Types of 3D Printing
There are many different technologies to do 3D printing. Each type of 3D Printing has its uses and needs to be matched with the right types of materials and input parameters. Since 2010, the American Society for Testing and Materials (ASTM) carried out the “ASTM F42 – Additive Manufacturing” standards. Those standards are used to classify the additive manufacturing processes into 7 categories.
Fused Deposition Modeling (FDM)
This is the most common and mainstream 3D printing method. Consumer-level 3D extrusion printers are available and very inexpensive; they have many options in the range of just a few hundred dollars. It is an additive process where the object is created by placing melted material layer by layer in a path (Figure1); thermoplastic polymers are the most common materials due to their ability to quickly melt and cool. This method allows for a myriad of materials to be used. Theoretically, any material that can be melted and cooled to a solid can be used with extrusion 3D printing.
For this process, the material is loaded into the printer, and it is passed on to the extrusion head once the nozzle reaches the correct temperature; the material melts in the nozzle, but the extrusion head allows the layers to be formed in any pattern (Figure 2). The strands are printed layer by layer, and they then cool ultimately creating a solid product. The platform moves down once the layer is complete in order to create a new layer which repeats until the object is done.
One drawback of this method as it applies to microfluidics is the resolution of these extrusion printers is typically not low enough to be effective for microfluidic channels, although recent improvements have placed resolution down to the micrometer scale. Additionally, during printing in this method, it is very difficult to get the layers to line up perfectly, making it difficult for this method to contain fluids.
In order to compensate for this drawback, there are several options available for finishing 3D models made using this technique such as exposing the model to a higher temperature so as to re-melt the layers together for a better fit, or using an appropriate chemical gas to partially dissolve the exposed outer layers and smooth them together. Current “affordable” 3D printers can achieve resolutions around 50 µm.
This method is a 3D printing method that uses photopolymerization, which is a process involved an exposure of UV light to liquid polymers to turn them into solids. The process usually is similar across the methods, listed below, involving 3D render, project UV onto liquid layer by layer, the process is repeated until all layers form and the liquid is drained away.
Stereolithography (SLA) and Digital Light Processing (DLP)
Stereolithography (SLA) is categorized into the vat photopolymerisation group by ASTM, which was introduced in 1982 by Chuck Hull. Upon printing, a thin layer of photopolymer solution is targeted by the light sources which can be a scanning laser or a digital light projector (DLP). Based on the printing part, the light source would trace the desired 2D cross section pattern. The photo-initiator would respond and initiate the polymerization reaction locally. After the entire 2D pattern were traced, the platform will be raised or lowered, depending on the printing style, allowing new layer to be printed. Then, the printing cycle continues until completion. The photopolymer solution contains monomer, oligomer, and photo-initiator. Based on the clear resin printing technology, the raw materials of the solution are urethane dimethacrylate (UDMA), methacrylate monomer(s) and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide. During the liquid resin photo-curing process, the resin is cured from liquid to solid through a chemical cross linking process. The SLA 3D printing process is shown in Figure 3.
Although the most commercially available type of printing style is Fused Deposition Modeling (FDM), it generally has poor printing resolution. In addition, the FDM printed parts have large void space and adhesion quality irregularity. Transparency and high resolution are fundamental for microfluidic applications. In some cases, certain rigidity is critical as well. Under such restrictions, the Stereolithography (SLA) suits the best, by using the commercially available clear resin from FormLabs. Its current SLA 3D printer have the xy-axis resolution of 25 μm.
DLP takes advantage of the same technology as SLA does, however, rather than having a laser cure the resin point by point, an entire 2D array of exposure if reflected onto the liquid resin layer, and the entire layer is cured simultaneously.
Powder Bed Fusion
Selective Laser Sintering (SLS)
In contrast to extrusion printing where the printed material is extruded from a moving tip, SLS involves using a bed of a powdered material and fusing together a layer of that material in a specific shape using a laser. The particles of powder are formed together from the laser beam. After adding another layer of the powder, it can then be fused on top of the previous layer, almost like a reverse CNC milling process in that an object is constructed by fusing powder together instead of shaving material off a starting block. Although this allows for a very high resolution, one drawback to this approach is that the unbound powder must be removed after the manufacturing process. For the scale of the small capillaries that microfluidics research necessitates, this makes this method somewhat unsuited to microfluidics channels. Using CO2 laser machining of poly(methyl methacrylate) (PMMA) Romoli et al., were able to adopt the technique to create 3D structures by printing several PMMA sheets and then bonding them together. In addition, this method is much harder to control, for it require a high input of energy of the laser beam and complete melting of the particles which in turn causes issues such as deformation.
Due to the extremely low cost and availability of ink jet printers, this technique promises to be extremely useful for developing low cost microfluidics solutions. When used in two dimensions, this method typically involves printing hydrophobic boundaries which might be made from wax or polymer onto hydrophilic paper. A recent publication described a technique wherein this method can be used for 3D printing using two different polymer inks. The first is made from SU-8, which forms the channels, while PMMA supports the structure during the curing process but is subsequently removed by washing. A relatively high vertical resolution of 4.6 µm has been reported using this method. The manufacturing steps involve printing an isolation layer. On top is printed a support material using PMMA. The microfluidics channel is then printed on top of the support material, and finally the support material is washed away.
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