Essentium PA Additive Manufacturing Filament is an unfilled aliphatic polyamide-based thermoplastic feedstock for fused filament fabrication and high-speed extrusion equipment. The product is supplied in desiccant-sealed spools; production formats include 1 kg and 4 kg spools, and the nominal filament diameter is 2.85 mm with an allowable deviation of ±0.05 mm. A 1.75 mm diameter option is specified only for low-throughput printers. The melt processing envelope is 250 °C to 270 °C at the nozzle, with a heated bed between 70 °C and 90 °C. When an actively controlled build chamber is used, a setpoint of 40 °C to 60 °C is required to manage warp. The feedstock is intended for direct-drive extruders and all-metal hot ends; bowden-tube feed paths are not recommended because polyamide softens and increases feed friction above 60 °C. Essentium PA differs from carbon- and glass-filled Essentium nylon grades because no fibre reinforcement is present, preserving ductile behaviour and eliminating the need for abrasion-resistant nozzle materials.

What Moisture Control Limit Governs Melt Stability?

Moisture is the limiting process variable rather than print speed or nozzle diameter. At 23 °C and 50% RH, unfilled PA6/6 reaches an equilibrium moisture content of approximately 2.5 wt% within 72 h when exposed to ambient air. Melt-phase moisture above 0.20 wt% hydrolyses the amide linkage at processing temperatures above 240 °C, producing viscosity loss, die swell instability, delamination, and spherical void formation in printed roads. Drying must be performed in a desiccant dryer at 80 °C for 4 h to 6 h, or in a vacuum oven at 100 °C for 4 h, to bring moisture below 0.02 wt%. The dry-state threshold is verified by Karl Fischer titration per ASTM D6869-03 or by thermogravimetric moisture analysis. Production experience on twin-screw extruders with L/D 32:1 shows that wet feedstock can be processed without visible splay only when devolatilization is available; fused filament machines do not provide devolatilization, so incoming spool dryness is required.

Moisture uptake and loss are diffusion-limited. For a 2.85 mm filament at 80 °C, the half-time for moisture desorption is typically between 30 min and 90 min; therefore, the 4 h drying cycle is deliberately conservative to account for spool packing density and dryer airflow. In production areas above 60% RH, an opened spool can exceed 0.08 wt% moisture within 2 h. This requires point-of-use dryers or sealed feed canisters on machines with extended idle periods. The moisture hysteresis of polyamide means that drying to 0.02 wt% does not permanently prevent re-absorption once the spool is returned to humid air.

When Controlled Enclosure Temperatures Are Unavailable, What Changes?

Polyamide crystallization generates volumetric shrinkage of 8% to 14% during cooling from the melt to room temperature. Without an enclosure at 45 °C to 60 °C, the temperature differential across a tall part creates a stress gradient that exceeds the yield stress of the partially crystallized layer, producing corner lifting and wall curl. Parts with length-to-width ratios above 3:1, or continuous spans longer than 150 mm, require a brim or raft even on heated glass. Field data from enclosed industrial printers indicate that a build chamber at 50 °C, combined with a bed at 80 °C, reduces corner lifting on unfilled polyamide to less than 2 mm on a 200 mm rectangular fixture; open-frame machines under identical bed conditions often exceed 5 mm of curl. If an enclosure is not available, part geometry should be restricted to low aspect-ratio sections, and infill density above 60% should be avoided because the larger frozen stress volume intensifies warpage.

Anisotropy is the dominant mechanical characteristic. With 0.2 mm layer height, four perimeters, and 45°/−45° raster on XY-oriented specimens, unfilled polyamide class materials produce tensile strength between 40 MPa and 60 MPa when tested to ASTM D638-14 in the dry-as-printed condition. Z-oriented tensile strength is typically 20 MPa to 35 MPa because interlayer fusion is incomplete and failure initiates at weld interfaces. Flexural modulus per ISO 178:2019 ranges from 1.2 GPa to 1.7 GPa. Conditioned specimens at 23 °C and 50% RH absorb moisture and can show notched Izod impact above 5 kJ/m² per ASTM D256-10, but tensile modulus may decrease by 15% to 25% relative to dry specimens. These combined properties support snap-fit arms, cable guides, and assembly fixtures that experience intermittent bending rather than continuous tensile creep. Continuous load-bearing applications exceeding 60 °C are better served by filled or high-temperature polyamide grades because unfilled PA enters the creep-sensitive regime.

Unfilled Polyamide Versus Carbon-Fibre and Glass-Filled Feedstocks

Essentium PA is differentiated from reinforced grades primarily by the absence of fibre. This absence allows use of brass or hardened steel nozzles; in carbon fibre-filled PA, the chopped fibre content causes rapid bore wear in brass nozzles and mandates hardened steel or ruby nozzle inserts. The mechanical consequence is toughness: unfilled PA retains elongation at break of 10% to 30% dry as printed, whereas carbon-filled PA typically falls below 3% and glass-filled PA below 5%. Stiffness moves in the opposite direction. Carbon-filled PA with 15 wt% chopped carbon fibre can reach tensile modulus of 8 GPa to 12 GPa, while 20 wt% glass-filled PA reaches 4 GPa to 7 GPa. Heat deflection temperature under 0.455 MPa is also lower for unfilled PA, commonly 60 °C to 80 °C per ASTM D648-18; carbon-filled PA may exceed 120 °C at the same stress. Therefore, Essentium PA is the appropriate unfilled polyamide selection when impact deformation and material flow are more important than absolute stiffness or high-temperature dimensional stability.

Crystallinity and solidification behaviour reinforce these differences. Unfilled polyamide develops a semi-crystalline microstructure with melting point near 255 °C to 265 °C; the cooling-rate control from a heated chamber affects the degree of crystallinity and subsequent moisture uptake. Rapid cooling suppresses crystalline order and lowers initial hardness but can improve interlayer toughness. In carbon-filled and glass-filled systems, fibres act as heterogeneous nucleation sites and increase the effective stiffness of the solidified road, but they also increase melt viscosity and reduce the processing window. This is why unfilled Essentium PA can be run through 0.4 mm nozzles at lower pressure than filled grades.

Solvent Exposure and Creep Boundaries Set the Upper Service Temperature

Unfilled polyamide swells and loses molecular weight in strong acids, phenols, chlorinated solvents, and certain aqueous salt solutions. Short-term contact with automotive fluids such as motor oil and grease at room temperature is generally tolerated in ASTM D543-21 immersion screening, but continuous immersion in hot water above 70 °C causes progressive hydrolysis. In dry service up to 50 °C, unfilled PA parts can retain more than 80% of initial tensile strength; above 80 °C, the design-limiting property becomes creep under sustained load. A moisture increase of 1.0 wt% can increase linear dimensions by 0.2% to 0.4% in unfilled PA, which must be accounted for when inspecting parts after conditioning. For dimensional control, parts should be stabilized at 23 °C and 50% RH per ISO 291 for 48 h before final measurement, or a scale factor based on the service humidity should be applied.

In applications where dimensional tolerance is tighter than ±0.2 mm, absorbed moisture is the largest source of non-repeatability. The coefficient of hygroscopic expansion for unfilled PA is on the order of 0.0025 mm/mm/%H₂O to 0.0040 mm/mm/%H₂O; therefore, a 1 wt% moisture change across a 100 mm feature produces a dimensional shift of 0.25 mm to 0.40 mm. This is not acceptable for precision assembly without humidity control.

Throughput is governed by shear-thinning melt behaviour. Capillary rheometry at 260 °C on unfilled PA6/66 shows apparent viscosity from 200 Pa·s to 500 Pa·s at 100 s^-1, falling to 40 Pa·s to 100 Pa·s at 1000 s^-1. The melt-flow index measured at 260 °C under 2.16 kg load according to ISO 1133-1:2022 is commonly between 10 g/10 min and 30 g/10 min for dry unfilled polyamide. Higher values after drying suggest molecular weight reduction from previous melt cycles. On a 0.4 mm nozzle, stable deposition is typically limited to volumetric rates below 10 mm³/s; increasing to 0.8 mm nozzles permits rates up to 30 mm³/s only when hot-end temperature is maintained at the upper range of 260 °C to 270 °C. Above 35 mm³/s, melt fracture and die swell are observed on direct-drive extrusion systems unless the material is dried below 0.02 wt% moisture and the melt residence time is kept below 15 min.

These setpoints are not independent. Raising the nozzle temperature to 270 °C without raising chamber temperature increases the thermal gradient and can make curl worse, even though melt viscosity decreases. Conversely, increasing chamber temperature above 60 °C may soften the print too early and cause sagging in unsupported overhangs. The processing window is therefore a coupled balance between melt temperature, chamber temperature, and deposition rate. Published data for this specific Essentium PA configuration is limited; production qualification should include a full factorial build at the three corner points of the processing envelope before transfer to manufacturing.

Storage and regrind policy affects lot-to-lot consistency. Sealed spools with desiccant retain process dryness for 12 months at 15 °C to 25 °C; opened filament should be consumed within 7 days or kept in a desiccant cabinet at 10% RH or lower. Regrind from printed purge shields and support structures is not recommended for direct re-extrusion into filament because polyamide undergoes thermal oxidative chain scission during repeated melting. If production economics require regrind, it should not exceed 15 wt% addition, and the melt should be filtered through a 200 µm screen pack on a compounding extruder with L/D 32:1. Because published data for this specific configuration is limited, each resulting lot must be qualified by tensile testing to ASTM D638-14 and impact testing to ASTM D256-10 before release for load-bearing fixtures.