21-04-2012, 04:03 PM
RAPID PROTOTYPING TECHNOLOGIES, APPLICATIONS AND PART DEPOSITION PLANNING
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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 ageold
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.
Stereolithography
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.
Selective Laser Sintering
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 away that only those grains, which are in direct contact with the beam,
are affected (Pham and Demov, 2001). 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
Fused Deposition Modeling
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 8. 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.
APPLICATIONS OF RP TECHNOLOGIES
RP technology has potential to reduce time required from conception to market up to 10-50
percent (Chua and Leong, 2000) 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 (Chua and Leong, 2000). 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.
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.