BUS606: Operations and Supply Chain Management

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:

1. Material
   
2. Energy
   
3. Machine and tool
   

4. 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:

1. Heat (electrical resistance, electron beam, etc.)
   
2. UV light (visible or laser)
   
3. 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:

1. Laser (valid to provide UV light and heat)
   
2. Electrical resistance
   
3. 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:

1. Vats and containers for photosensitive liquids or powder
   
2. Deposition or extrusion nozzles
   
3. 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.