BUS606: Operations and Supply Chain Management

3.1. Characteristics of the Models Manufactured Using the Addition of Material Technique

When the three-dimensional model is obtained using a reverse engineering process and the precision of the final model is determined by the scanning process, the virtual 3D modelling, and the additive manufacturing process. If an inverse engineering process has not been used, the models manufactured are determined by the virtual 3D modelling and the manufacturing process used.

We have already seen that during the manufacturing process the model is built by way of the depositing of layers on the x-y plane resulting in solid volume being acquired in the direction of the Z axis. This process is characterised by a volume error between the volume of the virtual 3D model and the volume of material obtained in the model and, therefore, the manufacturing precision is the result of superimposing different errors in the production of the model which affect the surface quality, the dimensional accuracy, and the final weight of the model.

The technology limitations that occur in this process are as follows: an error in the conversion of the 3D model into STL format (triangulation of the geometry), an error in the decomposition in layers of the 3D model (exact division of the thickness), stepping effect error (orthogonal deposition of the material by layers), and, finally, model infill error (Figure 5).

Figure 5 Technology limitations in the rapid prototyping process.

There are studies which show that the models obtained using rapid prototyping techniques have an average error in the majority of the 0.05 mm dimensions with respect to the original model or of modelling control in 3D.

The additive manufacturing of three-dimensional models with an aesthetic (visual) or assembly objective is achieved using techniques that involve the layer-upon-layer addition of plastic materials, while functional models or those capable of withstanding mechanical testing must be manufactured mainly in metal and, in some cases, in a polymeric material that is subjected to a postproduction hardening process.

Studies undertaken by different research institutes show that those products manufactured in metal using additive technologies provide the same or better mechanical performances than the same products manufactured using conventional processes. The resistance to corrosion of products manufactured using additive technologies is similar where the same level of surface finish is involved.

One objective during research is that of obtaining functional prototypes using polymeric materials capable of withstanding mechanical testing using rapid prototyping technologies. The advances made in the field of deposition materials and the subsequent finishing of the model may lead to functional prototypes that do not require the use of techniques based on rapid manufacturing (RM), thereby avoiding tooling costs (see Figure 6).

Figure 6 Function prototypes for testing.

3.2. Technologies and Decision Variables

Some of the technologies described above require the use of a material, known by the name of support material, the purpose of which is to hold any overhanging designed parts in place. Once the deposition process has been completed, the support material must be removed during an operation carried out subsequently to manufacture (postprocessing), and the technique used to remove it shall depend on the support material in question and, therefore, on the additive manufacturing technology employed. In some additive technologies there is no support material as the support function that is performed by the material that has not hardened.

With respect to the mechanical properties of the prototypes obtained via the addition of material, these are determined by the quality of the result of the fusion between layers and the properties of the material. The following parameters (DIN EN ISO 178/179/180/527/2039) must be established in order to analyse the mechanical properties of the materials used in the different additive manufacturing methods: elastic modulus, breaking stress, elongation, flexural modulus, impact strength, compressive strength, and melting point (Table 11).

Table 11 The principal mechanical and thermal properties of the functional materials used in AM.

The choice of the most suitable technique for each type of prototype is based on the definition of the objective behind the production of the prototype: aesthetic, functional, investigational, or visual if the purpose is only to check the external appearance of the item designed. When it comes to making this choice an analysis may be planned based on a study of the possible variables: technology, resolution and precision, materials, software, the mechanical properties of the material (traction, compression, impact, softening, and density), surface finish, production time, cost, maximum dimensions of the item or model, posthardening requirements, guarantee, noise, CE certification, operational temperature, electrical connections and consumption, interface (network, hardware, software and exchange formats), weight, and spares and consumables (Table 12).

Table 12 Decision variables when choosing a prototyping technology.

Of all the aforementioned study variables, those habitually taken into account when choosing the prototyping technology are resolution-precision, the mechanical and thermal properties of the material, surface finish, production time, and the cost of the prototype.

With respect to the evaluation of the different prototyping technologies in accordance with the cost of the prototype, the fact that the comparison of technologies is restricted by the type of machine used must be taken into account. It is commonly thought that those prototyping machines based on stereolithography (SL) and selective laser sintering (SS) can be applied to the industrial production of prototypes, while all the others are seen as being machines that can be used professionally, but not in situations where the main objective is production. The manufacturers are currently offering domestic or desktop, professional, and industrial rapid prototyping machines, and therefore the costs incurred by using these machines must be offset by the performance levels shown above and by the production levels that can be obtained.

$C_p=C_e+C_m+C_t+C_a$

where

If this calculation model is transferred to a specific case, observe the following example of the cost of a prototype generated using the MD technique in a professional grade machine that can be easily adapted to any additive technology. In this case, as can be seen, there is no finishing cost (Table 13).

Table 13 Calculation of the prototyping cost using material deposition (MD) technique.

3.3. The Benefits and Disadvantages of Additive Manufacturing

The processes used to manufacture conventional parts and components are influenced by a series of limitations related to the obtaining of certain shapes, such as curved holes, mould release angles, or preventing tools from coming into contact with geometrically complex pieces. And then there is the fact that some manufacturing processes do not comply with a company's commitment to a sustainable production process by involving the residues related with the use of cooling liquids.

1. The geometrical complexity of the part to be manufactured. Elegant geometrical forms, hollow interiors, internal channels, variable thicknesses, irregular shapes, etc. can easily be reproduced based on the geometrical template obtained from a 3D CAD.
   
2. The customisation of the part to be manufactured. Products that are exactly identical or completely different can be obtained without any notable influence on the process and without additional costs. This customisation represents one of the main current trends in the development of products with a high added value, and the mass application thereof is one of the paradigms pursued by the industrial sectors in developed countries, which see it as being the key to their sustainability.
   

These two characteristics can provide massive benefits in different industrial sectors:

1. Lightweight Products. They enable the manufacture of products designed for a specific function and with made-to-measure features, e.g.: lighter for reasons of weight savings, strength or costs. Some of the additive manufacturing techniques are capable of filling a model with different degrees of porosity without a change of material.
   
2. Multimaterial Products. They make it possible to manufacture a product using several materials simultaneously in the same solid. This means that the technique overcomes one of the current limitations with respect to the weight/mechanical strength ratio by the introduction of new functionalities or the lowering of production costs.
   
3. Ergonomic Products. The design of the components can achieve a greater degree of interaction with the user by adapting to the exact anthropometric characteristics of each individual (prostheses) without necessarily affecting the manufacturing costs.
   
4. Integrated Mechanisms. They make it possible to manufacture a mechanism that is totally embedded in the finished item without the need for subsequent assembly and adjustments, e.g., a journal bearing, a roller bearing, a spring and its support, and a screwed-on worm gear.
   

As far as the production of industrial components is concerned, the following must be highlighted as obvious benefits:

1. A reduction of the time it takes new designs to reach the market: when additive manufacturing is used as a manufacturing technique of the end product and not only in the production of prototypes, many of the current launch and validation phases can be drastically shortened. Another advantage is that it provides great flexibility when it comes to responding to the continuous changes in market demand.
   
2. Short production runs: the size of the production run can be minimal to the extent of being on a per unit basis while hardly influencing manufacturing costs (if and when the depreciation of the equipment is not considered). One of the characteristics that make this possible is the lack of a need for tooling, which represents a considerable advantage with respect to the conventional manufacturing methods.
   
3. A reduction of assembly errors and their associated costs: ready assembled components can be obtained with the only subsequent operation being the quality control inspection.
   
4. A reduction of tool investment costs: tools do not form part of the additive manufacturing process. This represents a great deal of flexibility as regards adapting to the market and a reduction, or even elimination, of the associated costs (toolmaking, stoppages due to referred changes, maintenance, and inspection).
   
5. Hybrid processes: it is always possible to combine different manufacturing processes. In this case combining additive manufacturing processes with conventional processes might be interesting to make the most of the advantages offered by both. For example, it might be extremely beneficial to combine additive manufacturing technology with mechanised material removal in order to improve surface quality via a reduction of the "stepping effect" produced by the additive manufacturing technologies. Hybridisation can also occur in the opposite direction, in other words manufacturing using subtractive methods starting with a block before adding, by way of additive manufacturing, those especially complicated characteristics which generate high value.
   
6. Optimum usage of materials: material wastage is reduced to a minimum. Any waste material can be easily recycled.
   
7. A more sustainable manufacturing process: toxic chemical products are not directly used in appreciable quantities.
   

However, additive manufacturing technologies do have a number of drawbacks which must be borne in mind when choosing the technology best adapted to the requirements of the product to be manufactured.

1. Additive layer manufacturing produces what is known as the stepping effect. The disadvantages of this phenomenon include complicating the shaping of geometrical curves and an extremely rough surface finish. This effect means that shafts and holes must typically be manufactured with their circular cross-section in plan. If they are not, the roundness of the piece would not be acceptable. On the other hand, and putting roundness to one side, positioning the piece in another way might be useful depending on the application in question; it would be interesting to manufacture an overturned sliding axis in such a way that no 'interlocking' occurs.
   
2. With respect to some technologies, the manufacturing operation itself can be slow, thereby making it particularly suitable for small production runs. When the production run reaches a certain size it may well be appropriate to use a conventional technology despite the fact that, as has been seen above, these technologies have a number of limitations, especially geometrically speaking.
   
3. The materials used in some of the technologies might not be suitable for the product to be manufactured.
   
4. The depositing of layers produces anisotropic materials. Given the fact that many industrial components are subjected to forces that put the material under stress and that they are so sized as to use the minimum amount of material, it is possible that the performance of the components with respect to the forces they must withstand while in service results inadequate.
   
5. The tolerances obtained using the majority of the additive manufacturing methods are still higher than those achieved using other manufacturing methods such as those based on the removal of material.
   

3.4. By Sector Innovation with Additive Manufacturing

Both in innovation and in research, advances are going to be defined by acquiring new materials, more precise, and less costly equipment and also by seeking out new sectors for 3D printing.

An interesting proposal in this field is that presented by Wong and his team in 2012. Six sectors are analysed: lightweight machines, architectural modelling, medical applications, improving the manufacturing of fuel cells, and additive manufacturing for hobbyist and additive manufacturing in art. We have no doubt that these were the sectors of innovation five years ago. However, our analyses show us that nowadays the additive manufacturing sectors where innovation can really be seen are as follows: consumer products, automotive industry, medicine and medical engineering, aviation industry, architecture, construction, and food.

The degree to which additive manufacturing is used in different sectors is shown in Figure 7.

Figure 7 The use of additive manufacturing in the different sectors.

A review of the sectors in which additive manufacturing is currently used is presented as follows.

3.4.1. Consumer-Electronic Products

This sector uses additive manufacturing to obtain prototypes and models of a multitude of articles for the home, sports equipment, toys, etc. It is the number one customer of those additive manufacturing technologies that enable the direct digital manufacture of finished components of high geometrical complexity and that require customisation.

As soon as materials that are both flexible and strong even when thin become available, it shall be possible to manufacture consumer products such as clothes and footwear using additive techniques. The deposition of conductive materials via the printing of passive circuit components such as resisters, condensers and coils, diodes, organic light emitting diodes (OLEDs) and circuit interconnections can only benefit the production of electronic devices and components.

3.4.2. Motor Vehicles

In this sector additive manufacturing is being used to create prototypes that enable the validation of engineering processes and, above all, functional and aesthetic component design processes. The production of finished parts is not yet a reality, with the technique only being used in the customising of certain elements in one-off vehicles. The hope is that the development of new materials and their application of large, high-speed machines will favour the use of additive manufacturing in conjunction with the highly demanding production criteria inherent to this sector.

3.4.3. Medical/Dental

The application of additive manufacturing in the medical/dental sector enables physical 3D models to be obtained from processed medical images (3D scans, TAC) for application in different specialist areas.

The use of additive rapid prototyping technologies enables preoperative planning processes, the production of prostheses, and the preparation of surgical templates and guides to be carried out with a higher diagnostic quality and greater surgical safety in less time and more cheaply than is possible using conventional manufacturing techniques. In the case of specific and customised implants optimum planning of the surgical process and a reduction of operating times has already been achieved.

We must not omit today's 3D printing of living cells, bioprinting, in which a lot of resources are being invested and which we trust will soon present some very interesting results.

3.4.4. Aerospace

This market requires additive manufacturing to respond to high mechanical and thermal performance demands, weight reduction, and minimum losses of material as regards certain components with respect to both polymeric and metallic materials, primary titanium, and nickel alloys. The selective sintering of powdered metals has become a manufacturing, repair, and maintenance solution for certain components, e.g., turbine blades, as well as for the manufacture of high added value aeronautical tooling.

3.4.5. Architecture

The manufacture of mock-ups and prototypes within the architecture and construction sector was, and still is carried out on a significant handicraft basis. The development of assisted design systems, with its resulting progress towards solid modelling systems and the current BIM systems with respect to building, has enabled the production of highly attractive quality digital mock-ups, infographics, and virtual animation of plans and projects. However, the same cannot yet be said about the physical mock-ups obtained from that digital model of the plan using additive mock-up and prototype construction machines. 3D printing could well become an essential piece of equipment in the studios of architects and designers. In Figures 8 and 9, we can see a number of examples of how these techniques are applied and they clearly show the great potential of additive manufacturing.

Figure 8 AM applications.

Figure 9 Another AM applications.

3.4.6. Food

Even when the catering industry is incorporating new 3D printing techniques for food, perhaps it is a good idea to discuss the advances being made in this field with regard to the food-health combination, that is, when 3D printing systems are used to measure out food and structure patients' diets.

However, naturally, in the field of catering and food in general, major advances are being achieved, which are already in operation in prestigious restaurants or in the production of desserts and sweets.

3.5. By Sector Investigation with Additive Manufacturing

In the field of research, that is, in the field of the work that is being carried out today in research centres and universities, both public and private, there are three approaches that stand out: materials, equipment, and new fields of applicability.

In the field of new materials, a lot of progress is being made on biocompatible materials, compound materials, and metals, each one within a clearly identified field of development. In these fields, materials with good mechanical properties are also being sought; however, at the same time, progress is being made on the development of elastic materials, which open up a whole range of possibilities for 3D printing that is yet to be quantified.

In terms of new equipment, greater precision and lower cost are clearly what is being sought after, in order to make this technology competitive with regard to other conventional technologies.

With regard to the new fields of applicability in which, without a doubt, great progress will be made over the coming years in terms of printing metal materials, we resolve the problem of the dispersion of metallic liquid when it reaches molten state, and in terms of bioprinting, we develop new applications with regard to living tissues, not only in bones and cartilage, where advances have already been made, but also in other tissues of the human body, including the viscera and muscles.

3.6. The RepRap Community and Free Software

In 2004 Adrian Bowyer founded RepRap, an open-code initiative for building a 3D printer capable of printing out most of its own components, at Bath University. The vision of this project is to make the manufacture of low-cost distribution units available to people all over the world, thereby enabling them to create their own products on a daily basis.

As the term 'Fused Deposition Modelling' had already been registered by Stratasys, the RepRap Community has coined the term 'Fused Filament Fabrication (FFF)', which can be used by anybody without any restriction whatever (under a version 2 GPL licence). Under these terms and conditions anybody can distribute and modify the RepRap machine, but they must respect the modifications made under this licence. In other words, all changes must remain in the public domain. As the machine is both free and open-code; anybody can, without having to pay any fees whatever, build an unlimited number of copies for themselves or for anyone else using the selfsame RepRap machines to manufacture the plastic parts of the copies (thereby making it self-replicating, Figure 10).

Figure 10 Example of RepRap machines and parts for replication.

Although we know that the RepRap movement has played and continues to play a very important role in the development of additive manufacturing, it is interesting to discuss here that it was not initially accepted by the academic community, as it was considered a subject of minor importance. Only a few years later the subject was accepted, however not as a phenomenon in itself, rather under the consideration of a machine that could replicate itself, something that every manufacturing professional knows has been possible since the 19th century, with milling machines.

There is an increasing number of meetings of "experts" who analyse what the 'printing' of physical objects using 3D printers will represent, which is now being seen as one of the great industrial revolutions of the next few years, and there has even been talk of the Third Industrial Revolution. The #Redada sessions are a meeting that allows users and professionals to exchange ideas and analyse the possibilities open to them, as in the case of Video 34, "#Redada 18 Madrid: The Challenges of 3D Printing", which features a debate about the social trends and the aspects related to culture, civil rights, and technology.

3.7. User and Exchange Communities

The social importance of this technology has been enormous and, as has already been said, it is developing in leaps and bounds, thereby enabling engineering students and professionals who specialise in these particular techniques from all over the world to experiment with their creations and perfect them prior to the production of full scale versions.

Accessibility to the technology links a series of communities of users and developers who exchange their know-how and experiences in order to continue perfecting the printing system and open up new and previously unimaginable fields in the process.

Alongside this, these communities of users have developed a series of platforms for exchanging existing 3D models for downloading and printing, thus broaching a far from ludicrous idea that involves manufacturers making 3D models of ex-catalogue parts of their products available to users. To a certain extent Google Earth has already done this by allowing the community of Google Sketch Up users to upload models of buildings from all over the world in their exact location for everybody to enjoy. The fact that the future holds countless possibilities cannot be too highly stressed. There are model upload and download banks such as the English language platform Thingiverse and its Spanish counterpart Rascomras. Many more exist, and their number increases by the day, some more creative than others, such as The Pirate Bay (a community for the downloading of all types of audiovisual material which has incorporated a new section for 3D models).

The Clone Wars Projects seek to spread the word about RepRap technology while at the same time contributing new designs and innovative research channels, but not so much along self-replication lines.

A series of workshops have existed all over the world for a number of years now. Known as FabLabs (Fabrication Laboratories), they are being promoted by the Center of Bits and Atoms (CBA) of the Massachusetts Institute of Technology (MIT), at which a lot of hard work is being done on this technological revolution in light of the social changes it is bringing about. They are equipped with a series of computer-controlled machines "for building (almost) anything": 3D printers, laser cutters, CNC (computerized numerical control) routers and an electronics laboratory (among many others that vary from workshop to workshop). Worldwide, with respect to the number of Fablabs Spain ranks fourth with 7-8 behind the USA, with more than thirty, The Netherlands (9), France (8), and ahead of Germany (6).