Evonik VESTAKEEP i4 3DF-T is a natural-colour polyether ether ketone monofilament for fused filament fabrication of high-temperature and medical components. The product is manufactured from VESTAKEEP i4 G resin, a medium-viscosity unfilled PEEK grade that meets ASTM F2026 for raw polyether ether ketone feedstock intended for surgical implant applications. The filament is supplied in two diameters, 1.75 mm and 2.85 mm, on sealed spools with net weights of 250 g and 500 g. The base resin density is 1.30 g/cm³ when tested to ISO 1183-1. The melting peak is 340 °C and the glass transition temperature is 143 °C, both determined by differential scanning calorimetry per ISO 11357-1 and ISO 11357-3. These values place the material in the semicrystalline high-temperature thermoplastics class and require extrusion hardware that can sustain melt temperatures above 400 °C. The filament is not a direct drop-in for amorphous 3D-printing polymers: it must be dried before processing, deposited into a heated chamber, and typically annealed if maximum crystallinity and dimensional stability are required. The product designation separates the filament format from granulate; the i4 suffix identifies the medium-viscosity base resin, while the 3DF-T suffix indicates a filament product with controlled diameter, ovality, and spool winding for reliable FFF feeding.

What Melt-Viscosity and Thermodynamic Values Govern Nozzle Deposition?

Extrusion of VESTAKEEP i4 3DF-T through a 0.4 mm hardened steel or tungsten carbide nozzle is constrained by the medium-viscosity melt behaviour of the i4 base resin. Evonik technical documentation for VESTAKEEP i4 G reports a melt volume-flow rate of approximately 25 cm³/10 min at 380 °C and 5 kg load when tested to ISO 1133-1. This is higher than high-viscosity PEEK grades such as VESTAKEEP 4000 G but lower than melt-extrusion grades optimized for high-throughput fused granulate fabrication. The practical consequence is that the filament can be extruded at linear deposition speeds typical of high-temperature FFF machines without excessive melt pressure, provided the hot end can maintain a continuous setpoint between 400 °C and 430 °C. A heated build plate is required in the range of 130 °C to 150 °C, and a closed build chamber should be held at a temperature that prevents rapid cooling of the deposited bead below the glass transition. Machine-specific chamber setpoints from 90 °C to 150 °C are encountered; published data for the exact minimum chamber setpoint for VESTAKEEP i4 3DF-T is limited, and the optimal value depends on part cross-section, layer time, and whether the printer uses active side heaters or passive enclosure heating. Hardened steel or tungsten carbide nozzles are preferred over brass when long campaigns are run because PEEK melt at 400 °C accelerates copper-alloy wear in carbon-filled grades; for this unfilled grade the primary nozzle concern is thermal stability of the thermistor and heater block rather than abrasive wear. The extruder drive system should be capable of maintaining filament feed force without buckling the filament; a constrained or dual-drive filament path is commonly specified on production high-temperature printers with actively heated chambers. The viscosity data also govern die swell and road width control: the first layer height is typically set at 0.20 mm to 0.25 mm for a 0.4 mm nozzle, and extrusion multipliers are reduced if the melt temperature is raised above the 400 °C reference setpoint because viscosity falls with increasing temperature. The residence time of the melt inside the hot end should be minimized because PEEK can degrade if held at 430 °C for extended periods; short melt zones and hardened nozzles reduce the risk of gel formation and black specks.

Prior to extrusion, the filament must be dried to prevent hydrolytic degradation and steam-induced porosity in the deposited road. PEEK has a low equilibrium moisture uptake, but filament wound onto spools can acquire surface moisture during shipping and handling at relative humidity above 60%. Drying in a desiccant or vacuum oven at 150 °C for 3 h is a typical pre-processing condition for PEEK filament, with the dried spool transferred directly to a dry-feed enclosure maintained below -40 °C dew point. Printed parts that exhibit intermittent surface pits, steam bubbles at the nozzle, or a hissing sound during extrusion are usually indicators of residual moisture. The spool should not be left uncovered in an uncontrolled production room for more than 4 h after drying; if longer exposure occurs, re-drying is required. The winding tension and layer-to-layer spooling pattern of VESTAKEEP i4 3DF-T are specified to avoid filament cross-over and buckling in long-bowden or direct-drive feed paths. Excessively tight feed-path bends can cause elastic energy storage and slip at the drive gear; for 1.75 mm filament, the machine-specific minimum bend radius supplied by the printer builder should be followed rather than a universal value. A filament run-out sensor and an enclosed spool holder with desiccant pack reduce batch-to-batch moisture variation in production.

Thermal Transitions Alone Do Not Prevent Z-Axis Delamination

The melting point of 340 °C and glass transition of 143 °C define the thermal window, but they do not guarantee sufficient interlayer fusion in FFF parts. In the molten state, the i4 base resin flows at the extrudate surface and begins to cool immediately after deposition. If the chamber temperature is too low, the previously deposited layer remains below the glass transition, and chain diffusion across the weld interface is incomplete. The result is a plane of weakness that is measurable as a reduction in tensile strength when specimens are printed in the Z orientation and tested to ASTM D638-14 or ISO 527-2. Published data for the specific Z-direction properties of VESTAKEEP i4 3DF-T is limited, but fused filament fabrication of PEEK is known to produce anisotropic mechanical behaviour; the interlayer bond is the critical process variable, not the bulk resin tensile strength. Annealing at 200 °C for 2 h is commonly used to increase crystallinity, relieve residual stress, and restore a proportion of the lost interlayer strength. The annealing cycle should be applied after printing with the part fixtured or supported, because PEEK parts can distort as the residual stress relaxes. Crystallization kinetics also influence this step: PEEK crystallizes slowly from the melt, and the cooling rate in the build chamber controls whether the as-printed matrix is predominantly amorphous or semicrystalline. A chamber held near 150 °C slows the cooling rate and shifts the crystallinity toward the equilibrium semicrystalline state, whereas a chamber below the glass transition freezes in a largely amorphous structure that later sinters and changes dimensions during annealing. This is why the same nominal print temperature can produce different final crystallinity and different mechanical response from one printer to another.

Chemical Resistance, Moisture Uptake, and Sterilization Workflow Boundaries

The unfilled PEEK chemistry of VESTAKEEP i4 3DF-T provides broad resistance to aqueous process fluids, aliphatic and aromatic hydrocarbons, alcohols, esters, and many halogenated solvents under ambient conditions; specific chemical compatibility of printed parts should be verified by immersion testing per ISO 175. The base resin absorbs less than 0.5% water at saturation when tested to ISO 62, which supports repeated steam sterilization cycles. Steam sterilization at 134 °C for minimum hold times of 18 min should be performed on printed parts only after annealing, because residual amorphous zones can undergo secondary crystallization and small dimensional shifts during the first autoclave exposure. Gamma irradiation at 25 kGy to 50 kGy is also applied to PEEK medical devices; mechanical evaluation per ISO 527-2 after irradiation is required because the dose rate and oxygen exposure affect the measured retention. Ethylene oxide sterilization is acceptable provided that the printed part is fully degassed after the cycle and the process validation confirms residual gas limits. Concentrated sulfuric acid and concentrated nitric acid attack PEEK at elevated temperatures and are outside the operational compatibility envelope. The use of strong oxidizing acids or combinations of chlorinated solvents with high temperatures should be excluded from cleaning protocols because the ketone and ether linkages in the polymer backbone are susceptible to chain scission. Machining coolants containing chlorinated paraffins should be removed before annealing or sterilization because thermal decomposition of the coolant at 400 °C is a potential source of surface contamination. The chemical inertness of the base resin does not eliminate the need for post-printing cleaning validation under the applicable medical-device quality system.

Support removal and machining are governed by the semicrystalline nature of the printed matrix. VESTAKEEP i4 3DF-T parts can be machined on standard CNC mills using carbide tooling; water-based coolant is preferred to dry machining because PEEK has low thermal conductivity and can generate localized heat at the cut surface. Drilling and tapping should be performed after annealing, because the as-printed amorphous regions can smear and produce burrs at the hole edge. Support structures printed in PEEK are difficult to remove; breakaway supports are practical only if the support interface is designed with a small contact area. Soluble support materials are generally incompatible with the high chamber temperatures required for PEEK, so mechanical removal is the standard route. Sanding and polishing require gentle pressure because PEEK is notch-sensitive; a sharp machined surface or scratch can reduce notched impact strength measured to ISO 179-1/1eA. Surface roughness and residual stress after machining should be checked before cleaning and packaging.

Because the filament is intended for medical and implant manufacturing, the regulatory documentation chain matters as much as the thermal and mechanical data. VESTAKEEP i4 G, the base resin for VESTAKEEP i4 3DF-T, is specified against ASTM F2026, which establishes chemical identity, density, tensile properties, and melt flow requirements for PEEK used in surgical implants. The medical-device manufacturer remains responsible for final biocompatibility evaluation under ISO 10993-1:2018 with endpoints selected according to contact duration and tissue type; cytotoxicity testing to ISO 10993-5 and sensitization or irritation testing to ISO 10993-10 are common for implant-grade PEEK, but material certification alone does not exempt the final device from testing. For devices sold in Europe, the finished implant must comply with EU MDR 2017/745; for the United States, the device must comply with 21 CFR Part 820 and applicable premarket requirements. For industrial and non-implant applications, documentation is frequently evaluated under REACH and RoHS obligations, but the applicability depends on the final article and its intended market. The filament format introduces additional process-controlled variables—extrusion temperature, build orientation, layer height, annealing temperature, and chamber atmosphere—that are not present in injection-moulded PEEK. Therefore, the quality system must treat the filament lot, printer lot, and build cycle as interacting variables. A compliance checklist for the documentation chain that typically accompanies the material supply is provided below.

Representative base resin values reported by Evonik for VESTAKEEP i4 G include tensile modulus of 4100 MPa, tensile stress at yield of 100 MPa, elongation at yield of 5%, elongation at break of 20%, Charpy notched impact strength of 6.0 kJ/m² to ISO 179-1/1eA. These values apply to injection-moulded test plaques and are not automatically transferred to FFF parts without build-orientation correction.

When the Filament Must Be Distinguished from VESTAKEEP i4 G Granulate and Other PEEK Grades

The distinction between VESTAKEEP i4 3DF-T and VESTAKEEP i4 G granulate is process-format based, not bulk-chemistry based. The i4 G granulate is supplied for injection moulding and other melt-processing methods; the i4 3DF-T filament is converted from the same resin chemistry into a controlled-diameter filament for FFF. The conversion step adds spool winding, diameter tolerance, and moisture control requirements that do not apply to granulate. VESTAKEEP i4 3DF-T should not be ground and fed as granulate into an injection moulding machine, because the winding process and thermal history of the filament are not validated for injection moulding process control. Conversely, i4 G granulate cannot be directly used in a filament-fed FFF machine without a pellet-fed extruder head and filament winding system. When compared to other Evonik PEEK grades such as VESTAKEEP 4000 G or high-viscosity grades, the i4 medium-viscosity designation shortens the time required for interlayer wetting but reduces the melt strength needed for very large unsupported horizontal spans; in FFF, these properties are balanced by nozzle temperature, chamber heating, and deposition strategy. When compared to carbon-fibre-reinforced PEEK filaments, the unfilled i4 3DF-T has lower wear on machine hardware but lower modulus and lower dimensional stability at elevated load; if compressive or flexural stiffness is the controlling design variable, carbon-filled grades should be evaluated with the appropriate flexural test method such as ISO 178 or ASTM D790. Published data for direct mechanical comparisons between VESTAKEEP i4 3DF-T and other PEEK filaments under identical FFF processing conditions is limited, so any substitution should be preceded by a designed experiment using the same printer, toolpath, chamber temperature, and annealing cycle.