Injection molders replacing 30 wt% of the polymer phase in a 30 wt% glass-fiber reinforced poly(ethylene terephthalate) compound with sorted post-industrial recycled flake routinely measure retention of tensile strength, flexural modulus, and notched impact energy against a virgin-only control. The material system assessed here is defined as a short-glass-fiber reinforced PET in which 30 wt% of the PET matrix originates from post-industrial process scrap; the total recycled polymer fraction in the finished compound therefore approximates 21 wt%. Test specimens are dried and injection molded under a shared thermal history, conditioned for at least 24 h at 23 °C and 50% relative humidity in accordance with ISO 291:2008, and tested using ASTM D638-14 for tensile properties, ISO 178:2019 for flexural properties, ISO 179-1:2023 for Charpy impact, and ISO 75-2:2013 for deflection temperature under load. Representative production-scale values for a 30 wt% short-glass-fiber reinforced virgin PET compound produced on a 40:1 L/D co-rotating twin-screw extruder are 152 MPa tensile stress at break, 10.2 GPa tensile modulus, 220 MPa flexural strength, 8.8 kJ/m² notched Charpy impact energy at 23 °C, and 223 °C deflection temperature under 1.82 MPa. When a properly dried 30% recycled polymer fraction is introduced, the same compound typically retains 92% to 97% of tensile strength and 95% to 99% of tensile modulus, provided the intrinsic viscosity of the compounded matrix remains above 0.60 dL/g and the glass fiber content is held at 30 ±1 wt%. The retention gap widens under suboptimal drying or uncontrolled melt residence because poly(ethylene terephthalate) undergoes hydrolytic chain scission and the fiber-matrix interphase loses the chain length required for stress transfer; this effect is reflected more strongly in impact energy retention than in fiber-dominated modulus retention.

What Limits Retention When 30% Recycled Feedstock Is Introduced?

The dominant mechanism limiting mechanical property retention with recycled feedstock is not the glass fiber itself but hydrolytic degradation of the recycled PET fraction during melt processing. Recycled PET flake enters the process with higher and more variable moisture than virgin pellets; moisture levels in post-industrial flake are commonly measured between 0.08% and 0.35% before drying, whereas virgin PET pellets are typically dried to below 0.005% before compounding. Hydrolysis follows a chain-cleavage mechanism in which the ester linkage of the PET repeat unit is attacked by water, reducing number-average molecular weight and liberating carboxylic acid end groups that accelerate further degradation. A drop in intrinsic viscosity from 0.68 dL/g to 0.58 dL/g during compounding, measurable by ISO 1628-5:2021, lowers notched impact retention more than tensile modulus because impact crack propagation depends on matrix ductility and interfacial debonding, whereas modulus is dominated by fiber volume fraction and fiber length. The practical drying boundary for a 30% recycled feedstock is a desiccant dryer with a -40 °C dew-point air supply, a residence time of 4 h to 6 h at 160 °C to 180 °C, and in-line moisture verification below 50 ppm. On production-scale twin-screw extrusion lines with L/D ratios from 40:1 to 48:1, the recycled fraction is usually introduced at the main feed throat with the virgin PET, while chopped glass fiber is side-fed downstream after the polyester has melted to limit fiber breakage. Vacuum venting at two zones with a vacuum of -0.08 MPa to -0.09 MPa assists in removal of volatile contaminants but cannot reverse molecular weight loss once hydrolysis has occurred. A second practical limitation is polyolefin contamination in recycled flake; concentrations above 0.5 wt% create immiscible domains that act as crack initiation sites and preferentially reduce multiaxial impact retention. The combination of moisture control, contamination sorting, and intrinsic viscosity stabilization is therefore the principal technical lever for achieving mechanical properties that approach virgin glass-fiber reinforced PET.

Thick-wall automotive sensor housings molded from this compound require a tool surface temperature between 120 °C and 135 °C to obtain sufficient crystallinity for dimensional stability and to prevent the recycled PET fraction from quench-freezing into a low-strength amorphous state. On hydraulic injection molding machines with clamping force between 1,500 kN and 3,000 kN, the barrel profile is typically set from 265 °C to 290 °C, with screw back pressure maintained at 0.5 MPa to 1.0 MPa and injection speeds set to avoid jetting or fiber orientation defects. Under these conditions, the heat deflection temperature of the 30% recycled polymer compound remains within 4 °C to 6 °C of the virgin baseline, and the tensile strength at weld lines typically retains 88% to 93% of the non-weld-line value. The weld line is a critical defect site because glass fibers do not bridge the weld plane effectively, and the lower molecular weight of the recycled fraction can reduce re-entanglement across the merging flow front. Molders compensate by maintaining a resin cushion above 5 mm, using sequential valve-gate sequencing on hot-runner tools to reposition weld lines away from high-stress regions, and pre-drying the material for 4 h to 6 h at 170 °C when the ambient relative humidity exceeds 40%. Failure to pre-dry under humid conditions produces surface splay, an acetaldehyde odor, and a measurable drop in notched impact energy because internal moisture converts to steam during plastication and hydrolyzes the ester backbone. Published multi-lot data for this exact 30% recycled feedstock configuration under combined 85 °C and 85% relative humidity aging are limited; available single-lot evaluations indicate that tensile modulus retention remains above 90% after 1,000 h, but notched impact retention can fall to 65% to 75% because hydrolysis continues at the fiber surface and creates microcracks near the interface.

When Barrel Residence Time Exceeds 8 Minutes at 290°C, Impact Retention Falls Below 80%

Residence time in injection molding is not a single setpoint but a distribution, and the tail of that distribution controls the property cliff-edge. At melt temperatures above 290 °C and cumulative residence times above 8 min, the notched Charpy impact energy of a 30% recycled polymer compound can fall from 8.8 kJ/m² to below 6.4 kJ/m², corresponding to a retention below 80% relative to the virgin control. The degradation pathway proceeds through thermal scission and hydrolysis, both of which reduce the molecular weight of the matrix and sharpen the ductile-to-brittle transition. A compound with a matrix intrinsic viscosity of 0.61 dL/g may still pass tensile strength and flexural modulus specifications at 23 °C while failing notched impact at -20 °C because the shorter chains cannot dissipate energy through shear yielding ahead of a notch. This is why impact retention is the most sensitive mechanical indicator of recycled feedstock quality. The processing window for glass-fiber reinforced PET with 30% recycled polymer is therefore bounded at the upper end by melt temperature, at the lower end by incomplete fiber wetting and high orientation, and at the middle by residence time distribution. Barrel settings above 295 °C are generally not recommended, even for thin-wall applications, because the additional flow length gained from lower melt viscosity is traded against faster chain scission and increased acetaldehyde evolution. Hot-runner manifold temperatures above 300 °C are more damaging than screw barrel temperatures because the polymer is held at high temperature under low shear, and no amount of back pressure can restore lost molecular weight. The preferred boundary is a melt temperature of 285 °C to 290 °C at the nozzle, a barrel residence time below 8 min, and a shot size that occupies 40% to 70% of the available barrel capacity to avoid excessive staging of molten material.

Downstream side-feeding of 4.5 mm chopped glass strands after the polyester has melted is standard on co-rotating twin-screw extruders with 40:1 L/D to 48:1 L/D because it reduces the total shear history on the glass fiber and preserves number-average fiber length. In a typical production configuration, the virgin PET and dried recycled flake are fed into the first barrel section at a throughput rate of 300 kg/h to 600 kg/h, the polymer is melted through barrel zones set from 260 °C to 285 °C, and the chopped glass is introduced through a side-feeder at approximately 60% of the barrel length. The screw design uses kneading blocks after the side-feeder to disperse the glass, followed by low-shear conveying elements and two vacuum vents operated at -0.08 MPa to -0.09 MPa. The melt is then filtered through a screen pack with screen openings of 150 µm to 250 µm to remove metal particles, aluminum flake, and crosslinked gel contaminants that are otherwise invisible after pelletizing but become visible as surface defects and crack initiators in molded parts. Filtration pressures above 12 MPa at the screen changer indicate either excessive polyolefin contamination or degraded PET gel, and the production line must be stopped before the screen pack ruptures and allows contaminants to bypass the filter. Pelletizing is accomplished by underwater pelletizers with die water temperatures held at 20 °C to 30 °C, and the resulting pellets are dried in a desiccant dryer at 140 °C to 160 °C for 2 h to 3 h before packaging in moisture-barrier bags. Each lot is tested for intrinsic viscosity by ISO 1628-5:2021, glass fiber content by ISO 1172:2019, melt volume-flow rate at 285 °C under 2.16 kg by ISO 1133-1:2022, and residual moisture by ISO 15512:2019. A melt volume-flow rate above 25 cm³/10 min at 285 °C and 2.16 kg frequently indicates excessive molecular weight loss and correlates with a batch-to-batch notched impact reduction of more than 10%.

Fiber Length Reduction, IV Decay, and the 30 Percent Feedstock Threshold

Fiber length measurements after pyrolysis at 600 °C for 1 h followed by optical microscopy image analysis according to ISO 22314:2006 show that a 30 wt% glass-fiber reinforced PET compound enters the injection molding process with a number-average fiber length of 0.8 mm to 1.2 mm but typically leaves the molded part with a number-average fiber length of 0.25 mm to 0.40 mm because of fiber-fiber abrasion, barrel-wall shear, and gate-induced orientation. This reduction in fiber length is slightly less severe when the matrix contains 30% recycled PET because the lower melt viscosity reduces hydrodynamic shear on the brittle glass fibers, but the benefit is partially offset by reduced matrix fracture toughness. The mechanical consequence is a trade-off: tensile modulus remains high because even short fibers above the critical length can transfer stress, while tensile strength and impact energy depend on the matrix ability to redistribute stress around fiber ends. Intrinsic viscosity decay during compounding follows a nonlinear trend; the first 0.05 dL/g to 0.08 dL/g of loss occurs during plastication and side-feeding, and further loss is governed by screw speed, fill level, and vacuum efficiency. Screw speeds above 400 min⁻¹ on 40:1 L/D machines can generate enough viscous dissipation to raise the local melt temperature above 300 °C even when barrel setpoints are below 285 °C, and this is often the hidden cause of impact retention failure in compounds that otherwise pass inlet moisture and feedstock viscosity checks. The 30% recycled feedstock threshold is not a thermodynamic boundary but a practical control boundary because lot-to-lot flake variability in molecular weight, moisture, and contamination becomes statistically more difficult to compensate above this fraction without exceeding the mechanical property tolerance band. At 30% recycled content, the incoming recycled flake intrinsic viscosity must be held between 0.65 dL/g and 0.72 dL/g to ensure the compounded matrix remains above 0.60 dL/g, and the flake must be screened to remove polyolefin closures, metal fragments, and PVC before entering the main feed stream.

After molding, the short-term mechanical property retention of glass-fiber reinforced PET with 30% recycled feedstock is frequently acceptable even when the recycled fraction is slightly over-dried or mildly degraded, but long-term property retention under hot and humid service conditions is more discriminating. Exposure to 85 °C and 85% relative humidity for 1,000 h causes water absorption in the matrix, interfacial debonding, and a gradual drop in notched impact energy that is not fully captured by room-temperature tensile tests. The glass transition temperature of PET is approximately 75 °C to 80 °C, and at 85 °C the amorphous regions have sufficient segmental mobility for recrystallization, which increases stiffness and embrittles the matrix. Recycled PET fractions containing residual polyolefin contaminants are especially vulnerable to interfacial cavitation because the polyethylene or polypropylene domains have low water permeability and can act as stress concentrators during cyclic thermal expansion. Molded components that must meet automotive under-hood requirements are typically specified for heat deflection temperature by ISO 75-2:2013, notched Charpy impact by ISO 179-1:2023, and tensile properties by ISO 527-2:2012 after heat aging for 1,000 h at 120 °C. For this condition, the recycled polymer compound is pre-dried to below 50 ppm moisture, processed at a melt temperature no higher than 290 °C, molded with a tool temperature of 125 °C to 140 °C, and tested without annealing unless the application explicitly requires annealing to complete crystallization. Published data for this specific configuration under simultaneous heat and moisture cycling are limited, and single-lot results should not be extrapolated to different recycled flake sources without a full comparative test program. Incompatibilities include ammonium polyphosphate-based flame retardants, which release acidic phosphate species at processing temperatures and accelerate hydrolysis of the recycled PET fraction, and certain high-acid post-consumer recycled flake sources that consume stabilizer packages designed for bottle-grade virgin PET. A secondary operation such as post-molding annealing at 150 °C for 2 h can raise crystallinity and improve heat deflection temperature by 2 °C to 5 °C, but the same annealing step may reduce notched impact energy by 5% to 10% and must be validated against the final part drawing before release.

After compounding, injection molding trials on production equipment with shot sizes of 200 g to 600 g have shown that the highest-impact retention occurs when the mold is filled through a fan gate or a tab gate rather than a pin gate, because the abrupt restriction of a pin gate raises local shear rate above 100,000 s⁻¹ and breaks glass fibers near the gate region. The same trials indicate that weld lines formed at the meeting of two flow fronts reduce the effective fiber bridging across the knit line and can lower tensile strength at the weld line to 50% to 70% of the bulk value, which is a larger reduction than the retention penalty introduced by 30% recycled feedstock itself. For parts with high clamp force requirements, molders have used sequential valve-gate systems with independent nozzle temperature control to move weld lines to low-stress regions or to heal the weld line with a short pressure-hold stage after the second gate opens. Because glass fiber orientation is strongly influenced by mold filling, the recycled compound also exhibits anisotropic shrinkage: 0.2% to 0.4% in the flow direction and 0.5% to 0.9% transverse to flow under the processing conditions described above. These shrinkage ranges must be corrected on the tool drawing before the recycled material is qualified, because the slight differences in melt viscosity and crystallization rate can shift the shrinkage relative to a virgin control by 0.05% to 0.15%. The mechanical property retention data therefore cannot be evaluated in isolation; a 30% recycled feedstock may pass tensile and impact testing while failing dimensional capability studies if the tool was cut for virgin material and the molder has not compensated for the slightly lower melt viscosity and faster crystallization of the recycled phase. Batch-to-batch variance in post-industrial flake remains the most persistent manufacturing concern, and incoming lots with intrinsic viscosity below 0.65 dL/g, moisture above 0.30%, or polyolefin contamination above 0.5 wt% should be quarantined or blended downward below the 30% threshold until the compounding line can be rebalanced to hold the compounded matrix above 0.60 dL/g and the molded Charpy impact retention above 80%. Avoid re-compounding glass-fiber reinforced PET containing recycled feedstock with ammonium polyphosphate flame retardants, because the acid-generating retardant accelerates hydrolytic chain scission at processing temperatures and can produce a brittle part that passes flexural modulus but fails impact and weld-line strength. Operation outside the stated melt temperature and moisture boundaries does not announce itself through visible degradation; instead, the first indication is a reduction in notched impact energy at -20 °C or a drop in weld-line tensile strength below the part drawing limit, which is why the processing boundary table is more useful as a production audit tool than as a post-molding test requirement.