Selective laser sintering

At an assumed 30% fresh / 70% recovered by mass, a 10.0 kg charge needs 3.0 kg fresh and 7.0 kg recovered. If eligible recovered inventory after the build is 8.5 kg, using it all requires 3.6429 kg fresh. Check recovery eligibility and machine capacity. Adding 30% of recovered mass would yield only 23.08% fresh in the mixture.

In polymer SLS, unfused powder supports successive recoated layers. EOS describes laser melting or fusion. Keep polymer powder and thermal conditions separate from metal powder-bed and resin routes. EOS, SLS printer technology overview, undated

Account for nested-part clearance, fragile features, cooling and depowdering.

Define a material and process combination

Specify supplier and powder grade, machine, validated parameter set, software/settings revision, layer thickness, atmosphere, and powder history. PA12, PA11, filled polyamides, and elastomer powders are different feedstocks; a shared nominal polymer family does not make powders interchangeable between systems.

Atmosphere requirements are also specific. Formlabs' Choosing the right powder guidance distinguishes powders recommended for air from those recommended for nitrogen and explains that atmosphere can affect powder aging, appearance, and refresh requirements. Its recommendations are qualified boundaries for the named materials and equipment, not an instruction to run every SLS polymer in nitrogen or in room air. Formlabs powder-selection guidance, undated

Maintain lot identity and separate incompatible powders. Resolve storage, moisture exposure, sieving, contamination controls, and permitted mixing with the material supplier. Unfused powder has experienced a thermal cycle; “not incorporated into a part” does not mean “identical to virgin powder.” Formlabs validates its material settings at particular refresh rates and cautions that performance outside those boundaries is not guaranteed. Formlabs, refresh-rate guidance

Powder refresh and recovery are different quantities

A refresh fraction describes the fresh powder incorporated into the prepared feed mixture. Recovery yield describes how much usable powder is actually collected after a build. Part mass, screening rejects, handling losses, and quarantined powder must be accounted for separately.

Formlabs Nylon 12 Powder FLP12G01, TDS V2, revised October 4, 2026, lists a recommended 30% refresh rate. That percentage is not a promise that 70% of every build's charged powder will be recoverable. It applies to that product's workflow, not every nylon. Formlabs Nylon 12 Powder TDS, storage and handling

Worked example: reconcile the mixture and inventory

This original hypothetical example assumes an approved 30% fresh / 70% recovered mixture by mass, a 10.0 kg prepared charge, and 8.0 kg of eligible recovered powder available:

• Fresh powder needed = 0.30 × 10.0 = 3.0 kg.
• Recovered powder used = 0.70 × 10.0 = 7.0 kg.
• Eligible recovered inventory remaining = 8.0 − 7.0 = 1.0 kg.

Now assume that build produces 2.0 kg of accepted parts, 0.2 kg of rejected parts, 7.5 kg of recovered eligible powder, and 0.3 kg of combined rejects/losses. The mass balance is 2.0 + 0.2 + 7.5 + 0.3 = 10.0 kg. Including the earlier unused inventory, eligible recovered stock is then 8.5 kg.

At the same assumed ratio, consuming all 8.5 kg would require a 12.1429 kg mixture and 3.6429 kg fresh powder, subject to machine capacity and powder eligibility. Adding 30% of the recovered mass would give only 23.08% fresh in the resulting mixture. These are planning calculations, not permission to reuse contaminated or over-aged powder.

Thermal history, packing, and cooling

The build includes warm-up, layer processing, and controlled cooling; laser exposure time alone is not the manufacturing cycle. Part position and neighboring geometry influence heat flow. More densely packing a chamber can alter local cooling and powder-cake behavior rather than merely spreading a fixed cost over more parts.

Formlabs' orientation guidance connects location and orientation with cooling rate and warpage. It recommends uniform distribution, attention to thin/slender features, and consistent orientation of mating surfaces. Those recommendations concern its SLS workflow; qualify placement and spacing for the actual material and machine. “No supports” is not evidence that orientation is irrelevant. Formlabs, Model orientation best practices for SLS printing, undated

Its warping guidance identifies hot unpacking, slender geometry, uneven wall thickness, boundary placement, and bed-temperature control as possible contributors. Follow the correct equipment procedure: transferring a build chamber for further cooling and unpacking parts are distinct operations. Do not adopt one manufacturer's temperature or cooling time as a universal SLS rule. Formlabs, Warping (SLS), undated

Design for depowdering and finishing

Provide escape paths for powder from cavities, channels, and nested assemblies. Confirm that openings are accessible to the intended cleaning method and that fragile walls can survive extraction and handling. A hole visible in CAD does not establish that a tortuous internal passage can be cleared.

Formlabs' hollowing guidance explicitly requires drainage openings to prevent trapped material in SLS models. Its separate warnings about suction cups concern SLA resin printing and should not be transferred as an SLS failure mechanism. Minimum walls and openings must come from the applicable printer/material design rules. Formlabs, Hollowing models in PreForm, undated

Define the delivered condition: depowdered, blasted, dyed, smoothed, coated, or machined as applicable. Agree which dimensions and surfaces are evaluated after those operations. Specify cleanliness of functional channels separately from external cosmetic appearance; a nominally complete depowdering operation is not evidence of a particular particulate limit.

Inspect properties in their stated condition

The Formlabs Nylon 12 V2 TDS reports elongation at break of 8% in X/Y and 5% in Z, using ASTM D638-14. Its footnote identifies Fuse 1 specimens conditioned for 48 hours at 23°C and 50% relative humidity. Its heat-deflection results also depend on load; they are not continuous-service temperature ratings. Geometry, orientation, and temperature can change achieved properties. Formlabs Nylon 12 Powder TDS, properties and footnotes

As another bounded example, EOS's PA 2200 Top Quality process sheet lists 60 µm layers and dry nominal strain at break of 20% X/Y and 7% Z, under ISO 527-1/-2. The retrieved page states last changed September 2, 2025; status October 30, 2026. Those values belong to that parameter set; they do not describe every PA 2200 process, and a layer increment is not a dimensional tolerance. EOS explicitly limits the sheet's use as a guarantee or standalone design basis. EOS PA 2200 Top Quality PDS

Diagnose and release finished parts

Investigate incomplete fusion, rough or collapsed surfaces, layer shifts, dimensional error, and warpage against actual powder history, thermal control, optics, and recoating condition. Formlabs notes that contaminated laser windows can obstruct fusion and contaminated IR sensing can produce incorrect temperature feedback. A visually acceptable exterior does not establish internal integrity or application performance. Formlabs, Diagnosing a print failure (SLS), undated

Agree on dimensional sampling, measurement conditioning, cosmetic boundaries, functional fit, mechanical tests, and any leak or cleanliness requirement. Identify build position and orientation for representative specimens. Record processing and finishing history so later builds can be compared meaningfully.

For powder handling, use the actual SDS and equipment instructions. The retrieved EOS PA 2200 GB SDS version 4.2.0, issued January 31, 2024, calls for dust control, ventilation, dry closed storage, ignition-source control, and precautions against static discharge; it identifies explosive dust/air mixtures as a hazard. These are product-specific handling instructions, not a generic regulatory exposure limit for every SLS powder or jurisdiction. EOS PA 2200 SDS, sections 6–8

Related reference

Manufacturing processes