Ste Rio Lithography

8/12/2019 Ste Rio Lithography

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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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