How do I choose between plastic and metal for CNC milled parts?
How Do I Choose Between Plastic and Metal for CNC Milled Parts?
Choose by Failure Mode, Then Check the Exact Grade
Choose metal for a CNC milled part when load, stiffness, durable threads, pressure, heat, or metallic finishing controls the design; choose engineering plastic when low weight, electrical insulation, low friction, or compatible chemical service matters more. Base the decision on the part’s mechanical requirements, cost targets, thermal or chemical exposure, and design complexity. A material-family name is not enough: compare the exact grade, condition, stock form, wall geometry, load duration, temperature, humidity, finish, and inspection state. ASTM D638-22 notes that plastic tensile results depend on specimen preparation, test speed, environment, and load-time scale. Metal tensile data also need grade, heat treatment, test method, and service limits. Put those conditions in the RFQ before comparing datasheet values.
Use Metal When Stiffness, Threads, Heat, or Pressure Governs
High Strength and Load-Bearing Requirements: Select a qualified metal grade when the part carries structural load, clamps another component, retains threaded preload, resists impact, or must limit long-term deflection. Aluminum 6061 can suit weight-sensitive, moderately loaded parts in an appropriate temper. Stainless Steel 304 can add stiffness, toughness, and corrosion resistance in compatible environments. Titanium Ti-6Al-4V offers high specific strength, but machining cost, galling, contact stress, and finishing still need review. Use the final grade for tests that approve torque, pressure, fatigue, or permanent load.
Thermal and Chemical Resistance: Metal is the better starting category when pressure, sterilization, sustained heat, fuel, oil, or impact excludes available polymers. Projects in aerospace, oil and gas, or medical devices may also require traceability, cleaning compatibility, and controlled inspection records. No metal is universally resistant: identify the fluid, concentration, temperature, pressure, exposure time, cleaning cycle, and governing material specification.
Dimensional Stability and Surface Precision: Metal often provides a more stable datum system for bearing seats, sealing lands, fine threads, and clamped assemblies, but thin walls can still move after roughing or unclamping. Finishes such as anodizing, polishing, and PVD coating may add or remove material. Define masking, edge condition, and whether each critical dimension is accepted before or after finishing.
Use Plastic When Insulation, Weight, Friction, or Chemical Service Governs
Lightweight Applications: Select plastic when low mass, electrical isolation, quiet contact, or low friction outweighs metal stiffness. ABS can suit housings and concept parts within its temperature and chemical limits. POM/Delrin can suit dimensionally controlled sliding parts where its grade and environment are compatible. PEEK extends the heat and chemical range, but resin grade, reinforcement, certification, and cost must match the application.
Cost-Sensitive Prototypes or Low-Stress Parts: Plastic can reduce prototype cost for covers, fixtures, electrical housings, and low-load geometry checks, but it is not automatically cheaper. Stock size, high-performance resin price, flatness, burr control, scrap risk, and conditioning can reverse the comparison. A plastic prototype should not approve metal thread strength, heat response, coating allowance, or long-term wear. Mark geometry-only tests when production may change to metal.
Insulation and Chemical Resistance: Plastic is a strong candidate for electrical isolation, non-metallic contact, and chemicals compatible with the exact polymer grade. Check moisture absorption, creep, stress cracking, heat deflection, additives, and cleaning media. Clamping can elastically distort a plastic blank, and the part may move after release. State conditioning time, measurement temperature, humidity, load duration, critical datums, and the required inspection state.
Compare the Failure Risk and Validation Plan
Criteria | Metal Decision | Plastic Decision |
|---|---|---|
Strength & Stiffness | Prefer for structural load, durable threads, clamps, and pressure; verify grade and condition. | Use for lower loads after checking creep, deflection, stress concentration, and service time. |
Weight | Aluminum or titanium can reduce mass while retaining metallic interfaces. | Usually lowest mass; confirm that wall thickness and stiffness remain adequate. |
Total part cost | Include alloy price, tool wear, finishing, inspection, and removed stock. | Include resin grade, stock size, conditioning, flatness control, and scrap risk. |
Machinability | Stable on many alloys; hard or heat-resistant grades raise force, heat, and tool wear. | Cutting force can be low, but heat, clamping, chip control, and burrs govern quality. |
Tolerance & Surface Finish | Inspect datum shift, thin-wall movement, deburring, and finish-sensitive dimensions. | Inspect after defined thermal and moisture conditioning and after fixture release. |
Environmental Resistance | Match alloy and finish to fluid, heat, pressure, cleaning, and galvanic contact. | Match resin and additives to chemicals, temperature, UV, moisture, and load duration. |
Define What the RFQ Must Prove
A material selection review should connect function, exact grade, stock condition, machining route, tolerance plan, and post-processing. State whether the order is for prototyping or mass production, then provide the load case, service environment, quantity, finish, critical dimensions, datum scheme, conditioning state, and inspection method. If both categories remain feasible, request separate risk notes for creep, corrosion, temperature, threads, coating, and test validity instead of comparing raw material price alone.
Use the related service pages to verify available material families, then attach a controlled drawing that marks functional, cosmetic, sealing, threaded, and finish-sensitive surfaces. The RFQ should state whether substitution is allowed, which tests require final production material, and which prototype checks are geometry-only.
