A POM datasheet lists melting point 175 °C, heat deflection temperature 110 °C, and a glass transition near -60 °C. The three numbers do not agree, and they are not supposed to. Pick the wrong one as your design limit and a part meant to last for years will sag, warp, or creep in its first hot week.
Marco, a product engineer at a tier-one automotive supplier, learned the heat deflection temperature vs melting point difference the expensive way. A bracket kept deflecting during a 130 °C oven-bake cycle. The acetal grade met every spec on paper because the team had designed to its melting point. The failure had nothing to do with melting, and it cost a retool.
This guide decodes the temperature numbers on an engineering-plastic datasheet: heat deflection temperature vs melting point vs glass transition temperature. You get a comparison table, design rules, and a way to verify those numbers when you buy resin. Our engineering plastic temperature guide charts these properties across the materials we supply, and our team can pull the manufacturer datasheet for any grade you are evaluating. Request a quote and we respond within 24 hours.
What Is the Difference Between Heat Deflection Temperature and Melting Point?
Melting point (Tm) is the temperature at which the crystalline regions of a semi-crystalline plastic change phase from solid to liquid. It is a molecular property and mainly a processing parameter. Heat deflection temperature (HDT) is a short-term test of how much a loaded plastic bar deflects as it is heated. It applies to every thermoplastic, and it almost always sits far below the melting point.
The phase-change distinction matters. In a semi-crystalline polymer such as POM, PA66, or HDPE, ordered crystals absorb heat until, at Tm, they collapse into a flowing melt. Load and geometry play no part.
Heat deflection temperature is a different idea. A test bar is loaded in the middle and heated in an oil bath at 120 °C per hour. The temperature at a set deflection, 0.25 mm under ASTM D648 or the ISO 75 equivalent, is the HDT. Two loads are reported, 0.45 MPa (66 psi) and 1.8 MPa (264 psi); the heavier the load, the lower the failure temperature.
So the two numbers answer two questions. Melting point tells you how to process the resin, since you melt it roughly 20 to 30 °C above Tm. Heat deflection temperature tells you how much load it can carry while it is hot.
One is chemistry. The other is structural engineering.
The Three Thermal Numbers on a Datasheet (and a Fourth You Should Know)
Every engineering-plastic datasheet lists a block of thermal properties. They all look like temperature limits. They are not. Each controls a different decision.
Melting Point (Tm): The Phase-Change Number
Melting point applies only to semi-crystalline plastics: POM near 165 °C (copolymer) to about 175 °C (homopolymer), PA66 near 260 °C, and HDPE near 130 to 135 °C. Measured by DSC (ISO 11357 / ASTM D3418), Tm appears as a sharp melt endotherm.
Its real job is processing. Barrel and nozzle temperatures sit above Tm so the melt flows. Our guide to engineering plastic processing temperatures covers how far above each material to run it. Do not read Tm as a service ceiling; no supplier rates a part to survive near its melt temperature.
Glass Transition Temperature (Tg): The Softening Number
Below Tm, the amorphous regions of a polymer have their own transition. The glass transition temperature (Tg) is where the polymer backbone gains enough energy for large-scale segmental motion. Below it, the material is glassy. Above it, modulus drops and the material turns rubbery.
For semi-crystalline plastics, Tg is a subtle event, such as POM near -60 °C. For amorphous plastics it is the headline number, because they have no true melting point and soften through Tg instead: PC near 147 °C, PMMA and ABS near 105 °C.
Heat Deflection Temperature (HDT): The Load-Bearing Number
HDT is the value most likely to appear in a real component spec, because it tests load and temperature together. A plastics supplier’s material temperature ratings guide describes the test: a bar deflects a fixed amount under a set stress while the temperature rises. The 1.8 MPa value governs structural and snap-fit parts; the 0.45 MPa value applies to lightly loaded parts and runs higher. Note the load on the drawing, such as “HDT 110 °C at 1.8 MPa.” For grade-level detail, see our polycarbonate heat deflection temperature guide.
Vicat and Continuous Service Temperature: The Other Two Rows
Two more rows appear nearby. The Vicat softening temperature (ISO 306 / ASTM D1525) measures softening by needle penetration and is mainly a cross-check. The number for sustained parts is the continuous service temperature, often a UL Relative Thermal Index (RTI, UL 746B) tied to roughly ten years of heat aging at 50 percent property retention. It runs far below Tm: unreinforced POM near 80 to 120 °C, standard PMMA near 65 to 70 °C.
MatWeb tabulates such values across many polymers but rarely says which to design to.
Does Polycarbonate Have a Melting Point?
No. Polycarbonate is amorphous, so it has no true melting point. It softens gradually through its glass transition near 147 °C and flows well above that.
Queries for “polycarbonate melting point” reflect that confusion. The relevant ceiling is Tg, covered in our polycarbonate glass transition temperature guide.
Does ABS Have a Melting Point?
No. ABS is amorphous too. It softens through its SAN-phase glass transition near 105 °C, so its thermal ceiling is Tg, not Tm.
That is why ABS datasheets never list a melting point. Our ABS melt temperature article explains the processing window amorphous ABS actually uses.
Amorphous vs. Semi-Crystalline: Why Some Plastics Don’t Melt
The most useful distinction on a datasheet is whether the polymer is semi-crystalline or amorphous, because it decides which thermal numbers exist.
- Semi-crystalline plastics (POM, PA66, HDPE, PP, PET, PPS) pack part of their chains into ordered crystals. They have a sharp melting point from those regions, plus a glass transition from the amorphous regions between them.
- Amorphous plastics (PC, PMMA, ABS) have no ordered regions, so no true melting point. Above Tg they soften continuously rather than melting at one temperature.
That is why a “melting point vs service temperature” comparison behaves so differently across families. For a semi-crystalline grade, Tm is real but says little about service limits. For an amorphous grade, Tm does not exist; Tg is the honest reference, though a loaded part still fails below it via HDT. In short, comparing glass transition temperature vs melting point compares amorphous with semi-crystalline behavior.
Lena, a design engineer at an optical shop, met this choosing a lens housing near a bank of LEDs. Neither candidate, PC nor acrylic, listed a melting point, so her usual first check gave nothing. What mattered was Tg under the housing’s mounting load.
PC held its shape through a Tg near 147 °C with an HDT near 125 to 135 °C. PMMA softened near 105 °C and deflected near 90 to 100 °C under the same 1.8 MPa load. The comparison decided the grade.
Heat Deflection Temperature vs Melting Point vs Glass Transition: Comparison Table
This table consolidates the three metrics for the six plastics we most often supply, plus two glass-reinforced rows. Values are typical starting points for standard grades. Treat them as ranges and verify against the specific grade’s datasheet; PA66 is quoted dry-as-molded because absorbed moisture changes its numbers.
| Material | Class | Tm (°C) | Tg (°C) | HDT 1.8 MPa (°C) | HDT 0.45 MPa (°C) | The number to design to |
|---|---|---|---|---|---|---|
| POM (acetal) | Semi-crystalline | ~165 (copoly) / ~175 (homo) | ~ -60 | ~100-110 | ~158-160 | HDT for short-term hot load; continuous rating for sustained service |
| PA66 (nylon 66, dry) | Semi-crystalline | ~260 | ~50-65 | ~70-100 | ~190-200 | Continuous rating; HDT when part carries load in heat |
| PA66-GF30 | Semi-crystalline | ~260 | ~50-65 | ~235-250 | ~255 | HDT 1.8 MPa, the connector and underhood spec gate |
| HDPE | Semi-crystalline | ~130-135 | ~ -110 to -120 | ~40-80 | ~75-90 | Continuous ceiling near 82 °C for sustained use |
| PC (polycarbonate) | Amorphous (no true Tm) | None | ~147 | ~125-135 | ~138-142 | Tg as the sustained ceiling; HDT for loaded parts |
| PMMA (acrylic) | Amorphous (no true Tm) | None | ~105 | ~90-100 | ~95-105 | Continuous rating near 65-70 °C for sustained use |
| ABS | Amorphous (no true Tm) | None | ~105 | ~75-105 | ~95-110 | HDT for short-term load; keep sustained service lower |
Three patterns in this table matter most.
Why HDT Is So Much Lower Than Melting Point
Compare POM: melting point near 175 °C, HDT near 100 to 110 °C at 1.8 MPa. That gap is not a flaw in the material. Melting needs far more thermal energy than the modest softening that makes a loaded bar deflect, so a stressed part loses dimensional stability long before its molecules melt.
Glass Fiber Raises HDT but Barely Moves Tm
Now compare unfilled PA66 with PA66-GF30. Tm stays near 260 °C in both. HDT jumps from roughly 70 to 100 °C up to about 235 to 250 °C. Glass stiffens the compound so the bar resists deflection, but it does not change the temperature at which the nylon crystals melt.
Tm is a molecular constant. HDT is an engineering variable.
Priya, a process engineer at a connector molder, watched a housing fail its heat-resistance check three times. The drawing specified unfilled PA66, and a 260 °C melting point looked safe. The housing carried terminal load during lead-free reflow, and the unfilled part deflected before peak temperature.
A 30 percent glass-filled PA66, with that same melting point, lifted the 1.8 MPa HDT from about 90 °C to 240 °C. It passed the next run.
Glass Transition Where It Bites: Dry PA66 in a Warm Environment
PA66’s dry Tg near 50 to 65 °C sits close to underhood temperatures, and the resin is hygroscopic: absorbed moisture pushes Tg toward ambient. A part specified on dry-as-molded data can soften in service simply by taking up water. This is why our PA66 melting point guide stresses conditioning when you compare datasheets. The logic runs through the other rows: POM temperature behavior, HDPE melting point, and the amorphous PMMA glass transition temperature page.
When to Design to Tm, Tg, or HDT
Rules of thumb are dangerous here; a framework is not.
Melting Point: Process, Don’t Design
Tm selects the processing window, not the service envelope. If a design limit is being justified with a melting point, stop and find the HDT and the continuous-service rating instead.
Glass Transition: The Ceiling for Amorphous, the In-Between Line for Semi-Crystalline
For amorphous plastics, Tg is the honest sustained ceiling, and even then a loaded part fails below it. For semi-crystalline plastics, Tg is the low-temperature impact boundary and the line where stiffness drops between the glassy and rubbery states.
HDT: The Short-Term Load-Bearing Check
HDT matches the real failure mode for hot, loaded events: solder reflow, paint and oven bake cycles, hot assembly, underhood heat soak. If the part carries load while hot, compare its stress to the 1.8 MPa HDT and apply margin.
Continuous Service Temperature: The Real Long-Term Number
For parts that run hot for years, the continuous-service rating or UL RTI governs. Design to roughly 10 to 20 °C below the governing limit, and never extrapolate a short-term HDT to a decade of service. HDT finishes in minutes; creep and aging are not captured in it.
In order, then: define the load and its duration; choose short-term (HDT) or long-term (continuous rating); match the material class to the number that exists for it; add a 10 to 20 °C safety margin; and verify against the full datasheet before the PO.
One common spec mistake deserves a warning: quoting the 0.45 MPa HDT for a structural part. A snap-fit or bracket under real stress must use the 1.8 MPa value, which can run 30 to 80 °C lower. A Port Plastics thermal properties explainer makes the same point about reading HDT with its load attached.
What Moves HDT (and Leaves Melting Point Alone)?
If Tm is a phase-change constant, HDT is a lever that compounding and environment pull. Knowing the levers tells you when a datasheet value will hold.
Glass Fiber Reinforcement: The Biggest Lever
Thirty percent glass can raise PA66 HDT past 235 °C, and glass-reinforced POM reaches roughly 160 to 168 °C against about 100 to 110 °C unfilled. Tm barely moves. When a review says “we need more temperature,” reinforcement is usually the fastest answer.
Flame-Retardant and Impact-Modifier Packages: They Can Lower HDT
Additives that do not stiffen the matrix can pull HDT down. Flame-retardant packages and rubber modifiers often soften the compound, so a V-0 grade may carry a lower HDT than its unfilled base. A flame-retarded polycarbonate can run 5 to 15 °C below the standard grade at the same load.
Moisture: The Hygroscopic Variable
For PA66 and other polyamides, absorbed water depresses Tg and stiffness and shifts the numbers a dry-as-molded datasheet promises. Test and design around the conditioned state when the part will see humidity.
Molding-Induced Stress and Annealing
Residual stress from flow and packing can lower a molded part’s effective HDT below the datasheet bar value. Annealing relieves that stress and can recover several degrees. This is why a part sometimes passes at room temperature and fails when hot and loaded.
Resin Quality: The Silent Variable
Off-spec resin, regrind, and counterfeit lots marketed as branded grades can shift thermal baselines. A cheap “acetal” lot can under-deliver on HDT while its melting point and melt-flow index look fine on paper. Tm drifts less than HDT, which makes HDT the more sensitive tell of a bad lot.
Why the Number on the Datasheet Is Only as Good as the Resin in the Bag
A spec written against a datasheet is only as safe as the resin that arrives.
Off-Spec and Regrind Resin Shift Thermal Baselines
Marco’s team traced their bracket failure to a mixed lot of regrind slipped in by a lower-cost trader. The melt temperature matched, so incoming QC passed. The HDT did not hold under load, because the regrind had lost molecular weight and was not the certified grade.
What to Verify on a Certificate of Analysis
A Certificate of Analysis (COA) is your first line of defense. Confirm these fields before you accept a lot:
- The exact grade designation matches the manufacturer’s published grade.
- The lot or batch number, so the material is traceable.
- The melting-point baseline where the manufacturer reports it by DSC.
- The melt-flow index, which flags molecular-weight drift that shifts HDT.
- Density and filler content, because a short glass load changes HDT.
- RoHS and REACH statements, if your market requires them.
Cross-check the thermal lines against the manufacturer’s datasheet rather than accepting them at face value.
How Yifuhui Supplies Thermally Consistent Branded Resin
At Suzhou Yifuhui, we supply branded prime resin with documented thermal baselines and traceable lots, so the number on the datasheet is the number in the bag. That means BASF Ultraform and Celanese Hostaform POM, Polyplastics DURACON acetal, BASF Ultramid PA66 including glass-reinforced grades for HDT-critical connector and underhood specs, and Covestro Makrolon polycarbonate. Every batch ships with a COA, and our Suzhou warehouse sits minutes from the Port of Shanghai for efficient export.
Need a glass-reinforced PA66 whose 1.8 MPa HDT genuinely clears 230 °C? See our PA66 resin supplier page. For a hot, loaded acetal bracket, start at our POM resin page. For transparent parts where Tg, not Tm, is the ceiling, review our polycarbonate resin grades.
Conclusion
Every datasheet hands you several temperature numbers and expects you to know which one controls your part. Now you do.
Melting point is for processing. Glass transition is the softening line, and for amorphous plastics like PC, PMMA, and ABS it replaces a melting point that does not exist. Heat deflection temperature is the short-term, load-bearing check for events such as oven bakes and underhood heat soak. The continuous-service rating is the number you design long-lived parts to.
The heat deflection temperature vs melting point gap is not a flaw in the material. It is the difference between chemistry and structural engineering, and picking the wrong side of it is one of the most expensive mistakes in plastic part design.
Before your next spec, confirm which number governs your load and duration, then make sure the resin behind it can deliver. When you need a grade whose Tm and HDT baselines you can trust batch after batch, request a quote for branded prime POM, PA66, or PC with COA documentation: 25 kg minimum order, FOB Shanghai, response within 24 hours. Contact our technical team and we will help you select the grade for the number that really matters. To learn more about the Glass Transition Temperature of HDPE, please refer to our accompanying guide.
Frequently Asked Questions
Is Heat Deflection Temperature the Same as Melting Point?
No. Melting point is a phase change of the crystalline regions of a semi-crystalline plastic. Heat deflection temperature is a short-term deflection test under load. They rarely align.
Why Is Nylon’s HDT So Low When Its Melting Point Is 260 °C?
HDT measures stiffness under load, not melting. Unfilled PA66 deflects at 70 to 100 °C under 1.8 MPa; glass lifts HDT toward 235 to 250 °C while Tm stays at 260 °C.
Can I Use a Plastic Above Its HDT?
Only for short, lightly loaded exposure, and never for sustained service. HDT is a minutes-scale dynamic test; long-term aging follows the continuous-service rating.
What Is the Continuous Service Temperature of POM, PA66, and PC?
Unreinforced POM and PA66 rate near 80 to 120 °C depending on grade, with heat-stabilized and glass-reinforced versions higher. Polycarbonate’s ceiling tracks its Tg near 147 °C, derated for load. Confirm on the grade’s datasheet.
