User login

You are here

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[1]. More in depth description about AM can be found in a recent technical book [2]. 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[3]. 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[4]. 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 [8]. 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[9]. 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[12]. 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)[13]. 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[13], a pyramid, or an airplane (origami)[14]. 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[15]. 

Summary

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.  


References

1. Scott, J., et al., Additive Manufacturing: Status and Opportunities. 2012.

2. Gibson, I., D.W. Rosen, and B. Stucker, Additive manufacturing technologies : rapid prototyping to direct digital manufacturing. 2010, London ; New York: Springer. xxii, 459 p.

3. http://www.aligntech.com/.

4. Jacobs, P.F. and D.T. Reid, Rapid prototyping & manufacturing : fundamentals of stereolithography. 1st ed. 1992, Dearborn, MI: Society of Manufacturing Engineers in cooperation with the Computer and Automated Systems Association of SME. 434 p.

5. Shim, J., et al., Buckling-induced encapsulation of structured elastic shells under pressure. Proc Natl Acad Sci U S A, 2012. 109(16): p. 5978-83.

6. Lazarus, A., H.C.B. Florijn, and P.M. Reis, Geometry-Induced Rigidity in Nonspherical Pressurized Elastic Shells. Physical Review Letters, 2012. 109(14).

7. Nasto, A., et al., Localization of deformation in thin shells under indentation. Soft Matter, 2013. 9(29): p. 6796-6803.

8. Babaee, S., et al., 3D Soft Metamaterials with Negative Poisson's Ratio. Adv Mater, 2013. 25(36): p. 5044-9.

9. Li, Y., et al., Wrinkling of Interfacial Layers in Stratified Composites. Advanced Engineering Materials, 2013. 15(10): p. 921-926.

10. Pandey, S., B. Gupta, and A. Nahata, Terahertz plasmonic waveguides created via 3D printing. Optics Express, 2013. 21(21): p. 24422-24430.

11. Sanchis, L., et al., Three-Dimensional Axisymmetric Cloak Based on the Cancellation of Acoustic Scattering from a Sphere. Physical Review Letters, 2013. 110(12): p. 124301.

12. Tibbits, S., The emergence of "4D printing" TED Talks 2013, Ted Talks.

13. Ge, Q., H.J. Qi, and M.L. Dunn, Active materials by four-dimension printing. Applied Physics Letters, 2013. 103(13).

14. Ge, Q., et al., Active Origami by 4D Printing. Smart Materials & Structures, 2014: p. Under revision.

15. http://www.news.pitt.edu/news/entering-new-dimension-4d-printing. 2013.

Comments

Dear Jerry,

 

thank you very much for the timely and excellent intro/review of the state of 3D printing.  

 Following up on the challenge of printing polymer-metal highly contrable micstructures, what do you think would be the most promising approach (-es) to achive this goal. And what would be the characteristic microstructure length scale?

Thank you!

Stephan 

Hi Stephan,

These are good questions. However, i don't have good answers.

For the first one, I think laser sintering is promising. It is common nowadays to sinter silver, gold, copper nanoparticles at ~100C. There are several demonstration s of sintering sliver wires on polymer surface. However, one critical issue with sintering is that it takes a few minutes to sinter, which significantly slows down the printing.

For the second one, the feature size depends on the printing methods, the nature of the microstructures (holes or material heterogeneity), sometimes, even the shape or orientation of the microstructure. For the 3D printer in our lab, we can print structures of a few hundred microns. But sometimes, features of a few millimeters cannot be printed if they are placed in certain orientations.

Jerry

Hi Jerry,

Thanks for the great introduction to 3D printing! Following up on Stephan's question, you mentioned that it can be difficult to print features of a few millimeters. What is the limitation (beyond time) for printing structures with larger feature sizes? Also, what are typical flaw sizes in these systems? Do they scale with the feature size? Can they be annealed out (or mitigated by some other mechanism)? On a related note, do printed materials in general have lower strength (e.g. due to larger flaw sizes) than the same material produced by other processing techniques? What about other mechanical properties relative to their unprinted counterparts? Sorry for the barrage of questions but the topic is too interesting!

Thanks,
Matt

Hi Matt,

Great and tough questions! We have studied some of the printed materials from our 3D printer, but not comprehensively. So I will use my intuition and try my best. Please correct me if I am wrong.

Regarding the limitation for printing structures with larger feature sizes, if time is not a concern, or if someone wants to use a 3D printer with a resolution of 0.05mm to make a part of about two meters, one potential issue might be system control because the machine is running so long time it may drift. Special control might be needed. In addition, the shape distortion due to material shrinkage during printing can be magnified when the feature size is too large. 

Regarding flaw sizes, I believe they should scale with printer feature size, such as the sizes of laser, layer thickness, line-width (if raster laser is used), powder size etc. For example, in solid state sintering, the defects are about the size of the powders used. For photopolymerization based 3D printing, gas generation during photopolymerization could be a problem. The size of gas bubbles is in sub-micron range. For these types of defects, I cannot find a reason why these defects scale with size of a structure (such as a hole). Regarding the methods for mitigating these defects, they can be reduced by selecting different binding agents, or composition of photopolymers, or different processing conditions. 

I believe that printed materials in general have lower strength; but I did see some conference reports that the material sometimes becomes slightly stronger. I am not sure why. We don’t have direct experience with metal 3D printer (too expensive). For photopolymerization based, I am sure the printed material will be different from the bulk made. But we haven’t compared them as we don’t have the unprinted material (one can certainly open a cartridge and photopolymerize some; but it is too expense, as a cartridge costs about $1000Smile). One interesting thing we found that the printed polymers become anisotropic, which is not difficult to understand as they are actually layered composites (due to layer-by-layer process). 

Best,

Jerry

Lifeng Wang's picture

Hi Jerry, 

Thank you for this excellent introduction! I worried a lot about the printing quality, such as the interface strength and internal flaws, when I used a multi-material 3D printer (Objet Connex 500) to design complex 3D interpenetrating phase composite materials (co-continuous hard/soft composites). However, it turns to be very good if the feature size is larger than 0.5mm. We have achieved excellent combination of stiffness, strength and energy absorption. And the interface is surprisingly close to perfect connection. Fractures are observed in the hard material instead of at the interface.  Please see

L. F. Wang, J. Lau, E. L. Thomas, and M. C. Boyce, “Co-Continuous Composite Materials for Stiffness, Strength and Energy Dissipation”, Advanced Materials 23, 1524-1529 (2011).  

All the best,

Lifeng

Hi Lifeng,

Thanks for the comment and sharing your experience.

Yes, we had the same observations that the bonding between two different materials can be very good. We also did some study in interface fracture and found that when hard and soft materials are printed together, the fracture occurred at the soft side. However, we recently also found some cases that the materials were very weak and the quality was really bad. We are still trying to figure out what is going on. Your paper with Mary (which is a very interesting one) is very helpful for us.

Best,
Jerry     

Lihua Jin's picture

Hi Jerry,

Thanks a lot for the timely review.

What are the criteria of the choice of materials for different 3D printing methods? For example, I think for the material extrusion process, the rheology of the material must be very important. For the photopolymerization process, what prevents you from using various photo-crosslinkable polymers, but enforces you to use that super expensive type? If you home-build the stereolithography, can you use various photo-crosslinkable polymers?

Lihua

Hi Lihua,

Very good questions. I believe your questions are related to material processibilities. The choice of printable materials usually depend on the processing techniques, processing conditions, etc. For polymers (I don't know much about metals, some other people may add), viscosity seems to be one of the common parameters. In extrusion method, high temperature viscosity is a determining factor. If the viscosity is too low, the polymer melt will flow freely; if the viscosity is too high, it will need lots of energy to push them out of the nozzle. These same issues also exist in photopolymerization based method. The jet-based photopolymerization technique typically requires relatively low viscosity so that monomer solutions can be spayed; such a requirement limits the choice of polymers. In addition, volume shrinkage during photopolymerization is a big concern; monomers need to be stabilized before they are used; presence of oxygen can be good and bad for radical  based photopolymerization polymers, which are the most commonly used; light penetration into the material also needs to be controlled. Therefore, there are lots of additives, such stabilizers, photo absorbers, etc., in monomer solutions in commercial 3D printers. Many 3D printer companies have a strong focus on new material development.

Of course, the importance of the above mentioned issues depends on what we need. As we are looking for higher resolutions, more and multiple printable materials, we will see more of these issues.

Best,
Jerry
 

Subscribe to Comments for "Journal Club Theme of April 2014: Challenges and Opportunities in 3D Printing: A Mini Introduction"

Recent comments

More comments

Syndicate

Subscribe to Syndicate