Understanding Material Behavior in CNC Machined Plastic Parts

When a CNC machine starts cutting plastic, the sound is different from metal—softer, sometimes squeaky, and far less forgiving.
We’ve seen many plastic parts fail not because of poor machining accuracy, but because material behavior was misunderstood at the design or process stage. If you are sourcing or engineering CNC machined plastic parts, understanding how plastics actually behave under cutting forces, heat, and stress is essential.

In this article, I’ll break down real machining behavior of common CNC plastics, based on shop-floor experience and measured outcomes—not theory alone.

Why Material Behavior Matters in CNC Machined Plastic Parts

Plastic is not “easy metal.”
Unlike aluminum or steel, plastics respond dramatically to:

• Cutting temperature

• Tool geometry

• Internal stress release

• Moisture absorption

In our production tests, identical CNC programs produced dimensional deviations of up to 0.15 mm simply by switching from Delrin (POM) to Nylon 6. That difference alone can cause assembly failure in precision housings.

Understanding material behavior allows you to:

• Reduce scrap rates

• Improve dimensional stability

• Select the right plastic for the real application, not just datasheets

Common CNC Plastics and Their Machining Characteristics

POM (Delrin / Acetal): Dimensional Stability First

POM is often our default choice for precision plastic CNC parts.

Observed behavior in machining:

• Low thermal expansion

• Minimal burr formation

• Excellent surface finish without polishing

Measured results (internal tests):

• Flatness deviation typically ≤ 0.03 mm on 150 mm parts

• Tool wear is low, even in long runs

Best for: gears, bushings, precision mechanical parts
Watch out: internal stress in low-quality extruded stock

Nylon (PA6 / PA66): Tough but Unstable

Nylon is strong and impact-resistant, but behavior changes with environment.

Real-world issues we encounter:

• Absorbs moisture (up to 2–3%)

• Post-machining size growth over 24–72 hours

• Tends to “spring back” after cutting

In one enclosure project, a Nylon PA6 part grew 0.08 mm in bore diameter after humidity exposure—enough to loosen a press-fit bearing.

Best for: load-bearing parts, wear surfaces
Solution: machine oversize and condition before final finishing

ABS: Easy to Cut, Hard to Control Heat

ABS machines smoothly but softens quickly.

Machining observations:

• Chips melt if feed is too slow

• Sharp tools are mandatory

• Surface finish degrades with heat buildup

We typically increase feed rates by 15–20% compared to POM to prevent edge melting.

Best for: housings, prototypes
Limitation: lower mechanical strength

PTFE (Teflon): Low Friction, Low Rigidity

PTFE behaves more like soft wax than engineering plastic.

What actually happens in CNC machining:

• Severe deformation under clamping force

• Threads tend to relax after machining

• Requires extremely sharp tools and light passes

Best practice: use custom fixtures and allow post-machining relaxation time before inspection.

How CNC Cutting Parameters Affect Plastic Behavior

Heat Generation Is the Real Enemy

Plastics don’t dissipate heat like metals.
We’ve measured surface temperatures exceeding 90°C during aggressive milling—enough to permanently deform ABS and Nylon.

Practical solutions we use:

• High spindle speed + high feed (chip evacuation)

• Polished carbide tools

• Air blast instead of flood coolant (for most plastics)

Tool Geometry Makes or Breaks Plastic Parts

For plastic CNC machining, we never reuse metal-optimized tools.

Preferred tool features:

• Sharp cutting edge (no honed edge)

• High rake angle

• Fewer flutes (1–2 flutes for soft plastics)

This alone reduces burrs by over 60% in our production runs.

Design Tips to Improve CNC Machined Plastic Part Performance

If you’re still in the design phase, these adjustments save cost later:

• Avoid thin walls (<1.5 mm) in Nylon or ABS

• Add fillets to reduce stress concentration

• Use through-holes instead of blind holes where possible

• Allow ±0.05–0.1 mm tolerance for moisture-sensitive plastics

We often see redesigns reduce machining time by 18–25% with no functional compromise.

Choosing the Right Plastic Based on Application (Quick Guide)