Choosing CNC plastic materials affects both part quality and production cost. After machining thousands of precision plastic components, small material decisions make huge differences in dimensional accuracy and tool life.
Start with POM (Delrin) for tight-tolerance parts, nylon 6/6 for structural strength, and ABS for general-purpose housings. Engineering plastics like PEEK cost 10x more but offer superior temperature and chemical resistance. Match material properties to your specific performance requirements, not theoretical maximums.
Learn how plastics perform in CNC machining, which hold tight tolerances, and how to specify the right grade to prevent costly production delays.
Table of Contents
How do I choose between commodity and engineering plastics?
Use this decision tree: Operating temperature >85°C? → Engineering plastics. Tolerances tighter than ±0.02mm? → POM or PEEK. Budget-sensitive general parts? → ABS or polypropylene. Chemical exposure beyond standard cleaners? → Engineering grades. The key is matching material capabilities to actual performance requirements, not theoretical maximums.
The temperature threshold is critical: ABS handles continuous exposure around 70-85°C, while POM withstands up to 100°C continuous and 140°C short-term. For precision work, POM “machines like a metal, holding tight dimensional tolerances” while commodity plastics drift under machining heat. ABS works well for electronics housings and rapid prototypes where cost matters more than extreme performance.
Engineering plastics cost 2-5x more than commodity grades, so the upgrade only makes sense when commodity materials would actually fail. PEEK is “one of the more expensive CNC machining plastics on the market” but justified for extreme temperature or chemical exposure.
Design Takeaway: Start with ABS for general housings under 80°C, upgrade to POM when you need precision or higher temperatures, and reserve PEEK for extreme conditions where its premium cost delivers measurable value.
Which plastic machines best for tight tolerances - POM, nylon, or PEEK?
Need ±0.01mm precision? → POM. Budget-sensitive general parts? → Avoid nylon for tight tolerances. Extreme environment + precision? → PEEK.
Tolerance Capability by Part Size:
- Small parts (<50mm): POM ±0.005mm, Nylon ±0.015mm
- Medium brackets (50-100mm): POM ±0.01mm, Nylon ±0.025mm
- Large housings (>100mm): POM ±0.02mm, Nylon ±0.05mm+
POM machines like a metal, holding tight dimensional tolerances without distortion” while nylon is “notoriously more difficult to machine precisely” due to heat buildup causing melting. On precision work, we consistently achieve ±0.008mm flatness on POM faceplates while identical nylon parts warp beyond ±0.035mm from cutting heat, verified using CMM measurement per ISO 2768 standards.
Specifying tight tolerances (±0.001″) typically costs 3-4x more than standard tolerance parts (±0.005″) due to slower feeds and finishing passes. Tighter tolerances drive up manufacturing costs exponentially – precision tolerance parts require specialized inspection and longer cycle times.
Design Takeaway: Use POM for features requiring ±0.02mm or tighter per ISO 2768-f fine tolerances. Keep ±0.05mm as default and tighten only critical assembly dimensions. Reserve PEEK for extreme temperature environments where POM would soften.
Which plastics can replace metal in structural parts?
Structural loads >80 MPa tensile? → Consider staying with metal. Moderate loads (50-80 MPa) + weight critical? → Glass-filled nylon with thicker sections. Cost-sensitive? → Keep aluminum.
Real Application Examples:
- Automotive: Glass-filled nylon used in intake manifolds, steering wheels, door handles, and under-hood components
- Industrial: Structural parts, valves, fittings, pumps, and conveyor components
- Electronics: Battery standoffs, mounting brackets, and structural supports in electric vehicles
Glass-filled nylon reaches 130-150 MPa tensile strength with 30% glass content, 50-70% stronger than unfilled nylon. 30% glass-filled nylon provides maximum tensile strength of 12,000 PSI (83 MPa) for structural applications. However, glass additives make material “more brittle” with fracture risk under impact loads.
Design considerations require 40-60% thicker walls versus metal equivalents to achieve comparable stiffness. ASTM D638 testing validates actual tensile strength for your specific load requirements rather than relying on general specifications.
Design Takeaway: Glass-filled nylon succeeds in automotive and industrial applications where weight reduction and heat resistance justify redesign costs. Validate loads with material testing and add structural ribs rather than just increasing wall thickness for cost-effective designs.
ABS vs PC vs nylon: which handles impact and heat better?
High impact + heat >100°C? → Polycarbonate. Moderate impact + cost-sensitive? → ABS. Chemical resistance + continuous use? → Nylon. Each material excels in different temperature-impact combinations for CNC applications.
Heat Deflection Temperature Comparison:
- ABS: HDT about 98°C, good impact strength, lowest cost
- Polycarbonate: HDT around 135°C, excellent impact resistance, transparent
- Nylon: HDT 70-120°C depending on grade, superior chemical/wear resistance
ABS offers “good impact resistance” but polycarbonate is “superior in these terms and has higher temperature resistance” though “ABS is cheaper and easier to CNC-machine.” Polycarbonate costs “35-40% more expensive” than ABS but justifies the premium in demanding applications.
Wall Thickness Impact: Thin walls (<2mm) reduce effective temperature limits by 10-15°C due to heat concentration during use. Polycarbonate maintains strength better in thin sections, while ABS becomes more brittle. Nylon requires minimum 3mm walls for reliable performance in high-temperature applications.
Design Takeaway: Choose ABS for general housings under 95°C with standard wall thickness. Select polycarbonate for safety applications above 100°C or thin-wall designs. Reserve nylon for thick-section wear parts where chemical resistance matters.
Which clear plastics resist yellowing and scratching?
Need long-term outdoor clarity? → Acrylic for UV resistance. High-impact optical parts? → Polycarbonate with UV coating. Budget displays? → Acrylic but expect brittleness.
Optical Performance Data:
- Acrylic (PMMA): 92% light transmittance, does not yellow under UV exposure
- Polycarbonate: 90% light transmittance, yellows without UV stabilizers
- Minimum thickness: 3mm for good optical quality after machining
Acrylic “does not turn yellow over time when exposed to UV light” while “unmodified polycarbonate” yellows without protection. Wall thickness considerations: Thinner sections (<3mm) show more visible tool marks and require additional polishing. Thick sections (>10mm) may develop internal stress that affects optical clarity.
Processing Cost Impact: “Post-machining polishing operations may be required for either material, especially if optical clarity is desired.” Vapor polishing adds 30-40% to machining cost but essential for true transparency. Flame polishing (acrylic only) costs 20% less than vapor polishing.
Design Takeaway: Specify acrylic for outdoor applications requiring long-term clarity. Choose polycarbonate for impact-critical optical parts. Budget extra 25-40% for polishing when transparency is required – machined surfaces alone won’t achieve optical quality.
Food-grade plastics: HDPE vs PP vs POM for easy cleaning?
Need chemical resistance + frequent cleaning? → HDPE. High-temperature wash cycles? → PP. Smooth surfaces + precision parts? → POM. Each material offers different cleaning advantages for CNC machined components.
Cleaning Performance Comparison:
- HDPE: Resists bleach/sanitizers, withstands aggressive scrubbing, lowest cost
- PP: Handles hot water cycles up to 100°C, autoclave-compatible, flexible
- POM: Smoothest machined surface finish, non-porous, dimensional stability
Surface roughness standards per ISO 4287 range from 25 μm to 0.025 μm with POM typically achieving Ra 1.6 µm standard finish, reducing bacterial adhesion compared to textured surfaces. Recommended wall thickness for plastic parts should be 1.5mm minimum (absolute minimum 1mm) to prevent stress cracking during cleaning cycles.
Wall Thickness for Durability: For plastics, standard minimum wall thickness is 1.5mm to stop warping and maintain structural integrity during cleaning. HDPE requires thicker sections (2-3mm) for repeated chemical exposure. PP maintains flexibility but may warp during hot wash cycles without adequate support.
Design Takeaway: Choose HDPE for equipment requiring frequent chemical cleaning and cost-effectiveness. Select PP for parts exposed to high-temperature sterilization cycles. Use POM when smooth surfaces (Ra ≤1.6 µm) and dimensional stability matter more than chemical resistance.
How do I specify plastic grades and tolerances to avoid production delays?
Standard parts? → “ISO 2768-m tolerances, specify material grade only.” Precision needs? → “±0.01mm callout on critical dimensions only.” Special grades? → “Include specific trade names and property requirements.”
Drawing Callout Examples:
- Material: “POM-C (Delrin 570)” not just “POM”
- Tolerances: “ISO 2768 Medium (M) for plastics” as standard, ±0.01 on critical features
- Surface finish: “Ra 3.2 µm standard” unless finer finish required
- Special requirements: “UV-stabilized grade” or “Glass-filled 30%”
Over-specifying tolerances accounts for “25-40% of unnecessary manufacturing costs” in development projects. “Tighter tolerances can be achieved but is only recommended for critical features.” Clear specifications reduce quote turnaround from 5-7 days to 2-3 days by eliminating clarification rounds.
Quote Comparison Guidelines: When specs differ between suppliers, standardize material callouts and tolerance requirements across quotes. Request breakdown of setup costs versus piece prices. Include application context (“equipment housing,” “precision assembly”) to help suppliers optimize recommendations.
Design Takeaway: Use specific material designations with trade names when possible. Apply ISO 2768-m as default tolerance per industry standards. Clear specifications save 2-3 days per quote cycle and enable accurate cost comparisons between suppliers.
How do I prevent warping and chipping when CNC machining plastics?
Thin walls warping? → Support with fixtures, use minimum 1.5mm thickness. Material chipping? → Sharp carbide tools, climb milling. Parts moving during cuts? → Improve workholding, reduce cutting forces.
Warping Prevention by Geometry:
- Thin sections: Minimum 1.5mm wall thickness for plastic parts to avoid vibration and warping
- Large spans: Maintain minimum 0.06″ (1.5mm) wall thickness to prevent movement during machining
- Support structures: Add ribs every 20-25mm on unsupported walls
Walls thinner than 0.020″ (0.51mm) “risk breakage during machining, and flex or warp afterwards.” Sharp tools prevent “melting, smearing, and gumming” that causes poor edge quality and dimensional problems.
Fixture Design Guidelines: Thin walls deflect when cutting forces are applied, causing chatter and vibration which leads to dimensional errors and poor surface finish. Use soft jaw materials and distribute clamping forces across larger areas. Support thin walls near cutting areas and allow thermal expansion.
Expected Warping: Parts may fall “outside standard fabrication tolerances of plus or minus five thousandths of an inch” when walls are too thin. Plan for ±0.05mm deviation on unsupported walls over 50mm spans.
Design Takeaway: Follow ISO and industry guidelines for minimum wall thickness. Add structural ribs to thin walls every 20-25mm spacing per standard practice. Consider machining-induced stresses when setting tolerances on thin features.
Conclusion
Selecting the right plastic material balances performance requirements with manufacturing constraints and cost. Start with commodity grades like ABS for general applications, upgrade to engineering plastics only when necessary, and specify clear tolerances and surface finishes to avoid delays. Contact us to explore manufacturing solutions tailored to your plastic component requirements.
Frequently Asked Questions
Only specify Ra values when functionally necessary. Standard machined finish is Ra 3.2 µm – adequate for most applications. Specifying Ra 1.6 µm or finer increases machining costs 20-40% due to additional operations and longer cycle times.
Include specific material grades (POM-C, Nylon 6/6-GF30), apply ISO 2768-m default tolerances, and specify your application upfront. Clear specifications reduce quote turnaround from 5-7 days to 2-3 days by eliminating clarification rounds.
POM (Delrin) consistently achieves ±0.01mm on features up to 100mm length using standard CNC equipment. Its high thermal conductivity prevents heat buildup during machining, while nylon typically drifts to ±0.03mm or worse due to thermal instability.
ABS offers the best balance of cost, machinability, and performance for temperatures under 95°C. It machines cleanly with standard tooling and holds ±0.05mm tolerances reliably, making it ideal for electronics housings and rapid prototypes before moving to production.
Upgrade to engineering plastics when operating temperatures exceed 85°C continuously, chemical resistance beyond standard cleaners is required, or precision tolerances tighter than ±0.02mm are critical to function. Otherwise, commodity plastics like ABS save 30-50% in material costs.
Maintain 1.5mm minimum wall thickness for plastic parts per ISO guidelines. Thinner walls risk warping during cutting and may fall outside standard ±0.005″ tolerance capabilities. Add supporting ribs every 20-25mm on thin-wall designs.
