Knowing how to read a plastic datasheet separates buyers who compare grades on paper from buyers who discover mismatches at the loading dock. A datasheet packs density, flow, stiffness, impact, heat and flame data into one page, but every number carries a test method, a conditioning state and a margin behind it. This guide walks through the fields that matter most, the fine print that changes their meaning, and the red flags that should trigger a second look. Most of it is sourcing basics that experienced buyers apply without thinking, written down so a new team can repeat it.
How to Read a Plastic Datasheet: Start With the Property Table
The property table is the heart of the document. Read it column by column, and note the unit next to every value.
| Field | What it tells a buyer | Common standards |
|---|---|---|
| Melt flow rate, g/10 min | How easily the melt fills a thin-walled cavity at a stated temperature and load | ISO 1133, ASTM D1238 |
| Density, g/cm3 | Mass per volume, the basis for part weight and material cost per piece | ISO 1183, ASTM D792 |
| Tensile strength and elongation, MPa and % | Stress at yield or break and how far the grade stretches before failure | ISO 527, ASTM D638 |
| Flexural modulus, MPa | Stiffness in bending, useful for clips, housings and covers | ISO 178, ASTM D790 |
| Notched impact, kJ/m2 | Energy to break a notched bar; always check whether the notch is stated | ISO 180, ASTM D256 |
| Heat deflection temperature, C | Temperature at which the bar deflects under load, quoted at 0.45 or 1.8 MPa | ISO 75, ASTM D648 |
| UL94 flame rating | Burning behaviour at a stated thickness; V-0 at 1.6 mm is not V-0 at 0.8 mm | UL 94 |
| Mould shrinkage, % | Dimensional change on cooling; drives cavity steel sizing | ISO 294-4, ASTM D955 |
Typical Values Are Not a Specification
Most datasheets publish typical values, not specification limits. A typical tensile strength describes the middle of a normal production distribution; it does not guarantee that every lot lands there. When a part fails in service, a typical value will not settle the argument. Ask the supplier which values are guaranteed limits, and require a certificate of analysis per lot for the properties that drive your function, such as melt flow, impact or colour. This is also where a datasheet specification turns into a purchase-order line item: copy the guaranteed properties into the contract, not the typical ones.
Check the Test Method and Conditioning
The same resin can look strong or weak depending on how it was tested. Impact values differ sharply between notched and unnotched bars, and between ISO and ASTM methods. Hygroscopic grades such as PA and PBT behave differently dry as moulded versus conditioned at 23 C and 50 percent relative humidity; a datasheet should state which state the numbers describe. Heat deflection temperature is meaningless without the load, and a UL94 rating is meaningless without the thickness. When comparing two grades, align the method, the load, the conditioning and the units before comparing the numbers themselves.
Red Flags in Supplier Datasheets
- No test method or standard cited next to a property
- Values with no units, or mixed units on the same page
- A flame rating quoted without the thickness at which it was measured
- Impact data that does not say whether the bar was notched
- Claims of equivalence to a branded grade with no property table behind them
- Food contact wording that implies approval; regulators do not approve resins, and compliance stays with the supplier
What to Ask for Beyond the Datasheet
A datasheet describes a grade family; your parts come from one lot. Ask for the certificate of analysis for each lot, the statement of compliance covering RoHS and REACH SVHC, the food contact declaration where relevant, and the safety data sheet. For colour-matched parts, agree a tolerance such as delta E under 1 against a retained master panel. If a grade is new to your programme, order a pilot lot and run the moulding trial before locking the specification.
Use the same discipline when comparing materials side by side in our PPS vs PBT resin guide and ABS properties guide, browse the plastic materials catalogue, or shortlist suppliers in the supplier directory.
