Plastics and metals may both be machinable materials, but they behave very differently under the cutting tool. Engineers and purchasing managers who understand these differences can provide better specifications and avoid common dimensional issues that arise during CNC machining plastic parts.
This article covers the practical considerations machine shops face when working with plastics, including thermal behavior, material flexibility, workholding challenges, and dimensional stability throughout the machining process.
Thermal Expansion and Heat Management
One of the most significant differences between plastics and metals is how they respond to heat generated during machining.
Coefficient of Thermal Expansion
Most engineering plastics have thermal expansion coefficients that are five to ten times higher than metals like aluminum or steel. When a plastic part heats up during machining, it expands considerably. When it cools back to room temperature, it contracts.
This creates a challenge: if you machine a plastic part while it's warm and measure it immediately, the dimensions may appear correct. Hours later, after the part has fully cooled, the dimensions can be significantly different.
Heat Generation During Cutting
Plastics are poor thermal conductors compared to metals. When cutting aluminum, heat dissipates quickly through the material and into the machine table. With plastics, heat concentrates at the cutting zone.
High cutting speeds or aggressive feeds can cause localized melting, surface burning, or dimensional distortion. Machinists working with plastics typically use:
- Lower cutting speeds than would be used for metals
- Sharp tooling to minimize friction
- Adequate coolant or air blast to remove heat
- Interrupted cuts to allow cooling between passes
- Climb milling to reduce heat buildup
Some plastics, particularly those with low melting points, may require pauses between operations to allow the part to return to ambient temperature before final dimensions are achieved.
Material Flexibility and Deflection
Plastics are generally much more flexible than metals, which affects how they respond to cutting forces and clamping pressure.
Cutting Force Deflection
When a cutting tool engages a thin-walled plastic feature, the material can deflect away from the tool rather than being cleanly cut. This results in poor dimensional accuracy, chatter marks, or inconsistent wall thickness.
Strategies to minimize deflection include:
- Using very sharp tools to reduce cutting forces
- Taking lighter depth of cut
- Supporting thin walls with temporary backing or fixtures
- Machining features in a sequence that maintains rigidity as long as possible
- Leaving extra stock on flexible features until final passes
Clamping Pressure Considerations
Plastics can deform under clamping pressure. Over-tightening a vise or fixture can crush the material, creating localized compression that springs back after the part is released. The result is a part that was dimensionally correct in the fixture but out of tolerance once removed.
Experienced machinists use lower clamping forces with plastics and often employ soft jaws, padded contact points, or custom fixtures that distribute pressure over larger areas.
Workholding Challenges
The combination of thermal expansion and flexibility makes workholding more complex for plastics than metals.
Fixture Design
Standard metal-cutting fixtures often don't work well for plastics. Custom fixtures may be needed for production runs, especially when tight tolerances are required.
Effective plastic workholding often includes:
- Larger contact areas to distribute clamping force
- Soft or compliant contact surfaces
- Support under thin sections
- Allowance for thermal expansion during the cut
- Minimal overhang to reduce deflection
Multiple Setups
Complex plastic parts may require more setups than equivalent metal parts because each setup must account for material flexibility and the need to support features being machined.
This can affect both cycle time and cost, particularly for low-volume or prototype work where custom fixturing may not be justified.
Dimensional Stability Over Time
Unlike metals, many plastics continue to change dimensionally after machining is complete.
Stress Relief
Plastic stock material often contains internal stresses from the manufacturing process, whether it was extruded, cast, or molded. Machining removes material and releases these stresses, which can cause the part to warp or change shape hours or days after machining.
Some shops pre-stress-relieve plastic stock by rough machining and allowing parts to stabilize before final machining. This adds time but improves dimensional consistency.
Moisture Absorption
Certain plastics, particularly nylons, absorb moisture from the air. As they absorb moisture, they swell. A nylon part machined to tight tolerances in a climate-controlled shop may grow slightly when exposed to humid conditions.
For critical applications, it's important to specify whether dimensions apply to the as-machined condition or after the part has reached equilibrium with its operating environment.
Temperature Sensitivity in Service
Plastic parts used in applications with temperature variation will expand and contract more than metal parts in the same environment. Designers need to account for this in tolerances and fits, especially for assemblies combining plastic and metal components.
Tolerance Considerations
The behaviors described above affect what tolerances are practical and economical when CNC machining plastic parts.
Achievable Tolerances
Tight tolerances are achievable with plastics, but they require more careful process control than metals. Tolerances in the range of ±0.002" to ±0.005" are common for many plastic machining applications. Tighter tolerances are possible but may require temperature-controlled environments, stress-relieved stock, and additional process steps.
Tolerance Specification
When specifying tolerances for plastic parts, consider:
- Which dimensions are truly critical to function
- Whether the part will be measured immediately after machining or after stabilization
- The operating temperature range of the application
- Environmental factors like humidity
- How the part interfaces with other components
Applying the tightest possible tolerance to every dimension increases cost without necessarily improving function. The drawing typically serves as the controlling specification, so clear communication of critical dimensions and tolerances helps the machine shop focus effort where it matters most.
Material Selection Impact
Not all plastics behave the same way. Material selection significantly affects machinability and dimensional stability.
Common Machinable Plastics
Some plastics machine more predictably than others:
- Acetal (Delrin): Excellent dimensional stability, low moisture absorption, machines cleanly
- PEEK: High-performance, stable, but expensive and requires careful speeds and feeds
- Polycarbonate: Tough and clear, but prone to stress cracking if not handled carefully
- Nylon: Strong and wear-resistant, but absorbs moisture and can be difficult to hold tight tolerances
- PTFE (Teflon): Very soft and flexible, requires specialized workholding and sharp tools
- UHMW: Extremely tough, but flexible and difficult to machine to tight tolerances
Each material has different thermal properties, flexibility, and stability characteristics. Specifying the right material for the application is as important as the machining process itself.
Providing Complete RFQ Information
When requesting quotes for CNC machined plastic parts, complete information helps the machine shop provide an accurate quote and identify potential issues early.
Essential RFQ Information
A complete request for quote should include:
- Engineering drawings or CAD files: STEP files are preferred when available, as they eliminate interpretation errors
- Material specification: Include grade and any special requirements like color or certifications
- Quantity: Both initial and potential ongoing volume
- Tolerances: General tolerances and any critical dimensions with tighter requirements
- Surface finish requirements: Specify if particular surfaces need specific finishes
- Required completion date: Realistic lead time expectations help with scheduling
- Inspection requirements: Note if dimensional reports or other documentation are needed
- Special notes: Any application-specific concerns, assembly requirements, or environmental conditions
Prototype Versus Production Considerations
The approach to machining plastic parts often differs between prototype and production quantities.
For smaller prototype quantities, the focus is typically on validating design and fit. Tolerances may be relaxed slightly, and standard workholding methods are often sufficient. The goal is quick turnaround to support the design iteration process.
Production quantities may justify investment in custom fixtures, pre-stressed stock, or process refinements that improve consistency and reduce cycle time. If a prototype will eventually move to production, mentioning this in the initial RFQ helps the shop consider manufacturing efficiency from the start.
RFQ Checklist for Plastic Parts
Before submitting your request for quote, verify you've included:
- Drawing or STEP file
- Plastic material type and grade
- Quantity needed
- Critical tolerances clearly marked
- Surface finish requirements
- Required delivery date
- Any special inspection or documentation needs
- Application notes if relevant to material selection or process
Request a Quote for Your Plastic Machining Project
If you have a project that requires CNC machining plastic parts, Anco Precision can review your requirements and provide a quote. We work with a range of engineering plastics and understand the process considerations that ensure dimensional accuracy.
Send us your drawing or STEP file when available, along with material specification, quantity, tolerances or critical requirements, and your required delivery date. We'll review your project and respond with pricing and lead time.
Contact Anco Precision to discuss your plastic machining needs.