Ordering CNC machined plastic parts is fundamentally different from ordering metal parts. The shops that do it well are not the same as the shops that do aluminum well. The materials behave differently, the quality risks are different, and the cost drivers follow different rules.
We machine and ship plastic parts to customers in 20+ countries. Here's what buyers need to know — but often don't — before placing an order.
Plastic machining is its own discipline
A shop that machines aluminum and steel all day might cut your PEEK parts competently. Or they might not. The differences that matter:
- Plastics expand 5-10x more with temperature than metals. A shop measuring parts in an un-air-conditioned inspection area at 30°C is giving you dimensions that won't match your 20°C QC lab.
- Plastics don't conduct heat. The cutting zone gets hot and stays hot. Feeds, speeds, and coolant strategies that work for aluminum will melt or smear a plastic surface.
- Plastics creep and stress-relax. A PEEK part machined to ±0.01mm today might not be ±0.01mm next week if internal stress wasn't relieved.
- Burrs are different. Metal burrs are rigid and break off cleanly. Plastic burrs are flexible and tear rather than shear. Deburring plastic is a different skill.
Ask your shop: "How many plastic parts do you run per month? What's your process for thermal management during inspection?"
Material: you have options
For CNC machined engineering plastics, the material comes in standard stock shapes — rods, plates, tubes. Most common diameters and thicknesses are in stock. Unusual sizes have longer lead times.
Virgin vs filled grades: Fillers (glass, carbon, PTFE, graphite) modify properties but also modify machinability. Glass-filled grades eat tools faster. Carbon-filled grades are conductive and leave a residue. Confirm your shop has experience with your specific grade.
Imported vs domestic resin: For PEEK, Victrex (UK) and Solvay (Belgium) are the gold standard. Chinese PEEK resin from Jilin Joinature or Panjin Zhongrun now rivals imported quality at the top end, at 20-30% lower cost. The choice depends on your end-customer requirements — some specs name a specific resin, others accept equivalent grades.
Certifications: For medical, ISO 13485 and material traceability back to the resin lot. For aerospace, AS9100 and full material certs. For general industrial, ISO 9001 is usually sufficient. Don't pay for certifications you don't need, but don't skip them if your application requires them.
Design rules for lower cost
The same design-for-manufacturability rules that apply to metal parts mostly apply to plastics — with a few twists:
- Internal corner radii: Even more important in plastics. Small radii require small tools. Small tools deflect more in plastic than in metal. Keep internal radii above 3mm wherever possible.
- Wall thickness: Keep unsupported walls above 1.5mm for PEEK and PEI, above 2.5mm for PTFE and softer plastics. Thin walls vibrate during machining and warp from residual stress.
- Threaded holes: Threaded inserts (Heli-Coil or similar) are recommended for repeated assembly/disassembly. Direct-threaded plastic holes wear and strip over time. For PEEK, direct threading can work for low-cycle applications.
- Undercuts: Avoid them unless absolutely necessary. Internal O-ring grooves, snap-ring grooves, and back-side features need specialty tooling and add cost. If you must have one, keep it to a single tool geometry across the part.
The quote: what drives cost
The biggest cost drivers in plastic CNC machining, in order:
- Tolerances. Every decimal place matters. ±0.25mm general tolerance is cheap. ±0.05mm adds 30-50%. ±0.01mm doubles the cost or more.
- Material grade. Unfilled PEEK costs less than glass-filled. PTFE costs less than PAI. Choose the cheapest grade that meets requirements.
- Quantity. The biggest cost drop is between 1 and 10 parts (setup amortization). Between 50 and 500, the curve flattens.
- Setup count. Every re-fixturing adds labor and tolerance stack-up. Design for the fewest setups.
- Secondary operations. Threading, anodizing (for aluminum), passivation, polishing, marking — each adds cost and lead time. Bundle what you can, eliminate what you don't need.
Quality: what to require
At minimum, every shipment should include a dimensional inspection report for critical features. For production quantities, request a Certificate of Analysis that includes:
- Material grade and lot number
- Melt flow index (for PEEK, PPS)
- Density (confirming correct filler content)
- Key mechanical properties if critical
For first articles, request a full FAI including all drawing dimensions. This catches issues before they become production problems.
Find a partner, not a vendor
The best plastic machining relationships are long-term. The shop learns your parts, your tolerances, your packaging preferences, your documentation requirements. You learn their capabilities, their lead times, their communication style.
The first order with a new shop costs more — in your time, in risk, in back-and-forth. The tenth order costs less because the learning curve is behind you. A 5% price difference from a new shop rarely justifies the switching cost unless your current supplier is failing on quality or delivery.
We work with buyers worldwide on precision plastic parts. Send us your drawing and we'll quote it with a DFM review — specific suggestions to reduce cost without sacrificing function, based on what we see working every day.