Polycarbonate (PC) is an amorphous thermoplastic whose unfilled fracture behavior under ASTM D256-10 is usually ductile: notched Izod values from 600 to 850 J/m and tensile elongation at break above 80 % under ISO 527-2:2012. The addition of chopped E-glass fiber raises tensile modulus and heat deflection temperature but suppresses the matrix’s ability to yield at the notch tip. This trade-off is the basis of the phrase Glass-Filled Polycarbonate: Why GF30 and Impact Toughness Cannot Both Win. At 30 wt% fiber loading, the compound enters a stiffness-dominated region in which notched impact strength falls by roughly 75 to 85 % relative to the unfilled resin, while tensile modulus rises from 2.3–2.4 GPa to 8.0–9.5 GPa.

When 30 wt% Chopped Strand Fills an Amorphous Polycarbonate Matrix, Notched Izod Falls Below 150 J/m

At 30 wt% glass loading, a representative commercial GF30 PC compound exhibits a notched Izod impact strength of 90 to 160 J/m when tested under ASTM D256-10 at 23 °C and 50 % RH after conditioning per ISO 291:2008. The same compound typically retains an ultimate tensile stress of 110 to 130 MPa and a tensile elongation at break of 2.0 to 4.0 % under ISO 527-2:2012. The abrupt transition from ductile hinge break to brittle crack propagation is associated with fiber-end stress concentrations and restricted micro-shear band formation. It is not a linear dilution effect: moving from 10 wt% to 20 wt% glass often reduces notched Izod by approximately 35 to 50 %, but the further increase to 30 wt% can produce an additional 50 to 60 % reduction because the fiber-to-fiber spacing falls below the critical damage-zone size of the PC matrix.

High-shear dispersion of 4.5 mm chopped E-glass strands into a PC melt is typically performed on a co-rotating twin-screw extruder with an L/D ratio of 40:1 and barrel temperatures from 260 to 300 °C. At 400 rpm screw speed and a throughput of 35 to 50 kg/h on a 40 mm machine, the residence time is kept below 45 s to limit thermal degradation; even so, fiber length attrition reduces the initial 4.5 mm strand to a number-average fiber length of 250 to 400 μm in the pellet. This length reduction is significant because the critical fiber length for effective stress transfer in PC is approximately 180 to 260 μm, depending on interfacial shear strength. A shorter average fiber length reduces notched impact further because short fibers fail by pull-out rather than fracture, and pull-out energy is consumed over a smaller embedded length.

Melt mass-flow rate measured at 300 °C under 1.2 kg load per ISO 1133-1:2022 falls from 12 to 18 g/10 min for unfilled PC to 4 to 8 g/10 min for GF30 PC. The increase in melt viscosity at shear rates below 100 s⁻¹ is particularly severe, which limits flow into thin ribs. Injection molders compensate with melt temperatures of 300 to 320 °C, but the upper bound is constrained by molecular weight degradation in PC and by outgassing of aminosilane sizes at the feed throat. Pre-drying is mandatory: residual moisture above 0.02 % after 4 h at 120 °C in a desiccant dryer with a dew point of -40 °C causes hydrolytic molecular weight loss and surface splay. GF30 PC is less forgiving than unfilled PC because the sizing layer on glass can carry moisture and because higher melt temperatures magnify hydrolysis kinetics.

Does Fiber Content Above 20 wt% Narrow the Hot Runner Processing Window to Less Than ±5 °C?

Thermal uniformity at the machine nozzle is one limiting constraint. For an unfilled PC grade, a melt-temperature variation of ±8 °C may be tolerable without visible defects. In GF30 PC, the combination of high viscosity and shear heating produces local melt-temperature spikes at the check ring, and a deviation of ±5 °C across the shot can be sufficient to create both cold-slag weld lines and brown streaks from degraded sizing. In hot runner systems, the temperature error in each drop should be controlled to ±2 °C at 290 °C using heated sprue bushings and valve-gate tips with embedded thermocouples.

Batch-to-batch variance in fiber length distribution is a second processing limit. When the retained glass fiber length falls from 320 μm to 240 μm because of a worn screw element, notched Izod can fall by 20 to 30 % even though tensile modulus remains within specification. The processing window is therefore not defined solely by barrel set points; it includes screw wear, screw speed, and back pressure. A production-scale check requires measuring ash content per ASTM D5630-22 and melt viscosity per ISO 1133-1:2022 after every 200 h of screw service.

Injection molding of GF30 PC components introduces anisotropic fiber orientation that dominates local mechanical response. Short glass fibers orient preferentially in the flow direction in the shear layer and more randomly in the core. A differential shrinkage between flow direction and transverse direction is observable in ISO 294-4:2018 plaques: flow-direction mold shrinkage may be 0.10 to 0.25 %, while transverse shrinkage may be 0.35 to 0.55 %. This anisotropy distorts flat parts and produces high residual stress at sharp corners. The tensile strength transverse to flow can be 40 to 50 % lower than the flow-direction value, and the notched Izod transverse to flow can drop to 60 to 80 J/m even when the datasheet value is 120 J/m.

In a multi-gate mold using 0.8 mm pinpoint gates, the weld line represents a plane of fiber orientation perpendicular to the flow front. Weld-line tensile strength in GF30 PC is commonly 50 to 65 % of the virgin material value under ISO 527-2:2012. The corresponding notched impact energy at the weld line may fall below 50 J/m. This is not a defect but a structural consequence of fiber discontinuity. Mold-filling simulation with anisotropic thermal conductivity can predict the location but not the full strength loss; validation must be performed on production tools with actual cavity-pressure sensors.

Thermal Stability Limits for Glass-Filled Polycarbonate Under Load

Under ISO 75-2:2013 method A at 1.8 MPa, GF30 PC exhibits a heat deflection temperature between 138 and 145 °C, compared with 125 to 130 °C for unfilled PC. This increase is one reason designers specify GF30 for automotive under-hood brackets and electrical enclosures. However, the coefficient of linear thermal expansion measured per ISO 11359-2:1999 is anisotropic: 25 to 35 μm/(m·°C) in the flow direction versus 55 to 70 μm/(m·°C) transverse. This mismatch creates warpage in flat parts when experienced over a temperature swing from -40 to 120 °C.

Long-term thermal aging at 120 °C for 1000 h typically reduces notched Izod in GF30 PC by an additional 10 to 20 %, while tensile modulus remains within ±5 %. The loss is caused by oxidation at the fiber-matrix interface rather than bulk molecular weight loss. Sizing degradation is detectable as a drop in interfacial shear strength measured by single-fiber pull-out, but published data for this specific configuration is limited. Components exposed to hot air above 110 °C should therefore be evaluated with notched impact tests after ageing per ISO 188:2011 and not solely by tensile retention.

GF30 PC is frequently selected for electrical enclosures requiring dimensional stability under IEC 60695-2-12 glow-wire testing at 850 °C for 30 s. In such parts, the grade must simultaneously maintain a glow-wire ignition temperature above 850 °C, reduce thickness shrinkage below 0.2 %, and pass a notched impact requirement after molding. These demands pull in opposite directions. Increasing glass content from 20 wt% to 30 wt% improves resistance to deformation at high temperature, but the same change lowers weld-line impact energy and raises the probability of brittle failure at snap-fit features. Mold trials on a 120-ton hydraulic injection molding machine with a 5.0 mm cold runner and two 1.2 mm tunnel gates demonstrate that flow-direction specimens pass a 40 J/m weld-line Izod criterion only when melt temperature is held at 310 °C and the mold temperature is held at 100 °C; lower mold temperatures create frozen skin layers that reduce weld-line strength by 15 to 20 %.