Metal Replacement Plastics: Glass- and Carbon-Filled Polyamides

Metal replacement is one of the oldest arguments for engineering plastics, and still one of the most misunderstood. A die-cast bracket redrawn in nylon with the same walls will be lighter. It will also be floppy, and it may creep out of position within a season. Conversions work when the part is rebuilt around what reinforced polyamide does well: complex shapes in one shot, integrated clips and bosses, no corrosion, no machining and no paint.

Start with the numbers

Specific modulus is tensile modulus divided by density. Polymer values are typical figures from published producer datasheets; metal values from standard reference tables. Full sources below.

Two things stand out. No reinforced polymer matches steel or aluminium on stiffness per kilogram; carbon fibre comes closest, and only when dry. And the aliphatic polyamides lose stiffness as they absorb moisture, while the semi-aromatic grade holds 14 GPa conditioned.

Geometry does the work

A plate's bending stiffness scales with its modulus multiplied by the cube of its thickness. That cube is the whole game. To match a steel plate in bending, a conditioned PA66 GF30 wall needs to be about three times thicker. Because the polymer is so light, the result still weighs roughly half as much.

Against aluminium the arithmetic is tighter. The same estimate gives a PA66 GF30 wall about twice as thick and roughly the same mass. Conditioned PA66 CF30 needs a wall 1.7 times as thick and comes out about 20% lighter. So the real savings come from ribs, box sections and folding several metal parts into one moulding. These are flat-plate estimates that ignore differences in Poisson's ratio; once the concept is fixed, finite-element analysis with anisotropic properties takes over.

Design rules that change when the metal goes

Use the properties the part will actually see

Dry, room-temperature data flatter every polyamide. The PA6 GF30 grade from our PA66 vs PA6 comparison falls from 9,600 MPa to 4,500 MPa tensile modulus at 80 °C, dry. Under sustained load, the PA66 GF30 grade's 1,000-hour creep modulus (conditioned, strain up to 0.5%) is 5,300 MPa against 7,200 MPa short-term. The PA6T/6I grade still shows 12,500 MPa at 80 °C and 10,000 MPa at 120 °C, dry.

Fibre orientation is the new grain direction

Glass fibres align with the flow, and so do the properties. The PA66 GF30 grade expands at 28 × 10⁻⁶/K along the flow and 88 × 10⁻⁶/K across it. Gate position, weld lines and wall transitions decide where the strong direction falls. Run a fill simulation before the tool is cut and steer the fibres along the load path.

Joints and fasteners

Bolted joints relax as the polymer creeps under preload. Put metal compression limiters under bolt heads, and treat thread-forming screws as a design decision rather than a default. Check thermal mismatch wherever the part meets steel or aluminium: across the flow, glass-filled PA66 moves almost four times as much as aluminium per degree.

Carbon fibre conducts

A published 30% carbon-fibre PA66 datasheet lists a surface resistivity of around 10³ Ω. That helps with static dissipation but rules the grade out where the part has to insulate, and a wet joint against aluminium invites galvanic corrosion. Carbon-fibre grades are also less forgiving of notches: 7 kJ/m² notched Charpy, against 11 kJ/m² for the glass-filled PA66, both dry.

Where conversions usually pay

• Covers and housings with integrated mounting features. Cylinder-head covers are a good example, where high glass content, heat resistance and dimensional stability all matter.
• Brackets, pedals and manifolds; BASF lists manifolds and pedals among the uses of its glass-filled PA6.
• Gears, bushes, rollers and slide rails, where low friction and corrosion resistance beat a machined metal part. For sliding contact, also look at POM acetal.
• Pump and fluid-handling parts, where semi-aromatic grades keep their stiffness when wet.
• Brackets and housings in battery-electric vehicles; see plastics in electric vehicles.

Mapping to OSAMIDE grades

All sit in the OSAMIDE family. When an application falls between grades, we can trial a formulation within 48–72 hours of a complete request, then scale from a lab batch on our 75 kg/hr R&D line to an industrial trial. The steps are set out in custom compound development and on our capabilities page.

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