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RAPID PROTOTYPING
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RP-DEFINITION:
A family of fabrication processes developed to make
engineering prototypes in minimum lead times based on aCAD model of the item.
What is prototype?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.
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Traditional method is machining
Can require significant lead-timesseveral weeks,
depending on part complexity and difficulty in ordering
materials.
RP allows a part to be made in hours or days, given that a
computer model of the part has been generated on a CAD
system
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Necessity of RP:Product designers want to have a physical model of a
new part or product design rather than just a computermodel or line drawing
Creating a prototype is an integral step in design.
A virtual prototype (a CAD model of the part) may notbe sufficient for the designer to visualize the partadequately.
Using RP to make the prototype, the designer can seeand feel the part and assess its merits andshortcomings
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RPTwo Basic Categories:
1. Material removal RP - machining, using a dedicated CNCmachine that is available to the design department on shortnotice Starting material is often wax
Easy to machine and it can be melted and resolidified
The CNC machines are often small - called desktopmachining.
2. Material addition RP - adds layers of material one at a
time to build the solid part from bottom to top
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BASIC PROCESS STEPS IN RP PROCESSES
1. Geometric modeling - model the component on a CAD
system to define its enclosed volume
2. Tessellation of the geometric model - the CAD model isconverted into a computerized format that approximates its
surfaces by facets (triangles or polygons)
3. Slicing of the model into layers - computerized model is
sliced into closely-spaced parallel horizontal layers
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Tessellation of a typical surface of CAD model
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Classification of RP Technologies
The RP classification used here is based on the
form of the starting material:
1. Liquid-based
2. Solid-based
3. Powder-based
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Liquid-Based Rapid Prototyping Systems
Starting material is a liquid
Includes the following processes:
o
Stereo lithographyo Solid ground curing
o Droplet deposition manufacturing
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Stereo lithography
SLA is a form of additive fabrication which allows you tophysically evaluate a prototype before moving into mass
production.
Stereo lithography, also known as 3-D layering or 3-D printing,allows you to create solid, plastic, three-dimensional (3-D) objects
from CAD drawings in a matter of hours.
Stereo lithography is a common rapid manufacturing and rapid
prototyping technology for producing parts with high accuracy
and good surface finish. A device that performs stereo lithography
is called an SLA or Stereo lithography Apparatus.
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Stereo lithography (STL)
A SL machine consists of a build platform (substrate), which ismounted in a vat of resin and a UV Helium-Cadmium or Argon ion
laser.
In this process photosensitive liquid resin which forms a solid
polymer when exposed to ultraviolet light is used as a fundamentalconcept.
Due to the absorption and scattering of beam, the reaction only
takes place near the surface and voxels of solid polymeric resin are
formed.
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.
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(1) at the start of the process, in which the initial layer is
added to the platform; and (2) after several layers have been
added so that the part geometry gradually takes form.
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Each layer is 0.076 mm to 0.50 mm (0.003 in to
0.020 in.) thick
Thinner layers provide better resolution and
more intricate shapes; but processing time is
longerStarting materials are liquid monomers
Polymerization occurs on exposure to UV light
produced by laser scanning beamScanning speeds ~ 500 to 2500 mm/s
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Part Build Time in STL
Time to complete single layer
d
i
i T
vD
AT
where Ti= time to complete layer i;
Ai= area of layer i;
v= average scanning speed of the laser beam at the surface;D= diameter of the spot size, assumed circular; and
Td= delay time between layers to reposition the worktable
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Once the Tivalues have been determined for
all layers, then the build cycle time is:
i
n
i
ic TT
1
where Tc= STL build cycle time; and nl= number of
layers used to approximate the part
Time to build a part ranges from one hour for small
parts of simple geometry up to several dozen hours
for complex parts
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RP Applications
Applications of rapid prototyping can be classified
into three categories:
1. Design
2. Engineering analysis and planning
3. Tooling and manufacturing
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Designers are able to confirm their design by building a real
physical model in minimum time using RP
Design benefits of RP:
Reduced lead times to produce prototypes
Improved ability to visualize part geometry
Early detection of design errors
Increased capability to compute mass properties
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Manufacturing Applications:
1. Small batches of plastic parts that could not beeconomically molded by injection molding because of the
high mold cost.
2. Parts with intricate internal geometries that could not be
made using conventional technologies without assembly.
3. One-of-a-kind parts such as bone replacements that mustbe made to correct size for each user
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THE END
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