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65. RAPID PROTOTYPING

•The key concept is RAPID - generally this is an all-in-one-step production of a part geometry.

•Parts are used for,

-prototypes to allow fast review of part shape, simple assembly, aesthetics, manufacturability, etc.

-low volume production - very small numbers of parts can be made using this technology.

•General advantages,

-reduce prototype/production times from months to weeks or days

-a physical model is easier to “sell” to customers and management

-physical models are easier to check for errors. Graphical methods often result in cluttered views

-avoids the high cost of prototype tooling, and allows (more) design iterations

-prototypes costs can be lower than production types

•General disadvantages,

-very expensive capital costs

-tolerances are generally >.005”

-primary materials are specialized, and other steps are required to produce metal parts

65.1 STL FILE FORMAT

•Originally developed for use with stereolithography, but now used by many other processes.

•The standard for Rapid prototyping systems

•Basically connected 3D triangles

•3D smooth surfaces are tessellated into triangles. the higher the degree of tessellation the closer the surface approximates the smooth surface.

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sphere

coarse tessellation

fine tessellation

•The triangles are defined

1.with a direction of nodes defined clockwise for the out direction

2.with similar nodes at the corners of triangles. If the triangles don’t overlap, the model will have gaps and be invalid.

• A general approach to determining a rapid prototype slice is to use a ray projection through the collection of polygons. When the ray strikes a triangle it is in/out of the solid. (This is a simple geometrical problem.) A set of lines constitute a slice.

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a hollow box

an STL tessellated box

3. Fire a “ray” through the triangles, and find intersections

ray

to infinity

For the stereolithography machine we need to develop a “scan line” for the laser. When the laser scan-line (the ray) is “in” the part, the laser is on, and thus developing the light hardening polymer.

	off	on	off		on	off

laser scans across object

• If polygon vertices don’t match up, then there will be gaps between the polygons. This can result in non contact, that leads to material added/subtracted unexpectedly.