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How to read a filament datasheet before you trust the MPa figure

Nozzle4 · · 5 min read

Bambu Lab lists its PETG Basic at 51 MPa tensile strength. eSun lists its PETG at 34.77 MPa. Both numbers come from bars printed at 100 percent infill, both sheets cite a tensile standard, and both spools are sold as ordinary PETG for ordinary parts. Before deciding that one is a third weaker than the other, read what is printed next to the number. A large part of the gap between two filament datasheets can come from how the bar was made and pulled, and the sheets usually give away enough to spot it.

These are the checks we run on every datasheet before a figure goes into one of our comparisons.

The standard and the bar

Two tensile standards cover almost every filament sheet. ISO 527 is the default in Europe and most of Asia; ASTM D638 is the North American habit. The preferred ISO bar, type 1A, is 170 mm long and 4 mm thick. The preferred ASTM bar, type I, is 165 mm long and 3.2 mm thick. For most rigid plastics the two methods land close together, but they are not considered technically equivalent, and for materials with little or no linear region in the stress-strain curve the results can drift apart. Several sheets add GB/T 1040, the Chinese national method, next to ISO or in place of it. eSun’s July 2025 PETG sheet cites GB/T methods only.

Then look for the drawing. Overture’s PETG sheet (version 1.1, January 2024) labels its tensile test “ASTM D638 (ISO 527, GB/T 1040)”, but the specimen it draws is 115 mm long with a 6 mm wide narrow section. That is ASTM’s type IV bar, which the standard describes as the specimen for comparing rigid and soft plastics, and it is a different shape from both the ISO 1A and the ASTM type I. The test is valid. Putting three standards in one bracket suggests an equivalence the drawing does not support.

Printed or moulded

This moves numbers more than anything else, and some sheets never say which one they used. A resin maker tests injection-moulded bars, solid all the way through. A filament brand that prints its bars is testing a printed structure, and the result depends on layer bonding and on the machine that laid the layers down.

Extrudr’s PETG sheet (July 2025) lists 51 MPa yield stress, 4 percent elongation at yield and 28 percent nominal elongation at break. It also gives shrinkage to ISO 294-4, a standard written for injection-moulded test specimens. It does not say how its tensile bars were made. The printed PETG bars in the other datasheets we collected break at between 5 and 11 percent elongation in the flat direction. A 28 percent figure is what a moulded bar looks like. It may be correct for the resin in the spool, and it is still not a number to design a printed bracket around.

Which way the bar was printed

Printed parts are weakest when the load pulls the layers apart. A useful sheet gives two tensile values, one for a bar printed flat (X-Y) and one for a bar printed standing up so the layers stack along the direction of pull (Z). Polymaker’s PolyLite PETG goes from 50.8 MPa flat to 42.8 MPa in Z. Bambu Lab’s PETG Basic drops from 51 to 35 MPa. eSun’s PETG goes from 34.77 to 28.65 MPa.

Prusament’s PETG sheet (version 1.1, February 2022) splits it differently. Its “vertical xz” bar stands on its long edge, so the layers still run the length of the bar, and it reports 50 MPa against 47 MPa flat. Layer bonding gets its own line, 18 MPa interlayer adhesion, measured with a Prusa Polymers in-house method. None of these numbers is wrong. None of them belongs in the same column as another brand’s Z value either.

The heat number and its load

HDT to ISO 75 heats a bar under a constant bending stress and records the temperature at which it deflects by a set amount. Check the load first. The result at 0.45 MPa (method B) is never lower than at 1.8 MPa (method A), and on most sheets it is a few degrees higher. Polymaker gives both for PolyLite PETG, 78 °C and 75 °C. Bambu Lab’s PETG Basic gives 71 °C and 68 °C. Prusament lists 68 °C at both loads. Sunlu gives 70 ± 5 °C at 0.45 MPa only, and eSun gives 64 °C at 0.45 MPa only.

Vicat is a separate test, a loaded needle with a 1 mm² tip pushed into the surface until it sinks 1 mm (ISO 306), and it does not track HDT in any fixed way. Polymaker’s PETG shows 84 °C Vicat against 78 °C HDT at 0.45 MPa. Bambu Lab’s PETG Basic shows 69 °C Vicat against 71 °C. A sheet that prints only a Vicat value has not told you the HDT.

Specimen conditions and the ± sign

Every brand prints its test bars on its own terms. Bambu Lab uses an X1-Carbon at 255 °C and 150 mm/s. Polymaker prints at 240 °C with the part cooling fan off. For its earlier PETG HF sheet, Bambu Lab annealed and dried the bars at 75 °C for 8 hours before testing, while advising customers not to anneal their PETG HF prints.

Those settings move results. In 2022 CNC Kitchen printed the same clear PETG with default settings and again with settings tuned for transparency (0.12 mm layers, 15 mm/s, no cooling). Standing tensile bars went from 34 MPa to 44 MPa, and a version with 30 percent cooling passed 80 percent of the flat-bar strength.

Last, read the tolerance. Polymaker quotes 50.8 ± 0.9 MPa. Sunlu quotes 48 ± 10 MPa, a band of about 20 percent either side. That can be honest reporting of a variable product or of a variable test, and in both cases the lower bound is the figure to design with.

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