The product designated PLA-SC-BLK-175-1000 is a black, semi-crystalline polylactic acid 3D printing filament extruded for fused filament fabrication. The product code encodes a nominal diameter of 1.75 mm and a 1000 g net spool. Continuous melt extrusion on a twin-screw compounding line with an L/D ratio of 32:1, melt filtration through a 100 µm screen pack, water quenching at 30–40 °C, laser diameter scanning, and closed-loop spool winding are used to maintain ovality within ±0.03 mm. Density under ISO 1183-1:2019 is 1.24 g/cm³. Melt flow rate at 210 °C and 2.16 kg piston load under ISO 1133-1:2022 is 6 g/10 min. Differential scanning calorimetry under ISO 11357-3:2018 places the glass transition at 55–60 °C and the melting endotherm between 150–165 °C. The black color is produced with a carbon black masterbatch added at a loading below 2.0 wt%; this filler lowers surface resistivity and alters crystallization kinetics relative to unfilled natural PLA.

Incoming PLA resin is specified with a D-isomer content of 2–5%; higher D-isomer ratios reduce crystallization rate and shift the grade toward amorphous behavior. Pellet moisture is controlled below 250 ppm by Karl Fischer titration under ISO 15512:2019 before melt extrusion. Carbon black dispersion is monitored by filter pressure value; an increase of 0.4–0.8 bar/g across a 100 µm screen indicates sufficient dispersion without excessive agglomerates. Poor dispersion produces visible surface roughness and nozzle clogging in 0.25 mm orifices.

Filament diameter is continuously monitored by dual-axis laser micrometers at a scan rate of 1000 Hz. The water quench bath is held at 30–40 °C because lower quench temperatures increase amorphous orientation and produce brittle filament; higher temperatures allow sag before solidification. Ovality drift above ±0.03 mm occurs when the puller speed deviates by 1–2% from melt pump output. Batch-to-batch melt flow rate variation from 5–7 g/10 min is within the production window; outside this window, bowden-style extruders show intermittent over-extrusion and under-extrusion. The spool is wound with a constant tension of 0.5–1.0 N to prevent cross-winding and filament crossover that causes mid-print tangle failure.

Layer Adhesion, Nozzle Backpressure, and Extrusion Parameters

At a 0.40 mm brass nozzle, the black semi-crystalline grade exhibits 0.8–1.2 MPa higher backpressure than unfilled PLA at the same volumetric throughput because dispersed carbon black particles increase melt viscosity. Apparent melt viscosity measured under ISO 11443:2014 at 210 °C and 100 s⁻¹ is 250–400 Pa·s, which is 10–20% above natural PLA of equivalent melt flow index. Capillary rheometry indicates a shear-thinning exponent of 0.4–0.6 between 10 s⁻¹ and 1000 s⁻¹; this non-Newtonian response means that reducing the nozzle orifice from 0.40 mm to 0.25 mm disproportionately increases backpressure. Nozzle temperature must be maintained at 200–220 °C. Below 195 °C, incomplete melting and insufficient interlayer diffusion reduce z-axis tensile strength by 30–40%; above 230 °C, PLA undergoes thermal hydrolysis and chain scission, increasing melt flow index and causing uncontrolled filament drool. The recommended volumetric throughput is 10–15 mm³/s for a 0.40 mm orifice, keeping melt pressure below 12 MPa. For a 0.25 mm nozzle, the volumetric throughput should be reduced to 4–6 mm³/s because shear rate increases and melt pressure can exceed 20 MPa. Carbon black is abrasive; hardened steel or ruby nozzles are specified for continuous runs exceeding 500 h. On brass or copper nozzles, orifice widening of 0.02–0.05 mm is observed after 200–300 h of continuous use, producing dimensional error in thin-wall sections.

Residence time in the hotend is another limiting variable. At 210 °C, PLA undergoes measurable molecular weight reduction after 10 min in the melt; at 230 °C, the same reduction occurs within 2–3 min. Long retraction distances above 6 mm in all-metal hotends pull molten material into cooler zones and create plugging after 12–24 h. A direct-drive extruder with a polished stainless steel heat break is preferred over a bowden system when cycling between 195 °C and 210 °C in production.

Build plate adhesion is controlled with a heated glass or polyetherimide substrate at 50–60 °C. No heated chamber is required for parts with an xy footprint below 80 mm; larger parts develop edge lifting when ambient temperature falls below 20 °C. First layer height is set to 0.20 mm, first layer width to 120%, and initial print speed to 20 mm/s. Subsequent layers at 60–80 mm/s produce z-axis tensile strength of 18–25 MPa, approximately 40–50% of in-plane tensile strength. This anisotropy is consistent with fusion-line mechanics in fused filament fabrication and is not a unique defect of the black semi-crystalline grade.

How Does Carbon Black Loading Alter Crystallization Kinetics?

Isothermal calorimetry under ISO 11357-7:2015 indicates that fine-particle carbon black acts as a heterogeneous nucleating agent. The cold crystallization peak shifts 3–7 °C lower than unfilled PLA, and the half-time of isothermal crystallization at 100 °C is shortened by 20–30%. As-printed crystallinity remains below 5% because the melt is quenched rapidly in fused filament fabrication; after annealing at 100 °C for 30 min, the crystalline fraction reaches 25–35% by differential scanning calorimetry. Published data for this exact carbon black masterbatch grade is limited; the stated behavior reflects PLA compounds with carbon black loadings below 2.0 wt%. Higher loadings increase melt viscosity and reduce interlayer fusion at standard nozzle temperatures. The nucleating effect is not uniformly beneficial: at extruder temperatures below 200 °C, premature cold crystallization during first-layer deposition can reduce contact temperature at the layer interface. The practical consequence is that semi-crystalline black PLA can develop greater heat resistance than natural amorphous PLA when subjected to post-print annealing, but it remains more brittle than high-elongation copolyester materials.

Tensile specimens printed with 100% rectilinear infill, 3 perimeters, and 0.20 mm layer height were conditioned at 23 °C and 50% RH for 48 h before testing. Tensile strength under ASTM D638-14 Type IV was 50–58 MPa, tensile modulus 3.4–3.6 GPa, and elongation at break 3–6%. Notched Izod impact under ISO 180:2019 was 3–5 kJ/m². Flexural modulus under ISO 178:2019 at 23 °C was 3.0–3.4 GPa. Hardness by Shore D durometer under ISO 868:2003 was 80–85. These values position the material above amorphous PLA in stiffness and below PETG and ABS in impact energy absorption.

Relative to ABS, the black semi-crystalline PLA exhibits lower warpage and does not require a heated chamber for build lengths below 150 mm. Relative to PETG, it has lower elongation at break and notched impact strength, but higher tensile modulus and cleaner support removal. Compared with natural amorphous PLA, the carbon black grade has a slightly higher melt viscosity, improved dimensional stability after annealing, and a lower tendency to stress-whiten; however, the black surface masks crack initiation during visual inspection. Compared with carbon-fiber-filled PLA, the black semi-crystalline grade has lower tensile modulus, less nozzle abrasion, and lower electrical conductivity. Compared with mineral-filled PLA, it has lower density and lower melt viscosity at equivalent filler loading. These distinctions are relevant when selecting a filament for functional prototypes that require post-print annealing rather than continuous service under load.

When Semi-Crystalline Black PLA Is Annealed in Enclosed Chambers

Annealing at 100–110 °C for 30–60 min in a forced-air or oil bath raises the heat deflection temperature at 0.455 MPa from 55–62 °C to 85–120 °C, depending on crystalline fraction. The part must be supported because PLA softens before crystallization; unsupported annealing causes sagging above 70 °C. Dimensional change during annealing is anisotropic, with global shrinkage of 0.3–0.8% along the z-axis and 0.1–0.3% in the x-y plane. The processing window is limited to ±5 °C for complex geometries with wall thickness below 2 mm. At 115 °C, thin sections distort before full recrystallization occurs. Annealing is therefore specified only for components with uniform wall thickness and no trapped cavities.

For parts requiring post-anneal dimensional accuracy, a scale factor of 1.003–1.008 in the z-axis and 1.001–1.003 in the x-y plane is applied before annealing. The scale factor must be determined on a geometry-specific basis because corners densify more quickly than thick sections. Annealing fixtures made from aluminum plate at 10 mm thickness reduce warpage by improving thermal contact and constraining flatness. Differential scanning calorimetry after annealing shows a melting peak between 165–175 °C because thicker lamellae form during slow crystallization; this shift is accompanied by an increase in the measured heat deflection temperature.

Unopened spools are sealed with desiccant and a moisture barrier film. PLA absorbs atmospheric water; after 24 h at 60% RH the filament surface reaches 0.3–0.5 wt% moisture. Hydrolysis is autocatalytic in PLA; wet filament processed above 200 °C undergoes chain scission that lowers molecular weight and creates steam voids at the nozzle. Extrusion of wet filament produces audible popping, increases diameter variability to ±0.10 mm, and reduces tensile strength by 10–20%. Drying in a forced-air oven at 45–55 °C for 4–6 h restores processing stability. Drying above 60 °C is not specified because spooled PLA can soften and block adjacent windings. The hydrolysis rate approximately doubles for every 10 °C above 60 °C when moisture is present. For production use, a dry-box maintained below 20% RH is required; a dew-point sensor and desiccant regeneration system are more reliable than visual indicators. Batch logs should record ambient dew point and drying time because moisture uptake is a function of absolute humidity, not relative humidity alone.

Under EU RoHS Directive 2011/65/EU, the carbon black masterbatch and PLA matrix are below restricted substance limits; compliance screening for lead, cadmium, mercury, and hexavalent chromium is performed under IEC 62321-5:2013. The product is not intended for direct food contact unless the printed and annealed article is verified under FDA 21 CFR 175.300 or equivalent migration testing for the finished part; the filament alone does not constitute a food-contact compliance statement. The grade should not be processed with amine-based additives or masterbatches that accelerate PLA hydrolysis. Continuous melt temperatures above 230 °C are outside the operational boundary and increase lactide monomer concentration in the melt.