Glass-fibre reinforcement turns flexible polyamide into a structural material that replaces metal in many assemblies. We break down what happens inside a glass-filled compound, why 30 % glass became the industry norm, and how to choose the reinforcement level for a specific part.
Definition
Glass-fibre reinforcement (glass fiber, GF) is the introduction of short or long glass fibres (typically 10–17 µm in diameter) into the thermoplastic polyamide matrix. Under ISO 1043, such a compound is designated as "PA6-GF30", where the number is the mass fraction of glass in percent. The result is an anisotropic composite with significantly higher stiffness, strength and heat resistance than the base polyamide, but at a noticeably lower price than carbon-fibre (CF) equivalents. This is precisely why glass-filled polyamide is the most widely used structural thermoplastic in mechanical engineering.
Chemistry and Structure
Picture rebar in concrete: thin glass rods take up the load, while the polyamide matrix holds them in place and transfers force to them. Each fibre is essentially drawn borosilicate (E-glass) glass with an elastic modulus of about 72 GPa. For comparison, the modulus of pure PA6 is about 3 GPa. When such fibres make up 30 % of the matrix by mass, the overall stiffness of the composite increases several-fold.
The key to strength is not only the amount of glass but also the interface between fibre and matrix. Glass fibre is coated with a special sizing agent (sizing) — most often silane-based, which chemically "couples" the glass to the polyamide. Without quality sizing, the fibre would simply slip within the matrix under load, and the entire reinforcement would fail to work. The second important factor is the orientation of the fibres: during moulding they line up along the melt flow, so the part comes out stronger in the flow direction and weaker across it. This is a fundamental property that the designer must account for as early as the placement of the mould gates.
Properties
| Parameter | Pure PA6 | PA6 GF30 | PA6 GF50 | Method |
|---|---|---|---|---|
| Density, g/cm³ | 1.13 | 1.36 | 1.57 | ISO 1183 |
| Tensile modulus, GPa | 3.0 | 9–10 | 15–16 | ISO 527 |
| Tensile strength, MPa | 80 | 170–185 | 220–240 | ISO 527 |
| Elongation at break, % | 50 | 3–4 | 2–3 | ISO 527 |
| HDT at 1.8 MPa, °C | ~60 | 200–215 | 215–220 | ISO 75-2 |
| Coeff. of linear expansion, 10⁻₆/K | 90 | ~30 | ~22 | ISO 11359 |
| Water absorption, 24 h, % | 1.6 | 1.1 | 0.9 | ISO 62 |
The values are typical for the class of glass-filled PA6 at the corresponding reinforcement level, not a data sheet for a specific grade. Confirm exact data for your part against the TDS and on a trial moulding.
Note: reinforcement raises HDT (heat deflection under load) several-fold — from about 60 °C for pure PA6 to over 200 °C. This is exactly what makes glass-filled polyamide suitable for engine-bay parts and housings that operate near heat sources.
Why Exactly 30 %
Why does GF30 dominate the catalogues of every manufacturer? It is the balance point. Up to ~30 %, each percent of glass appreciably adds stiffness and strength while processing remains manageable and part surface acceptable. Above 30 %, the return on each percent diminishes while problems grow: equipment wears faster, glass becomes more visible on the surface, warpage intensifies. GF30 delivers roughly triple the modulus at a moderate price — which is why it became the "workhorse". GF50 is chosen when stiffness is critical and one is willing to accept more difficult processing for its sake; GF10–GF15 is used when only light reinforcement is needed without loss of impact strength.
How It Is Produced
Glass-filled polyamide is manufactured by compounding on a twin-screw extruder. Polyamide pellets are melted in the first screw zone, then glass-fibre roving is fed through a side dosing unit (side-feeder), where it is broken up in the melt into short segments (typically 200–400 µm for short-fibre, SGF, grades). The melt with fibres is homogenised, extruded as strands, cooled in a water bath and cut into pellets.
For long-fibre versions (LGF, fibre length in the pellet 10–12 mm), pultrusion is used: the roving is impregnated with melt and drawn out as a continuous ribbon, which is cut into long pellets. LGF provides better impact strength and higher residual fibre length in the finished part. Glass-filled polyamides appeared as far back as the mid-20th century and quickly displaced metal in many assemblies — from power-tool housings to automotive parts.
Typical Applications
- Automotive engineering. Intake manifolds, brackets, housings in the engine bay, pedal assemblies. GF30–GF35 with heat stabilisation — for continuous operation at 130–180 °C.
- Electrical engineering. Circuit-breaker housings, terminal blocks, coil bobbins — thanks to heat resistance and arc resistance.
- Pumps and hydraulics. Impellers, housings, fittings that replace metal castings.
- Gearbox housings and robotics. GF50 for maximum stiffness where a part carries load in place of metal.
- Furniture fittings and building profiles. Reinforced hinges, brackets, thermal breaks in window systems.
- Household appliances. Housing and load-bearing parts that operate under load and heat.
Comparison: GF vs CF and Reinforcement Levels
| Property | PA6 GF30 | PA6 GF50 | PA66 CF30 (carbon-nylon) |
|---|---|---|---|
| Tensile modulus, GPa | 9–10 | 15–16 | ~25 |
| Density, g/cm³ | 1.36 | 1.57 | 1.30 |
| Electrical conductivity | insulator | insulator | ESD |
| Base price (relative) | 1× | 1.2–1.4× | ~3.5× |
The conclusion, without exaggeration: glass fibre is the rational choice when stiffness and heat resistance are needed at a moderate price, and weight and electrical conductivity are not decisive. Carbon fibre (carbon-nylon) delivers roughly three times the specific modulus and reduced weight, but costs several times more — it is chosen where every gram is critical (drones, portable equipment, sports). The choice between GF30 and GF50 is simple: more glass means more stiffness, but more difficult processing and lower impact strength.
Designation in Industry
The ISO 1043-1/-2 standard governs the notation: PA6-GF30 — polyamide 6 with 30 % glass fibre; the filler-type code "GF" (glass fiber) or simply "G". An example of a full designation is PA66-GF30 V0: PA66 with 30 % glass and a UL94 V-0 flammability rating. For long-fibre grades, "L" is added: PA6-LGF40. In the American tradition, "N6-GF30" (N = Nylon) is encountered. The reinforcement level in the designation is always the mass fraction, not the volume fraction (by volume there is less glass, because it is denser than the polymer).
Limitations Worth Knowing About in Advance
- Anisotropy and warpage. Because of fibre orientation, the part shrinks differently along and across the flow — this is the main cause of warpage in flat parts. It is accounted for at the mould design stage.
- Glass on the surface. Fibres can "show through", reducing gloss; for visible parts this is a limitation.
- Abrasiveness to equipment. Glass wears the screw, barrel and hot runner — a production run requires wear-resistant configurations.
- Weld lines (weld lines). Where melt flows merge, fibres are oriented unfavourably — strength is lower there.
- Pellet moisture sensitivity. Like any polyamide, the glass-filled grade needs drying before moulding.
- Impact strength. Reinforcement increases stiffness but reduces the ability to absorb impact; for impact loads there are modified (HI) grades.
What to Check Before a Production Launch
- Pellet moisture. Dry per the recommended schedule — wet polyamide produces bubbles and a drop in strength.
- Fibre orientation and gates. Check (preferably by fill simulation) where the weld lines will run and where warpage will settle.
- Dimensional stability after conditioning. Polyamide "breathes" moisture — check tolerances on conditioned samples.
- Wear of the mould, screw, hot runner. For a long production run, assess the abrasive load.
- Strength in the weld-line zone. If a weld falls in a loaded zone — test it separately.
- Impact behaviour at low temperature. If the part operates in cold conditions — an HI grade may be needed.
- Batch consistency. Control residual fibre length and mechanics between batches.
Material Wizard Grades in This Category
Material Wizard produces glass-filled polyamides under the Examid® brand across several bases and reinforcement levels to its own formulation. If you have a sample or a specification from another manufacturer, we will select a comparable material produced to our formulation.
Examid® PA6 GF30Base grade for series mouldingModulus 9–10 GPa · HDT ~210 °C · the "workhorse"Request supply terms → Examid® PA6 GF50 R10Maximum glass — maximum stiffnessModulus ~16 GPa · metal replacement in load-bearing partsRequest supply terms → Examid® PA66 GF30Heat-stabilised for the engine bayContinuous operation 130–180 °C · automotive, electrical engineeringRequest supply terms →All grades can be purchased with delivery across Ukraine. The price depends on the base, reinforcement level, batch volume and lead times — check with a specialist: we will suggest a grade for your specific part and current availability. An overview of the full range is on the glass-filled polyamides hub.
Expert Breakdown: 5 Questions Worth Asking Before Choosing a Glass-Filled Polyamide
1. What is the operating temperature of the part under load? Up to ~120 °C, PA6 GF30 is often sufficient; for 130–180 °C continuous — heat-stabilised PA66 GF30. This determines the base choice.
2. Does the part carry a constant load or experience impacts? For static stiffness — standard GF; for impacts, especially in cold conditions — a modified impact-resistant (HI) grade.
3. Is dimensional accuracy critical? If so — build in conditioning and account for anisotropic shrinkage as early as the mould design.
4. Where will the weld lines run? Gate placement determines whether a weak weld falls in a loaded zone. This question is resolved before the mould is built.
5. What mould life are you planning? Glass is abrasive; large production runs require wear-resistant steels and screw coatings — this affects the project economics.
FAQ
How is glass-filled polyamide better than pure polyamide?
Above all in stiffness and heat resistance. Reinforcement with 30 % glass raises the modulus from roughly 3 to 9–10 GPa, and HDT from ~60 to over 200 °C. The part stops "flowing" under load and heat. The trade-off is lower impact strength, anisotropy (different properties along and across) and abrasiveness to equipment. For structural parts, this is almost always a worthwhile exchange.
Why exactly 30 % glass, and not more?
30 % is the balance point between the property gain and processing difficulty. Up to 30 %, each percent of glass appreciably adds stiffness; above that, the return falls while problems (warpage, equipment wear, glass on the surface) grow. GF50 is chosen only when stiffness is critical and one is willing to complicate production for its sake.
What is warpage and how is it reduced?
Warpage is the distortion of a part due to uneven shrinkage: along the fibre orientation, polyamide shrinks less than across it. It is reduced by well-planned gate placement, uniform wall thickness, rib symmetry and a correct mould temperature regime. Part of the problem is solved as early as the mould fill simulation stage.
How does GF differ from CF (carbon-nylon)?
Glass fibre is cheaper and delivers good stiffness, but it is heavier and does not conduct current. Carbon fibre (carbon-nylon) is lighter, roughly three times stiffer by specific modulus and has ESD properties, but costs several times more. GF is for structural parts in general mechanical engineering; CF is for cases where weight is critical or antistatic behaviour is needed (drones, portable electronics).
Can glass-filled polyamide replace metal?
In many assemblies — yes. PA6 GF50 with a modulus of about 16 GPa successfully replaces aluminium and zinc castings in gearbox housings, brackets and pumps, gaining in weight, corrosion resistance and tooling cost. But it is an engineering decision: it requires recalculating stiffness for the material's anisotropy and testing on the specific geometry.
Material Wizard (Derazhnia, Kharkiv) — a Ukrainian manufacturer and supplier of engineering polymers: technical grade selection, samples for testing, warehouses in Derazhnia and Kharkiv. Check with a specialist — we will suggest a glass-filled polyamide grade for your specific task and support the testing.
See also: What is polyamide 6 (PA6) · PA66 vs PA6: how it is better · What is polyamide 12 (PA12)