Additive Metal Technology: Research into Decision Making Guides

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Additive Metal Technology: Research into Decision Making Guides

1.0 Abstract

In this research prospectus, the background and significance of Additive Manufacture (AM) were described. Unsolved problems are introduced for the future work summary. Two case studies were discussed. The first case study – Plastic Profile Extrusion Die Design in AM Way has been summarized as the preliminary result for the last year research. The ideas of second case study – Medical Instruments & Implants Design in AM Way has been introduced as well for the future work. An algorithm for technology selection is going to be developed in the future work.

2.0 Introduction

2.1 Background and Significance of Additive Manufacturing (AM)

Additive manufacturing (AM) is a rapidly growing manufacturing technology in recent decades. According to ASTM [1]: “AM can be defined as a collection of technologies able to join materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies”. This definition can be applied to all classes of materials including metals, ceramics, polymers, composites, and biological systems[2].

Additive manufacturing has received much attention for the advantages of free feature constrains, less waste of material , complexity- free and little pre-process time[3]. For short, the additive manufacturing is the process that generate model by directly using a three-dimension Computer-Aided Design (3D CAD). AM technology significantly simplifies the process of generating complicated-feature 3D models. On the other side, in traditional 3-axis CNC milling, these complex features require a large amount of time and efforts from an experienced technician on detailed analysis of the part geometry to determine some critical parameters like which tools should be used, which tool path is the best and what additional fixtures are required. In additional, AM technology even makes some features like undercut enclosures, and sharp internal corners that cannot be machined possible to be manufactured.AM technology generates parts directly from 3D solid model by selectively curing, depositing or consolidating materials in successive layers which significantly simplify the process of generating complicated-feature parts.

The basic idea of how AM works is that parts are made by stacking material layer by layer; the material is added on the plate and the next layer will be added on the previous one and so on; each layer is the thin cross-section which is sliced and derived with a certain finite thickness from the original 3D CAD data. We assume that every single layer is a flat plate. The accumulation of layers introduces a concept called step effect as illustrated by Fig.1 which results parts generated by AM technology an approximation to the designed models; so it is obviously that the thinner each layer is, the closer the generated part will be to the original design.

   

(a)                                     (b)                                      (c)

Fig.1 CAD image of a teacup with further images showing the effects of building using different layer thicknesses[4]; (a) 3D CAD model in Software, (b) 3D printed cup with coarse layer, (c) 3D printed cup with fine layer

This AM technology has brought significant effect on rapid prototyping. Rapid Prototyping referred to as a Solid Freeform Fabrication which can rapidly create models directly from digital data in hours or days with little need for human intervention. Many researches have been done to discuss the methodology and economic advantages of using AM technology to create functional prototypes.[3, 5-7] We can use AM technology to rapidly create a prototype of the product before mass production to confirm the functionalities and feasibility of this design without complicated process planning, fixture fabrication or mold fabrication which can save both time and money in design process.

Metal Additive Manufacturing (MAM) is a process using metal as material to additively manufacture parts. This is a relatively new technology; however Metal Additive Manufacturing (MAM) has had a tremendous impact in reimagining the design and manufacture of products in a number of industries. The use of Metal-AM technology has also been applied on making functional parts in some specific areas. In order to make a part, achievement of the function of this model is the most significant and difficult. The functional models created by Rapid Prototyping can be used for extensive tests, and based on the results of these tests; the new products can reach market in the shortest time which is very profitable and desirable. Today, they are used for a wider range of other applications and creating very small batches of parts. General Electric has been using Direct Metal Laser Melting (DMLM) technology which is one of the AM technologies to manufacture 3D-printed parts for the engine GE9X, the world’s biggest commercial jet engine (Fig.2) [8]. AM technology is also implemented to medical implants and instruments. Because most of the implants and some of the instruments have to have some features fit to an individual’s body, like organs and skeletons, which makes these features very variable in sizes and shapes, these features have to be customized for every single patient. The customization and unstandardized shape for every patient makes the mass manufacture for these parts costly or even not possible.[9] Furthermore, these features (see Fig.3) could be very complicated which is not able to be done easily like a standardized part. All these reasons make AM technology perfectly compatible to the medical area.

Fig.2 GE9X Engine (Referred from https://qz.com/667477/ge-fires-up-worlds-largest-commercial-jet-engine-using-3d-printed-metal-parts/)

Fig.3 Knee Replacement Implant (Referred from https://bonesmart.org/knee/choosing-a-knee-implant-prosthesis/)

AM technology looks like a very universal technology to one-run product; however, the impact of AM technology on real manufacture for functional parts is still limited. Admittedly, AM technology has numerous advantages over the conventional technologies as we discussed above, but AM technology still has its own disadvantages, and there are some other considerations need to be figured out before or during AM process. The rough surface condition is one of the major issues with the AM technology; the surface of the product after additively manufactured is not sufficiently smooth for most of the functional parts, and a post-process for polishing the surfaces is probably required in most cases. Because of the complicated geometry, the polishing process can be difficult and costly. Secondly, the appropriate manufacturing orientation needs to be determined before manufacture. Because the part is manufactured layer by layer successively, each layer has to be supported by the layer underneath it. To build overhanging features correctly, the use of sacrificed supports is necessary to support and hold overhanging features from collapse. [10] A properly oriented part during AM can efficiently reduce the amount of wasted material, save processing time, and save effort and time on removing the supports. Especially for the metal-AM, the removal of sacrificed supports is tedious and very time-consuming, which also increases the possibility to damage the part. Furthermore, the residual thermal stress in the manufactured part is also needed to be considered. Because the metal powder is sintered by the laser beam with very high temperature in extremely short time, the expected temperature changing rate is very high compare to the conventional manufacture operations, like CNC. Rapid heating of materials followed with rapid solidifying of the product can result in residual stresses that can cause part warpage, cracks and other undesired effects on the final part. [11]  At last but not least, the dimensional accuracy of the additively manufactured parts is not able to be hold within very tight tolerances  compare to the conventional CNC. There are more challenges need to be solved in the future, see Table 1. All these existing challenges of Metal-AM prevent more exposures of this technology to the other manufacturing areas.

Table 1 Summarized Advantages and Challenges of AM Technology[12]

2.2 Description and Significance of Unsolved Problem

In the future work, an algorithm needs to be developed for technology selection. The standardized approach of determining if a part needs to be additively manufactured is going to be introduced. All the parameters for making decision should be pulled together. A few more case studies need to be included to verify the feasibility of the developed algorithm. Furthermore, the possibilities of applying AM technology to the established technology, like case study 1, are going to be discussed for improving the product quality and manufacturability.

Before manufacturing a desired part in the subtractive methods, we have to plan the manufacturing process. We need to know the order of manufacture steps, and also the technology we using for each feature. Similarly, the technology selection is also very important for the application of AM technology. Although the AM technology is capable of making every feature, it is not reasonable to make every part in AM; just like it is not reasonable to use CNC to saw a big cylindrical part. We cannot use the AM technology for every single part. We have to determine which part is more suitable and reasonable to be additively manufactured as opposed to established manufacturing methods.  A wrong chosen technology may cause unnecessary costs and efforts, and also manufacture products with poor quality. See Fig.4 for the two sample parts; the first one is a solid block, and the second one is a hollow block connected with ribs. If we are using the AM technology to make the first part, it can be done, but the subtractive methods can make the manufacturing of this first block much easier, more efficient, and even more dimensionally accurate than using the AM technology. However, the second part is more reasonable to be done in the AM technology, especially when the block size is very small which makes the driller of 3-axis CNC unable to drill through the block without damaging the ribs or break the miller later because of the small radius of driller and miller.

  

(a)                                                                     (b)

Fig. 4 3D Model of Two Sample Parts (a) Solid Block, (b) Hollow Block connected with Ribs

The sample in Fig.4 might be very distinguishable for the technology selection; however it’s not always the case. Sometimes the geometry can be very complex, and there could be hundreds of unstandardized features included. This can take an experienced designer very long time to decide the technology selection. Furthermore, as the AM technology is stepping into the new era of manufacturing functional parts, there are more and more manufacturers going to explore the possibility of using AM to replace some of the established technologies, and different approaches for selecting technology are going to be implemented in their own factories. For the reasons above, a standardized and automatic approach to determine the technology selection is becoming highly demanded.

Exploring the new applications of AM technology is very significant. Some manufacture methods have been used for very long time, but the technology of AM has only been developed for decades, and the AM technology is developing in incredible speed. There are more material can be manufactured in AM technology, running speed is much higher than decades ago, and the cost of material and machine are reduced than years ago.[13, 14] It is a good timing to explore if this new technology can replace any old technologies for making the production rate higher and better-quality products.

3.0 Evaluate Applications of Metal-AM on Real Industries

3.1 Case Study 1: Plastic Profile Extrusion Dies

3.1.1 Abstract

In the field of plastic technology, plastic extrusion technology is one of the most widespread and important technologies. Many plastic products you can see in our daily life are made by this technology, such as pipes, tubes, cable coatings, plastic bags and bottles. Meanwhile, plastic extrusion is also a technology with very long history. We have been using it for nearly two centuries. Among all the plastic extrusion processes, plastic profile extrusion is the well-known significant and difficult one to design and manufacture. The main uses include window framing, tubing, and piping, and so on.

Profile extrusion is one of the major methods of plastic modeling and processing. The polymer melt is forced to pass through the flow channel in the extrusion die by great back pressure to get the desired profile geometry, which makes the profile extrusion a continuous, and universal plastic manufacturing process. There are literally infinite profiles can be produced by extrusion process, but the material to be extruded for profile extrusion is mainly concentrated on uPVC which has very good chemical resistance, excellent weather ability, self-extinguishing (flame retardant), and good insulation properties.

Although an extruder is assembled by dozens of components, the most influential component to the final product is the extrusion die. See Fig.5 for a sample of extrusion die. The extrusion die has three main functions, distributing polymer melt flow, stabilizing polymer melt flow and shaping the extrudate. Obviously, a better die design can make plastic products have better performance however the cross-section of the extruded part is not really the same as the die profile which makes die design a very complex and time-consuming process. Furthermore, with the complexity of profiles and combination of materials, the design for a perfect die becomes very hard even for an experienced designer. In industry, ‘trial and error’ method is applied for extrusion die optimization because of the large amount of variables involved during plastic extrusion manufacturing and the complicated geometry of an extrusion die. For each of these reworking iterations, the die needs to be disassembled and remounted onto the machine, and then the resulting profile needs to be examined again. During the entire reworking process, the extrusion machine has to be stopped and restarted which is very consuming for both time and money.

Fig. 5 Demonstration of Sample Extrusion Dies (Referred from http://www.pri-plastics.com/extrusion)

3.1.2 Introduction to the Current Technology

Most of companies, 2D Computer-Aided Design software are mainly used instead of 3D software because the manufacturing technology we commonly used to make extrusion dies is wire-EDM (Electrical Discharging Machinery). See Fig.6 for the real wire-EDM machine on floor. The software we have been using a lot to program the path of wire is called Esprit. With this CAM software, we only need two contours for up and bottom trajectory of the wire even when cut a complicated tapered feature. After we tell the software all the parameters, the manufacturing G-codes will be generated automatically. 3D solid modeling can be used to program wire-EDM as well, but using 2D CAD is more efficient which makes 3D modeling software not very essential unless a flow simulation is required.

Fig. 6 Demonstration of Wire-EDM Machine on Floor

As we can see in Fig.7, the work flow for designing and manufacturing an extrusion die is complicated. It starts from the profile data, and then using 2D or 3D CAD approaches to design the out frame of the die. If we would like to skip the flow simulation, the 2D file will be sufficient, however, if a flow simulation is required, the 3D CAD model is necessary for geometry mesh. Then we can run the first loop to modify the geometry of die until we get a good result of simulation. After the manufacture of die done, we can put the die onto the machine to actually try, if the result is not good enough we have to modify the die until we get good-quality product. This second loop will be more costly in terms of time and money, so we want to keep the times of trials as few as possible. After the trial done, we can start production and keep all the parameters constant.

Fig.7 Flow Chart of Die Manufacturing in Conventional Way

Since this current technology has been used and developed for a very long history, the advantages of this technology are very obvious and numerous.  Wire-EDM can cut electric-conducted metal very efficiently and precisely with a smooth finishing surface. The surface condition of flow channel should be as smooth as possible, and the roughness of the whole flow channel is supposed to be uniform. Meanwhile, the dimensional accuracy is highly required. These requirements can be fulfilled by wire-EDM technology. Furthermore, the modifications on the die can be done if we get a wrong-dimensioned product, even though this might be costly in many aspects.

In Fig.8 for the 3D model of flow channel geometry and cross-section of it, we can see that the flow channel is composed by different regions. In the conventional way of manufacturing extrusion die, wire-EDM is the major technology and this manufacturing operation works greatly for straight and tapered cuts within the angle of 35° for most types of machines. See Fig. 9 for the 3D model of Extrusion Die assembly. Another detail worthy to be mentioned in Fig.8 is that the flow channel in each plate is composite of straight lines, and for the purpose of minimizing cross flow in the flow channel, the transition angle of lines should be less than 20° as a rule of thumb. However, the flow direction still changed abruptly at the transition points. Hence we have to place a straight-channel plate right after to stabilize the flow again. Furthermore, spiral turbulence will occur at these transition points that make these points as potential possibilities of burning material. In addition, if there is a mismatch of plates found at connecting points, the material flow will hit the flat mismatch and create stronger spiral turbulence, and material will stay in the die longer than it should be which may cause a degradation of material.

Adapter Zone

Pre- Landing Zone

Transition Points

Landing Zone

Transition Zone

(a)                                                                                               (b)

Fig. 8 (a) Flow Channel Geometry in Conventional Approach (b) Cross-section View of Flow Channel Geometry in Conventional Approach

Fig.9 3D Assembly Model of Traditional Die Design (a) Assembly Model, (b) Exploded View of Assembly Model

3.1.3 Introduction to Metal Additive Manufacture (Metal-AM) in Plastic Profile Extrusion

Although the technology of Plastic Profile Extrusion has been developed for centuries, there are some points still can be improved with Metal-AM technology to get a better quality of product. The work flow for designing in Metal-AM way is changed. As we can see in Fig. 10, the option of using 2D drawing to manufacture the extrusion die has been removed for the necessary of flow simulation. After we get the profile data, it is necessary to develop a 3D CAD model with every single feature in flow channel because the Metal-AM software can only recognize the 3D model. Finally the Metal-AM machine begins to 3D print the die in one-piece layer by layer. Here we have to concern that the physical trail-and-error method is eliminated. We cannot make any modifications to the die after the die is done. It brings more pressure to the flow simulation. We have to make sure the simulation is absolutely right before we manufacture the die. Otherwise a huge waste is going to happen.

Fig.10 Flow Chart of Die Manufacturing in Metal-AM Way

On the other hand, the Metal-AM way can bring some features that cannot be manufactured by wire-EDM technology which can significantly bring up the rate of production. Comparing to the Fig.8, we can see in Fig. 11 that the landing zone didn’t change. However instead of using 45° chamfer for abruptly converging material flow before landing, we used a smoothly curved surface which is tangent to the landing zone. This modification will not only convergent material flow better, but also make the material flow stabilized. The third zone is transition zone. All the surface of transition zone has the same curvature as the last surface. This change is also for the better flow pattern. The last zone is adapter zone. We made the same modification to the surface in this zone to keep the curvature the same, in another word, make the surface smoothly tangent to the last surface. See Fig.12 for the comparison of flow channels in the conventional way and AM way.

See Fig. 13 for the 3D solid model of extrusion die designed in AM method, and its cross-sections. This die is created in one-piece. Compare to Fig.9, it has the advantage of preventing mismatches on the connecting planes.

Smoothly Curved Surface

(a)                                                                         (b)

Fig. 11 (a) Flow Channel Geometry in Metal-AM Approach     (b) Cross-section View of Flow Channel  Geometry in Metal-AM Approach

                

(a)                                                                                     (b)

Fig.12 Wireframe of the Extrusion Die, (a) Conventional Way, (b) AM way

     

(a)                                                    (b)                                                     (c)

Fig. 13 3D Model of Extrusion Die Designed in AM Method. (a) One-Piece Die, (b) Side Cross-Section, (c) Top Cross-Section

3.1.4 VBA Code to Automate the Plastic Extrusion Die Designs

Visual Basic Application (VBA) is a Visual Basic based language which is associated to many applications, such as SolidWorks, CREO, and Microsoft Office software. I am using VBA to program a Macro (Add-in software) in SolidWorks to automate the design of extrusion die in the traditional way for saving time and effort from engineers. Actually, the profile extrusion design is a very tedious and time-consuming job after the profile data is given; the thing we need to do is to solve an Inverse Engineering problem which is possible to be done by the software automatically. This Macro starts from a modified 2D profile, and then using offset and rescale to get all the following profiles, and loft all the 2D profiles to a solid flow channel, and subtract the flow channel from the work piece we would like to use at last.  In the future, this program can be extended to the design in Metal-AM way as well.

3.1.5 Future Work for Case Study 1

What I showed above in Case Study 1 is only a preliminary work we have done, and the plan for the future work is to use CFD package, ANSYS POLYFLOW, to numerically analyze the material flow in order to verify our expectation of the new flow channel geometry will bring higher production rate, more stable flow with uniformly distributed flow velocity all the way thorough flow channel, especially at the moment of exiting die. Furthermore, we are using Selective Laser Sintering (SLS) machine to fabricate this die in real part and run an extruder with this new extrusion die to double check our expectation and the result we got from numerical analysis. See Fig. 14 for the SLS machine on floor.

Fig. 14 A Selective Laser Sintering (SLS) Machine on Floor

In the next stage, we are going to explore the possibility of applying the AM technology on the exterior shape for better heat distribution. The 3D model of the preliminary idea is shown in Fig. 15. As we can see in Fig. 15, the interior flow channel is the same; however, instead of using a straight cylinder as the exterior feature, this constant wall thickness of the die can offer better heat distribution thorough out the die which will definitely provide better quality of product. However, regarding this improvement on thermal distribution, we still have to consider some other parameters which may affect the product quality and safety issue, for example, the wall thickness need to be considered for the high flow pressure may cause serious human injury.

(a)                                           (b)                                      (c)

Fig.15 3D Model of New Extrusion Die for Better Thermal Distribution (a) Model, (b) Side Cross-Section, (c) Top Cross-Section

3.2 Case Study 2: Medical Instruments and Implants

Medical area is always one of the greatest influenced areas by AM technology in all the industries because some of the standardized instruments and implants are not sufficient to some patients. At present, although the impact of 3D printing on medicine remain small[15].  3D printing is currently a $700 million industry, with only $11 million (1.6%) invested in medical applications.[15] In the next 10 years, however, 3Dprinting is expected to grow into an $8.9 billion industry, with $1.9 billion (21%) projected to be spent on medical applications. [15]. Medical uses for 3D printing, both actual and potential, can be organized into several broad categories, including: tissue and organ fabrication; creation of customized prosthetics, implants, and anatomical models; and pharmaceutical research regarding drug dosage forms, delivery, and discovery.[16] There are many benefits that can be brought by using AM technology, including: the customization and personalization of medical products, drugs, and equipment; cost-effectiveness; increased productivity; the democratization of design and manufacturing; and enhanced collaboration.[9, 15, 17-19].

The application of the AM technology on implants has been widely researched and developed. The greatest advantage of AM technology to medical applications is the freedom of manufacturing customized implants and equipment.[9] Because of the complex geometries of the implants and instruments can be, the operation with the conventional manufacturing methods is not applicable. Implants and prostheses can be made in nearly any imaginable geometry through the translation of x-ray, MRI, or CT scans into digital .stl 3D print files. [9, 16, 19] In this way, it is possible to make the manufacture of the implants and prostheses within 24 hours. [9, 14, 20] Previously, surgeons had to perform bone graft surgeries or use scalpels and drills to modify implants by shaving pieces of metal and plastic to a desired shape, size, and fit. [9, 20] Custom-made implants and instruments can have great positive effects on both patients and physicians, in terms of time required for surgery, patient recovery time, and the success of surgery or the implant. [17] And in the future, it is possible that the custom-made medical parts will completely replace the standard medical parts.

The better cost-efficiency is another benefit offered by the AM technology.[15] At present, the mass production of standard surgery parts still remain the advantage of less expensive, however, the cost of additively manufactured medical parts become more competitive than ever, especially for low volume of production and one-run production. [15] For the AM technology, every single part costs similar price no matter if it’s the first one or the last one which can be a significant advantage because the modifications on surgical parts can be common.

Producing in AM technology can also enhance productivity. A normal size of medical custom-made part can be done in AM technology within several hours.[17] On the contrary, the conventional methods of manufacturing can take much longer time on milling, forging and delivery. Furthermore, dimensional accuracy, surface condition and reliability are improving rapidly.

There is few cases have been done in the area of medical instruments, however the implants have been explored from many companies and researches. For example, the spine supporter can help solve back pain which is affecting thousands of people. (Fig. 16). A custom-made supporter can be much more compatible to patient than a standard part. The design for medical instruments and implants of my case study is still during planning stage, however, I can say there is a big potential in this area.

Fig. 16 3D Model of Spine Supporter (Referred from http://www.stryker.com/en-us/products/Spine/index.htm)

4.0 Discussions and Conclusions

The AM technology is a very potential and rapidly developing technology. Its capabilities and limitations have been discussed in the previous sections.  An algorithm of making decision for technology selection needs to be developed. The case study for the Plastic Extrusion Tooling is being developed and further verifications needs to be included. Second case study is still in planning stage, and further study will be introduced in the following papers.

5.0 Future Work

An algorithm of making decision for technology selection needs to be developed, and then some cases need to be studied for the verification of this algorithm. The possibility of application of AM technology on the established technology should be further discussed. In the first case study, the polymer flow needs to be numerically analyzed to verify our expectation of additively manufactured extrusion die by ANSYS POLYFLOW software. A real die needs to be fabricated in AM technology and should be implemented on a real plastic extruder to double check our expectation and analysis result from CFD. The application of AM technology on the exterior surface of extrusion die for a better thermal distribution ought to be researched. The research of second case study for medical instruments and implants need to be included.

 

 

 

6.0 Reference

1. ASTM, Standard Terminology for Additive Manufacturing Technologies. ASTM F2792 – 12e1, 2012.

2. Frazier, W.E., Metal Additive Manufacturing: A Review. Journal of Materials Engineering and Performance, 2014. 23(6): p. 1917-1928.

3. Frank, M.C., Joshi, S., and Wysk, R.A, RAPID PROTOTYPING AS AN INTEGRATED PRODUCT/PROCESS DEVELOPMENT TOOL AN OVERVIEW OF ISSUES AND ECONOMICS. Journal of Chinese Institute of Industrial Engineers, 2003. 20.

4. Gibson, I., D.W. Rosen, and B. Stucker, Additive Manufacturing Technologies – Rapid Prototyping to Direct Digital Manufacturing. 2010.

5. Luo, X., Li, Y., and Frank, M.C., A finishing cutter selection algorithm for additive/subtractive rapid pattern manufacturing. International Journal of Advanced Manufacturing Technology, 2013.

6. Yan, X. and P. Gu, A review of rapid prototyping technologies and systems. Computer-Aided Design, 1996. 28(4): p. 307-318.

7. Kruth, J.-P., M.-C. Leu, and T. Nakagawa, Progress in additive manufacturing and rapid prototyping. CIRP Annals-Manufacturing Technology, 1998. 47(2): p. 525-540.

8. Coren, M.J., GE fired up world’s largest commercial jet engine using 3D-printed metal parts. April 25, 2016.

9. Banks, J., Adding value in additive manufacturing: Researchers in the United Kingdom and Europe look to 3D printing for customization. IEEE pulse, 2013. 4(6): p. 22-26.

10. K.Mumtaz, P.V.a.N.H., A METHOD TO ELIMINATE ANCHORS/SUPPORTS FROM DIRECTLY LASER MELTED METAL POWDER BED PROCESSES. 2011.

11. Kruth, P.M.J.-P., Residual stresses in selective laser sintering and selective laser melting. Rapid Prototyping Journal, 2006.

12. Ford, S. and M. Despeisse, Additive manufacturing and sustainability: an exploratory study of the advantages and challenges. Journal of Cleaner Production, 2016. 137: p. 1573-1587.

13. Hoy, M.B., 3D printing: making things at the library. Medical reference services quarterly, 2013. 32(1): p. 93-99.

14. Lipson, H., New world of 3-D printing offers” completely new ways of thinking”: Q&A with author, engineer, and 3-D printing expert Hod Lipson. IEEE pulse, 2012. 4(6): p. 12-14.

15. Carl Schubert, M.C.v.L., Larry A Donoso, Innovations in 3D printing A 3D overview from optics to organs. The British journal of ophthalmology, 2013. 98(2).

16. Klein GT, L.Y., Wang MY., 3D printing and neurosurgery—ready for prime time? World Neurosurg, 2013. 80(3-4) p. 233-235.

17. Mertz, L., Dream it, design it, print it in 3-D: What can 3-D printing do for you? IEEE Pulse, 2013. 4(6)(15-21).

18. Ursan, I.D., L. Chiu, and A. Pierce, Three-dimensional drug printing: a structured review. Journal of the American Pharmacists Association, 2013. 53(2): p. 136-144.

19. Gross, B.C., et al., Evaluation of 3D printing and its potential impact on biotechnology and the chemical sciences. Anal Chem, 2014. 86(7): p. 3240-53.

20. Bartlett, S., Printing organs on demand. The Lancet Respiratory Medicine, 2013. 1(9): p. 684.

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