(5)The high manufacturing cost The RPM is an integration of high technologies。 The technical cost is high。 At present, it is used for single or low mass pro- duction, the average fixed cost is high。 And the special prepared materials are usually much more expensive than other available industrial materials。 For example, the Viper Si2 SL-equipment of 3D System Corporation in 2001 costed 180 000 USD; and its special resin was 200-250 USD/kg。

4Applications and New Progress

After two decades of development, RPM have stepped into the maturity。 Many new technics emerged out, and a lot of measures were developed to promote the RPM。 Though it was originally used as an auxiliary method for developing the new product in the process of de- sign, test, and assembly, now it can directly form func- tional entities like metal parts。 It is also applied in nano-/mirco- fabrication and biomanufacturing as a powerful forming method。

4。1Curved-surface layered manufacturing

The layered error for curved surface, known as step ef- fect, is the principle error of RPM technology。 It prominently affects the precision and surface quality especially when the angle between the horizontal plane and the surface is small。 To decrease this error, besides reducing the layered height, which is restrained by the materials and technics, optimizing the direction of forming is another acceptable method, in which proc- ess it is needed to be layered along curved surfaces in- stead of being easily layered along parallel planes。

Curved-surface-layered manufacturing forms a part along a series of layered curved surfaces。 The layered algorithm of constructing the layered curved surfaces, including the key frames and transition frames from a CAD model, is one key technology。 The other is the

forming control technology。 First one is the controlling of the nozzle moving track and the nozzle pose in forming process。 To form curved surface, it is up- graded from traditional 2-axis plane scanning to 3-axis 3-D scanning, and the pose of nozzle should be con- trolled to avoid the formed part from damage。 Second one is the material delivery control。 The on-demand material should be determined according to the real- time nozzle parameters to eliminate over-stack and lack-stack。

This technology can be only fitted for certain RPM technics like FDM。 The material preparation process of technics such as SL, LOM, and 3DP, produces a plane material layer for the next step processing, making them impossible for manufacturing along curved sur- faces。 The forming material of FDM is added on- demand, and the extrusion velocity and axis-moving velocity can be controlled at real time, what makes it suitable for curved-surface layered forming。 Actually, our team had used a MEM-300-II (Yinhua, China) to complete the first curved-surface layered prototype。

Besides the curved-surface layered algorithm, there are also some other new technologies in CAD model dispersing process such as rapid layered algorithm of STL, direct layered algorithm of CAD model, and self- adaptive layered algorithm。 With these improvement, the errors in layered process can be greatly eliminated, the information process efficiency can be improved, and the integration between front CAD and end RPM can be further upgraded。

4。2Machines for concept prototype

The RPM was brought out as a method to manufacture concept prototypes, what may be used in  processes such as design, test, and assembly。 After 2 decades of progress, many types of commercial machines in this field have been developed。 All of these equipments are similar in some common features as follows: (1) rapid forming velocity; (2) able of dealing with multi-kind materials  (for  continuous-gradient-colored  products);

(3) able to be controlled by PC through standard com- municating interfaces; (4) small like a desktop  device;

(5)low manufacturing cost, low price; (6) green, pollution-free, and noise-free in manufacturing。 And the representative commercialized equipments are as follows: