CNC Plastic Machining: A Comprehensive Guide on Materials, Benefits, Applications, and Challenges

CNC machining is a highly versatile manufacturing technology that can accurately create complex, functional parts from various materials, including plastics. Plastics offer numerous beneficial properties that make them indispensable in various applications. This article explores plastic CNC machining, the properties, benefits, applications, and challenges of CNC plastics, and the capabilities and challenges of plastic machining.

Can Plastics be CNC machined?

Why Does Plastic CNC Machining Matter?

Benefits and Applications of CNC Plastic Machining

Common CNC Machinable Plastics

CNC Machining Plastics Post-Processing Options

Plastic CNC Machining: Challenges and Factors to Consider

Conclusion

Why Choose Geomiq

Can Plastics be CNC machined?

While CNC machining is usually associated with metals and wood, plastic CNC machining is common in manufacturing. Plastics, in this context, refer to thermoplastics, thermosets, and elastomers. CNC machining is compatible with any material that is rigid enough to withstand cutting forces, including plastics.

Various types of plastics exist with varying properties. However, they are not all machinable plastics. The material's rigidity is the primary consideration for plastic machining suitability.  Rigid plastics with a Shore D hardness of 50 and above are machinable. These materials include:

• ABS (Acrylonitrile Butadiene Styrene)
• PC (Polycarbonate)
• PEEK (Polyether Ether Ketone)
• POM (Polyoxymethylene)
• Acrylic
• Nylon

Flexible and elastic plastics are more challenging to machine given their elasticity, tendency to deform under cutting forces, and resistance to tear and abrasion. Plastic CNC machining requires a minimum Shore A hardness of 70 for such materials. These include TPU (Thermoplastic polyurethane) and EPU (Elastomeric polyurethane). We will explore popular CNC machinable plastics later on in the article.

Why Does Plastic CNC Machining Matter?

Hundreds of materials are compatible with CNC machining, so why bother with CNC plastic? Plastics have many unique properties that are not obtainable in other engineering materials. These properties, crucial in numerous industrial and domestic applications, are mixed and matched across various plastic types. Hard yet elastic plastics, as well as plastics that are “soft” but resistant to the harshest of chemicals, exist. There are also plastics, such as Kevlar, that you can fold in a roll but are tough enough to stop bullets. The availability of plastic types that meet different very specific requirements makes this material indispensable in manufacturing.

Furthermore, plastics are synthesisable, making them relatively low-cost and readily available in an extensive range of compositions. In addition to the numerous generic plastics such as ABS, PC, PEEK, and PE (Polyethylene), material manufacturers synthesise countless commercially available proprietary plastics. Technically speaking, thousands of plastic materials exist.

CNC machined Nylon 6 

Manufacturers create plastic products in various ways. Formative technologies such as moulding, extrusion, and thermoforming exist in various forms. Several Additive 3D printing services also exist. So why CNC machine plastics? Like other manufacturing processes, CNC machining offers a unique blend of advantages for creating plastic parts. CNC machining creates versatile, highly accurate, complex parts with intricate features and geometries. This manufacturing process can create custom one-off parts as well as batch productions.

Injection moulded ABS part

Comparing injection moulding, 3D printing, and CNC machining plastics, moulding offers the lowest cost per unit for batch productions but has limited accuracy and design capabilities. Considering the tooling cost, it is also not cost-effective for one-off parts.

SLS 3D printed nylon 12 part

On the other hand, 3D printing has very few, if any, design limitations. It is excellent for one-off parts but is the most expensive for batch manufacturing, as economies of scale barely apply. Lastly, CNC plastic machining is exceptionally accurate, can produce highly complex geometries, and is very suitable for batch manufacturing. The table below compares plastic machining with other manufacturing methods.

From the table above, each manufacturing process is most appropriate in different scenarios. Not sure whether to choose between CNC machining and 3D printing? See our CNC vs 3D printing article for a thorough comparison. In summary, there are three major reasons why CNC plastic machining is ubiquitous.

• Plastics material properties
• CNC machining capabilities
• Plastic availability and cost

By combining CNC machining capabilities with plastics' material properties, manufacturers can create a wide variety of highly capable, functional products. 

Benefits and Applications of CNC Plastic Machining

CNC plastics are used in a wide range of domestic and industrial products across numerous industries. This widespread usage is due to the various beneficial properties of plastics that make them excellent materials for different applications. Before diving into the applications of CNC-machined plastic components, it is essential to understand the properties of plastics that make them indispensable in domestic and industrial applications.

Properties of CNC Plastics

Plastics have unique properties that are not obtainable in other materials. One unique advantage is that different plastics have a unique mix of these properties at different levels, making it possible to find a CNC plastic to meet unique requirements. However, this also means proper material selection is necessary to select the suitable CNC plastic for your application. The following are some beneficial properties of plastics.

• Lightweight: Plastics are generally lightweight, with the heaviest plastic, PTFE (Polytetrafluoroethylene), having a density of only 2.2 g/cm³. This property makes them ideal for applications where low weight is essential for part functionality and efficiency.
• Elasticity: Most plastics can return to their original shape after deformation. This property varies by plastic type and is very high in elastomers. It allows plastic parts to bend, undergo stress, and deform without breaking.
• Corrosion and chemical resistance: Plastics are resistant to various chemicals, making them ideal for parts in harsh chemical environments. However, this resistance varies between different plastic types, so using the right plastic type for the specific chemical is essential. Furthermore, plastics do not rust or corrode, as these are chemical processes exclusive to metals.
• Wear and tear resistance: Many plastics are wear and tear-resistant, making them suitable for applications where durability is crucial. Certain plastics, like Nylon (PA), Delrin (POM), and Polyethylene, are known for their excellent wear resistance, low friction, and ability to withstand repeated mechanical stress.
• Impact resistance: Plastics’ elasticity makes them resistant to impact. This property varies between plastics, but elastomers typically have the highest impact resistance.
• Electrical Insulation: Plastics are generally excellent electrical insulators due to their high resistivities. This property facilitates safe interaction between electrically conductive components.
• Optical properties: Plastics have both transparent and opaque options at varying levels. Polymers’ interaction with light depends on various factors, including crystallinity, additives, and the synthesisation process. Transparent plastics include PS, ABS, acrylic, PC, and PEI. Plastics are also very receptive to pigments and dyes and can be synthesised in countless colours.
• Thermal insulation: Due to their atomic structures and lack of free-moving electrons, plastics are poor conductors of heat. Some plastics are also heat resistant, with polybenzimidazole capable of withstanding temperatures above 500⁰C. Other heat-resistant plastics include PEI (ULTEM®), PEEK, and PTFE (Teflon), with melting points of 219°C, 371°C, and 327°C, respectively.
• Biocompatibility: Certain plastics, such as PEEK and medical-grade silicones, are biocompatible, meaning they can be safely used in contact with biological tissues, making them suitable for medical devices and implants.
• Durability: Plastics can last very long periods as they hardly scratch, are impact-resistant, resistant to wear and tear, cannot corrode, and are resistant to many chemicals. Note that the durability of different plastics varies under different conditions.
• Recyclability: Many plastics can be recycled, helping reduce waste and environmental impact. Thermoplastics, in particular, can be remelted and reshaped multiple times, contributing to sustainability efforts.

Applications of CNC Plastic

Plastics are some of the most ubiquitous materials in the world today. Chances are, a glance around your surroundings would reveal at least a few plastic products made from different manufacturing processes. CNC plastic parts have become indispensable in numerous applications. This ubiquity results from a combination of CNC machining capabilities and plastics' inherent material properties, creating a match that meets specific needs.

For example, a project requires a one-off lightweight and acid-resistant component. The component features complex geometries, and extremely high accuracy is crucial to its functionality. Based on the project requirements, plastic (such as PEEK) and CNC machining are the most suitable materials and manufacturing processes. Some of the applications of CNC plastic parts are explored below.

Moving and load-bearing components

Parts such as gears, bushings, bearings, and conveyor system guides, which are subject to frequent motion and loading, benefit from CNC plastics' wear and tear resistance. Nylon, POM, and HDPE are common materials for such parts.

CNC plastic machined gear (POM)

Sealing

Elastic CNC plastic parts create leak-proof seals in joints and assemblies. These materials deform under compressive and shear stresses, expanding to fill gaps between joints. Typical CNC plastic sealing applications include:

• Elastomeric O-rings for hydraulic, pneumatic, and plumbing systems to prevent fluid or gas leaks.
• Teflon gaskets are used in flanges and mechanical joints to seal connections in engines, pipelines, and valves.
• Nylon valve seats that create tight seals within valves to control fluid flow in chemical processing, plumbing, and industrial systems.
• PEEK sealing washers that create a reliable barrier between fasteners and the surfaces they secure.

CNC machined PEEK sealing ring

Medical devices

CNC plastics are used to manufacture medical prosthetics, orthotics, and implants. The widespread application of medical CNC machining plastics is due to the customisation capability of CNC machining and the biocompatibility, durability, and high strength-to-weight ratio of plastics. Manufacturers also use CNC plastics in batch production of medical tools, instruments, and equipment. The image below is a prosthetic hand by Taska prosthetics featuring CNC plastic machined components.

Electrical components

Due to their high resistivities, CNC plastic electrical components ensure the safe operation of electrical systems by preventing contact between conductive elements within the system. They also facilitate safe handling of these systems by providing a non-conductive barrier. CNC plastic electrical components include cable glands, couplings, and PCB enclosures.

CNC plastic electronic housing

Fluid control components

Due to their high strength-to-weight ratio, leakage prevention characteristics, and resistance to an extensive range of substances, CNC plastic components are ubiquitous in fluid control systems. Specific examples include:

• Valve Bodies, machined from PVC, PVDF, or PTFE, control fluid flow in piping systems, offering chemical resistance and durability in harsh environments.
• Pump Impellers made from Nylon, PPS, or Acetal (POM) are used in pumps to move fluids efficiently, benefiting from these plastics' wear and corrosion resistance.
• Industrial systems commonly use manifolds machined from acrylic, polycarbonate, or PEEK to control the flow of liquids or gases through multiple channels. 
• PTFE and PEEK seals ensure tight control over fluid flow by preventing leaks in valves, especially in chemical processing and high-temperature applications.
• Precision nozzles machined from PTFE or PEEK control the direction and flow of fluids in spray systems or fluid dispensing equipment.

CNC-machined Ultem plastic manifold

Thermal insulation

Thermal insulating and heat-resistant CNC machined plastics are used in various applications and components, with the unifying condition being high-temperature environments. These components include parts in fluid control, mechanical, and electrical systems with elevated temperatures, such as heat exchangers, HVAC systems, boilers, and combustion engines.

CNC plastic PEEK component for high-temperature systems 

Chemical and marine environments

Like their heat-resistant counterparts, chemical-resistant CNC plastic parts are also used in numerous components across various applications with chemical exposure. Examples include impellers, gears, fluid control parts, and mechanical engine components. These parts are standard CNC machined components but manufactured from chemical-resistant plastics. Plastics vary significantly in resistance to different chemical types and under different conditions. Consult chemical resistance charts to select the correct plastic for the specific chemical.

CNC plastic polysulfone manifold for a mineral acid processing system

Transparent and opaque components

CNC plastic parts interact with light differently and have both transparent and opaque applications. Some of these applications are as follows:

• Optical Lenses machined from acrylic or polycarbonate, are used in cameras, optical instruments, and protective eyewear due to their excellent clarity and light transmission
• Transparent display covers made from polycarbonate or acrylic protect screens for electronic devices such as smartphones, tablets, and control panels while offering durability and scratch resistance.
• Clear polycarbonate guards are used in industrial machinery to provide visibility while protecting operators from moving parts or debris.
• Light diffusers, machined from acrylic, are used in lighting systems. They allow even light distribution while maintaining high transparency.

Protective parts and casings

CNC Plastics' high impact resistance and weatherability make them excellent for manufacturing protective items. These items include personal protective equipment such as custom-made sports guards, helmets, sports guards, and protective shields, commonly made from PC and ABS. CNC plastics are also machined to create barriers, such as high-impact plastic bumpers that absorb collision energy in vehicles and rugged protective housings for relatively fragile components.

Common CNC Machinable Plastics

The table below contains the most common CNC machining plastics, their properties and their typical applications.

CNC Plastic Machining Post-Processing Options

Post-processing is often necessary after CNC machining plastic parts to achieve the desired surface finish, precision, or functional characteristics. Here are some standard post-processing options for CNC-machined plastic components:

Sanding and Polishing

After CNC machining, plastic parts may have rough edges, visible tool marks, or burrs. Sanding deburrs the part, smooths the surface, and removes imperfections. Different grits of sandpaper, from coarse to fine, are progressively used to achieve the desired finish. Manufacturers also use sanding to prepare a part for coating.

For plastics that can achieve a high-gloss finish, polishing can be used after sanding to create this finish. Methods include mechanical polishing, using polishing compounds and pads, or flame polishing. Polishing also enhances the properties of transparent parts.

Bead Blasting

This process uses fine beads (usually glass, plastic, or ceramic) propelled at high pressure to create a uniform, matte surface finish on CNC plastic parts. It smooths surfaces, deburrs parts, improves grip, and enhances aesthetics. Bead blasting is particularly useful for parts requiring a consistent texture or improved visual appeal. Care must be taken with softer plastics, as they may deform under high pressure.

Vapour Polishing 

Vapour polishing is a polishing technique that improves surface roughness and enhances clarity. The part is exposed to a solvent vapour, such as acetone, that slightly melts the part’s outermost surface, smoothing the surface and removing any machining marks. This process is often applied to transparent plastics to achieve an optically clear finish in industries where optical clarity is essential, such as medical devices and lenses.

CNC plastic vapour polishing

Annealing

Annealing is a heat treatment process that relieves internal stresses in the CNC plastic part that may have built up during CNC machining. Stress relief is particularly important for load-bearing or stressed parts, as it reduces the likelihood of cracking, warping, or dimensional instability during use. Operators slowly heat the part to a specific temperature, hold it there for a period, and then allow it to cool gradually.

Colouring and Coating

Plastics are usually dyed in specific colours during production to create coloured-through workpieces. However, machined CNC plastic parts, especially white ones, can be painted or dyed to improve appearance, change colour, or provide additional protection. Surface preparation (such as priming or sanding) is often required to improve bonding.

In addition, manufacturers can apply functional coatings to enhance chemical, UV, or wear resistance or to achieve specific surface finishes. For example, PTFE coatings can improve a part’s performance in demanding environments. Similarly, special coatings can add a matte or textured feel, often used to improve grip or reduce reflections.

Marking and Texturing

Marking and texturing encompass various techniques to add visual features to CNC plastic parts. After CNC machining, machinists can add logos, labels, serial numbers, barcodes, or graphics to plastic parts using silk-screening, pad printing, and Laser etching.

Laser-etched CNC plastic

Plating and Metallisation

CNC plastic parts can undergo plating to improve functionality or aesthetics using plating materials and techniques such as electroless nickel plating, chrome plating, and gold plating.

Nickel-plated CNC plastic

Post-Machining Manufacturing

Post-machining manufacturing involves processes that are not part of the machining process but are required to create the finished product. These processes usually involve assembly or the addition of features. Assembly techniques include adhesive bonding, fitting, welding, screwing, and many others, while additional features include thread tapping, inserts, and heat forming.

Plastic CNC Machining: Challenges and Factors to Consider

Plastic machining generally follows the same principles as machining metals. However, there are important differences due to the material properties of polymers. Here are some aspects of machining plastics to note as a machinist, manufacturer, or customer ordering CNC plastic parts.

Tooling

Polymers, especially fibre-reinforced or glass-filled types, can be abrasive to cutting tools, leading to faster tool wear. In some cases, polymers can also cause material build-up on the tool (gumming), leading to poor finishes and reduced precision. Using appropriate hard, scratch-resistant tools and regular tool maintenance (sharpening and cleaning) can help mitigate these issues. 

Material Expansion

Plastics can expand or contract more significantly with temperature changes, so controlling heat during plastic machining is crucial to maintain tolerances. Their low thermal conductivity makes this even more important. Cooling fluids or compressed air is necessary when machining plastics to regulate temperature.

Chip Formation

Plastics tend to form long, stringy chips rather than the short, segmented chips common with metals. These chips can clog the machine and reduce efficiency. Effective chip management, such as air blowers or vacuum systems, is essential to maintain clean cutting paths and avoid overheating or tool damage.

Vibration and Flexibility

Plastics are typically more flexible than metals, meaning they vibrate more during plastic machining. Excessive vibration can cause inaccuracies or poor surface finishes. Workholding and fixturing should be optimised to minimise movement, and slower cutting speeds might be necessary for certain parts.

Surface Finish

Plastics, especially hard rigid ones, can be more susceptible to burrs and poor surface finishes. The right cutting tools, speeds, and finishes can help achieve smoother results, but post-processing finishing operations may be required.

Coolants and Lubricants

Coolants and lubricants are sometimes necessary to prevent material overheating, reduce friction, and enhance tool life. However, not all coolants suit every polymer, as some plastics can absorb moisture or react with certain chemicals, leading to warping or degradation. Always select coolants that suit the characteristics of the particular CNC plastic.

Machining Parameters

Plastics are softer and more prone to melting compared to metals. Thus, cutting parameters—such as speeds, feeds, and depth—need to be carefully adjusted to achieve optimal plastic machining. Plastics typically require fast speeds and light continuous speeds.

Design Considerations

Due to plastics' unique properties, designers need to make special considerations when designing a plastic part. In addition to the general CNC design rules, here are some plastic-specific design considerations.

• Maintain a minimum wall thickness: Thin walls are susceptible to damage, such as bending and warping, due to vibrations during machining. Given their flexibility, this problem is more pronounced in CNC plastic parts. Design walls with a minimum thickness of 1.5 mm to mitigate these issues.
• Avoid excessively tight tolerances: Plastics are susceptible to shrinking, warping, vibrations, and other occurrences that impact dimensional stability. While CNC machining can achieve tight tolerances, and machinists can mitigate most of these issues, we recommend specifying standard tolerances unless tight tolerances are crucial.
• Reinforce thin features: Incorporate reinforcements such as ribs and tabs to provide support and prevent flexing in thin, fragile features.
• Account for specific material properties: It is important to consider the properties of the specific CNC plastic in your design. For example, softer plastics, like polyethene or PVC, may deform under high cutting forces. Therefore, avoid features that require heavy cuts or sharp transitions when using this material. Similarly, brittle plastics like acrylic or polystyrene are prone to cracking. Avoid sharp edges, narrow cuts, and thin walls for these materials. Introduce fillets wherever possible to reduce stress concentrations.
• Avoid overly fine details: CNC machining plastics can produce relatively rough surfaces. While post-processing can improve surface roughness, these processes can impact fine details such as lettering.

Conclusion

CNC plastic machining is a vital manufacturing process that leverages the unique properties of various plastics and the advanced capabilities of CNC machining to produce highly accurate and complex components across numerous industries. With the ability to create custom parts and balance cost and performance, CNC plastic machining offers a compelling alternative to traditional manufacturing methods.

Why Choose Geomiq

Need Plastic CNC machining services? Geomiq is your ultimate partner for all your CNC machining needs. We offer over 50 industrial-grade plastics. You can also request custom materials. Whether for one-off parts or bulk production, we combine state-of-the-art CNC equipment, advanced machining techniques, and stringent quality assurance to ensure you receive accurate, high-quality, error-free parts in as little as three days - the first time, every time.

Simply Upload your design to our instant quoting platform and select your desired specifications to get started. You’ll receive a tailored instant quote, complete with pricing and lead times. You can also contact us to discuss your project with our team of manufacturing professionals.