Understanding Plastic Melting Temperature for Better Manufacturing
Understanding Plastic Melting Temperature
In polymer science and plastics engineering, melting temperature (Tm) plays a pivotal role in how a material behaves under heat, particularly during forming processes such as injection molding, extrusion, or thermoforming. However, the term “melting temperature” doesn’t apply equally across all plastics — especially when distinguishing between amorphous and semi-crystalline materials.
Melting Temperature (Tm) vs. Glass Transition Temperature (Tg)
- Melting Temperature (Tm) refers to the point at which a semi-crystalline plastic transitions from a solid crystalline structure into a viscous or fluid state. This is a sharply defined temperature, much like melting ice into water.
- Glass Transition Temperature (Tg), on the other hand, applies to amorphous plastics. At Tg, the polymer doesn’t melt but instead transitions from a rigid, glassy state to a rubbery or leathery one. The molecular chains gain enough mobility to deform more easily, although the material remains solid in appearance.
Example:
ABS plastic does not have a clear Tm because it is amorphous. Instead, it has a Tg of approximately 105°C, beyond which it softens considerably. Nylon 6, a semi-crystalline polymer, has a well-defined Tm of around 220°C.
Why Melting Point Matters
Understanding the correct thermal behavior of a plastic is essential for:
- Mold Design and Tooling: Tool temperatures must be calibrated to align with Tm or Tg to ensure proper flow, fill, and cooling.
- Dimensional Stability: Parts used near or above Tg may warp, creep, or lose shape over time.
- Post-Processing and Recycling: The ability to reprocess plastics depends on achieving consistent melting without degrading the polymer chains.
Ignoring these thresholds can lead to incomplete molding, voids, poor surface finish, or even burnt material due to thermal degradation.
Amorphous vs. Semi-Crystalline Plastics
| Property | Amorphous Plastics | Semi-Crystalline Plastics |
|---|---|---|
| Melting Behavior | No defined Tm; softens at Tg | Sharp Tm |
| Transparency | Usually transparent | Often opaque |
| Dimensional Stability | Better at lower temperatures | Better at higher temperatures |
| Examples | ABS, PC, PS, PVC | Nylon, PEEK, HDPE, PP |
| Processing Ease | Easier to thermoform | Requires controlled cooling |
Suggested Illustration: Thermal profile chart showing Tg and Tm zones for different polymer types.
Melting Temperatures of Common Plastics
The following table summarizes the typical melting or softening points for various common plastics.
| Material | Melting Temp. (°C) | Type | Notes |
|---|---|---|---|
| ABS | ~105 (Tg) | Amorphous | No sharp melting point; easy to shape |
| PLA | 150–160 | Semi-crystalline | Biodegradable; brittle under high heat |
| HDPE | 120–130 | Semi-crystalline | Excellent chemical resistance |
| Nylon 6 | ~220 | Semi-crystalline | Absorbs moisture; requires drying |
| Polycarbonate (PC) | ~145 (Tg) | Amorphous | Strong and tough; Tg is critical for molding |
| PEEK | ~343 | Semi-crystalline | High-performance; used in aerospace, medical |
Tip: These values may vary slightly depending on formulation, supplier, and the presence of additives. Always refer to datasheets from resin manufacturers when precision is required.
Factors That Affect Plastic Melting and Processing
Even with published melting or transition temperatures, actual processing results may vary. Several factors can shift or influence how a plastic behaves thermally during production.
Additives and Fillers
Common additives like flame retardants, UV stabilizers, plasticizers, and colorants can alter the plastic’s thermal characteristics.
- Fire retardants may raise the effective decomposition temperature, but reduce flowability.
- Glass fiber fillers often increase stiffness and thermal resistance but may also narrow the processing window.
Recommendation: Always re-test melt flow index (MFI) for modified plastics before full-scale molding.
Moisture Content
Some plastics are hygroscopic, meaning they absorb moisture from the air. Materials like nylon and polycarbonate must be properly dried before processing.
- Moisture trapped inside the polymer will vaporize at high heat, causing:
- Bubbles or voids in molded parts
- Splayed surface finish
- Hydrolytic degradation of polymer chains
Drying Guidelines:
Nylon 6 should be dried at 80°C for 4–6 hours. PC at 120°C for similar durations.
Processing Conditions
Factors like heating rate, barrel temperature, mold temperature, and cooling time play a significant role in determining the success of thermal processing.
- Overheating can cause polymer degradation (browning, gas emission).
- Insufficient heating may lead to incomplete mold filling or weld lines.
- Rapid cooling in semi-crystalline materials reduces crystallinity → softer parts.
Tip: Use gradual, multi-zone heating in injection barrels and maintain mold temperatures near Tg or Tm for dimensional accuracy.
Crystallinity and Molecular Structure
A polymer’s internal arrangement—amorphous coils vs. ordered lamellae—affects:
- Thermal conductivity (crystalline > amorphous)
- Melt viscosity
- Shrinkage behavior during cooling
Materials like PEEK exhibit high crystallinity and need controlled slow cooling to avoid warping and internal stress. Amorphous plastics like PC cool faster but are more susceptible to softening at elevated service temperatures.
How to Select the Right Plastic Based on Melting Temperature
Choosing the correct plastic for a component or product requires careful consideration of its thermal properties—particularly the melting temperature (Tm) or glass transition temperature (Tg). These thermal thresholds determine the temperatures at which a plastic will soften, flow, or degrade, and they directly impact the part’s long-term performance, especially in high-temperature environments.
Below are four key factors that should guide material selection based on melting behavior.
Application Temperature Range
The most important starting point is understanding the operating temperature range of your application. Melting temperature defines when a semi-crystalline plastic becomes fully fluid, while glass transition temperature marks the point at which amorphous plastics soften. If your part will experience continuous exposure to elevated temperatures, select a material with a safe service temperature at least 20–30°C below its Tg or Tm.
For example, polycarbonate (PC) has a Tg of around 145°C and performs well in applications up to about 120°C. In contrast, PEEK, with a Tm of 343°C, can maintain mechanical integrity even at 250°C, making it suitable for aerospace or oil and gas environments.
Avoid pushing plastics near their softening point, as even modest overexposure can lead to creep, warping, or stress relaxation over time.
Load-Bearing and Creep Resistance
When parts are under load in a high-temperature setting, it’s crucial to consider creep resistance—a material’s ability to maintain dimensional stability under constant mechanical stress.
Amorphous plastics, although easier to mold and often more impact-resistant, generally have poorer high-temperature load performance. Semi-crystalline polymers like nylon 6/6 or PPS provide better resistance to deformation under prolonged mechanical load due to their crystalline structure, which offers greater molecular packing and thermal stability.
For structural parts such as brackets, bushings, or housings used in heated environments, choose materials with both a high melting point and documented stress-relaxation performance.
Environmental Exposure (UV, Chemicals, Moisture)
Thermal performance is not the only consideration. The operating environment can accelerate thermal degradation or reduce a plastic’s effective service temperature.
- UV exposure can weaken polymers like ABS or polystyrene, especially at elevated temperatures. Stabilized polycarbonates or UV-grade acrylics may be preferable in outdoor applications.
- Chemical exposure, such as to oils, fuels, or cleaning agents, can compromise structural integrity at heat. Plastics like PTFE or PEEK offer strong chemical resistance at high temperatures.
- Moisture absorption, especially in nylons and polyesters, can cause parts to lose mechanical precision or expand in humid or hot environments. Materials must be dried before molding and may need protective coatings or alternative formulations for long-term outdoor use.
Understanding all environmental factors allows engineers to narrow the selection to materials that offer stable performance over time.
Safety and Compliance (Food-Grade, Medical Use)
Applications involving food contact, medical devices, or consumer electronics often require plastics that meet strict compliance standards in addition to having adequate thermal properties.
- For food-grade applications, materials like PPSU or polypropylene may be certified under FDA or EU food contact regulations and offer resistance to boiling water or steam sterilization.
- Medical-grade materials, such as PEEK, may be required to withstand autoclave sterilization cycles at 121–134°C without deformation, leaching, or degradation.
- Electronic components may require UL 94 flammability ratings, especially V-0 classifications for insulation and enclosures exposed to thermal buildup.
In these cases, thermal thresholds must be validated alongside compliance documentation, and verified through suppliers with appropriate certifications.
CSMFG’s Expertise in High-Temperature Plastic Fabrication
At CSMFG, we understand the critical role that thermal performance plays in plastic part manufacturing. Whether you’re working on a high-heat industrial component or a precision medical device, our team provides the expertise, tools, and infrastructure to ensure your parts perform reliably under stress.
Custom Injection Molding with Heat-Resistant Plastics
We maintain strict temperature control throughout the molding process to prevent degradation and ensure optimal crystallinity and dimensional accuracy. For each project, we assess tool material compatibility, gate design, and cooling flow to suit the thermal behavior of the selected material.
CNC Machining for Semi-Crystalline Plastics
For lower volume runs, prototyping, or tight-tolerance components, CNC machining offers a flexible alternative to molding. Our advanced machining capabilities support high-performance thermoplastics with high melting points and exceptional stability.
Semi-crystalline materials like nylon 6/6 can be machined into complex geometries with consistent finishes and tight dimensional control, even after exposure to heat.
Mold Design and Material Consultation
Selecting the right plastic is only half the equation. We support clients in matching resin grade to part geometry and use-case, optimizing mold design for thermal efficiency, and recommending venting and cooling layouts to prevent warpage.
Our engineers provide DFM (design for manufacturability) feedback early in the process to minimize downstream thermal failures.
Global Supply Chain & Certification
CSMFG partners with qualified resin suppliers and maintains comprehensive documentation for RoHS, REACH, UL 94, and FDA compliance. We support traceability, third-party testing, and material certification. Leveraging our ISO 9001 expertise, we ensure materials like ISO 10993-compliant PEEK or V-0-rated PPS meet your project’s stringent global standards.
FAQs About Plastic Melting Temperature
What’s the difference between melting temperature and glass transition temperature?
Melting temperature (Tm) is when semi-crystalline plastics become fluid, while glass transition temperature (Tg) is when amorphous plastics soften. Tg is a range; Tm is a fixed point.
Can I use ABS in high-temperature applications above 100°C?
ABS has a Tg of around 105°C and begins to soften beyond this point. For applications above 100°C, consider using PC or heat-stabilized ABS blends with caution.
What plastic has the highest melting point?
PEEK has one of the highest Tm values among thermoplastics at around 343°C. It’s widely used in aerospace, automotive, and high-pressure environments.
Why did my plastic part deform during autoclave sterilization?
If the part is made from a plastic with a Tg below the sterilization temperature, it may soften and warp. Use materials like PPSU or PEEK designed for autoclave cycles.
Can I 3D print high-temperature plastics?
Yes, materials like PEEK and PEI (Ultem) can be 3D printed, but they require high-temperature-capable printers with heated chambers and nozzles above 400°C.
Does CSMFG support PEEK or other engineering-grade plastics?
Yes, we routinely mold and machine PEEK, PPS, PSU, and other heat-resistant polymers for critical applications. Our team can advise on grade, sourcing, and processing methods.
What processing methods suit high-melting plastics best?
High-temperature plastics require specialized injection molding equipment with barrel and mold temperature control. For low-volume or complex parts, CNC machining is often preferred.
If you’re ready to explore high-performance plastics for your application, contact CSMFG today for expert consultation and a custom manufacturing solution tailored to your needs.