A New View to 3D Printing-Process Applications and Limitations

3D printing (3DP),?as the name suggests, is a manufacturing process that enables physical or 3D realization of a computerized model, referred as CAD (Computer-Aided Design). It is a non-conventional technique, which basically means that it is significantly younger than other manufacturing processes like casting, welding, forming and machining, but still old enough to achieve ground breaking research work.

3D printing is done layer-by-layer, where the whole CAD model is disassembled into various slices of very small thickness. These slices or lamellas, marks the transition from 3D to 2D and are deposited one at a time. These slices are than stacked over each other and joined to form the complete 3D solid. Because of this layered/deposition nature, 3DP is also known as?Layered Manufacturing/Additive Manufacturing.?3DP saves design iteration time and reduces the overall ‘time to market’ period. Therefore, it is also called as?Rapid Prototyping.?

There are various 3DP techniques available today but all follow the same basic process flow comprising of?CAD, Slicing?and?Deposition.?The starting element is the CAD file which is a software based representation of a physical object. Many commercial softwares are available for the same. Some of the popular ones are SolidWorks, SpaceClaim, PTC Creo, Catia, AutoCAD etc. Also, the most common file extension is?.stl.?Standard Tessellation Language or STL is a file format that saves the CAD model in the form of simplest 2D structures, triangles. For example, the CAD model of a cube will have 12 triangles (right-angled, isosceles), 2 on each face, in the STL format. After geometry finalization, the next step is slicing.?

As mentioned before, the CAD model is various 2D lamellas called slices. These slices when stacked and joined over each other form the complete CAD model. The input for the slicing step is the CAD file (usually STL) and the output is a 3D printer readable file called G-code. It should be noted that the part orientation is mentioned in the slicing step. In other words, changing the build orientation, changes the slicing and the slices. For example, a solid cylinder when 3D printed upright (bottom circular face touching the build plate) will have circular slices and it will have rectangular slices when it is printed in a lateral position (curved surface touching the build plate). There are various open source softwares available for slicing 3D models. Some of them are Cura and Slic3r.

The final step is the deposition or material addition. The 3D printer reads the G-code while given by the slicing software and deposits the material accordingly. G-codes are basically the coordinates of the nozzle movement throughout the CAD model considering various parameters like speed, deposition rate, dwell time etc. The deposition mechanism and the movements are subjected to the type of 3D printer. There are various types of commercially available 3D printers based on various techniques like SLS (Selective Laser Sintering), SLM (Selective Laser Melting), SLA (Stereo Lithography Apparatus), DLP (Digital Light Processing), MJP (Multi Jet Printing), FDM (Fused Deposition Modeling), LOM (Laminated Object Manufacturing), EB 3DP (Electron Beam 3D Printing), LENS (Laser Engineered Net Shaping), SGC (Solid Ground Curing), HLM (Hybrid Layer Manufacturing) etc. All these techniques have their dedicated process characteristics, applications and limitations.

3D printing, as a whole has a lot of ‘not possible by other processes’ applications ranging over material flexibility to geometrical impossibilities. Multi-material components i.e. parts made up of more than one material can be easily fabricated using 3D printing. It should be noted that multi-material components are different than alloys. Alloys are materials made up of more than one kind of element with a homogeneous distribution of the elements throughout. Multi-material components are tailor made i.e. having the desired element present at the desired location, thus, inducing inhomogeneity. Fabrication of a solid mufti-material rod with one end made of magnetic material and the other end made of non-magnetic material is highly applicable in nuclear applications and can be easily fabricated using LENS 3D printing.?LENS 3D printing can also help in ‘repair’ based applications, where a small singular part needs to be repaired like impeller blades. Apart from multi-material components, functionally gradient materials (FGMs) can also be made using 3D printing. FGMs are materials with tailor made internal structures, thereby providing, varying density and material distribution. With FGMs materialistic manipulation of addition and subtraction can be done wherever necessary. FGMs help in material saving and optimization. Techniques like FDM, SLA, SLS, SLM, LENS etc. can fabricate FGMs with multi-material components also. 3D printing is very versatile from the geometric point of view also. Impossible geometries like hollow spheres can be easily produced using techniques like FDM and DLP, which otherwise is impossible by other techniques. Owning to the above features, 3D printing has immense applications in various domains like aerospace, biomedical, industrial engineering, machining, rapid prototyping, rapid tooling, die manufacturing, design iterations, optimizations, physical realization, 3D modeling etc.

Although being the manufacturing technique of the future, 3D printing is susceptible to various limitations and bottlenecks that hinder its usage to prototyping only. Because of these hindrances, 3D printing is limited to single production and not batch production and that it’s why it is not used on an assembly line. There are 4 primary bottlenecks of 3D printing which are strength, cost, time and surface finish.

3D printed parts are low in strength as compared to other manufacturing counter parts. This can be attributed to the layered nature of additive manufacturing. The adhesion strength between two consecutive layers is less than the strength within a layer. Because of this, the failure occurs at the joining of the layers and not within a layer. This layered nature also adds to the directional sense of the strength. In other words, 3D printed parts have directional strength, making them highly anisotropic. The manufacturer can exploit this feature to have directional strength to the parts.?

Cost is another bottleneck of 3D printing that limits its usage. While other process gave an inverse relationship between the parts produced and cost of each part, 3D printing have a constant linear profile, i.e. the printing cost is independent of the number of parts produced. This limitation conjures 3D printing as the costliest approach for manufacturing.?

The total printing time also follows a similar linear trend as the printing cost. Although called rapid prototyping, in a sense that it reduces the design iteration time, 3D printing is the slowest manufacturing process. What would just take minutes in process like machining, will take hours in 3D printing to manufacture.?

Poor surface finish is another limitation of 3D printing. The stacked layer lines are easily visible through the naked eye and can be felt by human touch, resulting in ‘mm’ range roughness, which is 10-100 times rougher than the finish produced in other manufacturing processes. Although using prost-processing techniques like chemical vapor bath, the roughness can be reduced to achieve a smoother finish.?

The above bottlenecks, limits the usage of 3D printing for prototyping purposes only and cannot be extended for production. That’s why 3D printing is called Rapid Prototyping and not Rapid Production Technique or Rapid Manufacturing.?

Latest advancements in 3D printing aim to eliminate these bottlenecks. A team of researchers at MIT have developed a photopolymer resin based 3D printing technique that literally prints in the matter of seconds! Advancements have been made in the slicing algorithm to enable 3D slices than the conventional 2D lamellar slices. This helps in reducing the stair-step error that causes surface roughness in 3D printing. Various materials and their various forms have been explored in order to achieve the economically cheapest raw material. For example various FDM systems have been developed that uses grains as raw materials (which are ten times cheaper) instead of the conventional wire form. Also with the advent of hybridization (integration of additive and subtractive approach) and techniques like HLM, faster and stronger 3D printed parts can be fabricated.?

With a thought for betterment and continuous advancement, the current bottlenecks can be overcome and will present 3D printing as the most widely applicable manufacturing technique, ultimately making it the manufacturing of the future.

(Check out my original article at: https://memeiitb303483862.wordpress.com/2020/08/06/a-new-view-to-3d-printing-process-applications-and-limitations/)