EOS HT-23, specified as a 23% carbon-fiber-reinforced polyetheretherketone, is a high-temperature selective laser sintering feedstock. The material combines a PEEK matrix with short carbon fiber reinforcement to increase stiffness and reduce in-plane thermal expansion relative to unfilled PEEK. Incoming powder is characterised by ISO 1133-1:2022 melt-flow-rate measurement, ASTM D3418-21 differential scanning calorimetry, and ISO 1183-1:2019 density determination. Because published third-party mechanical data for this exact EOS configuration are limited, orientation-dependent mechanical properties must be qualified on the intended laser-sintering platform before serial production.

What Controls the Crystallization Window in High-Temperature Laser Sintering?

PEEK is a semicrystalline polymer with a melting endotherm near 343°C and a cooling crystallization exotherm near 280–300°C at 10 K/min under ASTM D3418-21. The powder-bed temperature is maintained between these transitions to suppress edge curl and prevent premature crystallization while avoiding particle coalescence in the feed zone. The 23% carbon-fiber loading increases composite thermal conductivity, which can narrow the usable temperature window because heat is conducted away from the melt pool more rapidly. In practice, powder-bed surface temperature is held within ±5°C of the set point. Failure to hold this tolerance produces visible curl in the first 2–3 layers and Z-direction interlaminar porosity.

The carbon-fiber filler also increases absorption of the 10.6 µm CO₂ laser radiation used in polymer laser sintering. Compared with unfilled PEEK, the same geometrical scan pattern may require lower effective laser energy density to reach the same melt pool temperature. Recalibration is required when changing between unfilled PEEK and HT-23 on the same machine, particularly for scan speed, laser power, and beam offset.

Before processing, the powder is dried at 120–150°C for 4–6 h in a desiccant or vacuum dryer to reduce absorbed moisture below 0.02% by weight. Residual moisture generates steam during laser exposure, producing voids and surface pitting. Drying time is extended when ambient relative humidity exceeds 60%. In a production setting, the powder feed hopper is maintained under dry nitrogen to prevent moisture regain.

Processing Equipment Constraints and Inert Atmosphere Requirements

The build chamber is purged with nitrogen to hold oxygen below 0.1% by volume. At powder-bed temperatures above 300°C, oxidative degradation causes carbonyl formation, chain scission, discoloration, and reduced interlayer fusion. Because carbon fiber masks some discoloration, melt-flow-rate testing is used to detect thermal degradation rather than visual inspection alone.

Carbon-fiber reinforcement is abrasive. Recoater blades accumulate wear more rapidly than with unfilled PEEK, and fiber fines can contaminate linear rails and optical windows. Hardened steel or ceramic-coated recoater blades are used, with blade geometry inspected after each build. Grounded dust extraction and antistatic bars prevent carbon-fiber fines from creating layer defects caused by electrostatic attraction.

The higher melt viscosity of carbon-filled PEEK reduces particle coalescence. Insufficient laser energy density leaves residual porosity and low Z-direction strength; excessive energy density causes matrix degradation and blackened powder. Batch-to-batch variation in fiber length and particle size distribution changes melt flow and surface finish. Incoming lots should be checked by laser diffraction per ISO 13320:2020 and melt-flow-rate testing per ISO 1133-1:2022.

After laser exposure, the powder cake is cooled under nitrogen at a controlled rate. Removing a part directly from a 330°C powder bed into ambient air creates thermal shock and can open interlayer boundaries. Annealing at 200–250°C for 2–4 h in nitrogen increases crystallinity and relaxes frozen-in stress. Dimensional change during annealing is compensated in the CAD offset and verified with ISO 129-1:2018 dimensional inspection.

In production-scale builds of manifolds, seal retainers, and electrical connector bodies, the primary bottleneck has been depowdering of blind cavities and channels below 2.0 mm. Carbon-fiber fines pack into internal volumes and form sintered bridges if not removed before annealing. Drain apertures, split-part designs, and oil-free compressed-air depowdering are used on the production line to prevent hard carbon deposits in subsequent builds.

Mechanical Property, CTE, and Failure Mode Shifts at 23% Carbon Fiber

SLS parts are orthotropic, and HT-23 is no exception. Tensile specimens prepared to ISO 527-2:2012 and flexural specimens prepared to ISO 178:2019 should be built in both X-Y and Z orientations. The 23% carbon-fiber reinforcement raises tensile and flexural modulus while reducing elongation at break. In the X-Y plane, fibers align during recoating and produce higher stiffness; in the Z orientation, tensile strength is governed by interlayer fusion and may be 30–60% lower than X-Y strength. The exact ratio depends on layer thickness, scan speed, and powder-bed temperature.

The theoretical density of a fully dense 23% carbon-fiber PEEK composite calculated from constituent densities is approximately 1.41 g/cm³. Laser-sintered parts may show slightly lower measured density by ISO 1183-1:2019 due to residual porosity, which also lowers Z-direction mechanical performance. Because carbon fiber density is near 1.8 g/cm³ and PEEK matrix density is near 1.30 g/cm³, the 23% weight fraction corresponds to a fiber volume fraction of roughly 17–18%.

PEEK glass transition temperature is near 143°C; the carbon filler does not eliminate property loss above this point but reduces creep and modulus drop relative to unfilled PEEK. Coefficient of linear thermal expansion measured by ISO 11359-2:1999 is lower in the fiber-aligned plane than in the Z direction. This anisotropy is critical for metal-to-polymer interfaces and thermal cycling. Notch sensitivity increases with fiber content; sharp internal corners should be radiused and thin sections below 0.8 mm should be validated with sacrificial builds because crack propagation along the fiber-matrix interface is faster than in unfilled PEEK.

When HT-23 Replaces Unfilled PEEK or PA12-CF in Load-Bearing Manifolds

Relative to unfilled PEEK, HT-23 provides lower thermal expansion, higher modulus, and improved creep resistance at elevated temperature, but elongation at break and thin-wall ductility are reduced. Unfilled PEEK remains preferable where electrical insulation, high elongation, or unfilled-grade processability is required. The selection should be based on ISO 527-2:2012 tensile tests at the service temperature rather than on room-temperature values alone.

Relative to glass-filled PEEK, carbon fiber gives lower density at equivalent filler volume and can improve sliding wear and reduce surface resistivity. Glass-filled PEEK is often electrically insulating and may exhibit lower notch sensitivity. Wear performance is quantified by ASTM D5963-22 abrasion tests; the ranking between carbon-filled and glass-filled grades depends on counterface material, contact pressure, and lubrication condition.

Relative to PA12 carbon-filled, the PEEK matrix raises continuous-use temperature to approximately 250°C and provides greater resistance to hydrocarbon fuels and hydraulic fluids. PA12-CF is easier to process and less expensive, but its mechanical properties decline at temperatures beyond the PA12 service range. The comparison must be made using ISO 175:2010 chemical immersion data and heat-deflection temperature measured by ISO 75-2:2013.

For chemical exposure, concentrated sulfuric acid, concentrated nitric acid, and strong oxidizing halogens are outside the usual service envelope of PEEK and should be excluded unless published compatibility data for the specific chemical, stress level, and temperature exist. Immersion testing is performed according to ASTM D543-20 or ISO 175:2010 with production-equivalent surface roughness and residual porosity.

For aerospace and medical applications, validation is performed under program-specific standards. Biocompatibility evaluations follow ISO 10993-1:2018, and food-contact or medical-body-contact use requires supplier certification under FDA 21 CFR 177.2415 where applicable. No compliance claim is made from the datasheet alone; resin composition, additive migration, and post-processing residues must be evaluated on final parts.

Powder handling, sieving, and reuse must be integrated into the manufacturing sequence. Used powder from high-temperature builds is sieved through a 125 µm stainless steel mesh and mixed with fresh material at a defined refresh fraction established by melt-flow-rate and bulk-density testing rather than by colour alone. Compressed air used for depowdering should be oil-free and water-free; hydrocarbon contamination changes local laser absorption and produces surface char in subsequent builds. Carbon-fiber dust requires local exhaust ventilation and antistatic grounding during sieving and powder transfer. These handling controls are required to maintain lot-to-lot consistency in a material already constrained by a narrow sintering window.