3D Printing and Additive Manufacturing

Kirubanidhi Jebabalan¹, *, R. Rajasekaran², Milon Selvam Dennison³

¹ Department of Mechanical Engineering, Division of Materials Technology, Technical University of Liberec, Czech Republic

² Department of Mechanical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam, India

³ Department of Mechanical Engineering, Kampala International University, Western Campus, Uganda

Abstract

At present, the requirement for new product development and upgrading of the existing product have become inevitable in the manufacturing scenario. The manufacturing sectors are striving hard to sustain in the global market, hence they are continuously seeking rapid manufacturing technologies for developing new products as there is a demand for innovative designs with enhanced features. Conventional manufacturing technologies have certain shortcomings, such as long production times, and are inherent to material wastage due to the subtractive nature of the processes. To meet the demand, it is necessary to accelerate the product development process. The time spent on the design, manufacturing and testing of a product has to be shortened. To emphasize the part representation (or) to rapidly create a system, the prototyping part is ‘Rapid Prototyping’ (RP), and the technology is ‘Additive Manufacturing’ (AM); it is also popularly known as ‘3D Printing’. AM is a novel manufacturing technology as the products are fabricated by adding successive layers of material with the aid of a computer. A Computer Aided Design (CAD) model is created and exported as a Standard Triangle Language (STL) file that is readable by an AM machine. There are many techniques available, which can be categorized according to their raw material. This chapter comprehensively reviews the AM techniques, the applications and the various materials used to produce the AM component.

Keywords: 3D printing, Additive manufacturing, Ceramics, Computer-aided design, Direct energy deposition, Functionally graded materials, Fused deposition modeling, Laminated object manufacturing, Manufacturing, Metals and alloys, Nanocomposites, New product, Polymers, Powder bed fusion, Rapid manufacturing, Rapid prototyping, Selective laser sintering, Solid state sintering, Stereolithography, Sustainability.

* Corresponding author Kirubanidhi Jebabalan: Department of Mechanical Engineering, Division of Materials Technology, Technical University of Liberec, Czech Republic; Tel: +420774503844; E-mail: kiruprod@gmail.com.

INTRODUCTION

Mass customization is the future of ‘manufacturing’, and the technique which enables this is known as rapid manufacturing. It is the new order of the day where the engineer does not compromise on the cost and quality, yet achieves customer satisfaction. The best example is designing a disabled-friendly car seat for the entry of physically challenged people who want to enter and exit the car on their own without the help of another person. The manufacturing process here is called rapid manufacturing, and the technology followed to achieve this is called Additive Manufacturing (AM) [1, 2]. There have been numerous cases where AM has played an important role. For instance, in the medical field, a person who has lost a limb can get back the limb manufactured through reverse engineering techniques with the help of a CAD model functional prototyping. Depending on the use of the limb, a proper material will be chosen; for example, silicon material can be chosen for the ear, since it is more flexible. For a better understanding of our readers, the process of generating a customized product for the end-user is known as rapid manufacturing, but the route employed is Additive Manufacturing. Thus, we will see how a process has evolved over a period of time from producing polymer-type prototypes to metal-type prototypes. In the heart of this revolution lies the device 3D printer. Additive Manufacturing (AM) is popularly known as layered manufacturing [1]. AM is a layered built automated fabrication process that is used to build a 3-Dimensional object from 3D CAD data, ushering the fourth generation of the industrial revolution, which integrates computer and physical processes better known as Cyber-Physical integration. AM provides the third vertical that gives completion to the manufacturing process. There are three verticals, such as subtractive vertical, constant volume vertical and additive vertical. In the subtractive pillar, we have conventional and unconventional machining processes like milling, turning, etc. In the constant volume, we have metal forming and metal casting, and in the additive pillar, we have joining (welding) and AM.

AM is an evolving technology that arrived on the scene nearly 3 decades back in 1987; during that time, it was popularly known as Rapid Prototyping (RPT). It is called a young technology because the standardization for the process was given only in 2009 by the American Society for Mechanical Engineers (ASME) in cooperation with the American Society for Testing and Materials (ASTM). In the autumn of 2009, only the ASME conveyed a subcommittee ‘F-42’ and coined the term ‘Additive Manufacturing’, which was mainly a layer-based technology, and gave ‘F2792-12a’ standard. But this standard has been withdrawn as of 2015, with no other standardization being assigned to it [1]. Additive manufacturing contains three stages; they are 1) Preprocessing, 2) Processing and 3) Post-processing. Once the 3D image is captured, then it is subjected into slices by the computer. Finally, the contour information is obtained by using the special kind of software and physical processing layering technique. Here the customer or the end-user can give their input at any time. The slice thickness of a component can be varied in each layer while intra thickness varying is in the early stage of research. We will showyou how a product attains a final shape using a flowchart representation shown in Fig. (1).

Fig. (1))

Process flow in additive manufacturing.

AM is characterized by ‘Rapid Prototyping’ and ‘Rapid Manufacturing’, which are two main areas of this technology. Rapid prototyping refers to the production of prototypes intended for the specific application, whereas rapid manufacturing is used when there is a need for the final product. Rapid prototyping has two sub-stages, such as solid imaging and concept modeling. In these stages, the 3D image of the product is captured, and it is followed by functional prototyping; this is applied to allow examining and authenticating one or more critical remote functions of the final product or to give any alterations to the model [3].

Next is rapid manufacturing, which is a route for delivering the final product. The final product obtained through this process is called an AM product. Rapid manufacturing has two sub-levels, namely direct manufacturing which is a positive process and direct tooling which is a negative process. The major difference is when the product from the prototype becomes the finished product, it is called direct manufacturing which is positive, and if there is an involvement of die-like molds, it is called direct tooling, which is negative in nature [3]. The relationship and various sub-processes of AM are portrayed in Fig. (2).

Fig. (2))

Relationship and various sub-processes of additive manufacturing.

To summarize the introduction on AM, our readers will be able to understand the technology behind the process, the evolution of the process, and how this technology has penetrated the various levels of applications, ranging from a dental implant to an aircraft component. AM is an ever-growing field of research; our future discussion will also span the materials used in the various processes employed to achieve the final product and its application in various sectors along with advantages and limitations.

3D PRINTING PROCESS

3D Printing is the process of developing a three-dimensional object. Here our readers need to understand the difference between the ordinary printing technique employed at offices which transfers 2D laminate information in one sheet of paper, whereas the 3D printing process is used to generate or fabricate 3D products. This technology was first developed by ‘Z Corporation’ with 24 colors and a resolution of 600 dots per inch (dpi). In this topic, we travel from prototyping to the final product development. The word printing has extensively evolved from desktop printing to industrial electronic printing. This technology has rapidly developed to such an extent where electronic circuits can be printed in garments. These technological advancements have bypassed the stage of research mostly and have come to direct application, especially in the field of electronics and optics. The creation of 3D objects is becoming a promising area of study, which has been widely studied and applied in recent days.

As discussed, printing is used to fabricate 3D parts. There are two technologies involved: 1) Direct part printing, and 2) Binder printing technology.

In the direct part printing, we can directly obtain the product where all the material is distributed directly from the printing head. In the binder printing technology, the material is built one over the other to develop a 3D product. When we talk about binder printing, the entire block is not released; instead, binders are released from which we develop the 3D component. There are certainly technical challenges in the material development sector when polymers, metals and ceramics are used for printing. The readers must understand why printing technology is more talked about; it is a well-established technological process that can be used for mass production and it is economical too. In recent years, optical fibers have been woven into clothes, and once light passes through, we can see the clothes glowing in different shades of light. Printing has also moved to the next level, especially in the field of medicine, where doctors and engineers have combined to form tissue printing and organ printing. When the technology develops, people usually talk about different types of machinery but here with the development of printing technology, the quality of the product and consistency in production have been adapted in AM.

The Binder printing process is a 3D printing technology where the binder is printed into a powder bed to form a component. When one layer is printed, the powder bed is lowered, and a new layer of powder is spread on it. This process is continuously repeated until the desired part is obtained such that we can obtain 10 to 15 parts in one go. A range of parts is completed because the printing head contains several ejection nozzles.

To understand this process, the schematic setup of a 3D printer is given in Fig. (3). The powder is spread on the top of the movable platform and the lead screw attachment in the setup enables the ‘to and fro’ movement across the z-axis. The powder is spread on the top of the table evenly using an inkjet print head from which binder droplets fall on the table. Once the binder and the powder mix, they agglomerate, and the process is continuous until the final part is obtained, which is called the green part since it possesses less strength. For increasing the strength, the part is immersed in polymers so that the core is strengthened. The finished part is left on the powder bed for a certain period of time, mostly for the binder to set. For post-processing, the finished part is taken and annealed. The temperature along with the time is set according to the binder used so that the binder properly joins with the powder [4]. If you need to color your product, the color can be added to the binder, and we get a colorful product.

Fig. (3))

Schematic diagram of the printing process.

THE PROCESSING ROUTE FOR ADDITIVE MANUFACTURING

AM was initially seen as suitable for the production of polymer parts through a process called stereolithography. Stereolithography was one of the earliest processes developed as early as 1988. At this time, the technology was limited to concept modeling and rapid prototyping, but through constant cutting-edge research, the possibilities of achieving near net-shaped metallic components have been achieved. From manufacturing polymer components up to manufacturing metallic components, the AM technology has come a long way for a component to be joined [5]. We will see the various processes and the factors involved in AM technology in the upcoming section.

Stereolithography

Stereolithography is a form of 3D printing where we use liquid photopolymer; the process of polymerization takes place (photochemical reaction) to produce polymer parts. This technique is used in the production of precise and comprehensive polymer parts. In the early 1980s, Japanese Scientist Hideo Kadama used Ultraviolet (UV) Rays as a source to cure photosensitive polymers [6]. But this technology was patented only in 1984 by Chuck Hull, the founder of ‘3D Systems’ when he filed a patent for this process. He used curable UV rays to stack thin slices of materials. In this process, UV rays are passed from the bottom to the top layer. The schematic representation of the stereolithographic process is shown in Fig. (4). The component to be cured is placed on an elevated platform with a support base and inserted in a tank placed with photosensitive monomer liquid with a thin film of liquid at the top. The power source used in this process is a pulsed gas laser that produces UV beams. When the UV beam falls on the thin liquid monomers, the elevated platform that supports the component is slightly lowered; this, in turn, exposes the fresh layer of liquid monomer. This procedure is repeated at the end of the process, and the finished product is obtained which is then subjected to curing after which excess resin is removed.

Fig. (4))

Stereolithography (SL) process.

When a process starts from a liquid solution, a solid product is obtained at the end of the process. It goes through solidification, and it can be done by 1) Free surface mode which usually occurs when the resin/air interface takes place, 2) Fixed surface mode where the resin is stored in a container with a transparent glass reaction between resin/glass causes solidification. There are two process variants, namely 1) Scanning lithography and 2) Projection lithography. The major limitation of this process is the expensive equipment and materials.

Powder-Based Process

The powder-based process is of three types: 1) Polymer-based, 2) Ceramic based, and 3) Metal-based. The polymer has a low melting point, metal has a medium melting point and ceramic has a higher melting point. From the melting point, one can conclude that if the end-user wants a polymer-based material, then the end-user has to use less heat. While processing a polymer, it does not undergo the melting state but a visco-elastic state (the combination of elastic and viscous behavior). There are a few processes under this category, like Selective laser sintering and Solid-state sintering. All the powder-based processes follow the same set of basic processing routes. More than one heat source induces fusion between powder particles. We will see each process one by one. The word sintering pre-existed even before AM was invented, which means fusing powder before undergoing the melting stage.

Selective Laser Sintering (SLS)

From the name itself, we can find out that the heat source used in Selective Laser Sintering (SLS) is a Powder Based Fusion (PBF) process. It was the first commercialized PBF process which was developed at the University of Texas at Austin by three scientists, namely Dave, Carl Deckard and Joe [7]. The technology was later acquired by 3D Systems in 2001 [8]. SLS was originally developed for plastic prototypes with the use of pointwise laser scanning technique which is somewhat similar to the stereolithography technique where the laser source is vector-based. With the advancement in technology, it was later extended to metals and ceramics with the additional use of thermal sources. We have been able to achieve a layer-wise fusion of materials as a result of technological advancement [9]. The schematic diagram of the process is shown in Fig. (5).

In this process, there is a table that is divided into three segments in that the two segments are feed cartridges which are the sources of input, and the single segment is the middle one known as the build platform. The powder is the input, and powder material ranges from polymer to ceramics based on the feed cartridges. The material is transferred from the cartridges to the powder bed. Infrared heaters are placed above the table to maintain the temperatures of the parts to be formed and to preheat the powder, thereby minimizing the requirement of the laser. The CO2 laser is the source used in this process, and the role of the roller is to spread the powder having thicknesses less than 100μm across the built area.

Fig. (5))

Selective laser sintering (SLS) process.

Now coming to the process, the powder is deposited on the platform and the focused laser beam is used to sinter the powder within small volumes across the layers. The laser beam is adjusted by the galvo mirror, which can move in the x-y direction that helps the powder to be thermally fused. After completing a particular layer, the platform is lowered. A new layer of powder is introduced, and the excess powder present is leveled out with the help of the counter-rotating roller. The laser beam is again used to scan the 2D object slice by slice, and this process is repeated until the product is formed. The product that has been formed will have an unequal temperature, so it is allowed to cool down so that the parts can uniformly come to an ambient temperature, and we can handle it once the finished part is removed from the powder bed.

Solid-State Sintering

This process takes place at a temperature between half of the total melting temperature and the melting temperature. The word ‘solid’ indicates that the powder fuses without undergoing the melting stage. The mechanism for solid-state sintering is diffusion between powder particles.

The governing equation of this process is given as Es = σs*SA, where the surface energy (Es) is proportional to the total particle surface area (SA) (‘σs’ is the surface energy per unit area for a particular material at a particular temperature and atmospheric condition).

When the particles fuse at an elevated temperature, there is a rapid decline in the surface area that leads to a decrease in surface energy. When a heat source is supplied, the un-sintered free particles will agglomerate. When the temperature of the particle increases to half of the absolute melting temperature, it causes the free energy to decrease over the surface area, resulting in a decrease of pore size and an increase in neck size as the sintering progresses. The driving force for the sintering process is directly related to the surface area to volume ratio due to increase in surface area to volume ratio; it results in the increase of free energy which is the driving force due to the reduced surface area experience, thus sintering occurs at a lower temperature than larger particles. Limitations in this process begin when the heating temperature approaches the melting temperature of the powder; at this point, the fusion of powders becomes very slow [9, 10].

Extrusion Process

A solid material when squeezed through a nozzle comes out in a visco-elastic state. The extrusion process can be imagined like toothpaste coming out of its container. The extrusion also follows a similar principle if the pressure applied in the process is uniform, and then the extruded material is called the path. The material that comes out in a semi-solid state solidifies over a period of time and remaind in its net shape. There are seven important steps or key features that have to be followed for an extrusion process, as shown in Fig. (6).

Fig. (6))

Extrusion process [11].

When we use the extrusion process, there are two factors to be considered; one is temperature and the other is inducing chemical change. The temperature used in the conventional extrusion process is used to liquefy the material, and the liquefied material goes and bonds with the already existing material, leading to solidification. In the chemically induced process, certain curing agents are added to alter the solidification time, and this kind of process is used mostly in a biochemical application where biocompatibility is required as the component has to work along with living tissues [9].

In the following section, we are going to see various processes that employ the extrusion principle.

Fused Deposit Modelling (FDM)

In the late 1980s and early 90s, companies were looking for an alternative for the already existing stereolithography process in the exploding 3D printing business market. The company which commercialized and patented this process in the year 1992 is Stratasys 3D Printing Solutions, Eden Prairie, USA [12]. FDM is suitable for polymers that are amorphous in nature. The polymers that are crystalline in nature are more suited for the PBF process since amorphous polymers do not have a specific melting point when extruded under high temperatures. They can maintain their shape, and thus, can solidify very quickly. The most used material is a type of thermoplastic polymer known as Acrylonitrile Butadiene Styrene (ABS) [9]. Understanding the extrusion process through mathematics or physics is highly impossible because most of the terms are non-linear in nature [13]. The schematic diagram of the process is shown in Fig. (7). A spool of thermoplastic wire of diameter 300 µm is supplied continuously through the nozzle. The material is mostly ABS, the polymer that is fed through a spool in a liquid state. The heated up nozzle ejects hot strands of viscous wires. The nozzle is moved with the help of a computer-based mechanism along the x-y axis. Each cross-section of the component is produced by melting the wire, which subsequently solidifies on cooling. The size and shape of the component are determined by the nozzle and the rollers are usually used to push the extrusion rod. The major advantage of processing polymers is the economic limitation, and this process has a lower printing speed than stereolithography.

1. Sheet Stacking Process

It is a transformation process from Rapid Prototyping to Rapid Manufacturing.

Fig. (7))

Fused deposition modelling (FDM).

The sheet is a 2-Dimensional laminate, and it can be polymer-based, ceramic, composite or paper. It can be used to manufacture components with unrestricted complexity, i.e. drafted directly from the computer. These processes can be further classified based on the various mechanism used to achieve bonding between layers, such as 1) Gluing, 2) Clamping, 3) Thermal bonding and 4) Ultrasonic welding. In this type, we are going to discussabout the process known as Laminated Object Manufacturing.

Laminated Object Manufacturing (LOM)

It was one of the earliest AM techniques developed in 1991 by Helisys Inc., USA. Before the American ‘Helisys’, a similar kind of machine was manufactured by a Japanese company called ‘Kira’ which had a machine known as a solid center machine. The major difference between Helisys and Kira is Kira’s company employed blades for cutting out profiles. These blades were driven by a 2D plotter drive. The Japanese make machine used heat-activated adhesive along with laser printing technology to bond the layers together [14]. An Israeli company known as ‘Solido’ used plotter drives to bond the layers [15].

The CO2 laser was used as the power source. The main objective was to cut sheets that are supplied through a continuous roller. These sheets get piled up one over the other to get the final product. The layer of sheets that are stacked one over the other have to be bonded; for these, heat-activated resins are used. These resins are glued on the surface of sheets when sheets are stacked and when the heat is supplied, the resins get activated, leading to the formation of a block of sheets. The rough unfinished blocks are hatched, and this leads to the excess removal of materials, thereby revealing the final product.

The key parameters of this process are rollers, temperature, laser source and hatch size. These parameters have to be optimized to prevent delamination of sheets [16]. Sheets of materials are cut according to the required dimensions. The sheets are either pre-cut or they are rolled. Then a new sheet is placed on the platform and glued underneath a laser beam that traverses across the top portion of the sheet and cuts the desired profile on each sheet which represents the computer-drafted model of the part. We use CO2 laser here because the wavelength of the laser plays an important factor when there is an interaction between states of matter; in this case, it is solid and laser [9]. Then the section to be removed is chopped out so that the cutout scrap remains and supports the remaining build-in platform. Another sheet is placed on it, and the process is repeated till the final product comes out in the form of a rectangular block containing the required material. The scrap is filtered out from the required material. Since the machine is versatile in nature, the sheets can be cut and stacked or vice-versa [9]. The biggest limitation of this process is removing scrap parts which are quite laborious. The schematic diagram of the Laminated Object Manufacturing is shown in Fig. (8).

Fig. (8))

Laminated object manufacturing (LOM).

Wire and Arc Additive Manufacturing (WAAM)

The two most prevalent processes in manufacturing metal parts through AM technologies are Direct Energy Deposition (DED) and Powder Bed Fusion (PBF). DED technologies in the aerospace sector include electron beam