Additive Manufacturing Technologies

Historical and Current Framework

Many different additive manufacturing (AM) technologies enable the production of prototypes and fully functional artefacts. Although very different in solution, principle, and embodiment, significant functional commonality exists among the technologies.

In order to enter the subject from its origins, before proposing a classification or analyzing the different technologies and their advantages and disadvantages, a chronological analysis of facts will be carried out that will allow us to later base the conclusions. This chronological analysis will be based on dates of publication of works, dates of application for patents, and dates of acceptance of these patents, being aware that in any case the dates in which the developments were reached are always prior to those dates of a public nature.

Without a doubt, the milestone that marked the beginning of additive manufacturing took place on 9th March 1983, when Charles W. Hull successfully printed a teacup on the first additive manufacturing system: the stereolithography apparatus SLA-1, which he himself built.

From then on, there were several advances that paved the way for what is today known as additive manufacturing (Table 1). From a chronological point of view, the most relevant are as follows:

Table 1: Key inventions in additive manufacturing (ordered by publication of patent).

Technology	Inventors	Patent	Development centre	Request for patent	Publication of patent	Principle of operation
Stereolithography  SL	Charles W. Hull	Method and apparatus for production of three-dimensional objects by stereolithography	3D Systems	08.08.1984	12.02.1986	Photopolymerization of a photosensitive resin using UV light
Selective Sintering  SS	Carl R. Deckard	Method and apparatus for producing parts by selective sintering	University of Texas	17.10.1986	21.04.1988	Selective sintering of powder (fusion – solidification using laser)
Material Deposition  MD	Scott S. Crump	Apparatus and method for creating three-dimensional objects	Stratasys, Inc.	30.10.1989	01.05.1991	Deposition of material, using a nozzle, in plastic state (heated by electrical resistance)
Jet Prototyping (injection)  JP	Emanuel M. Sachs;  John S. Haggerty;  Michael J. Cima;  Paul A. Williams	Three-dimensional printing techniques	Massachusetts Inst. Technology	08.12.1989	09.06.1991	Injection of binding agent and coloured ink on a bed of powdered material
Laminated Manufacturing (cutting)  LM	Feygin, Michael;  Pak, Sung Sik	Forming integral objects from laminations - Apparatus for forming an integral object from laminations	Helisys, Inc.	05.10.1988	18.04.1996	Cutting and gluing of laminations with the geometry determined for each layer

As an evolution of Hull's work based on photopolymerization, other processes have been developed:

1. Solid Creation System (SCS). Developed by Sony Corporation, JSR Corporation and D-MEC Corporation in 1990.
2. Solid Object Ultraviolet Laser Printer (SOUP) Developed by CMET Inc. in 1990.
3. Solid Ground Curing (SGC) developed by Cubital Ltd. in 1991
4. Inkjet Rapid Prototyping (IRP), the parts are formed by injecting a photopolymer drop by drop which is then cured using ultraviolet light. Developed by Object Geometries Ltd. in 2000, under the name Polyjet.

As an evolution of Deckard's work based on sintering, other processes have been developed:

1. Direct metal laser sintering (DMLS), where the base material is metal powder and the grains are bonded by sintering, without the grains being fully fused together.
   
2. Selective laser melting (SLM), where metal powder is fully fused together and so the process is not sintering but rather melting.
   

As an evolution of Crump's work on fused deposition modeling, other processes have been developed:

1. Metal deposition (MD), where a metal filler material (powder jet or wire) is deposited by a nozzle following the path marked out by the G-code in the  .stl or  .amf file.
   
2. Fused Filament Fabrication (FFF), name from the RepRap community, an open community at RepRap.org, founded by Adrian Bowyer at the University of Bath in 2004.
   

At this point, hardfacing processes using numerical control (NC) should be mentioned, which are predecessors of fused deposition modeling and prior to the work of Crump, with the difference being that they were not based on electronic files generated by means of solid modeling systems. Although additive manufacturing could date back to automated welding systems, where a robotic arm controlled by numerical control (G-code) deposited material in welding or hardfacing operations (which may be a similar case), it was not until 1983 that this G-code was used to control a laser that "solidifies" a resin and builds a part using a virtual model (solid model and  .stl file).

As an evolution of the work of Sachs and his team on the injection of binding agent or base material, other processes have been developed:

1. MultiJet Modeling System (MJM) developed by 3D Systems Inc. in 1999, with multiple heads in parallel that move along one axis.
   
2. ModelMaker and Pattern Master, by Solidscape, with one single print head that moves along two axes.
   
3. ProMetal, division of Extrude Hone Corporation, process that binds together steel powder and then infiltrates molten bronze to produce a part that is 40% steel and 60% bronze.
   

Finally, as an evolution of the work of Feygin and his team based on cutting laminations, other processes have been developed:

1. Selective Deposition Lamination (SDL) Invented in 2003 by MacCormack. The SDL technique works by depositing an adhesive in the area required, both the model and the support, and a blade that cuts the outline of the layer.
   

It is interesting to point out that there are current processes based on more than one of the contributions stated or on integrated processes. This is the case of polyjet modeling (PJM), which is said to be a combination of stereolithography and injection.

Nowadays, the volume of processes, technologies and initialisms is so high that there is no extensive classification system in operation. In Table 2 we can see a list of initialisms used in this field, which is by no means exhaustive, which gives us an idea of how technology is evolving.

Table 2: A sea of initialisms

3DB	three-dimensional bioplotter	LPD	laser powder deposition
3DP	three-dimensional printing	LPF	laser powder fusion
AF	additive fabrication	LPS	liquid-phase sintering
ALPD	automated laser powder deposition	LRF	laser rapid forming
AM	additive manufacturing	LS	laser sintering
BM	biomanufacturing	M3D	maskless mesoscale material deposition
CAM-LEM	computer-aided manufacturing of laminated engineering materials	MD	metal deposition
DCM	direct composite manufacturing	MD	material deposition
DIPC	direct inkjet printing of ceramics	MEM	melted extrusion manufacturing
DLC	direct laser casting	MIM	material increase manufacturing
DLF	directed light fabrication	MJM	multijet modeling system
DLP	digital light processing	MJS	multiphase jet solidification
DMD	direct metal deposition	MS	mask sintering
DMLS	direct metal laser sintering	M-SL	microstereolithography
EBM	electron beam melting	PBF	powder bed fusion
EBW	electron beam welding	PJM	poly jet modeling
EP	electrophotographic printing	RFP	rapid freeze prototyping
ERP	electrophotographic rapid printing	RM	rapid manufacturing
FDC	fused deposition of ceramics	RP	rapid prototyping
FDM	fused deposition modeling	RPM	rapid prototyping and manufacturing
FFEF	freeze-form extrusion fabrication	RT	rapid tooling
FFF	fused filament fabrication	RTM	rapid tool maker
FLM	fused layer modeling	SALDVI	selective area laser deposition and vapor infiltration
FLM	fused layer manufacturing	SCS	solid creation system
HSS	high speed sintering	SDL	selective deposition lamination
IJP-A	aqueous direct inkjet printing	SDM	shape deposition manufacturing
IJP-UV	UV direct inkjet printing	SFC	solid film curing
IJP-W	hot-melt direct inkjet printing	SFF	solid free-form fabrication
IRP	inkjet rapid prototyping	SGC	solid ground curing
JP	jet prototyping	SHS	selective heat sintering
LAM	laser additive manufacturing	SL	stereolithography
LC	laser cladding	SLA	stereolithography apparatus
LCVD	laser chemical vapor deposition	SL-C	stereolithography of ceramics
LDC	laser direct casting	SLM	selective laser melting
LDM	low-temperature deposition manufacturing	SLP	solid laser diode plotter
LENS	laser engineered net shaping	SLS	selective laser sintering
LFFF	laser free form fabrication	SLSM	selective laser sintering of metals
LLM	layer laminated manufacturing	SOUP	solid object ultra-violet laser printer
LM	laminated manufacturing	SS	selective sintering
LMD	laser material deposition	SSM-SFF	semisolid metal solid freeform fabrication
LMF	laser metal forming	SSS	solid-state sintering
LOM	layered object manufacturing	UC	ultrasonic consolidation
LOM	laminated object manufacturing	UOC	ultrasonic object consolidation

In order to highlight the basic pillars of additive manufacturing in Table 2, the abbreviations referring to the technologies discussed at the beginning of this introduction have been highlighted in italic.

Given the large number of initiatives and processes that are developed and patented every day, there is no doubt that additive manufacturing is a technology that will set the standards for many productive processes in the short and medium term. One more proof of this is that the European Union has decided that manufacturing in general and additive manufacturing in particular shall be one of the key tools to tackle some of the European challenges and their subsequent objectives, above all economic growth and the creation of added value and high-quality jobs. This decision is generating the setting up of research and innovation support and promotion programmes aimed at achieving a situation in which additive manufacturing enables the provision of both high-value products and competitive services.

All over the world the additive manufacturing industry is beginning to respond to global, national, and regional standardisation needs via a series of working groups in which the European Union is a key participant: the ISO/TC 261, Additive manufacturing; the ASTM Committee F42 on Additive Manufacturing Technologies; the CEN/TC 438, additive manufacturing; or the AEN/CTN 116, Sistemas industriales automatizados.

The ISO standard (ISO 52900) defines additive fabrication as follows:

The terms habitually used in conjunction with additive manufacturing have been evolving at the same pace as the technological developments, and it is convenient to establish a framework of reference that enables an analysis to be carried out of the developments made and of the standardisation required for the future:

1. "Desktop manufacturing", perhaps the first name, in line with the names at the time (1980s) such as desktop computer, desktop design.
   
2. "Rapid Prototyping". This was the first term used to describe the creation of 3D objects by way of the layer-upon-layer method. The technologies that currently exist enable the manufacture of objects that can be considered as being somewhat more than "prototypes".
   
3. "Rapid tooling". When it became clear that the additive manufacturing system not only enabled us to build prototypes, but also moulds, matrices and tools, this name began to be used to differentiate it from rapid prototyping.
   
4. "3D Printing". This is the most commonly used term. The term "low-cost 3D printing" is frequently coined when we use printers that domestic or semi-professional users can afford.
   
5. "Freeform Fabrication". Is a collection of manufacturing technologies with which parts can be created without the need for part-specific tooling. A computerized model of the part is designed. It is sliced computationally, and layer information is sent to a fabricator that reproduces the layer in a real material.
   
6. "Additive Manufacturing". This is the most recent term applied and it is used to describe the technology in general. It is commonly used when referring to industrial component manufacturing applications and high-performance professional and industrial equipment.
   

An evolution can be seen in this sequence from obtaining prototypes (with purely aesthetic and geometric goals at the beginning) to simple functional parts, tools, and moulds, to obtain complex functional parts such as those currently obtained via additive manufacturing in the metal industry.

The difference between these techniques is reduced to the application for which the additive manufacturing technology is used. This means that a rapid prototyping technique can be used as a possible technology for the rapid manufacturing of elements, tools, or moulds. It answers the following questions: how would it be possible to manufacture a ball joint fully mounted in its housing without having to manufacture the elements separately prior to their assembly? And is the mass or individual production of this unit possible at an affordable price (Figure 1)?

A characteristic common to the different additive manufacturing techniques is that of the need for a minimum number of phases in the manufacturing process starting with the development of the "idea" by the designer through to obtaining the finished product (Figure 2).

Figure 2 Phases of the additive manufacturing process.

Obviously, the scheme proposed by Yan and his team is a generalization since not in all AM processes G-Codes are created, and preprocessing is not contemplated (necessary in some processes).

In the process described above the designer can perform the entire product manufacturing operation from start to finish. The involvement of another technician is not necessary for carrying out any complementary operations. However, it must be taken into account that, during the process and prior to manufacture, the designer must know the determining factors of the end product in order to be able to select the most suitable manufacturing technique, make the necessary modifications to the geometrical data file (stl or amf file), and review the NC code. The designer must, therefore, have a full overview of and the necessary training in all of the phases of the process.

This article contains a full and up-to-date description of the benefits and drawbacks of the most important manufacturing methodologies and processes that exist within the scope of the additive manufacturing concept, of the characteristics of the models manufactured and their associated costs, of the functional models, the applications, and the sectors of influence. At the end, there is a reference to the RepRap community and free software due the importance they acquired last years.

It is important to establish the variables that currently support the implementation of additive manufacturing and its relation to the basic principles of each technology, which allow detecting the advantages and limitations of each of them. In Figure 3, the relationship between the main variables that intervene in the development of additive manufacturing is shown: technologies, materials, type of models, associated costs, and visual appearance. The current scope of these variables and their evolution can be seen in the subdivision that is presented in a circular diagram.

Figure 3 Table of contents.