Key takeaways
- The GFnn suffix states glass loading by weight, not by volume — PA66-GF30 is 30% glass by mass, which is only about 17% by volume, and density rises to roughly 1.36 g/cm³ versus 1.14 for the natural resin.
- Glass reinforcement lifts stiffness, strength and heat deflection temperature dramatically — flexural modulus can rise three- to fourfold and HDT at 1.8 MPa (ASTM D648 / ISO 75) can jump to within 10–20°C of the melting point for semi-crystalline resins — but elongation, impact and dimensional stability fall.
- Fibre-filled grades are anisotropic: properties measured along flow can be 30–50% higher than across flow, and that mismatch drives differential shrinkage, warpage and weld-line weakness that an isotropic mineral or bead filler does not.
- Glass fibre is the choice for load-bearing stiffness and HDT; mineral fillers such as talc (T) or glass beads (GB) trade some of that uplift for lower warpage, tighter dimensional control and a smoother surface — read the ISO 1043-2 code, not just the percentage.
A spec sheet that reads PA66-GF30 is telling you four things at once: the base polymer is polyamide 6.6, the reinforcement is glass fibre, the loading is 30% by weight, and almost every mechanical number on the page has been transformed relative to the natural resin. Reading that suffix correctly is the difference between buying a part that holds load near 150°C and one that creeps at 80°C — or between a flat moulding and one that bows out of tolerance on the first shot.
Reinforced compounds are the workhorses of the engineering-plastics shelf, and the nomenclature is standardised but easy to misread. The percentage is mass, not volume. The headline modulus is usually measured along the flow, not across it. And two grades carrying the same number on the label can behave very differently depending on whether the filler is fibre, mineral or bead. This is the buyer's guide to reading the whole code — not just the number — before you commit to a load-bearing engineering grade.
Filler and reinforcement codes follow ISO 1043-2. The two-letter code names the material, the single-letter prefix (where used) names the form, and the number is the content in percent by mass. So GF30 is 30 wt% glass in fibre form; GB20 is 20 wt% glass in bead form; MD40 is 40 wt% mineral, unspecified form; T20 is 20 wt% talc. A grade marked PA6-(GF+MD)-35 is a hybrid — glass fibre plus mineral totalling 35%. The bracketing and order matter: read the full string, because it tells you whether you are buying anisotropic fibre stiffness or a more balanced filled compound.
| Code | Material | Typical form | What it buys |
|---|---|---|---|
| GF | Glass fibre | Chopped strand | Maximum stiffness, strength, HDT uplift; anisotropic |
| GB | Glass beads | Solid spheres | Isotropic stiffness, low warpage, good surface |
| CF | Carbon fibre | Chopped strand | Highest modulus, light weight, conductivity; premium cost |
| T | Talc | Platelet mineral | Stiffness and HDT at low cost; reduced warpage vs GF |
| MD | Mineral, unspecified | Powder/platelet | General reinforcement; isotropic shrinkage |
| GF+MD | Glass + mineral hybrid | Mixed | Balanced stiffness vs flatness; common in production grades |
One number trips up most first-time buyers: weight versus volume. E-glass has a density near 2.54 g/cm³ against roughly 1.14 g/cm³ for natural PA66, so a 30 wt% loading is only about 17 vol%. That is why GF30 grades feel heavy — overall density climbs to roughly 1.36 g/cm³ — and why glass content should always be back-checked against the ash residue (ISO 3451-1) line on the certificate of analysis.
Glass fibre is added for three properties above all: flexural modulus, tensile strength and heat deflection temperature. For a semi-crystalline base at 30% loading, flexural modulus (ASTM D790 / ISO 178) typically rises three- to fourfold, tensile strength (ASTM D638 / ISO 527) roughly doubles, and HDT measured at 1.8 MPa (ASTM D648 / ISO 75) moves from somewhere near the glass transition to within 10–20°C of the crystalline melting point. The mechanism is straightforward: the glass carries load and constrains the polymer chains from softening, so the compound holds its shape far closer to its melt.
| Property (standard) | PA66 natural | PA66-GF30 | PA66-GF50 |
|---|---|---|---|
| Density, g/cm³ (ISO 1183) | ~1.14 | ~1.36 | ~1.57 |
| Tensile strength, MPa (ISO 527) | ~80 | ~180 | ~230 |
| Flexural modulus, GPa (ISO 178) | ~2.8 | ~9 | ~15 |
| HDT @ 1.8 MPa, °C (ISO 75) | ~70 | ~245 | ~250 |
| Tensile elongation, % (ISO 527) | >30 | ~3 | ~2 |
| Notched Izod, kJ/m² (ISO 180) | ~5 | ~10 | ~13 |
The table also shows the cost of that uplift. Elongation at break collapses from tens of percent to low single digits, and the part becomes notch-sensitive — a sharp internal corner that a natural grade would tolerate can initiate a crack in a filled one. Above roughly 35–40% loading the modulus gains flatten while processing difficulty, abrasion on the screw and barrel, and surface roughness all rise. GF50 exists for stiffness-critical structural parts, but it is not a free lunch; for many applications GF30 sits at the sweet spot of property gain versus processability. The same logic applies whether the base is PA6 or PA66, PBT, PP or a polycarbonate blend.
The percentage on the label is weight; the modulus is along flow; the elongation is gone. Read all three before you size the part.
The single most important behaviour to understand about fibre-reinforced grades is anisotropy. Chopped glass fibres orient with the polymer flow during injection, so the compound is stiffer and stronger along the flow direction than across it — the gap is commonly 30–50%, and a datasheet that quotes a single modulus figure is almost always quoting the flow direction. That directional mismatch carries straight through to shrinkage: a GF grade may shrink only 0.2–0.5% along flow but 0.8–1.2% across it, and that differential is what drives warpage.
- Mould shrinkage — fibre-filled grades shrink far less than unfilled (often 0.2–0.5% vs 1.0–2.0% for natural PA), but the along/across gap is the problem, not the absolute value.
- Warpage — differential shrinkage in flat, thin or asymmetric sections releases as bow or twist; predict it with mould-flow analysis, do not discover it on the first tool trial.
- Weld (knit) lines — where two melt fronts meet, fibres lie parallel to the line rather than crossing it, so local strength can fall by a third to a half; keep weld lines off load paths.
- Gate and rib design — gate location sets fibre orientation and therefore where the part is strong; ribs and bosses concentrate weld lines and must be checked.
- Surface finish — fibres near the surface read through as a matte, slightly rough texture; if Class-A finish matters, consider glass beads or a mineral fill.
None of this is a reason to avoid glass fibre — it is a reason to design for it. The grades that fail in the field are almost always the ones specified as if they were isotropic. Treat the across-flow numbers and the weld-line strength as the real design limits.
Glass fibre maximises stiffness, strength and HDT per unit weight, but it brings anisotropy, warpage, weld-line sensitivity and a rougher surface. Mineral fillers — talc, mica, wollastonite — and glass beads behave very differently: roughly isotropic, they shrink evenly, warp little, hold tighter tolerances and give a smoother finish, at the cost of lower peak modulus and strength. The decision is an engineering trade, not a hierarchy.
| Driver | Favours glass fibre (GF) | Favours mineral / beads (T, MD, GB) |
|---|---|---|
| Stiffness & strength | High — load-bearing parts | Moderate |
| HDT uplift | Large | Moderate |
| Warpage / flatness | Worse (anisotropic) | Better (isotropic) |
| Dimensional tolerance | Looser, direction-dependent | Tighter, uniform |
| Surface finish | Rougher, fibre read-through | Smoother |
| Weight | Heavier per % stiffness | Mineral heavier; beads similar |
| Typical use | Brackets, housings under load, structural | Flat panels, covers, tight-tolerance trim |
In practice many production grades hedge with a hybrid glass-plus-mineral fill — enough fibre for the stiffness and HDT target, enough mineral to pull the warpage back into tolerance. That is exactly why the full ISO 1043-2 code string matters more than the headline percentage.
When you specify a reinforced grade, lock down four things on the order: the base polymer and its viscosity, the exact filler code and loading (verified by ash content), the directional property basis of the datasheet, and the regrind/recycled content if any. Get those onto the spec and the certificate and the grade will behave on the press the way it behaved on the datasheet. The OmniaStrata desk can source verified PA, PBT, PP and PC reinforced compounds to a written spec — talk to us about your grade requirement or see our engineering-plastics service.
Frequently asked
Questions on the desk
What does the GF30 in PA66-GF30 actually mean?
GF stands for glass fibre and 30 is the glass content as a percentage of total weight, per ISO 1043-2 nomenclature — so PA66-GF30 is a polyamide 6.6 compound containing 30% chopped glass fibre by mass. Note this is weight, not volume; because E-glass is roughly 2.2 times denser than the resin, 30 wt% is only about 17 vol%. Always confirm the figure is verified by ash content (ISO 3451-1) on the certificate of analysis.
How much stiffer does glass fibre make a grade?
For a typical 30% loading, flexural modulus (ASTM D790 / ISO 178) commonly rises three- to fourfold over the unfilled base resin, and tensile strength roughly doubles. Heat deflection temperature at 1.8 MPa climbs sharply — for semi-crystalline resins like PA and PBT it can move from near the glass transition to within 10–20°C of the melting point. The trade-off is a large loss of elongation at break and a more brittle, notch-sensitive part.
Why do glass-filled parts warp?
Chopped glass fibres align with the polymer flow during moulding, so the compound shrinks much less along the flow direction than across it. That differential shrinkage builds in internal stress that releases as warpage, especially in flat or thin sections. Mineral fillers and glass beads are roughly isotropic and shrink evenly, which is why they are chosen where flatness matters more than maximum stiffness.
What is a weld line and why does it matter in reinforced grades?
A weld (knit) line forms where two melt fronts meet — around a hole, boss or multi-gated cavity. At that interface the glass fibres lie parallel to the weld rather than bridging it, so the local strength can drop substantially versus the bulk material, commonly by a third to a half. Reinforced grades are far more sensitive to this than unfilled resins, so weld-line position must be designed away from load paths and confirmed in mould-flow analysis.
When should I specify mineral fill instead of glass fibre?
Choose mineral fillers — talc (T), mica, wollastonite or glass beads (GB) — when dimensional stability, low warpage, surface finish or isotropic behaviour outrank peak stiffness and HDT. Glass fibre wins on load-bearing strength, modulus and heat resistance per unit weight. Many production grades use a hybrid glass-plus-mineral fill to balance stiffness against flatness; read the ISO 1043-2 code string to see exactly what is in the compound.
General market commentary from the OmniaStrata desk, provided for information only. It is not legal, financial, tax, or trading advice, and it is not an offer or a commitment to any terms. Figures such as price ranges, spreads, financing costs, and credit periods are illustrative market context, not OmniaStrata's rates or terms. Actual contract terms — including price, payment instrument, credit, insurance, and Incoterms — are agreed in writing on a per-transaction basis and at OmniaStrata's discretion. Market conditions change; figures reflect the publication date.