Same Property Name, Different Numbers — Can You Compare Two Material Datasheets?
Not until the unit and the test method match. 496 kgf/cm² is about 48.6 MPa, so a supplier quoting 496 is not 18 times stronger than one quoting 27 — it is a unit, not a miracle. ASTM D638 and ISO 527 also use different specimens and speeds, so even an identical material can return different numbers, and a JIS film test is neither of them. A number without its test method is just a number.
The engineering problem
Two datasheets. Same property. One says 27 MPa, the other says 496. Which material is stronger? Wrong question.
We sat with four supplier TDS for thermoformed tray materials, side by side. The same property — tensile strength — was reported four different ways: kgf/cm² from a Vietnamese mill, MPa per ASTM from a US resin maker, MPa per ISO 527 from China, and JIS K7127 from Japan.
Buyers put these quotations next to each other and compare the numbers. The numbers are not comparable.
What the four datasheets actually said
Supplier names anonymised. Values reproduced as printed on the original documents.
| Supplier | Material | Property | Value as printed | Unit | Test method |
|---|---|---|---|---|---|
| A | PET sheet | Tensile strength at yield | 496 | kgf/cm² | ASTM D638 |
| A | PET sheet | Tensile strength at break | 281 | kgf/cm² | ASTM D638 |
| B | HDPE resin | Tensile strength at yield | 27 | MPa | ASTM D638 |
| C | PS, black conductive | Tensile strength at yield | 21.6 | MPa | ISO 527 |
| C | PS, black conductive | Tensile strength at break | 18.9 | MPa | ISO 527 |
| D | HIPS conductive sheet | Yield strength MD / TD | 20.3 / 21.5 | MPa | JIS K7127-1989 |
Three questions before you compare any two TDS
1. Same unit?
496 kgf/cm² is about 48.6 MPa, using 1 kgf/cm² ≈ 0.0981 MPa. The supplier quoting 496 is not 18 times stronger than the one quoting 27 — it is a unit, not a miracle.
2. Same test method?
ASTM D638 and ISO 527 use different specimens and speeds. Even an identical material can return different numbers. A JIS film test is neither of them.
3. Is the number even possible?
One TDS in our library reports impact strength in MPa, under ASTM D256 — a test that reports J/m. When the unit is impossible, question everything else on the page.
What buyers should ask their supplier
- Report the same property, by the same test method, in the same unit
- Better still, have the material tested to your specification
- Where a value looks unusual, ask which document it came from before ranking suppliers on it
Engineering note
The fix costs one email. A number without its test method is just a number.
Good packaging should be practical, not just presentable.
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