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Journal Club Theme of April 2014: Challenges and Opportunities in 3D Printing: A Mini Introduction
3D printing, or Additive Manufacturing (AM), has drawn significant attention from both general public and research community in recent years. Although the first prototype of 3D printing (called rapid prototyping) emerged about three decades ago and has gone through significant development, the recent explosion in its popularity appears to be mainly due to intensive media reports. However, I personally feel what is truly underneath this increasing popularity is the development of 3D printing has reached a critical point where the revolutionary transition of 3D printing from a prototyping tool (which is cool but few people take it seriously) to a high fidelity manufacturing powerhouse is occurring, which will dramatically change how materials, parts, or devices are made in the future. As will be outlined below, 3D printing is a very broad concept and there are many dramatically different 3D printing technologies. In addition, 3D printing research is part of the large manufacturing research community, which consists of many extremely innovative people. Therefore, I will briefly introduce the different 3D printing technologies then comment on the challenges and potential opportunities, which I am sure will be incomplete. I hope this can lead to more in-depth discussions.
3D Printing Methods
A good starting point to learn the state-of-the-art and its challenges of 3D printing, or additive manufacturing, is to look at a report published by IDA Science and Technology Policy Institute. More in depth description about AM can be found in a recent technical book . There are many different ways to categorize the AM technologies. But commonly encountered AM technologies include:
Powder Bed Fusion (PBF): Material powders, typical coated with binding agents to lower the processing temperature, are pre-laid in the processing bed and exposed to a raster laser to build a layer where the powders are fused together either by melting or by sintering; after one layer is built, the platform lowers down by one-layer thickness, another layer is then built on top of the previous layer. Both metallic and polymeric parts can be built by using PBF. The representative PBF method is selective laser sintering (SLS) where the high energy from laser beam melts the binding agents and then sinters the powders into a solid part. The processing temperature due to the laser beam can be below melting temperatures thus the processed material does not melt. Because of this, the choice of printable materials is huge, including most of metals and polymers. The quality of parts is generally high, but at the price of $0.5M for a typical basic model SLS machine. Because powders are pre-laid into the processing bed, it is difficult to create composites with controllable microstructures using this approach.
Photopolymerization: In this method, photo-curable polymers are photopolymerized in a layer-by-layer manner to create part. There are two methods based on how pre-polymer resins are deployed. The first one is known as stereolithography (SL) where a vat of prepolymer resin is exposed to light (either laser or projected pattern) that cures the polymer. In the second approach, the prepolymer solution is sprayed on the surface by a jet, followed by a light irradiation to cure the resin. In both methods, after one layer is done, the supporting platform is lowered and the next layer is built on top the previous layer. Both methods can build high quality polymer products. The SL approach is relatively simple and most of the labs can build one at the cost of less than $10k. But as it can be seen, SL operates in a similar way as PBF, therefore is difficult to create composites with controllable microstructures. The advantage of the second approach is that multiple jets (or PolyJet) can be used to spray resins thus offer the possibility of creating materials with very well controlled heterogeneities. The jet-based machine is typically more expensive, ranging from ~$10k to ~$100k. Because photoploymerization normally occurs at room temperature and cells can survive a short time exposure of light, photopolymerization based methods are particularly popular for tissue engineering, such as organ printing.
Material Extrusion: In this approach, a thermoplastic polymer is heated to above its melting temperature, extruded through a nozzle, then cooled down. A part is then formed in a line-by-line then layer-by-layer manner. The machines are relatively low cost and are therefore popular. For example, a MakerBot 3D printer costs around $2k. But of course, the quality of the product is low. One can easily see lines (due to line-by-line plastic extrusion) by eyes.
Other methods, such as material jetting, binder jetting, beam deposition, sheet laminating, are not discussed here.
Applications of 3D Printing
Despite the disadvantages discussed above, the 3D printing has found its own niche areas of applications where a highly customized product is necessary. For example, Aligntech has used 3D printing to help ~2M patients for their orthodontic needs. In addition, 3D printing is used to manufacture parts in obsolete machines, which may have been out of production for many years and the original drawings for the design are gone. Nowadays, AM has $1.3B in worldwide sales of materials, equipment, and services in 2010 and is poised to exceed $3B by 2016.
Challenges and Opportunities
There are several challenges that 3D printing has to overcome before it can be widely used at industrial manufacturing level. To list a few:
Printing speed: The layer-by-layer processing method is intrinsically a slow process, especially when one wants to reduce the layer thickness for higher resolution. For example, using the PolyJet 3D printer (ObJet 260 Connex) in our lab, a 1mm layer needs about 10min to print; thus a small part of 2cm thick takes about 3 hours to print.
Instrument and operational cost: As discussed above, a typical SLS machine costs above ~$0.5M. For the 3D printer in our lab, it costs above $160k. In addition, the printable materials are expensive. The price for the resin for our printer is about $1000/kg. The high cost and slow output of individual printers are one of the limiting factors for its wide industrial usage.
Printing quality: The quality of printed parts is another major concern for its wide application. For example, surface finish from SLS method is low; mechanical behavior of the sintered material is different from that of the material from melts; the printed part may have a slightly distorted shape because of complicated processing conditions. In the past, the AM community also developed standards to evaluate the printed shape. For example, printing a suspended cantilever as a test.
Physics of printing process: Most 3D printing methods listed above involves very complicated and highly coupled thermo, chemical, and mechanical processes. For example, in the photopolymerization method, photopolymerization itself is a very complicated chemo-thermo-mechanical process where a liquid resin is transformed into a gel then a rubber then a glass-like solid very quickly, normally within a few or a few tens seconds. Photopolymerization is also accompanied by heat generation as well as large volume shrinking. Theoretical investigation of the whole manufacturing process is very limited.
Properties of printed materials: Our knowledge about printed material in general is very limited, although people know that the material might be anisotropic, the interface between layers might be weak. Systematic investigation of properties of printed materials is still needed.
Multi-material printing: As mentioned above, most AM cannot print composites with well controlled heterogeneity. Photopolymerization based 3D printing using polyjet technique allows us to print two types of polymers simultaneously but it limits to polymers only. Printing metals with polymers is highly desirable but is very difficult to achieve.
Design: Using 3D printers to realize or to assist the realization of materials/structures with path-breaking behaviors through innovative designs becomes increasingly popular. For example, Bertoldi and Reis used 3D printing to create molds for making elastic shells[5-7], with interesting behaviors, including 3D negative Poissonís ratios . 3D printing has also been used to directly fabricate heterogeneous materials. For another example, Boyce group fabricated soft multi-material polymer composites to study the he formation of wrinkled interfaces. 3D printing was used to create metamaterials [10, 11] because of its easiness to create or control heterogeneities in the materials.
4D Printing: 4D printing is a new concept and is to use a 3D printer to print a part made of active materials so that the printed parts can further change its shape (the 4th dimension (time) of shape formation). The concept was initially proposed by Tibbits. In Tibbitsí work, the active material swells when immersed in water; by spatially and temporally controlling the activation of these materials, the shape can change in a controlled manner. About the same time, our group has been working with Dunn group to use shape memory polymers and multi-material printing capability from our 3D printer to create printed active composites (PACs). We have demonstrated by carefully placing shape memory polymer fibers at different locations in a polymer matrix, a strip can bend, coil, and twist; a thin flat sheet can change into a 3D contour face with continuously changing curvatures; a thin flat sheet can also be folded into different 3D structures, such as a box, a pyramid, or an airplane (origami). Our work demonstrated that mechanics analysis should be used to guide the design of 4D printing process. Recently, a group of researchers, including Balazs at U. Pittsburg, Lewis at Harvard, and Nuzzo at UIUC has won an ARO grant to pursue hydrogel based 4D printing.
3D printing, or additive manufacturing, is a rapid development field. Although there are still lots of challenges, 3D printing has its niche areas of applications, which are expanding very fast. This, in turn, demands more fundamental research to address the challenges. Mechanics will play a critical role in addressing both fundamental and application problems for the current and future 3D printing technologies.
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