The Best Plastics for CNC Machining: Strength, Stability, and Cost

When it comes to CNC machining, most people think of metals first, particularly Machining Stainless Steel. But here's something that might surprise you: plastics are actually one of the most versatile and cost-effective materials we work with at our facility. Whether you're prototyping a new product, creating components for consumer goods, or manufacturing specialized parts for industrial applications, choosing the right plastic can make all the difference.

In this article, I'm going to walk you through the best plastics for CNC machining, what makes them tick, and how to choose the right one for your project.

The Quick Answer: Top Plastics for CNC Machining

If you're in a hurry, here's what you need to know:

• Acrylic (PMMA) – Best for clarity and optical applications; excellent machinability
• Polyether Ether Ketone (PEEK) – Superior strength and heat resistance; premium cost
• Polyoxymethylene (POM/Delrin) – Best all-rounder for dimensional stability and machinability
• Polycarbonate (PC) – Impact resistant; good for protective components
• Polyvinyl Chloride (PVC) – Budget-friendly; good for non-critical applications
• Nylon (PA) – Excellent for wear-resistant components; good strength-to-weight ratio
• Teflon (PTFE) – Superior lubricity and chemical resistance; limited machinability
• Polystyrene (PS) – Lightweight and cost-effective; limited strength

Now let's dive deeper into what makes each of these materials special and when you should use them.

Understanding Plastic Machinability: Why It Matters

Before we discuss specific options, let's talk about what distinct qualities categorize certain polymers as high-quality machinable plastics.

Machinability refers to how easily a material can be cut, shaped, and finished. Several factors influence this:

• Hardness and rigidity – Materials that are too soft will melt and clog; too hard and they'll chip
• Thermal properties – Plastics generate heat during machining, and some handle this better than others
• Chip formation – The way material breaks away during cutting affects surface finish and tool life
• Homogeneity – Uniform material properties mean consistent results across the part

Materials that rank high in machinability include Acrylic, POM, and Polystyrene. Materials that require more expertise and specialized tooling include PEEK, PTFE, and some nylons.

The Best Plastics for CNC Machining: Detailed Breakdown

1. Acrylic (PMMA) – The Transparent Champion

Best For: Optical components, signage, displays, protective shields, light covers

Acrylic is hands-down one of the easiest plastics to machine. It's rigid, has excellent optical clarity, and machines beautifully with sharp cutting edges.

Advantages:

• Outstanding machinability – produces clean, chip-free cuts
• Crystal-clear transparency with excellent light transmission (92% of light passes through)
• Available in numerous colors and finishes
• Good chemical resistance to most acids and bases
• Excellent surface finish with minimal post-processing
• Cost-effective for high-volume production

Limitations:

• Relatively brittle – prone to cracking if not handled carefully
• Lower impact strength compared to polycarbonate
• Moderate temperature resistance (up to about 70°C continuously)
• Sensitive to stress cracking from certain solvents

Typical Applications: LED light diffusers, protective covers for displays, custom brackets, aquarium components, decorative panels

Cost: Low to moderate – usually $5-15 per pound, depending on grade and color

2. Polyoxymethylene (POM/Delrin) – The All-Rounder

Best For: Precision mechanical components, bearings, gears, sliding parts

If there's a "Goldilocks" plastic for CNC machining, it's POM (also sold under the brand name Delrin by DuPont). This material is engineered for precision machining and performs exceptionally across the board.

Advantages:

• Exceptional dimensional stability – tolerances stay put after machining
• Excellent machinability with sharp, clean cuts
• Outstanding wear resistance – perfect for components that slide or bear loads
• Low friction coefficient – natural lubricity
• High stiffness and rigidity
• Good chemical resistance to most organic solvents
• Tight dimensional tolerances achievable consistently

Limitations:

• More expensive than acrylic or standard nylons
• Moisture absorption can cause slight dimensional changes if not properly conditioned
• Temperature limit around 80-90°C continuous operation
• Not transparent or translucent (opaque only)

Typical Applications: Precision gears, pump components, bearing sleeves, timing mechanisms, medical device components, replaceable wear parts

Cost: Moderate – typically $15-25 per pound for virgin material

3. PEEK (Polyether Ether Ketone) – The Premium Performer

Best For: Aerospace, medical devices, high-temperature applications, chemical processing

PEEK is the premium choice in the plastic world. It's what you spec when cost isn't the primary concern and performance is absolutely critical.

Advantages:

• Exceptional temperature resistance – continuous use up to 250°C (some grades to 300°C+)
• Outstanding chemical resistance – resistant to nearly all industrial chemicals
• Excellent mechanical strength across a wide temperature range
• Self-extinguishing and low smoke emission (important for aerospace and medical)
• Excellent machinability despite its high performance profile
• Biocompatible grades available for medical implants
• Outstanding fatigue resistance

Limitations:

• Significant cost – typically $40-80+ per pound
• Slower machining speeds than standard plastics
• Requires proper tooling and expertise
• Overkill for many standard applications

Typical Applications: Aerospace bushings, medical implants, high-temperature seals, chemical processing components, subsea connectors, surgical instruments

Cost: High – $40-80+ per pound depending on grade and supplier

4. Polycarbonate (PC) – The Impact Resistant

Best For: Impact-resistant applications, protective equipment, transparent load-bearing components

Polycarbonate is significantly tougher than acrylic, though slightly less transparent. It's the material of choice when you need something that won't shatter under stress.

Advantages:

• Exceptional impact resistance – 200 times stronger than glass
• Good transparency (though slightly less clear than acrylic)
• Higher temperature resistance than acrylic (up to about 120°C)
• Excellent for transparent load-bearing applications
• Good UV resistance available (UV-stabilized grades)
• Available in various tints

Limitations:

• More difficult to machine than acrylic – can melt if cutting speeds aren't optimized
• Slightly lower optical clarity than acrylic
• Sensitive to scratching during machining
• More expensive than acrylic
• Can stress-crack with certain solvents

Typical Applications: Safety windows, protective shields, lens components, transparent enclosures, transparent structural components

Cost: Moderate to moderate-high – typically $10-20 per pound

5. Nylon (PA) – The Workhorse

Best For: Functional parts, wear surfaces, snap fits, bushings, gears

Nylon comes in several varieties (PA6, PA66, PA11, PA12), and each has slightly different properties. It's incredibly common in production parts because it balances performance with reasonable cost.

Advantages:

• Excellent wear resistance and low friction
• Good impact strength and toughness
• Self-lubricating properties reduce friction
• Strong tensile strength
• Can be reinforced with glass or carbon fiber for enhanced properties
• Good chemical resistance
• Reasonably easy to machine (with right approach)
• Cost-effective for production

Limitations:

• Absorbs moisture, which can affect dimensions and properties
• Must be conditioned before final machining for precision work
• Some grades can be stringy when machining
• Lower rigidity than POM or acrylic
• Not suitable for high-temperature applications (typically max 80°C continuous)

Typical Applications: Gears, bushings, bearings, snap fits, connectors, flexible components, wear surfaces

Cost: Low to moderate – $6-15 per pound for standard grades

6. Polyvinyl Chloride (PVC) – The Budget Option

Best For: Non-critical functional parts, prototype testing, consumer goods, utility components

PVC is everywhere in manufacturing because it's inexpensive and surprisingly versatile. It comes in rigid and flexible varieties; we typically work with rigid PVC for CNC machining.

Advantages:

• Very cost-effective
• Good chemical resistance to acids, bases, and many solvents
• Adequate rigidity for non-critical applications
• Decent machinability with proper techniques
• Good availability in various grades and colors
• Flame-resistant without additives

Limitations:

• Lower mechanical strength than many alternatives
• Thermal properties are mediocre
• Can release chlorine gas if overheated during machining
• Not suitable for food contact in many jurisdictions
• Requires careful tool selection to avoid melting
• Environmental concerns in manufacturing

Typical Applications: Prototype parts, test fixtures, valve bodies, decorative items, prototype consumer goods

Cost: Very low – typically $3-8 per pound

7. PTFE (Teflon) – The Slippery One

Best For: Low-friction components, chemical processing, non-stick surfaces, seals

PTFE is famous for its non-stick properties and exceptional chemical resistance. However, it's also notoriously challenging to machine.

Advantages:

• Unsurpassed chemical resistance – resistant to virtually all chemicals
• Exceptional low-friction coefficient
• Wide temperature range (-200°C to +260°C continuous)
• Self-lubricating – no other lubricant needed
• Non-toxic and FDA-compliant for food contact
• Excellent electrical insulation properties

Limitations:

• Very difficult to machine – requires specialized techniques and tooling
• Cold-flows under pressure over time
• Low creep resistance compared to other high-performance plastics
• Most expensive option for many applications
• Difficult to achieve tight tolerances
• Generates significant chips during machining
• Slower machining speeds required

Typical Applications: Seals and gaskets, non-stick liners, low-friction bearings, chemical reactor components, electrical insulation

Cost: High – typically $30-60+ per pound

8. Polystyrene (PS) – The Lightweight

Best For: Prototypes, lightweight components, transparent applications, cost-sensitive projects

Polystyrene is surprisingly easy to machine and very affordable. It's ideal for prototyping and non-structural applications.

Advantages:

• Extremely cost-effective
• Excellent machinability – machines like a dream
• Can be transparent or opaque
• Easy to achieve good surface finishes
• Low density – lightweight parts
• Good stiffness for its weight
• Easy to post-process (paint, glue, coat)

Limitations:

• Low impact resistance – brittle at room temperature
• Low temperature resistance – softens around 80°C
• Not suitable for structural applications
• Limited chemical resistance
• Hygroscopic – absorbs water
• Environmental concerns

Typical Applications: Prototype parts, mockups, lightweight covers, demonstration models, non-structural housings

Cost: Very low – typically $2-6 per pound

How to Choose the Right Plastic for Your Project

Here's my practical framework for selecting the right material:

Step 1: Define Your Requirements

• What are the functional requirements? (Load-bearing, temperature, chemical exposure, etc.)
• What aesthetic requirements? (Transparent, color, finish?)
• What are your dimensional tolerance needs?
• What's your timeline and budget?

Step 2: Consider Your Environment

• Operating temperature range
• Chemical exposure
• Humidity and moisture
• UV exposure
• Mechanical stresses (impact, wear, friction)

Step 3: Evaluate Production Constraints

• Machining complexity – does your design need a material that's easy to machine?
• Volume – high volumes might benefit from slightly slower-machining materials with better mechanical properties
• Lead time – some materials are more readily available than others
• Expertise – some materials require specialized knowledge and equipment

Step 4: Calculate Total Cost

• Don't just look at material cost per pound
• Consider machining time (harder materials = longer machine time)
• Factor in tool wear (some materials wear tools faster)
• Consider post-processing and finishing needs
• Account for waste and scrap

Real-World Example

Let me give you a concrete example. A client recently came to us needing 500 components for a consumer appliance. The parts needed to:

• Maintain precise tolerances through temperature changes
• Be mechanically sound (moderate load-bearing)
• Be cost-competitive
• Machine reasonably quickly for the volume

We recommended POM (Delrin) instead of the nylon they'd been considering. While nylon was cheaper per pound (18), here's why POM made sense:

• Better dimensional stability meant fewer rejects after machining
• Faster machining speeds reduced per-part machining time by 15%
• Superior wear resistance meant the parts would last longer in customer hands
• Better reputation for reliability = fewer returns and warranty issues

When we calculated the actual piece cost including machining, scrap, and warranty considerations, POM was actually competitive with nylon and provided better overall value.

Machining Tips for Each Material

• Acrylic & Polystyrene: Use sharp tools, keep speeds relatively high, go slow to prevent melting
• POM (Delrin): Excellent with standard carbide tools; maintains tight tolerances beautifully
• Nylon: Dry carefully after receiving material; condition before precision machining if strict tolerances needed
• Polycarbonate: Lower speeds than acrylic to prevent melting; use coolant carefully to avoid stress cracking
• PEEK: Slow speeds, patience, and quality tooling; worth investing in expertise
• PTFE: Specialized techniques; consider outsourcing if unfamiliar

Final Thoughts

Choosing the right plastic for CNC machining isn't just about picking the cheapest option or the one you've used before. It's about understanding your requirements, evaluating the material properties against those requirements, and calculating the true cost including machining, quality, and performance.

At our facility, we've machined virtually every type of plastic commercially available. We've made parts that had to be optically perfect, parts that had to endure extreme temperatures, and parts that simply needed to be affordable. Each material has its place.

If you're unsure about which plastic is right for your project, we're here to help. Feel free to reach out with your specifications, and we'll recommend the optimal material and machining approach for your specific application.

The right plastic can mean the difference between a project that's a headache and one that's a smooth success.