CNC Machining Plastic Parts: How to Prevent Melting

CNC Machining Plastic Parts: How to Prevent Melting During Processing?

Plastic CNC machining looks easy — until your parts start melting, warping, or sticking to the tool.

If you’ve ever seen:

• edges turning glossy
• chips becoming gummy strings
• dimensions drifting out of tolerance
• or parts deforming after machining

you’re facing heat buildup, the #1 failure in CNC plastic parts.

After machining 2,000+ plastic components monthly (ABS, POM, Nylon, PEEK, Acrylic) for medical, electronics, and automation customers, we’ve tested dozens of parameters and tooling methods.

In this guide, I’ll share real shop-floor fixes, tested cutting data, and proven setups we use daily to stop plastic melting completely.

Why Do Plastic Parts Melt in CNC Machining?

Unlike metal, plastic doesn’t cut — it softens first.

Most engineering plastics have:

• low melting point (120–220°C)
• poor heat dissipation
• high friction coefficient

So heat stays at the cutting zone.

3 Root Causes We See in Production

In 80% of customer-sent defective samples, the issue was wrong cutting parameters, not material.

Step-by-Step: How We Prevent Melting

Step 1 – Use Sharp Tools Designed for Plastic

Standard metal tools = wrong geometry.

We use:

• ✔ Single-flute or 2-flute end mills
• ✔ Polished flutes
• ✔ Large rake angle
• ✔ DLC or TiB2 coating

Why?

Fewer flutes = larger chip space = less friction.

Factory Result:

Switching from 4-flute to 1-flute tools reduced surface temperature 38% (infrared test).

Step 2 – Increase Feed Rate, Don’t Slow Down

Many beginners slow feed to “protect" plastic.

This actually causes melting.

Correct logic:
Cut fast enough to form chips immediately

Our proven parameter range:

These settings reduced part deformation rate from 12% → under 2% in our workshop.

Step 3 – Use Air Blast Instead of Flood Coolant

Flood coolant can:

• swell plastics
• cause cracking
• leave stains

We prefer:

• ✔ compressed air
• ✔ vortex cold air gun
• ✔ light mist only for PEEK/PTFE

Air removes chips + cools surface simultaneously.

Chip evacuation = 50% of heat control.

Step 4 – Reduce Depth of Cut

Deep passes trap heat inside material.

Better strategy:

• shallow cuts
• multiple passes

Example from production:

Before:
DOC = 3mm → melting edges

After:
DOC = 0.8–1.2mm → perfect finish

Cycle time increased only 6%, but scrap dropped 70%.

Material-Specific Machining Tips

ABS

• Easy to melt
• Keep high feed
• Avoid tool dwell

Acrylic (PMMA)

• Cracks easily
• Use O-flute only
• Never pause spindle

Nylon (PA)

• Absorbs moisture
• Bake at 70°C for 4h before machining

POM (Delrin)

• Best machining plastic
• Stable, low friction
• Ideal for precision gears

PEEK

• Expensive material
• Mist cooling mandatory
• Lower speed, higher torque

Common Problems & Quick Fix Table

Real Case Study

Customer: Medical housing (ABS)

Problem:
Edges melted, tolerance ±0.15mm (requirement ±0.05mm)

Our adjustments:

• 4-flute → 1-flute O-tool
• Feed 1200 → 4200 mm/min
• Added air blast
• DOC 2.5 → 1mm

Result:

• tolerance ±0.03mm
• cycle time -18%
• zero scrap after 500 pcs

FAQ

Why does plastic melt during CNC machining?

Because low heat conductivity causes friction heat to accumulate faster than it dissipates.

Is coolant necessary for plastic machining?

Usually no. Air blast is better. Flood coolant may damage material.

What flute count is best?

1–2 flute tools provide better chip evacuation and lower heat.

Which plastic machines best?

POM (Delrin) is the most stable and easiest.