How to read a datasheet

A datasheet is the first document you should pull for any grade you are considering — and one of the most frequently misread. Most selection mistakes trace back to taking a datasheet number at face value without checking the test method, conditions, or specimen state behind it. This guide walks through a typical engineering-resin datasheet section by section.

1. Know what a datasheet is (and is not)

Datasheet values are typical values measured on standardized test specimens under controlled conditions. They are not minimum guaranteed values, and they are not measured on your part. Injection-molded test bars are small, well-packed, and conditioned in a lab; your production part has weld lines, orientation, ribs, gates, and real-world thermal history. Treat datasheet numbers as a comparison tool between grades, never as a performance promise for a finished component.

2. Mechanical properties

• Tensile strength / stress at break (MPa): the stress at which a specimen fails in tension. Compare only values measured at the same test speed and temperature.
• Elongation at break (%): ductile materials like PC can stretch 100%+ before breaking; brittle, glass-filled grades may break below 3%.
• Flexural modulus (MPa): stiffness in bending — usually the best single number for comparing rigidity between grades.
• Izod/Charpy impact (kJ/m² or J/m): always check whether the value is notched or unnotched. Unnotched values can be 10x higher and hide real notch sensitivity.

Glass-fiber reinforcement raises modulus and strength while cutting impact and elongation. Two grades with identical fiber content can still differ significantly depending on fiber length and coupling chemistry.

3. Thermal properties

• Heat Deflection Temperature (HDT): the temperature at which a loaded bar deflects 0.25 mm (or 0.2%, depending on method). Reported at 1.8 MPa or 0.45 MPa load — the 0.45 MPa number is always higher, so verify which load is used before comparing grades.
• Vicat softening temperature: the point at which a flat-ended needle penetrates 1 mm. Useful for amorphous resins, less meaningful for filled grades.
• RTI / continuous service temperature (UL 746B): the maximum temperature for long-term use, often the most honest thermal number on the sheet.
• Melting point (Tm) vs glass transition (Tg): Tm applies to semi-crystalline resins (PA, PBT, POM); Tg governs stiffness loss in amorphous resins (PC, ABS).

For load-bearing parts, HDT at 1.8 MPa and RTI matter far more than the headline Vicat number.

4. Flammability and electrical

UL94 ratings (HB, V-2, V-1, V-0) are always thickness-specific: a grade rated V-0 at 1.5 mm may only be HB at 0.8 mm. Check the thinnest rated thickness and whether the rating covers your wall stock. For appliances, also look for Glow Wire Ignition Temperature (GWIT) and Glow Wire Flammability Index (GWFI). Compare Limiting Oxygen Index (LOI) and Comparative Tracking Index (CTI) only when electrical creepage or insulation is a design concern.

5. Processing properties

• Melt Flow Rate (MFR, g/10 min): higher MFR means easier flow into thin walls, but usually lower mechanical performance. MFR values are only comparable within the same resin family at the same test load.
• Mold shrinkage (%): semi-crystalline resins shrink more (PA66 ~1.5%, PBT ~1.7%) and shrink unevenly with fiber fill; amorphous resins (PC, ABS) shrink less (~0.5–0.7%) and more predictably.
• Drying requirements: hygroscopic resins like PA, PC, PET and PBT must be dried before molding or they hydrolyze and lose properties.

6. The traps to watch for

• ISO vs ASTM methods: the same property measured under ISO and ASTM is not directly comparable (impact methods differ most).
• Conditioned vs dry-as-molded: nylons absorb moisture that raises impact by 2–3x while cutting stiffness. Always check whether values are DAM or at 50% RH conditioning.
• Directionality: glass-filled grades report machine-direction values; transverse properties can be 30–50% lower.
• Single-temperature data: a tensile number at 23°C says nothing about behavior at −20°C or 80°C. Ask for multi-temperature curves when the part runs hot or cold.

Worked through a datasheet and still unsure? Send us the datasheet together with your application requirements — our engineering team reviews selections daily, and ResinIQ can shortlist candidate grades in minutes.