Chapter 2Rapid Prototyping review.docx

1、Chapter 2Rapid Prototyping review2.1 Rapid Prototyping and ApplicationsRapid prototyping (RP) is a group of techniques used to quickly fabricate physical models and parts using three-dimensional computer aided design (CAD). The part is generated using layer by layer deposition of materials.1 RP has

2、been developed to reduce the product development time and to reduce the cost of manufacturing. As a result, rapid prototyping offers the potential to revolutionize the product design and manufacturing industry.Different types of rapid prototyping methods have been developed since 1980s. RP processes

3、 can be categorized in terms of the state of materials before forming the parts like liquids, powders or solid sheets or the way by which layers are created such as lasers, hot rollers or binders.One of the early patents in rapid prototyping was issued to Charles Hull in 1986 for inventing stereolit

4、hography (SLA). SLA is considered to be the first RP technique worldwide. It is a process to making polymers harden under ultra-violet light and setting up the apparatus for making an object by layer deposition. The first object that he built was a cup of 5cm tall and it took him months to fabricate

5、 it. Two years later, Hull co-founded 3D Systems Corporation, one of the largest and most prolific organizations operating in the 3D printing sector today.2 During this process, curable materials like ultraviolet (UV) curable materials, are fabricated layer by layer to form the object. The component

6、 is fabricated on a build platform which is submerged in a UV curable liquid photopolymer vat. The beam of UV light is directed by x and y scanning mirrors to shine on the cross-section of liquid and the liquid exposed to it will be cured to create a layer. Once a layer is complete, the build platfo

7、rm is moved down by the thickness of one layer to make room for creating next layer. This process is repeated until the final part is formed. The completed part is carefully removed from liquid and separated from the platform. The excess resin is removed by chemical bath and the part is cured again

8、under UV light.3 Fused Deposition Modelling (FDM) was developed by Scott Crump in the late 1980s and the first FDM system was launched by Stratasys Inc in early 1990s.4 It is a layer additive manufacturing process that use thermoplastic materials or metal wires to produce parts. The heated thermopla

9、stic filaments are extruded from the nozzle that moves in the horizontal x-y plane while the build platform moves down in the Z axis after one layer is built.5 FDM gains increasing use because of its safe and convenient fabrication process, low cost of materials and the availability of thermoplastic

10、s.Laminated Object Manufacturing (LOM) is a rapid prototyping process that layers of heat sensitive materials like adhesive coated paper, plastic, or metal laminates are successively bonded together to form products. In this process, layers of materials are glued together by heated roller and cut to

11、 shape with a laser cutter. Compared with other RP technologies, LOM has some advantages due to peculiarity of the process.6 LOM could produce parts with large dimensions. Besides, the LOM parts have low internal tensions which prevent distortion, shrinking and deformation. LOM produce wood-like pro

12、perties when working with paper. However, the part complexity is limited.Selective laser sintering (SLS) was invented in the late 1980s by The University of Texas at Austin.7 It is a rapid prototyping process that allows to generate complex 3D parts by fusing successive layers of powder materials on

13、 top of surface using a high power laser. The powders can be sintered directly or indirectly with the mix of a thermoplastic binder, depending on the properties of the powders. A beam deflection system makes the laser beam scan each layer according to the corresponding cross section of the component

14、 as calculated from a Computer Aided Design (CAD) model.8 SLS can be used to process almost any type of materials including metals, thermoplastic polymers and elastomers, provided that the material is available in the form of powders and that powder particles tend to fuse when heat is provided. It a

15、lso has the advantage to produce parts with high density. When a layer is completed, the bed containing the powders is lowered one-layer thickness to create the next layer. This process repeated until the part is created. The sintered powders form the part while the unsintered powders remains in pla

16、ce and provide structure support. After the component is built, the unsintered powders will be cleaned away and may be recycled.9Laser Engineered Net Shaping (LENS) was developed at Sandia National Laboratories, as shown in Fig. 2.1. The LENS system consisted of a nozzle feeding powders to depositio

17、n surface, which creates a converging powder stream. Meanwhile the laser beam is focused on the converging point to melt the targeted surface. The part is created layer by layer. After solidifies, the new part is created or the existing parts are repaired. The deposits carried out in the atmosphere

18、of argon. The motion control of the deposit could be from the CAD file or programmed manually. This method has the ability to process more complex geometries due to the 3 or 5 axis systems flexible for the nozzle moves. Parts achieved with LENS usually have a net shape geometry but with a rough fini

19、shing. Post-processing both thermally to relieve the internal stress and mechanically to get the desired resolution are needed.10Fig. 2.1 LENS process (Courtesy of Welding Journal)Three-Dimensional printing (3DP) is a powder based rapid prototype technology that developed by Massachusetts Institute

20、of Technology in 1990s.11 It is also called binder jetting. In this process, each layer starts with a thin distribution of powder spread over the surface of a powder bed. The process is similar to the process of SLS, but instead of using a laser to sinter materials, an ink-jet head deposits a liquid

21、, which is a binder to binds the materials. A binder material selectively joins particles to fuse the powders together by an inject-printing head. After one layer is formed, the piston that supports the powder bed moves down in the height of one layer so that next powder layer can be spread and fuse

22、d. 3DP system has high building speeds and easy to handle. For metals, 3DP is essentially a powder metallurgy process, so porosity is major problem. Sometimes infiltration is needed to decrease porosity. Infiltration refers to use a second metal, which is a lower melting temperature alloy to printed

23、 structure to achieve dense materials.RP systems are effective in reducing time and cost of fabricating new products; however, the part accuracy, surface finish, variety of materials in most RP systems need to be improved. A new form of hybrid RP systems, combines material deposition (RP) and Comput

24、er Numeric Control (CNC) machining in a single station was put forward to find the solution to those problems.12 Shape Deposition Manufacturing (SDM) is a hybrid process that involves depositing materials in a near-net shape on a substrate and then removing unwanted materials using CNC machining.In

25、this process, microcasting, which is a weld-based process was used to deposit the materials. The part is then transferred to the shaping station for machining, where the desired accuracy and finish is achieved. Stress relief could be used to control residual stress. The robotized pallet system is us

26、ed to transfer parts between stations. The support material is embedded or deposited and shaped.13 By varying different materials in the deposition process, the material properties can be changed. The process cycle of SDM is shown in Fig. 2.1.Fig. 2.2 The process cycle of SDM with additive and subtr

27、active process14Solvent welding freeform fabrication technique (SWIFT) is a hybrid RP process that based on solvent welding and CNC contour machining.15 This process is applicable for thermoplastic materials such as polystyrene, which is solvent weldable and available in sheet form. For each layer,

28、a laser printer printed a thin film high density polyethylene (HDPE). HDPE is a thermoplastic material which not soluble to acetone and therefore serves as a solvent mask. After masking, acetone solvent is applied to the bottom side of the sheet and pressed to the existing stack of sheets where solv

29、ent welding take place. Acetone breaks the van der Waals bonds between polymer chains and thus dissolves the surface of the thermoplastic sheet. The dissolved interfaces of each sheet blend together and form new polymer chains between sheets. Computer-aided manufacture (CAM) of laminated engineering

30、 materials (LEMS) is a hybrid freeform fabrication process for fabricating laminated parts from sheet metals. Laser cutting is applied to materials sheet stock to cut slices. The resulting part slices are extracted from the stock and to assembly the component by achieving intimate interlayer contact

31、 and subsequent sintering processes.16The main purpose of hybrid processes is to overcome the limitations of rapid prototyping systems, to improve the quality of the finishes products, especially in surface finish and roughness. The hybrid processes are also expected to improve the cutting rate and

32、materials removal rate and thus reduce the process cost.14 2.2 Laminated ToolingRapid Tooling (RT) refers to a process that uses RP model to quick create a mold or uses the RP process fabricate a tool for a limited volume of prototypes. RT is a natural extension of Rapid Prototyping. Compared with c

33、onventional tooling, RT provides a significant increase in speed and reduction in cost. RT is broadly classified as soft tooling and hard tooling and also as indirect tooling and direct tooling. The short run and lower volume of parts is produced by soft tooling from silicon, epoxy resins or low melting point alloys. The long run and higher volu