Open almost any plastic datasheet and you will find two temperature numbers sitting next to each other. “Softening point: 140 °C. Melting point: 175 °C.” They look like two points on the same slope, as if the plastic softens a little and then melts a little later.
They’re not. They’re two different kinds of number, measured two different ways, answering two different questions. Treat them as the same thing and you’ll eventually specify a part that sags under load, or reject a perfectly good resin for looking wrong on paper.
This guide draws the softening point vs melting point line clearly. You’ll learn what a softening point actually measures, how the Vicat test defines it, and which number belongs on a specification for a loaded part, a process, or a long-term service application.
We supply engineering plastics from Suzhou, near the Port of Shanghai, and buyers ask us this question most weeks. If you’d rather skip ahead to your specific material, our plastic melting point chart holds the reference values.
Softening Point vs Melting Point: The Direct Answer
A melting point (Tm) is the temperature at which a semi-crystalline plastic’s crystalline regions melt, a thermodynamic transition measured by DSC. A softening point is the temperature at which the plastic deforms a set amount under a set load, an empirical result measured by Vicat or by heat deflection. One is a material property; the other is a test outcome. They are not interchangeable, and a softening point means nothing without the method that produced it.
That single distinction explains why the two numbers sit so far apart on a datasheet, and why “softening point” and “melting point” get used as synonyms online when they should not be. If you have seen the question written the other way around, as melting point vs softening point, the answer is unchanged: they are different kinds of number, from different methods, that happen to sit on the same datasheet.
If you would like the softer concept itself unpacked first, our guide to whether plastic has a melting point sets out the three ways a plastic responds to heat. This article stays on the comparison between these two specific numbers.
What Is a Softening Point?
A softening point is the temperature at which a plastic specimen deforms a defined amount under a defined load. Nothing about that definition is fixed by nature. Someone decided how big the load would be, how fast to heat the sample, and how much deformation counts. Change any of those, and the “softening point” changes.
Vicat softening temperature: the standard method
The most common softening test is the Vicat softening temperature (VST), defined under ISO 306 and ASTM D1525. A flat-ended needle with a 1 mm² circular cross-section presses into the specimen under a set load, while the specimen is heated at a set rate. The Vicat temperature is the point at which the needle has penetrated 1 mm. Test-equipment maker Zwickroell publishes the full apparatus and procedure.
The two standards recognize four methods, and they give different answers for the same plastic:
| Method | Load | Heating rate |
|---|---|---|
| A50 | 10 N | 50 °C/h |
| B50 | 50 N | 50 °C/h |
| A120 | 10 N | 120 °C/h |
| B120 | 50 N | 120 °C/h |
This isn’t a technicality. The same material can read roughly 151 °C under A50 (10 N) and about 92 °C under B50 (50 N). That’s a spread of almost 60 degrees, produced by a change of load alone. A supplier who quotes “softening point 140 °C” without naming the method hasn’t given you a specification. They’ve given you a number.
Other ways to measure softening
Vicat is not the only softening test. The Mettler softening point (ASTM D3104) heats a disc until a steel ball drops through it, and it is common for amorphous polymers such as hot-melt adhesives. Some laboratories use thermomechanical analysis (TMA), covered by JIS K7196. Heat deflection temperature (HDT), defined under ISO 75 and ASTM D648, resists the temptation to collapse everything into one term; it measures deflection of a loaded bar rather than needle penetration.
The reason a “softening point” exists at all is that many plastics have no definite melting point. Whenever a material makes the solid-to-soft transition gradually instead of snapping through a phase change, engineers need an agreed, repeatable way to put a number on “soft enough to matter.” Vicat is that agreement.
How to interpret a softening point
Vicat chose a procedure that works for the two families of plastic, but the physical meaning differs by family.
- For an amorphous plastic, the softening point sits close to the glass transition temperature (Tg). The number tracks the polymer’s genuine softening threshold.
- For a semi-crystalline plastic, the softening point sits below its melting point but reasonably close to it.
That family difference is the whole reason a comparison exists, and it’s worth understanding properly. The IUPAC Gold Book definition states the relationship plainly: for an amorphous polymer the softening temperature sits near Tg, and for a crystalline polymer it sits below but close to Tm. The full framework lives in our article on amorphous versus semi-crystalline plastics; here it matters only as the key to reading the two numbers side by side.
What Is a Melting Point (Tm)?
A melting point is different in kind. It’s the temperature at which the crystalline regions of a semi-crystalline plastic lose their order and the material absorbs a burst of energy. That energy absorption shows up as an endothermic peak on a differential scanning calorimetry (DSC) scan, the method defined under ASTM D3418 and ISO 11357.
Two things follow from that. Only semi-crystalline plastics have a melting point; amorphous plastics don’t. And the melting point is a property of the material, measured rather than defined. For a given grade, it barely moves from one competent laboratory to the next.
Why even a melting point is a range, not one sharp number
Here is where the intuition breaks down in the other direction. Ice melts at exactly 0 °C. A semi-crystalline plastic melts across a span of several degrees, because its crystals are not all the same size and perfection. The smaller, less perfect crystals melt first, and the largest, most perfect ones melt last. LDPE is the extreme case, with a melting range wide enough to span tens of degrees.
So “melting point” is shorthand for the peak of a melting range. It is still a thermodynamic event and still a material constant, but it is not the razor-sharp transition you learned for water. If you want to see where those melting ranges fall for everyday and engineering plastics, our practical guide to what temperature plastic melts lays them out.
The Key Differences: Softening Point vs Melting Point
Set the two side by side and the differences stop being academic.
| Softening point | Melting point (Tm) | |
|---|---|---|
| What it measures | Onset of deformation under load | Melting of crystalline regions |
| Nature | Empirical, test-defined | Thermodynamic transition |
| Test method | Vicat (ISO 306 / ASTM D1525), HDT, Mettler, TMA | DSC (ISO 11357 / ASTM D3418) |
| Load dependent? | Yes, changes with load and rate | No, a constant for a grade |
| Applies to | Amorphous and semi-crystalline | Semi-crystalline only |
| Typical value | Lower | Higher |
The two aren’t directly connected. Softening happens when a loaded specimen deflects enough for a needle or a bar to travel a set distance. That can occur far below the temperature at which any crystal actually melts. A plastic shelf that bows under a heavy box on a warm summer day softened. It didn’t come close to melting.
That gap is exactly why engineers lean on a third number, heat deflection temperature, for load-bearing decisions. Our guide to heat deflection temperature vs melting point works through that comparison with a real failed part. Here, it is enough to notice that softening and melting sit at different points on the curve, defining different failure modes.
The Full Temperature Picture: Softening, Vicat, Tg, HDT and Tm
A datasheet can carry four or five thermal numbers for one grade. Here they are together, as starting points for common engineering plastics. Treat these as typical values for unmodified grades and confirm every one against the specific grade datasheet.
| Material | Class | Tg (°C) | Softening / Vicat (°C) | HDT 1.8 MPa (°C) | Tm (°C) | Design number to use |
|---|---|---|---|---|---|---|
| POM (acetal) | Semi-crystalline | ~ -60 | ~150–160 (Vicat A50) | ~99–110 | ~165–175 | HDT for loaded parts |
| PA66 (nylon 66) | Semi-crystalline | ~50–65 dry | ~250 (Vicat, dry) | ~70–100 | ~255–265 | HDT; Tg rises with moisture |
| HDPE | Semi-crystalline | ~ -110 | ~112–132 (Vicat) | ~40–80 | ~125–137 | HDT |
| PC (polycarbonate) | Amorphous | ~147 | ~145–150 (Vicat B50) | ~125–135 | No true Tm | HDT / Tg |
| PMMA (acrylic) | Amorphous | ~105 | ~100–110 (Vicat) | ~90–100 | No true Tm | HDT / Tg |
| ABS | Amorphous | ~105 | ~100–105 (Vicat B50) | ~75–105 | No true Tm | HDT / Tg |
Read the table the way a specifier reads it. The melting point column is dominated by processing, not by service. The softening and HDT columns are the ones that describe what a part does under heat and load. The amorphous rows confess the truth outright: no melting point at all, only a softening threshold near Tg. Acrylic is a clear example, and our guide to the PMMA melting point explains what actually happens to it as the temperature climbs.
Each material has a deeper write-up if you are specifying, not just comparing: POM melting point, PA66 melting point, polycarbonate melting point, and ABS melting temperature. For the broad thermal and processing view across all of them, see our engineering plastic temperature guide.
Which number should you actually specify?
Match the number to the job.
- A loaded part in short-term heat (a bracket near a warm motor, a part through a paint bake): specify HDT or softening, and always state the load.
- A semi-crystalline process (injection barrel setpoints, drying): specify Tm, because melt temperature sits 20 to 30 °C above it.
- A long-term service limit: specify neither. Use continuous service temperature or the UL relative thermal index, because thermal aging is a slower question than any single test can answer.
One rule ties it together: never read a short-term softening or HDT value as a long-term service temperature. They answer different questions on different timescales.
Why Softening and Melting Values Shift Between Grades
Neither number is fixed for “POM” or “HDPE” as a family. Both move with the grade, and knowing which levers move them keeps you from specifying by family name alone.
Crystallinity and density
More crystallinity means more order to break down, which lifts both the softening point and the melting point. This is why HDPE softens higher than LDPE, and why density is printed on every polyethylene datasheet. A higher-density grade is a higher-softening grade. To see how much those values move across real polyethylene grades, our grade-by-grade HDPE melting point guide breaks the numbers down for processors and buyers.
Comonomer and molecular structure
POM homopolymer melts around 175 °C; the copolymer melts a little lower, near 165 °C. Nylon 6 and nylon 66 sit about 40 °C apart even though both are polyamides. The backbone decides the baseline, and the grade tunes it.
Additives, fillers, and reinforcement
This is where the two numbers finally part company. Glass fibre lifts HDT dramatically, nudges the softening point, and leaves the melting point essentially untouched. A 30% glass-filled PA66 can hold a load to well over 200 °C at 1.8 MPa, while its melting point stays put near 260 °C. That contrast is the cleanest proof that HDT and softening are engineering measures while Tm is a phase-change constant. Flame retardants and impact modifiers bend the softening number too, usually downward.
Moisture
For a hygroscopic plastic such as PA66, absorbed water is a thermal variable. Water plasticizes the polymer and depresses both its Tg and its practical softening. A “dry” datasheet and a “conditioned” datasheet can quote different softening points for the same grade, and neither is wrong.
How to Verify Softening and Melting Data When You Buy Resin
Tomás learned the cost of an unspecified number when his connector housing failed a heat-soak test. The datasheet had quoted a softening point, and the number looked comfortable against his requirement. What it did not say was that the figure had come from a 10 N Vicat measurement, while the application, loaded and mounted, behaved closer to a 50 N result. The grade was fine. The specification was not, because a softening point without its method is incomplete.
That is the practical reason the softening-versus-melting distinction matters to buyers, not just to engineers. Both numbers are only as good as the grade and the documentation behind them. Off-spec, regrind, or counterfeit lots can shift the softening and melting baselines, and a missing method hides the drift.
When a lot arrives, check the grade designation against the manufacturer’s name, the lot number, the Vicat value with its method stated, the melting point by DSC, the melt flow index, and the density. Compare all of it against the manufacturer datasheet. If a supplier can’t show lot-traceable documentation, their softening point is a claim, not a specification.
We hold certified inventory of branded engineering resins in Suzhou, with a manufacturer-issued COA on every lot, branded prime and compounded grades, a 25 kg minimum order so smaller buyers can qualify a grade without a container commitment, and FOB Shanghai pricing with a standard 7 to 14 day lead time. Our POM resin and PA66 resin pages list the grades we stock. If you’re choosing against a thermal requirement, send us the service temperature and the load, and we’ll tell you which number governs your part, and whether a different material is the better answer. Request a quote and we respond within 24 hours.
Conclusion: Two Numbers, Two Jobs
Keep four things from this guide.
The softening point vs melting point distinction comes down to two different kinds of number. The melting point is a thermodynamic transition, measured by DSC, and belongs to semi-crystalline plastics. The softening point is an empirical result, defined by a test load and a heating rate, and it only means something when its method travels with it.
The softening point is always lower, and the reason is physical: a loaded specimen deforms long before its crystals melt. For an amorphous plastic the softening point sits near Tg; for a semi-crystalline plastic it sits below Tm.
Neither number, on its own, tells you how hot a part can run. For a loaded part, specify HDT or softening with the load stated. For a process, specify Tm. For a long-term limit, specify continuous service temperature.
The numbers also belong to the grade, not the family. Comonomer, crystallinity, fillers, and moisture all move them, which is why a datasheet and a certificate of analysis belong in the same conversation. If you want a certified grade with documented, repeatable thermal values, request a quote and we’ll send the datasheets that match your application. Branded prime resin, COA on every lot, 25 kg minimum order, FOB Shanghai, response within 24 hours.
Frequently Asked Questions
Is the softening point the same as the melting point?
No. A melting point is a thermodynamic transition, the temperature at which a semi-crystalline plastic’s crystals melt, measured by DSC. A softening point is an empirical test result, the temperature at which the plastic deforms a set amount under a set load, measured by Vicat or heat deflection. They measure different things.
Is Vicat the same as melting point?
No. Vicat (ISO 306 / ASTM D1525) measures needle penetration under load, so a Vicat softening temperature is an empirical softening point, not a melting point. For an amorphous plastic the Vicat value sits near Tg; for a semi-crystalline plastic it sits below, but close to, the melting point.
Why is the softening point lower than the melting point?
Because softening is a deformation under load, not a phase change. A loaded specimen can deflect and fail long before its crystals melt. That is why the softening point or HDT is the number that governs a loaded part, while the melting point sits higher and mainly governs processing.
What is the softening point of plastic?
It is the temperature at which a plastic specimen deforms a defined amount under a defined load. Common methods include the Vicat softening point (ISO 306 / ASTM D1525), heat deflection temperature (ISO 75 / ASTM D648), and the Mettler softening point (ASTM D3104). The value must always be quoted with its method, because load and heating rate change the result.
How is Vicat softening temperature measured?
A flat-ended needle with a 1 mm² cross-section presses into a specimen under a set load, while the specimen is heated at a set rate. The Vicat temperature is the point at which the needle penetrates 1 mm. The four standard methods combine a 10 N or 50 N load with a 50 °C/h or 120 °C/h heating rate.
Which is a better design number, softening point or melting point?
For a part that carries a load in service, the softening or HDT value is the useful one, provided you state the load. The melting point governs processing. For a long-term service limit, use neither: specify continuous service temperature or the UL relative thermal index.
Do all plastics have a softening point?
Almost all do in practice, because softening tests were designed for materials without a definite melting point. The exception is a highly crystalline resin, whose melting peak dominates the transition so completely that no independent softening point can be singled out.
What is the softening point of POM, PA66, or PC?
For typical unmodified grades, POM softens around 150 to 160 °C (Vicat A50), PA66 around 250 °C measured dry, and PC around 145 to 150 °C (Vicat B50), though PC has no true melting point at all. Always confirm against the specific grade datasheet, since fillers and moisture move these values.
