engineering-skills/additive-manufacturing/references/post-processing.md

AM Post-Processing — Complete Technical Guide

Principle: post-processing is part of the process, not an afterthought

In AM, final properties depend on the post-processing sequence as much as on the printing process. Plan post-processing BEFORE printing (it affects orientation, tolerances, and geometry).

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Metal AM — Sequence and Treatments

General LPBF sequence (mandatory)

1. STRESS RELIEF (on the build plate, before any other operation)
2. Removal from build plate (EDM wire cutting / saw / milling)
3. Support removal (manual + tools + milling where necessary)
4. Heat treatment / aging (if required by the material)
5. HIP (if critical application)
6. Post-machining of critical surfaces
7. Surface finishing
8. Inspection / quality control

Stress Relief — Conditions by alloy

Alloy	Temperature	Time	Atmosphere	Purpose
AlSi10Mg	270–300°C	2h	Air / Argon	Reduce residual stresses; does not alter microstructure
Ti-6Al-4V	600–650°C	2–4h	Vacuum / Argon	Critical — without this, severe distortions upon removal
316L	900–1050°C	1–2h	Vacuum / Argon	Solution annealing + stress relief
17-4PH	1040°C (solution) + 480°C (H900)	1h + 1h	Vacuum	Mandatory aging sequence
Inconel 718	980°C + 720°C + 620°C	1h + 8h + 8h	Vacuum	Full precipitation hardening sequence
Inconel 625	1050°C	2h	Vacuum	Stress relief only, no precipitation

HIP (Hot Isostatic Pressing)

• Purpose: Closes residual porosity (gas-phase pores, lack of fusion) → mechanical properties closer to wrought material
• Typical conditions:
  • Ti-6Al-4V: 900°C / 100–200 MPa / 2–4h / Argon
  • IN718: 1170°C / 175 MPa / 4h
  • AlSi10Mg: 500°C / 100 MPa / 3h (rarely economically justified)
• When mandatory: Biomedical (implants), aerospace fatigue-critical, pressure-bearing components
• Effect on microstructure: May coarsen the AS-built microstructure → compensate with post-HIP heat treatment
• Cost: €500–2000/batch depending on size and material

Post-Machining Metal AM

• Functional surfaces (seats, precision holes, sealing surfaces) → ALWAYS post-machine
• Recommended machining allowance in design phase: 0.5–1.5mm on critical surfaces
• Techniques: CNC milling, turning, grinding (for critical surfaces), EDM (for hard-to-reach features)
• Warning: AM microstructure may differ from wrought material → cutting parameters may vary

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Metal Surface Finishing

Methods and achievable Ra

Method	Achievable Ra	Cost	Notes
As-built LPBF	5–25 µm	—	Baseline; depends on orientation
Shot peening	3–8 µm	Low	Introduces surface compression → improves fatigue
Sand/bead blasting	3–10 µm	Low	Uniform finish, matting
Vibratory finishing	1–4 µm	Medium	Good for batches of small parts
Barrel tumbling	1–3 µm	Medium	Parts without sharp edges
Electropolishing	0.5–2 µm	Medium-High	Excellent for 316L; possible for Ti; difficult for Al
CNC machining	0.1–1.6 µm	High	Specific surfaces, accessible geometries
Grinding / lapping	0.01–0.5 µm	High	Sealing surfaces, mating interfaces
Laser polishing	1–5 µm	High	Internal surfaces (channels), inaccessible areas
Chemical etching	Varies	Low	Removal of oxidised surface layer, especially Ti

Shot peening — specific notes

• Significantly improves fatigue resistance (compressive residual stress)
• Standard AMS 2430 for aerospace
• Parameters: shot size S110–S230, pressure 2–4 bar, coverage 100–200%

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Polymer AM — Post-Processing

FDM Post-Processing

Operation	Purpose	Materials	Notes
Support removal	Access to geometries	All	Mechanical (pliers/cutters) or dissolution (PVA in water, HIPS in limonene)
Sanding	Surface finishing	PLA, ABS, ASA, PETG	Grits: 120 → 220 → 400 → 800 progressive; wet sanding for best results
Primer + filler	Covering layer lines	All	Primer surfacer + sanding before painting
Acetone smoothing	Partial surface dissolution	ABS ONLY	Ra from ~30µm to ~2–5µm; caution: modifies dimensions ±0.1–0.3mm
IPA smoothing (XTC-3D resin)	Surface encapsulation	All	Adds ~0.3–0.5mm — account for this in tolerances
Painting	Aesthetics, UV protection	All (with primer)	Polyurethane or acrylic paints
Annealing	Reduction of internal stresses, HDT	PLA, PETG, ABS	PLA: 60–80°C / 1h; PETG: 80°C / 2h; improves thermal resistance
Heat-set inserts	Reliable metal threads	All thermoplastics	M2–M12; insert with soldering iron/heat station; pull-out strength >>printed thread
Epoxy impregnation	Impermeability, rigidity	All	Low-viscosity resins (West System, Smooth-On) penetrate the structure

SLS / MJF Post-Processing

Operation	Purpose	Notes
Breakout + cleaning	Remove excess powder	Shot/sand blasting standard; compressed air for internals
Bead blasting	Uniform finish, improved Ra	Ra from 12µm → 6–8µm; uniform satin appearance
Dyeing	Uniform colouring	PA12: excellent dyeing capability (standard black or colours with dedicated dyes); hot process 80–95°C
Vibratory finishing	Ra < 4µm	Good for batches; caution with thin features
SLS coating (e.g. Ceracoat, Duracoat)	Impermeability + colour	Specific coatings for SLS; slightly increases Ra
Impregnation	Impermeability	Epoxy resins or low-viscosity cyanoacrylate
Painting	Aesthetics	With PA-specific primer
Machining	Critical tolerances	PA12 machines well; watch for cutting heat

SLA / DLP / MSLA Post-Processing — MANDATORY

MANDATORY SEQUENCE:
1. Removal from the build plate
2. Wash in IPA (10–15 min agitation or dedicated washing machine)
   → IPA 90%+ for optimal cleaning
   → Alternative: Form Wash solution or equivalent
3. Air drying (5–10 min) for IPA evaporation
4. UV post-curing (MANDATORY)
   → 405nm, 900–1200 mJ/cm² or follow manufacturer specifications
   → Time: 15–60 min depending on resin and part size
   → Elevated temperatures (40–60°C) accelerate and improve uniformity
5. Support removal (post-curing for standard resins; pre-curing for flexible resins)
6. Optional finishing: sanding, painting, coating

Warning: Uncured resin is toxic — mandatory PPE (nitrile gloves, goggles, ventilation)

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Ceramic AM — Post-Processing

SLA/DLP ceramics (Lithoz, 3DCeram)

1. Wash (IPA or supplier-specific solvent)
2. UV post-curing (same as standard SLA)
3. Thermal DEBINDING: 200–600°C / slow ramp (1–5°C/min) to eliminate organic binder
   → Critical phase: ramp too fast → cracking
4. PRE-SINTERING (brown body): ~1000°C / 2h → brittle but handleable ceramic
5. Final SINTERING:
   → Alumina: 1550–1600°C / 2h
   → Zirconia: 1450–1550°C / 2h
   → Shrinkage: 20–25% linear (compensate in CAD)
6. Optional HIP for dense zirconia (density >99.9%)
7. Grinding/lapping for functional surfaces

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Inspection and Qualification

Inspection methods for AM parts

Method	Applies to	Detects	When to use
Visual + dimensional (CMM)	All	Dimensions, tolerances	Standard, always
CT scan (tomography)	Metal AM, ceramics	Internal porosity, cracks, inclusions	Critical parts, biomedical, aerospace
Ultrasound (UT)	Metals	Cracks, delaminations	Large parts
X-ray radiography	Metals	Porosity, defects	Cost-effective alternative to CT
Hardness (HV, HRC)	Metals	Heat treatment state	Verify post-HT
Metallography	Metals (coupon)	Microstructure, porosity	Initial process qualification
Tensile testing	All (coupon)	Rm, E, elongation	Batch qualification
Profilometer	All	Ra, Rz	Verify surface finish

Acceptable porosity by application

Application	Max acceptable porosity
Prototypes / non-structural	<5%
Structural parts (non-critical)	<1%
Fatigue-critical applications	<0.1% (HIP often required)
Biomedical (implants)	<0.05% (HIP mandatory)
Pressure vessels / sealings	<0.01% → HIP + CT scan