Plastic Melting Point Chart: A Guide for CNC Machining Materials

When you’re machining plastic parts, few things are as frustrating as watching a perfect surface turn to a gummy mess because the material melted on the cutter. I’ve seen it happen more times than I care to count – often because someone chose speeds and feeds based only on a melting point number they found online. The truth is, melting point is important, but it’s only one piece of the puzzle. In this guide, I’ll walk you through what melting point really means for CNC machining, give you a practical chart you can use, and share the real-world thermal limits that matter on the shop floor.

At PlasticCNCPro, we machine dozens of different thermoplastics every week – from common ABS and nylon to high-performance PEEK and PEI. Understanding how heat affects each material is the difference between a good part and a reject. Let’s dive in.

What Is the Melting Point of Plastic?

Definition: Solid to Liquid – But It’s Not That Simple

Melting point is the temperature at which a crystalline thermoplastic transitions from a solid to a liquid. For semi-crystalline plastics like nylon or POM, this is a sharp, well-defined temperature. But for amorphous plastics like ABS or polycarbonate, there is no true melting point – instead, they gradually soften as they pass through their glass transition temperature (Tg). That’s why you’ll see “melting range” or “processing temperature” for many plastics rather than a single number.

Thermoset vs. Thermoplastic: Why Machining Works for One but Not the Other

Thermosets are cross-linked polymers that, once cured, cannot be re-melted. If you heat a thermoset too much during machining, it doesn’t melt – it degrades, chars, or burns. That’s why CNC machining of thermosets is rare and tricky. Thermoplastics, on the other hand, can be melted and re-solidified multiple times. This makes them ideal for machining: you can remove material without the risk of permanent chemical change (as long as you stay below degradation temperature). Nearly everything we machine at PlasticCNCPro is a thermoplastic.

Semi-Crystalline vs. Amorphous: What Matters for Machinability

• Semi-crystalline plastics (e.g., nylon, POM, PEEK) have a sharp melting point. They also have a higher latent heat of fusion, meaning they absorb more energy before melting. This makes them more forgiving during machining – you can push a little harder before things go wrong. But once you cross that melting point, chips can weld to the tool instantly.
• Amorphous plastics (e.g., ABS, PC, PMMA) have no sharp melt. They soften gradually above Tg. This softening can cause the material to deflect under tool pressure, leading to poor surface finish and dimensional drift. For these materials, controlling the cutting temperature below Tg is critical.

Plastic Melting Point Chart for CNC Machining

Below is a chart of plastics we commonly machine. I’ve included melting point (or range), glass transition temperature (Tg), heat deflection temperature (HDT at 1.82 MPa), and a recommended maximum cutting temperature when machining without coolant.

| Plastic | Melting Point (°C) | Tg (°C) | HDT @ 1.82 MPa (°C) | Max Cutting Temp (°C, no coolant) |

|--------|-------------------|---------|---------------------|-----------------------------------|

| ABS | None (amorphous) | 105 | 95 | 85 |

| Nylon 6/6 (PA66) | 265 | 50 | 90 | 80 |

| POM (Acetal) | 175 | -60 | 110 | 100 |

| Polycarbonate (PC) | None (amorphous) | 147 | 132 | 120 |

| PEEK | 343 | 143 | 150 | 140 |

| PTFE (Teflon) | 327 | -97 | 55 | 50 |

| Polyethylene (HDPE) | 130 | -110 | 45 | 40 |

| Polypropylene (PP) | 165 | -10 | 55 | 50 |

| PVC (rigid) | None (amorphous) | 80 | 70 | 65 |

| PMMA (Acrylic) | None (amorphous) | 105 | 95 | 85 |

| PET | 255 | 70 | 75 | 70 |

| PEI (Ultem) | None (amorphous) | 217 | 198 | 180 |

| PPS | 285 | 90 | 135 | 120 |

| PSU | None (amorphous) | 187 | 174 | 160 |

| PVDF | 177 | -40 | 110 | 100 |

How to Read the Chart

Notice that HDT is always much lower than melting point for semi-crystalline materials. For example, Nylon 6/6 melts at 265°C, but under a load of 1.82 MPa, it softens at only 90°C. When you’re cutting nylon, the tool pressure can cause local deformation long before the bulk material reaches melting temperature. Always use HDT as your upper temperature limit for machining, not the melting point.

For amorphous plastics, Tg is the key number. Once you exceed Tg, the plastic becomes rubbery and dimensionally unstable. Keep cutting temperature at least 10–20°C below Tg for best results.

Why Melting Point Matters in CNC Machining

Thermal Expansion During Cutting

Every cut generates frictional heat. When that heat raises the material temperature close to its melting point (or Tg for amorphous plastics), the plastic softens locally. Soft plastic sticks to the tool, causing the infamous “built-up edge” that ruins surface finish and can break the cutter. I’ve machined plenty of HDPE parts where a dull tool would literally melt the chip into a string that wrapped around the spindle. The fix was always sharper tooling and lower feed rates.

Heat Generation and Tool Selection

Low-melting-point plastics like polyethylene and polypropylene require sharp, polished tools with positive rake angles. A dull tool generates more heat and will quickly exceed the material’s softening point. For high-melting-point plastics like PEEK or PEI, you can run higher spindle speeds because the material can tolerate more heat, but you still need effective chip evacuation to prevent recutting of hot chips.

Effect on Dimensional Accuracy

When a part gets hot during machining, it expands. If it then cools unevenly (especially with thin walls or complex geometries), it warps. This is a major issue with plastics that have high coefficients of thermal expansion, like nylon or polypropylene. By keeping the part below its HDT during cutting, you minimize thermal expansion and get more stable dimensions.

Chip Formation and Surface Finish

Plastics with low melting points tend to produce long, stringy, gummy chips (especially PE, PP, and some nylons). These chips can wrap around the tool and damage the surface. High-melt plastics like PEEK or PPS chip more like metals – short, discontinuous chips that are easy to clear. Understanding chip behavior helps you choose the right tool path and chip-breaking strategies.

Key Factors That Influence Plastic Melting Point in Machining Context

Internal Factors

• Molecular weight: Higher molecular weight usually raises melting point and melt viscosity.
• Crystallinity: More crystalline regions mean a sharper, higher melting point. A highly crystalline POM will melt at 175°C, while a less crystalline batch might soften earlier.
• Additives: Fillers like glass fiber or carbon fiber increase the effective melting point and heat resistance. Plasticizers lower it. Flame retardants can change thermal degradation behavior.

External Factors

• Cooling rate from previous processing: A rapidly cooled injection-molded part may have lower crystallinity and a different melting response than a slowly cooled one.
• Moisture content: Nylons absorb water, which acts as a plasticizer, lowering the melting point and Tg. Always dry nylon before machining.
• Thermal history: Repeated heating and cooling can change the crystalline structure. Machining itself can induce local annealing, altering the material’s response.

Processing History

Injection-molded parts often have a “skin-core” morphology – the surface cools quickly (less crystalline), while the core cools slowly (more crystalline). When you machine away the skin, you expose the core, which may have different melting behavior. This is one reason prototype machined parts sometimes behave differently than production parts.

Heat Deflection Temperature (HDT) vs. Melting Point

Why HDT Is More Critical for CNC

Melting point tells you when the material becomes liquid. HDT tells you when it begins to soften under load – which is exactly what happens when a cutting tool pushes against the material. If the part temperature exceeds HDT, the plastic can deform, deflect, or even collapse under tool pressure. For CNC, HDT is far more relevant than melting point.

Comparison Table

| Plastic | Melting Point (°C) | HDT @ 1.82 MPa (°C) |

|--------|-------------------|---------------------|

| Nylon 6/6 | 265 | 90 |

| POM | 175 | 110 |

| PEEK | 343 | 150 |

| HDPE | 130 | 45 |

| PVC (rigid) | None | 70 |

Notice the huge gap for nylon. At 100°C, nylon is still 165°C below its melting point, but it’s already 10°C above its HDT. That means the part will deform under tool load even though it’s nowhere near melting.

Practical Advice

Use the HDT value from the chart as your absolute upper limit for cutting temperature. If you cannot keep the part below HDT with coolant (flood, mist, or air blast), you must reduce feeds and speeds to lower the heat generation. For amorphous plastics, use Tg as your limit.

Best Practices for Machining Plastics Based on Melting Point

Coolant Strategy

• Low-melting-point plastics (melting <150°C): Use mist or air blast. Flood coolant can cause thermal shock, leading to cracking or swelling (especially with nylon). Air blast also helps evacuate gummy chips.
• High-melting-point plastics (>250°C): Flood coolant is acceptable as long as it’s chemically compatible (no water-reactive plastics like some filled grades). For PEEK, PEI, and PPS, flood coolant significantly extends tool life.

Feeds and Speeds Guidelines

Keep the cutting temperature 30–50°C below the melting point (or below Tg for amorphous). General starting parameters:

| Plastic Group | Surface Speed (SFM) | Feed per Tooth (in/tooth) |

|--------------|-------------------|--------------------------|

| Polyolefins (PE, PP) | 100–200 | 0.002–0.005 |

| Engineering plastics (Nylon, POM, PC) | 200–400 | 0.003–0.008 |

| High-temp plastics (PEEK, PEI, PPS) | 300–500 | 0.004–0.010 |

These are starting points – always adjust based on chip color (white/clear chips indicate cool cutting; yellow/brown chips mean too hot).

Tool Geometry

• Use positive rake angles (10–20°) to shear rather than push the material.
• Polished flutes reduce friction and heat buildup.
• For low-melt plastics, single-flute or two-flute tools are best to avoid chip packing.
• For abrasive high-melt plastics (glass-filled PEEK, PEI), use carbide or PCD tools. HSS tools dull quickly.

Clamping and Fixturing

Plastics expand roughly 10 times more than metals per degree Celsius. Account for thermal expansion in your fixture design. For high-melt plastics, consider pre-heating the part to a temperature near the expected cutting temperature (but still below HDT) to reduce thermal shock and warpage.

High-Temperature Plastics for Advanced CNC Applications

Overview

The demand for high-temperature plastics in aerospace, medical, and electronics is growing fast. These materials can withstand higher machining heat without softening, but they bring their own challenges.

• PEEK: Melts at 343°C, HDT ~150°C. Excellent chemical resistance and mechanical strength. Requires sharp tools and sufficient cooling. We often run PEEK at 400–500 SFM with flood coolant.
• PEI (Ultem): Amorphous, Tg 217°C, HDT 198°C. Machines nicely with polished carbide tools. Very stable dimensions.
• PPS: Melts at 285°C, HDT 135°C. Often glass-filled, which is abrasive. Use PCD inserts for longer tool life.
• PTFE: Melts at 327°C but softens much earlier (HDT 55°C). Extremely difficult to machine – chips are soft and stringy, and the material deforms easily. We use very sharp tools, low feeds, and a lot of air blast.

Machining Challenges

High thermal stability doesn’t mean easy machining. These plastics often have high melting temperatures, but they also have high melt viscosity, meaning the chips can be tough and abrasive. Always use sharp tools, avoid dry machining without chip evacuation, and monitor tool wear closely.

Trends

• Recycled high-temp thermoplastics: Increasingly available (e.g., recycled PEEK), but may have lower consistency. Test scrap first.
• Carbon-fiber-reinforced versions: Extremely abrasive – require diamond-coated tools.
• Antimicrobial grades: For medical and food contact parts – often have slightly different thermal properties due to additives.

Frequently Asked Questions

What is the melting point of plastic?

“Plastic” covers a huge range of materials. Melting points vary from about 100°C for LDPE to over 340°C for PEEK. The chart in this article gives you the most common ones for CNC machining.

What temperature will plastic melt?

It depends on the plastic. But for CNC, you rarely reach the melting point – you first hit the softening point (Tg or HDT). For example, ABS softens around 105°C; PEEK softens around 143°C.

What is the melting point of PVC?

PVC is amorphous – it has no sharp melting point. Its processing temperature is about 160–210°C, but during machining, you must stay below 90°C to avoid thermal degradation and release of acidic gases.

How does plastic softening relate to its melting point?

For amorphous plastics, softening begins at Tg. For semi-crystalline plastics, softening begins near the melting point, but stiffness drops significantly at HDT, which is much lower than the melting point.

Will plastic melt at 95 degrees?

Some plastics will. LDPE melts at 105°C, so at 95°C it will be very soft but not fully liquid. Others like nylon or PEEK remain completely solid at 95°C. Use the chart to check.

Where can I find a comprehensive chart of melting points for different plastics?

The chart in this article is a good starting point for CNC machining. For more detailed thermal data, refer to material data sheets from resin suppliers or standards like ASTM D3418.

What factors influence the melting point of plastics?

Internal factors: molecular weight, crystallinity, additives. External factors: cooling rate, moisture, thermal history. All of these can shift the effective melting point by 10–20°C.

Why do plastics have relatively low melting points compared to metals?

Plastics are held together by weaker van der Waals forces and hydrogen bonds between polymer chains, whereas metals have strong metallic bonds. That difference makes plastics melt at much lower temperatures.

Conclusion

Melting point is an essential property for understanding plastic behavior, but it’s not the number you should chase on the CNC machine. For practical machining, heat deflection temperature and glass transition temperature are far more important. They tell you when the material will soften, deform, or stick to your tool.

Use the chart in this article as a quick reference, but always test on scrap material first. Every batch of plastic can behave a little differently. If you have a challenging application or need to machine a plastic you haven’t worked with before, reach out. We at PlasticCNCPro have machined hundreds of different plastic grades and can help you select the right material, feeds, speeds, and tooling.

Send us your drawing, 3D file, or project requirements. We’ll review the design, material, and tolerances, and give you a quotation with production recommendations. Whether you need a prototype or repeat production, we’re here to make custom plastic CNC machining simpler and more dependable.