PA6-GF vs PA6-CF: Toughness, Stiffness and Nozzle Wear

PA6-CF is normally the better starting point when a printed part must resist bending, while PA6-GF can make more sense when the design needs a less stiffness-dominated balance of rigidity, durability, weight and cost. That distinction is not universal: fiber percentage, fiber length, moisture condition, print direction and annealing can change the result enough to reverse a simple “GF for toughness, CF for stiffness” rule.

Both materials use PA6 as the polymer matrix, but their short fibers change how the molten filament flows, shrinks and transfers load. The comparison is therefore less about choosing between two unrelated nylons and more about deciding which reinforced PA6 failure pattern is easier to manage in the intended part.

The Fiber Changes How the Part Fails

Strength, stiffness and toughness describe different responses. A material can carry a high tensile load yet flex more than the application allows. Another can resist bending very well but crack when a thin corner receives an impact. Calling either filament simply “stronger” hides the distinction that matters during part design.

Stiffness controls elastic movement. It matters when a camera angle must remain fixed, a sensor gap cannot change, a motor shaft must stay aligned or a fixture must locate a workpiece repeatedly. A PA6-CF grade with a high tensile or bending modulus is often favored in these cases because the part moves less before reaching its strength limit.

Toughness describes how much energy a part can absorb before fracture. It matters around impact zones, notches, screw bosses, thin transitions and parts that may be dropped. Toughness cannot be inferred from fiber type alone because the PA6 formulation, fiber-matrix bonding, test direction, notch geometry and moisture level all influence the measured result.

A same-manufacturer comparison shows why generic labels should not replace grade data. Fiberon PA6-GF25, containing 25% glass fiber by weight, lists a dry XY Young’s modulus of 5,356.9 MPa and dry notched Charpy impact strength of 10.0 kJ/m²[a].

The corresponding Fiberon PA6-CF20 grade, containing 20% carbon fiber, lists a dry Young’s modulus of 8,636 MPa and dry notched Charpy impact strength of 11.0 kJ/m²[b]. In this product pair, the carbon-filled grade is not only stiffer; it also has a slightly higher published dry notched-impact value. That does not establish a rule for every PA6-CF and PA6-GF filament, but it does disprove the idea that the GF label automatically identifies the tougher option.

Dry Stiffness Is Not the In-Service Stiffness

PA6 absorbs moisture from its environment. The fibers reduce some forms of movement and shrinkage, but they do not seal the nylon matrix. As moisture enters the polymer, it acts as a plasticizer: stiffness and tensile strength usually fall, while elongation and impact response can rise. A part tested immediately after drying can therefore behave differently after weeks in a humid workshop.

The effect is large enough to change material rankings. In the Fiberon data, PA6-GF25 Young’s modulus falls from 5,356.9 MPa in the dry condition to 1,793.6 MPa after the supplier’s wet-conditioning procedure. Its notched Charpy value moves in the opposite direction, from 10.0 to 28.0 kJ/m². PA6-CF20 moves from 8,636 to 2,508 MPa in Young’s modulus, while its notched Charpy value rises from 11.0 to 35.6 kJ/m².

This does not mean wet filament should be printed. Moisture in the spool during extrusion can cause bubbling, rough surfaces, unstable flow and weaker bonding. The useful distinction is between dry filament during printing and a finished part conditioned by its service environment.

A dry-room fixture, an outdoor equipment bracket and a coolant-area machine guard should not be evaluated from the same modulus value. Tight fits, bearing seats, bolt spacing and alignment surfaces should be checked after the part has reached a moisture condition that resembles actual use.

This is particularly important when a PA6-CF part is chosen because of its low deflection. The dry-data advantage may remain, but the working part can still become far more compliant than the first bench test suggests. For a continuously loaded assembly, creep testing under expected temperature and humidity is more informative than comparing only dry tensile strength.

Fiber Direction Can Outweigh the GF/CF Label

Short fibers tend to align with the extruded road as material passes through the nozzle. This helps explain why reinforced PA6 can be highly rigid within the printed plane while remaining weaker across layer interfaces. The fibers strengthen the deposited path; they do not form continuous reinforcement running vertically through the complete model.

A long bracket printed flat may use PA6-CF effectively because the main fibers and perimeters follow the load path. The same bracket printed upright can place the highest tensile stress across layer boundaries. In that orientation, changing from GF to CF may provide less benefit than rotating the model, enlarging the root radius or redesigning the joint.

Stress concentration becomes more important as stiffness rises. Sharp internal corners, abrupt wall changes, countersunk screws and thin sections beside thick bosses can force a rigid composite to fail locally. A generous fillet or gradual thickness transition may improve the part more than moving to a filament with a higher modulus.

Holes also need separate treatment. A tightly torqued fastener can create radial stress around a printed bore, while a threaded insert can split layers if the surrounding wall is too thin. PA6-CF can hold a fixture accurately, but its extra stiffness should be paired with adequate edge distance, washers, controlled torque and enough material around inserts.

Nozzle Wear Can Erase the Dimensional Advantage

Both glass-filled and carbon-filled PA6 are abrasive. A printer may continue extruding after a soft nozzle begins to wear, so the first symptom is not always a failed print. The opening gradually grows or loses its circular shape, changing extrusion width, corner definition, hole clearance and wall thickness.

This creates a direct contradiction in tolerance-sensitive work: PA6-CF may be selected because it holds a rigid geometry, yet a worn nozzle can introduce process variation before the part leaves the printer. The nozzle becomes part of the dimensional tolerance stack.

Glass fiber should not be treated as the nozzle-friendly alternative. Polymaker states that a brass nozzle has a lifespan of about nine printing hours with its PA6-GF25 formulation and recommends a more durable nozzle[c]. That figure is specific to the supplier’s material and test conditions, but it illustrates how quickly a GF compound can consume a standard brass orifice.

For PA6-CF20, the manufacturer recommends a wear-resistant nozzle such as hardened steel or a ruby-tipped design[d]. The practical rule is therefore the same for both materials: the printer should be configured as an abrasive-filament system rather than temporarily equipped for one difficult spool.

A larger nozzle is often easier to manage with fiber-filled materials because it gives short fibers and the polymer melt a less restrictive flow path. A 0.6 mm wear-resistant nozzle is a practical starting point for thick functional walls, faster deposition and reduced clog sensitivity. A 0.4 mm nozzle can still be appropriate when the filament manufacturer supports it and the model depends on smaller details.

Nozzle diameter does not determine abrasion resistance. A large brass nozzle can still wear rapidly, while a properly manufactured wear-resistant 0.4 mm nozzle may remain stable for much longer. Material loading, fiber shape, flow rate, total kilograms printed and nozzle construction all affect service life.

Print Setup and Annealing Change the Comparison

The printability difference between PA6-GF and PA6-CF is often smaller than the difference between two commercial formulations. Some grades are designed for a cool build plate and open chamber, while others need a warm enclosed environment. A generic “all nylon requires the hottest possible chamber” profile can work against a material engineered around controlled low-bed-temperature crystallization.

For the current Fiberon PA6-GF25 and PA6-CF20 examples, the manufacturer lists a 280–300°C nozzle range and a 40–50°C build plate range, with no heated chamber required. These values should not be copied to a PA6 composite from another supplier without checking its documentation.

Annealing can raise crystallinity and improve heat performance, but it may also alter dimensions. A flat plate can bow, a circular bore can become oval and a press fit can tighten or loosen. Parts requiring annealing should be designed around the post-annealed state rather than measured only when they first leave the build plate.

The same-manufacturer example also shows why published heat values need their test conditions. PA6-GF25 is listed with an HDT of 191°C at 0.45 MPa, while PA6-CF20 is listed at 215°C under the same nominal load level. Those values describe prepared test specimens, not a guarantee that a thin printed bracket will carry its full room-temperature load at those temperatures.

Heat exposure combines with moisture, load and geometry. A motor mount near a warm enclosure wall may survive the temperature without melting but still creep enough to lose belt alignment. The material decision should therefore consider working deflection at temperature rather than treating HDT as a continuous-use rating.

Choosing by Failure Mode, Geometry and Maintenance

The useful question is not whether glass fiber or carbon fiber is better in isolation. It is which material reduces the most likely failure without creating a harder problem elsewhere. The table below treats material choice as a part-level decision rather than a ranking of filament labels.

PA6-CF is the clearer choice when a designer can identify excessive movement as the expected failure. PA6-GF becomes more attractive when the part needs a broader balance of reinforcement, durability and production cost. The most reliable selection comes from matching a specific technical data sheet, conditioned test part and nozzle-maintenance process to the real geometry rather than selecting by the letters printed on the spool.