Top Defects in CNC Machined Plastic Parts and Fixes

If you regularly source CNC machined plastic parts, you’ve probably faced this situation:

You receive parts that look perfect on drawings — but in real use they crack, warp, shrink, or fail tolerance checks.

We machine over 120,000+ plastic components per month in our shop (ABS, POM, Nylon, PEEK, PTFE, PC), and more than 70% of quality complaints come from only a handful of recurring defects.

The good news?

Almost all plastic machining defects are predictable and preventable — if you understand the root cause.

In this guide, I’ll share:

• Real factory defects we see weekly

• Measured test data from production

• Proven fixes we use on our CNC floor

• A buyer-friendly troubleshooting checklist

So you can reduce scrap, avoid rework, and lower cost per part.

This is the #1 defect in CNC machined plastic components.

Material: POM (Delrin)
Plate size: 300 * 250 * 15 mm
After finishing:
Flatness changed 0.05 mm → 0.42 mm within 24 hours

Customer rejected the batch.

• Internal stress from extrusion sheets

• Heat buildup during milling

• Uneven material removal

• Thin wall design (<2 mm)

✅ Rough → rest → finish machining
(leave 0.3–0.5 mm stock, rest 8–12 hours)

✅ Symmetrical cutting on both sides
✅ Use sharp polished tools
✅ Reduce spindle speed 20–30%
✅ Prefer cast sheets over extruded

For precision parts, we now pre-anneal plastic plates at 80–120°C for 2–4 hours, reducing deformation by ≈60%.

If edges look glossy or smeared, the material is melting, not cutting.

• ABS

• Acrylic (PMMA)

• Nylon

• Polycarbonate

Plastic has low thermal conductivity, so heat accumulates quickly.

Unlike metal, it softens before chips separate.

✅ Lower spindle speed
✅ Increase feed rate
✅ Use single-flute O-flute cutters
✅ Add air blast or mist cooling
✅ Avoid dull tools

Switching to O-flute cutters improved surface finish Ra from 3.2 → 0.8 μm in our tests.

This problem frustrates many buyers.

Parts measure OK today — tomorrow they shrink or expand.

Nylon example:

• Dry size: 50.00 mm

• After 48h humidity: 50.18 mm

That’s +0.36% growth.

Plastic expands 5–10* more than aluminum.

✅ Pre-dry hygroscopic materials

• Nylon: 80°C for 4–6 hours

✅ Machine in temperature-controlled room
(23 ± 2°C)

✅ Design larger tolerances for plastic
Avoid ±0.01 mm — unrealistic

Always ask your supplier:

“Do you pre-dry nylon/POM before machining?"

If not, expect instability.

Usually tooling related.

• Standard metal end mills

• Too small chip load

• Dull edges

Plastic needs cutting, not rubbing.

✅ Polished carbide or diamond-coated tools
✅ Larger chip load
✅ Fewer flutes
✅ Climb milling only

Seen often in:

• PC

• PMMA

• ABS

• Sharp corners

• No fillets

• Overtightening screws

• Stress from machining

✅ Add R ≥ 0.5–1 mm fillet
✅ Increase hole diameter tolerance
✅ Use brass inserts
✅ Avoid self-tapping screws

After adding inserts, one customer reduced cracks from 18% → <1%.