The short answer
The glass transition (Tg) is the temperature range where the amorphous part of a plastic changes from rigid and glassy to rubbery or leathery. The melting point (Tm) is where crystals melt. Amorphous plastics such as polycarbonate have only a Tg (about 145–150°C / 293–302°F for PC) and soften gradually above it. Semi-crystalline plastics have both: nylon 66, for example, has a Tg of about 50–70°C (122–158°F) when dry and melts at 260–265°C (500–509°F).
These two temperatures explain why one resin turns brittle in the cold, why another sags in a hot car, and why mold temperature matters so much for some resins and not for others.
Definitions
Glass transition (Tg): below Tg, the chain segments in amorphous regions are frozen in place and the material is hard and glassy. Above it they can move, and stiffness falls, by a large factor in an unfilled amorphous resin. It is a range of several degrees rather than a sharp point, and it shifts with how fast the sample is heated or loaded.
Melting point (Tm): the temperature at which crystalline regions melt. Only semi-crystalline plastics have one. Above Tm the material is a viscous melt that can be molded. Amorphous resins have no Tm; they simply get softer and more fluid above Tg. For more on the two structures, see amorphous vs. semi-crystalline plastics.
How they are measured
Differential scanning calorimetry (DSC, ASTM D3418 or ISO 11357) heats a few milligrams of resin and records heat flow. Tg shows up as a step and Tm as a peak. Dynamic mechanical analysis (DMA) flexes a small bar while heating it and shows Tg as a drop in stiffness. DMA values, especially the tan delta peak, often read several degrees higher than DSC.
Reported numbers also depend on whether the onset or the midpoint of the step is quoted. PEEK is a good example: its Tg is 143°C (289°F) as an onset and roughly 147–150°C (297–302°F) as a midpoint. When two sources disagree by a few degrees, this is often why.
Typical Tg and Tm of common plastics
| Resin | Structure | Glass transition (Tg) | Melting point (Tm) |
|---|---|---|---|
| LDPE | Semi-crystalline | About −120°C / −184°F (reported values vary widely) | 105–115°C / 221–239°F |
| HDPE | Semi-crystalline | About −120°C / −184°F (reported values vary widely) | 125–135°C / 257–275°F |
| PP homopolymer | Semi-crystalline | About −20 to 0°C / −4 to 32°F | 160–165°C / 320–329°F |
| Acetal (POM) | Semi-crystalline | About −75 to −60°C / −103 to −76°F | About 165°C / 329°F copolymer; 175°C / 347°F homopolymer |
| Nylon 6 (PA6) | Semi-crystalline | About 40–60°C / 104–140°F dry; lower when moist | About 220°C / 428°F |
| Nylon 66 (PA66) | Semi-crystalline | About 50–70°C / 122–158°F dry; lower when moist | 260–265°C / 500–509°F |
| PBT | Semi-crystalline | About 40–60°C / 104–140°F | 220–230°C / 428–446°F |
| PET | Semi-crystalline (can be molded amorphous) | About 70–80°C / 158–176°F | 245–260°C / 473–500°F |
| PPS | Semi-crystalline | About 85–100°C / 185–212°F | 275–290°C / 527–554°F |
| PEEK | Semi-crystalline | About 143–150°C / 289–302°F | About 343°C / 649°F |
| PTFE | Semi-crystalline | Reported values disagree | About 327°C / 621°F (not melt-processed like other resins) |
| Polystyrene (GPPS) | Amorphous | About 90–100°C / 194–212°F | None |
| ABS | Amorphous | About 100–110°C / 212–230°F (SAN phase) | None |
| Acrylic (PMMA) | Amorphous | About 100–110°C / 212–230°F | None |
| Rigid PVC | Largely amorphous | About 75–85°C / 167–185°F | No sharp melting point |
| Polycarbonate (PC) | Amorphous | About 145–150°C / 293–302°F | None |
| Polysulfone (PSU) | Amorphous | About 185–190°C / 365–374°F | None |
| PEI | Amorphous | About 215–217°C / 419–423°F | None |
Typical literature and producer values, mostly by DSC; grades vary with molecular weight, comonomer, moisture and test method. Melting points match our material pages where they publish one. Check the data sheet for your grade.
How Tg affects stiffness and toughness
An amorphous resin is used below its Tg. As it approaches Tg it loses stiffness quickly, which is why the heat deflection temperature of polycarbonate, PSU and PEI sits somewhat below Tg. See HDT vs. continuous use temperature.
Semi-crystalline resins behave according to where Tg sits relative to room temperature:
- Tg below room temperature (PE, PP, acetal): the amorphous regions are rubbery in normal use, so the material is tough and somewhat flexible. Polyethylene stays tough far below freezing. PP homopolymer turns brittle as it approaches its Tg near 0°C (32°F), which is why impact copolymers exist.
- Tg above room temperature (PET, PPS, PEEK, dry nylon): the material is stiffer at room temperature and steps down in stiffness as it passes Tg. Glass fiber reduces that step.
Moisture lowers nylon's Tg
Nylon absorbs water from the air, and the water acts as a plasticizer. Dry nylon 6 has a Tg around 50°C (122°F). Conditioned at 50% relative humidity, it falls to around room temperature, and fully saturated nylon 6 can drop below 0°C (32°F). That is why conditioned nylon is tougher and more flexible than dry-as-molded nylon, and why many data sheets list both. See nylon and moisture.
What Tg and Tm mean for processing
Melt temperature: semi-crystalline resins are molded well above Tm, for example nylon 66 at about 275–305°C (527–581°F). Amorphous resins are molded far above Tg, for example polycarbonate at about 280–345°C (536–653°F).
Mold temperature: an amorphous part must cool below Tg to set, so the mold runs well below Tg. A semi-crystalline part needs time above Tg for crystals to grow. PP, PE and acetal have a Tg below room temperature, so they crystallize even in a cool mold. PET, PPS and PEEK have a high Tg, so they need hot molds; PEEK is molded at 170–200°C (338–392°F). A part molded too cold keeps crystallizing later and shrinks in service. Starting points are in processing parameters.
Annealing: heating a finished part to a temperature below Tg (amorphous) or between Tg and Tm (semi-crystalline) relieves molded-in stress and, for semi-crystalline parts, completes crystallization so dimensions stay put. Times and temperatures are grade-specific; follow the producer's guidance.
What to send us for a quote
- The part's lowest and highest service temperatures
- Whether it sees moisture, load or impact at those temperatures
- The grade you run now, or the resin family
- Quantity and ship-to city
We aim to send your quote within one to three business days. Unfamiliar terms are defined in the resin glossary.
Common questions
What is the difference between glass transition and melting point?
The glass transition (Tg) is where the amorphous part of a plastic changes from rigid and glassy to rubbery or leathery. The melting point (Tm) is where crystals melt and the material becomes a liquid. Only semi-crystalline plastics have a true melting point.
Does polycarbonate have a melting point?
No. Polycarbonate is amorphous, so it has a glass transition of about 145–150°C (293–302°F) and then softens gradually. It is molded at about 280–345°C (536–653°F).
Why does nylon's glass transition change?
Water absorbed from the air acts as a plasticizer and lowers nylon's Tg. Dry nylon 6 has a Tg around 50°C (122°F); conditioned at 50% relative humidity it falls to around room temperature, and fully saturated it can drop below 0°C (32°F).
Is Tg the maximum use temperature of a plastic?
For amorphous plastics, Tg is a hard ceiling for load-bearing use, and parts are normally used well below it. For semi-crystalline plastics, service above Tg is common, but stiffness is lower and creep is higher there; the practical limit depends on load, filler and time.
How are Tg and Tm measured?
Usually by differential scanning calorimetry (DSC, ASTM D3418 or ISO 11357), which shows Tg as a step and Tm as a peak in heat flow. Dynamic mechanical analysis (DMA) shows Tg as a drop in stiffness and often reads several degrees higher than DSC.
What is the melting point of polypropylene?
Typical PP homopolymer melts at about 160–165°C (320–329°F). Random and impact copolymers melt somewhat lower.
Tell us the part and its temperatures
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
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