Product designation Covestro Addigy PPU 77A identifies a thermoplastic polyurethane powder supplied for selective laser sintering and laser-based powder-bed fusion processes. The grade name carries a nominal indentation hardness of 77 Shore A as determined by ISO 7619-1, placing it among flexible elastomer powders rather than rigid polyamide feedstocks. Representative technical data from supplier literature list a density of 1.07–1.10 g/cm³ under ISO 1183-1, tensile strength in the range of 8–9 MPa under ISO 37, and tear strength of 30–35 kN/m under ISO 34-1. Elongation at break typically exceeds 300% in the XY build direction. The powder is handled as a dry, free-flowing polymer medium at layer thicknesses of 0.10–0.12 mm. Because the powder bed itself supports overhangs, no separate support material is required for most part geometries, which is one operational difference from vat photopolymerization and filament extrusion.

What distinguishes this grade from rigid polyamide and higher-hardness TPU powders?

Comparative evaluation against unfilled polyamide 12 SLS media shows a fundamentally different mechanical response. PA12 powders typically produce parts with tensile modulus in the 1500–1800 MPa range and elongation at break of 15–25%, whereas this polyurethane grade remains elastomeric with tensile modulus below 100 MPa and elongation above 300%. The difference arises from the segmented polyurethane chain architecture: soft polyol domains lower modulus and increase recovery, while hard urethane domains contribute tensile strength and thermal resistance. Higher-hardness TPU powders at 90 Shore A or 95 Shore A show higher tensile modulus and reduced viscoelastic damping but can preserve similar powder-bed processing behavior. Table 1 summarizes representative comparative values from supplier technical literature.

Mechanical anisotropy between XY and Z directions remains measurable. Builds produced with 0.10 mm layers on 10.6 µm CO₂ laser systems show Z-direction elongation commonly 25–40% lower than XY values because interlayer coalescence is limited by the narrow melting window of the polyurethane. Flexural fatigue testing of lattice structures built from this powder has been conducted using coupon geometries derived from ISO 527-2 and ASTM D638; however, lot-specific fatigue data are limited. Designers therefore apply a knock-down factor to Z-direction tensile properties when qualifying thin hinge features or living-spring geometries.

When powder-bed temperature control drifts beyond the sintering window

The process window for this grade is narrower than that of semi-crystalline polyamide powders. Polyurethane powder develops adequate melt coalescence only when the part bed is held between the onset of melting and the onset of crystallization. A temperature offset of 2–3 °C above the control plateau can cause excessive melt viscosity reduction, resulting in edge curl, part growth, and surface gloss. A drop of similar magnitude lowers interlayer adhesion and produces brittle interlayer fracture under ISO 34-1 tear testing. Production machines therefore require closed-loop part-bed thermography and powder surface temperature control in 1 °C increments. Laser energy density is adjusted as (P)/(v·s), where P is laser power, v is scan speed, and s is scan spacing. For this elastomer powder, starting energy densities below 0.10 J/mm² are typical; excessive energy density causes dimensional drift, over-bright surfaces, and evolution of volatile degradation products. Table 2 lists representative starting parameters for flexible TPU powders, not a grade-specific specification.

The powder bed is not uniformly heated across large build platforms. In production-scale equipment, regional temperature differences of 3–5 °C have been observed between the center and corner zones during preheating. These gradients affect melt coalescence and produce anisotropic shrinkage. Calibration builds should include a full-height tensile bar grid to map XY and Z property variation before committing to production layouts. Process failure modes include edge curl in thick rigid sections adjacent to thin elastomeric zones, orange-peel surface defects from insufficient energy density, and interlayer delamination from premature cooling after part completion. Parts removed before the part-cake temperature falls below 40–50 °C can exhibit permanent warpage and lower crystallinity.

Drying is a separate process boundary. Polyurethane powders absorb surface moisture that can depress melt flow during sintering and increase void content. After storage at relative humidity above 60%, a dehumidified oven at 70–80 °C for 4–6 hours is commonly used for moisture conditioning. Drying above 85 °C is not recommended because particle surface softening can accelerate powder agglomeration and reduce flowability. Moisture content should be monitored by coulometric Karl Fischer titration or equivalent method before starting a build batch.

A direct application area is the production of fatigue-resistant lattice structures for footwear, orthotic interfaces, protective pads, and flexible hose or duct elements. Parts built with 2–3 mm wall thickness and 0.10–0.12 mm layers have been used in functional prototypes of midsole inserts and padded guards. Hardness of 77 Shore A allows conformability while retaining resistance to compression set. Under ISO 815-1, compression set after 22 h at 70 °C is typically below 25–35%, depending on post-build conditioning. In orthotic shells and prosthetic liner channels, the unsintered powder supports undercuts and complex lattice cavities. No solvent dissolution step is required, but depowdering of blind channels smaller than 2 mm is difficult and requires compressed air at 0.4–0.6 MPa plus manual brushing. Published data for end-use load cycles on this specific powder are limited; fatigue life is therefore validated through printed coupon testing rather than extrapolated datasheet values.

Powder refresh ratio, recoater blade shear, and lot-to-lot particle size variation

In continuous production, virgin powder is mixed with reclaimed overflow and part-cake powder. Uncontrolled refresh ratios above 50% reclaimed material can shift particle size distribution and reduce flowability because recycled polyurethane particle surfaces are partially sintered and less spherical than virgin material. Recoater speed and blade edge condition influence layer density; a worn blade can reduce packed density by 3–5%, producing porosity and low interlayer tear strength. Lot-to-lot variation in median particle size of ±10 µm is possible. Laser diffraction testing under ISO 13320 or sieve analysis should be used to monitor incoming batches. Production data indicate that maintaining a refresh ratio of 30–50% virgin powder and using hardened recoater blades above 50 HRC reduces build failures in long unattended operations. Published equipment-specific field data for this exact grade are limited; process validation on the intended SLS platform is therefore required before serial production.

Blending with foreign powders is not recommended. Mixing this polyurethane powder with unmodified polyamide 12 powder or with TPU grades having different hard-segment melting ranges produces heterogeneous melt fronts and weak interlayer interfaces. The result is reduced tear strength and higher variability in elongation at break. If a hardness between 77 Shore A and another TPU grade is needed, the sintered part geometry should be adjusted instead of dry-blending powders, because particle-level fusion is composition-sensitive.

Regulatory and handling boundaries must be confirmed against current Covestro material safety and product datasheets. The powder is not represented as a food-contact or implantable medical grade unless the processor obtains specific compliance certification under FDA 21 CFR or ISO 10993 protocols. Thermal decomposition during laser processing can release isocyanate-derived volatiles; exhaust and filtration should be sized for polyurethane laser processing. Powder disposal, workplace exposure limits, and air emissions require local regulatory review and, where applicable, REACH compliance documentation. Parts should not be placed into service until post-build conditioning, moisture content, and Z-direction mechanical testing have been completed against the targeted application load profile.