Rapid Pro To Typing

Rapid Prototyping (RP) Introduction From art to part is the main theme of rapid prototyping. A CAD model of the part is sliced and downloaded to a rapid prototyping system to get a three-dimensional print of the part. Rapid prototyping was first introduced with stereolithography from 3-d Systems in 1988. Since then, there has been numerous type of rapid prototyping system being introduced into the market. However, in most cases, due to the limitation imposed on the technology as a result of the available materials and systems, rapid prototyping is utilized to shorten the art to part cycle rather than using the rapid prototyping part as a functional part. This stream of applications includes design verification, reverse engineering, sand casting and investment casting. When the RP was first introduced the prediction were that it would sweep through industries. The result however was that the rapid prototyping equipment produced only plastic models which cannot be used as functional parts. The first rapid prototyping models were brittle objects made fairly loose tolerances. The technology continued to evolve in the areas of process adopted, materials used as well as the role of rapid prototyping model is expected to play. Engineers no longer build rapid prototyping models simply to check geometry against prints. In this lecture we will cover the process used, the system running the process and the available rapid prototyping materials. Types of Rapid Prototyping There are several types of RP available in the market currently. We will look at the most widely known rapid prototyping system and their process in this lecture. The systems are:

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Stereolithography (SLA) Fused Deposition Modeling (FDM) Solid Ground Curing (SGC) Selection Laser Sintering (SLS) Laminated Object Manufacturing (LOM)

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Stereolithography (SLA) First introduced in 1988, the stereolithography (SLA) used a laser beam to solidify the layer of resin. In the process, the designs created on a CAD system is converted into .STL format. In this .STL format, the object is sliced into two-dimensional cross-section with a slice thickness ranging from 0.0015Ó to 0.005Ó. The work area is spread with a thin layer of photopolymer in which each crosssectional layer of the model is formed using a laser beam to solidify the layer of resin, in figure 1. As each layer solidifies, an elevator platform lowers the work piece to allow another layer of resin to be applied. A wiper smoothes the layer to a proper thickness and the next layer is drawn, adhering to the previous layer. The process is repeated until the part is completed. Each layer is not fully cured by laser, but instead with a thin wall and a honeycomb internal structure which trap the uncured resin. The top and bottom surfaces of the part are fully cured by repeated passes made by the laser in a pattern of overlapping lines called skin-fill. Approximately 96 % of the part are solidified before the post-curing process. Final curing is achieved by placing the part in the curing oven, which floods the part with UV light to complete polymer solidification process. Advantages: 1. Unattended operation This system is fully automated that is doesnÕt require any attention during the operation. 2. Good accuracy The SLA able to produce a very high accuracy ≈ 0.001Ó 3. High detail SLA is capable of producing an intricate design with high precision. 4. Surface finish The Òprinted partÓ has a very smooth surface. 5. Industry presence SLA was the first RP to be introduced into the market. It constituted 71% of the worldwide market based on sales. Disadvantages: 1. Post-curing This process requires post-curing to complete the solidification process. Some area of the parts did not solidify during the laser beaming process. 2. War page The part may melt if exposed to high temperature for a period of time.

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Figure.1 The StereoLithography Process

3. Limited material This process is limited by the use of polymers. Only certain polymers can be used for the process. 4. Supports Support is always needed when the parts being ÒprintedÓ. Often the removal of these supports can be difficult.

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Fused Deposition Modeling (FDM) FDM consist of main 3-D modeler, ProtoSlice software and silicon Graphics workstation. The process starts with the creation of the CAD design and it is sliced into horizontal cross-section by ProtoSlice software. These cross-sections are then downloaded into FDM machines. The modeler consist of a fixtureless base, a spool containing filament material and head which heats and deposits the RP material. The material is transferred from the spool to the head where it is heated to one degree above the solidification point. The melted material is extruded through the head and deposited onto a fixtureless foundation, shown in figure 2. The head move across the part in a pattern set by the cross-sectional CAD data. Material is deposited in layers where it is adhered to the previous layer by thermal diffusion until the part is built-up. The successive laminations occur within a layer thickness range of 0.001Ó to 0.030Ó and wall thickness range of 0.009Ó to 0.25Ó. The operation temperature is in the range of 180 to 220 degrees Fahrenheit. Advantages: 1. Post ÐCuring Unlike SLS, FDM does not require any post-curing. The wax solidifies during the operation. 2. Temperature The operation temperature is in the range of 180 Ð220 degrees Fahrenheit, making it safe for office use. There is no worry of exposure to toxic chemicals, lasers or liquid polymer. 3. Wax The process uses no powder and therefore there is no messy cleanup.

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Figure.2 The Fused Deposition Modelind Process Laminated Object Manufacturing (LOM) The process started with the sliced data of a #D model created on CAD system, The laser used this data to cut the outline of the slice out of a layer of the material being used to produced the model shown in figure 3. Once the layer is completely cut, another layer of material is bonded to the top of the layer that was just cut. This process is repeated until the model is complete and then separated from the excess material.

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Advantages: 1. Materials LOM uses varieties of thin sheet included paper, plastic and composites. The sheets are coated with heat sensitive adhesive, which enable the sheet to be bonded layer by layer by hot compression to form the part. 2. Properties The parts created by LOM system are durable structures, which have a wide variety of applications. The parts created may be sanded, polished, coated and painted. 3. Post-curing Since LOM parts use papers, there is no need for post-curing. 4. Cost There is no need of special polymer or wax to build the part. Thus this reduce the cost of buying these special polymer. 5. Stress Since the part is bonded layer by layer, there is no induced stress in the parts. Disadvantages: 1. Quality The process has the wood like quality on the surface finish and it is sensitive to humid / wet environment. 2. Strength The parts are weak when subjected to stress in the direction perpendicular to lamination 3. Geometry Geometries of the parts that can be produced by the process are limited by the size of access hole which are required to remove excess material from the part.

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Figure.3 The Laminated Object Manufacturing Process

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Solid Ground Curing (SGC) The process involves two simultaneous operations, the mask plotter and model grower cycle, shown in figure 4. In the mask plotter, the image of a cross-section of CAD data or slice is transformed to mask generator. The mask plate is electrostatically charged and the image is developed on the plate using electrostatic toner producing a mask of the slice. As the mask is prepared, a thin layer of liquid photopolymer is spread on the work piece area as the first step of the model grower cycle. When the mask is ready and positioned over the workspace, a shutter is opened and strong flash of ultra-violet light is exposed to the mask. The areas on the layer exposed to the light through the mask are fully cured instantly forming a solid layer of resin in the pattern of the mask. The mask plate is then cleaned of the pattern and a new image charged to the plate. On the work piece, all non-solidified material is wipe off and collected for reuse. Melted wax is then used to fill the cavities of the model after the photopolymer is removed. A cooled plate is then lowered onto the work piece instantly solidifying the wax and forming a solid support structure of the model. The work piece is passed under a milling, which mills the layer down to precise thickness thereby producing a flat surface for the next layer of photopolymer to adhere to. The work piece is then lower by one layer height and the process is repeated until the part is completed. Advantages: 1. Post ÐCuring Unlike SLS, SGC does not require any post-curing. The resin/wax solidifies during the operation. 2. Warpage The entire layer is cured during the operation and there is no need for the part to go to oven for further solidification. This reduces the chance of the part to change it state. 3. Mechanical Assemblies Little assemblies is needed, since the process is controlled by the computers. 4. Support No support is needed in the operation as the part is being held on a fixtureless base. 5. Predictable speed With the constant speed of the mask, it is relatively easy to predict the build time of the part. 6. Nesting

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Disadvantages: 1. Excess waste There is a waste of the wax due to the heating and solidifying of the wax during the operation. 2. Attended operation. SGC requires a technician to monitor the whole process. 3. Wax removal There is an additional work after the process is done: removing the wax. Wax builds up during the operation need to be removed before continuing for another process. 4. Materials This process is limited by the use of polymers. Only certain polymers can be used for the process. 5. Cost FDM is the largest and the most expensive RP system in the market. Thus it is not economical to own for private use or for occasion operation.

Figure.4 The Solid Ground Curing Process

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Selective Laser Sintering (SLS) SLS operates the same basic layer y layer principle as SLA, however it used a powder instead of liquid. The process starts by downloading the sliced data of the 3-D model into the SLS system. Within the SinterStation 2000, a compartment wall consists of infrared heat panels, which act to heat the powdered material to just below the material melting point. The workspace consists of a platform that lowers the model as each successive layer is added. A powder cartridge supplied the powder material used to produce the part, and a roller is used to distribute the material evenly across the workspace. A thin layer of powder material is spread evenly across the workspace. The laser traces the pattern of the slice, heating and fusing the material it come in contact with. Careful modulation of the laser beam assures that the surrounding powdered material remains unaffected. After one layer of the designed part is formed, another layer of powder material is spread and then distributed by the roller. The movable platform lowers the work piece to allow for solidification of the next layer. The process is repeated until the part is completed. After the part is completed, excess powder is removed by a jet of air.

Advantages: 1. Materials SLS has the ability to work with varieties of material including possible ceramic and metal parts. 2. Post-curing Since the working piece for SLS is the powder, this eliminates the need for post-curing. 3. Support The unsintered material surrounding the part acts as a support structure for the part hereby eliminating the need for external support Disadvantages 1. Densities The process has unpredictable densities of the produced parts. 2. Surface finish The produced parts have a rough surface finish. 3. Setup It is difficult to change setup from one working material to another.

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Figure.5 The Selective Laser Sintering Process Applications 1. Design Verification Ø The design verification consists of form, fit and function and this type of application require a physical model but it does not have to be the same material for the final production part.

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Ø This application has the ability to compare designs, incorporate design changes, and obtain design approval without making a prototype. Ø The prototype is used as a quick and efficient which means Òshow and Tell Ò or Òproof of concept.Ó Ø Provided answer quickly and feedback to the designers and also enables to make iterations prior to the final product. Ø Removes ambiguities that present in two dimensional renderings. Ø The disadvantages in this application is the expensive tool 2. Manufacturing Producibility studies Ø Required jigs and/or fixtures to perform the manufacturing function. Ø The setup can not be performed until the parts are manufactured and take times if the part requires hard tooling. Ø This application is very cost effective when the prototype has been approved and made. 3. Mold Making Ø The prototype has to be fabricated from some material instead of rapid prototype plastic. Ø Normally, the model shops make a precision model, or master pattern where the mold will be fabricated. Ø The main applications for this applications in the mold making industry are in the areas of silicon and spray metal molds. Ø The silicon molding is soft tooling process which used to produce 10 Ð 15 parts made from polyurethane and investment casting wax. Ø This application process is relatively inexpensive Ø The disadvantage of this process is limited to simple part like producing a two part mold. Ø The high surface finish and high dimensional accuracy can be accomplished by tooling the mold. 4. Casting Ø This application involves investment casting and sand casting Ø Rapid prototype can create a mold which wax patterns can be fabricated and used in the lost wax process. Ø This application use to prove the castability of a part geometry and aid in obtaining the cast shrink factors. Ø The disadvantage of this rapid prototypes is that SLA and LOM models cannot be melted out of the ceramic shell using a steam autoclave, because it burned out at very high temperatures that require a longer processing time.

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Ø The application has high cost and the surface finish is not compatible to hard tooling for the rapid prototyping technology.

5. Reverse Engineering Ø This is the process of reconstructing the design information for a previously designed part starting from a product until its final form. Ø The part is scanned to build the CAD model. Ø The various application for this application such as: Medicine (prostheses, plastic surgery) which combines reverse engineering and rapid prototyping for building artificial body parts.

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