A model, by definition, is not identical to the object it represents. The model may be built to a different scale, or from another material, or with less accuracy, etc. These changes might be made to reduce the cost and time required to build the model, or to make the model more illustrative.
Regardless of the reason for these changes, the result is that the model will not function exactly like the original device. The designer, therefore, must decide what aspects of the device are important and what can be sacrificed. This usually depends on the purpose of the model. Is it an aesthetic model? A teaching tool? A form-fit model? Each has unique requirements. Color and surface finish may be most important for the appearance model, whereas dimensional accuracy is essential for a form-fit test prototype. Model design, therefore, must consider the intended purpose of the model.
In addition, the design must consider what operating conditions (stresses, temperatures, chemical interaction, etc.) the model will experience. For example, a plastic model will be less prone to humidity-induced dimensional changes than a paper-based prototype (such as the LOM produces). As another example, most layer-manufactured objects are stronger in the direction parallel to the layers than normal to them.
At this point, you should design your model. Determine its purpose and then specify exactly will be required of it. With this information in hand, proceed to Step 3 to select the most appropriate RP technique.
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