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ABSTRACT
In recent years, rapid prototyping technology (RPT) has been implemented in many spheres of
industry, particularly in the area of product development. Existing processes provide the
capability to rapidly produce a tangible solid part, directly from three dimensional CAD data
from a range of materials such as photo curable resin, powders and paper. This Report gives
an overview of the growth and trend of the technology, areas of applications and its
significant benefits to manufacturing industries.
The quality through frugal engineering concepts has gained prime importance to the foundries
around the world. Although simulation packages have been used as a tool to achieve quality
and productivity, the use of Rapid Prototyping (RP) is not yet fully practiced in foundries in
this concern. This Report provides a detailed survey of the fabrication of a complex master
pattern (impeller casting) using rapid prototyping technique besides presenting its scope and
benefits compared to the current practices. The main objective of this work is to enlighten the
foundry engineers to make use of rapid prototyping technology (fused deposition modelling)
to eliminate the material wastage in pattern production, reduce the cost and lead-time in the
production of accurate complex castings of an acceptable surface quality.
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 modelled 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.
Rapid Prototyping (RP) is a term most commonly used to describe a variety of processes,
which are aimed at quickly creating three-dimensional physical parts from virtual 3D
computer models using automated machines. The parts are built directly from the 3D CAD
model and can match that model very closely (within the precision limits of the chosen
process).
The development of rapid prototyping (RP) gave the consumer the ability to form prototype or
a component of a prototype that can be directly used in assemblies and product testing for
short or medium production with the least time consumed. Since the first rapid prototyping
machine (RPM) was introduced in 1988, the rapid growth of this technique provides wide
choices of machines with different performance and credit in consideration. RP is a layer
technology which is widely used in industries.
1.2 Differences between conventional machining and rapid prototyping:
Conventional machining can produce prototypes using metal removal method with almost any
type of engineering materials however there are only a limited type of materials for any
particular type of RP&M systems. RP&M systems are usually used for making one or a few
sample of prototype, while the number of prototype produced in conventional machining can
be adjusted as required. The basic working principle of RP technologies is totally different
from conventional machining processes which build up a solid by means of addition approach
like construction of the building. Hence the RP also started from foundation then gradually increase in height until it reached the top layer.
LITERATURE REVIEW
2.1 INTRODUCTION
There are basically three stages of building physical RP models based on the CAD data,
namely the Pre-processing, building and the post processing. Whatever the CAD model is
generated by solid modeling or surface modeling approaches, most of RP machine accept only
STL file as digital data input in which most commercial Engineering CAD system is capable
to convert 3D CAD model into STL format. The STL model is then slice into layers data set
and transfer to machine for building model. Completed RP model is then performed
corresponding post-processed operations such as cleaning, post-curing and infiltration.
Pre-processing:
The first step in the RP&M process is virtually identical for all of the various systems, and
involves the generation of a three-dimensional computer-aided design model of the object. A
good, preferably solid CAD modeling or a total enclosed surface water tight CAD model is a
key component of success RP processing. The CAD file is then translated into a triangulation
tessellated STL format, which is the standard of the RP&M field. Figure 3.2 shows a typical
example of STL model which is composed of triangles and each triangle is described by a unit
normal vector direction and three points representing the vertices of the triangle.
Verification and fixing of STL file:
When one create 3D surface model using common surface modeller, there is very little
concern on the orientation of the surface normal, as most of the tool path generation
algorithms can detect the material side correctly. However, when one generate STL data using
these surface models, many converters just use the normal data straight from the NURBS
surfaces – free form surfaces, thus the STL files generated are not useable without repair.
Triangles in an STL file must all mate with other triangle at the vertex and must be properly
oriented to indicate which side of the triangle contains mass. Many STL viewers like magics
RP from Materialist read a STL file to analysis and to correct, the connectivity and gap in the
three-dimensional triangle matrix. This process is totally automatic and simple to operate.
2.2.3 Part orientation:
Part orientation has a significant effect on the final part quality and prototyping cost. The
switching between individual layers takes a significant part of the overall building time and hence must be properly optimized hence to reduce to building time and the building cost. The
part should be orientated with minimum height in order to reduce the number of layers. For
processes that need supports structures, part orientation should also be optimized such that it
would require minimal support hence reduce to building time and the building material. The
ultimate strength of the part will be affected by its orientation within the print box.
Furthermore, staircase effect will be appeared on the near flat curve surface. Hence proper
orientation can produce a smooth external curve surface of the prototype. In addition the
minimum wall thickness of the part can only be built in some particular orientation.
2.2.4 Support Generation & Editing:
The rapid prototyping systems usually bundled with software which allows the automatic
creation and editing of the supports. The software will initially generate the supports for all
overhang regions based on the default support parameters. After the creation, the support
structure can be individually modify, edit, delete or add based on the part geometry. The
region by region editing or customization for supports generation has strengthened the
essential support and also minimizing the building of unnecessary supports.
2.2.5 Slicing (Layer thickness):
A STL model used for RP contains a collection of planar triangular surfaces. These faces
define an approximate boundary for the object. Horizontal layers of equal thickness are
produced while slicing to produce the outer boundary of the part slice curve. Then void and
solid region of the slice curve can be identified and proper fill pattern can be created for part
filling. Typical layer thickness of commonly RP system is ranged from 0.05mm to 0.15mm.
2.2.6 Part building:
The prototype can be built in the RP machine according to the tool path and control codes
generated by the software of the RP system. By laser scanning, disposition, sintering, etc and
under limited working envelope with closed control of the processing environment the
machine will start the build at the bottom layer. Subsequent layer is added after the
completion of the previous layer until the final layer was build. Hence, RP process also refers
to layer manufacturing.
2.2.7 Post processing:
Once the last layer on the part has been built, the prototype need to have undergo some post
processing processes such as removal of support, cleaning, depowder, drain excessive resin, post curing in SLA, infiltration of resin/wax, etc. The post processing is aimed to clean and
reinforce the green part.
Additional surface finish procedures such as sanding, sand blasting, painting or even
electroplating are normally employed for cosmetic prototypes.
RAPID PROTOTYPING PROCESSES
Few important RP processes namely Stereo lithography (SL), Selective Laser Sintering (SLS),
Fused Deposition Modeling (FDM) and Laminated Object Manufacturing (LOM) are
described.
3.1. STEREO LITHOGRAPHY
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.
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.
ADVANTAGES AND DISADVANTAGES OF RPT
4.1 General Advantages of RPT[w-1]:
Almost any shape or geometric feature can be produced.
ÿ Reduction in time and cost (could range 50 –90%. Wohler)
ÿ Errors and flaws can be detected at an early stage.
ÿ RP/RM can be used in different industries and fields of life (medicine, art and
architecture, marketing..)
ÿ Discussions with the customer can start at an early stage.
ÿ Assemblies can be made directly in one go.
ÿ Material waste is reduced.
ÿ No tooling is necessary.
ÿ The designers and the machinery can be in separate places.
4.2 Disadvantages of RPT[w-1]:
ÿ The price of machinery and materials.
ÿ The surface is usually rougher than machined surfaces.
ÿ Some materials are brittle.
ÿ The strength of RP-parts are weaker in z-direction than in other
RPT APPLICATIONS
No single rapid prototyping technology is dominant in medical applications and they can be
used in the most fields of medicine [P-6].
a) Design and development of medical devices and instrumentation.
This is the field where applications of RP show the best results. It specially applies to hearing
aids but also to other surgical aid tools.
b) Great improvements to the fields of prosthetics and implantation.
RP techniques are very useful in making prostheses and implants for years. The ability to
quickly fit prosthesis to a patient's unique proportions is a great advantage. The techniques are
also used for making hip sockets, knee joints and spinal implants for quite some time. Both
the release of and the improvement of the properties of used materials have had a significant
influence on the quality of prostheses and implants made by RP. One interesting example is
maxillofacial prostheses of an ear which is obtained by creating a wax cast by laser sintering
of a plaster cast of existing ear. Due to RP technologies it is very easy to manufacture custom
implants. The made model could be used as a negative or a master model of the custom
implant. Many researchers explored new applications of RP in this field.
c) Planning and explaining complex surgical operations.
This is very important role of RP technologies in medicine which enable presurgery planning.
The use of 3D medical models helps the surgeon to plan and perform complex surgical
procedures and simulations and gives him an opportunity to study the bony structures of the
patient before the surgery, to increase surgical precision, to reduce time of procedures and risk
during surgery as well as costs (thus making surgery more efficient). The possibility to mark
different structures in different colours (due to segmentation technique) in a 3D physical
model can be very useful for surgery planning and better understanding of the problem as well
as for teaching purpose. This is especially important in cancer surgery where tumour tissue
can be clearly distinguished from healthy tissue by different colour. Surgical planning is most
often done with Stereolithography (SLA) where the made model has high accuracy, transparency but limited number of colours and 3DP (for more colored models, presentation
of FEA results).