This kind of technics is suitable for large pieces of solid part with high accuracy, needs no additional sup- port in forming process。 The prototype pieces are with a hardwood texture。 But the post-processing is usually time-consuming, and produces more waste。

SLS forms parts by piling up layers of laser- sintering metal or non-metallic powder。 This forming machine basically contains a laser implement (CO2 la- ser or YAG laser with a general near-infrared band wavelength of 1。06 μm, and a power of 50-200 W), an optical system (used to focus and orientate the laser beam), a function generator and an industrial personal computer (controlling laser beam to scan in the X-Y plane), a hopper, and a spreading system (loading pow- der and spreading the powder surface to flat)。 This technics was first developed in 1989 by CR Dechard in Texas University at Austin。 And later, it was commer- cialized as Sinterstation by DTM Corporation in     US。

The other similar is EOSint (EOS, Germany)。

The forming materials include wax powder, metal powder, polystyrene (PS), engineering plastics (ABS), polycarbonate (PC), nylon (PA), coated sand, and coated ceramic powder。 In recent years, composite powder with a diameter of 50-125 μm is often used。 The effect of laser on powder material has two stages。 First one is the absorption and reflection of the powder surface。 In this stage the absorption of laser energy is related with the laser wavelength and the powder sur- face state。 Second one is the heat transferring in pow- der material。 In this stage the powder heating and cool- ing process are very soon, and the material parameters vary rapidly with the rise of temperature,  what  is a very complicated and unstable thermal conduction process。 Thus, the powder material should be of good thermoplastic ability, a certain degree of thermal con- ductivity, and adequate bond strength。

This technics can directly form parts without addi- tional support。 And it has an extensive material re- source。 However, the accuracy is not very high。

2。2Technics enabled by jetting/extrusion

The representatives are FDM, ink jet  printing (IJP), and 3DP。

FDM is one of the most popular RPM technics。 It forms a part by extruding melting viscous slurry-like materials out and deposited them on the base plane or previous formed part。 Then, the material conglutinates to adjacent material and cools in open air。 The  nozzle is very important in FDM machine。 It is responsible for melting materials and extruding them out precisely on- demand。 It was first successfully developed by Dr。 Scott Crump in 1988。 And Stratasys Corporation in US introduced the first commercial FDM series equip- ments。 The other similars are MEM series (Yinhua, China)。

This technics usually uses filamentous thermoplastic materials such as ABS, wax, and nylon。 The material is heated and melted in the nozzle, and then extruded out for accumulating forming。

IJP builds part by shooting/jetting material droplet on-demand from piezoelectric ink-jet printing nozzles。 And high forming velocity  can  be  achieved  by  a liner array of ink-jet nozzles。 The representative com- mercial  machines  are  Actua  (3D  System,  US)   and

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ModelMaker (Sanders, US)。

The forming materials are mainly wax and wax-like thermoplastic polymers with a medium strength be- tween wax and technical thermoplastics。

3DP uses ink-jet printing nozzles to jet binder drop- lets onto solid powder surface。 After one layer is formed, the next layer of powder is spreaded on the former layer。 The forming velocity can be accelerated by providing a linear nozzle array。 This technics was first invented by Prof。 Sachs in MIT, then commercial- ized by Soligen Corporation in US in the name of DSPC (direct shell production casting) used for manu- facturing of ceramic shells and cores for casting。 The other similar machines are 3DP (Extruhone, US), and 3DP (Z-corp, US)。