Why Two Grades Both Labelled PA6 GF30 Can Differ by More Than 15% in Tensile Strength

Overview: A Grade Name Is Not a Specification

Modified plastics have no unified grade naming standard. In the label "PA6 GF30" only two things are agreed: the base resin is PA6, and the glass loading is around 30%.

At least four things are not agreed:

• whether the glass fibre is long or short, and what fibre length survives compounding;
• which coupling agent system bonds the fibre to the resin, and how strong that interface is;
• the molecular weight and viscosity grade of the PA6 base resin;
• whether the figures were measured dry as moulded or conditioned, and at what test temperature.

Any one of those four being different produces a visible difference in measured tensile strength. Where all four differ at once, a gap of more than 15% is normal; the absence of a gap is the case that should be questioned.

So laying two TDS sheets that both read "PA6 GF30" side by side and comparing numbers is already a flawed move. You believe you are comparing one and the same thing, when in fact you are comparing two micro-sections of two different formulations.

Four Sources of the Difference

Four sources of the difference:

1. Fibre length retention: the fibre is cut both during compounding and during injection moulding. The length that survives decides reinforcement efficiency. At the same 30% glass loading, a grade with higher retained length sits a class higher in tensile strength and flexural modulus; cut too short, the reinforcement is discounted and the material behaves closer to "PA6 filled with 30% glass powder". This item is normally not on the TDS. What you can ask is whether the supplier has measured the fibre length distribution in a moulded part.
2. Interfacial bonding (coupling agent system): the bond between glass fibre and PA6 runs through the coupling agent. If the interface is weak, stress is not transferred into the fibre and the reinforcing effect does not appear. Interfacial bonding affects impact strength more than it affects tensile strength, particularly at low temperature. A common finding: two grades sit close together on notched impact at 23 degrees C and differ by a factor of two at -30 degrees C - low temperature amplifies the difference in the coupling system.
3. Molecular weight and viscosity grade of the base resin: within PA6 too, matrix strength, flowability and toughness differ by viscosity grade and therefore by molecular weight. A high-viscosity matrix is tough but harder to process; a low-viscosity one processes well but loses toughness. This item usually moves together with melt flow rate (MFR / MI): high MFR generally means low base viscosity. If the MFR figures on two TDS sheets differ substantially, the mechanical properties are not on the same level, even where the stated glass loading is identical.
4. Test conditions and specimen state: this is the item most likely to make two data sets appear to contradict each other. Three variables have to be aligned - they are listed below.

• Specimen state: PA6 absorbs moisture. Between dry as moulded and conditioned to equilibrium at 23 degrees C / 50% RH, differences of more than 30% in tensile strength are common, and impact moves in the opposite direction. A figure without the state stated cannot be compared.
• Test temperature: notched impact at 23 degrees C and at -30 degrees C are two different things. Low-temperature service requires low-temperature data.
• HDT load: heat deflection temperature is quoted at two loads, 0.45 MPa and 1.80 MPa. The same material typically reads 20-40 degrees C lower at 1.80 MPa. Compare only values taken at the same load.

What the 15% Figure Actually Means

15% is not a tolerance band. It is a measured observation on different formulations carrying the same grade name.

What that means in practice: when you hold a TDS reading "tensile strength 180 MPa", you cannot conclude that the material delivered to you will reach 180 MPa, and you equally cannot conclude that any material quoting 180 MPa is interchangeable with it. The real situation is that the figure falls inside a band, and that band can be wider than 15%.

What the band means for you depends on the part:

• Non-load-bearing parts with design margin: a 15% difference is usually absorbed inside the margin and does not affect service.
• Structural parts designed to a lower strength limit: verify against the measured lower value, not against the TDS typical value.
• Existing tool, frozen dimensions: shrinkage matters more than strength. A 0.2% difference in shrinkage can push a large part out of tolerance.

If you want a compound developed against your own measured data rather than against a grade name, send the TDS or your measured values. We state which items we meet and which carry a deviation before any sampling starts.

Batch Consistency: Where Switches Fail More Often Than on Grade Differences

Differences between grades are static - one round of testing settles them. Differences between batches are dynamic and usually surface at the third or fifth delivery.

Three things show whether a supplier has its batches under control:

1. whether a COA comes with every batch, or only once at sampling stage. At production volume, every batch needs one;
2. whether the COA gives measured values or "reference values". Measured values are traceable to the batch, reference values are not;
3. whether a range for batch-to-batch variation has been agreed. Melt flow rate, ash content and colour difference are the three control items - how they are controlled should be fixed at quotation stage rather than left at "roughly the same".

The items CPlastics controls between batches in compounds are melt flow rate, ash content (filled and reinforced systems) and colour difference, and a COA carrying measured values ships with every batch. Specific control limits are still to be confirmed: until they are backed by measured statistics, we do not commit to figures. COAs from several recent consecutive batches can be supplied for comparison on request.

Four Checks a Buyer Can Run Without a Laboratory

These four do not need a laboratory of your own:

1. Re-measure with the test conditions aligned: test both materials to the same standard, in the same specimen state, at the same temperature. This step removes differences that were never real.
2. Measure the part, not the granulate: mould the real part and test that. Good granulate data does not mean good part performance.
3. Keep a retained sample: keep material from the first approved batch and compare later batches against it under identical conditions. It is the simplest way to detect batch drift.
4. Verify across three consecutive batches: one clean batch proves nothing. Three stable ones in a row do.

Risk notes

Treat "PA6 GF30" as a description, not as a specification. When you request a quotation, state the test standard, the specimen state and the test temperature alongside the property targets.

Before releasing a batch, ask for: a complete TDS, the COA for the actual batch, and the compliance statements your target market requires (RoHS, REACH, UL and so on). More on documentation: frequently asked questions on compound sourcing. Company and plant details are on the about page.