3d Printing

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28 February 2016

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1. Introduction
2. History Of 3D Printing
3. Current 3D Printing Technologies
4. The Basic Process Of 3D Printing
5. 3D Printing VS Conventional Technologies
6. Applications
7. Conclusion


3Dimensions printing is a method of converting a virtual 3D model into a physical object. 3D printing is a category of rapid prototyping technology. 3D printers typically work by printing successive layers on top of the previous to build up a three dimensional object. The past decade has witnessed the emergence of new manufacturing technologies that build parts on a layer-by-layer basis. Using these technologies, manufacturing time for parts of virtually any complexity is reduced considerably. In other words, it is rapid. Rapid Prototyping Technologies and Rapid Manufacturing offer great potential for producing models and unique parts for manufacturing industry. A few years ago, to get some prototyping work done for a product or design you are working on, you are required to spend a lot of man-hours just to come up with the model. Those hours will be spent creating miniature parts of your design using wood and then gluing all those parts together painstakingly. Prototyping is, at the very least, time-consuming and extremely tedious. These days, however, you can take the tediousness and the time investment out of your prototyping tasks through rapid prototyping or 3d printing. 3D printing is a revolutionary method for creating 3D models with the use of inkjet technology. Many engineers have even dubbed 3D printing as the process of creating something out of nothing. Thus, the reliability of products can be increased; investment of time and money is less risky. Not everything that is thinkable today is already workable or available at a reasonable price, but this technology is fast evolving and the better the challenges, the better for this developing process. The term Rapid prototyping (RP) refers to a class of technologies that can automatically construct physical models from Computer-Aided Design (CAD) data. It is a free form fabrication technique by which a total object of prescribed shape, dimension and finish can be directly generated from the CAD based geometrical model stored in a computer, with little human intervention. Rapid prototyping is an “additive” process, combining layers of paper, wax, or plastic to create a solid object. In contrast, most machining processes (milling, drilling, grinding, etc.) are “subtractive” processes that remove material from a solid block. RP’s additive nature allows it to create objects with complicated internal features that cannot be manufactured by other means. In addition to prototypes, RP techniques can also be used to make tooling (referred to as rapid tooling) and even production-quality parts (rapid manufacturing). For small production runs and complicated objects, rapid prototyping is often the best manufacturing process available. Of course, “rapid” is a relative term. Most prototypes require from three to seventy-two hours to build, depending on the size and complexity of the object. This may seem slow, but it is much faster than the weeks or months required to make a prototype by traditional means such as machining. These dramatic time savings allow manufacturers to bring products to market faster and more cheaply.


Imagine a future in which a device connected to a computer can print a solid object. A future in which we can have tangible goods as well as intangible services delivered to our desktops or highstreet shops over the Internet. And a future in which the everyday “atomization” of virtual objects into hard reality has turned the mass pre-production and stock-holding of a wide range of goods and spare parts into no more than an historical legacy. Such a future may sound like it is being plucked from the worlds of Star Trek. However, whilst transporter devices that can instantaneously deliver us to remote locations may remain a fantasy, 3D printers capable of outputting physical objects have been in development for over two decades. What’s more, several 3D printers are already on the market. Available from companies including Fortus, 3D Systems, Solid Scape, ZCorp, and Desktop Factory, these amazing devices produce solid, 3D objects from computer data in roughly the same way that 2D printers take our digital images and output hardcopy photos. History Of 3D Printing

The technology for printing physical 3D objects from digital data was first developed by Charles Hull in 1984. He named the technique as Stereo lithography and obtained a patent for the technique in 1986.While Stereo lithography systems had become popular by the end of 1980s, other similar technologies such as Fused Deposition Modeling (FDM) and Selective Laser Sintering (SLS) were introduced.In 1993, Massachusetts Institute of Technology (MIT) patented another technology, named “3 Dimensional Printing techniques”, which is similar to the inkjet technology used in 2D Printers.In 1996, three major products, “Genisys” from Stratasys, “Actua 2100” from 3D Systems and “Z402” from Z Corporation were introduced.In 2005, Z Corp. launched a breakthrough product, named Spectrum Z510, whichwas the first high definition color 3D Printer in the market.Another breakthrough in 3D Printing occurred in 2006 with the initiation of an open source project, named Reprap, which was aimed at developing a self-replicating 3D printer. 3. Current 3D Printing Technologies

Most commercially available rapid prototyping machines use one of six techniques. At present, trade restrictions severely limit the import/export of rapid prototyping machines, so this guide only covers systems available in the U.S. 3.1 Stereo lithography

Patented in 1986, stereolithography started the rapid prototyping revolution. The technique builds three-dimensional models from liquid photosensitive polymers that solidify when exposed to ultraviolet light. As shown in the figure below, the model is built upon a platform situated just below the surface in a vat of liquid epoxy or acrylate resin. A low-power highly focused UV laser traces out the first layer, solidifying the model’s cross section while leaving excess areas liquid. Next, an elevator incrementally lowers the platform into the liquid polymer. A sweeper re-coats the solidified layer with liquid, and the laser traces the second layer atop the first. This process is repeated until the prototype is complete. Afterwards, the solid part is removed from the vat and rinsed clean of excess liquid. Supports are broken off and the model is then placed in an ultraviolet oven for complete curing. Because it was the first technique, stereolithography is regarded as a benchmark by which other technologies are judged. Early stereolithography prototypes were fairly brittle and prone to curing-induced warpage and distortion, but recent modifications have largely corrected these problems.

Fig 2.1:Stereo lithography
3.2 Laminated Object Manufacturing
In this technique, developed by Helisys of Torrance, CA, layers of adhesive-coated sheet material are bonded together to form a prototype.. As shown in the figure below.
Fig2.2: Schematic diagram of laminated object manufacturing. A feeder/collector mechanism advances the sheet over the build platform, where a base has been constructed from paper and double-sided foam tape. Next, a heated roller applies pressure to bond the paper to the base. A focused laser cuts the outline of the first layer into the paper and then cross-hatches the excess area (the negative space in the prototype). Cross-hatching breaks up the extra material, making it easier to remove during post-processing. During the build, the excess material provides excellent support for overhangs and thin-walled sections. After the first layer is cut, the platform lowers out of the way and fresh material is advanced. The platform rises to slightly below the previous height, the roller bonds the second layer to the first, and the laser cuts the second layer. This process is repeated as needed to build the part, which will have a wood-like texture. Because the models are made of paper, they must be sealed and finished with paint or varnish to prevent moisture damage. Helisys developed several new sheet materials, including plastic, water-repellent paper, and ceramic and metal powder tapes. The powder tapes produce a “green” part that must be sintered for maximum strength. As of 2001, Helisys is no longer in business.

3.3 Selective Laser Sintering
Developed by Carl Deckard for his master’s thesis at the University of Texas, selective laser sintering was patented in 1989. The technique, shown in Fig, uses a laser beam to selectively fuse powdered materials, such as nylon, elastomer, and metal, into a solid object. Parts are built upon a platform which sits just below the surface in a bin of the heat-fusable powder. A laser traces the pattern of the first layer, sintering it together. The platform is lowered by the height of the next layer and powder is reapplied. This process continues until the part is complete. Excess powder in each layer helps to support the part during the build. SLS machines are produced by DTM of Austin, TX.

Fig 2.3: Schematic diagram of selective laser sintering.

3.4 Fused Deposition Modeling
In this technique, filaments of heated thermoplastic are extruded from a tip that moves in the x-y plane. Like a baker decorating a cake, the controlled extrusion head deposits very thin beads of material onto the build platform to form the first layer. Fig2.4: schematic diagram of fused deposition modeling. The platform is maintained at a lower temperature, so that the thermoplastic quickly hardens. After the platform lowers, the extrusion head deposits a second layer upon the first. Supports are built along the way, fastened to the part either with a second, weaker material or with a perforated junction.

3.5 Solid Ground Curing

Developed by Cubital, solid ground curing (SGC) is somewhat similar to stereolithography (SLA) in that both use ultraviolet light to selectively harden photosensitive polymers. Unlike SLA, SGC cures an entire layer at a time. Figure 5 depicts solid ground curing, which is also known as the solider process. First, photosensitive resin is sprayed on the build platform. Next, the machine develops a photomask (like a stencil) of the layer to be built. This photomask is printed on a glass plate above the build platform using an electrostatic process similar to that found in photocopiers. The mask is then exposed to UV light, which only passes through the transparent portions of the mask to selectively harden the shape of the current layer.

Fig 2.5: Schematic diagram of solid ground curing.
After the layer is cured, the machine vacuums up the excess liquid resin and sprays wax in its place to support the model during the build. The top surface is milled flat, and then the process repeats to build the next layer. When the part is complete, it must be de-waxed by immersing it in a solvent bath. SGC machines are distributed in the U.S. by Cubital America Inc. of Troy, MI. The machines are quite big and can produce large models.

3.6 3D Printing

Ink-Jet Printing refers to an entire class of machines that employ ink-jet technology. The first was 3D Printing (3DP), developed at MIT and licensed to Soligen Corporation, Extrude Hone, and others. The ZCorp 3D printer, produced by Z Corporation of Burlington, MA is an example of this technology. As shown in Figure 6a, parts are built upon a platform situated in a bin full of powder material. An ink-jet printing head selectively deposits or “prints” a binder fluid to fuse the powder together in the desired areas. Unbound powder remains to support the part. The platform is lowered, more powder added and leveled, and the process repeated. When finished, the green part is then removed from the unbound powder, and excess unbound powder is blown off. Finished parts can be infiltrated with wax, CA glue, or other sealants to improve durability and surface finish. Typical layer thicknesses are on the order of 0.1 mm. This process is very fast, and produces parts with a slightly grainy surface. ZCorp uses two different materials, a starch based powder (not as strong, but can be burned out, for investment casting applications) and a ceramic powder. Machines with 4 color printing capability are available. 3D Systems’ version of the ink-jet based system is called the Thermo-Jet or Multi-Jet Printer. It uses a linear array of print heads to rapidly produce thermoplastic models (Figure 6d). If the part is narrow enough, the print head can deposit an entire layer in one pass. Otherwise, the head makes several passes. Sanders Prototype of Wilton, NH uses a different ink-jet technique in its Model Maker line of concept modelers. The machines use two ink-jets (see Figure 6c). One dispenses low-melt thermoplastic to make the model, while the other prints wax to form supports. After each layer, a cutting tool mills the top surface to uniform height. This yields extremely good accuracy, allowing the machines to be used in the jewelry industry. Ballistic particle manufacturing, depicted in Figure 6b, was developed by BPM Inc., which has since gone out of business.

Fig 2.6: Schematic diagrams of ink-jet techniques.
4. The Basic Process of 3D Printing
Although several rapid prototyping techniques exist, all employ the same basic five-step process. The steps are: 1. Create a CAD model of the design
2. Convert the CAD model to STL format
3. Slice the STL file into thin cross-sectional layers
4. Construct the model one layer atop another
5. Clean and finish the model
CAD Model Creation: First, the object to be built is modeled using a Computer-Aided Design (CAD) software package. Solid modelers, such as Pro/ENGINEER, tend to represent 3-D objects more accurately than wire-frame modelers such as AutoCAD, and will therefore yield better results. The designer can use a pre-existing CAD file or may wish to create one expressly for prototyping purposes. This process is identical for all of the RP build techniques. Conversion to STL Format: The various CAD packages use a number of different algorithms to represent solid objects. To establish consistency, the STL (stereolithography, the first RP technique) format has been adopted as the standard of the rapid prototyping industry. The second step, therefore, is to convert the CAD file into STL format. This format represents a three-dimensional surface as an assembly of planar triangles, “like the facets of a cut jewel.” 6 The file contains the coordinates of the vertices and the direction of the outward normal of each triangle. Because STL files use planar elements, they cannot represent curved surfaces exactly. Increasing the number of triangles improves the approximation, but at the cost of bigger file size. Large, complicated files require more time to pre-process and build, so the designer must balance accuracy with manageability to produce a useful STL file. Since the STL format is universal, this process is identical for all of the RP build techniques. Slice the STL File: In the third step, a pre-processing program prepares the STL file to be built. Several programs are available, and most allow the user to adjust the size, location and orientation of the model. Build orientation is important for several reasons. First, properties of rapid prototypes vary from one coordinate direction to another. For example, prototypes are usually weaker and less accurate in the z (vertical) direction than in the x-y plane. In addition, part orientation partially determines the amount of time required to build the model. Placing the shortest dimension in the z direction reduces the number of layers, thereby shortening build time. The pre-processing software slices the STL model into a number of layers from 0.01 mm to 0.7 mm thick, depending on the build technique. The program may also generate an auxiliary structure to support the model during the build. Supports are useful for delicate features such as overhangs, internal cavities, and thin-walled sections. Each RP machine manufacturer supplies their own proprietary pre-processing software.

Layer by Layer Construction: The fourth step is the actual construction of the part. Using one of several techniques (described in the next section) RP machines build one layer at a time from polymers, paper, or powdered metal. Most machines are fairly autonomous, needing little human intervention.

Fig 3.2 Layer by Layer Construction
Clean and Finish: The final step is post-processing. This involves removing the prototype from the machine and detaching any supports. Some photosensitive materials need to be fully cured before use. Prototypes may also require minor cleaning and surface treatment. Sanding, sealing, and/or painting the model will improve its appearance and durability.

3D printing VS Conventional Technologies
3DP does not—and will not—replace completely conventional technologies such NC and high-speed milling, or even hand-made parts. Rather, one should regard 3DP as one more option in the toolkit for manufacturing parts. Figure depicts a rough comparison between 3DP and milling regarding the costs and time of manufacturing one part as a function of part complexity10. It is assumed, evidently, that the part can be manufactured by either technology such that the material and tolerance requirements are met.

Fig5.1: 3DP vs. conventional machining

Application of 3D printers
The concept of custom manufacturing is exciting to nearly everyone, but it always seems to be something that will happen in the “future”. Gibson was right and the following list of applications for 3D printers show the truth in the saying “The future is here. It’s just not evenly distributed yet.” The following items are all available for purchase or are being used in industry now. We are still a long way from Replicators like the ones from Star Trek: The Next Generation, but we probably won’t have to wait til the 24th century either.

1. Art

3D printing allows artists to create objects that would be incredibly difficult, costly, or time intensive using traditional processes. These sculptures by Bathsheba Grossman are exquisitely complex and manufactured using a laser sintering process.

2. Action Figures

Blood Elves and band mates can both be brought to life using 3D printers. These two were created using Zcorp. machines which apply glue ink and powder in fine layers slowly creating a replica of one of your characters. Figure Prints allows you to create characters from Warcraft, Rock band and Spore printing services are coming soon. A number of other sites allow you to pull data from Second Life and your own 3D programs.

3. Jewelry

Jewelry makers were some of the first to use 3D printing in their manufacturing process, however they do not use metal printers, but rather ones that use wax. In a process called “investment casting” a piece of jewelry is sculpted or printed out of wax. Plaster is then poured on either side. Molten metal is poured onto the wax which melts out leaving a metal version of your wax sculpt in its place in the plaster. This piece is then finished and polished by a jeweler. Many independent jewelers have been using high tech printers in their businesses and an innovative company called Paragon Lake has combined this process with web based design tools to offer an infinite inventory to the masses of jewelry stores.

4. Hearing Aids

3D printers can also make things more functional. In the case of hearing aids a cast of your ear canal is made. The casting is digitized using a 3D scanner and a perfect replica of your ear is printed from that ensuring a great fit and improving the quality.

5. Prototypes

Prototyping in product development is currently the biggest use of 3D printing technology. These machines allow designers and engineers to test out ideas for dimensional products cheaply before committing to expensive tooling and manufacturing processes.

6. Home Decor

Home goods are structurally simple but endlessly decorative and are perfect matches for 3D printing. This service, called “Shapeways Creator” allows you to create products like this lamp with any selection of words that have relevance to you (wedding vows, a favorite poem, etc.). Another company called JuJups allows you to make a customized picture frame using intelligent design tools and a zCorp printer.

7. Models

Sales folks lives get much easier when you can have models like this of your product printed up for show and tell.

8. Components/Manufacturing

Many of the examples so far are somewhat gimmicky or decorative, However in some industries 3D printing is displacing traditional manufacturing entirely. In the left hand picture a surgical knee replacement implant has been designed and manufactured to fit a patient’s joint perfectly. On the right, high tolerance engine parts were printed using a process called “Electron Beam Melting” and finished with traditional machining processes. While not the norm these uses begin to suggest what is possible in medicine and industry.

9. Medicine

3D World of Warcraft characters are cool, but these tools have the power to help save lives. Surgeons are using 3d printers to print body parts for reference before complicated surgeries. Other 3D printers are used to create bone grafts for patients who have suffered traumatic injuries. Looking further in the future scientist are working on PRINTING replacement organs. Personal Fabrication indeed!

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