Blog

A technical data sheet (TDS) looks like a simple table of numbers. In reality, half of all material-selection mistakes are born right here — when two numbers are compared as though they were measured the same way. Let's look at how to read a TDS so the figures work for the engineer, not against them.

A typical situation: an engineer opens two technical data sheets, sees “strength 180 MPa” in one and “160 MPa” in the other — and concludes that the first material is stronger. Six months later the part cracks in cold weather, the “weaker” alternative would have performed without issue, and the 20 MPa difference decided nothing at all. The problem is not the materials — the problem is that the TDS was read like a price list rather than a test report. Every number in the data sheet is tied to a standard, to conditions and to the state of the specimen. Without that context, the figures are not comparable. This article is a map of how to read a polymer TDS: what stands behind each line, in what units, to which standard, and exactly where the traps of interpretation hide.

A TDS is not a property of the material — it is a record of conditions

The key idea to start from: a value in a TDS is not “a property of the material in itself,” but the result of a specific test to a specific standard on a specific specimen under specific conditions. It's like a 100 metre time: the figure only makes sense once you know whether it was a level track or an uphill run, a tailwind or a headwind.

That's why sound TDS reading always works in pairs: “number + method.” In a good data sheet, next to every value there is a reference to a standard (ISO, ASTM, IEC or UL) and the conditions: temperature, load, specimen state (dry or conditioned), cut direction. If these notes are missing, that in itself is a signal that the data sheet is incomplete, and its numbers should be treated with caution. Let's go through the key blocks of the data sheet one by one.

Mechanics: strength, modulus and elongation are three different things

The most common confusion in a TDS is between strength, modulus and elongation. All three are measured in a single tensile test to ISO 527 (the counterpart of ASTM D638), but they describe different behaviour of the material.

Tensile strength (tensile strength, MPa) is the stress at which the specimen fails or reaches the yield point. In plain terms: how hard you can pull before it breaks.

Tensile modulus (tensile modulus, MPa or GPa) is stiffness, that is, how strongly the material resists deformation in the elastic region. It is the slope of the initial section of the stress–strain curve. A material can be very stiff (high modulus) yet brittle — and vice versa.

Elongation at break (elongation at break, %) is how far the specimen stretched before it broke. It is a measure of ductility and an indirect indicator of toughness.

An analogy: picture three fishing rods. Strength is the size of fish it can hold before it snaps. Modulus is how much it bends under that same fish (stiff or “a whip”). Elongation is how far you can bend it into an arc before it cracks. These are independent things: a stiff rod is not necessarily a strong one.

Trap #1: comparing numbers measured to different standards. Values to ISO 527 and ASTM D638 are close, but the test speed, specimen geometry and the way the modulus is calculated differ, so a direct comparison down to tenths is incorrect. Always check to which standard both figures were obtained.

Trap #2: confusing modulus and strength. “Modulus 10 GPa” and “strength 100 MPa” are not about the same thing. For a stiff part that must not flex, the modulus is critical; for a part that carries a peak load, it is the strength. Glass-filled polyamides sharply raise the modulus, but their elongation at break drops several-fold — the part becomes stiff, yet less tolerant of impacts and overloads.

Heat resistance: four different “temperatures” that must not be confused

The second most common failure is to take any temperature from a TDS as “the operating temperature of the material.” In reality a data sheet may carry at least four different temperatures, and each means something of its own.

HDT (heat deflection temperature, ISO 75, the counterpart of ASTM D648) is the temperature at which a specimen under a fixed flexural load deflects by a set amount (0.25 mm per ISO 75). The key detail: HDT always comes with a note on the load — method A (1.8 MPa), B (0.45 MPa) or C (8.0 MPa). HDT/A and HDT/B for the same material can differ by tens of degrees, so HDT values may be compared only for the same method.

Vicat softening temperature (VST, ISO 306) is the temperature at which a flat needle under load (10 N in method A or 50 N in method B) penetrates the specimen by 1 mm. This is a different test from HDT, with different physics, and its values are not interchangeable with HDT.

Tg (glass transition temperature) and Tm (melting temperature) are no longer mechanical deflection tests but phase transitions, usually determined by DSC (ISO 11357). Tg is the temperature at which the amorphous phase passes from a “glassy” to a “rubbery” state; above Tg the stiffness drops noticeably. Tm is the melting temperature of the crystalline phase, above which the material flows. For amorphous polymers one speaks of Tg; for semi-crystalline ones (like most polyamides) both matter.

Standing apart is RTI (relative thermal index, per the UL 746B methodology) — this is no longer a one-off temperature but an estimate of the temperature at which the material can operate long-term (for years) while retaining a defined fraction of its properties. RTI is almost always lower than HDT, and it is RTI, not HDT, that comes closest to the notion of “maximum operating temperature for long-term service.”

Trap #3: taking HDT as the operating temperature. HDT is a short-term deflection test point, not a guarantee of long-term service. A part that briefly withstands 200 °C by HDT may not survive years at 150 °C because of thermal ageing. For long-term duty, look at RTI and thermal-ageing data, not at HDT.

Melt flow: what MFR shows — and what it does not

MFR (melt mass-flow rate) and MVR (melt volume-flow rate) to ISO 1133 (the counterpart of ASTM D1238) show how many grams (MFR, g/10 min) or cubic centimetres (MVR) of melt flow through a standard die in 10 minutes under a set temperature and load. It is a simple measure of flowability: a higher MFR means a thinner melt.

Here lies one of the subtlest traps. MFR is an indicator of processability and an indirect measure of molecular weight, but it is NOT a mechanical property. It does not tell you how strong the part will be. Two polyamides with the same MFR can have different mechanical properties. And conversely: high flowability is convenient for thin-wall moulding, but on its own it does not make a material “better.”

Trap #4: comparing MFR under different conditions. MFR only makes sense together with the test conditions — temperature and load mass (for example, 275 °C / 5 kg). One and the same MFR value at different temperatures and loads describes entirely different materials. You can only compare MFR figures taken under identical conditions.

Water: why for polyamides the “dry” and “conditioned” numbers are two different materials

This is a critically important block precisely for polyamides. Polyamides (nylons) are hygroscopic: they absorb moisture from the air, and moisture acts as a plasticiser — it lowers stiffness and strength, raises impact toughness and changes the part's dimensions.

That's why in a good polyamide TDS the mechanical properties are often given in two states: “dry, straight from moulding” (dry / dry-as-molded) and “conditioned” (conditioned) — that is, after being held in a standard atmosphere (23 °C, 50 % relative humidity per ISO 291) or up to saturation. The difference can be dramatic: the modulus of dry PA6 can fall by tens of percent after absorbing moisture.

Trap #5: comparing the dry number of one material with the conditioned number of another. This is the most insidious mistake in polyamide TDSs. If one data sheet shows the modulus in the dry state and another in the conditioned state, the comparison is meaningless. Always check the state in which a characteristic is given, and compare “dry with dry,” “conditioned with conditioned.” Water absorption, incidentally, is measured to ISO 62 (the counterpart of ASTM D570), and a TDS usually gives two figures — after 24 hours of immersion and at saturation.

Impact toughness: why Charpy and Izod are not directly comparable

Impact toughness is measured by two “families” of methods: Charpy (ISO 179) and Izod (ISO 180) — plus the American ASTM D256 (Izod). Both strike the specimen with a pendulum, but the geometry differs: in Charpy the specimen lies horizontally on two supports and the blow falls opposite the notch; in Izod the specimen is clamped vertically as a cantilever with the notch turned toward the pendulum.

The main point for reading a TDS: notched and unnotched are different numbers, and they are not comparable. A notch concentrates stress and sharply lowers the measured toughness. Charpy notched and Charpy unnotched values for the same material can differ several-fold. Likewise, you cannot directly juxtapose Charpy with Izod, or ISO with ASTM — different units and geometry.

Trap #6: juxtaposing impact figures from different methods. Seeing “impact toughness 8” in one TDS and “6” in another, first check: is it Charpy or Izod, notched or unnotched, to ISO or ASTM, at what temperature? Often the “lower” figure actually describes the stricter test.

One table: parameter — standard — unit — what must not be confused

Designation: how to decode PA66-GF30 in the data sheet header

Abbreviations in a TDS are not arbitrary. Base polymers are designated to ISO 1043-1, fillers and reinforcing materials to ISO 1043-2, and the way products are marked for identification and recycling to ISO 11469. So an entry such as PA66-GF30 reads unambiguously: base polymer PA66 (polyamide 66), filler GF (glass fiber), content 30 %. Similarly, CF is carbon fiber, MD is mineral filler, GB is glass beads. The letter denoting a flame retardant is governed by ISO 1043-4. Knowing this system gives the engineer half the information about a material at once, even before reading the properties table.

Checklist: what to look at in a TDS before selecting a material

A practical minimum worth running through every time:

1. Is there a standard next to every number? No reference to ISO/ASTM/IEC/UL — treat the figure with caution.
2. In what state are the mechanics given? For polyamides — dry or conditioned. Compare identical states.
3. Which method for HDT? A (1.8 MPa), B (0.45 MPa) or C (8.0 MPa). Compare only identical ones.
4. Impact toughness — notched or unnotched? Charpy or Izod? At what temperature?
5. Under what conditions is MFR? Temperature and load must be the same for both materials.
6. Is there an RTI or thermal-ageing data? For long-term duty this matters more than HDT.
7. Water absorption at 24 h and at saturation — both figures are critical for dimensional stability.
8. Specimen cut direction — along or across the flow. For filled grades anisotropy is significant.
9. Date and revision of the TDS — grade properties are updated; take the current version.
10. Are you comparing apples with apples? One standard, one state, one set of conditions — otherwise the comparison is invalid.

How this relates to Material Wizard materials

Material Wizard supplies engineering polyamides and high-performance polymers together with technical support: we help you read a TDS for a specific task, verify the test conditions and select a grade so that the figures from the data sheet match the real service regime of the part. Below are two typical entry points.

Both grades can be purchased with delivery across Ukraine; for exact properties for your geometry and processing regime, consult a Material Wizard specialist. We work with production sites in Derazhnia and Kharkiv and support material selection from the data sheet all the way to testing on the specific part. A related topic — glass-fiber reinforcement of polyamides.

Expert breakdown: 5 questions about reading a TDS

Can you compare the strength of two materials if the TDS lists different standards?

Directly — no. Values to ISO 527 and ASTM D638 are obtained at different speeds and specimen shapes, so a difference of a few percent may be an artefact of the method rather than a property of the material. It is correct to compare figures taken to one standard and in one specimen state.

What do “HDT/A” and “HDT/B” mean — why is one number higher than the other?

It is the same deflection-under-load test to ISO 75, but with a different load: method A — 1.8 MPa, method B — 0.45 MPa. Under the smaller load (B) the material “holds” a higher temperature, so HDT/B is usually higher. HDT values may be compared only for the same method.

Why are a polyamide's mechanical properties given in two states in the TDS?

Polyamides absorb moisture, which acts as a plasticiser and substantially changes stiffness, strength and dimensions. That's why the data sheet gives characteristics in the “dry” state (straight from moulding) and in the “conditioned” state (after holding at standard humidity). For a real part operating in a humid environment, the conditioned value is closer.

Higher MFR — is that a better material?

No. MFR to ISO 1133 shows only melt flowability and, indirectly, molecular weight. It is handy for assessing processability (thin-wall moulding), but it is not a measure of strength or durability. A material with a lower MFR may well be mechanically better for your task.

Which is more reliable for estimating the maximum operating temperature — HDT or RTI?

For long-term service — RTI (per the UL 746B methodology), because it accounts for thermal ageing over years. HDT describes only short-term deflection under load and is usually much higher than the real long-term operating temperature of service.