RAPID PROTOTYPING TECHNOLOGY AND ITS APPLICATIONS

 Vinay.K

v  STUDENT  OF FINAL YEAR B.E

Govt. Engineering College, Kushalnagar-571234

ABSTRACT

Rapid Prototyping (RP) technology and methods have been around for over twenty years. As this technology has matured the base price of these units has decreased as well. RP are now cost effective in graduate schools, technical schools, secondary and primary educational facilities. New improvements in the technology have made RP units available to almost any school in our country. This paper evaluates some of the current low cost RP units available and provides recommendations for those schools seeking to implement such technology in the class room.

INTRODUCTION                                                                     

Prototyping or model making is one of the important steps to finalize a product design. It helps in conceptualization of a design. Before the start of full production a prototype is usually fabricated and tested. Manual prototyping by a skilled craftsman has been an age old practice for many centuries. Second phase of prototyping started around mid-1970s, when a soft prototype modeled by 3D curves and surfaces could be stressed in virtual environment, simulated and tested with exact material and other properties. Third and the latest trend of prototyping, i.e., Rapid Prototyping (RP) by layer-by-layer material deposition, started during early 1980s with the enormous growth in Computer Aided Design and Manufacturing (CAD/CAM) technologies when almost unambiguous solid models with knitted information of edges and surfaces could define a product and also manufacture it by CNC machining.

RP technology and systems have been around for a number of years. This technology has found application in industry, governmental engineering laboratories, manufacturing facilities and all types of schools. As the sophistication of this technology has increased the base price of these units has decreased. RP are now cost effective in graduate schools, technical schools and secondary and primary educational facilities. New improvements in the technology have made RP units available to almost any school in our country.

 The historical development of RP and related technologies is presented in table 1.

Table.1

Year of inception	Technology
1770	Mechanization
1946	First computer
1952	First Numerical Control (NC) machine tool
1960	First commercial laser
1961	First commercial Robot
1963	First interactive graphics system (early version of Computer Aided Design)
1988	First commercial Rapid Prototyping system

Table 1: Historical development of Rapid Prototyping and related technologies      

 BASIC PRINCIPLE OF RAPID PROTOTYPING PROCESSES

RP process belong to the generative (or additive) production processes unlike subtractive or forming processes such as lathing, milling, grinding or coining etc. in which form is  shaped by material removal or plastic deformation. In all commercial RP processes, the part is fabricated by deposition of layers contoured in a (x-y) plane two dimensionally. The third dimension (z) results from single layers being stacked up on top of each other, but not as a continuous z-coordinate. Therefore, the prototypes are very exact on the x-y plane but have stair-stepping effect in z-direction. If model is deposited with very fine layers, i.e., smaller z-stepping, model looks like original. RP can be classified into two fundamental process steps namely generation of mathematical layer information and generation of physical layer model. Typical process chain of various RP systems is shown in figure 1.

The terminology used in the rapid prototyping industry is not completely consistent. Some differentiate “RP” from “3D Printing” Theoretically RP includes more expensive machines and manufacturing staff. “3D printing” is thus focused on lower-cost systems and technologies.

          

                     Figure 1:- RP process chain showing fundamental process steps

In a RP process an object or model is first created electronically in a CAD file. The CAD file is then converted to a STL file format. The RP machine or some intermediate computer slices the STL file to generate the object electronically in layers. Finally the RP machine physically produces the model in layers from the layered “slice” file. The layers may be produced by build material in filaments, droplets, or laminates.

 RP build materials include paper laminates, powders, thermo-plastics, photopolymers, and other special materials. All of these materials are solidified, joined, melted, welded, or hardened by one of a number of specific methods.

   CLASSIFICATION OF RAPID PROTOTYPING

The main RP types are:

Ø  Selective Laser Sintering [SLS]

Ø  Laminated Object Manufacturing [LOM]

Ø  Stereolithography [SLA]

Ø  Fused Deposition Modeling [FDM]

Table:-2

RAPID PROTOTYPING METHODS	BUILD MATERIALS
Selective Laser Sintering [SLS]	Metal powers, Thermoplastics
Laminated Object Manufacturing [LOM]	Special paper
Stereolithography [SLA]	Photopolymer
Fused Deposition Modeling [FDM]	Thermoplastics

v Selective Laser Sintering

Selective Laser sintering [SLS] was developed at the University of Texas in the 1980s. In this process a high power laser is used to fuse material (e.g. plastics, metal, and ceramic) in layers. See Figure 2. The new backed bed of material for each layer is deposited on the previous fused material from a roller and a bin of unprocessed powder. Support material is not required in the SLS process. Due to the materials used the final object built by this method can be stronger than from other RP methods.

In Selective Laser Sintering (SLS) process, fine polymeric powder like polystyrene, polycarbonate or polyamide etc. (20 to 100 micrometer diameter) is spread on the substrate using a roller. Before starting CO2 laser scanning for sintering of a slice the temperature of the entire bed is raised just below its melting point by infrared heating in order to minimize thermal distortion (curling) and facilitate fusion to the previous layer. The laser is modulated in such a way that only those grains, which are in direct contact with the beam, are affected. Once laser scanning cures a slice, bed is lowered and powder feed chamber is raised so that a covering of powder can be spread evenly over the build area by counter rotating roller. In this process support structures are not required as the unsintered powder remains at the places of support structure. It is cleaned away and can be recycled once the model is complete. The schematic diagram of a typical SLS apparatus is given in figure 2.

                                   

                                     Fig:-2 Selective Laser Sintering

v  Laminated Object Manufacturing (LOM)/Paper systems

In this method layered paper is first adhered to a moveable base. A CO2 laser cuts out the outline of the first layer of the object, the support material is cut in cross-hatch fashion. The table translated down and new paper (or plastic) is adhered over the previous layer. See Figure 3. The laser then cuts out the next layer of the object. After completion the support material can be removed with a pick. The final object has the look and feel of wood. LOM may be less expensive than other methods but the final object may be susceptible to shrinkage/warpage if the exterior surfaces are not sealed.

Typical system of Laminated Object Manufacturing (LOM) has been shown in figure 3. It can be seen from the figure that the slices are cut in required contour from roll of material by using a 25-50 watt CO2 laser beam. A new slice is bonded to previously deposited slice by using a hot roller, which activates a heat sensitive adhesive. Apart from the slice unwanted material is also hatched in rectangles to facilitate its later removal but remains in place during the build to act as supports. Once one slice is completed platform can be lowered and roll of material can be advanced by winding this excess onto a second roller until a fresh area of the sheet lies over the part. After completion of the part they are sealed with a urethane lacquer, silicone fluid or epoxy resin to prevent later distortion of the paper prototype through water absorption.

In this process, materials that are relatively cheaper like paper, plastic roll etc. can be used.  Parts of fiber-reinforced glass ceramics can be produced. Large models can be produced and the building speed is 5-10 times as compared to other RP processes. The limitation of the process included fabrication of hollow models with undercuts and reentrant features.  Large amount of scrap is formed. There remains danger of fire hazards and drops of the molten materials formed during the cutting also need to be removed.

         Fig:-3 Laminated Object Manufacturing (LOM)/Paper systems

v Stereolithography

Stereolithography (or SLA) was coined and developed by Chuck Hull in about 1985. This method is widely used today. In this method a light – sensitive resin in a vat is photo-cured by a UV laser. The table in the vat moves downward as each layer is built. Each layer is about 0.001 to - 0.007 inch. Ventilation of the vat is usually required due to the adverse resin vapors.  Accuracy in the z-direction may suffer if there is no milling in this direction.

In this process photosensitive liquid resin which forms a solid polymer when exposed to ultraviolet light is used as a fundamental concept. Due to the absorption and scattering of beam, the reaction only takes place near the surface and voxels of solid polymeric resin are formed. A SL machine consists of a build platform (substrate), which is mounted in a vat of resin and a UV Helium-Cadmium or Argon ion laser. The laser scans the first layer and  platform is then lowered equal to one slice thickness and left for short time (dip-delay) so  that liquid polymer settles to a flat and even surface and inhibit bubble formation. The new slice is then scanned. Schematic diagram of a typical Stereolithography apparatus is shown in figure 4.                             In new SL systems, a blade spreads resin on the part as the blade traverses the vat. This ensures smoother surface and reduced recoating time. It also reduces trapped volumes which are sometimes formed due to excessive polymerization at the ends of the slices and an island of liquid resin having thickness more than slice thickness is formed. Once the complete part is deposited, it is removed from the vat and then excess resin is drained. It may take long time due to high viscosity of liquid resin. The green part is then post-cured in an UV oven after removing support structures.

Overhangs or cantilever walls need support structures as a green layer has relatively low stability and strength. These overhangs etc. are supported if they exceed a certain size or angle, i.e., build orientation. The main functions of these structures are to support projecting parts and also to pull other parts down which due to shrinkage tends to curl up . These support structures are generated during data processing and due to these data grows heavily specially with STL files, as cuboid shaped support element need information about at least twelve triangles. A solid support is very difficult to remove later and may damage the model. Therefore a new support structure called fine point was developed by 3D Systems and is company s trademark. Build strategies have been developed to increase build speed and to decrease amount of resin by depositing the parts with a higher proportion of hollow volume. These strategies are devised as these models are used for making cavities for precision castings. Here walls are designed hollow connected by rod-type bridging elements and skin is introduced that close the model at the top and the bottom. These models require openings to drain out uncured resin.

                                      Fig:-4 Stereolithography

v Fused Deposition Modeling (FDM).

 This process was invented and developed by Stratasys. In the FDM procedure a filament of thermoplastic is metered through a heated injection head to lay down a cylinder of hardening plastic on a build-plate. The heated head follows a tool path as prescribed by the software. Support structure may be required. See Figure 5. This is much like a “hot glue gun” technique.

In Fused Deposition Modeling (FDM) process a movable (x-y movement) nozzle on to a substrate deposits thread of molten polymeric material. The build material is heated slightly above (approximately 0.5 C) its melting temperature so that it solidifies within a very short time (approximately 0.1 s) after extrusion and cold-welds to the previous layer as shown in figure 5. Various important factors need to be considered and are steady nozzle and material extrusion rates, addition of support structures for overhanging features  and speed of the nozzle head, which affects the slice thickness. More recent FDM systems include two nozzles, one for part material and other for support material. The support material is relatively of poor quality and can be broken easily once the complete part is deposited and is removed from substrate. In more recent FDM technology, water-soluble support structure material is used. Support structure can be deposited with lesser density as compared to part density by providing air gaps between two consecutive roads.

                                                                                                                                                                     

                              Fig:-5 Fused Deposition Modeling (FDM)

 APPLICATIONS OF RP TECHNOLOGIES

RP technology has potential to reduce time required from conception to market up to 10-50 percent as shown in figure 10. It has abilities of enhancing and improving product development while at the same time reducing costs due to major breakthrough in manufacturing. Although poor surface finish, limited strength and accuracy are the limitations of RP models, it can deposit a part of any degree of complexity theoretically. Therefore, RP technologies are successfully used by various industries like aerospace, automotive, jewelry, coin making, tableware, saddletrees, biomedical etc. It is used to fabricate concept models, functional models, patterns for investment and vacuum casting, medical models and models for engineering analysis. Various typical applications of RP are summarized in figure 6.

Fig:-6 Application of Rapid Prototyping

SUMMARY

This paper provides an overview of RP technology in brief and emphasizes on their ability to shorten the product design and development process. Classification of RP processes and details of few important processes is given. The description of various stages of data preparation and model building has been presented. An attempt has been made to include some important factors to be considered before starting part deposition for proper utilization of potentials of RP processes.

REFERENCES

·         Chua, C.K., Leong, K.F. (2000) Rapid Prototyping: Principles and Applications in Manufacturing, World Scientific.

·         Gebhardt, A., (2003) Rapid Prototyping, Hanser Gardner Publications, Inc., Cincinnati.

·         Pandey, P.M., Reddy N.V., Dhande, S.G. (2003a) Slicing Procedures in Layered Manufacturing: A Review, Rapid Prototyping Journal, 9(5), pp. 274-288.

·         Pandey, P.M., Reddy, N.V., Dhande, S.G. (2003b) Real Time Adaptive Slicing for  Fused Deposition Modelling, International Journal of Machine Tools and Manufacture,  43(1), pp 61-71.

·         Pandey, P.M., Reddy, N.V., Dhande, S.G. (2004a) Part Deposition Orientation Studies  in Layered Manufacturing, Proceeding of International Conference on Advanced  Manufacturing Technology, pp. 907-912.

·         Pandey, P.M., Thrimurthullu, K., Reddy, N.V. (2004b) Optimal Part Deposition  Orientation in FDM using Multi-Criteria GA, International Journal of Production  Research, 42(19), pp. 4069-4089.

·         Thrimurthullu, K., Pandey, P.M., Reddy, N.V. (2004) Part Deposition Orientation in  Fused Deposition Modeling, International Journal of Machine Tools and Manufacture,  2004, 44, pp. 585-594.

Williams, R.E., Komaragiri., S.N., Melton, V.L., Bishu, R.R. (1996) Investigation of  the Effect of Various Build Methods on the Performance of Rapid Prototyping  (Stereolithography),