Last spring, a product engineer at a European medical-device OEM approved polycarbonate for a sterilization-tray handle. The datasheet listed a polycarbonate glass transition temperature of 147°C, and the autoclave ran at 134°C. That should have been comfortable, she reasoned. The trays passed their first fifty cycles. By cycle 200, the handles had bowed enough to jam the lids, and the hospital flagged the lot for quarantine.
Here is what the datasheet did not say: 147°C is a molecular transition, not a service limit. Polycarbonate has no melting point, and its glass transition temperature tells you where stiffness begins to collapse, not where a loaded part can safely operate.
If you have read “glass transition temperature 147°C” and treated it as a green light up to 147°C, you’re in good company. It’s the single most common PC specification error we see at Suzhou Yifuhui. This guide explains what the polycarbonate glass transition temperature actually means, how it differs from heat deflection temperature (HDT) and processing temperature, which Covestro Makrolon PC grades are built for sustained heat, and what to verify on a Certificate of Analysis (COA) when you source the material. By the end, you’ll specify PC from the right number and source it with the right documentation.
[Need a Makrolon grade recommendation for your operating temperature? Contact Yifuhui’s technical team →]
What Is the Glass Transition Temperature of Polycarbonate?
The Direct Answer
Standard bisphenol A polycarbonate (PC) has a glass transition temperature (Tg) of approximately 147°C (297°F; 420 K). PC is an amorphous thermoplastic, so it has no true melting point. It softens gradually above its Tg and begins to flow as a viscous liquid above roughly 155°C. Reported values range from 140°C to 155°C depending on molecular weight, additive package, and test method.
Why Polycarbonate Has No True Melting Point
Polycarbonate belongs to a different family than POM, PA66, or HDPE. Those are semi-crystalline polymers. Their molecular chains pack into ordered crystalline regions, so they have a sharp, repeatable melting point (Tm) that shows up as a clear endotherm in differential scanning calorimetry (DSC). Heating a POM gear past its melting point changes it from a solid to a liquid over a narrow temperature band.
PC is amorphous. Its bulky aromatic carbonate backbone can’t pack into a regular crystal lattice, so there’s no crystalline region to melt. Instead, the material passes through a glass transition: below ~147°C the chains are frozen in a rigid, glassy state; above it they gain mobility and the material softens like a stiff rubber. This is why the “melting point” quoted for polycarbonate is really a processing temperature, not a true phase change. When someone asks about the polycarbonate melting point vs Tg, the accurate answer is that PC only has a Tg, and the melt-flow temperature used in molding sits far above it.
The Role of the Aromatic Backbone
The rigidity of PC’s glassy state comes from the bisphenol A (BPA) molecular unit. The stiff aromatic rings in the backbone limit chain rotation, which raises the glass transition temperature to ~147°C. Compare that with ABS and PMMA, whose more flexible chains give a Tg around 105°C. This aromatic stiffness is also why PC combines high impact strength, dimensional stability, and heat tolerance in one material. It is the same reason PC holds optical clarity up to relatively high service temperatures, and the reason it costs more than commodity styrenics.
Tg vs. HDT vs. Processing Temperature: The Three Numbers That Confuse Engineers
A polycarbonate datasheet lists several temperatures, and treating them as interchangeable causes most of the failed parts we see. There are three distinct numbers, and each answers a different question.
Glass Transition Temperature (Tg): ~147°C
Tg is a molecular property. It describes when the polymer chains transition from a glassy to a rubbery state, and it is measured by DSC (ASTM D3418 / ISO 11357) or dynamic mechanical analysis (DMA). It’s consistent across most standard PC grades. Critically, Tg is not a service limit. It tells you when the material starts to soften at the molecular level, not when a part will fail under load.
Heat Deflection Temperature (HDT): 128–140°C at 1.8 MPa
HDT is measured under mechanical stress. At the standard engineering load of 1.8 MPa (ASTM D648 / ISO 75), polycarbonate deflects between roughly 128°C and 140°C. At the lower 0.45 MPa load it holds closer to 140°C. HDT sits below Tg because real parts operate under load, and stress accelerates softening. For brackets, housings, structural clips, and any component carrying a sustained load, HDT is the number that governs service temperature. If you need the deeper breakdown, our polycarbonate heat deflection temperature article covers it in detail.
Processing Temperature: 260–320°C
Injection molders process polycarbonate at 260–320°C, which seems absurd next to a 147°C Tg. The reason is viscosity. PC doesn’t melt in the crystalline sense, but it must be heated far above Tg to reach the low viscosity needed to fill thin walls and complex geometries. High-viscosity grades run at the top of the window, up to 330°C.
The upper limit matters. Above roughly 320°C, PC begins to thermally degrade: it yellows, releases carbon dioxide, and its molecular weight drops, which pulls down mechanical properties. In flame-retardant grades, over-enthusiastic barrel temperatures also impair the FR additive package, which can cost you a UL94 rating.
Polycarbonate Temperature Metrics at a Glance
| Metric | Typical Value | Test Standard / Note |
|---|---|---|
| Glass transition temperature (Tg) | ~147°C (140–155°C by grade) | DSC, ASTM D3418 / ISO 11357 |
| Heat deflection temperature (HDT) | ~128–140°C @ 1.8 MPa; ~140°C @ 0.45 MPa | ASTM D648 / ISO 75 |
| Vicat softening temperature | ~147°C (grade-dependent) | ASTM D1525 / ISO 306 |
| Melt / processing temperature | 260–320°C (to 330°C for high-viscosity grades) | Injection molding window |
| Mold temperature | 80–120°C (80–100°C for optical clarity) | Higher end reduces internal stress |
| Continuous service temperature | ~115–130°C | Design rule: keep 40–50°C below Tg |
| Thermal degradation onset | ~320°C | Prolonged exposure is harmful |
| Pre-drying | 120°C for 2–3 h; residual moisture ≤0.02% | PC is hygroscopic; dry before molding |
Values are grade-dependent. Always verify against the manufacturer’s published datasheet for the specific grade you specify.
Ana, a processing engineer at a Taiwan electronics molder, learned the degradation limit the hard way. She ran Covestro Makrolon 6557 at 335°C to shave eight seconds off her cycle time. The first batch looked fine. By the third day, the housings had a visible yellow cast, and a follow-up UL94 screening on the molded parts showed the V-0 rating had drifted. The resin was genuine and the mold was sound. The problem was a barrel temperature 15°C over the datasheet ceiling. She re-set the profile to 300°C and extended cycle time slightly, and the yellowing disappeared. The lesson: PC’s Tg tells you nothing about the melt window, but the melt window can destroy the material if you ignore it.
How Polycarbonate’s 147°C Tg Compares to Other Engineering Plastics
Engineers choosing between PC, POM, PA66, ABS, and PMMA need a thermal map, not just a single number. This table compares the key temperature metrics across the materials Yifuhui supplies.
| Material | Tg (°C) | HDT @ 1.8 MPa (°C) | Melt / Processing (°C) | Continuous Service (°C) |
|---|---|---|---|---|
| PC | ~147 | ~128–140 | 260–320 | ~115–130 |
| POM (copolymer) | ~-60 | ~95–110 | 180–230 | ~82–115 |
| PA66 | ~50–70 | ~70–105 (dry) | 260–300 | ~95–120 |
| ABS | ~105 | ~85–100 | 200–250 | ~60–80 |
| PMMA | ~105 | ~85–95 | 170–275 | ~70–77 |
Two caveats keep this table honest. First, POM and PA66 are semi-crystalline, so their Tg is less relevant than their sharp melting points (roughly 165–175°C for POM, 260–265°C for PA66). Their HDT and continuous-service figures reflect the crystalline structure, which is why they hold stiffness near their melting point. Second, service limits assume the part carries some load. Unloaded PC can tolerate temperatures closer to Tg; any sustained load drops the practical ceiling toward HDT.
PC’s high Tg (~147°C) is its advantage over ABS and PMMA for heat, which is why it wins in enclosures, glazing, and lighting. But if your application involves sustained loads above ~130°C, exposure to solvents or strong bases that attack PC, or a need for low friction in a gear or bearing, POM or PA66 is often the better material. As a distributor, we would rather route you to the right resin than sell you the wrong one. For precision mechanical parts, see our POM resin supplier guide; for sustained-load and chemical-exposed applications, compare with our PA66 resin supplier pages. The full cross-material chart lives in our engineering plastic temperature guide.
How Polycarbonate’s Glass Transition Temperature Is Measured
Tg is not pulled from a table. It is measured, and the method affects the number. Knowing which measurement produced the value on your datasheet helps you compare grades fairly.
Differential Scanning Calorimetry (DSC)
DSC is the standard method, defined in ASTM D3418 and ISO 11357. A small sample is heated at a controlled rate while the instrument records heat flow. At the glass transition, the heat-flow curve shows a step change in slope as the material’s heat capacity increases. That step marks Tg. For BPA polycarbonate, DSC typically reports 140–155°C, with the exact value shifting with heating rate, molecular weight, and thermal history.
Dynamic Mechanical Analysis (DMA)
DMA measures the drop in storage modulus as the polymer passes from glassy to rubbery. Because it applies oscillating stress, DMA often reports a slightly different value than DSC, and it gives additional information about damping behavior. A DMA-based Tg for PC may come in a few degrees higher than a DSC value on the same lot. Neither is “wrong”; they probe different responses to the same transition.
What Shifts the Measured Tg
Moisture lowers the measured Tg of PC, which is why pre-drying matters for both processing and testing. Additives shift it too: flame retardants and UV stabilizers can move the reported value by a few degrees, and glass fillers raise the effective heat resistance even when the matrix Tg stays similar. Cooling and heating rates in the test also matter. Grade-aware buyers should not over-index on a one-degree difference between datasheets and instead check the range against the test method listed. One real-world example: 3D-printing PC filament grades report lower Tg values (around 113°C for some brands) than injection-molding grades, because the filament chemistry and additive package differ. A printed PC part is not the same thermal animal as a molded PC part.
Covestro Makrolon Grades and High-Temperature Performance
Not all polycarbonate behaves identically at temperature. Covestro’s Makrolon portfolio spans general-purpose, flame-retardant, and reinforced grades, each with a thermal profile matched to an application. This is where grade selection becomes a real decision, not a datasheet guess.
General-Purpose Grades: Makrolon 2407 and 2805
Makrolon 2407 is a high-flow, UV-stabilized grade with excellent optical clarity. It carries the standard Tg (~147°C) and an HDT around 128°C at 1.8 MPa, and it suits outdoor lighting lenses, transparent covers, and displays. Makrolon 2805 is a medium-viscosity general-purpose grade with very good dimensional stability, widely used in industrial housings, medical device components, and automotive interior trim. Both offer the classic PC set: high impact strength, roughly 88–90% light transmission, and forgiving processability. For most consumer and industrial enclosures that see moderate heat, these are the workhorses.
Flame-Retardant Grades: Makrolon 6555 and 6557
Makrolon 6555 achieves UL94 V-0 at 1.5 mm while retaining useful optical properties. Makrolon 6557 adds UV stabilization to the flame-retardant package. Both hold a Tg near 145°C and a Vicat softening temperature around 147°C, but their practical service limit is governed by HDT (roughly 130°C at 1.8 MPa) and the thermal-aging profile of the FR additives. These are the grades for connector housings, switch components, and battery enclosures where fire safety and heat tolerance are both non-negotiable. Process them with discipline: exceeding ~320°C impairs the flame-retardant response, exactly as Ana discovered.
High-Viscosity, High-Heat, and Reinforced Grades: 8025, 8035, 8325, 9415, 9125
When Tg and unfilled HDT are not enough, reinforced grades push the practical thermal envelope higher. Makrolon 8025 and 8035 are glass-fiber-reinforced grades (typically 20% glass) with substantially higher stiffness and HDT values reaching 145°C or more. They are the specification for structural brackets, underhood components, and parts under sustained mechanical load at elevated temperature. Makrolon 8325 is a high-flow, glass-filled grade optimized for thin-wall molding in thermally demanding applications. Makrolon 9415 is a mineral-filled, high-heat grade with low coefficient of linear thermal expansion (CLTE), preferred where thermal cycling must not compromise tolerance retention. High-viscosity grades like Makrolon 3105 and 3258 run at melt windows of 290–330°C and suit large or thick-wall parts.
At Yifuhui, we stock Makrolon grades 2407, 2805, 6555, 6557, RE6717, 8025, 8035, 8325, and 9415, with manufacturer COA documentation on every batch. If your application demands a specific thermal baseline, tell us the operating temperature, the load, and the regulatory requirements, and we will identify the grade that fits.
Design and Processing Implications of PC’s Tg
The glass transition temperature of polycarbonate shapes how you mold it, not just how you specify it. Four practical rules follow from the science.
Mold Temperature and Internal Stress
Run a mold temperature of 80–120°C. The high end matters more than most processors realize: a hotter mold slows the skin formation, reduces frozen-in stress and birefringence, and yields more dimensionally stable optical parts. If you mold clear PC parts and see stress cracking at edges or around inserts, raise the mold temperature before you change anything else.
Avoiding Thermal Degradation
Keep the melt at or below 320°C and shorten residence time. If a machine must idle, purge the barrel. Because PC is hygroscopic, dry the resin at 120°C for 2–3 hours to a residual moisture of 0.02% or less before molding; even small moisture pickup hydrolyzes the polymer and drops impact strength and molecular weight.
Annealing Considerations
For parts that will see sustained heat in service, annealing below Tg relieves molded-in stress. A typical cycle is 120–130°C for a defined dwell time, followed by slow cooling. Annealing reduces the risk of creep and stress cracking in high-service-temperature applications and improves dimensional stability. It’s a low-cost step that designers often skip and then regret in warranty.
The 40–50°C Margin Rule
Design for a continuous service temperature roughly 40–50°C below Tg, which puts standard polycarbonate around 115–130°C. Short excursions to ~135°C are tolerable; sustained exposure above that invites creep under load, dimensional drift, and loss of optical stability. If your part must hold a dimension or carry a load indefinitely at high temperature, treat HDT as your ceiling and the margin rule as your design discipline.
Sourcing Consistency: Why Batch Quality Affects Thermal Performance
Thermal properties aren’t a promise printed on a datasheet; they are a property of the specific lot you receive. This is where procurement and material science meet, and it’s the part of PC sourcing that trips up the most buyers.
Off-spec, regrind-contaminated, or counterfeit polycarbonate can show a measurably reduced Tg and HDT. Regrind blending raises MFI and shortens molecular chains, which lowers the glass transition and heat resistance even when the part looks identical. Mislabeled commodity PC sold as a branded grade is worse: the thermal and mechanical properties may not match the brand at all, and there is no compliant datasheet behind it.
A Certificate of Analysis (COA) is your verification tool. Every reputable branded-PC shipment carries one, issued by the manufacturer against a specific production lot. When you receive a COA, check five fields:
- Grade designation and manufacturer name (a real Covestro grade, not an invented number)
- Melt flow index (MFI), measured per ISO 1133 / ASTM D1238, cross-referenced against the datasheet
- Density (around 1.20 g/cm³ for standard PC)
- Tg, HDT, or Vicat baselines where the manufacturer reports them
- Lot or batch number, traceable to the manufacturer’s production records
If the MFI on the COA deviates significantly from the datasheet range, or the lot number format does not look like the manufacturer’s, ask questions before the material ships. A supplier who responds defensively to COA verification is a red flag.
Rafael, a quality manager at a Spanish appliance manufacturer, caught exactly this on an inbound lot. A spot-market trader had supplied a COA with an MFI of 18 g/10 min on a grade specified at 10. He ran an in-house check, confirmed the deviation, and rejected the batch before it reached the molding line. The substitution would have shifted his cycle times and pulled the enclosures below their HDT requirement. The cost of the test was a fraction of the rework it prevented.
This is why Yifuhui supplies only traceable branded prime resin. Every shipment of Makrolon PC ships with a manufacturer-issued COA, an MSDS, and full export documentation, and our MOQ starts at 25 kg with FOB Shanghai as the standard term. If you want to verify thermal consistency before committing to production volume, a 25 kg trial from a single lot gives you a real reference sample at qualification scale.
Conclusion
Polycarbonate’s glass transition temperature of ~147°C is one of the most useful numbers on its datasheet, and one of the most misused. The polycarbonate glass transition temperature is a molecular ceiling: it defines where PC begins to soften, not where it is safe to operate. HDT (~128–140°C at 1.8 MPa) sets the loaded-service limit, processing runs at 260–320°C to overcome viscosity, and good design keeps continuous service roughly 40–50°C below Tg.
The practical takeaway for engineers and buyers is simple. Specify from HDT, process within the melt window, verify the grade against the manufacturer datasheet, and confirm the lot with a COA. When you do that, PC is a remarkably capable material: transparent, impact-resistant, dimensionally stable, and thermally honest.
When you’re ready to source, we make verification easy. Yifuhui stocks Covestro Makrolon 2407, 2805, 6555, 6557, RE6717, 8025, 8035, 8325, and 9415, all with manufacturer COA documentation, a 25 kg minimum order, and FOB Shanghai export. To learn more about ABS Melting Temperature: Complete Processing and Grade Guide, please click to refer to our accompanying guide.
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Frequently Asked Questions
What is the glass transition temperature of polycarbonate?
Standard bisphenol A polycarbonate has a glass transition temperature of approximately 147°C (297°F; 420 K). Reported values range from 140°C to 155°C depending on grade, molecular weight, and test method.
Does polycarbonate have a melting point?
No. Polycarbonate is an amorphous polymer, so it has no true melting point. It softens gradually above its glass transition temperature and flows as a viscous liquid above roughly 155°C.
What temperature does polycarbonate soften at?
PC begins to soften at its glass transition temperature, around 147°C, and flows above about 155°C. Below 147°C it remains rigid and glassy.
What is the difference between polycarbonate Tg and melting point?
The polycarbonate melting point vs Tg distinction comes down to crystallinity. Semi-crystalline polymers like POM or PA66 have a sharp melting point (Tm); amorphous PC has only a glass transition (Tg) at ~147°C and no melt endotherm.
What is the difference between Tg and HDT?
Tg is a molecular transition measured without load. HDT is measured under a standard load (1.8 MPa) and reflects how a real part deflects at temperature. For PC, HDT (~128–140°C) is the practical loaded-service limit and always sits below Tg.
What is the maximum service temperature of polycarbonate?
Standard PC is designed for continuous service around 115–130°C, roughly 40–50°C below its 147°C Tg. Short excursions to ~135°C are possible, but sustained load at higher temperatures invites creep and distortion.
What temperature should I mold polycarbonate at?
Melt temperature of 260–320°C (to 330°C for high-viscosity grades) with a mold temperature of 80–120°C. Dry the resin at 120°C for 2–3 hours to ≤0.02% residual moisture before molding, and do not exceed ~320°C.
Is polycarbonate heat resistant?
Yes, relative to other transparent and amorphous thermoplastics. Its ~147°C Tg beats ABS and PMMA, and its HDT of 128–140°C supports enclosure and lighting applications. It is not a high-temperature engineering plastic like PPS or PEEK; sustained service is limited to roughly 115–130°C.
How is polycarbonate Tg measured?
By differential scanning calorimetry (DSC, ASTM D3418 / ISO 11357) or dynamic mechanical analysis (DMA). DSC observes a step change in heat flow; DMA observes the modulus drop. Values from the two methods can differ by a few degrees.
Is polycarbonate or POM more heat resistant?
For sustained loaded service, PC’s higher Tg (~147°C vs ~-60°C for POM copolymer) and higher HDT give it the edge in heat tolerance without chemical exposure. POM wins on chemical resistance, low friction, and fatigue performance, and its semi-crystalline structure holds stiffness near its melting point. The right choice depends on the application.
