Book summary “Additive Manufacturing Technologies”: summary

Introduction

You face constant pressure to shorten product development cycles while increasing geometric complexity. Conventional manufacturing methods like machining and molding limit your designs through draft angles, tooling costs, and assembly requirements. You struggle to produce highly customized components without paying prohibitive tooling penalties or accepting long lead times.

Additive manufacturing transforms digital CAD models directly into physical end-use components through layer-based construction.

Ian Gibson structures Additive Manufacturing Technologies to guide you from fundamental principles to industrial applications. You will examine the generalized process chain and explore specific material processing technologies including photopolymerization, powder bed fusion, extrusion, sheet lamination, and beam deposition. You will also learn advanced design rules and direct digital manufacturing strategies.

• How do you select the optimal process for your specific material and geometry?
• What design freedoms and constraints govern layer-based fabrication?
• How can you transition from prototyping to mass customization?

1. Introduction and Generalized Process Chain

Additive manufacturing builds parts layer by layer directly from digital files, offering complete geometric freedom compared to conventional subtractive machining.

Industry terminology has evolved from Rapid Prototyping to Additive Manufacturing because these technologies are increasingly used to produce final end-use parts rather than just initial prototypes. Early applications focused on creating quick visualization models, but improvements in machine accuracy, material properties, and high-power lasers for direct metal fabrication enabled functional parts. Consensus standards from ASTM International's Technical Committee officially adopted Additive Manufacturing to reflect this fundamental layer-based additive principle.

Additive manufacturing operates on the What You See Is What You Build principle, translating virtual solid model data directly into physical models. Virtual solid model data is broken down into a series of two-dimensional cross-sections of a finite thickness. These cross-sections are combined layer by layer in the machine, faithfully reproducing geometry without needing adjustments for conventional manufacturing constraints like tooling, undercuts, draft angles, or other features.

The Eight Steps in Additive Manufacture

The generic Additive Manufacturing process follows an eight-step pipeline starting from a virtual 3D CAD model and culminating in the final application or finished product. Every AM workflow begins with a 3D solid or surface CAD model or reverse engineering data, which is converted into a standard STL file format.

You can follow these exact stages to execute a successful build:

• Step 1: CAD - create a 3D solid or surface representation using professional CAD software or reverse engineering equipment.
• Step 2: Conversion to STL - export the model into the STL file format describing external closed surfaces.
• Step 3: Transfer to AM Machine and STL File Manipulation - transfer the file and position, orient, and scale it correctly for building.
• Step 4: Machine Setup - configure build parameters including material constraints, energy source, and layer thickness.
• Step 5: Build - execute the automated layer-by-layer fabrication with superficial monitoring.
• Step 6: Removal - take parts out of the machine after verifying safe operating temperatures and stationary mechanisms.
• Step 7: Postprocessing - clean parts, remove supporting features, and perform manual manipulation.
• Step 8: Application - use the parts directly or apply additional treatments like priming, painting, and assembly.

Distinction Between AM and CNC Machining

Additive Manufacturing and Computer Numerical Controlled machining differ fundamentally because AM adds material layer-by-layer with high geometric freedom, whereas CNC subtracts material from a solid block and faces tool accessibility constraints. CNC machining works exceptionally well for hard, brittle materials like steels and produces homogeneous, predictable properties, but it requires extensive multi-stage setup and process planning. AM can build complex internal features, undercuts, and enclosed geometries in a single step without tool path clashes, though AM parts may exhibit voids or anisotropy based on build orientation.

Machining a ship inside a bottle requires making them separately and assembling them, whereas AM can build both the ship and the bottle simultaneously in one process.

2. Photopolymerization Processes

Photopolymerization uses focused ultraviolet energy to cure liquid resins layer by layer, driven by precise laser vectors or mask projections.

Acrylate resins polymerize rapidly via free radicals but exhibit high shrinkage of 5 to 20 percent and curling. Epoxy resins polymerize via cationic ring-opening with much lower shrinkage of 1 to 2 percent, higher strength, and no oxygen inhibition, but suffer from slow photospeed and brittleness. Commercial resins combine both to form interpenetrating polymer networks.

Vector Scan SL

Vector scan stereolithography uses a focused UV laser beam to trace 2D cross-sectional contours point-by-point in a vat of liquid resin. Parts are built bottom-up on a platform lowered into a resin vat. A recoater blade spreads a fresh resin layer before each subsequent slice is traced by galvanometer-steered laser optics. Modern machines utilize solid-state frequency-tripled Nd:YVO4 lasers emitting in the UV range.

3D Systems introduced commercial SL systems like the SLA-250 using a 325 nm helium-cadmium laser, later replaced by advanced models such as the iPro SLA Centers and SLA Viper Si2.

The interaction between laser energy and photopolymer resin is mathematically governed by the Beer-Lambert law, establishing a working curve that links exposure to cure depth. Cure depth is proportional to the natural logarithm of maximum exposure on the centerline of the scanned laser beam. The slope of the working curve gives the penetration depth Dp, and the x-intercept gives the critical exposure Ec.

Mask Projection Photopolymerization Technologies

Mask projection photopolymerization cures entire part cross-sections simultaneously using dynamic masks like digital micromirror devices, drastically increasing build speed. Instead of tracing vectors with a laser, an entire layer bitmap is projected onto the resin surface via a UV lamp or light source. Systems often build upside down through a transparent window, eliminating the need for a mechanical recoater blade.

EnvisionTEC commercialized MPSL systems such as the Perfactory starting in 2003, and 3D Systems introduced the inexpensive V-Flash machine in 2008.

You must execute these steps when preparing and building parts:

• Input data from an STL file created from CAD or reverse engineering data.
• Perform part preparation by specifying support structures and machine parameters.
• Execute layer preparation by dividing the STL model into slices and translating into machine language.
• Perform laser scanning to solidify each slice.
• Clean, post-cure, and finish the part by removing supports and sanding.

3. Powder Bed Fusion Processes

Powder bed fusion uses thermal sources to selectively fuse powdered materials layer by layer, utilizing mechanisms such as solid-state sintering, chemically-induced binding, liquid-phase sintering, or full melting.

Powder bed fusion processes spread thin layers of powder across a build platform maintained at an elevated temperature to build parts.

Selective Laser Sintering (SLS), developed at the University of Texas at Austin, was the first commercialized powder bed fusion process. A thermal source like a CO2 laser, fiber laser, or electron beam fuses the prescribed cross-section with ~0.1 mm layer thickness typically spread across the build area. Loose surrounding powder acts as a natural support, eliminating the need for secondary supports in polymer systems.

Solid-state sintering occurs between half the absolute melting temperature and the melting point via diffusion, though it is slow. Chemically-induced sintering uses thermal reactions between powders or gases, commonly used for ceramics. Liquid-phase sintering (LPS) melts a lower-melting constituent to bind higher-melting particles together. Full melting completely melts the material zone to achieve high density.

Laser processing of silicon carbide (SiC) in the presence of oxygen forms silicon dioxide (SiO2) to bind a composite together.

Powder Fusion Mechanisms

PBF machine architectures vary widely by thermal source, ranging from low-temperature polymer laser systems to high-temperature metal Selective Laser Melting (SLM) and Electron Beam Melting (EBM) systems.

Optimizing process parameters requires balancing accuracy, surface finish, density, and residual stresses.

Low-temperature systems use CO2 lasers and 0.1–3.0% oxygen level maintained in nitrogen atmospheres to process polymers like nylon. Metal systems (SLM) utilize fiber or Nd:YAG lasers tuned to higher metal absorptivity wavelengths in a low-oxygen inert atmosphere, requiring parts to be rigidly attached to base plates to manage residual stresses. Electron Beam Melting (EBM) uses a vacuum environment and magnetic deflection coils to achieve rapid, high-energy scanning and higher bed temperatures with 1 kW and higher beam energies available.

As particle size decreases, interparticle friction and electrostatic forces increase, causing powders to lose flowability. Comparing a spoonful of granulated sugar to fine flour illustrates the loss of flowability in smaller particles. You must follow these principles for powder handling:

• Ensure the powder reservoir has sufficient volume for maximum build height without pausing.
• Transport the correct volume of powder to cover the previous layer without wasteful excess.
• Spread the powder to form a smooth, thin, repeatable layer.
• Ensure powder spreading does not create excessive shear forces that disturb previously processed layers.

For polymers like nylon polyamide, effective particle size and molecular weight change during processing. A fraction-based mixture might consist of 1/3 unused powder, 1/3 overflow/feed powder, and 1/3 build platform powder. You must apply these practices for recycling powders:

• Determine a target melt flow index (MFI) based on experience.
• Test used powders (part-bed and overflow/feed) and unused powder.
• Create a well-blended mixture of unused and used powder, and test it.
• Iterate the mixture blending until the target MFI is achieved.

Test tracks made in -150/+75 micron M2 steel powder demonstrate distinct track types ranging from continuous flat tops to broken balls depending on power and velocity combinations. You can optimize energy density with these steps:

• Run a series of experiments for a given material, laser spot size, and machine configuration to determine the minimum applied energy necessary for adequate fusion.
• Maximize build speed by utilizing the fastest combination of laser power, scan rate, and scan spacing based on the applied energy equation.

4. Extrusion-Based Systems and Printing

Extrusion-based additive manufacturing forces semi-solid material through a nozzle via applied pressure, creating continuous roads that solidify and bond. You rely on a reservoir, a liquefaction mechanism, a nozzle, and a plotting system capable of horizontal scanning and vertical indexing. Materials achieve stability using temperature change via a molten state or chemical change through drying, curing, or a reaction agent.

Extrusion systems and 3D printing deposit thermoplastic filaments, wax, or binder droplets to fabricate solid components rapidly.

Fused Deposition Modeling from Stratasys

Fused Deposition Modeling, developed by Stratasys, is the most common extrusion-based AM technology, utilizing polymer filaments fed into a heating chamber. Scott Crump received the initial FDM patent in 1992. The process provides strong polymer parts but suffers from slower build speeds due to plotting head inertia and vector-based path requirements. Stratasys commercialized low-cost systems through Dimension and high-end industrial systems through Fortus.

Stratasys machine lines range from the uPrint, with a build envelope of 6 inch by 8 inch by 8 inch, up to the FORTUS 900mc, featuring a 36 inch by 24 inch by 36 inch envelope and a layer thickness of 0.076 mm. You can use 0.25 and 0.33 mm layer thickness settings on mid-range Dimension machines and a 0.178 mm minimum layer thickness for the Elite machine. Dimension low-cost machines start at 15,000 USD, while top-of-the-range FORTUS machines cost 400,000 USD.

Three-Dimensional Printing

Droplet formation in printing systems is categorized into continuous stream and drop-on-demand modes, with piezoelectric drop-on-demand preferred for polymers to avoid thermal degradation. Continuous mode ejects a pressurized fluid column that breaks into uniform droplets via Rayleigh instability, requiring electrostatic charging and deflection fields. Drop-on-demand mode expels individual droplets using thermal, piezoelectric, or acoustic pressure pulses only when needed. Typical continuous systems operate at 80-100 kHz frequency to generate 150 um droplets, whereas drop-on-demand systems achieve deposition rates of 0-2000 drops per second with droplet sizes ranging from 25 to 120 um.

The Three-Dimensional Printing process developed at MIT fabricates parts by selectively jetting liquid binder into a powder bed layer by layer. Binder droplets form spherical agglomerates with powder particles and bond layers together. ExOne markets 3DP machines that build metal parts by printing binder into stainless steel powder beds, burning off the binder, sintering, and infiltrating with bronze. The process achieves a typical process accuracy of ±0.125 mm for ExOne metal binder printing with 80 um binder droplets. You must follow these steps when executing the process:

• Print binder into a powder bed to define part cross sections.
• Lower the powder bed.
• Spread a new layer of powder using a counter-rotating rolling mechanism.
• Repeat the printing and recoating cycle until the part is completed.
• Remove the finished part from the powder bed after the binder sets.
• Remove unbound powder using pressurized air.
• Infiltrate the part with an infiltrant to enhance strength and mechanical properties.

Green parts removed directly from metal powder beds achieve 50% relative density. Bond-then-form adhesive processes laminate material to the substrate first and then cut it to shape layer by layer. Helisys Inc. and Cubic Technologies serviced LOM systems using CO2 lasers, while Solidimension developed systems for laminating PVC sheets using an x-y plotter and anti-glue pens. You use paper thicknesses from 0.07 to 0.2 mm as lamination build material.

5. Sheet Lamination and Beam Deposition

Sheet lamination and beam deposition bond foils or feed materials directly into dynamic energy melt pools to build high-performance metal structures.

Ultrasonic Consolidation

Ultrasonic Consolidation (UC) is a hybrid sheet lamination process combining ultrasonic metal seam welding and CNC milling. A rotating sonotrode travels along a thin metal foil while applying a normal force and oscillating transversely at a constant frequency. Each level consists of multiple foils laid side-by-side and trimmed by CNC milling to achieve complex 3D shapes without high-temperature melting. Solidica Inc. commercialized the UC process in 2000, and it was used to fabricate honeycomb structures from Al 3003 foils on a base plate.

You execute UC through specific mechanical steps:

• Bolt a rigidly held base plate onto a heated platen
• Deposit multiple metal foils side-by-side to form a layer
• Apply normal force and 20 kHz ultrasonic oscillation via a rotating sonotrode to bond foils
• Repeat deposition procedure until four layers are placed to form one level
• Trim excess tape ends and machine internal and external features using CNC milling

Solid-state bonding in Ultrasonic Consolidation relies on nascent metal forces, plastic deformation, and acoustic softening to achieve metallurgical bonds. Janaki Ram et al. evaluated linear welding density (LWD) using metallographic cross-sections of Al 3003 samples showing unbonded regions. You optimize results by following these procedures:

• Optimize controllable parameters including oscillation amplitude, normal force, travel speed, and preheat temperature
• Remove surface oxides using chemical stripping or mechanical techniques for poorly weldable materials
• Arrange successive layers with a 50 percent overlap to reduce mechanical property degradation from gaps

Introduction to Beam Deposition

Beam deposition processes build three-dimensional objects by melting and depositing material from powder or wire feedstock using a focused energy beam rather than melting pre-laid powder beds. Unlike powder bed fusion techniques, beam deposition techniques melt materials dynamically as they are being fed into a traveling melt pool created by a focused laser, electron beam, or plasma arc. Each pass forms a solidified track, and adjacent lines make up layers, requiring either multi-axis heads or support materials for complex geometries. Commercial implementations include systems like LENS, DLF, and DMD.

High solidification cooling rates in beam deposition create unique microstructural advantages, though they can also induce residual stresses and cracking in incompatible materials.

Cooling rates from 10^3 to 10^5 °C/s suppress diffusion-controlled phase transformations, form supersaturated or nonequilibrium solutions, and refine microstructures. Researchers at Utah State University demonstrated that depositing Ti on CoCrMo without cracking requires placing the focal plane above the substrate, using low laser power, and applying a rapid scan rate to suppress brittle intermetallics. Beam deposition requires operators to determine application-specific process parameters because machines are sold as flexible platforms rather than pre-programmed units. You operate these systems using active interventions:

• Monitor melt pool size, shape, and temperature using sophisticated accessory equipment.
• Dynamically change travel speed based on sensor feedback to control deposit thickness.
• Monitor and control melt pool size by dynamically changing laser power to regulate solidification rate and microstructure.

6. Design for Additive Manufacturing

Traditional design for manufacture focuses on eliminating manufacturing difficulties and minimizing costs within conventional constraints, whereas design for additive manufacturing seeks to maximize product performance by exploiting AM design freedom. While traditional DFM restricts design spaces to what is manufacturable via conventional machining, molding, or casting, AM allows designers to disregard those constraints and focus on optimal shapes, hierarchies, and material compositions.

Design for additive manufacturing exploits shape, hierarchical, functional, and material complexity to maximize product performance.

Additive manufacturing provides four unique capabilities that enable revolutionary product designs: shape complexity, hierarchical complexity, functional complexity, and material complexity. These capabilities allow the creation of virtually any shape, multi-scale structures from micro to macro, fully functional assemblies built in one run, and point-by-point material composition control.

Hierarchical Structures

Hierarchical structures such as lattice mesostructures provide high strength-to-weight and stiffness-to-weight ratios superior to stochastic foams. Designed cellular materials with characteristic cell lengths between 0.1 and 10 mm allow material placement only where needed. Lattice strength scales linearly with volumetric density rather than exponentially like foams because lattice elements undergo stretching and compression instead of cell wall bending.

A cover plate for an aerospace structure was redesigned with an octet lattice structure, increasing plate thickness to 9 mm and reducing skin thickness to 1.5 mm while utilizing 14,960 beam elements. The original cover plate measured 300 x 350 mm and had a 3 mm thickness.

Additive manufacturing enables the direct fabrication of fully operational mechanisms and the in situ embedding of components. By controlling layer clearances and pausing builds to insert components, AM machines can produce kinematic joints and complex devices without post-fabrication assembly.

A Georgia Tech researcher built a 1:4 scale model of an SLA-250 machine inside an SLA-250 machine, incorporating 11 embedded components and multiple sliding and rotating contact joints after a 75-hour build time. The model dimensions measured 150 x 150 x 260 mm.

7. Direct Digital Manufacturing and Applications

Direct digital manufacturing replaces mass production with mass customization, enabling on-demand production and complex medical solutions.

Direct Digital Manufacturing is the use of additive manufacturing technologies to produce end-use components, often serving as mass customization rather than mass production. You can bypass traditional high-volume mass production constraints by exploiting geometric complexity to manufacture customized parts economically in low volumes. Align Technology and hearing aid manufacturers like Siemens and Phonak use DDM to produce thousands of unique, body-fitting medical devices daily. Between 1997 and March 2009, Align Technology created 44 million aligners.

Align Technology Case Study

Align Technology achieves mass customization of orthodontic aligners by engineering a distributed workflow from dental impressions to stereolithography mold production and thermoforming. You convert patient mouth impressions into digital 3D models via laser scanning, manipulate individual teeth positions using customized software, print custom molds on SL machines, and thermoform clear plastic sheets over them. Align Technology operates SLA-7000 machines 24 hours a day, producing about 100 aligner molds per build and reaching a total production capacity of 40,000 unique aligners daily.

To produce aligners in this workflow, you follow several steps:

• take an impression of the patient mouth with dental clay and ship to Align Technology;
• scan the impression using a laser digitizer and convert the point cloud into a tessellation;
• separate the tessellation into gums and individual teeth, then position each tooth into the desired final location;
• divide the motion of each tooth into a series of treatments representing different aligners;
• build aligner molds in stereolithography machines and fabricate aligners by thermal forming clear plastic over the molds.

Medical Applications and Scaffolds

Tissue engineering uses AM to fabricate porous biocompatible scaffolds designed to host living cells for tissue regeneration. Extrusion-based techniques and printing build porous structures that allow cell introduction, ingrowth, and micro-porosity attachment via fibrils. Materials like polycaprolactone and tri-calcium phosphate act as biodegradable matrices that encourage bone and cartilage regeneration. Hybrid scaffolds composed of a Polycaprolactone layer for cartilage and a Polycaprolactone/Tri-calcium phosphate layer for bone were implanted into a rabbit, revealing subchondral bone and cartilage-like tissue formation after 6 months.

10 Key Ideas

1. Additive manufacturing transforms digital CAD models directly into physical end-use components through layer-based construction without needing conventional tooling adjustments.
2. The eight-step AM workflow begins with a 3D CAD model, moves through STL conversion and slicing, and culminates in automated building, removal, and postprocessing.
3. Photopolymerization uses focused ultraviolet energy or mask projections to cure liquid acrylate and epoxy resins layer by layer.
4. Powder bed fusion spreads thin layers of powder across a heated platform and uses thermal sources like lasers or electron beams to selectively fuse materials.
5. Extrusion systems and 3D printing deposit thermoplastic filaments or binder droplets through nozzles to fabricate solid components and green parts rapidly.
6. Sheet lamination and beam deposition bond foils or feed materials directly into dynamic energy melt pools to build high-performance metal structures.
7. Design for additive manufacturing exploits shape, hierarchical, functional, and material complexity to maximize product performance beyond conventional constraints.
8. Lattice mesostructures provide high strength-to-weight ratios because their elements undergo stretching and compression instead of cell wall bending.
9. Direct digital manufacturing replaces mass production with mass customization, enabling on-demand production and complex medical solutions like hearing aids and orthodontic aligners.
10. Tissue engineering uses additive manufacturing to fabricate porous biocompatible scaffolds designed to host living cells for tissue regeneration.