CNC Machining of ABS, PC, ACRYLIC, POM, and PEEK in the Prototype Industry: Characteristics and Painting Considerations

CNC (Computer Numerical Control) machining is a widely used manufacturing process in the prototype industry due to its precision, versatility, and efficiency. It is particularly valuable for processing engineering plastics such as ABS, PC (Polycarbonate), ACRYLIC (PMMA), POM (Polyoxymethylene), and PEEK (Polyether Ether Ketone). Each of these materials has unique properties that make them suitable for different applications. Additionally, post-processing techniques like spray painting require careful consideration to ensure optimal adhesion and finish quality.

This article explores the machining characteristics of these materials and provides essential guidelines for spray painting CNC-machined plastic prototypes.

1. CNC Machining Characteristics of Common Prototyping Plastics

(1) ABS (Acrylonitrile Butadiene Styrene)

Properties:

• Good mechanical strength and impact resistance

• Excellent machinability with minimal warping

• Moderate heat resistance (up to ~85°C)

• Low cost and widely available

CNC Machining Considerations:

• Easy to cut, drill, and mill with standard carbide tools

• Produces smooth surface finishes

• Minimal risk of cracking or melting if proper speeds and feeds are used

Applications: Automotive parts, consumer electronics, housings, and prototypes.

(2) PC (Polycarbonate)

Properties:

• Extremely high impact resistance (nearly unbreakable)

• Good optical clarity (if transparent grades are used)

• High heat resistance (~135°C)

• Prone to stress cracking if machined improperly

CNC Machining Considerations:

• Requires sharp tools to prevent chipping and cracking

• Low cutting speeds are recommended to avoid heat buildup

• Coolant or air blasting helps reduce thermal stress

Applications: Safety goggles, transparent covers, medical devices, and automotive components.

(3) ACRYLIC (PMMA – Polymethyl Methacrylate)

Properties:

• Excellent optical clarity and light transmission

• Hard and scratch-resistant but brittle

• Moderate chemical resistance

• Prone to cracking under stress

CNC Machining Considerations:

• Requires slow feed rates to prevent cracking

• Diamond-cutting tools yield the best surface finish

• Polishing may be needed to restore clarity after machining

Applications: Display cases, lenses, signage, and light diffusers.

(4) POM (Polyoxymethylene / Delrin®)

Properties:

• High stiffness and low friction (self-lubricating)

• Excellent dimensional stability

• Resistant to chemicals and moisture

• Poor adhesion for painting

CNC Machining Considerations:

• Produces clean cuts with minimal burrs

• Sharp tools are essential to avoid melting

• No coolant is typically needed

Applications: Gears, bearings, sliding parts, and precision mechanical components.

(5) PEEK (Polyether Ether Ketone)

Properties:

• Exceptional heat resistance (up to 250°C)

• High chemical and wear resistance

• Excellent mechanical strength

• Expensive compared to other plastics

CNC Machining Considerations:

• Requires high-speed machining with sharp carbide tools

• Coolant is recommended to prevent overheating

• Generates fine chips that must be properly evacuated

Applications: Aerospace, medical implants, high-temperature seals, and industrial components.

2. Spray Painting Considerations for CNC-Machined Plastic Prototypes

Spray painting enhances the aesthetics and functionality of plastic prototypes but requires careful preparation to ensure proper adhesion and durability. Below are key considerations for different plastics:

(1) Surface Preparation

• Cleaning: Remove dust, oils, and machining residues using isopropyl alcohol or specialized plastic cleaners.

• Sanding: Lightly sanding (e.g., 400-600 grit) improves paint adhesion, especially for smooth plastics like POM and PEEK.

• Priming: Use a plastic-specific primer to enhance paint bonding, particularly for low-surface-energy plastics (e.g., POM, PP).

(2) Material-Specific Painting Tips

• ABS: Bonds well with most paints; no primer is strictly necessary but recommended for best results.

• PC: Requires adhesion promoters due to its smooth surface. Avoid solvent-based paints that may cause stress cracking.

• ACRYLIC: Can be painted directly but benefits from a primer for long-term durability.

• POM: Extremely difficult to paint; plasma treatment or specialized primers (e.g., etching primers) are often required.

• PEEK: Similar to POM, requires surface activation (flame treatment, plasma, or adhesion promoters).

(3) Paint Selection

• Acrylic Paints: Good for general-purpose applications.

• Polyurethane Paints: Offer better durability and chemical resistance.

• Epoxy Paints: Suitable for high-wear applications but may require curing.

(4) Application Techniques

• Spray Gun vs. Aerosol: Spray guns provide better control, while aerosol cans are convenient for small projects.

• Multiple Thin Coats: Prevents runs and ensures even coverage.

• Curing: Follow manufacturer recommendations for drying times and temperature conditions.

(5) Post-Painting Finishing

• Clear Coating: Adds gloss and protection.

• Buffing/Polishing: Enhances the final appearance.

Conclusion

CNC machining of ABS, PC, ACRYLIC, POM, and PEEK offers diverse solutions for rapid prototyping, each with distinct advantages. Understanding their machining behaviors ensures high-quality parts, while proper spray painting techniques enhance both aesthetics and performance. By selecting the right material, optimizing machining parameters, and following best practices in surface preparation and painting, manufacturers can achieve professional-grade prototypes that meet functional and visual requirements.

For industries ranging from automotive to medical devices, mastering these processes is essential for delivering high-performance CNC machining plastic components efficiently.