How Rapid Prototype systems work
Posted by Robert Kiser on Mon, Jan 05, 2009 @ 06:51 PM
Rapid prototype systems use Stereolithography files, or STL for short, to replicate the 3-Dimensional computer aided design image into a physical hand held part. Stereolithography files consist of many groups of triangles called "facets". The smoother the surface of the part, the more triangles which make up the part. This is termed "surface resolution". If you were to draw a square box on a piece of paper, then draw a straight line inside the box from one corner to the other, you would see two triangles inside the box. This 2-Dimensional bitmap image you just drew actually now has a resolution in theory. If you were to then draw another line from the other corner to the other, you would see four triangles. You just increased the resolution of your 2D bitmap image. The more triangles, the higher the surface resolution, or "smoothness" of the surface. At the same time, the higher you go in resolution (add more triangles) the larger the file size becomes. There is a point where the surface resolution is okay and the smoothness of the surface doesn't need a rise in the number of triangles as this would only be overkill and harder for the rapid prototype applications software to process, especially during the slicing process. If not enough triangles (or "facets") are present, then the surface appears jagged and resolution must be increased. Most default settings in Computer Aided Design solid modeling packages output smooth surface resolutions, while some do not. Even some of the more expensive software packages output rough surface resolutions by default, which needs to be a thing of the past. The PolyJet 3D printer technology is a printer, so high resolution physical parts are built that are near the actual surface quality of the computer Stereolithography image.
3-Dimensional Stereolithography files are then imported into the rapid prototype system's application software package where a visual simulation of the actual build platform is viewed by the operator. The 3D file is then manipulated inside the simulation and oriented for fastest and most accurate build parameters. Once the platform simulation is finished, the simulation is of course saved and the "build" is started. The next step the application software takes is a process called "slicing". The parts are electronically sliced into 2-Dimensional bitmap images and stored in an area on the computer. When the rapid prototype system is warmed up and ready, one bitmap is shipped into the control electronics, and in the case of the PolyJet 3D printer, this single bitmap is printed down onto the actual system build medium, or "build tray". This first bitmap printed is the very bottom "bitmap slice" of the part/parts on the tray. You may wonder how thick the bitmap slice is. Answer is, in theory it doesn't matter because the PolyJet 3D printer system's mechanical parameters are what control the actual thickness of the bitmap slice to be built, not the application software. Dependent on the thickness of the slice, as determined by the manufacturer of the rapid prototype system, the build tray moves downward after lay down of the first bitmap by the amount of the physical bitmap slice built mechanically on the medium (build tray). Then, in comes the next 2-Dimensional electronic bitmap slice into the electronics and this bitmap slice is laid down on top of the previous and fused together. The build platform then moves down by the manufacturer's specified amount again (called layer thickness), and the next bitmap is laid down on top of the previous. This is basically how rapid prototyping works and is a process called "additive manufacturing". You may hear the term "Direct Digital Manufacturing", but in the end, the term also relates to the final process called "additive manufacturing".
