Heat Deflection Temperature (HDT): ASTM D648 and ISO 75, the 2 Standard Loads, and the 6 Additive Families That Raise or Lower It

Heat deflection temperature (HDT) is the temperature at which a standard plastic bar, loaded in bending at 0.455 or 1.82 MPa and heated at 2 °C per minute, deflects by 0.25 mm, and 6 additive families are bought or blamed for moving it: reinforcing fibres, mineral fillers and nucleating agents raise it, while impact modifiers, plasticizers and liquid flame retardants lower it. Additives move the number further than a processing change does: 4.7 wt% of montmorillonite clay lifted nylon 6 to an HDT of 152 °C, 87 °C above the neat polymer, in the work Kojima, Usuki, Kawasumi and Okada published from Toyota Central R&D Labs in 1993. So which additive moves HDT in which plastic, and why does the same filler barely shift it in another?

Fibres and platy fillers stiffen the bar, and nucleating agents raise its crystallinity. Rubber impact modifiers, plasticizers and aryl phosphate flame retardants soften the bar instead, and the lower side has its own sourced number: 20 wt% of an aryl bisphosphate left a PC/ABS blend at an HDT of 72.6 °C in a 2024 study in the journal Materials. The size of every effect depends on the polymer, because a semi-crystalline plastic gives a stiffening additive a temperature window that an amorphous plastic lacks.

This page follows one order: the three names of the test and its 0.25 mm end point, the ASTM D648-18 and ISO 75 methods with their 2 loads, the 6 additive families, the amorphous and semi-crystalline mechanism, values per polymer, a 7-step route to a higher HDT, and the limits of the result. Six families of plastic additives are bought partly or wholly for what they do to this one number.

The 10 facts below define a heat deflection temperature result.

ASTM D648-18 is the edition current on 24 October 2026. ISO 75 parts are given without an edition year because the ISO text was not available to us.

What Is Heat Deflection Temperature?#

Heat deflection temperature is the temperature at which a plastic bar under a fixed bending load reaches a fixed deflection, which ASTM D648-18 itself calls the temperature at which "an arbitrary deformation occurs" when specimens are subjected to "an arbitrary set of testing conditions". Both the stress and the 0.25 mm deflection are set by convention rather than by any physical transition in the polymer, so the result is a short-term, comparative reading of stiffness at temperature.

If the load and the deflection are both arbitrary, why does every engineering data sheet still print an HDT? The number ranks grades against each other under identical conditions, which is exactly the "control and development work" ASTM D648-18 says the method suits. Compounders read it to compare a filled grade with its base, quality labs to check incoming lots, and formulators to screen a new filler or nucleator.

HDT, heat distortion temperature and DTUL: three names for one test#

Heat deflection temperature, heat distortion temperature and deflection temperature under load (DTUL) are three names for the same measurement, and a data sheet that uses any of them is reporting the deflection temperature of a loaded bar under ASTM D648 or ISO 75. The three names in circulation are listed below.

• Heat deflection temperature (HDT): the common name on data sheets and in ISO-based literature.
• Heat distortion temperature: the older name for the same test, still used in polymer handbooks and search engines.
• Deflection temperature under load (DTUL): the wording closest to the ASTM D648-18 title, used in US resin databases.

The standard and the stress printed next to the number change the result; the name does not. ISO 75 labels its load levels with letters, which is where the notation HDT/A and HDT/B on European data sheets comes from, and a heat distortion temperature quoted without its load level or stress cannot be placed on either scale.

What the 0.25 mm deflection measures: stiffness at temperature, not melting#

The 0.25 mm end point measures how far a plastic's stiffness has fallen as it heats under a set load, not the temperature at which it melts, so a part can lose shape at its HDT and still be tens of degrees below its melting point. HDT is read, not calculated: no formula turns a polymer's composition into its HDT, and the stress formula that sets the weight belongs to the standard itself.

A plastic has 4 different heat limits, and each is a different test. The HDT is a whole bar softening under a bending load in a short temperature ramp. The Vicat softening temperature is a loaded needle sinking into the surface under ISO 306 or ASTM D1525-25. The glass transition and melting temperatures are read without any load on a DSC: rigid PVC's glass transition is 82 °C and isotactic polypropylene melts at 160 to 166 °C. The fourth limit is the long-term rating from oven ageing under ASTM D3012, ISO 188 or UL 746B.

Plotted against temperature, the deflection of an HDT bar stays near zero, then turns upward as the modulus falls, and the HDT is the point at which that curve crosses the 0.25 mm line.

Is heat deflection temperature the same as glass transition temperature?#

No: the glass transition temperature is a property of the polymer read without load on a DSC, while heat deflection temperature is the point at which a loaded bar bends 0.25 mm, so the two coincide only roughly, and only in amorphous plastics. The glass transition temperature itself is defined in the glossary. Amorphous plastics such as polycarbonate (glass transition 147 °C) lose their stiffness at the glass transition, so their HDT sits just below it. Semi-crystalline plastics keep a crystalline skeleton above the glass transition, which is why their HDT depends so strongly on crystallinity and fibre content, and why the comparison only holds when HDT is compared with HDT and glass transition with glass transition.

How Is HDT Measured? ASTM D648 and ISO 75#

HDT is measured by loading a moulded bar in three-point bending, immersing it in a heat-transfer liquid, raising the temperature at 2 °C per minute and recording the temperature at which the bar has deflected 0.25 mm, under ASTM D648-18 with the bar on its edge or under ISO 75 with the bar lying flat. HDT is one of the 6 mechanical and thermo-mechanical methods among those for testing plastic additives, next to the Vicat softening point, impact strength, tensile testing, Shore hardness and thermal conductivity.

The test set-up: a loaded bar in a heated bath#

An HDT test runs in 4 steps: the bar is placed on two supports, a weight sets a fixed bending stress at mid-span, the bath heats at 2 °C per minute, and the temperature is read when the mid-span deflection reaches 0.25 mm. The 4 steps of the procedure are listed below in the order a laboratory runs them.

1. Mould or cut a bar at least 3 mm thick and condition it as the material specification requires.
2. Place it on two supports, on its edge for ASTM D648-18 or flat for ISO 75, and apply the weight that gives the chosen stress at mid-span.
3. Immerse it in a heat-transfer liquid, such as a silicone or mineral oil, and heat at 2 °C per minute.
4. Record the temperature at which the bar has deflected 0.25 mm.

The bar's moulding history is part of the result. Specimens are moulded to ASTM D4703-24 or ISO 293:2023, two practices that are similar but no longer equivalent, so the moulding standard belongs on the report next to the test standard. Under ASTM D648-18 Note 1, sheet between 1 and 3 mm thick can be tested as a composite of laminae at least 3 mm thick. Where a material specification sets its own conditioning or specimen rules, ASTM D648-18 clause 5.2 gives the specification precedence over the method.

The 2 standard loads: 0.45 MPa and 1.8 MPa#

HDT is run at 2 standard loads: a low stress of 0.455 MPa (66 psi) in ASTM D648-18 or 0.45 MPa in ISO 75, and a high stress of 1.82 MPa (264 psi) in ASTM D648-18 or 1.80 MPa in ISO 75.

The two loads, the values each standard uses and the way data sheets typically apply them are compared below.

The usage column reflects common data-sheet practice, not a rule in either standard; a supplier can quote either stress for any grade.

ISO 75 adds a third stress of 8.00 MPa for very stiff materials and labels its three load levels with letters, while ASTM D648-18 uses its own method letters for a different purpose. A letter alone therefore means different things under the two standards. An HDT should always be quoted with its stress in MPa, never with the method letter alone.

Why does one plastic have two HDT values?#

One plastic has two HDT values because the bar needs less softening to deflect 0.25 mm under 1.82 MPa than under 0.455 MPa, so the high-stress HDT is always the lower of the two. The two values are two different numbers on the same specimen geometry, not a range and not a tolerance. Never compare an HDT at 0.45 MPa with one at 1.8 MPa, and treat a data sheet that prints a single HDT without its stress as a number that cannot be compared with anything.

ASTM D648-18 and the ISO 75 parts: editions, specimens and scope#

The current ASTM method is ASTM D648-18, "Standard Test Method for Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position", last updated on 23 April 2018, and the ISO method is split into ISO 75-1 for the general procedure and ISO 75-2 for plastics. ASTM D648-18 supersedes D648-16 and the earlier 2007, 2006, 2004, 2001 and 2000 editions, so a data sheet that still quotes D648-01 is citing a withdrawn edition.

The two methods measure the same property with 5 differences that stop their results being interchangeable.

Editions are the versions current on 24 October 2026. Where no ISO edition is shown, the ISO text was not available to us and only the part number is stated.

ASTM D648-18 applies to moulded and sheet materials 3 mm (1/8 in.) or thicker that are rigid or semirigid at normal temperature, which is why flexible PVC carries no HDT at all, a point the plasticizer section returns to. ISO 75 has a third part, ISO 75-3, for high-strength thermosetting laminates and long-fibre reinforced plastics, which sit outside the scope of ISO 75-2.

Is ASTM D648 equivalent to ISO 75?#

No: ASTM D648-18 states in its own notes that it covers the same subject as ISO 75-1 and ISO 75-2 but differs from them technically, and that results from the two should not be compared. A specification written to one standard is therefore not met by a result from the other, so a supplier's data sheet and an incoming-goods check have to name the same standard and the same stress. The most visible reason is the bar itself: ASTM loads it on its edge and ISO loads it flat, so the same compound is bent across a different section.

How Do Additives Change Heat Deflection Temperature? 3 Families Raise It, 3 Lower It#

Six additive families change heat deflection temperature: reinforcing fibres, mineral fillers and nucleating agents raise it, and impact modifiers, plasticizers and liquid flame retardants lower it. If three families raise HDT, why does a talc-filled or glass-filled grade gain so much more in polypropylene or nylon than in polycarbonate? The answer lies in the polymer's crystallinity, which the mechanism section further down explains; this section covers what each family does and the sourced example behind it.

The 6 families, the direction each moves HDT, the mechanism and the sourced example are compared below.

Values come from the cited study or supplier at the stated loading in the stated polymer. An HDT from one grade, one specimen and one stress does not transfer to another.

1. Reinforcing fibres: glass, carbon and aramid#

Reinforcing fibres raise HDT more than any other additive family because a network of fibres stiffer than the polymer keeps carrying the bending load after the matrix around it has begun to soften. The effect is strongest in semi-crystalline polymers and holds at the high stress of 1.82 MPa as well as at the low one. Loadings, sizing and selection for all reinforcing fibers for plastics sit on the reinforcement hub.

A fibre transfers load from the matrix by shear along its length, so it carries its full share only above a critical length, the relationship Kelly and Tyson set out in 1965 on the shear-lag model Cox published in 1952. Short glass loses length in the extruder and the mould, which is why long-fibre thermoplastics are sold as 10 to 12 mm pellets and keep at least 6.35 mm of fibre in the moulded part. So which fibre is chosen when glass is not enough? The three reinforcing fibres used in thermoplastic compounds are listed below.

• Glass fibre: the default, E-glass at a modulus of 76.0 GPa, a tensile strength of 3,445 MPa and a density of 2.58 g/cm3, chopped into PA, PP and PBT compounds at loadings that start around 10 wt%.
• Carbon fibre: filaments of 5 to 10 µm, mostly from a PAN precursor, stiffer and lighter than glass.
• Aramid fibre: an aromatic polyamide in which at least 85 % of the amide linkages sit on two aromatic rings, with para-aramid melting above 500 °C.

Glass fibre carries a sizing of 0.5 to 2.0 wt% that couples it to the matrix, and Clariant states that flame-retarded polyamides carry 10 to 50 % glass; GF30 is the grade name of the 30 wt% version. Carbon fibre is chosen when weight or electrical conductivity matters as much as heat, because the same stiffness comes with a lower density than glass. Grades and precursors are compared under carbon fiber reinforced plastics, the page for that fibre.

The gain has a cost in shape. Fibres orient in the flow direction during moulding, so HDT and stiffness differ along and across the flow, and a part with uneven orientation can warp as it cools.

Aramid is bought for wear in POM and PTFE compounds, often next to glass, rather than for HDT; wear grades and the para and meta types are covered under aramid fibres in plastics.

2. Mineral fillers and nanoclay: talc, wollastonite, mica and montmorillonite#

Mineral fillers raise HDT in proportion to their aspect ratio: platy talc and mica and needle-shaped wollastonite stiffen a compound far more per weight percent than blocky calcium carbonate, because a long, thin particle carries more of the load the Halpin-Tsai equations assign to it. Halpin and Kardos set out those equations in Polymer Engineering and Science in 1976. Aspect ratio, density and loading for every mineral are tabulated under fillers for plastics.

The four minerals that raise HDT most per unit of loading are listed below, each with its aspect ratio.

• Talc: platelets with an aspect ratio of 5 to 40, loaded at 10 to 40 wt% in polypropylene (the T10 to T40 grades) for stiffness and HDT, and a nucleating agent for PP as a side effect.
• Wollastonite: needles with an aspect ratio of 5 to 30, which raise strength, modulus and HDT in the way a short fibre does.
• Mica: platelets with an aspect ratio of 20 to 100, used where flatness and dimensional stability matter.
• Montmorillonite (nanoclay): silicate layers about 1 nm thick that reach an aspect ratio of 100 to 1,000 once exfoliated, stiffening a polymer at loadings below 5 wt%.

Platelets raise stiffness and HDT with less warpage than fibres, while calcium carbonate, with an aspect ratio near 1 to 3, raises HDT less than talc or glass fibre.

The most-cited HDT result in additive science comes from nanoclay: Kojima, Usuki, Kawasumi and Okada at Toyota Central R&D Labs reported in the Journal of Materials Research in 1993 that a nylon 6-clay hybrid with 4.7 wt% montmorillonite reached an HDT of 152 °C, 87 °C above the neat nylon 6, with strength and modulus up and impact strength comparable to the unfilled polymer. That 87 °C rise from less than 5 wt% of additive compares with the 10 to 40 wt% of talc in plastics such as T20 and T40 polypropylene; the T grades and talc's nucleating side effect are on talc in plastics.

A particle only stiffens the compound if the interface holds, which is the job of a coupling agent such as a silane on glass or wollastonite, or a maleic-anhydride-grafted polymer in polyolefins. Acidic mineral surfaces such as talc and clays also adsorb phenolic antioxidant, so a filled grade needs a stronger stabilizer package than the unfilled grade it came from, or the filled grade loses its stabilization faster than the base resin did.

3. Nucleating agents: higher crystallinity, higher HDT#

Nucleating agents raise HDT because more, smaller crystals form when the melt cools, and a polymer with a higher crystalline fraction stays stiff to a higher temperature under the same load. Polypropylene is the reference case for nucleating agents for polypropylene, whose crystallinity runs from 30 to 60 %. Gahleitner and co-workers (2011) list what nucleation buys in PP: a higher crystallization temperature, a shorter cycle time, a higher flexural modulus and HDT, and more isotropic shrinkage.

The supplier evidence points the same way. ADEKA states that ADK STAB NA-11 (CAS 85209-91-2), a sodium phosphate salt that melts above 400 °C, raises stiffness, HDT, crystallization temperature and clarity. ADEKA also reports that 0.1 wt% of its NA-27 can let a PP part be 10 % thinner at equal load, about 9 kg of lightweighting per 90 kg of PP in a vehicle. Neither figure is a measured HDT, and the FDA limit for NA-11 under 21 CFR 178.3295, 0.30 wt% in polypropylene (0.10 wt% for conditions of use A to H), is a legal maximum rather than a dosage.

Beta nucleation trades the other way. Beta-PP is markedly tougher than alpha-PP, as Varga reviewed in 2002, so a beta-nucleated grade is bought for impact strength rather than for HDT; industrial pipe practice combines 0.02 % alpha nucleator with 0.2 % beta nucleator. Beta nucleating agents trade stiffness for toughness, and pigments can do the reverse by accident: phthalocyanine pigments nucleate PP and cause warpage in coloured parts.

4. Impact modifiers: the stiffness the rubber phase costs#

Impact modifiers lower HDT because the soft rubber particles that absorb impact energy also lower the modulus of the whole compound, which is the price a formulator pays for toughness.

The named instances differ by polymer: MBS, acrylic core-shell modifiers and CPE in PVC, and EPR and polyolefin elastomers (POE) in polypropylene. In rigid PVC the effective impact-modifier window is about 3 to 10 phr: below 3 phr the modifier has no measurable strength effect, and above 10 phr the further gain is insignificant while stiffness keeps falling; CPE in PVC and CPVC runs at 1 to 10 phr, with 2.5 to 7.0 phr preferred. Butadiene-based modifiers such as MBS are also not UV-stable, which matters for outdoor parts. How soft the rubber itself is shows in one supplier figure: Dow states a Vicat softening temperature of 45 °C and a density of 0.870 g/cm3 for ENGAGE 8100, the modifier on its own, not a compound containing it. The rubber types and their phr windows are compared under impact modifiers, and an HDT result belongs next to an impact result, because the two move in opposite directions.

5. Plasticizers: why flexible PVC has no HDT#

Flexible PVC has no HDT because plasticizers lower the glass transition so far below room temperature that the material is no longer rigid or semirigid, and ASTM D648-18 applies only to rigid and semirigid plastics. A flexible PVC compound carries 5 to 65 wt% plasticizer, so the test is never run on it.

Plasticizers lower HDT long before that point, because they lower the glass transition that sets the HDT of an amorphous polymer such as PVC, which has a glass transition of 82 °C unplasticized. The exception sits at low levels: below about 15 phr, antiplasticization can make PVC stiffer rather than softer, a threshold reported in secondary literature rather than in a standard. Flexible PVC is judged on Shore hardness instead of HDT, and plasticizer efficiency is compared at Shore A 80 or at 50 phr, with DEHP set to a reference factor of 1.00; efficiency and the phr conventions are set out under plasticizers for plastics.

6. Flame retardants: liquid aryl phosphates against mineral hydroxides#

Flame retardants move HDT in opposite directions: liquid aryl phosphates such as BDP and RDP lower it, because they plasticise the polymer as well as suppressing the flame, while mineral hydroxides such as ATH and MDH behave like fillers at the loadings they need. The trade-offs of every class of flame retardants for plastics are compared on the family hub. Aryl phosphates act mainly in the gas phase, and BDP adds char in the polycarbonate phase of a blend, but both remain small molecules dissolved in the polymer, where they lower the glass transition.

The sourced case comes from a 2024 study in Materials (volume 17, article 5682): 20 wt% of an aryl bisphosphate in PC/ABS, with 0.4 wt% PTFE as an anti-drip agent, reached UL 94 V-0 and an LOI of 25.4 but left an HDT of 72.6 °C.

Mineral hydroxides need loadings in the order of 185.7 phr of MDH for an LOI of 30.2 in halogen-free PP, which turns the compound into a highly filled system in which stiffness rises and elongation falls. No HDT value is sourced for such a compound, but the direction is the opposite of the liquid aryl phosphates among the phosphorus flame retardants.

Why Do Additives Raise HDT More in Semi-Crystalline Than in Amorphous Plastics?#

Additives raise HDT more in semi-crystalline plastics because their crystals keep the material solid between the glass transition and the melting point, so a stiffening additive has a temperature window of tens of degrees to work in, while an amorphous plastic loses its stiffness at one glass transition that no filler can move far. How much of that window a polymer has depends on its crystallinity, 30 to 60 % in industrial polypropylene. The deflection curves earlier on this page show the two cases side by side.

Amorphous plastics: the glass transition sets the ceiling#

In an amorphous plastic the glass transition sets the ceiling for HDT: polycarbonate, with a glass transition of 147 °C, cannot be filled or reinforced to an HDT far above it, while an additive that lowers the glass transition, such as a liquid aryl phosphate, pulls the HDT down with it. The whole polymer softens across one transition, so a fibre or filler raises the modulus below that transition but has no crystalline phase to work with above it.

The glass transition temperatures of 6 amorphous plastics in our source set are listed below.

Neat-polymer reference values; grade, comonomer and additive package move them.

The PC/ABS result shows the ceiling working in reverse: a blend of a 147 °C and an about 105 °C glass reached an HDT of only 72.6 °C once 20 wt% of an aryl bisphosphate had lowered its glass transition. For an amorphous plastic, the additive question is less "what raises HDT" than "what avoids lowering the glass transition", which is why plasticizing flame retardants and impact modifiers are audited first in these polymers.

Semi-crystalline plastics: glass fibre lifts HDT toward the melting point#

In a semi-crystalline plastic glass fibre lifts HDT toward the melting point, because above the glass transition the crystals and the fibres together keep carrying the load that the softened amorphous phase can no longer bear. Sizing, fibre grades and loadings are on glass fiber reinforced plastics, the reinforcement node that applies this mechanism to PA, PP and PBT.

The glass transition and melting temperatures of 8 semi-crystalline plastics in our source set are listed below.

Where a cell says "not in our source set", no verified value exists and none is invented.

Nylon 6 is the clearest case. Its glass transition is 47 °C and it melts at 215 to 218 °C, so the unfilled polymer softens under load far below its melting point, and 4.7 wt% of exfoliated clay moved its HDT to 152 °C. Absorbed moisture lowers polyamide stiffness, and glass reduces that effect without removing it, so the conditioning state belongs on every polyamide HDT report; moisture, shrinkage and the property changes glass brings are on glass-filled nylon.

What HDT Do Common Plastics Reach With and Without Additives?#

The verified HDT values in our source set are 3, and each one comes from an additive study: nylon 6 with 4.7 wt% montmorillonite at 152 °C, PC/ABS with 20 wt% aryl bisphosphate at 72.6 °C, and PLA rising from about 60 °C to as much as 190 °C through stereocomplex crystallization. This page prints no generic value table per polymer, because the value tables on the first page of search results cite no source for any row, and grade, stress, specimen orientation and conditioning move HDT by tens of degrees, so one number per polymer would be wrong for most grades. The grade's own data sheet, read with its stress and its standard, is the number to use.

The 3 verified data points, with the additive, loading and source behind each, are compared below.

"Not stated in our source" means the stress used is not recorded in the material we checked, not that the value is unreliable; quote these values as additive effects, not as grade specifications.

Polypropylene: homopolymer, nucleated, talc-filled and glass-filled#

Polypropylene's HDT typically rises in a fixed order of additive packages: an unfilled homopolymer is lowest, a nucleated grade is higher, a talc-filled T20 to T40 compound is higher again, and a glass-filled grade is highest, all still below the 160 to 166 °C at which isotactic PP melts. The full PP package, filler and nucleator included, is on additives for polypropylene. Talc-filled PP is used for automotive interior and under-hood parts, and it needs a stronger antioxidant package because talc adsorbs phenolic stabilizer.

Polypropylene starts to deform under load at its HDT, and for a given grade that temperature is printed on the data sheet with its stress. Our source set holds no sourced HDT value for a PP grade, so this page gives the order of the packages and the 160 to 166 °C melting-point ceiling rather than a number. Coupled long and short glass in PP is covered under glass-filled polypropylene.

Polyamides: PA6, PA66 and the nanoclay benchmark#

Polyamides gain more HDT from reinforcement than any other common plastic, because nylon 6 softens under load above its 47 °C glass transition yet does not melt until 215 to 218 °C, which leaves a window of more than 160 °C for glass or clay to fill. PA66 melts higher still, at 264 °C, and Clariant's flame-retarded polyamides carry 10 to 50 % glass to use it. Heat stabilizers, nigrosine and the glass-fibre package are on additives for nylon (polyamide).

The nanoclay route reached the same territory with far less additive: the 152 °C HDT of the 1993 Toyota nylon 6-clay hybrid came from 4.7 wt% of exfoliated montmorillonite, against the 30 wt% of glass in a GF30 grade. Exfoliation and the Toyota hybrid are covered under nanoclay (organoclay) in plastics.

PLA: from about 60 °C to 190 °C with stereocomplex crystals#

PLA's HDT runs from about 60 °C, just below its 60 to 65 °C glass transition, to as much as 190 °C when PLLA and PDLA crystallize together as a stereocomplex, so the additive and the mould temperature decide which end of that range a part reaches. Its 130 to 180 °C melting range only becomes useful once the part has crystallized. The whole PLA package is on additives for PLA.

A nucleating agent alone is not enough: PLA needs a nucleator plus a hot mould or an annealing step to crystallize, and 3 wt% of stereocomplex cuts the crystallization half-time drastically. The DSC data that rank PLA nucleators belong to the DSC method rather than to HDT, and grade-by-grade data sits on nucleating agents for PLA.

Polycarbonate and PC/ABS: the flame-retardant penalty#

PC/ABS shows the clearest flame-retardant penalty on HDT: a blend whose two phases have glass transitions of 147 °C and about 105 °C fell to an HDT of 72.6 °C with 20 wt% of an aryl bisphosphate, the loading that gave it UL 94 V-0. Anything that plasticises polycarbonate costs HDT directly. The flame-retardant, UV and release package for these resins is on additives for polycarbonate and PC/ABS, where the choice between plasticising and non-plasticising flame retardants starts.

Rigid PVC and CPVC#

Rigid PVC is limited by its 82 °C glass transition, and chlorinated PVC (CPVC), with 63 to 69 % chlorine, raises the softening range to 106 to 115 °C and the maximum service temperature to 93 °C (200 °F), which is why CPVC is chosen for hot-water service rather than PVC. Stabilizer, impact modifier and lubricant levels for rigid grades are on additives for PVC. The 106 to 115 °C range is a Vicat B and glass-transition range, not an HDT, and the 3 to 10 phr of impact modifier a rigid PVC compound carries lowers its stiffness further.

How to Raise the HDT of a Plastic Compound in 7 Steps#

Raise the HDT of a compound in 7 steps, from fixing the target to confirming it on the right bar. The 7 steps are listed below in the order a formulator works through them.

1. Fix the target as an HDT at a named stress and standard, for example at 1.82 MPa under ASTM D648-18, never as a bare temperature.
2. Check the polymer's ceiling: an amorphous polymer stays near its glass transition, while a semi-crystalline polymer can climb toward its melting point.
3. Raise crystallinity first where the polymer allows it, with a nucleating agent or, for PLA, with a nucleator plus a hot mould or annealing.
4. Add a high-aspect-ratio filler or a reinforcing fibre: talc or wollastonite where warpage matters, glass where the highest HDT is needed.
5. Couple the filler to the matrix and strengthen the antioxidant package, because acidic mineral surfaces adsorb phenolic antioxidant.
6. Audit everything that lowers stiffness: impact modifier above the level the impact target needs, plasticizer, and liquid aryl phosphate flame retardants.
7. Confirm on bars moulded to ASTM D4703-24 or ISO 293:2023 and tested to the same standard and stress as the specification, then run the impact test, because the stiffness gained is toughness lost.

Step 6 matters because the three lowering families are added for a different target (toughness, flexibility or a fire rating), and their cost in stiffness only appears when the bar goes into the bath.

The end point is set by the product specification, not by this page, because our sources hold no numeric HDT requirement for any application. A specification should name the stress, the standard and the moulding practice before it names a temperature. To shortlist a stiffening package from the polymer and the function rather than from the test, use the additive finder by polymer and function.

What HDT Cannot Tell You#

HDT cannot answer 5 questions that engineers regularly put to it, and ASTM D648-18 says as much in its own scope, which calls the result the temperature of an arbitrary deformation under arbitrary conditions.

The 5 questions HDT cannot answer, and the test that can, are compared below.

None of these limits makes the test less useful for its real job, ranking one compound against another under identical conditions, but each rules out a reading that data sheets invite. ASTM adds a condition of its own: HDT data are not to be used to predict behaviour at elevated temperature unless time, temperature, loading method and fibre stress are similar to those of the test.

Is HDT the maximum service temperature of a plastic?#

No: HDT is a short-term reading taken while the temperature climbs at 2 °C per minute, and ASTM D648-18 states that its data are not intended for predicting endurance at elevated temperatures, so a maximum service temperature comes from long-term ageing tests instead. A part in service sits at temperature for months or years, under loads that may be lower or higher than the test stress, while the HDT bar spends only minutes near its end point. CPVC illustrates the gap with sourced values: its maximum service temperature of 93 °C sits more than 10 °C below its 106 to 115 °C softening range. A service limit is therefore always a separate rating, read from oven ageing and supplier creep data rather than from the HDT.

Which Other Tests Are Read Alongside HDT?#

An HDT result is read next to 5 other tests, because a higher heat deflection temperature always costs something elsewhere, in toughness, in surface softening or in long-term ageing, and each of those is measured on a different instrument. The complete map of testing plastic additives and additive-dependent properties shows which test answers which question.

HDT vs Vicat softening point#

HDT bends a whole bar under load, while the Vicat softening point pushes a 1 mm2 flat-ended needle 1 mm into the surface under 10 N (A50) or 50 N (B50), so Vicat reports where the surface softens and HDT reports where the part loses stiffness. Vicat runs under ISO 306 or ASTM D1525-25 at 50 or 120 °C/h, ISO 10350 standardises on B50, and D1525 is not recommended for flexible PVC, the same limit HDT has. Method, loads and rates are on Vicat softening point.

DSC: the crystallinity and Tg behind the HDT number#

A DSC explains an HDT result rather than replacing it: it reads the glass transition, the melting point and the crystallization temperature that a nucleating agent raises, which are the reasons behind a higher or lower heat deflection temperature. A nucleator that lifts the crystallization temperature usually lifts HDT with it, and a plasticizer that lowers the glass transition on a DSC scan lowers HDT in the bath. The cooling programme and the nucleation-efficiency scale are on DSC testing.

Impact strength and tensile modulus: what a higher HDT costs#

Every additive that raises HDT by adding stiffness, whether a fibre, a platy filler or a nucleator, tends to cost impact strength, so an HDT gain is only accepted once an Izod or Charpy test confirms the part is still tough enough. Izod runs under ASTM D256-26 in J/m on a 63.5 x 12.7 x 3.2 mm notched bar, or under ISO 180 in kJ/m2, and Charpy under ISO 179; notch, specimen and unit conventions are on impact strength (Izod, Charpy).

Tensile modulus is the room-temperature stiffness that HDT extends to temperature, measured under ISO 527 or ASTM D638-22, two standards that are similar but not equivalent. ASTM D638-22 and ISO 527 are compared under tensile testing of plastics.

Long-term heat ageing and RTI: the service-temperature tests#

Long-term heat ageing answers the question HDT cannot, how long a compound keeps its properties at temperature, by holding specimens in an oven for days or thousands of hours under ASTM D3012, ISO 188 or UL 746B. Recycled polypropylene at 150 °C embrittles in about 25 days unstabilised against 37 to 42 days with 0.2 to 0.4 % of XP 2121, and the additives that decide this result are antioxidants and heat stabilizers, not the stiffening families that set HDT. Oven methods and the UL 746B index are on long-term heat aging and RTI.

Filler and glass content: checking what is in the bar#

An HDT result is only as reliable as the filler or glass content behind it, so incoming checks confirm the loading by ashing the compound under ASTM D5630-22 or ISO 3451 before the HDT bar is moulded. Both cover ash contents of 0.01 % and above in materials stable to 900 °C and exclude halogenated polymers; Procedures A and B are compared under ash content and filler content testing.

Fibre length decides how much of a glass loading actually reinforces, following the critical-length rule of Kelly and Tyson, so a compound with the right ash content can still miss its HDT if the fibres broke during moulding. Residual length after moulding is covered under fibre length and glass content measurement.

How hot is too hot for plastic?#

There is no single temperature that is too hot for plastic, because each plastic has 4 limits, and two common ones soften well below 100 °C: PLA's glass transition is 60 to 65 °C and rigid PVC's is 82 °C. A question about "400 degrees" depends on the unit: 400 °F is 204 °C, above the 160 to 166 °C melting point of isotactic PP, while 400 °C is above the 343 °C melting point of PEEK, the highest-melting thermoplastic in our source set. Moulding happens far above any HDT: engineering plastics are processed at 240 to 320 °C, above their melting points.

What does HDT mean for 3D-printing filaments?#

For a 3D-printing filament, HDT means the same test on a bar of the filament's polymer, and a printed PLA part starts to sag near its 60 to 65 °C glass transition unless it has been crystallized, which in PLA takes a nucleating agent plus annealing. A filament data sheet should state whether the HDT bar was moulded or printed and at which stress, because without both the number cannot be compared; nucleators and annealing for printed PLA are on additives for 3D printing filaments.

Where can you get HDT testing done?#

HDT testing is offered by accredited plastics laboratories and by compounders with their own instruments, and the order has to name 4 conditions, because each one changes the result. The 4 conditions an HDT test order has to name are listed below.

• Standard: ASTM D648-18 or ISO 75-2, never "either".
• Stress: 0.455 or 1.82 MPa (0.45 or 1.80 MPa under ISO 75).
• Specimen: orientation (edgewise or flatwise), thickness and conditioning.
• Moulding standard: ASTM D4703-24 or ISO 293:2023 for the bars.

This page quotes no test price, because no verified price figure exists in our sources.

---