A number of experts have proposed RP classification schemes that are based on the physics of the processes. For the purposes of this chapter, a classification scheme is presented that is based on the operation of the machines themselves. The classification is done as a matrix, as shown in Fig. 9.1. The two axes of the matrix are Imaging Strategy and Imaging Mechanism.
Imaging strategy, the horizontal axis of Fig. 9.1, is the approach used to define the image of a single layer and has direct analogy to the methods used to create images in graphics printing. One approach is to use a raster, where the image is created as a series of contiguous or overlapping straight line segments. This is the method used to create TV images and is the predominant method for printing graphics (desktop computer printers all work with a raster strategy).
The alternative method is to draw at least the outline of the image with vector motions, as illustrated by the arrows in Fig. 9.2. The interior could be filled with a vector motion or, more commonly, with a raster fill pattern, also as illustrated in Fig. 9.2. Flatbed pen plotters employ vector motions to draw images. The fundamental trade-off here is between speed and precision. A raster approach is faster, since no changes in direction are required and it can often be done in parallel (with a line of pens, figuratively speaking). However, a raster pattern produces only an approximation of the outline of the part, with discretization errors apparent on any edge that is not parallel to the raster motion. Such "aliasing" is evident in Fig. 9.2. This situation is avoided by the vector approach.
Imaging mechanism, the vertical axis of Fig. 9.1, is a classification of the mechanism used to define the 3D geometry of the part. In one case, all three axes are defined by mechanical motion. For example, a 2D mechanical motion using an x-y gantry system is used to define the geometry of each layer, and the third axis is effected by dropping the piston down vertically. In the alternative case, each 2D slice image is defined optically, and the third axis is due to a mechanical motion. The most common optical imaging technique is the use of galvanometer mirrors. One might argue that galvo mirrors are mechanical devices as well; however, they have a distinct set of performance issues associated with them, and their use dictates a fundamentally different machine design as compared to mechanical slides.
The matrix of Fig. 9.1 shows representative types of equipment from most of the RP machine vendors and research groups known at the time of this JTEC/WTEC study. Note that the machines divide roughly evenly between two axes optical and all axes mechanical. However, use of vector outline predominates over raster only (this is in sharp contrast to desktop computer printers, where raster dominates).