CNC machining plastics is the fastest way to get functional parts in production-grade materials when molding tooling is not justified. The selection process is the same as for metals: define the operating environment, map it to a material family, then confirm the exact grade against the manufacturer's data sheet. The grade, not the family, decides the result, because a glass-filled nylon and an unfilled nylon machine and behave differently even though both are "nylon." This guide gives an environment-material matrix, the application boundaries of each common family, and the machining and tolerance behavior you need to specify correctly.
The Selection Process: Environment First, Grade Second
Run the part through four environment questions before opening the material list.
- Temperature: continuous and peak service temperature, and whether the part sees thermal cycling.
- Chemistry: oils, solvents, cleaning agents, and moisture that will contact the part.
- Load: sustained load, impact, friction, and whether creep will matter.
- Tolerance: how much dimensional movement the assembly can absorb over temperature and humidity.
The answers narrow the family, and the family narrows the grade list. Then the data sheet, not the catalog description, confirms the choice: tensile strength at the service temperature, water absorption, coefficient of thermal expansion, and chemical resistance are the fields that matter.
Environment-Material Matrix
| Environment or requirement | Candidate materials | Selection notes |
|---|---|---|
| High temperature, chemicals, strength | PEEK | Verify continuous temperature and chemical data; cost is justified only when required |
| Stable precision, low moisture absorption | Acetal (POM) | Holds dimensions; first choice for bushings, gears, spacers |
| Toughness, wear, impact | Nylon (PA6, PA66) | Moisture changes dimensions; account for equilibrium |
| Low friction, chemical inertness | PTFE | Soft and creeping; needs support and sharp tooling |
| Impact, transparency, general enclosure duty | Polycarbonate (PC) | Tough; verify chemical and UV resistance |
| General purpose, easy machining | ABS | Moderate properties; low cost, easy to process |
| Food contact or medical use | Grade-specific | Compliance belongs to the formulation, not the family |
The matrix is a starting point. If two families both survive the environment, choose by dimensional stability, friction, and cost; if the environment eliminates every family on the list, the requirement is beyond machined plastic and the design should be reconsidered.
Material Families and Application Boundaries
Acetal (POM). The machining workhorse: it machines cleanly, holds tight tolerances, absorbs almost no moisture, and resists wear, which makes it the default for bushings, gears, spacers, and precision components. The boundary is temperature and strong chemicals: acetal is not a high-temperature material, and aggressive acids can attack it, so confirm the service conditions before defaulting to it.
Nylon (PA). Tough, wear-resistant, and chemically tolerant, used in gears, rollers, and wear components. The boundary is moisture: nylon absorbs water, and the absorbed moisture changes its dimensions and properties. A machined nylon part that measures perfectly at delivery can grow or shrink as it equilibrates with the environment, so confirm the grade, PA6 or PA66, and set tolerances with the movement in mind. Glass-filled nylon adds stiffness but wears tools faster and can be more brittle.
PEEK. The high-end choice for high temperature, strength, chemical resistance, and low outgassing, valuable in aerospace, medical, semiconductor, and oil-and-gas applications. PEEK machines stiff and clean but is expensive and tough on tooling, so the material and machining costs are both high. The boundary is economic: PEEK is wasted on parts that acetal or nylon could handle, and the manufacturer's data should confirm the exact grade's continuous temperature and chemical resistance before the cost is committed.
PTFE. Soft, slippery, and chemically inert, excellent for seals and bearings, but low in strength and prone to creep under sustained load. Machining PTFE needs sharp tools, light cuts, and support while cutting, because the material deflects and smears. The boundary is mechanical: PTFE is chosen for friction and corrosion, not for load-bearing structure.
Polycarbonate and ABS. PC offers impact strength and transparency; ABS is a general-purpose, easy-machining option. Both suit covers and enclosures, with the boundary being temperature and chemical exposure: PC is sensitive to some solvents and UV without protection, and ABS softens at relatively low temperatures.
Machining Behavior by Grade
Plastics deflect more than metals, so thin walls and unsupported sections move under cutting forces; sharp tools, light cuts, and proper support are the standard recipe. Heat is the second difference: plastics soften as they heat, so feeds and speeds are set to remove material without melting the surface, and coolant or air blast is matched to the grade. Some plastics absorb coolant or swell, and others melt and smear at high speeds, so the supplier should match the coolant and parameters to the specific material.
Surface finish and burrs also differ by grade. Acetal and PC machine to a clean finish; nylon can fuzz at edges; PTFE tears without razor-sharp tooling. The finish specification should be confirmed against the grade, and sharp edges should be called out for deburring rather than assumed.
Tolerances and Environmental Behavior
Plastic tolerances are wider than metal tolerances because the material moves: temperature changes size, moisture changes nylon, and machined surfaces relax over time. A ±0.01 mm callout that is routine in aluminum may be unrealistic in nylon or PTFE, so the tolerance scheme should reference the material's expansion, absorption, and creep behavior. The tolerance reference is a useful starting point, but the achievable range belongs to the specific grade, geometry, and supplier.
State the measurement conditions on the drawing. A dimension measured at 20 °C on a dry part differs from the same dimension at 40 °C in a humid environment, and a contact probe can compress a soft surface. Confirm the inspection method and temperature with the supplier, and account for creep on loaded parts, because a plastic part that relaxes in service fails no matter how well it machined.
How to Read the Data Sheet Before Selecting
Confirm these fields on the specific grade, and note the conditions under which the data was measured, because properties change with temperature and humidity.
- Tensile strength and modulus at the service temperature
- Maximum continuous service temperature
- Water absorption and dimensional change at equilibrium
- Coefficient of thermal expansion
- Chemical resistance to the fluids in the application
- Hardness or wear behavior if the part rubs against another surface
- Creep data if the part carries sustained load
The acetal data sheet is an example of the detail a manufacturer publishes for one family; the same discipline applies to every grade you evaluate.
6CProto Engineering Team
Select the plastic by the environment, then confirm the machining behavior. Acetal for stable precision parts, nylon for toughness with moisture accounted for, PEEK when temperature and chemicals demand it, and PTFE when friction and corrosion rule. Confirm the exact grade, the tolerance, and the measurement conditions during DFM, because a plastic part that cannot hold its dimensions in service will fail no matter how well it machines.
Conclusion
Match the grade to the environment: acetal for precision and stability, nylon for toughness, PEEK for high performance, and PTFE for friction and chemical resistance. Confirm the exact grade against the manufacturer's data sheet, then set tolerances and measurement conditions that respect the material's movement. A machined plastic part selected and specified correctly performs like the production material it represents.
FAQs
What is the best plastic for CNC machining?
Acetal is the most common choice for precision parts because it machines cleanly and holds dimensions with low moisture absorption. The best material still depends on the environment, load, and temperature, so map the requirements to the family before choosing.
Why does my machined nylon part change size?
Nylon absorbs moisture, and the absorbed water changes its dimensions and properties. Confirm the grade, PA6 or PA66, and set tolerances with the moisture equilibrium in mind, including the humidity where the part will actually operate.
When is PEEK worth its cost?
When the application needs high continuous temperature, strength, chemical resistance, or low outgassing that standard plastics cannot provide. For general parts, acetal or nylon delivers most of the function at a fraction of the cost, so confirm the requirement against the data sheet before committing.
What tolerances can machined plastic hold?
It depends on the grade, geometry, and measurement conditions. Acetal holds tighter tolerances than nylon or PTFE, and plastics move with temperature and moisture, so the drawing should specify the measurement temperature and humidity and confirm the achievable range with the supplier.
Sources
- 6CProto CNC Machining Services
- 6CProto Standards and Tolerances
- 6CProto Medical Manufacturing
- Victrex – PEEK polymer product range
- Ensinger – POM (Acetal) material data