3D printing deals with processes that involves joining materials together or solidifying them using computers to come up with three-dimensional objects . The materials that are used in this process vary from liquid molecules to grains of certain powder which are added layer by layer. While in the past, 3D printing was only used for the purposes of prototyping and only considered suitable for functional units, they have found a wide range of applications nowadays. In today’s world enhanced precision, repeatability and material ranges have enhanced to such an extent that 3D printing is becoming more of a standard practice in industrial manufacturing. More and more industries are finding it suitable to use this technology in their manufacturing processes. The term ‘additive manufacturing’ is being repeatedly used synonymously with 3D printing and rightly so. Using a digital 3D model or a CAD file, 3D printing can produce objects having quite complex shapes and sophisticated geometries .
Fused deposition modelling (FDM) is the most commonly used 3D printing technology. It is a material extrusion technique. With the increasing application of metal parts in the 3D printing regime Metal Powder bed fusion has gained prominence in recent years . Other techniques include casting and forging processes. In 3D printing, material is usually added layer-by-layer and is guided by a computer-aided design (CAD) model to get the final product.
- Working of 3D Printers:
3D printing works on a basic principle of adding layer upon layer to transform a digital computer-aided model into a physical three-dimension product. This gives 3D printing its other name: Additive Manufacturing. Also, this is what differentiates 3D printing from traditional manufacturing techniques of subtractive and formative manufacturing processes. 3D printing neither utilizes CNC machines nor does it need molds for its manufacturing process. This makes 3D printing free from the need of using any special tool like molds or cutting tools. It only requires a build platform on which product is developed using the basic principle previously discussed . This technique of manufacturing leads 3D printing to have its own set of flexibilities and limitations.
The process of 3D printing starts with a digital model designed on a computer. This model is then broken down into instructions, by printer’s software to the machine language version and instructions understandable by the printer. After this step, various 3D printing techniques vary in their process of manufacturing the product. While FDM utilizes plastics, melts them down, and ejects them using a nozzle to add layers to the product, large industrial SLS machines take advantage of lasers guns to melt metal. These techniques also vary by the type of material they use. There is a large variety of materials being used in 3D printing. They range from soft plastics to metal sheets. Consequently, the products manufactured by these techniques also vary in their properties. Some have the rigidity of metals while others are made of plastics. Some are flexible and rubbery, and others are transparent like glass .
Time taken by the printers in the manufacturing process also varies based on the type of printer, the material used, and the size of the final product. So, it takes from four to 18 hours to complete the whole process. One of the drawbacks of 3D printed products is that they are not ready-to-use most of the time, just after printing. They often require (manual) efforts to get surface finishing and to make them use-worthy.
Types of 3D Printing Techniques:
Following are broad categories of 3D printing techniques:
- FDM: Fused Deposition Modelling
- SLA & DLP: Stereolithography & Digital Light Processing
- SLS: Selective Laser Sintering
- MJ: Material Jetting
- BJ: Binder Jetting
A brief description, as well as the pros and cons of each of these, are given below:
FDM: Fused Deposition Modelling
In Fused Deposition Modelling a printer is loaded with a reel of filament which is fed to the extrusion head equipped with a nozzle. This nozzle has a heating mechanism that helps in melting the filament. Once the nozzle is heated up to the required temperature, the filament is pushed through the nozzle to get the filament melted while passing through it. There is an extrusion head in the printer that moves according to the instructions and lays down the melted filament at the required positions precisely. Once the melted material lands its position, it solidifies and cools downs, and a layer is completed. When one layer is completed the printer moves to the next layer and the process is repeated multiple times until the finished product is obtained. Finally, post-processing is required. It includes finishing touches like smoothing the surface and removing support structures.
It is one of the most cost-effective 3D printing techniques to produce thermoplastic products and prototypes . Due to the high availability of the materials used in this printing, it has the shortest turnaround times. However, it has got its own limitations. While it is a very fast printing technique, it is not very accurate, and of high resolution. It requires post-processing to remove visible layers. It has the lowest dimensional accuracy and due to plastic material used in the manufacturing process, the products have intrinsic anisotropic properties . It makes them weaker and unsuitable for high stress and industrial applications.
SLA & DLP: Stereolithography & Digital Light Processing
Stereolithography and digital light processing, as their names indicate, are two similar techniques that employ the use of ultraviolet light in their manufacturing process. the UV light source is used to solidify liquid material in a container to get the final product. While SLP makes use of a single laser source in its operation, DLP relies on a digital light projector in the manufacturing . Post-processing is done after printing in order to enhance the strength of the material as well as the removal of residual resins.
There are numerous advantages of these techniques. Products manufactured using this technology generally possess high dimensional accuracy and details and great visual characteristics which makes them suitable for prototypes. Also, a large variety of liquid materials including biocompatible resins are available that are well suited to various industrial applications. However, other than visual characteristics, they don’t possess much physical strength and are highly unsuitable for outdoor usage . They are sensitive to direct sunlight and subject to degradation under UV light.
SLS: Selective Laser Sintering
In selective laser sintering technique polymer powder is used. A bin of this polymer powder is heated to achieve the temperature just below the melting point of the material. A thin layer of powder is deposited afterwards on to the platform used for manufacturing. Thickness of this layer is around 0.1mm. Then a laser comes into action that utilizes carbon dioxide. This laser scans the surface on which powder is sprinkled and binds the particles together to get the layer. In this way, powder is sprinkled and scanned and bound by carbon dioxide laser and the process is repeated . Once the printing is done, the container is allowed to cool. Final product is cleaned from unsintered powder and post-processing is done to improve visuals. Polish or dye may also be applied afterwards.
Products manufactured by this 3D printing technique are usually rich in isotropic mechanical properties that make them well-suited for functional prototypes. Also, this technique can handle complex geometries very easily . However, these benefits come at a higher cost compared to those techniques discussed previously and the use of powder might result in porous and grainy products.
Figure 3. Selective Laser Sintering
MJ: Material Jetting
Material Jetting is a 3D printing technology whereby a technique similar to inkjet printing is used. While a single layer of ink is deposited by an ink-jet printer, material jetting printing deposits multiple layers of the selected material onto the build platform in order to get the finished product. There are multiple jets that eject drops of the selected material onto the build platform. These drops are solidified using an ultraviolet light source . It also works layer by layer and when one layer is completed it moves on to the next layer and the process is repeated.
Material jetting is the most accurate 3D printing technique so far. It is not only capable of manufacturing multi-material products owing to its capability of having multiple jets, but also of producing multi-color products. It also results in intricate details and highly finished products having a mold-like appearance. However, these advantages do make it the most expensive 3D printing technology as well. Other disadvantages include degradation of products with time and a probability of brittleness in the final products.
Figure 4. Material Jetting
BJ: Binder Jetting
Binder jetting is a 3D printing technique with diverse capabilities and a variety of applications. It is capable of producing low-cost products as well as full-color products and mold production with the help of large sand casting. It is similar to material jetting in that it also works on same principle of jetting powder onto the build platform. It differs however, in the technique of solidifying those powder grains. While material jetting uses ultraviolet light source, binder jetting employs adhesive material whose droplets are sprinkled over the build platform to bind particles together. It also works layer-by-layer and once a layer is completed, the cycle repeats of ejecting powder and solidifying using adhesive repeats itself until the final layer is reached and product is manufactured . Initially, the product is very brittle and normally a thermal sintering is required at the end after removing the product from powder.
Binder jetting has its own unique advantages that somehow overshadow those of material jetting’s. It can produce the same products as material jetting but at a much lower cost. Objectives involving getting big manufacturing parts, complex geometries, great details, and low to medium batch productions can be easily achieved by this technique. However, the products are prone to porosity and require extensive post-processing to enhance visual and mechanical properties.
Figure 5 Binder Jetting
Types of 3D Printing Materials:
3D printing materials are dependent upon 3D printing techniques. Each technique utilizes different materials subject to the technology it uses. While plastics are the most commonly used by 3D printers, metals are also in use now. However, some 3D printing technologies can also make use of composites and ceramics. Following are the details regarding the plastics and metals used in 3D printing.
Plastics are the most commonly used materials in 3D printing owing their physical properties that make them suitable for printing not only prototypes but also functional products. Plastics can be broadly categorized in two types namely thermoplastics and thermosets . Thermoplastics are utilized by FDM and SLS 3D printing techniques whereas thermosets are being employed by SLA/DLP and material jetting technologies. Thermoplastics find their applications in functional products and thermosets in products having superior visual characteristics. Plastic materials used in 3D printing include PLA, ABS, Resins, Nylon, PETG, TPU, ASA and PEI.
Plastics are not suitable for applications that demand high tensile strength, hardness and ability to work in severe thermal conditions. It is where metals come into play and fulfil the needs of industry demanding such characteristics as mentioned above. While working with metals, topology optimization becomes a necessity to curb high costs incurred on printing and also to improve the performance . DMLS/SLM are the most widely used techniques for printing using metals. In recent years Binder Jetting has also gained importance due to its comparatively lower costs in application which are not that demanding. Binder jetting mostly utilizes stainless steel in its operations. As far as protype printing is concerned, extrusion-based techniques have become popular lately. Metals utilized in 3D printing include Stainless steel, Aluminum, Titanium, Cobalt-chrome and Nickel alloys.
Advantages and Disadvantages of 3D Printing:
3D printing, while becoming popular in recent years, has come head-to-head with traditional manufacturing technologies. If it offers some advantages, its capabilities are limited by some disadvantages as well and they are certain areas in which it has not been able to replace traditional manufacturing as of now. Following are some of the benefits and limitations of 3D printing.
Benefits of 3D Printing:
Complex geometries can be manufactured easily with the help of 3D printing many of which are not even possible with traditional manufacturing technologies. Interestingly, this added advantage of 3D printing comes with no extra costs involved during the printing process.
The traditional formative manufacturing processes require molds for each new design. These customized molds for each product result in higher costs. However, this is not the case with 3D printing since molds are not part of 3D printers. It also eliminates the necessity of large-scale manufacturing of products  in order to get the break-even and recoup the costs incurred on molds.
The capability of manufacturing complex geometrical shapes with almost zero initial costs makes 3D printing more customizable. In fact, every single item produced by 3D printers can be customized which is not the case with traditional manufacturing.
3D printing finds its vast utilization in manufacturing prototypes of products. In this aspect, no other manufacturing technology can compete with 3D printing, both in terms of cost and time.
Another advantage of 3D printing comes in the form of the flexibility of the material used during the printing process. While plastics are the most commonly used materials, metals are well-suited for industrial applications. Other than plastics and metals, composites and ceramics are also being used by 3D printers.
Limitations of 3D Printing:
Products manufactured with the help of 3D printing do not possess high tensile strength and degraded physical properties compared to the products manufactured by traditional manufacturing techniques. This limits the application of the products to prototypes and non-functional applications. 3D printing using metals is a better alternative to address the issue.
While traditional manufacturing is not cost-effective in small-scale manufacturing, 3D printing loses its significance in large-scale manufacturing as far as cost-effectiveness is concerned .
The accuracy achieved during 3D printing is subject to the process used during printing. This results in not-so-good tolerances. And in some techniques whereby plastics are used for printing, accuracy is also not that great in the final products.
Another major disadvantage of 3D printing is that it requires post-processing before making products worthy of testing and utilization. They require additional operations and finishing in order to have a quality finish and better visual characteristics.
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