Development of AM processes

Additive manufacturing has been developed for a wide range of different applications and materials. Additive manufacturing systems can be categorised in a variety of ways depending on the physics of the process, the source of energy, type of material, or size of parts [Kruth et al, 1998; Pham, 2001; Noorani, 2006]. According to the raw material used, additive manufacturing systems can be divided into: liquid-based, powder-based or solid-based processes, as shown in Figure 2-2 [Kruth et al, 1998; Pham, 2001; Noorani, 2006]. The basic processes and operation of the most common commercial AM systems based on this classification is reviewed in the following section.

Figure 2-2 AM process classification [Kruth et al, 1998; Pham, 2001; Noorani, 2006]

2.2.2.1 Liquid-based process

The stereolithography apparatus (SLA) is the most common liquid-based process, and was the first commercial implementation of the AM system introduced by 3D Systems in 1988 [Cee Kai, 2003; Noorani, 2006].

The process of a liquid-based system begins with the raw material, a photosensitive polymer in the liquid state which is then converted into a solid state through a curing process. A schematic view of the SLA process is illustrated in figure 2-3 [Upcraft and Fletcher, 2003], where the three-dimensional part‟s model is built in a vat. Based on the predefined path in slicing the model, the surface layer of resin is cured or solidified selectively by a UV (ultraviolet) laser beam to form a solid layer. When one layer of the part is formed, the elevation systems control causes the elevator table to lower the part, and then a new layer of liquid resin is swept over it and the process repeated until the complete part is built [Cee Kai, 2003; Noorani, 2006]. The part is then removed from the vat and cleaned of uncured material.

Additive Manufacturing

(AM)

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SLA technology can produce parts with a good surface finish, high degree of detail and thin walls. Drawbacks include a limited range of materials (photopolymer); and the needs for a support structure during processing and postcuring [Noorani, 2006].

Figure 2-3 Schematic view of stereolithography apparatus (SLA) [Upcraft and Fletcher, 2003]

2.2.2.2 Powder-based process

Powder-based processes are a special category of solid-based processes that use powder as raw material. A wide range of polymers, metals and ceramics may be used [Kruth et al, 2005b]. Three-dimensional printing (3DP) and selective laser sintering (SLS) processes are the most popular type in this category.

2.2.2.2.1 Three-dimensional printing (3DP) systems

This layer manufacturing technology was invented and patented by the Massachusetts Institute of Technology (MIT). It utilises ink-jet printing technology directly for the rapid and flexible production of prototypes, parts and tooling [Noorani, 2006]. Firstly, a thin layer of powder material is spread over the surface of the powder bed. Next, by using ink-jet printing technology, a binder material is “printed” into the powder where the part model is to be formed and the powder particles are selectively bonded to form the solid part layer. Then the platform is lowered, a fresh powder layer is spread and deposited over the previous solid layer, and the binder prints again. This process is

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continued until the part is completely developed [Noorani, 2006]. Figure 2-4 shows a basic schematic of three-dimensional printing (3DP) systems. For a metal part, the 3DP process is used firstly to build a green-part, which is then post-processed with debinding, sintering and infiltrating in a furnace.

Figure 2-4 Schematic view of 3DP systems [Upcraft and Fletcher, 2003]

2.2.2.2.2 Selective laser sintering (SLS)

Selective laser sintering (SLS) was developed and patented by the University of Texas at Austin and was commercialised by the DTM Corporation (later bought by 3D Systems) [Noorani, 2006].

Selective laser sintering (SLS) builds a 3D solid part model by laser-fusing or sintering a layer of powdered raw material. Firstly, the powder is transferred to the build cylinder platform from the feed powder container through a counter-rolling cylinder or blade. When a thin layer of the heat-fusible powder is spread and deposited over the build platform, a concentrated heating laser beam selectively sinters the powder, fusing the powder particles and forming a solid part. The powder is spread again for the next layer and the process is repeated, so that layers of powder are deposited and sintered until the part building is complete. Unsintered material remains loose and can act as a support for the next powder layer and object under fabrication. When the object is completely formed, the piston is raised to elevate the

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object. Excess powder is brushed away and final hand finishing may be carried out. Figure 2-5 illustrates a schematic of the SLS process.

Selective laser sintering is described in greater detail in section 2.3 below.

Figure 2-5 Schematic view of selective laser sintering (SLS) [Kruth et al, 2005a]

2.2.2.3 Solid-based process

Solid-based systems begin with the build material in a solid (non-powder) state, which may include material in the form of wire, a roll, laminates or pellets. The most common systems are fused deposition modelling (FDM) and multi-jet modelling (MJM) systems.

2.2.2.3.1 Fused deposition modelling (FDM)

Fused deposition modelling (FDM) technology was developed by the Stratasys company and a patent was issued in 1992 [Noorani, 2006]. FDM generates three- dimensional parts using an extrusion process. A schematic process is shown in figure 2-6. To develop a 3D solid part, a filament of thermoplastic is extruded from a heated dispensing nozzle and deposited onto a platform. The nozzle is laid across the X-Y plane to form a thin 2D layer. To build each layer, the dispensing nozzle deposits the outline of the layer first and then the next slice of the part is deposited. As each layer is built up with hot filament, it bonds to the material in the previous slice to develop a 3D solid part. To build up the support structure, when needed, another nozzle is used with different material [Pham, 2001; Noorani, 2006].

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There are certain advantages to FDM, such as the variety of materials which can be used its ease of material changeover and fast build speed on small/hollow geometries; but requiring no postcuring. However, the surface finish is not as good as in conventional parts there may/be weak properties in the Z axis and for large/dense parts building speed is slow [Noorani, 2006].

Figure 2-6 Schematic view of fused deposition modelling (FDM) [Upcraft and Fletcher, 2003]

2.2.2.3.2 Multi-jet modelling (MJM) systems

As shown in figure 2-7, the principle process of the multi-jet modelling (MJM) system is similar to the technology used in an ink-jet printer, but applied in three dimensions. To start the process, a small droplet of thermoplastic polymer material is sprayed through the print head of a linear array jets or spray nozzle. To build 3D solid parts layer-by-layer, the print head is moved in a similar fashion as a line printer. A moveable platform is used to build the part. This lowers after each layer is developed, where the hot droplet of material bonds to the previous layer of the part [Upcraft and Fletcher, 2003].

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Figure 2-7 Schematic view of multi jet modelling (MJM) [Upcraft and Fletcher, 2003]