Polycarbonate Heat Deflection Temperature: HDT Values, Grades & Selection Guide

An automotive lighting supplier in Germany with an expert designer clearly defined polycarbonate with an HDT of at least 130°C for the new LED headlamp housing. The Suzhou supplier sends a datasheet of 132°C.  Three months later, the polycarbonate lens goes out of shape near the bulb chamber, thus failing an ambient seal. The tier 1 provides the OEM and issues a recall notice by week six. The 132°C reading from the oven after a successful test gave a false impression. There was never an application so sensitive to the oven technique.

According to your specifying, sourcing, or qualification of polycarbonate resin, you most certainly are aware that heat distortion temperature can be found lurking near the very top of any PC datasheet. You also must fully understand this to be one of the most misinterpreted numbers in the engineering plastics industry. The following guide provides you with the actual heat distortion temperature values of specific polycarbonate grades which you badly need, separated by Covestro Makrolon® grades and, other than the lab proverbial guideline numbers, gives practical means of translating them to real material specifications. By the end, you will be able to spec the right PC grade, compare it to the lab test/Certificate of Analysis, and avoid a catastrophic thermal failure that turns a quiet production line into a recall investigation.

What Is Heat Deflection Temperature for Polycarbonate?

Polycarbonate heat deflection temperature (HDT) is the temperature where a standardized test bar deflects 0.25 mm under a specified load, typically 0.45 MPa or 1.82 MPa as per ISO 75 and ASTM D648. Unfilled PC has a heat deflection temperature of around 138–142 degrees Celsius at 0.45 MPa and 125–135 degrees Celsius at 1.82 MPa.

Indeed, it is the first answer that most engineers want. The two stress levels are of interest because they depict two extremes of actual service conditions. The 0.45 MPa value (66 psi) compares with a lightly loaded housing or cover. The 1.82 MPa value (264 psi) describes a structural component under sustained mechanical load. Failure to specify both values is a typical shortcut found on datasheets that can lead to problems when the wrong number is plugged into a specification document.

The way HDT is Measured by the Laboratory

The test procedure is standard. The polycarbonate bar is 127 mm long and 12.7 mm wide and is placed on two supports 100 mm apart in an oil bath heated uniformly. A weight corresponding to either 0.45 MPa or 1.82 MPa stress is placed on the specimen directly in the center of the support. The bath is heated at the rate of 2°C per minute. The heat deflection temperature for that stress is taken as the temperature at which the idiapontial bar would deflect 0.25 mm under load.

ISO 75 and ASTM D648 follow the same basic theory with minute procedural differences. Every major manufacturer of polycarbonates uses those standards; therefore, when a data sheet of Makrolon® has an HDT value on it, you should, in als general terms, rely on the methodology behind it. The critical issue is the stress to which the test specimen was subjected.

Why HDT Is Not the Maximum Service Temperature

The effective glass-transition temperature of polycarbonate glass is about 145 < 150 degrees. Above Tg, polymer chains have freedom to move and the material starts to behave as a viscous liquid itself, not as rigid solid. HDT measurements are always below Tg, but they should not be taken to mean a “safe” level of continuous in-service temperature for an actual item.

Polycarbonate parts suffer from creep, fatigue, oxidation, UV radiation, and moisture absorption over thousands of hours. A part rated at 130°C HDT will see creep at 95°C under continuous load over 10,000 hours. As a rule of thumb, Covestro recommends continuous service temperatures at least 30-40°C below the 1.82 MPa HDT value. Take note of the concept that HDT should be a shared reference point but not an absolute upper temperature threshold.

Heat Deflection Temperature of Polycarbonate: Typical Values

Among major suppliers, it had been favored that a particular grade of polycarbonate would provide standard unfilled behavior. The HDT values, due to the consistent Bisphenol-A polycarbonate chemistry established by Covestro, SABIC, Trinseo, and other manufacturers are in turn extremely close. Differentemente, values of those HTI stipulations may be lifed up or down due to the effects of special additives or fillers, reinforcement, or FR packages.

Look at the spread. The buyer specifying a “PC with HDT ≥ 130°C” without naming a load condition could lawfully obtain a compound from a full-flame resistant grade with HDT at 123°C under 1.82 MPa, up to a 30% glass filled grade with HDT at 148°C. The point here is that you must match the grade to the load condition a part will see. For a deeper understanding of polycarbonate flame retardants, please refer to our accompanying guide: Polycarbonate Flame Retardant: UL94 V-0 Grade Selection and Sourcing Guide

Ready to confirm which Makrolon grade matches your HDT specification? Send us your application details and we will return a grade recommendation with the full COA values from our current Suzhou stock. Request a Makrolon grade match →

HDT Across Covestro Makrolon Grades

Most of the engineering market’s requirements of polycarbonate’s heat deflection temperatures are met by Covestro’s Makrolon® portfolio. Just sacrificing grade-level selections, Yifuhui strategically stocks in Suzhou almost all general-purpose, flame-retardant, UV-stabilized, and glass-fiber-reinforced polycarbonates on the market so that buyers get top-grade in any category.

The above effect should be evaluated as imposed. Intra-batch variation due to the plant production typically remains as per 2-4°C. The value on your COA, however, will be within this tolerance band, making the necessity of a good COA even higher than what is given in the datasheet when validating a specific lot for production fruits.

Another story was presented when the sprouts from UL94V-0 connector housings began to cradle at 118 degrees when being shelved in for 1 hour by Marc R.’s Spanish electronics OEM last spring. And deep into the exploration of lot-level determinants, it was known what the supplier did. His vendor used to ship material to him as Makrolon 6555 with an HDT of, case in point, 124°C against a nominal of 125°C in published data, which is good enough for most projects but then, in summer, temperatures in the vicinity of 120°C were exceeded by radiant heat coming from the underhood-mounting bracket. Switching to Makrolon 8025 (10% GF) moved the HDT margin up 16°C and ensconced the part from one lot.

Factors That Affect Polycarbonate HDT in Service

HDT values on the datasheet belong to clean test bars at proper lab conditions; that is not the real world where your part sees residual molding stress, a load in different directions, its glass fiber orientation, interactions with other constituents, and more importantly, such effects can be felt even more in long-term thermal aging. Every one of the above items has the potential to depress the actual polycarbonate heat deflection temperature in service. For a deeper understanding of the glass transition temperature of polycarbonate, please refer to our accompanying guide: Polycarbonate Glass Transition Temperature: Why 147°C Matters for Grade Selection

Glass Fiber Reinforcement

Glass fiber raises HDT in polycarbonate considerably. A 30% glass-filled grade like Makrolon 8325 will run 15-20 °C higher at 1.82 MPa than its unfilled counterpart. Mechanistically, glass fibers carry the load that the polymer matrix would have otherwise directed to thermally weakened chain segments. The penalty is a drop in transparency, skeletal anisotropy, and the need for more difficult injection molding processing.

Flame-retardant Packages

UL94V-0 polycarbonate grades show HDT values slightly lower by approximately 2-6°C relative to their unfilled counterparts at the same load. The polymer plasticizers are added for the flame retardant chemistry, while Makrolon 6555 runs close to 125°C at 1.82 MPa as compared to a non-FR grade running at 130°C. That is a real engineering trade-off. No defect.

Mold stress and Annealing

A polycarbonate part immediately after injecting its carries some residual stress inflicted on it. These stresses are responsible for reducing the effective HDT in service as well as inciting creeping under load. This stress can be removed by annealing the part at 120–125°C for two to about four hours and increasing the effective HDT by 3–8°C. Rarely seen is this revealed on data sheets. These have gone unaddressed in the postings motivated by competition that flood every now and then, advertising their products versus the plaintiff. The procedure is the raw material of highly experienced injection molders to levy a somewhat enhanced thermal performance from the PC grade as a buffer; it works.

Geometry and Load Direction of the Part

The HDT lab bar is a very simple three-point bend specimen. Your part is not. A thin-walled light lens is going to behave distinctly from a thick structured bracket when loaded, and the glass-filled grades show remarkable anisotropy depending on the flow direction in which they are molded. If the stress focuses on geometry (sharper corners, ribbed structures, thinning at the gate area), the local effective HDT value may be lowered by 10–15°C as compared with the datasheet value. For any structural elevated PC application past 100°C, numerical modeling FEA is of much greater concern than the datasheets.

Polycarbonate HDT vs. Other Engineering Plastics

The heat deflection temperature of PC (polycarbonate) is better than that of some materials but worse than that of others. Now this needs a little explanation. That is, if the comparison is seriously contemplated by the layman (engineer), he would know exactly why he was to use PC, I think.

Polycarbonate wins on the combination of clarity, impact strength, and reasonable thermal stability. If your application demands HDT above 140°C unfilled, PC is usually the wrong choice, look at PPS, PEI, or PSU instead. For mechanical fatigue or precision gear applications, our Polycarbonate Properties guide covers when acetal outperforms polycarbonate despite the lower HDT.

How to Use HDT Values in Material Selection

Specifying a polycarbonate that satisfies a high-heat requirement is an intense art form. The unsystematic way to go about things—such as taking the highest temperature number from a datasheet and tacking it within a purchase order—is how the LED headlamp case underlines just how absurd such an assumption can be in light of all the people involved.

1. Ascertain what loads will really be working on your part in service. Will it be an unsupported cover (0.45 MPa equivalent stress)or a structural member under continuous mechanical loading(1.82 MPa equivalent)?
2. What HDT measurements indifferent load will be used in service? Choose the HDT corresponding to the load that matches your mechanical part, not the one reflecting the best pattern.
3. Incidentally, mind that each structure should have a safety margin in operating the temperature. Do not glass PC 30–40°C below 1.82 MPa HDT and then provide even lesser temperature margins in conditions of short-duration temperature cycles.
4. Inspect against the full datasheet of the manufacturer. HDT assessment is but a single feature. Double-check Vicat softening, CLTE, creep modulus, and long-term aging indicate.
5. The coa could be requested for every change of batches. The form specifically states nominal values and does not correspond to actual values.

The material referred to by Anna L. was a procurement lead in the medical device sector in the Czech Republic, with the specification of “PC, HDT 135°C minimum” required for housing that went through autoclave cycles. Originally, the suggestion was Makrolon 9415 at 135°C HDT (1.82 MPa), but after the review of the use condition of autoclave cycle, i.e., 121°C for 20 minutes per cycle for 200+ cycles per device lifetime was made and reviewed, The best safety margin against creep was the limiting factor to enhance quality. Therefore, we upgraded her to Makrolon 8035 (20% glass-filled) at 145°C HDT for the same period of mold execution time and hence, for better dimensional stability during repeated cycles. Hence, the original spec actually offered a number of possibilities. For a deeper understanding of polycarbonate density, please refer to our accompanying guide：Polycarbonate Density: Grade-Specific Values, Standards, and Sourcing Verification

Verifying Polycarbonate HDT in Your Supply Chain

A Mastercurve for polycarbonate HDT is subject to the supply chain all the way back to production. Substandard grade, contamination from regrind, or at a bad extreme, an undisclosed addition can make the HDT 10 degrees Celcius lower than the datasheet value Makrolon 6555 had. The tendencies of being a proper part of the Consequences are all negatively inclined, having warranty claims, OEM audit failures, and in some rare cases actually having to initiate a recall.

Almost all exceptions concerning purchasing stand by the doctrine that gives them Certificate of Analysis. A genuine Makrolon COA will list specifics of grade, lot, batch, and contain measure specifications among which HDT is always present, standard of 1.82 MPa (method ISO 75 part2). Every Covestro Makrolon® by Yifuhui shall be shipped following a manufacturer-provided COA, with a trace back to required material from Covestro and batches, the MSDS, and required compliance certificates (RoHS, REACH, UL94, FDA where the grade is compliant).

Buyers who have been burned before tend to ask three questions during qualification:

• Is the grade name on the COA a real Covestro grade? (Yes, cross-reference against the Covestro Makrolon Solutions Center datasheets.)
• Does the HDT value on the COA match the manufacturer datasheet within normal tolerance? (Typically within 2–4°C.)
• Can the lot number be verified against Covestro’s production records? (Yes, on request; Covestro’s regional sales team confirms lot authenticity for distributors with verified supply relationships.)

Frequently Asked Questions

What is the Heat Deflection Temperature of Polycarbonate?

According to ISO 75 and ASTM D648, the temperature is approximately 138-142°C at 0.46 MPa and 125-135°C at 1.46 MPa. Specific values depend on the reinforcing agents used, the percentage of glass loads and flame retardant chemical.

Does the HDT of Polycarbonate break down after melting?

Contrary to what may be the thoughts of some people, no. Polycarbonate is an amorphous polymer that does not melt under the right conditions. Its glass transition region is 145-150°C, above which the polymer softens. HDT is always below Tg and refers to softening under the load, not a change in phase.

At what temperature is polycarbonate deforming?

Normally, come load application, a polycarbonate piece will significantly begin to creep under load at 30-40°C below its 1.82 MPa HDT. For unfilled PC at 130°C HDT, continuous service deformation commences around 90-95°C over prolonged periods under load.

How does polycarbonate loaded with glass compare with standard polycarbonate in terms of the HDT?

The glass-filled polycarbonate enhances HDT significantly. This approached to around 148°C under 1.82 MPa condition for a 30% glass-filled grade such as Makrolon 8325, compared to around 128–130°C for the neat grade Makrolon 2407. The loss, however, is transparency, increased anisotropy, higher mold wear.

Can I operate above the HDT on polycarbonate?

For some short operational periods of minutes or even hours, certainly if the design margin allows. However, above its HDT, the material may not continue in long-term continuous service. The HDT, here, is not where the material simply stops functioning; instead higher loads may assist creep to work harder earlier causing dimensional failure. While working on a design, subtract a further 30–40°C for stressing inside the design expected service temperature.

The Procurement Takeaway

Polycarbonate heat deflection temperature is a rough metric data sheet-not the working limit itself. The working temperature window of the given polycarbonate grade itself is quite informative: it falls something like 30°-40°C below the 1.82 MPa HDT. Have the relative applications—those are the ones deciding the concerning temperature they count as servicing such apiece quality temperature to be asserted by the manufacturer.

The Yifuhui top-notch Makrolon grades possess the package to fulfill any kind of specification: 2407, 2805, 6555, 6557, RE6717, 8025, 8035, 8325, and 9415, complete with manufacturer-issued COA, 25 kg MOQ, and FOB Shanghai-before-sale export documentation. Do you have a polycarbonate grade at the hand vs nominal thermal requirement? Please state to us your application requirements, operating temperature, load condition, and regulation; we shall recommend a grade with the COA data available in our hand.

Request a Covestro Makrolon® Quote with Full COA → Response within 24 hours, 25 kg MOQ, FOB Shanghai.
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