1、- 1 - 中文 6600 字 SOUD FREEFORM FABRICATION ISFF) AND RAPID PROTOTYPING 1 1 SOUD FREEFORM FABRICATION ISFF)METHODS Several manufacturing processes are available to make the important transition from computer aided design(CAD)to a prototype part Several new technologies began to make their appearance a
2、fter 1987 In that year， stereolithography (SLA) was first introduced bv 3D Systems Inc， and over the next five years several rival methods also appeared TllS created the family of processes known as solid freeform fabrication fSFFl As with most new technologies at the beginning of the“market adoptio
3、n S shaped curves”in Chapter 2 the SFF domain iS accompanied by a rela tive amount of advertising“hype ”SFF processes are sometimes described as： Parts on demand From arttopart Desktop manufacturing Rapid prototyping At the time of this writing， stereolithography (SLA)， selective laser sintering (SL
4、S)， fused deposition modeling(FDM)， and layered object modeling(LOM) are being used on a day-to-day basis by commercial prototyping companies The three-dimensional(3 一 D)printing process in cornstarch， plastic， and ceramics is also being used commercially The methods lower on the list show promise b
5、ut do not seem to be in great use by third party prototyping houses to make their daily income Casting is a special case It is still used to make oneof-akind prototypes Furthermore， for batch runs in the 10 to 500 category it is a very cost effective method to use once an original mold has been made
6、 by a process such as stereolithography Machining is also used to make oneof a kind or several prototypes 1 1 1 Summary of SFF and Rapid Prototyping Processes In daily commercial use： Stereolithography (SLA) Selective laser sintering (SI S) - 2 - Laminated object modeling (LOM) Fused deposition mode
7、ling (FDM) More at the research and development(R&D)stage： 3 -D printing in cornstarch， plastic， or ceramic 3 -D printing with plastics followed by planarization using machining Solid ground curing(similar to SLA) Shape deposition modeling ra combination of addition and subtraction) Non-SFF(traditio
8、nal)： Machining Casting Comparisons done in the early 1990s by the Chrysler Corporation revealed that the SLA process was ahead of its rival nontraditional prototyping methods in terms of cost and accuracy(these studies excluded an evaluation of machining and casting) Following the technical descrip
9、tions in this chapter additional figures and tables are thus included to compare these costs and accuracies Over the last decade， SLA has further emerged as the most used SFF process,especially for the generation of the master patterns for casting and injection molding At the time of this writing， S
10、LS， FDM and LOM have the most visibility after SLA 1 1 2 The History of SFF Methods During the late 1970s,Mead and Conway(1980)created the groundwork for the fast prototyping of very large scale integrated(VLSll circuits Designers were encour aged to think in terms of five two-dimensional(2 D)patter
11、ns These patterns defined three stacked interconnection layers on a metal oxide semiconductor fMOSl wafer and their mutual connections through holes The patterns descry bed the actual geometrv of the connection runs and via holes that one would see when looking down onto the circuit chip， regardless
12、 of the exact process and number of masking steps that were used to implement the chip(see MOSIS,2000) Inspired by this success， beginning in the 1970s,several companies tried to create layered manufacturing for mechanical parts AISO by the mid 1980s,severa U.S government studies analyzed the possib
13、ilities of a“mechanical MOSIS”(Man ufacturing Studies Board， 1990；Bouldin， 1994； NSF workshop I， 1994， and II， 1995) The prospects for a mechanical MOSIS were thus frequently linked to the fabrication - 3 - processes in the lists mentioned(Ashley,1991， 1998； Heller， 1991； Kruth， 1991； Woo， 1992，1993
14、； Au and Wright， 1993； Kochan， 1993； Kai， 1994； UCLA， 1994； Wleiss and Prinz，1995； Cohen et a1， 1995； Dutta， 1995； Jacobs， 1992， 1996； Beaman et a1， 1997； Kumar et a1， 1998； Sachs et a1， 2000) The introduction of the first commercial SFF technology-stereolithography-was accompanied by the advent of
15、the STereoLithography ( STL)representation of a CAD obiect “ STUis a modified CAD format that suits a subsequent slicing oper action and the“downstream”laser scanning paths on a physical SLA FDM or SLS machine Is a soccer ball round? The answer depends on how carefully the balIis meas ured Nominally
16、,it is a perfect sphere However on closer inspection the leather is sewn together from about 20 little hexagonal patches and a few pentagonal patches to create the curvature In reality it is an approximation to a sphere Likewise the“ STLformat approximates the boundary surfaces of a CAD model by bre
17、aking it down into interconnected small triangles-a process called tes sellation Each triangle is represented by the x y z coordinates of each of its three ver tices,enumerated by the right hand rulethat is Counterclockwise (ccw) order as viewed from the outside of the bodv The vector normal to the
18、surfaee of each triangle is also specified 1his tessellated surface is stored as an“ STL file ”this file Perhaps containing up to 200， 000 triangles， is sent over the Internet to a prototyping shop As shown in Figure 4 1 this tessellated CAD model is then sliced like a stack of playing cards For 3D
19、Systemsmachines this is known as the SLI or sliced file Other rapid prototyping machines use the slicing technique but have their own file creation details and names Each slice for the imaginary soccer ball will thus be a circle However because of the tessellation procedure it will not be a perfect
20、circle The slicing action cuts through the triangles on the boundary Thus， each circular slice (or disc will actually be a multisided polygon running inside the“bounding circle ”The number of sides on this inner polygon is of course related to how finely divided the original tessellation was made In
21、side the SLA machine the laser first creates the outer boundary of each slice and then“weaves”across each slice in a hatching pattern to create the layer The number of slices and the style of the weaving pattern are chosen by each rapid pro Totyping shop Especially for SLA and SLS a certain amount of trial and error Or craftspersonship， begins to play a role at this
