In fused filament fabrication of semicrystalline polyaryletherketone components, diameter consistency of the feedstock and control over recrystallization between adjacent rasters determine whether z-direction interfaces function as load-bearing boundaries or as defect arrays. The product specified here is Ensinger TECAFIL PEKK natural – 1.75 mm – Filament Polyetherketoneketone. The term natural denotes an unpigmented formulation; the wound monofilament typically presents as a translucent amber to light brown strand. Polyetherketoneketone contains a higher ketone-to-ether ratio in the backbone than polyetheretherketone, which affects both melting behaviour and the rate of crystalline ordering after deposition.

The bulk material density reported for unfilled PEKK according to ISO 1183 is typically 1.28 g/cm³. Tensile properties obtained on compression-moulded or injection-moulded Type 1B specimens under ISO 527-2 are typically in the range of 90–100 MPa tensile stress at yield and 3.2–3.5 GPa tensile modulus. These values are not direct guarantees for additively manufactured parts, because raster direction, air gaps, chamber temperature, moisture, and annealing history alter the final part response. Commercial PEKK resins are frequently described by the ratio of terephthaloyl to isophthaloyl repeat units. Higher terephthaloyl content tends to raise the melt peak and increase crystallinity; higher isophthaloyl content lowers the melt peak and slows crystallisation. The supplied TECAFIL PEKK natural filament is therefore evaluated not only by diameter but also by its thermal transitions as received, because a shift in melting peak above or below the supplier control window may indicate a resin blend or molecular-weight change.

Filament dimensional conformance and incoming inspection

For a 1.75 mm feedstock, diameter drift outside ±0.05 mm changes feed rate and can generate melt-pressure fluctuation at the nozzle. Incoming inspection on direct-drive extruders with hardened steel feed zones should measure the strand with a two-axis laser micrometer at intervals no wider than 500 mm across the first 5 m of a new spool. A maximum ovality of 0.03 mm is used as a practical acceptance limit; above that threshold, the filament may alternately slip and overfeed in the drive gear. The diameter value is measured at 23 °C and 50 % relative humidity after the spool has equilibrated to avoid transient dimensional variation from moisture expansion.

Spool-to-spool variation in melt volume-flow rate is more difficult to detect visually. Resin lot drift can be screened by melt volume-flow rate testing under ISO 1133-1:2022 at 380 °C with a 5 kg load; a large viscosity shift changes die swell, strand width, and the extrusion multiplier required to produce a void-free raster. Batch-to-batch variation in 1.75 mm filament is therefore controlled by both dimensional metrology and rheological lot checks.

Moisture content is quantified by Karl Fischer coulometric titration according to ISO 15512. Unfilled PEKK reaches an equilibrium moisture uptake near 0.20 % by mass at 23 °C and 50 % relative humidity when tested under ISO 62. A residual moisture level above 0.04 % at the feed inlet will decompose into steam during extrusion, producing intra-raster porosity and audible outgassing at the nozzle.

Because residual moisture in polyetherketoneketone behaves as a volatilisation source at melt temperature, drying before extrusion is mandatory. The filament is dried in a convection dryer at 120 °C to 150 °C for 3–4 h; the drying air should have a dew point below −20 °C. After drying, the spool should be fed from a purged dry box or sealed holder with desiccant. At 150 °C drying, extended exposure beyond 6 h can cause strand embrittlement and surface discolouration, so timer-controlled cycles are preferred over indefinite standby. The dryer volume should allow at least one spool per 0.2 m³ to maintain uniform heat transfer; stacked spools in a static oven develop temperature gradients that leave the core material wet while the outer windings are over-dried.

During extrusion, the first observable consequence of incomplete drying is micro-void formation in the top rasters. In sectioned polished cross-sections evaluated under optical microscopy at 50× to 200×, steam-induced voids appear as spherical pores with diameters from 10 μm to 80 μm. These pores act as crack initiation sites in tensile testing under ISO 527-2 and reduce the apparent interlaminar toughness in mode-I fracture tests.

Interlayer strength in unfilled PEKK is bounded by melt diffusion and surface reactivation. At a nozzle temperature of 355 °C, the melt viscosity remains high enough to prevent neat filling of sharp corners unless the extrusion multiplier is increased by 3–5 % relative to standard amorphous filaments. Layer adhesion is improved when the previous layer remains above the material cold crystallisation temperature; this is why chamber or bed heating is required for structural parts. Data from instrumented tensile pull-off coupons printed in the z-orientation and tested under ISO 527-2 show a more consistent failure at rasters when the chamber is held above 90 °C, but published data for this specific configuration is limited.

What separates PEKK from PEEK and PEI in high-temperature service?

The primary difference in backbone chemistry is the ratio of ketone to ether linkages. PEKK has a higher ketone density, which raises the glass transition temperature but suppresses the crystallization rate relative to PEEK. This combination has processing implications: parts can be deposited with lower accumulated residual stress, but as-printed mechanical properties remain less stable until post-crystallization is completed. The comparison in Table 1 is based on typical published unfilled-grade data and is not a batch-specific certificate of conformance.

PEI exhibits a higher heat deflection temperature but its amorphous structure is more vulnerable to environmental stress cracking in chlorinated solvents and polar cleaning fluids. PEKK and PEEK both develop semicrystalline lamellae after annealing; their chemical resistance in hydrocarbon and acidic media is generally governed by the degree of crystallinity rather than by short-term thermal resistance alone. The lower melting peak of PEKK permits processing at similar or slightly reduced nozzle settings compared with PEEK, but the thermal window above the melt peak remains narrow.

For a component that must survive hot-wet conditions, PEKK is often selected over PEEK when lower crystallisation speed reduces warp in thick sections and when the application demands consistent compressive performance at 150 °C to 180 °C. PEI is selected for electrical or low-stress thermal applications where processing ease outweighs solvent resistance; PEI cannot be annealed into a semicrystalline state and therefore remains dimensionally stable but susceptible to stress-cracking agents. Compared with polyphenylsulfone and polyethersulfone, unfilled PEKK provides higher tensile modulus and better resistance to hot aliphatic hydrocarbons, but fewer amorphous-processing accommodations such as low-temperature nozzle and bed settings.

When a heated chamber is unavailable

The product is not restricted to heated-chamber machines, but the process boundary shifts. Supplier guidance for unfilled PEKK filament recommends a heated build chamber at 80 °C to 120 °C for sections exceeding 6 mm in thickness. When no chamber heater is present, the bed is maintained at 120 °C to 150 °C and the nozzle temperature is kept at the upper end of the 355–375 °C range to reduce melt viscosity and extend the interlayer wetting time. Below 60 °C chamber equivalent temperature, z-direction tensile strength typically falls below 35 MPa and delamination may occur at raster-to-raster stress concentrations during cool-down.

Open-build configurations require the print to be shielded from forced convection cooling; a peripheral skirt with a minimum height of 10 mm directs heat from the bed upward and reduces vertical thermal gradient. Build speeds above 40 mm/s in such configurations are associated with intermittent adhesion loss in corner radii, particularly when extrusion temperature is below 365 °C. In direct-drive extruders operating near 375 °C, dwell time in the transition zone should be kept below 2 min to limit chain scission. Purge with unfilled purge polymer or PEKK scrap after each print reduces carbonized residue at the nozzle tip. A hermetic hopper or dry feed path is used to avoid moisture regain during prints exceeding 8 h.

Chemical compatibility of unfilled PEKK is most relevant to applications that combine elevated temperature with hydrocarbon exposure. In oilfield equipment screening under ISO 23936-1:2022, PAEK semicrystalline materials are preferred over amorphous sulfone polymers for resistance to hot brine, hydrogen sulfide, and aliphatic hydrocarbons; however, this product should not be specified for continuous immersion in concentrated sulfuric acid, fuming nitric acid, or methylene chloride. Steam sterilisation of printed components at 134 °C and 2.1 bar is possible if the part is annealed to stabilise crystallinity; validation is performed under ISO 17665-1 on the finished geometry because internal voids and freeze-off defects alter moisture conduction.

Annealing after deposition is used to increase crystallinity and reduce residual stress. Typical post-print annealing for unfilled PEKK is conducted at 180 °C to 200 °C for 2 h, followed by slow cooling at 0.5 °C/min to below 100 °C. This step is mandatory when the intended service temperature exceeds 120 °C, because unannealed parts may continue crystallizing and induce geometric distortion during the first thermal excursion. The operational boundary is defined by the filament 1.75 mm geometry: standard 0.4 mm brass nozzles are not suitable for the required processing temperature; hardened steel or nickel-plated copper alloy nozzles with a nominal orifice of 0.4–0.6 mm are used to limit catalytic degradation from brass.

In high-temperature tooling and aerospace ducting applications, the product is qualified against part-specific performance standards rather than generic polymer datasheet values. Flammability, smoke, and toxicity screening may be required under 14 CFR 25.853 or sector-specific equivalents; published data for this specific configuration is limited, so each printed geometry must be verified on the final build orientation and thickness. The product should not be combined with additives or support materials containing amine-based surfactants that can accelerate stress-cracking in finished parts.