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    Additive Manufacturing

    Additive Manufacturing (AM) is a process that builds parts layer-by-layer directly from a digital model, rather than subtracting from a bulk material like conventional manufacturing processes. Additive manufacturing enables complex geometry, lightweighting, and part consolidation. 3D Printing is one category of additive manufacturing.

    3D Printing Process

    • 1. 3D CAD model is converted to STL (Standard Tessellation Language)
    • 2. Model is sliced into 2D layers
    • 3. Each layer is fabricated sequentially
    • 4. Support material may be required, depending on geometry and process.

    Callout

    Standard Tessellation Language (STL) represents surfaces using triangular facets

    Tessellation: surface represented by triangles

    image

    Advantages and Disadvantages

    There are a wide range of 3D Printing technologies that each have their own unique properties. Between them, you can optimize certain qualities with other tradeoffs, but these refer to 3D Printing as a whole.

    Advantages

    • Easy and quick to change part design and produce a new version (rapid prototyping)
    • Limited skills or training needed to make parts
    • Reduced individual manufacturing time with no prep time required
    • Cost of production unique parts in small quantities is relatively low
    • Some parts can be made pre-assembled

    Disadvantages

    • Part tolerances/surface finish:

    • Staircase Effect on sloped surfaces from slicing and layer thickness
    • Shrinkage or distortion of manufactured parts from uneven cooling
    • Holes come out smaller and posts bigger than CAD dimensions

  • Limited variety of possible materials

  • Post-processing of parts may be required

  • Cost and time does not improve with increased volume

  • Part size is generally limited

  • Porosity and mechanical property challenges

  • Material anisotropy, i.e. mechanical properties within layers vary from those between layers

    • Types of 3D Printing Processes

    1. Material Extrusion (MEX)
      • Fused Deposition Modeling (\( (FDM)^{TM} \))
      • Fused Filament Fabrication (FFF)
    2. Vat Polymerization (VAT)
      • Stereolithography (SLA)
      • Digital Light Processing (DLP)
      • Continuous Digital Light Processing (CLIP)
      • Liquid Crystal Display (LCD)
    3. Material Jetting
      • Poly Jet
      • Nanoparticle Jetting
      • Drop on Demand (DOD)
    4. Directed Energy Deposition (DED)
      • Laser Engineered Net Shaping (\( LENS^{TM} \))
      • Electron Beam Additive Manufacturing (EBAM)
      • Wire Arc Additive Manufacturing (WAAM)
      • Friction Stir Additive Manufacturing (FSAM)
    5. Powder Bed Fusion (PBF)
      • Selective Laser Sintering (SLS)
      • Selective Laser Melting (SLM)
      • Direct Metal Laser Sintering (DMLS)
      • Electron Beam Melting (EBM)
      • Selective Heat Sintering (SHS)
      • High-Speed Sintering
      • Multi-Jet Fusion (MJF)
    6. Binder Jetting
    7. Sheet Lamination
      • Laminate Object Manufacturing (LOM)
      • Selective Lamination Composite Object Manufacturing
      • Plastic Sheet Lamination
      • Selective Deposition Lamination
      • Composite Based Additive Manufacturing
      • Ultrasonic Consolidation
      • Ultrasonic Additive Manufacturing (UAM)

    Material Extrusion (MEX)

    This is what most people picture when they think of 3D Printing

    Also known as: Fused Deposition Modeling (FDM), Fused Filament Fabrication (FFF)

    • Heated thermoplastic filament extruded through a nozzle, layers fusing together in cooler air
    • Machine deposits layers by moving nozzle horizontally, tracing 2D cross sections, then moving vertically and repeating

    • 3+ DOF machines exist for non-planar slices, but much more complex and less common

  • Support structure needed for steeply overhanging parts

  • Main Materials: PLA, ABS, PC, Nylon

  • Typical layer thickness: \( 0.1 \)–\( 0.3 \) mm

  • Surface finish is generally not as good as other methods

    • MEX Numerical Example

    Given:

    • Nozzle diameter \( d = 0.4 \) mm
    • Layer height \( h = 0.25 \) mm
    • Extrusion speed \( v = 50 \) mm/s
    • Delay between layers = 5 s
    • Part: \( 30 \times 30 \times 30 \) mm cube

    Volumetric flow rate:

    $$ Q = v \cdot d \cdot h = 50 \times 0.4 \times 0.25 = \boxed{5\ \text{mm}^3/\text{s}} $$

    Number of layers:

    $$ N = \frac{30}{0.25} = 120 $$

    Total inactive time:

    $$ t_{\text{idle}} = 120 \times 5 = 600 \text{ s} = 10 \text{ min} $$

    (a) Solid Cube

    Volume:

    $$ V = 30^3 = 27{,}000 \ \text{mm}^3 $$

    Time to extrude:

    $$ t = \frac{V}{Q} = \frac{27{,}000}{5} = 5400 \text{ s} = 90 \text{ min} $$

    Total production time:

    $$ t_{\text{total}} = 90 + 10 = \boxed{100 \text{ min}} $$

    (b) 25% Infill

    Interior dimensions:

    $$ 30 - 2(0.4) = 29.2 \text{ mm}, \quad \text{height} \approx 29.5 \text{ mm} $$

    Interior volume:

    $$ V_{\text{int}} = 29.2^2 \times 29.5 $$

    Material used:

    $$ V_{\text{total}} = (30^3) - (29.2^2 \cdot 29.5)\cdot 0.75 = 8{,}135 \ \text{mm}^3 $$

    Extrusion time:

    $$ t = \frac{8{,}135}{5} = 1627.3 \text{ s} \approx 27.1 \text{ min} $$

    Total production time:

    $$ t_{\text{total}} = 27.1 + 10 = \boxed{37.1 \text{ min}} $$

    Traverse Path Method (Check)

    Tracks per layer:

    $$ \frac{30}{0.4} = 75 $$

    Travel distance per layer:

    $$ 75 \times 30 = 2250 \ \text{mm} $$

    Time per layer:

    $$ \frac{2250}{50} = 45 \text{ s} $$

    Total extrusion time:

    $$ 45 \times 120 = 5400 \text{ s} = 90 \text{ min} $$

    Total production time:

    $$ 90 + 10 = 100 \text{ min} $$

    Vat Polymerization (VAT)

    • UltraViolet laser cures UV-curable polymer (or resin) dissolved in a solvent- Liquid Deposition Process
    • Laser may be positioned above or below resin tank (top-down vs bottom-up)
    • Support structures required when overhang angle exceeds \( 45^\circ \), since cured resin is denser than uncured
    • SLA, DLP, CLIP are common variants
    • Typical layer thickness: \( 0.025 \)–\( 0.1 \) mm

    VAT Build Time Model

    Time for layer \( i \):

    $$ T_i = \frac{A_i}{vD} + t_d $$

    Total build time:

    $$ T = \sum_{i=1}^{n} T_i $$

    VAT Numerical Example

    A rectangular part with dimensions \( 20 \text{ mm} \times 10 \text{ mm} \times 10 \text{ mm} \) is fabricated using a vat photopolymerization process.

    Given:

    • Layer thickness \( t = 0.2 \) mm
    • Part height \( H = 10 \) mm
    • Laser scanning speed \( v = 1000 \) mm/s
    • Laser spot diameter \( D = 0.2 \) mm
    • Delay per layer (stage movement in \( z \)-direction) \( t_d = 10 \) s
    • Cross-sectional area per layer \( A = 200 \) mm\( ^2 \)
    Find:
    1. Number of layers
    2. Time to write one layer (including delay)
    3. Total fabrication time
    4. Volumetric method approximation for fabrication time
    Solution:
    1. Number of layers:
      2. $$ n = \frac{H}{t} = \frac{10}{0.2} = 50 $$
    2. Time to write one layer:
      3. $$ T_i = \frac{A}{v \cdot D} + t_d = \frac{200}{1000 \times 0.2} + 10 = 11 \ \text{s} $$
    3. Total fabrication time:
      4. $$ T = n \cdot T_i = 50 \times 11 = \boxed{550 \ \text{s}} $$
    4. Volumetric method:

      5. Total part volume:

      $$ V_{\text{part}} = A \cdot H = 200 \times 10 = 2000 \ \text{mm}^3 $$

      Volumetric build rate:

      $$ \dot{V} = D \cdot t \cdot v = (0.2)(0.2)(1000) $$

      Total time:

      $$ T = \frac{V_{\text{part}}}{D \cdot t \cdot v} + n \cdot t_d = \frac{2000}{(0.2)(0.2)(1000)} + 50 \times 10 = 50 + 500 = \boxed{550 \ \text{s}} $$

    Material Jetting

    • Individual droplets of photopolymer deposited layer-by-layer and UV-cured
    • Machines typically have 2 sets of print heads, one for main material and one for support, with multiple nozzles on each head
    • Uses viscous thermosetting resin that is cured by high intensity UV, with a secondary support material that is water-soluble.
    • Multiple materials and colors possible
    • Typical layer thickness ~0.02 mm

    Powder Bed Fusion (PBF)

    • Laser or electron beam selectively melts heat-fusible powder (thermoplastics, metals, or ceramics)
    • After each layer, a new layer of loose powder is spread across the surface
    • Unfused powder acts as support structure for overhanging parts, but powder in a 10 mm thick shell around the part is thermally damaged
    • Undamaged powder can be poured out of complete part to be reused
    • Typical layer thickness: \( 50 \)–\( 100\ \mu \)m

    Directed Energy Deposition (DED)

    • The material (usually a metal powder) and the energy for fusion (usually laser heating) are simultaneously focused at the same location
    • The metal powder is projected onto a surface where the laser directly melts the powder in place
    • Can be combined with in-situ machining, combining both machining and deposition in a single machine
    • Suitable for repair and large structures
    • Tool change or part shifting required, requires specialized toolpath planning

    Binder Jetting

    • Liquid adhesive is selectively deposited to join powder materials (gypsum or starch), which are deposited in thin layers
    • Parts are weak before post-processing

    Laminated Object Manufacturing

    • The input is a roll of thin sheet material (paper, plastic, or metal) coated with an adhesive
    • Successive layers are cut by a laser in the required shape and stacked to build up the part

    Design Guidance for AM

    • 1. Minimize unnecessary material
    • 2. Choose a print orientation that is aware of the direction of anisotropy, mechanical properties, surface finish, roundness of holes, support materials, etc.
    • 3. Reduce support structures
    • 4. Iterate steps 1-3

    Design Advisor.