2Representative Technics

Since 1986, more than ten main technics of RPM[6] technology were developed such as stereolithography (SL), laminated object manufacturing (LOM), selective laser sintering (SLS), selective laser melting (SLM), fused deposition modeling (FDM), ink jet printing (IJP), 3-D printing (3DP), patternless casting manufac-

turing  (PCM)[7],  and  electron  beam selective melting

(EBSM)。  There  are  many  classifications  for     these

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technics。 According to the enabling technology, there are two categories: (1) one is based on the high-energy beam (laser, electron beam) and (2) the other is based on the droplet jetting or slurry extrusion。

2。1Technics enabled by high-energy beam

The representative technics are SL, LOM, and SLS。 SL is based on the photopolymerization of liquid photo- sensitive resins induced by UV radiation。 This type of facility usually contains a liquid basin filled with liquid photosensitive resins, a mobile elevated workbench with a porous board, UV laser implements (He-Cd, ar- gon or solid state laser, power range from dozens to hundreds  of  mW,  wavelength  range  from   320 to 370 nm), and a scanning system (optical mirror array reflects the laser beam and focuses it onto the liquid surface, and performs the X-Y directional scanning movement)。 As issued above, Charles W Hull got the US patent of SL in 1986。 And in 1988, 3D System Corporation in US introduced the commercial SLA- 250, which is the world’s first RPM machine。  The other similar machines are SOUP (CMET, Japan), Ste- reos (EOS, Germany), and Auro (Yinhua, China)。

The forming materials for SL mainly contain free radical light-cured resin, cation light-cured resin, and mixed-type light-cured resin。 The resins are composed of further polymerisable prepolymer, active monomer, photoinitiator, and other auxiliary components。 Radi- ated by UV light, the photoinitiator would transit from ground state to excited state, then generate active free radical, triggering the polymerization curing reaction between prepolymers and active monomers。 The mixed-type light-cured resin is now mainly used。 It has low polymerization volume shrinkage and even has a small expansion, and can be designed to be no- contraction polymer。 It can provide polymer systems with short induction periods and stable polymerization rates。 It could also overcome the shortcomings that the free radicals would quickly be inactive and the polym- erization course would quickly stop after the disap- pearance of laser radiation。

This kind of technics is of high precision。 It can pro- duce very fine parts with good plastic-like surface quality。 However, the support material is needed。 And it is of high cost for the photopolymerizable resin and laser system。

LOM  uses laser  (or  knife) to cut  patterns  on   thin

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layers, then piles the layers together。 The basic struc- ture of the machine contains a paper feeding and roll- ing implement, a heat-presser, altimeter sensors, X-Y scanning system, mobile elevated workbench, and sometimes a CO2 laser system (power: 50 W)。 This technics was first invented by Michael Feygin in 1986。 And the commercial machines were later launched by Helisys Corporation in US。 The other similar machines are SAHP (Kira, Japan), SSM (Yinhua, China), and ZIPPY (Kinergy, Singapore)。

The forming materials are usually thin sheets like papers and polymer films。 And there are three common problems involved: the sheets, the hot-melt adhesives, and coating technology。 The performance of sheet should be ensured of certain tensile strength (prevent- ing it from being pulled off in forming process), good soakage ability (for good coating), hydrophobic (not easy to absorb water), little shrinkage (no deformation due to loss of water), easy to be polished (for smooth surface), and good stability (for long-term store)。 The main ingredients of hot-melt adhesives are EVA resins combined with tackifier, wax, and antioxidants。 And it should be with proper melting viscosity, mobility, con- traction, adhesion, infiltration, and thermal decomposi- tion temperature。 The coating technology includes uni- form coating and non-uniform coating。 The  former uses a simple device of slit-scraper while the latter in- cludes striped style and particles style, which can re- duce the stress concentration but are more expensive。