Process categories

AM has been officially defined by the ASTM F42 committee as “the process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to substractive manufacturing methods” [1]. It is the mature collective name of processes formerly described as ‘Rapid Prototyping’ or ‘Rapid Tooling’ [2]. It gained traction with the invention of stereolithography by Chuck Hull in 1986 [3]. Stereolithography allows production of polymer parts and was originally intented to serve as a fast way to create prototypes, hence the early name of ‘Rapid Prototyping’. Over time, with the maturation of the technology, new processes were created that also allowed processing of metals or even ceramics. Nowadays, state of the art parts are incorporated in commercial aircraft, inserted into the human body and applied in other safety critical environments, providing the best evidence of the economic viability of AM.

All AM processes share the basic working principle of adding material where needed rather than substracting it or using molds. However, they differ in the material that is added, the morphology of the material added, the way of supplying this material to where it needs to be and the consolidation mechanism.

The International Organisation for Standardization (ISO) has recently defined 7 classes into which all AM processes can be divided, listed in Table 1.1 [4].

The table details the feedstock type, working mechanism and a qualitative comparison based on cost, speed, resolution and performance.

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Table1.1:CategoriesofAMprocesses. CategoryNameMaterial

Resolu-Perfor- Material FWorkingMechanismCostSpeed tionmance orm MaterialExtrusionPolymerWireWarmextrusionofpolymer wirefedthroughaheated nozzle.

+++0-- MaterialJettingPolymerLiquidMaterialisdepositedin dropletformthroughheated nozzle 0-+- BinderJetting

Sand Metal Ceramic PowderBinderindropletform depositedonpowderbed, followedbydebinding, sinteringandinfiltration -+-- PowderBedFusionPolymer MetalPowderEnergysourcefullymelts powderparticlesintoplayer ofpowderbed

-----+++++ DirectedEnergy DepositionMetalPowder WirePowderorwireisfedintothe energysource,mountedona roboticarm

-++0++ VatPolymerisationPolymerLiquidUVlightpolymerizesliquid prepolymerlayerbylayer+0++(+)+ SheetLaminationPaper MetalSheetSheetcutoutsarestacked andbonded++------

MetalSheetSheetcutoutsarestackedandbonded++---• In the first category called Material Extrusion, a polymer wire (usually nylon) is fed through a heated nozzle that can move in the X and Y (and Z) direction. If the nozzle cannot move in the Z direction, the base plate onto which the material is deposited can move in the Z direction. The polymer is heated to lower the viscosity and enable smooth deposition of continuous lines. This mechanism is used by most cheap desktop 3D printers.

• During Material Jetting, the material is again passed through a heated nozzle, but it emerges as liquid droplets rather than as a semi-viscous continuous line. In a process very similar to 2D inkjet printing, the droplets are deposited first on a base plate and then on top of each other.

It is fairly easy to deposit multiple materials during the production of one part. The droplets solidify fast enough to produce a 3D part. This is one of the fewer used categories, with commercial machines and applications still to be announced. Stable droplets need to be produced at a workable temperature, and wax is the go-to material for this category.

• Binder jetting is a third category where droplets are deposited through a nozzle. But the droplets themselves are not the material from which the final product is made. Rather, the droplets are deposited on a layer of powder, effectively binding the powder particles together. By repeatedly applying a layer of powder and binding it, fragile ‘green’ 3D parts are built up. This process requires extensive post processing. The binder is first burned away and the powder is sintered together. The sintered part is then infiltrated by a liquid metal, often bronze, to produce a fully dense part. Since the final part usually consists of bronze and a different metal, the resulting mechanical properties are average at most, and these parts are often used for prototypes or sand molds. The advantage however is that it is not a high temperature process that would build up thermal stresses, so large metal parts can be made.

• In Powder Bed Fusion, a heat source is directed onto a metal or polymer powder bed, fully melting the powder particles and fusing them together.

The energy source is either a laser or an electron beam. The powder is deposited by a scraper, rake or roller moving over the powder bed.

Because of the high temperatures during the process, thermal stresses are built up, limiting the maximum size of producible parts. Loose, unmelted powder can act as a support for layers to be deposited on top, but support structures are often needed to conduct heat away and prevent sinking of the melt pool into the powder below. The SLM process belongs to this process category, as does Electron Beam Melting (EBM). Fully dense metal parts are possible, making this the process category with the widest applications in structural or functional applications.

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• Directed Energy Deposition or DED is similar to powder bed fusion, but the powder is sprayed co-axially into the energy beam, or a wire is fed into the melt pool created by the beam, which is mounted on a robot arm allowing free movement. The process creates a larger melt pool than those commonly found in powder bed fusion, so it cannot attain the high level of detail possible in powder bed fusion. The Laser Engineered Net Shaping or LENS process, also called laser cladding (LC) or laser metal deposition (LMD) is the most prominent technique in this category. The abbreviations LMD, LENS and LC will be used interchangeably in the remainder of this work.

• In Vat Polymerization or stereolithography, a liquid bath of prepolymer is illuminated with UV light from above, causing it to polymerize and thus solidify. The parts are attached to a base plate which gradually lowers into the liquid bath as the process continues. Bottom-up approaches exist as well, where a thin liquid layer is illuminated from below, and the base plate is pulled upwards and out of the liquid. This bottom up approach is the one used by an emerging type of desktop 3D printers that have a higher resolution than extruder based designs.

• The last category, called Sheet Lamination, is not often used and can only marginally be considered additive manufacturing. A sheet is first cut to the right shape (producing scrap on the way that needs to processed again) and stacked on top of other cutouts. The layers are bound to each other either via ultrasonic vibrations or glue.