Plastics and metals respond very differently to cutting forces, heat, and clamping pressure. If you've only worked with machined metal components, your first plastic machining project may produce unexpected results unless you account for these material differences. Understanding how plastics behave during CNC machining helps you design better parts, set realistic tolerances, and communicate more effectively with your machine shop.
Thermal Expansion and Heat Management
One of the most significant differences between CNC machining plastic parts and machining metals is how each material responds to heat generated during cutting.
Heat Generation During Cutting
Metal cutting generates substantial heat, but metals also conduct that heat away from the cutting zone relatively quickly. Aluminum, for example, has excellent thermal conductivity. Plastics, on the other hand, are thermal insulators. Heat generated at the tool-material interface stays localized, which can cause several problems:
- Material softening or melting at the cut surface
- Chip welding or re-adhering to the part
- Dimensional changes as the part heats and cools
- Surface finish degradation from heat-affected zones
Machinists compensate by using sharp tooling, slower cutting speeds, and sometimes air or coolant to manage heat buildup. The goal is to remove material efficiently while keeping temperatures low enough to maintain dimensional stability.
Thermal Expansion Coefficients
Plastics typically expand and contract much more than metals with temperature changes. The coefficient of thermal expansion for common engineering plastics can be five to ten times higher than aluminum or steel. This matters in two ways:
First, during machining, a plastic part may expand slightly from cutting heat. If you measure the part while it's still warm, dimensions may appear acceptable, but the part can shrink as it returns to room temperature. Experienced shops allow parts to stabilize before final inspection.
Second, if your finished part will operate in a temperature range significantly different from room temperature, you need to account for dimensional changes in your design. A plastic housing with tight-fitting metal inserts may crack if thermal expansion isn't considered.
Material Flexibility and Deflection
Most engineering plastics are significantly less rigid than metals. This flexibility affects both the machining process and the final part performance.
Deflection Under Cutting Forces
When a cutting tool engages a thin-walled plastic feature, the material may deflect away from the tool rather than being cleanly cut. This can result in:
- Dimensional inaccuracy on thin walls or features
- Chatter marks or poor surface finish
- Difficulty holding tight tolerances on flexible geometries
Machinists address this by using sharp tools with positive rake angles, taking lighter cuts, and sometimes providing temporary support for thin features during machining. In some cases, a feature that's easy to machine in aluminum becomes challenging or impractical in a flexible plastic.
Design Considerations
If you're converting a metal design to plastic, consider whether thin walls, long unsupported spans, or delicate features will behave differently. A 0.040-inch wall that's perfectly rigid in aluminum may flex noticeably in acetal or nylon. This doesn't mean the part can't be machined, but it may require design modifications or adjusted tolerance expectations.
Workholding Challenges
Securing plastic parts during machining requires a different approach than clamping metal components.
Clamping Pressure
Apply too much clamping force to a plastic part, and you may:
- Deform the part during machining
- Create stress concentrations that cause cracking later
- Leave permanent indentations from vise jaws or clamps
Machinists use lower clamping pressures, soft jaws, or custom fixtures to distribute forces more evenly. The challenge is finding the balance between secure workholding and avoiding part distortion.
Part Relaxation
Some plastics exhibit stress relaxation or creep. A part clamped firmly may slowly deform under sustained pressure, even at room temperature. This is particularly relevant for long machining operations or when a part remains fixtured between multiple setups.
Once released from the fixture, a stressed plastic part may also change shape slightly as internal stresses relieve. This is another reason why final dimensions should be verified after the part has been removed from workholding and allowed to stabilize.
Dimensional Stability Over Time
Unlike metals, many plastics continue to change dimensionally after machining.
Moisture Absorption
Hygroscopic plastics like nylon absorb moisture from the air. As they do, they swell dimensionally. A nylon part machined from dry stock may grow noticeably after exposure to humidity. Some shops machine nylon slightly undersize to account for this growth, but this requires knowing the part's operating environment.
If your application involves a hygroscopic plastic, communicate this to your machine shop. They may recommend pre-conditioning the material or adjusting dimensions to compensate for expected moisture uptake.
Stress Relief and Creep
Machining introduces stresses into plastic parts, just as it does with metals. However, plastics are more prone to stress relaxation over time. A part that measures correctly immediately after machining may drift slightly as internal stresses relieve over days or weeks.
For critical applications, some customers request a stress-relief cycle or allow parts to stabilize before final inspection. This is particularly important for parts with very tight tolerances or complex geometries.
Tolerance Expectations
All of these factors—thermal expansion, flexibility, workholding challenges, and dimensional stability—affect what tolerances are practical when CNC machining plastic parts.
Standard machining tolerances are achievable on most plastics, but extremely tight tolerances may be difficult or impossible to hold consistently, especially on thin or flexible features. If your design includes tolerances tighter than ±0.005 inches, discuss feasibility with your machine shop early in the process.
The drawing typically serves as the controlling specification, so clearly indicate which dimensions are critical and which have more flexibility. This helps the machinist prioritize where to focus attention and where standard tolerances are acceptable.
Material Selection Matters
Not all plastics machine the same way. Some general characteristics:
- Acetal (Delrin): Machines well with good dimensional stability and relatively low moisture absorption
- Nylon: Tougher and more flexible, but hygroscopic and prone to heat buildup
- PEEK: Excellent properties but requires careful heat management and sharp tooling
- Polycarbonate: Can be brittle during machining and sensitive to stress cracking
- PTFE (Teflon): Very soft and flexible, challenging to hold tight tolerances
- Acrylic: Machines cleanly but can crack if stressed or improperly clamped
If you're unsure which plastic is best for your application, a conversation with your machine shop can help. Material selection affects both machinability and part performance.
What to Include in Your RFQ
When requesting a quote for CNC machined plastic parts, providing complete information helps your machine shop give you an accurate quote and identify potential issues early.
Essential RFQ Information
- Engineering drawings or CAD files: A drawing or STEP file shows the part geometry and serves as the controlling specification
- Material specification: Include the specific plastic grade if known, or describe the application requirements
- Quantity: Both immediate needs and potential future volumes
- Tolerances: General tolerances and any critical dimensions requiring tighter control
- Surface finish requirements: Machined finish, specific roughness values, or cosmetic requirements
- Required completion date: Realistic lead time expectations help with scheduling
- Inspection requirements: Whether you need dimensional reports, material certifications, or other documentation
- Special notes: Operating environment, assembly requirements, or other context
Prototype Versus Production Considerations
Prototype quantities and production runs may be quoted differently. For smaller quantities, the setup time represents a larger portion of the total cost. For production runs, per-piece costs typically decrease, and investing in custom fixtures or optimized tooling paths may be worthwhile.
If you anticipate moving from prototype to production, mention this in your RFQ. It helps the shop understand the bigger picture and may influence their approach to fixturing and programming.
RFQ Checklist for Plastic Machining
Before submitting your request for quote, verify you've included:
- Drawing or STEP file with dimensions and tolerances
- Material type and grade
- Quantity needed
- Critical dimensions or features
- Surface finish requirements
- Required delivery date
- Any special inspection or documentation needs
- Operating environment or application context
- Future volume expectations if applicable
Working With Your Machine Shop
Good communication between designer and machinist leads to better parts. If you're new to plastic machining, ask questions. An experienced shop can often suggest design modifications that improve manufacturability without compromising function.
If your part has features that concern you—thin walls, tight tolerances, or complex geometries—point these out when requesting a quote. It's better to discuss potential challenges before machining begins than to discover problems during first article inspection.
Request a Quote for Your Plastic Machining Project
If you have a project that requires CNC machined plastic components, Anco Precision can help. We work with a variety of engineering plastics and understand how material properties affect machining and final part quality.
To request a quote, send us your drawing or STEP file, specify the material, let us know the quantity you need, identify any critical tolerances or requirements, and provide your required delivery date. We'll review your project and provide a detailed quote.
Whether you're prototyping a new design or need production quantities of plastic parts, we're ready to discuss your project requirements and answer any questions about material selection or design for manufacturability.