Thus in this background, a novel interdisciplinary subject of rapid RPM technology came out[1], which is the general term for all technologies based on the dis- persed-accumulated forming principle。 It disperses  the

process of forming a complicated 3-D  physical part into forming a series of two-dimensional layers, and then piles up the layers together。 This is a typical order- reduced and material-increased forming technology。 It can complete forming a 3-D entity with an arbitrary complicated structure directly controlled by its   digital

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CAD model。

This dispersed-accumulated forming principle has been mentioned long ago。 In 1892’s US patent (No。 473901), Blanther proposed a method to constitute a topographic map by layered manufacturing, and after 1950s hundreds of RPM-related patents were proposed。 However, the fundamental developments came out in 1980s。 The most known is that Charles W Hull was in- spired by ultraviolet (UV) light-curing resin and pro- posed the stereolithography (SL) technology (US pat- ent, No。 4575330) in 1986, based on which 3D System Corporation produced the first modern rapid prototyp- ing machine (SLA-250) in 1988。 Since then, tens of RPM  technics have emerged  out, and there  were  274

patents registered in US during 1986-1998 period。 As issued in Terry Wohlers’s annual RPM report[2], it has already achieved a lot in many fields。 And the GARPA (global alliance of rapid prototyping associations)  was

set up and have been promoting this technology into much wider and deeper applications。

1Principle

1。1Dispersed-accumulated forming

The discrete methodology means the method/theory that aims to analyze, process, and achieve the general or precise solutions of complicated things and systems by dispersing them into limited or unlimited simple things or sub-systems。 For example, the process of calculous is first differential of integer into units, then integral of units into integer。 Actually most of objects are composed of discrete units。 It is obvious that breaking an object down into limited or unlimited units, would make the processing of complex issues much more simple。

The dispersed-accumulated forming is actually the application of discrete methodology in the form- ing/shaping field。 For example, when a pyramid is  to be built, firstly how many pieces of stones, their shapes, their exact locations, and orders to be piled should be pre-designed, which is the dispersing process;  then these stones are produced and are orderly piled them up into a pyramid, which is the accumulating process。

In the process of dispersed-accumulated  forming, the manufacturing of a 3-D part is transformed into manufacturing of a series of simple units。 In some sense the unit itself has broken through the traditional manufacturing  concept:  it  can  be  either  a    material

YAN Yongnian (颜永年) et al：Rapid Prototyping and Manufacturing Technology … 3

droplet, or a cell, or even a protein molecule。

In a word, we can get the paths and the conditions for later accumulating through the dispersing process while through the accumulating process to pile up dis- persed materials to form a 3-D entity。 This type of forming principle is an advanced concept。 It is the in- tegration of discrete methodology and manufacturing science。 The ultimate evolvement of discrete is digital, and the ultimate evolvement of this forming method is digital manufacturing。

1。2Digital manufacturing

Usually the manufacturing process is composed of information process and physical process。 The application of computer technology in manufacturing industry has already realized the digital description   of