This technics needs no additional support, and can form an extensive range of materials that can be con- glutinated by available binders such as ceramics with binder, metals with binder, and polymers with binder。

3Features and Traditional Difficulties

Observed from Fig。 4, the novel forming method of RPM technology has a series of important features:

(1)The process is directly controlled by digital CAD models。 The process of CAD and the process of com- puter-aided manufacturing (CAM) were integrated to- gether; it needs the least or no human intervene。 It is an automated process of forming。

(2)The process is with a high degree of flexibility。 It can build arbitrary complicated 3-D physical parts us- ing a general machine without special fixtures and tools。

(3)The material forming process is integrated with the information process。 It is especially suitable for forming a part with non-homogeneous materials and gradient function or porosity。

(4)Compared with other forming methods, this technology is much more rapid in the process from the design to a formed part, and the cost has no relation with the manufacturing quantities。

(5)RPM is a highly integrated technology。 It bene- fits a lot from the other emerging high technologies such as CAD, NC, and material technology。

Tsinghua Science and Technology, June 2009, 14(S1): 1-12

Fig。 4    Flow chart of RPM process

As described above, in the early  developmental stage of RPM, it was mainly applied as a forming me- thod of product prototypes, in the invention process such as design test and assembly, known as rapid pro- totyping。 And yet it had to compete with the virtual re- ality (VR) and high speed milling (HSM) technology。 However, due to the distinct principle of dispersed- accumulated forming process, it can deal with non- homogeneous materials and complicated structures, what makes it more potential in further development and applications。

And the constraints that traditional RP  technics faced were developmental levels of available forming materials, forming precision, forming velocity, and manufacturing cost:

(1)The residual stress The dispersed-accumulated forming process is always accompanied with phase- change and temperature-varying process of materials, the residual stress is difficult to eliminate, and the de- formation and warpage after forming is easy to occur。 Measures such as pre-/post- warming may reduce it in some extent, but not totally satisfied。

(2)The limited types of  available  materials RPM technology is greatly dependent on the properties of the material itself。 The material should not only be suitable for dispersed-accumulated  forming process and postprocessing, but also have certain mechanical strength and some functions after forming。 At present, the most popular materials for RPM are mainly resin, wax, certain engineered plastic, and paper。 However, there was no mature technics for the widely used metal, ceramic, polymer, and biomaterial。

(3)The insufficient forming precision The preci- sion is restrained by the unit scale, known as the step effect determined by present insufficient level of material   dispersing。   And   the   low   precision needs

YAN Yongnian (颜永年) et al：Rapid Prototyping and Manufacturing Technology … 7

postprocessing to compensate the step effect which further consumes time and reduces the flexibility and rapidity。

(4)The conflict between precision and forming velocity This is the conflict between information and physical process。 The forming time is inversely propor- tional to the cube of dispersed unit scale。 Therefore, if the unit scale slightly decreases, the used time would significantly increases。 One available method is to use the nozzle array。