Sources and scope for this page: UL 746B — Polymeric Materials, Long Term Property Evaluations (UL does not year-stamp 746B; RTI values as published for each grade), covering the maximum continuous service temperature column; and ASTM D648-18, covering the deflection temperature under load column (one more source is tabulated below). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-11-08 · based on n/a — UL does not year-stamp 746B; RTI values as published for each grade
Quick Answer
Service Temperature by Plastic
| Material[1] | Glass transition °C[1] | Melting point °C[1] | Deflection temp @ 1.8 MPa °C[1] | Max continuous °C[2] | Max intermittent °C[2] | Note[1] |
|---|---|---|---|---|---|---|
| Rigid PVC # | 80 | none (amorphous) | 70 | 60 | 80 | Softens gradually; not a structural material when hot |
| Acrylic (PMMA) # | 105 | none (amorphous) | 82 | 65 | 90 | Tg governs; there is no crystalline phase |
| UHMW-PE # | −120 | 135 | 80 | 80 | 100 | Lowest ceiling of the common engineering plastics |
| POM-H (Delrin) # | −60 | 175 | 125 | 85 | 149 | DTUL is high relative to the continuous rating |
| POM-C (Acetal copolymer) # | −60 | 166 | 110 | 100 | 140 | Slightly lower melting point than POM-H |
| Nylon 6/6 # | 50 | 265 | 75 | 105 | 150 | DTUL at 1.8 MPa is only 75 °C despite the 265 °C melt |
| Polycarbonate # | 147 | none (amorphous) | 132 | 115 | 130 | Tough and transparent; not for hot water |
| PET-P (Ertalyte) # | 75 | 255 | 80 | 120 | 150 | Better wear than acetal at higher temperature |
| PEI (Ultem) # | 217 | none (amorphous) | 200 | 170 | 200 | Inherently UL94 V-0, no additives |
| PES (Polyethersulfone) # | 225 | none (amorphous) | 203 | 180 | 200 | Amorphous; transparent and steam-sterilisable |
| PPS # | 90 | 280 | 110 | 220 | 240 | High ceiling but brittle; DTUL at load is far below the melt |
| PEEK # | 143 | 343 | 152 | 250 | 300 | The highest continuous rating of the melt-processable plastics |
| PTFE # | −97 | 327 | 55 | 260 | 290 | Highest melting point here, lowest DTUL under load |
Continuous ratings follow the UL 746B relative thermal index where one is published, and supplier data-sheet ratings otherwise. The melting point column is not a service limit — it is the temperature at which the crystalline phase disappears under zero load. A part carrying load fails far below it, which is what the deflection temperature column measures. Amorphous plastics (PVC, acrylic, polycarbonate, PEI, PES) have no melting point at all; they soften progressively above their glass transition.
Which Temperature Number Actually Limits Your Part
Four different temperatures appear in this table, and using the wrong one is the most common material selection error:
Glass transition temperature (Tg) is where an amorphous plastic changes from glassy and stiff to rubbery and compliant. For an amorphous plastic — PVC, acrylic, polycarbonate, PEI, PES — this is the real ceiling, because stiffness falls off a cliff above it. For a semi-crystalline plastic it matters much less, because the crystalline regions keep the material stiff well past Tg. That is why nylon 6/6 has a Tg of only 50 °C yet works at 105 °C.
Melting point is the temperature at which the crystalline phase disappears. It is a materials-science number, not a design limit. A part held near its melting point has essentially no stiffness regardless of what the data sheet's room-temperature tensile strength says.
Deflection temperature under load (DTUL) is the useful one. It measures the temperature at which a standard specimen deflects a fixed amount while carrying a specified load — 1.8 MPa (264 psi) in the column above, or 0.46 MPa (66 psi) in the lighter test. It directly answers "at what temperature will my part start to sag under its own load?" Note PTFE: a melting point of 327 °C, the second highest here, but a DTUL of only 55 °C. The high melting point is irrelevant to a loaded part.
Maximum continuous service temperature is a long-term property, usually taken from the UL relative thermal index. It reflects how long the material retains half its original property value at that temperature — a measure of thermal ageing, not of instantaneous strength. It is typically higher than the DTUL, because a part can survive at a temperature where it would deflect if loaded.
Semi-Crystalline vs Amorphous
The table splits naturally into two groups, and the split predicts most of the differences in behaviour you will notice in the shop.
Semi-crystalline plastics — POM, nylon, PEEK, PPS, PET, UHMW-PE, PTFE — have ordered regions that hold the material stiff up to near the melting point. They are opaque or translucent, resist fatigue and wear well, do not stress-crack readily, and hold strength above Tg. They shrink noticeably on cooling from the melt and their machined surfaces can be slightly waxy or crystalline in appearance.
Amorphous plastics — PVC, acrylic, polycarbonate, PEI, PES, PSU — have randomly arranged chains with no crystalline order. They are usually transparent, shrink less and more predictably, machine to a glassier finish, and are stiffer at room temperature. Their weakness is that once past Tg their stiffness collapses rather than declining gradually, and many of them stress-crack in contact with solvents.
This is why the four amorphous materials in the table cluster in the middle: they are excellent at room temperature and unusable near their Tg. It is also why PEEK and PTFE sit at the top — both are semi-crystalline with very high melting points.
Frequently Asked Questions
- What is the maximum temperature for plastic?
- Among melt-processable unfilled plastics, PEEK is the practical ceiling at roughly 250 °C continuous and 300 °C intermittent. PTFE reaches 260 °C continuous but loses stiffness far earlier under load. Below them, PPS and PES work to about 220 °C and 180 °C respectively. Beyond PEEK the options stop being thermoplastics — you move to thermosets, polyimides, or metal.
- Why does nylon melt at 265 °C but only work to 105 °C?
- Because melting point and service temperature measure different things. Nylon 6/6 is semi-crystalline, so its stiffness is held up by the crystalline phase rather than by the polymer's glass transition — but that crystalline phase starts to soften and the material begins to creep and oxidise long before it melts. The deflection temperature of 75 °C under 1.8 MPa is the number that governs a loaded part.
- Can I use the melting point as a service limit?
- No, and doing so is the most common plastic selection error. The melting point is measured with no load applied. A part under any real load will deflect, creep or fail far below it — PTFE melts at 327 °C and deflects at 55 °C under 1.8 MPa. Use the deflection temperature under load for loaded parts, and subtract a safety margin.
- What does UL 746B actually rate?
- UL 746B determines a relative thermal index (RTI): the temperature at which the material is expected to retain half of a chosen property — typically tensile strength, impact strength or dielectric strength — after long-term exposure. It is an ageing rating, not a melting or deflection rating, and RTI values are listed separately for each property because a material can age differently in each.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | Published unfilled-grade property data sheets (thermal section) | reference | supplier data sheets as published 2024–2026 |
| [2] | UL 746B — Polymeric Materials, Long Term Property Evaluations | reference | n/a — UL does not year-stamp 746B; relative thermal index as published for the grade |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| UL 746B — Polymeric Materials, Long Term Property Evaluations | n/a — UL does not year-stamp 746B; RTI values as published for each grade | the maximum continuous service temperature column |
| ASTM D648 — Deflection Temperature of Plastics Under Flexural Load | ASTM D648-18 | the deflection temperature under load column |
| ASTM D3418 / D789 / D4591 — transition and melting temperatures | ASTM D3418-21, D789-19, D4591-17 | the glass transition and melting point columns |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Values are for unfilled, natural-grade resin. Glass- and mineral-filled grades raise the deflection temperature substantially — a 30% glass-filled nylon 6/6 deflects at 250 °C where the unfilled grade deflects at 75 °C — so a filled grade's data sheet is required for any elevated-temperature design. Continuous ratings describe thermal ageing, not instantaneous strength: a part at its continuous rating may still be far too hot to carry load.
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