Machining is a subtractive process, beginning with a solid piece of stock. The machinist must carefully remove material until the desired geometry is achieved. For complex part geometries, this is an exhaustive, time consuming, and expensive process. Some parts are even too complex to be machined.

Rapid Prototyping is a method in which the part is created by a layer-additive process. Using a specialized software, a 3-D CAD model is cut into very thin layers or cross-sections. Then, depending on the specific method used, the RP machine constructs the part layer by layer until a solid replica of the CAD model is generated. Material selection is also method specific.

The advantages of this process is clear: development of physical models can be accomplished in significantly less time as compared to the machining process. Some other applications of these technologies include development of molds and toolings. Additionally, in the medical field, the convergence of medical imaging, CAD, and RP has made it possible to quickly develop medical models.

What Are We Doing?

We are developing a virtual clearinghouse for parts that have been created by RP processes. Based on the information contained here, a third party with the appropriate equipment should be able to produce any of the catalogued parts. Additionally, the user should have the ability to modify the part by manipulating the original CAD object. This would require the use of the appropriate pre-processing software.

In this process, a filament of wax or plastic is supplied to a heated dispensing head. The head moves across the surface, building up the cross-section in molten wax or plastic in the 'green stage'. After fabrication, binder burn out is required to achieve the final hardened state. Where an in-fill or overhang is required, a second head can be employed to lay a plastic or wax lattice which is amenable to easy removal after manufacture.

Laminar Object Modelling

In this process, the laser cuts the slices from a sheet of paper which is then attached to previously cut layers. unlike other processes, only the outline of the shape need be cut, but areas which are to be free space in the final model are finely cross-hatched by the laser, producing a collection of small cubes in the final model. These provide support for material which may come on top, and can normally be removed at the end of the process. Occasionally, the unwanted cubes could become entirely enclosed within the model, and thus the process has to be interupted so that the extraneous material can be removed before enclosure occurs.

Selective Laser Sintering

This process employs a powder which can be fused by heat instead of a resin. The model is built on a platform which is situated within a horizontal platen. The platform is lowered a small amount so that a very shallow recess is formed between platform and platen. A roller then spreads the powder, the laser scans the surface, and the process is repeated. There is generally no need for additional supports to be provided for overhangs because unfused powder fulfils this function. No other process is required, and very little dressing of the mdel is neede. Currently, polycarbonate, nylon and wax powders are available, with metal powders under development. The models can be filed, carved, painted or sprayed.

Stereolithography

In this process, a laser draws a cross-section on the surface of a bath of photosensitive resin. The laser partially cures the resin and thus produces a thin layer of solid material. The process commences by positioning the support table just below the surface of the resin so that the first layer drawn adheres to the table. After each layer is completed, the table lowers by a small increment, exposing another thin layer of resin which can be cured. In this way a solid model slowly builds up in the bath of resin. In order for the model to be removed from the table, the first layer consists of a fine lattice of resin. As the model develops in the resin bath, overhanging sections can present considerable problems. To prevent the collapse of the structure, supports in the form of a lattice have to be provided for the layers and have to be removed from the object after the final curing. After the model is finished, it is removed from the bath, cleaned by removing excess resin, and then baked in an ultra-violet oven.

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