# 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).

---

## 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

---

## 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%

---

## 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)

---

## 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
```

---

## 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 |