Additive Manufacturing Technologies

Processes

2.1. Additive Manufacturing Technologies

Conventional component manufacturing processes are based on the use of high-capacity resources combined with control elements to achieve extremely high levels of precision and reliability. The use of computer systems during the design engineering, manufacturing, and simulation phases of a product in combination with other techniques based on mechatronics have succeeded in making production systems highly efficient.

However, a number of limitations still plague manufacturing processes. This is because on occasion we find ourselves being forced to use processes and tools that increase the end cost of the element in accordance with the size of the production run and the geometrical complexity of the component.

Transformation processes currently exist that enable us to extract, shape, fuse, and bind the base material of our component, and for the last few years we have also been able to deposit the material where it is needed; in other words, using a virtual 3D model it is possible to manufacture the component by adding the material in accordance with the solid volume designed into the model.

Current additive-type technologies are based on the dispersion-accumulation principle (Figure 4). The material or additives filler processes are those that involve the solidification of a material the original state of which is solid, liquid, or powder by way of the production of successive layers within a predetermined space using electronic processes. These methods are also known by the acronym MIM (Material Increase Manufacturing).

Figure 4 The dispersion-accumulation principle.

If we focus our attention on the application of the different manufacturing technologies used for obtaining rapid prototypes, the current technologies can be classified as additive (stereolithography, laser sintering, fused deposition modelling, etc.) and nonadditive (incremental forming, high-speed machining, pressure injection moulding, lost wax, laminating and contouring, etc.).

In Tables 3–8 the most relevant data with respect to the main additive manufacturing technologies are analysed.

Table 3 Stereolithography (SL).

Resin Solidification - Stereolithography
	The material undergoes point-to-point solidification due to the photopolymerisation laser being directed onto a 2D cross-section of the model (XY plane). The platform gradually descends (Z plane) in accordance with the height of the layer defined.
	Layer thickness varies from between 0.1 and 0.2 mm. The precision in SL is +/- 0.2% (min. +/- 0.2 mm). Maximum model size: 2100x700x800 mm.
	The following materials can be used (epoxy-acrylic resins: Poly1500, PP, TuskXC2700T / Tusk2700W, Tusk SolidGrey3000, Flex70B, NeXt, Protogen White, Xtreme, WaterClear. The properties of the material NeXt are: tensile modulus: 2370-2490 Mpa; tensile strength: 31-35 Mpa; elongation at rupture: 8-10%; flexural modulus: 2415-2525 Mpa; bending strength: 68-71 Mpa; impact strength: 47-52 J/m; deformation under load temperature: 48-57°C.
	Advantages and disadvantages The quality and surface finish are good or very good. Great precisions and transparent parts can be obtained. The equipment and materials are medium-high cost. They have problems to obtain pieces with cantilevers or internal holes due to the difficulty of removing the supports.

Table 4 Selective sintering/melting (SS).

Selective Laser Sintering
	A layer of powder is laid down and a CO₂ laser sinters it at the points selected on a 2D cross section of the model (XY plane). The platform gradually descends (Z plane) in accordance with the height of the layer defined.
	Precision is between +/3% (min. +/- 0.3 mm). The minimum layer thickness is 0.08 mm. Maximum model size 700x380x580 mm.
	The following materials can be used: Polyamide (PA), Glass filled polyamide (PA-GF), Alumide, PA 2241 FR, TPU 92A-1. Properties of the PA material: tensile modulus: 1650 Mpa; tensile strength: 22 Mpa; elongation at rupture: 20%; flexural modulus: 1500 Mpa; bending strength: - Mpa; impact strength: 53 J/m; deformation under load temperature: 86°C.
	Advantages and disadvantages Pieces of high quality and precision are obtained. A large quantity of sintering materials is available. They do not present problems to obtain pieces with cantilevers or internal holes because the own dust makes of support. The equipment and materials are medium-high cost.
Selective Laser Melting
	The print nozzle head is fitted with a CO₂ laser that is directed via a set of lenses onto the powdered material. The support structures are made of the same material as the model and must undergo a subsequent finishing or even machining process.
	The minimum layer thickness is 0.020 mm.
	The material can be: stainless steel, Co-Cr, Inconel 625-718, titanium Ti64. Properties of the material Co-Cr: ultimate creep ( $R_m$ )
	1050 Mpa; elongation (E): 14%; Young's modulus: 20 Gpa; Hardness 360 HV.
	Advantages and disadvantages Pieces of high quality and precision are obtained. There is a large amount of metallic materials to be sintered. Equipment and materials are expensive. They have problems to obtain pieces with cantilevers or internal holes due to the relative difficulty of removing the supports.

Table 5 Material deposition (MD).

Fused Deposition Modelling
	The wire is rolled around a coil and deposited using the thermal nozzle head that moves in accordance with the plane (XY). The platform gradually descends (Z plane) in accordance with the height of the layer defined.
	The layer thickness is: 0.13 - 0.25mm (for ABS); 0.18 - 0.25 mm (for ABSi); 0.18 - 0.25 mm (for PC); 0.25 mm (for PPSU). The maximum model size is 914x610x914 mm.
	The following thermoplastic materials can be used: ABS, ABSi, ABS-M30, ABS-ESD7, PC-ABS, PC-ISO and ULTEM 9085. The properties of ABS are: tensile modulus: 1627 Mpa; tensile strength: 22 Mpa; elongation at rupture: 6%; flexural modulus: 1834 Mpa; bending strength: 41 Mpa; impact strength: 107 J/m; deformation under load temperature: 76-90°C.
	Advantages and disadvantages Low medium cost equipment, available even in domestic environments. Possibility of eliminating supports by dissolution and obtaining very clean pieces. Pieces of quality and precision can be obtained but resorting to high cost equipment.

Table 6 Jet prototyping (JP).

Three Dimensional Printing – Glue Injection
	The model is built on a bed full of powdered model material. A nozzle head injects an agglutinate onto the surface of the bed and fuses the powder in accordance with the geometry of the 2D cross-section of the model. The powder is added and levelled using a roller. Once the process has been completed, the excess powder is sucked off the bed leaving the model clean. The model then has to be cured (hardened) using different coatings.
	Minimum layer thickness is between 0.013 and 0.076 mm.
	The material used can be ceramic, metal and polymers. The properties of the material zp150-Z-Bond are: tensile modulus: - Mpa; tensile strength: 14 Mpa; elongation at rupture: 0.2 %; flexural modulus: 7.2 Mpa; bending strength: 31 Mpa; deformation under load temperature: 112°C.
	Advantages and disadvantages Pieces of color are obtained, with great aesthetic quality. No supports are needed. It is not easy to obtain functional pieces due to the fragility of the pieces obtained. The cost of the equipment is medium-high.

Table 7 Laminated manufacturing (LM).

Selective Deposition Lamination
	Invented in 2003 by MacCormack, SDL must not be confused with laminated object manufacturing (LOM) technology. LOM uses a laser, laminated paper and an adhesive that fixes the model and support material. The SDL technique works by depositing an adhesive in the area required, both of the model and the support, and a blade that cuts the outline of the layer.
	LOM: The layer thickness is 0.165 mm. The maximum model size is 170x220x145 mm. SDL: Layer thickness corresponds to the thickness of the paper used plus that of the layer of adhesive.
	Sheets of PVC with the following properties are used: tensile strength: 40-50 Mpa; elongation at rupture: 30-100%; flexural modulus: 1200-200 Mpa; deformation under load temperature: 45-55°C.
	Advantages and disadvantages Pieces of high quality aesthetic objectives are obtained. The starting materials are low cost. The pieces obtained can be painted. The equipment is of average cost. No functional parts are obtained.

Table 8 Injection-polymerisation (JP-SL).

Resin Injection (Projection) and Ultraviolet Light Photopolymerisation
	A head with thousands of injectors deposits drops of liquid resin that are hardened using two UV ray lights fitted on the sides of the selfsame head. Two materials can be used simultaneously (bi-material pieces).
	The minimum layer thickness is 0.017 mm.
	The range of materials is extremely extensive and includes translucent resins, polypropylene, ABS and elastic resins. Properties of the SCI White (PolyJet) material: tensile modulus: 2500 Mpa; tensile strength: 58 Mpa; elongation at rupture: 10-25 %; flexural modulus: 2700 Mpa; bending strength: 93 Mpa; deformation under load temperature: 48°C.
	Advantages and disadvantages The quality and surface finish are good or very good. Great precisions and transparent parts can be obtained. Equipment and materials are expensive. They have problems to obtain pieces with cantilevers or internal holes due to the difficulty of removing the supports.

2.2. Classification

The classification of the additive manufacturing processes has been a controversial issue, even bordering on obsession, since the appearance of the first alternatives for obtaining pieces in these technologies. The first classifications took into account whether the starting material was solid, liquid, or powder, posing inconsequential doubts that contributed little to the academic community. The same problem was found in the professional field when the first commercial solutions began to appear, since the classifications that were proposed had little or no utility.

One of the pioneers in the classification of processes, hierarchizing them based on the starting material, is JP Kruth, who proposed an interesting classification in 1991. In the literature we can also find other classification alternatives based on the equipment used, to the process itself or to the transformation of materials.

The classification proposed by Williams and his team is very academic, but of little use from a practical point of view. However, the classification proposed by ASTM has some loopholes and disadvantages, and the same is the case with the classification proposed by ISO.

ASTM (ASTM F2792) also proposes the following groups: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat photopolymerization.

However, for example, it is not very coherent to separate the photopolymerization processes depending on whether they are carried out in a vat or with another alternative, such as injection. A difference is made between directed energy deposition and material extrusion when both are, without a doubt, deposition processes.

Moreover, it is not justifiable to distinguish between injection processes when an "adhesive" or a "material" is injected, since, at the end of the day, both materials will end up forming part of the prototype or final part, as is the case of the ProMetal system.

It is also interesting to see that there are two types of injection: (a) when an adhesive is injected (which ends up forming a "material" part of the product) and (b) when a "material" is injected.

Lastly, there are different processes, some very important, that do not fall into any of the groups in this classification, such as in the case of mask sintering or digital light processing, for example.

ISO proposed, in its 2010 working draft, the following ten processes: stereolithography, laser sintering, laser melting, fused layer modeling/manufacturing, multijet modeling, polyjet modeling, 3D printing, layer laminated manufacturing, mask sintering, and digital light processing.

There is no doubt that for the simple fact of defining ten processes, other important processes are left out. In this classification, it can be seen that the manufacturers' considerations have more of an influence than logic. In 2015 ISO assumes the ASTM classification with its standard ISO/ASTM 52900:2015 (ASTM F2792).

An additive manufacturing system is in itself a production system and, therefore, for the purposes of classification, the systematics of manufacturing processes should be used. In every manufacturing system, there must be four elements present:

Material
Energy
Machine and tool
Technology (know-how)

From the point of view of the material, we could opt for the classic classification of solids, liquids, powder, etc.; however, for both the engineer who wishes to manufacture the product and their customer, it is more important to expand upon the technical qualities of the material and, thus, we need to begin to classify the processes according to their ability to work with metal materials (with high melting points and which, therefore, require more energy in the process) or with other materials. And it is these technical qualities that we are going to focus on in this article.

From the point of view of energy, it is important to analyse what type of energy is required and how this energy is transmitted. With regards what type of energy is required, this may be as follows:

Heat (electrical resistance, electron beam, etc.)
UV light (visible or laser)
Chemical energy (for adhesion processes, chemical reactions, etc.)

With regard to how this energy reaches the material for successful transformation, this may be via the following:

Laser (valid to provide UV light and heat)
Electrical resistance
Electron beam

From the point of view of the machine, it is important to analyse the alternatives of smaller machines, suitable for offices, compared to industrial machines and as regards the tools, these shall include the following:

Vats and containers for photosensitive liquids or powder
Deposition or extrusion nozzles
Injectors

With regard to technology, the most important variable, it is necessary to know if it is available commercially or if it is only an option available in research centres, as in this case there will likely be a wait involved, although fortunately not long if the technology is valid.

In conclusion to all of this, Table 9 shows the classification system that takes these variables into account: material, energy, machine and tool, and technology.

Table 9 Classification matrix of the processes in additive manufacturing.

Material	Materials with a low melting point $1$	Resins, transparent materials (2) Flexible materials (3)	Ceramics (4) Powder (5) Sheets (6)	Metals and difficult materials (7)
Energy	Heat	Chemical energy	UV light (visible or laser)	Mechanical energy
Energy	Electrical resistance (a) Electron beam (b) High power laser (c)	Adhesives (d) Reagents (e)	Laser (f) or visible light (g)	Cutting tool (h)
Machine and tools	Low cost (such as RepRap) $1$	Office (2)	Professional (3)	Industrial (4)
Machine and tools	Vats and containers for photosensitive liquids or powder (a)	Deposition or extrusion nozzles (b)	Injectors (c)	Cutting tool (d)
Technology	In conceptual and development phase $1$	In experimental phase (2)	Available at research centre (3)	Available commercially (4)

To specify this classification, which can cover all additive manufacturing processes with simple approaches and that can evolve as technology evolves, we present the classification chain of the five processes analyzed in the introduction, the basic pillars of additive manufacturing (Table 10).

Table 10 Classification of the original processes in additive manufacturing.

Stereolithography (SL)	2 – f – 3 – a – 4
Selective Sintering (SS)	1 – c – 3 – a – 4
Material Deposition (MD)	1 – a – 3 – b – 4
Jet Prototyping (JP)	5 – d – 3 – c – 4
Laminated Manufacturing (LM)	6 – h – 3 – d – 4

For example, stereolithography (SL) is a process that uses resins as material (2); uses laser as energy (f); is a professional machine (3); uses vats and containers for photosensitive liquids as tools (a); and is available commercially (4). For this reason, stereolithography will have a classification: 2f3a4.

2.3. Analysis of the Environment

In order to carry out this study, we worked with leading additive manufacturing companies, both ones that are developing new processes and ones that are part of the market providing service. Contact was also established with researchers in technology centres and universities and their contribution has proved to be highly valuable.

Unsurprisingly, this study entails an exhaustive analysis of the most important literature in the field of additive manufacturing, a study that we cannot include in this paper in full due to the obvious space constraints. Nonetheless, a series of papers must be listed which, due to their special interest in the subject, provide information that has proved fundamental in producing this paper.

In the field of metal additive manufacturing, an analysis has been performed on processes using powder, whether powder injected, powder deposited in layers, or processes using wire.

As has been discussed, this paper has been approached from both a professional perspective (expanding upon the most interesting innovations in this field) and a research perspective. Therefore, papers that take into account the social impact of additive manufacturing have been analysed. Papers focusing on medical issues have also been analysed and papers have been located which expand upon design methodologies for additive manufacturing, the involvement of concurrent engineering, and the study of nonrigid materials. The fields of architecture and the automotive industry have also been addressed.

References have also been found relating to functional prototypes, which show the use that additive manufacturing still has in this regard, along with papers on very contrasting environments, such as the food industry. In the field of research, papers relating to studies of the processes or medical studies with living cells are worth mentioning.

Naturally, this study also considers the RepRap movement, as although technologically speaking it does not contribute much, it has undoubtedly played an essential role from both a social perspective and one of technology disseminations, and so it cannot be omitted in any serious study on additive manufacturing.