What is Rapid prototyping?• The first commercial rapid prototyping process was brought on the market in 1987.• Rapid prototyping is a technology which considerably speeds up the product development, by allowing corrections in early stages.• It consists of various manufacturing processes by which a solid physical model of part is made directly from 3D CAD model data without any special tooling.• It is also referred as computer automated manufacturing.• Now a days, more than 30 different processes (not all commercialized) with high accuracy and a large choice of materials exist.
Why Rapid prototyping?• Visualization.• Complex shapes can be build.• Decreases lead time.• Facilitates concurrent engineering.• No tooling.• Testing and evaluating the design.• First to market(product).• Patentable details.
DIFFERENCE BETWEEN RAPID PROTOTYPING AND TRADITIONAL FABRICATION RP Traditional1. RP processes are “additive”. Parts are built 1. Subtractive processes, as the name up by adding, depositing, or solidifying in a implies, create objects by removing horizontal layer-wise process. unwanted material from a large block or sheet in the form of chips.2. RP technologies are able to create one- 2. Part is made in assemblies. piece part geometries.3. Undercuts, overhangs, difficult shapes can 3. Useful for making simple shapes only. me made.4. No tooling required. 4. Tooling is always required (jigs and fixtures).5. Part can be fabricated in hours. 5. Take more time.6. Rough surface finish. 6. Good surface finish.7. Less human interaction. 7. More human interaction.8. Non dangerous in nature 8. Dangerous in nature. (dust free, noise free).
ADVANTAGES OF RAPID PROTOTYPING• No planning of process sequences.• No specific material for material handling.• No transportation between machines.• No need of jigs and fixtures.• High accuracy.• Tool for visualisation and product verification.• CAD data files can be manufactured in hours.
SOLID MODELING AND TESSELATION• Solid model is generated with the help of softwares (eg: pro engineers, solidworks, etc).• Surfaces of the CAD model are tessellated and STL file is exported.• Tessellation is piecewise approximation of surfaces of CAD model by using series of triangles.• STL file=An stl file is a standardized computer exchange file which contains a 3d model, it entirely composed of triangular faces, further the faces have assigned normals which indicates their orientation (inside/outside).• The name “STL” is taken from its extension .stl .
Slicing• After selecting part orientation, the tessellated model is sliced.• The pre-processing software slices the stl model into a no of layers from 0.01mm to 0.7mm thick, depending upon 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.
GENERATION OF LASER SCANING PATHS Deposition of Layers: various technologies for layer deposition are used.• Liquid based: Stereolithography• Powder based: Selective Laser Sintering• Solid based: Fused Deposition Modeling, Laminated Object Manufacturing etc.
Post processing/cleaning• 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.• painting the model will improve its appearance and durability.
METHODS OF RAPID PROTOTYPING• Shape deposition manufacturing(SDM)• Stereolithography(SLA)• Solid base curing(SBC)• Fused deposition modeling(FDM)• Ballistic particle manufacturing (BPM)• 3D Printing• Selective layer sintering(SLS)• Laminated object manufacturing(LOM)
Shape deposition manufacturing• Shape Deposition Manufacturing (SDM) is developed at Carnegie Mellon University in the early 1990s.• Deposition of material(eg: welding).• After deposition each layer is precisely shaped with a cnc milling machine.• Due to thermal deposition the internal residual stresses are build up, due to contraction of internal gradients of previous layers on the new layer, the internal stresses can lead to warpage, early failure of the material.• Shot peening is required to control the build up of stresses(small round metal spheres are called “shot”)are projected at high velocity against the surface of layer it imparts the compressive load on the upper layer, this process is used to balance the stress.
SDM• Special components can be embedded inside the structure.• The filler material is removed to get the finished part.
STEREOLITHOGRAPHY• Stereolithography started the rapid prototyping revolution. The technique builds three-dimensional models from liquid photosensitive polymers that solidify when exposed to ultraviolet light.• Elevator lowers hardened cross section below liquid surface.• Laser prints the next cross section directly on top of previous.• The self-adhesive property of the material causes each succeeding layer to bond to the previous one and thus form a complete, three-dimensional object out of many layers.• Objects which have overhangs or under cuts must be supported during the fabrication process by support structure.
SOLID BASE CURING• Cross section shape is printed onto a glass mask• Glass mask is positioned above photopolymer tank• UV lamp shines through mask onto photopolymer light only can pass through clear part, polymer solidifies there, polymer in masked areas remains liquid• All excess polymer is removed-part is again hit with UV light• Melted wax is spread over workpiece filling all spaces• Workpiece is precisely milled flat• Glass is erased and re-masked, workpiece is placed slightly below surface in photopolymer repeats• After fabricating part, wax is melted and removed• Note : No support or post cure needed
FDM• FDM is the second most widely used rapid prototyping technology, after stereolithography.• A plastic filament is unwound from a coil and supplies material to an extrusion nozzle.• The nozzle is heated to melt the plastic and has a mechanism which allows the flow of the melted plastic to be turned on and off.• The nozzle is mounted to an X-Y plotter type mechanism which traces out the part contours, There is a second extrusion nozzle for the support material (different from the model material).• Materials used mostly are ABS and PC(polycarbonate).
Ballistic particle manufacturing (BPM) Employs a technology called Digital Microsynthesis• Molten plastic is fed to a piezoelectric jetting mechanism, similar to those on inkjet printers.• A multi-axis controlled NC system shoots tiny droplets of material onto the target, using the jetting mechanism.• Small droplets freeze upon contact with the surface, forming the surface particle by particle. Process allows use of virtually any thermoplastic.
3D PRINTING• The process starts by depositing a layer of powder object material at the top of a fabrication chamber. To accomplish this, a measured quantity of powder is first dispensed from a similar supply chamber by moving a piston upward incrementally.• A roller or scraper then distributes and compresses the powder at the top of the fabrication chamber the jetting head subsequently deposit a liquid adhesive (binder) in a two dimensional pattern onto the layer of the powder. The binder bonds the powder particles together where it has been deposited, solidifying it to form a layer of the object.• Once a layer is completed, the fabrication piston moves down by one layer thickness, and the process is repeated until the entire object is formed within the powder bed. After completion, the object must be removed from the chamber, and excess powder is brushed off.• No external supports are required during fabrication since the powder bed supports overhangs.
SELECTIVE LAYER SINTERING(SLS)• SLS or Selective Laser Sintering is another process that begins with raw material in a powdered state. Only instead of binding with some extra solvent (as was done in 3-D Printing) powder particles are sintered together, by heating with a laser until adjacent particles sinter together.• So process begins with a cartridge feeding system spreading a thin layer of heat fusible (sinterable) powder into workspace container. This layer of powder is heated to just below its melting point.• A carbon dioxide laser then traces the cross sectional shape. Wherever the laser hits, enough extra heat is generated to sinter the particles together. Upon completion of one layer, the process can repeat.
LAMINATED OBJECT MANUFACTURING• A solid physical model is made by stacking layers of sheet stock, each an outline of the cross-sectional shape of a CAD model that is sliced into layers.• Starting material = sheet stock, such as paper, plastic, cellulose.• After cutting, excess material in the layer remains in place to support the part during building.