meet different needs: ABS is suitable for impact-resistant functional parts, acrylic (PMMA) for transparent components, nylon for high-wear mechanical parts, and PEEK or PEI for high-temperature, high-strength industrial applications.

Thus, whether for one-off prototyping or mass production of functional parts, plastic CNC machining offers a highly precise, durable, and cost-effective solution.

2.Which plastics are best for machining?

Thermoplastics are the most commonly used plastic polymers in CNC plastic machining. This is because they retain their original properties after being melted and reshaped.

To better understand and know which plastic polymers to use, you can check out our CNC material list. However, here are some common plastics that are suitable for many products.

ABS (Acrylonitrile-Butadiene-Styrene)

ABS is a lightweight, strong, and impact-resistant synthetic plastic. It has excellent processability, making CNC parts easy to mold and maintaining dimensional stability. However, ABS has limited UV resistance and may fade or become brittle with prolonged exposure to sunlight. Its heat resistance is also moderate, making it unsuitable for high-temperature environments.

Applications:

ABS is commonly used in automotive parts, toys, electronic product housings, and rapid prototyping. Due to its low cost and ease of processing, it is suitable for producing large quantities of lightweight parts or functional prototypes.

PC (Polycarbonate)

PC is a transparent and tough plastic that can withstand high shock and temperature and has excellent optical properties. However, care must be taken during machining to prevent surface scratches. Prolonged UV exposure can cause the material to age or discolor.

Applications:

It is widely used in protective eyewear, helmets, electronic device housings, automotive lighting, and architectural structures, where both strength and transparency are required.

PMMA (Acrylic)

PMMA is lightweight, transparent, and weather-resistant, making it a suitable alternative to glass. It has moderate hardness, but it is brittle and easily scratched when exposed to force, making it unsuitable for high mechanical stress.

Applications:

Suitable for signage, display cases, light housings, aquariums, as well as translucent windows and lenses. It is a top choice for parts requiring high visual quality.

POM (Polyoxymethylene)

POM offers high rigidity, wear resistance, and low friction, making it a very stable material for CNC machining. However, it has poor UV resistance and is susceptible to degradation in contact with strong acids or chlorine.

Applications:

It is suitable for parts requiring high precision and wear resistance, such as gears, bearings, fasteners, and automotive parts. It is a common material for mechanical structural components.

Nylon (PA)

Nylon offers high strength, wear resistance, and chemical resistance, with mechanical properties close to those of metal. However, its disadvantages are high water absorption, which can affect dimensional stability due to humidity fluctuations, and it can become brittle at low temperatures. Tungsten carbide tools are suitable for CNC machining.

Applications:

Commonly used in gears, bushings, conveyor belts, and mechanical parts. Also used in automotive components, textiles, and sports equipment, it is particularly well-suited for applications subject to friction and impact.

HDPE (High-Density Polyethylene)

HDPE offers high hardness, excellent impact, corrosion, and chemical resistance, good dimensional stability, and ease of processing. However, its heat resistance is moderate, making it unsuitable for high-temperature environments.

Applications:

Suitable for pipes, containers, pulleys, guide rails, and parts for food processing equipment. It is an ideal choice for lightly loaded mechanical parts.

LDPE (Low-Density Polyethylene)

LDPE is flexible, lightweight, and has moderate chemical resistance. It is easy to process, but has low heat resistance and can degrade and become brittle due to long-term UV exposure.

Applications:

Suitable for packaging, plastic bags, flexible tubing, and lightweight parts. It is often used in CNC machines requiring low loads and high flexibility.

PP (Polypropylene)

PP is lightweight, flexible, and chemically resistant, but is UV-sensitive and brittle at low temperatures. The cutting temperature must be controlled during processing to prevent part warping or deformation. It has moderate heat resistance.

Applications:

Food containers, pharmaceutical packaging, automotive parts, laboratory equipment, and industrial components are widely used in a variety of applications due to their low price and balanced performance.

PBT (Polybutylene Terephthalate)

PBT is tough, heat-resistant, chemical-resistant, and dimensionally stable, while also possessing electrical insulation properties. However, its disadvantage is that it can be brittle and crack easily at low temperatures or under prolonged UV light.

Applications:

Suitable for electrical connectors, household appliance parts, automotive components, and precision machinery parts, particularly for electrical insulation and high-heat-resistance applications.

PEI (Polyetherimide)

PEI offers high strength, high heat and chemical resistance, excellent dimensional stability, and the ability to maintain tight tolerances during machining, but at a higher cost. It is suitable for high-performance CNC machining.

Applications:

Aerospace parts, medical devices, electronic components, and high-heat-resistant industrial components.

PEEK (Polyetheretherketone)

PEEK offers high strength, corrosion resistance, high-temperature resistance, and excellent mechanical properties, but is expensive and difficult to machine, making it suitable for precision CNC machining.

Applications:

Aerospace parts, high-load automotive parts, medical implants, and high-performance industrial components.

PAI (Polyamide-imide)

PAI offers exceptional wear resistance, high-temperature resistance (up to 260°C), high dimensional stability, and superior mechanical properties. While difficult to process, it is suitable for parts with tight tolerances.

Applications:

Precision aerospace parts, high-temperature wear-resistant parts, high-performance automotive parts, and industrial machinery components.

PVC (Polyvinyl Chloride)

PVC is durable, chemically and corrosion-resistant, and flame-retardant, yet affordable. However, it produces harmful gases when burned, and some formulations may become brittle over time.

Applications:

Pipes, windows, flooring, cable sheathing, building components, and other CNC-finished products are widely used.

PTFE (Polytetrafluoroethylene/Teflon)

PTFE offers low friction, high-temperature and chemical resistance, and flexibility, but is difficult to process and relatively expensive. It is suitable for specialized industrial applications.

Applications:

Seals, gaskets, coated cookware, electrical insulation, low-friction mechanical parts, and chemical equipment.

UHMW (Ultra-High Molecular Weight Polyethylene)

UHMW offers high tensile strength, wear resistance, and corrosion resistance, but it suffers from poor dimensional stability and is difficult to process, typically requiring experienced operators.

Applications:

Slide rails, wear-resistant parts, industrial guide rails, and conveyor equipment parts, suitable for high-wear and impact-resistant applications.

CNC machining plastic material comparison chart

CNC Plastic Type	Key Characteristics	Cost	Machining Performance	Typical Applications
ABS	Lightweight, impact-resistant, easy to mold; not resistant to UV and high temperatures	Low	Easy to machine	Automotive parts, electronic housings, prototypes
PC (Polycarbonate)	Transparent, tough, impact-resistant; surface is easily scratched	Medium	Moderate	Protective covers, lighting fixtures, helmets
PMMA (Acrylic)	High transparency and gloss; relatively brittle	Medium	Easy to machine	Signage, display cases, aquariums
POM (Polyoxymethylene)	High rigidity, excellent wear resistance; fears UV and acidic environments	Medium	Easy to machine	Gears, bearings, fasteners
Nylon (PA)	High strength, wear-resistant, good toughness; prone to deformation after moisture absorption	Medium	Moderate to Difficult	Gears, conveyor belts, mechanical components
HDPE	Tough, chemical resistant, lightweight, stable machining; general heat resistance	Low	Easy to machine	Pipes, pulleys, containers
LDPE	Soft, elastic, good impact resistance; not heat resistant	Low	Easy to machine	Packaging bags, hoses
PP (Polypropylene)	Lightweight, chemical resistant, cost-effective; brittle at low temperatures	Low	Easy to machine	Containers, automotive parts, laboratory equipment
PBT	Dimensionally stable, good insulation; prone to cracking in low-temperature environments	Medium	Moderate	Electrical connectors, automotive parts
PEI	High strength, heat resistant, dimensionally stable; relatively high cost	High	Difficult to machine	Aerospace, medical, electronics
PEEK	Extremely high strength and temperature resistance, high-end engineering plastic	Very High	Difficult to machine	Aerospace, medical implants, industrial components
PAI	Wear-resistant, heat-resistant, excellent dimensional stability; difficult to machine	Very High	Difficult to machine	Aerospace precision parts, high-temperature components
PVC	Sturdy, durable, flame retardant, corrosion resistant; prone to aging and embrittlement	Low	Easy to machine	Pipes, window frames, cables
PTFE (Teflon)	Very low friction coefficient, excellent chemical resistance; high machining requirements	High	Difficult to machine	Seals, gaskets, insulating components
UHMW	Ultra-wear-resistant, strong impact resistance; dimensional accuracy is difficult to control	Medium	Moderate to Difficult	Slides, guides, conveying parts

3.How is plastic machined?

Plastic machining involves a variety of processes, depending on the material and the product being manufactured. Among the many processes used in popular rapid prototyping services, the following are three of the most popular plastic machining processes:

1) CNC Turning

CNC turning involves spinning a plastic polymer on a CNC lathe while a stationary CNC tool cuts away portions of it to form the desired shape. The process is well suited for processing a wide range of shapes, although the most common workpiece shape is cylindrical.

Choosing whether CNC turning is the right process requires you or the prototyping service to understand several factors, such as the desired production rate, the outer diameter of the plastic polymer, etc.

2) CNC Milling

CNC plastic milling is the opposite of CNC turning, as the tool rotates while the material is fixed. As with CNC turning, there is a high degree of precision and accuracy as well as uniform productivity due to the corresponding cutting speed of the tool. CNC milling is also suitable for machining flat surfaces and irregular shapes.

The process involves the use of a single-point cutting tool that works on multiple axes (3 axes is common). The number of axes supported determines the flexibility of the process and its suitability for manufacturing complex components.

3) CNC Drilling

CNC drilling involves drilling holes in a material using a cutting tool in the form of a drill. Depending on the type and shape of the drill, holes of varying cross-sections are formed in the material.

CNC machines used for drilling are versatile as they can also perform some milling and turning operations. They also come in different sizes and require experts to ensure cost-effectiveness when manufacturing plastic parts.

4.How to achieve a perfect finish on plastic parts?

The key to achieving a perfect finish in CNC plastic machining lies in the technology used and the understanding of the material properties. To avoid this situation with plastic products (i.e., to achieve a perfect surface finish), follow these guidelines:

1. Avoid using any cutting tools with complex geometries.
2. Do not exceed the recommended feed rate for the material being used.
3. The plastic material to be processed must be properly fixed. Improper material clamping can lead to vibrations, which in turn can result in imperfect plastic surfaces.
4. The cutting speed must also be within the recommended range and also varies for different plastics.

These factors depend on the understanding of the different properties of plastic materials. However, you can further improve the surface finish of any plastic product using finishing techniques such as annealing, sandblasting, powder coating, etc.

5.Why do people prefer CNC plastic machining over other methods?

Plastic polymers are often used with other rapid prototyping methods such as injection molding and 3D printing. However, people have recently started turning to CNC plastic machining due to its advantages.

Comparison of Plastic CNC Machining, 3D Printing, and Injection Molding：

Criteria	CNC Machining	3D Printing	Injection Moulding
One-off part	Expensive	Most cost-effective	Least cost-effective
Batch production	Cost reduces with production quantity	The cost per unit stays the same; least cost-effective for large-scale production	Cost reduces significantly with production quantity; most cost-effective for large-scale production
Part complexity and accuracy	Can produce highly complex parts (with some design limitations); offers the highest accuracy	Has little to no design limitations	Has relatively limited accuracy

All three methods are fast and common, and each has its own advantages, but many people now prefer CNC plastic machining capabilities for the following reasons:

1) Precision and Accuracy

CNC plastic machining has unparalleled precision and accuracy. Among the three rapid prototyping methods, the high precision and accuracy ensure high repeatability, as the quality of different plastic products made with this process is similar.

2) Suitable for complex designs

Plastic polymers can be used to make complex and simple products. CNC plastic machining is suitable for making plastic products with complex designs. Although 3D printing also has strong complex design capabilities, the precision and accuracy provided by CNC machines make this process more advantageous.

3) Tight tolerances

CNC plastic machining is the best of the three methods for making products with tight tolerances. This is also due to its high accuracy and precision, making it an important process for most plastic manufacturers.

4) Compatible with different plastic polymers

All methods are suitable for a wide range of materials. However, CNC plastic machining can handle a wider range of plastics well, especially hard plastics, without too many problems.

6.Major Challenges of Plastic CNC Machining

During CNC machining, plastics exhibit significantly different properties from metals. Plastics are lighter, softer, and have poor thermal conductivity, making them more susceptible to deformation, surface defects, and dimensional instability during machining. The following are some common challenges:

• Poor Surface Quality

Plastics have low strength and rigidity, making them susceptible to friction, pulling, or melting caused by the tool during machining. Improper control of cutting speed, tool angle, or chip evacuation can easily lead to scratches, drag marks, and inconsistent surface finish. This is particularly problematic with transparent materials such as PMMA or PC.

• Poor Dimensional Stability

Due to their high coefficient of thermal expansion, plastics expand when heated during machining and contract upon cooling, affecting dimensional accuracy. Furthermore, some plastics are hygroscopic, expanding upon absorbing moisture from the air, further impacting dimensional stability.

• Clamping Deformation

Plastics are highly elastic and easily deform during clamping. When the clamping force is released after machining, some parts of the material may not return to their original shape, resulting in dimensional or shape errors.

• Difficulty in Chip Evacuation

Cutting certain plastics produces light and highly sticky chips. If these chips are not removed promptly, they can easily clog the tool’s chip flutes, affecting cutting efficiency and increasing tool temperature.

• Thermal Deformation and Softening

Many plastics have low heat deformation temperatures. If heat is not adequately dissipated during processing, excessive temperatures can cause the plastic to soften, deform, or melt, damaging the part surface or affecting its dimensions.

• Chemical Reaction and Hygroscopicity

Different plastics have varying resistance to chemicals. Some polymers swell or become brittle after contact with certain solvents or coolants. Some materials, such as nylon (PA), are highly hygroscopic, and changes in humidity during processing and storage can affect their dimensional and mechanical properties.

• Optical and Appearance Issues

For transparent or translucent materials such as PC, PMMA, and ABS, the surface roughness after processing directly affects the optical quality. Poor surface finish can result in haze, whitening, or uneven light transmission in the finished part.

7.Tips for Plastic CNC Machining

In response to the above problems, the following measures can be taken in actual CNC machining to improve the accuracy and surface quality of the finished product:

(1) Control the clamping force

Plastics have low strength, so a smaller clamping force should be used during machining. The clamping system should apply pressure evenly to avoid deformation caused by local force. For parts with complex shapes or thin walls, special fixtures or flexible gaskets can be used to disperse stress.

(2) Select the right tool

The sharpness, angle and chip removal design of the tool have a significant impact on the machining quality. Tools with sharp edges, appropriate cutting angles and strong chip removal capabilities should be used to reduce friction and heat. If necessary, special plastic tools should be used to achieve better surface quality.

(3) Control cutting heat

Temperature control is the core of plastic CNC machining. High-speed cutting and low feed rate machining strategies can be used to reduce cutting time and friction heat. At the same time, a cooling system (such as compressed air or coolant) should be used to reduce the workpiece temperature and prevent thermal deformation.

(4) Maintain good chip removal conditions

Ensure that chips can be removed in a timely manner to avoid accumulation near the tool. Processing stability can be improved by optimizing the tool chip groove design or using high-pressure gas to assist chip removal.

(5) Understand material properties and optimize parameters

Different plastics vary greatly in hardness, thermal stability, and hygroscopicity. Before processing, you should fully understand the material properties and select the appropriate cutting speed, feed rate, and tool material. If necessary, conduct a small sample cut to verify the adaptability of the parameters.

8.Post-processing options for CNC plastic parts

(1) Grinding and polishing

Plastic parts may have burrs, knife marks or rough surfaces after processing. Grinding can remove these irregularities and make the surface smooth. A common method is to use sandpaper of different coarseness and fineness in sequence, gradually grinding from coarse to fine to achieve the desired finish.

Plastic parts that require high gloss or transparency can be further polished. Polishing methods include mechanical polishing, using polishing paste or polishing pads, and flame polishing or steam polishing for transparent parts, which can improve the visual effect and optical performance of the parts.

(2) Sandblasting

Sandblasting is a method of treating the surface of a part by spraying tiny particles (such as glass beads, plastic beads or ceramic particles) at high pressure. It can evenly remove surface burrs, create a matte or textured effect, and enhance the tactile and visual beauty of the part. Sandblasting is particularly suitable for parts that require a consistent surface texture or special decorative effects, but for softer plastics, the pressure needs to be controlled to avoid deformation.

(3) Steam polishing

Steam polishing is a post-processing technology specifically for transparent plastic parts. By exposing the surface of the part to solvent vapor (such as acetone vapor), the surface will melt slightly, thereby eliminating minor scratches and processing marks, and restoring the part to a smooth and transparent effect. This process is often used in medical devices, optical components or other applications that require high transparency.

(4) Coloring and coating

CNC machined plastic parts can be painted or dyed to improve their appearance or achieve a specific color. The surface usually needs to be polished or primed first to enhance the adhesion of the coating. In addition, functional coatings such as chemical resistant coatings, UV resistant coatings or wear-resistant coatings can be applied to enhance the performance of the part in specific environments. Some special coatings can also give the part a matte, textured or anti-slip effect to meet different industrial needs.

Customized post-processing

Based on the use and material characteristics of the part, Xavier can also provide customized post-processing services, such as local hardening, high temperature resistant coatings or special surface textures. If your project requires special processing, please communicate with us in advance to ensure that the part fully meets your requirements.

9.Application of CNC plastic parts

CNC plastic parts have a wide range of practical applications in modern manufacturing. Their advantage is that they can combine the material properties of plastics with the precision processing capabilities of CNC to meet the specific needs of different fields. The following are some common applications of CNC plastic parts:

(1) Moving and load-bearing parts

CNC plastics are very suitable for manufacturing moving and load-bearing parts because of their wear resistance, low friction, light weight and dimensional stability. For example:

• Gears, bushings, bearings: Nylon (PA), POM and HDPE are often used in transmission systems to ensure precision and wear resistance.
• Conveyor system guides: Highly wear-resistant plastics can reduce friction and increase mechanical life.

(2) Seals

The elasticity and chemical stability of plastics make them very suitable for sealing applications:

• O-rings, valve seats and gaskets: Nylon, PTFE, PEEK can be used in hydraulic, pneumatic systems or chemical pipelines to prevent liquid and gas leakage.

(3) Medical devices

CNC machining can achieve high-precision customization, and the biocompatibility and high strength-to-weight ratio of plastic materials make it an excellent choice for the medical field:

• Prosthetics, orthotics, implants: PEEK, nylon, and ABS can be processed into ergonomic parts.
• Medical tools and equipment components: They can be mass-produced to ensure dimensional consistency and durability.

(4) Electrical components

The insulating properties of plastics make CNC plastics irreplaceable in electrical systems:

• PCB housings, cable connectors, couplings: High resistivity materials (such as PC, PBT) prevent short circuits and protect electronic components.

(5) Fluid control components

CNC plastic parts are corrosion-resistant and pressure-resistant and can be used in various fluid delivery and control systems:

• Valve bodies and pump impellers: Made of PVC, PVDF, PTFE or nylon, POM for chemical piping and pump systems.
• Manifolds and nozzles: Made of acrylic, polycarbonate, or PEEK, suitable for chemical and high-temperature liquid environments.

(6) High temperature and heat insulation components

Heat-resistant plastics (such as PEEK, PEI) can be used in high temperature mechanical or electrical systems:

• Heat exchanger components, boilers and HVAC components: ensure dimensional stability and reliable performance at high temperatures.

(7) Chemical and marine environments

Chemically resistant CNC plastic parts are suitable for harsh environments:

• Impellers, gears, pump bodies and chemical machinery parts: PTFE, PVDF, POM and other materials maintain their performance in strong acid, alkali or salt water environments.

(8) Transparent and opaque components

Some plastics have good optical properties and can be used in visible or light-transmitting scenes:

• Optical lenses and display covers: Acrylic (PMMA) and polycarbonate (PC) are widely used in cameras, electronic equipment and industrial observation windows.
• Light diffusers and transparent protective covers: used in lighting systems or mechanical equipment, taking into account durability and visibility.

(9) Protective parts and housings

The impact resistance and weather resistance of CNC plastics make them suitable for protective parts:

• Personal protective equipment: helmets, protective gear, etc., commonly made of ABS or PC.
• Industrial protective covers and bumpers: Energy-absorbing design to protect machinery or vehicle parts from damage.

Choose Xavier for your plastic part CNC machining project.

If you’re looking for a reliable plastic CNC machining partner, Xavier is the right choice! We have extensive experience and comprehensive machining capabilities to meet a wide range of project needs. We offer over 50 industrial-grade plastics and support custom materials to ensure parts meet your exacting design requirements. Whether you’re working on a single prototype or high-volume production, we utilize advanced CNC equipment and proven machining processes to rigorously control every step of the process, ensuring part accuracy, surface quality, and dimensional consistency.

Our process is simple: simply upload your design file and select your specifications to receive a quick, customized quote. Our dedicated team is always available to provide technical support and help optimize your design and machining solutions, ensuring your plastic parts are molded with precision and reliability. Trust us with your project and you can be assured of high-quality, repeatable CNC plastic parts.

10.Conclusion

Plastic polymers are used to manufacture different industrial and household products. For some products, a high level of precision, accuracy, tight tolerances, etc. is required. Therefore, plastic CNC machining is the first choice for many people who want to work with durable and high-quality plastic polymers.

FAQs: