How to Choose Glass Fibre Content: What Do 15%, 30% and 50% Change?

Glass fibre reinforcement creates a load-bearing skeleton within the polymer matrix. As a result, stiffness (modulus of elasticity), tensile strength and heat deflection temperature increase markedly.

It also reduces shrinkage and improves dimensional stability, which matters particularly for tight-tolerance parts.

Every advantage has a trade-off. As fibre content increases the material becomes more brittle; toughness and elongation at break decrease. This can be critical for parts under impact loading.

Surface quality is also affected: at high fibre contents, fibre marks can become visible on the surface, which is a problem for visible parts.

Fibre is also abrasive; mould and screw-barrel wear increase and maintenance intervals shorten.

Glass fibres tend to align in the flow direction. This means the part behaves differently along and across the flow — shrinkage rates and strength values become direction dependent.

As a result, parts can warp. Gate location and flow design become more critical than expected at high fibre contents.

Low contents (roughly 10–20%) suit parts where improved dimensional stability and a moderate stiffness increase are wanted while toughness must be preserved.

Medium contents (around 30%) are the standard choice for most engineering applications: a balanced point between strength and toughness.

High contents (40% and above) are for structural parts targeting metal replacement, where maximum stiffness and thermal performance are required — but the design must be handled carefully with respect to warpage and brittleness.

The trends above are general; the actual modulus, strength and shrinkage values of each grade depend on the formulation and appear only in the official technical data sheet. The TDS of the relevant grade must be reviewed before deciding.

At Buteo Petrochemicals we have access to LG Chem's glass fibre reinforced PA, PBT, PPS and PP compounds. Share the loading conditions of your part and we will determine the right reinforcement content together and validate it with samples.