RAPID PROTOTYPING

Rapid prototyping takes virtual designs from computer aided design (CAD) or animation modeling software, transforms them into thin, virtual, horizontal cross-sections and then creates each cross-section in physical space, one after the next until the model is finished. It is a WYSIWYG process where the virtual model and the physical model correspond almost identically.

With additive fabrication, the machine reads in data from a CAD drawing and lays down successive layers of liquid, powder, or sheet material, and in this way builds up the model from a series of cross sections. These layers, which correspond to the virtual cross section from the CAD model, are joined together or fused automatically to create the final shape. The primary advantage to additive fabrication is its ability to create almost any shape or geometric feature.

The standard data interface between CAD software and the machines is the STL file format. An STL file approximates the shape of a part or assembly using triangular facets. Smaller facets produce a higher quality surface.

The word "rapid" is relative: construction of a model with contemporary methods can take from several hours to several days, depending on the method used and the size and complexity of the model. Additive systems for rapid prototyping can typically produce models in a few hours, although it can vary widely depending on the type of machine being used and the size and number of models being produced simultaneously.

Some solid freeform fabrication techniques use two materials in the course of constructing parts. The first material is the part material and the second is the support material (to support overhanging features during construction). The support material is later removed by heat or dissolved away with a solvent or water.

Traditional injection molding can be less expensive for manufacturing plastic products in high quantities, but additive fabrication can be faster and less expensive when producing relatively small quantities of parts.

Rapid prototyping is now entering the field of rapid manufacturing and it is believed by many experts that this is a "next level" technology. Pioneer companies have already claimed to master this new technique.

Technologies

A large number of competing technologies are available in the marketplace. As all are additive technologies, their main differences are found in the way layers are built to create parts. Some are melting or softening material to produce the layers (SLS, FDM) where others are laying liquid materials thermosets that are cured with different technologies. In the case of lamination systems, thin layers are cut to shape and joined together.

As of 2005, conventional rapid prototype machines cost around £25,000.

Selective Laser Sintering(SLS)

Selective laser sintering is an additive rapid manufacturing technique that uses a high power laser (for example, a carbon dioxide laser) to fuse small particles of plastic, metal, or ceramic powders into a mass representing a desired 3-dimensional object. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.

Compared to other rapid manufacturing methods, SLS can produce parts from a relatively wide range of commercially available powder materials, including polymers (nylon, also glass-filled or with other fillers, and polystyrene), metals (steel, titanium, alloy mixtures, and composites) and green sand. The physical process can be full melting, partial melting, or liquid-phase sintering. And, depending on the material, up to 100% density can be achieved with material properties comparable to those from conventional manufacturing methods. In many cases large numbers of parts can be packed within the powder bed, allowing very high productivity.

SLS is performed by machines called SLS systems; the most widely known model of which is the Sinterstation SLS system. SLS technology is in wide use around the world due to its ability to easily make very complex geometries directly from digital CAD data. While it began as a way to build prototype parts early in the design cycle, it is increasingly being used in limited run manufacturing to produce end-use parts. One less expected and rapidly growing application of SLS is its use in art.

A process similar to SLS was invented by R.F. Housholder who patented the concept in 1979 but did not commercialize it. SLS itself was developed and patented by Dr. Carl Deckard at the University of Texas at Austin in the mid-1980s.

Fused Deposition Modeling(FDM)

Fused deposition modeling, which is often referred to by its initials FDM, is a type of rapid prototyping or rapid manufacturing (RP) technology commonly used within engineering design. The technology was developed by S. Scott Crump in the late 1980s and was commercialized in 1990. The FDM technology is marketed commercially by Stratasys Inc.

Like most other RP processes (such as 3D Printing and stereolithography) FDM works on an "additive" principle by laying down material in layers. A plastic filament or metal wire is unwound from a coil and supplies material to an extrusion nozzle which can turn on and off the flow. The nozzle is heated to melt the material and can be moved in both horizontal and vertical directions by a numerically controlled mechanism, directly controlled by a Computer Aided Design software package. In a similar manner to stereolithography, the model is built up from layers as the material hardens immediately after extrusion from the nozzle.

Several materials are available with different trade-offs between strength and temperature. As well as Acrylonitrile butadiene styrene (ABS) polymer, the FDM technology can also be used with polycarbonates, polycaprolactone, polyphenylsulfones and waxes. A "water-soluble" material can be used for making temporary supports while manufacturing is in progress. Marketed under the name WaterWorks by Stratasys this soluble support material is actually dissolved in a heated sodium hydroxide solution with the assistance of ultrasonic agitation.

Stereolithography

Laminated Object Manufacturing (LOM)