Sealing SLS & MJF Parts for Fluids

The Problem

Why Raw SLS and MJF Parts Leak

The sintering process fuses powder particles but leaves a microscopic, interconnected porosity throughout the part. In functional applications involving pressurized fluids, pneumatics, or even passive liquid contact, this porosity allows slow weeping through walls—making post-processing non-negotiable for fluid-carrying assemblies.

Stage 1 Sintered Part Interconnected micro-voids throughout nylon matrix

Stage 2 Leak Path Exists Low-viscosity fluids weep through walls under pressure

Stage 3 Select Sealing Method Based on pressure rating, geometry, and use environment

Stage 4 Sealed & Qualified Pressure-tested, functional end-use part

Post-Processing Methods

Three Proven Approaches to Sealing Powder Bed Fusion Parts

Each method targets a different pressure requirement, geometry type, and production volume. Selecting the right approach at the design stage saves iteration cycles downstream.

Production Grade

Automated Vapor Smoothing

Parts are suspended in a sealed, computer-controlled chamber where a vaporized solvent briefly melts the outermost microscopic layer of the thermoplastic. The liquefied nylon flows into surface peaks and valleys, closing porosity before the solvent is evacuated and the part hardens. The result is an isotropic sealed skin with no added weight and minimal dimensional impact.

Seals internal cavities and complex external geometries uniformly
Reduces surface roughness from ~10 µm to under 3 µm
Does not degrade tensile strength or flexural modulus
Suitable for serial production runs requiring aesthetics + function

High Pressure

Epoxy Resin Impregnation

Parts are submerged in a low-viscosity epoxy bath inside a vacuum chamber. The vacuum draws trapped air from the interconnected pore network; when pressure is restored, the epoxy is driven deep into the nylon's internal cell matrix. After thermal cure, the epoxy cross-links inside the part structure—not just on its surface skin.

Eliminates internal voids rather than sealing the outer skin alone
Tested up to 50+ PSI depending on wall geometry
Adds chemical resistance to oils, hydraulic fluids, and fuels
Preferred for manifolds, cooling blocks, and pneumatic assemblies

Aesthetic + Seal

Industrial Surface Coatings

Following abrasive media blasting to clear residual powder, parts receive multiple thin cross-coats of high-solid polyurethane, commercial acrylic, or automotive-grade finish. Low-viscosity primers sink into surface pores to establish a watertight foundation before topcoats are applied for color, gloss, or UV protection.

Dual-purpose: fluid containment and color/finish specification
UV and outdoor weather resistance for long-term nylon protection
Matches specified product housing finishes (gloss, matte, RAL)
Best suited for structural housings at low pressure / splash exposure

Side-by-Side

Selecting the Right Sealing Method

Use this reference when specifying post-processing requirements on your engineering drawings or quoting requests.

Method	Best Application	Tolerance Impact	Pressure Rating
Vapor Smoothing	Serial production, intricate internal geometries, aesthetic finish required	Minimal — ~0.02 mm surface reduction	Moderate Standard fluid / air reservoirs
Epoxy Impregnation	High-pressure pneumatic / hydraulic manifolds, heavy industrial tooling	Negligible — deep internal penetration only	High 50+ PSI (geometry-dependent)
Industrial Coating	Large structural housings, outdoor UV-exposed components	Minor additive — +0.03 to +0.08 mm	Low Splash-proof / low-pressure only

Design for Manufacturing

CAD Guidelines for Watertight Powder Bed Fusion Parts

These parameters should be incorporated at the CAD stage. Retrofitting them after a first build is costly. Get them right before you upload.

Minimum Wall Thickness

Maintain at least 2.5 mm for any wall forming a fluid reservoir. Thinner boundaries increase the risk of localized micro-voids during sintering that no post-processing method can fully remediate.

Radiused Internal Corners

Replace sharp 90° internal junctions with fillets at a minimum 1.5 mm radius. Sharp corners create stress concentrations and uneven cooling during sintering, which can open micro-fissures invisible to the eye but active under pressure.

Powder Evacuation Ports

All enclosed chambers need at least two exit ports at a minimum 6 mm diameter to allow complete extraction of un-sintered nylon. Residual powder trapped inside absorbs sealing agents and blocks internal fluid channels.

Specify Post-Processing Early

Call out the sealing method on your engineering drawing or RFQ, not after build review. Vapor smoothing, epoxy, and coatings each carry different lead-time and tolerance implications that affect downstream assembly fit.

Sealing MJF & SLS Parts
for Fluid Containment