Plastic Melting Point: Why Materials Melt and Soften at Different Temperatures

In practice, a plastic part may soften, warp, or lose strength before reaching its plastic melting point. This happens because melting point, glass transition, softening temperature, and processing temperature describe different thermal behaviors. This guide explains these values and how they affect material selection and injection molding.

Plastic Melting Point Depends on Material Structure

There is no single melting point of plastic. Different polymer structures respond to heat in different ways.

Semi-crystalline thermoplastic materials such as PP, PE, PA, POM, and PEEK contain ordered crystalline regions and usually have a measurable melting temperature, known as Tm.

Amorphous plastics such as ABS, PC, PS, and PMMA do not have one sharp melting point. Instead, they gradually soften as their amorphous structure passes through the glass transition temperature, or Tg. Semi-crystalline plastics also have a Tg, but their Tm is usually more relevant when discussing melting.

The thermoset vs thermoplastic difference becomes especially clear after curing: thermosets do not remelt and will eventually degrade under excessive heat.

Material Type	Heating Behavior	Useful Thermal Value
Semi-crystalline	Shows a clearer melting transition	Tm, with Tg also relevant
Amorphous	Softens over a temperature range	Tg, Vicat, or HDT
Thermoset	Does not remelt after curing	Service or degradation limit

Plastic Melting Point Chart

The following plastic melting point chart provides typical reference data for commonly used materials. These figures are starting points only because resin grades, additives, fillers, test methods, and supplier formulations vary.

For amorphous plastics, the table shows Tg or a typical softening reference instead of a physical melting point. Processing temperature is listed separately.

Plastic	Structure	Thermal Value	Typical °C	Typical °F	Typical Melt Processing Range
LDPE	Semi-crystalline	Tm	105–115	221–239	160–220°C
HDPE	Semi-crystalline	Tm	125–135	257–275	180–250°C
PP	Semi-crystalline	Tm	155–165	311–329	200–260°C
Rigid PVC	Amorphous, heat-sensitive	Tg or Vicat	About 75–90	About 167–194	About 175–200°C
PS	Amorphous	Tg	95–105	203–221	180–260°C
ABS	Amorphous, multiphase	Main high-temperature Tg	About 100–110	About 212–230	220–260°C
POM-C	Semi-crystalline copolymer	Tm	About 165	About 329	About 190–210°C
POM-H / Delrin	Semi-crystalline homopolymer	Tm	About 177–178	About 351–352	About 210–220°C
PA6	Semi-crystalline	Tm	About 220	About 428	240–270°C
PA66	Semi-crystalline	Tm	About 260	About 500	270–290°C
PC	Amorphous	Tg	145–150	293–302	270–320°C
PMMA	Amorphous	Tg	About 100–120	About 212–248	220–270°C
PET	Semi-crystalline	Tm	245–260	473–500	260–295°C
PBT	Semi-crystalline	Tm	220–230	428–446	240–280°C
PLA	Grade-dependent, usually semi-crystalline	Tm	150–180	302–356	About 180–220°C
PTFE	Semi-crystalline	Tm	About 327	About 621	Not normally injection molded
PPS	Semi-crystalline	Tm	280–285	536–545	About 315–343°C
PEEK	Semi-crystalline	Tm	About 343	About 649	380–400°C

These values explain why a table may show both a melting temperature and a higher processing range. A polymer must normally flow well enough to fill the mold, so the recommended melt temperature may be above its physical Tm.

Always confirm the selected grade with the resin supplier’s technical data sheet. Fillers, copolymers, flame retardants, plasticizers, and other formulation changes can alter both processing and thermal performance.

What Happens at 40°C, 100°C, or 150°C?

At 40°C, melting isn’t really a concern for most plastics—though softer grades might creep or deform if they’re under load. At 100°C, it depends: some plastics hold up fine, while others start to soften, lose stiffness, or warp. And a part can become unusable well before the material actually melts. At 150°C, many commodity plastics are simply out of their depth, so heat-resistant plastic materials may need to be considered for sustained exposure. Some engineering plastics can still handle it, but you also need to consider exposure time, load, grade, moisture, and any chemicals in contact.

Melting Point Is Not Processing Temperature

A physical melting point describes a thermal transition within the polymer structure. A processing temperature describes the conditions needed to move the resin through an injection molding machine.

Using the two terms interchangeably can lead to unsuitable material choices or incorrect molding settings.

Melt and Mold Temperature

Melt temperature is the temperature of the plastic entering the mold. It must be high enough to provide suitable flow without causing excessive degradation.

In mold temperature control for injection molding, the mold surface temperature affects cooling, surface replication, crystallization, shrinkage, warpage, and cycle time, but it is not the material’s melting point.

Tg, HDT, and Service Temperature

Tg describes the glass transition of the amorphous regions within a polymer. It is especially important for amorphous plastics, although semi-crystalline plastics also have a Tg in addition to their melting temperature.

Vicat measures softening under specified test conditions, while HDT measures deformation under a defined load.

Service temperature describes how a material performs during actual use. A part may creep, warp, or lose stiffness long before it melts. For demanding applications, compare Tg, HDT, operating temperature, load, and chemical exposure rather than relying on Tm alone.

What Changes Plastic Melting Behavior?

The material name on a drawing is not a complete specification. Two grades from the same polymer family may have different flow behavior, crystallinity, additives, and thermal performance.

Resin Grade and Crystallinity

Homopolymers, copolymers, molecular weight, branching, and cooling history can all affect thermal behavior.

Semi-crystalline materials may also show different melting ranges depending on their chemical structure and degree of crystallinity. This is why PP, PE, PA, PET, and POM should not be assigned one universal temperature.

Grade-specific data is more reliable than a generic material-family value.

Additives and Material Condition

Glass fiber, mineral filler, impact modifiers, plasticizers, flame retardants, stabilizers, colorants, and recycled content can change stiffness, flow, shrinkage, crystallization, and heat deformation.

Moisture also affects processing. Hygroscopic materials such as PA, PC, PET, PBT, and PLA generally require proper drying. Incorrect material preparation can cause defects even when the machine temperatures appear suitable.

Why Plastic Melting Point Matters in Injection Molding

Understanding thermal behavior helps engineers establish a suitable processing window, but temperature cannot be considered separately from part design, tooling, and molding conditions.

Material Flow and Mold Filling

Melt temperature affects viscosity and the resin’s ability to fill runners, gates, thin walls, and detailed features.

If the temperature is too low, filling pressure may rise and short shots, flow marks, or weak weld lines may become more likely. Gate design, venting, injection speed, pressure, and wall thickness must also be considered.

Material Degradation and Appearance

Excessive temperature or residence time can cause discoloration, black specks, gas, odor, surface defects, or loss of material properties.

Heat-sensitive materials require tighter processing control; PVC injection molding in particular can have a relatively narrow processing window, while POM can also degrade if overheated.

Cooling and Dimensional Control

Mold temperature and cooling rate affect shrinkage, warpage in injection molded parts, dimensional stability, and cycle time.

The effect is especially important for semi-crystalline plastics because their crystal structure continues to develop as the material cools. Uneven cooling can therefore lead to inconsistent shrinkage across the part.

Equipment and Mold Requirements

High-temperature polymers may require more capable heaters, suitable screw and barrel components, controlled hot runners, insulation, and higher mold temperatures.

These requirements should be reviewed before tooling and molding equipment are selected. Temperature is only one possible cause of molding problems because material condition, part design, tooling, and process settings all work together.

Choosing Plastic for Heat Exposure

The melting point of common plastics is useful for early material screening, but it should not be the only selection criterion.

Confirm the following conditions:

1. Continuous operating temperature
2. Short-term temperature peaks
3. Mechanical load at elevated temperature
4. Contact with steam, oil, water, or chemicals
5. Dimensional stability requirements
6. Flammability and electrical requirements
7. Molding and drying conditions
8. Material cost and availability

A high melting point does not automatically mean better performance in every hot environment. Creep, moisture absorption, chemical resistance, impact, wall thickness, and assembly stress may matter more.

A structured injection molding material selection process can start with the polymer family, then compare actual commercial grades against supplier data and expected operating conditions.

FAQs

Will plastic melt at 100°C?

Not all plastics will—some soften, creep, or warp around that temperature, while others hold up just fine. Also, a part can fail by losing stiffness or dimensional stability long before it actually melts. So beyond temperature, you should also consider heat deflection, mechanical load, and how long the part is exposed to heat.

Is Melt Temperature the Same as Mold Temperature?

No. Melt temperature refers to the plastic entering the mold. Mold temperature refers to the cavity surface that shapes and cools the part. They affect flow, cooling, crystallization, surface quality, shrinkage, and cycle time in different ways.

Can Thermoset Plastic Be Melted Again?

No. After a thermoset has fully cured, its cross-linked structure cannot be melted and reshaped like a thermoplastic. Continued heating eventually causes scorching, decomposition, or chemical degradation rather than useful material flow.

Conclusion

Plastic melting point depends on polymer structure, formulation, and grade. Semi-crystalline plastics normally have a measurable Tm, while amorphous plastics soften over a temperature range. Melting point, molding temperature, and service temperature should not be treated as the same value.

A temperature chart is useful for an initial comparison, but the final choice should be based on the supplier’s technical data and the actual operating environment.

For projects where thermal behavior affects material or tooling decisions, HingTung provides plastic injection molding services covering material review, DFM, mold development, trial molding, production, and quality control.