Glass-fibre content matches — part still too weak: why GF30 is not always GF30

'GF30' feels like a hard specification: 30% glass fibre, 30% glass fibre, done. Then two lots that both say GF30 give you different stiffness, different impact — a part that passes with one and fails with the other. The percentage was never the whole story.

The problem

Two PA66 GF30 or PBT GF30 materials, identical glass loading on the datasheet, deliver measurably different mechanical results in the same tool. The buyer compared '30% GF' to '30% GF' and expected equivalence — the part proves they were not equivalent.

Why it happens

Reinforcement performance comes from far more than how much glass is in the compound:

• Fibre length — the residual fibre length after compounding and moulding drives strength and stiffness much more than the weight percentage.
• Fibre type & sizing — the glass grade and the coupling agent (sizing) decide how well the fibre bonds to the polymer — poor coupling, poor mechanics.
• Polymer matrix — the base resin's viscosity and quality affect how the fibre is wetted and held.
• Processing & fibre orientation — injection conditions and part geometry orient the fibres, so the same material can perform differently part to part.

How to compare it properly

Stop comparing on the glass percentage alone:

• Mechanical values on the TDS — compare tensile strength, modulus and impact — not just '% GF'.
• Coupling & grade — coupled vs uncoupled, and the specific reinforced grade, matter as much as the loading.
• Your actual requirement — define the mechanical property the part must hit, and match to that.
• Processing conditions — keep them consistent when you benchmark two materials, or you are comparing noise.

The number on the label is the starting point; the datasheet and the part decide.