Significant progress has been made in the further development of techniques intrinsic to additive manufacturing, but some of the challenges that remain to be addressed are listed below:85,86 1. Limited amount of materials available for use in additive manufacturing processes 2. Relatively poor accuracy of the part or component caused by the “stair-stepping” effect 3. Insufficient repeatability and consistency in the end part that is produced
4. A lack of both qualification and certification methodologies for additive manufacturing processes
Products having complex shapes can now be fabricated without tools, dies, or molds in an efficient and inexpensive manner and will meet all of the necessary functional requirements. By shortening the fabrication time, production costs are appreciably reduced. Research is currently in progress with the primary objective of expediting the transformation of 3D printing from rapid prototyping to additive manufacturing using advanced materials that have the following qualities to offer:
1. Material flexibility
2. Intrinsic ability to generate fine features (less than 100 àm) 3. High throughput
The key elements that exert an influence on additive manufacturing technology are shown in Figure 1.15. These elements are discussed in the ensuing subsections with particular emphasis on existing gaps and current needs.
1.5.1 deSign MethodSand StandaRdS
The unique capabilities of technologies related to additive manufacturing include the intrinsic abil- ity to (1) fabricate complex shapes, (2) tailor materials and properties, and (3) provide functional complexities. These attributes give designers the freedom to explore both novel and innovative
applications for the technology, but there is a need for new design tools that can better represent both the functions and material interactions of additive manufacturing. Below are a few of the design tools that must be developed:
Tools that can further aid designers in exploring designs made possible by additive manu- facturing, particularly with regard to representations of shapes, properties, processes, and other related variables
Methods for simultaneous product and process design coupled with multifunctional design Methods to assess life-cycle costs and the impact of additive manufacturing on both the components and products produced
The emphasis on design also requires that CAD systems be modified so as to overcome the limitations imposed by parametric boundary representations and solid modeling in representing complex geometries and multiple materials. One other important factor to be considered is that additive manufacturing calls for simulation capabilities for primitive shapes, materials, material compositions, and even functionally graded materials, to name a few. Additive manufacturing also requires multiple-scale modeling and inverse design capabilities to assist in navigating through complex process–structure–property relationships and improved finite element analysis software to put to effective and efficient use all of the available capabilities.
1.5.2 Modeling, MonitoRing, contRol, and pRoceSS innovation
In light of recent technological developments and advances, the modeling, sensing, and control of additive manufacturing techniques or processes can easily be considered to rank among the highest priorities for realizing the potential far-reaching applications of this technology. The modeling of additive manufacturing processes presents significant challenges; for example, to understand the transport phenomenon in additive manufacturing processes it is essential to model the temperature, stress, and composition history. Further, it is difficult to predict the microstructures and resultant fatigue properties arising as a direct consequence of the additive manufacturing process used. This difficulty can be ascribed to the extreme heat and cooling rates, which create fundamentally new regimes of material transformation. For the case of polymeric materials, melting and recrystalliza- tion have not as yet been adequately understood to allow the robust development of mathematical models. To do so, the physics of polymeric materials be better understood with the primary objec- tive of achieving better effective modeling. The development and availability of supercomputing
System integration
and cyber implementation
Design method and standards Modeling, monitoring, control, and process Materials development
and evaluation Characterization and
certification
fIgure 1.15 Key elements essential to ensuring overall effective functioning of additive manufacturing of metallic materials.85
have had a great impact on modeling efforts and related studies. The complex process models must often be reduced to lower-order models for use with real time parameters and resultant control of the additive manufacturing process. As of now, there exists a noticeable gap between high-fidelity modeling research and real-time efforts aimed at online process control.
Two of the noticeable challenges that must be considered with regard to additive manufacturing sensor processes include a lack of access to the build chamber and the need for intense computing power. The sensing of additive manufacturing processes requires the following:
1. In situ measurement of temperature, cooling rate, and residual stresses
2. Calibration of all optical sensors used for the purpose of high-accuracy measurements 3. In-process monitoring of not only the geometric dimensions but also the surface quality of
the finished layers
High-speed infrared thermography has primarily been used to obtain imaging data for the pre- diction of microstructures. This is done by determining the grain size that results from character- istics of the melt pool. For the purpose of process control, to put to effective use such information the images from the additive manufacturing processes have to be processed at a speed of at least 30 kHz. If this is to be achieved, then there exists the challenge of using the information gained for the following purposes:
1. Online process control for both material composition and phase transformation
2. Repair of existing defects, such as the presence of pinholes or porosity, microscopic cracks, and segregation
Careful integration of the control algorithms with the existing equipment being used for additive manufacturing through the control unit of the machine creates a significant barrier to cost-effective implementation of real-time process control of additive manufacturing. Also, addressing the need for improved throughput and the need for multi-material additive manufacturing fabrication capa- bilities requires the following:
1. Development of multiple-nozzle array print heads and machines capable of integrating multiple additive manufacturing processes
2. Subtractive and finishing processes intrinsic to the technique of conventional manufacturing A recent and noticeable additive manufacturing process innovation is in the domain of 3D bioprinting. The challenges related to the printing of 3D tissue scaffolds include the following:
1. Biophysical requirements specific to the scaffold’s structural integrity, strength stability, and degradation, as well as cell-specific pore shape, size, porosity, and interarchitecture 2. Biological requirements pertinent to both cell loading and spatial distribution, as well as
the attachment of cells, their growth, and the eventual formation of new tissues 3. Mass transport considerations with specific reference to topology and interconnectivity 4. Anatomical requirements with specific regard to anatomical compatibility and geometric
fitting
5. Manufacturability requirements with reference to the overall ability of the process (e.g., availability of biomaterials coupled with the feasibility of printing) and effects of the pro- cess (e.g., warping, distortion, and overall structural integrity)
Ongoing studies have shown that printing in vitro biological constructs imposes a requirement for the following:
1. Development of a new generation of biomaterials in the form of bioink for (a) dispensing with cells (a cell delivery medium), (b) growing with cells (support as an extracellular matrix), and (c) functioning with cells (in the role of biomolecules)
2. Developments in engineering (vs. developments in biology) to fill the gap created by a lack of knowledge in biology
3. Commercialization of bioprinting tools so as to be able to make 3D heterogeneous struc- tures in a viable, reliable, and reproducible manner
4. Four-dimensional bioprinting models (obtained by embedding time into the 3D bioprinting models), including stem cells having a controlled release of the biochemical molecules for exercising control of (a) complex tissues, (b) organs, (c) cellular machines, and (d) human- on-a-chip devices
1.5.3 MateRialS developMentand evaluation
Intense research and development in the specific domain of materials are necessary to 1. Broaden the selection of suitable or appropriate materials.
2. Prepare a database on the mechanical properties of the parts fabricated by additive manufacturing.
3. Determine any and all of the existing interactions occurring between materials and pro- cess parameters.
In the fields of metallurgical and materials engineering, it can take well over a decade to develop a new alloy and determine its various mechanical properties, such as mechanical strength, fatigue strength, and analysis. This time frame also applies to engineering new materials for the purpose of additive manufacturing—materials that have a variety of features to offer, ranging from high mechanical strength to high or improved corrosion resistance. Even with the currently available and preferred materials, noticeable advances are still needed in the preparation of raw material feedstock in the form of powder. Overall efforts in the field of materials development should include the development of both metallic and non-metallic inks that have the desired rheological properties so as to enable fairly good resolution on the order of submicrometers. For paste-extrusion-based additive manufacturing processes, the development of ceramic slurries is essential. Similarly, the development of a new generation of biomaterials to serve as cell delivery media or biomolecules is needed in the field of bioprinting.
1.5.4 chaRacteRizationand ceRtiFication
Actual production practices are much more rigorous than those traditionally used for prototyping purposes; therefore, certification is highly critical in a production-based environment, including certification of (1) equipment, (2) materials chosen and used, (3) personnel, (4) quality control, and (5) logistics. For a manufacturing process to be readily and easily adopted in industry circles, both repeatability and consistency of the manufactured parts are not only important but also essen- tial. With gradual advances in additive manufacturing technology, these attributes have become both necessary and required over the entire build volume and between builds for each machine.
Currently, the inability of additive manufacturing to guarantee material properties for a given pro- cess is inhibiting its adoption and restricting its use in industry. This is primarily because many of the companies involved lack confidence that the resulting manufactured parts will provide the mechanical properties and dimensional accuracy required to meet the needs of a specific applica- tion. The primary reason for this problem is that the prevailing additive manufacturing systems are based on a rapid prototyping machine architecture, which is quite different from the actual require- ments for fabricated parts having functional use.
A lack of well-established standards has contributed to the following:
1. The material data reported by the various companies are often not comparable.
2. Various users of the additive manufacturing technology utilize different process param- eters when operating their equipment consistent with their own preferences and needs.
3. There appears to exist minimal repeatability of results between suppliers and the service bureaus.
4. Only a few specifications are available that end users can reference.
1.5.5 integRationoF vaRiableSand theiR iMpleMentation
Successful implementation of additive manufacturing technology requires prudent integration of all of the available interdisciplinary knowledge. A good example is bioprinting, for which understand- ing the intrinsic interactions between materials and processes is essential to bringing about effective cooperation among the engineers and biologists. Such cooperation aids in improving our knowledge of the interactions between cells and the environment in a predominantly structural environment.
In addition to noticeable advances in the modeling of biological structures in three dimensions, the fourth dimension of time can be incorporated into models to predict cellular behavior. New 3D bioprinting equipment and tools will be necessary for the reliable and reproducible creation of heterogeneous biological structures.57,59,60
The utilization of online resources has become essential for both small- and medium-sized companies. Ensuring successful completion of the many manufacturing tasks requires the sup- port of both suppliers and business partners. Further, there is a need to connect the various manufacturers so they are in a position to share their resources and put them to effective use.
The emergence of “cloud manufacturing” has made it possible to share a pool of manufacturing resources, such as the machines commonly used in additive manufacturing; however, readily available cyber–physical systems that can be easily used for “cloud-based” additive manufactur- ing are in limited supply.58,59