Unit.

Introduction to Rapid Prototyping, Material, Applications, Limitations,

Classification of Rapid Manufacturing Process, Traditional Prototyping Vs

Rapid Prototyping.

Introduction:

The current marketplace is undergoing an accelerated pace of change that challenges

companies to innovate new techniques to rapidly respond to the ever- changing global environment.
A country's economy is highly dependent on the development of new products that are innovative
with shorter development time Organizations now fail or succeed based upon their ability to
respond quickly to changing customer demands and to utilize new innovative technologies.

Prototype:

It is the first or preliminary version of a product from which other forms are developed. It
is a model from which further models and eventually the final product will be derived.

Rapid Prototyping:

The term rapid prototyping (RP) refers to a class of technologies that can automatically
construct physical models from Computer-Aided Design (CAD) data.

It is a process for rapidly creating a system or part representation before final release or
commercialization.

It is a process for fabricating of a physical, three – dimensional part of arbitrary shape

directly from a numerical description (typically a CAD model) by a quick, totally automated
and highly flexible process.

Alternative names for RP:

Additive Manufacturing

Layer Manufacturing

Direct CAD Manufacturing

Solid Freeform Fabrication

Traditional Prototyping Vs. Rapid Prototyping:

 Traditional Prototyping Rapid Prototyping
It could include building a model from It could include building a model from
CLAY, carving from wood, bending wire thermoplastic, photopolymer, metals, paper,
titanium alloys etc.
meshing etc.
These methods are time consuming. These methods consume less time.

Lack the quality to serve its purpose. Gives better quality.

It can’t effectively evaluate the It can effectively evaluate the alternative

alternative design concepts in the design concepts in the product definition
product definition stage. stage.

Generally these methods are performed Generally these methods are performed
manually. automatically.

Increases product launch time. Reduces product launch time.

 Classification of Rapid Prototyping Systems

Fundamentally, the development of RP can be seen in four primary areas.

The Rapid Prototyping Wheel as shown in below figure depicts these four key aspects of
Rapid Prototyping. They are: Input, Method, Material and Applications.

The Rapid Prototyping Wheel

While there are many ways in which one can classify the numerous RP systems in the market, one
of the better ways is to classify RP systems broadly by the initial form of its material, i.e. the
material that the prototype or part is built with.

In this manner, all RP systems can be easily categorized into

(1) liquid-based (2) solid- based and (3) powder-based.

1.3.1 Liquid-based RP systems

Liquid-based RP systems have the initial form of its material in liquid state.

Through a process commonly known as curing, the liquid is converted into the solid state.

The following RP systems fall into this category:

1) 3D Systems’ Stereo lithography Apparatus (SLA)

2) Cubital’s Solid Ground Curing (SGC)

3) Sony’s Solid Creation System (SCS)

4) CMET’s Solid Object Ultraviolet-Laser Printer (SOUP)

5) Autostrade’s E-Darts

6) Teijin Seiki’s Soliform System

7) Meiko’s Rapid Prototyping System for the Jewelry Industry

8) Denken’s SLP

9) Mitsui’s COLAMM

10) Fockele & Schwarze’s LMS

11) Light Sculpting

12) Aaroflex

13) Rapid Freeze

14) Two Laser Beams

15) Micro fabrication

Each of these RP systems will be described in more detail in next chapters.

Following table shows some important RP systems and materials used for that

particular technology.
Table 1.2 RP systems and related base materials

Prototyping Technologies Base Materials

Selective laser sintering (SLS) Thermoplastics, Metals powders
Fused Deposition Modeling (FDM) Thermoplastics, Eutectic metals.
Stereo lithography (SLA) Photopolymer
Laminated Object Manufacturing (LOM) Paper
Electron Beam Melting (EBM) Titanium alloys
3D Printing (3DP) Various materials

Additive manufacturing Techniques:

1.Laser Engineered Net Shaping (LENS)

The LENSTM process builds components in an additive manner from powdered metals using a Nd:
YAG laser to fuse powder to a solid as shown in Figure 5.15. It is a freeform metal fabrication
process in which a fully dense metal component is formed. The LENSTM process comprises of the
following steps.

Steps

A deposition head supplies metal powder to the focus of a high powered Nd:YAG laser
beam to be melted. This laser is typically directed by fiber optics or precision angled mirrors.

The laser is focused on a particular spot by a series of lenses, and a motion system
underneath the platform moves horizontally and laterally as the laser beam traces the cross-
section of the part being produced.
The fabrication process takes place in a low-pressure argon chamber for oxygen-free operation in
the melting zone, ensuring that good adhesion is accomplished.

When a layer is completed, the deposition head moves up and continues with the next
layer. The process is repeated layer by layer until the part is completed. The entire process is
usually enclosed to isolate the process from the atmosphere. Generally, the prototypes need
additional finishing, but are fully dense products with good grain formation.

Principle

The LENS process is based on the following two principles:

A high powered Nd: YAG laser focused onto a metal substrate creates a molten
puddle on the substrate surface. Powder is then injected into the molten puddle to increase material
volume.

A “printing” motion system moves a platform horizontally and laterally as the laser beam
traces the cross-section of the part being produced. After formation of a layer of the part, the
machine’s powder delivery nozzle moves upwards prior to building next layer.

Advantages

Superior material properties. The LENS process is capable of producing fully dense
metal parts. Metal parts produced can also include embedded structures and superior material
properties. The microstructure produced is also relatively good.

Complex parts. Functional metal parts with complex features are the forte of the

LENS system.

Reduced post-processing requirements. Post-processing is minimized, thus reducing cycle

time.

Disadvantages

Limited materials. The process is currently narrowly focused to produce only metal parts.

Large physical unit size. The unit requires a relatively large area to house.

High power consumption. The laser system requires very high wattage.

2.Direct Metal Deposition (DMD)

A direct laser deposition (DLD) or direct metal deposition (DMD) process is a laser- assisted direct
metal manufacturing process that uses computer controlled lasers that, in hours, weld air blown
streams of metallic powders into custom parts and manufacturing molds. Some processes use wire
instead of powder, but the concept is similar.

A representative process is called the Laser Engineered Net Shaping (LENS) process. It uses CAD
file cross-sections to control the forming process developed by Optomec Inc. The DLD process can
be used throughout the entire product life-cycle for applications ranging from materials research to
functional prototyping to volume manufacturing.
An additional benefit is its unique ability to add material to existing components for service and
repair applications. Powder-metal particles are delivered in a gas stream into the focus of a laser to
form a molten pool of metal. It is a layer-by-layer additive rapid prototyping process. The DLD
process allows the production of parts, molds, and dies that are made out of the actual end-material,
such as aluminum or tool steel. In other words, this produces the high-temperature materials that
are difficult to make using the traditional RP processes.

The laser beam is moved back and forth across the part and creates a molten pool of metal where a
precise stream of metal powder is injected into the pool to increase its size. This process is the
hybrid of several technologies: lasers, CAD, CAM, sensors, and powder metallurgy. This process
also improves on other methods of metalworking in that there is no waste material or subtractive
processes necessary. It can also mix metals to specific standards and specifications in a manner that
has never been possible before.

Advantages:

The strength of DLD lies in the process’ ability to fabricate fully dense metal parts

with good metallurgical properties at reasonable speeds.

DLD is an efficient approach that reduces production costs and speeds time to market for
high-value components.

The DLD systems enable the fabrication of novel shapes, hollow structures, and
material gradients that are not otherwise feasible.

Disadvantages:

Since DLD is a freeform process, there is a limit to the overhang angle that can be built.
 The traditional DLD or RP processes are using three-axis tables, and thus support
structures are very often needed in building overhang parts. These structures are not desirable in
laser-based processes involving metals. One could use a high melting- point material to build the
support structures and use other processes, such as chemical etching, to remove the support material
afterward.

3.Sheet Lamination (Laminated Object Manufacturing (LOM):

There are two approaches of LOM process.

I. Cut and then paste

 Handling the cut pieces is difficult if not impossible since

 More than one piece may have to be handled for every layer
 Such pieces may be odd-shaped
 Paper being flexible further complicates handling
 A support mechanism will be required.
 Suitable for laminated tooling.

II. Paste and then cut

Handling is easy – indexing of the reel is all that is required.

The remaining stock acts as the support material.

The only drawback is the time-consuming decubing operation.

Suitable for paper-like flexible materials.

Steps

If multiple parts are to be made, one has to arrive at a cluster of optimal packing (an automatic
program for this is still not available!). It is preferable to pack as many pieces as possible in
processes such as LOM, SLS, SGC and 3DPrinitng.

The object/ cluster is positioned and oriented in the desired place. Some users tilt it by 10 to 15
deg. to avoid any surface becoming horizontal (why?).

Set the machine with the desired process parameters such as beam diameter, beam offset flag, grid
sizes, number of dummy layers, bridging gap between two cuts etc.

Load the paper roll of appropriate width.

Identify the location for the build on the table and feed it to the machine. Paste a double-sided
adhesive in that zone.

Each slice or layer is realized using the following steps:

The paper reel indexes by a fixed distance. It has adhesive at the bottom surface.
The table rises to the required height.
A hot roller (laminating tool) rolls over it causing it to stick to the previous layer.
The height is measured and it is passed on to the slicing software.
The loops of the slice are cut by the laser. It is possible to offset the laser beam by beam
radius in such a direction as to compensate for it.
This is followed by grid cutting around the bounding box of the stock. Note that the grids
of all layers coincide. Finally, a parting off cut is made.
The table lowers by a considerable distance so that the cut portion is stripped off from the
reel.
After all layers are made, the built volume is a rectangular block. This is parted off from
the table using a thin wire rope.
The unwanted material inside and surrounding are removed using hand tools. This is
called ‘decubing’. This operation takes several hours.

The part is finished and painted as required. It can be given a lacquer coat to prevent it from
absorbing moisture.

Advantages

 Only boundaries are to be addressed and not their interiors.

 It employs CO2 laser which is cheaper. No protective environment is required.
 Paper is very cheap.
 It gives strong wood-like parts. Ideal as patterns for casting

Limitations

 Grid cutting takes much more time than object cutting.

 Decubing also is time-consuming.
 Horizontal surfaces pose problems. Although it is solvable, it has not been done till date.

4. 3D Printing
Very similar to SLS except that a binder liquid is spayed in selected regions instead of laser. Raw
material is powder. Concept models can be prepared rapidly using a multi-jet multi-color spray over
starch (ZCorp). Green parts will require sintering inside another furnace.

When a binder is sprayed through thin nozzles on the selected region over a layer of powder, the
particles in that region stick together. The remaining powder acts as support as in the case of LOM.

Binder spray makes use of mechanical movement. However, use of multiple jets make it faster.
Explicit support structures are not required. A wide variety of powders can be used.

Steps

Raw material is powder.

The binder liquid is selectively deposited on the layer of powder.

This is followed by a curing after which unbound powder is separated.

4. Fused Deposition Modeling (FDM)
Molten material inside a hot chamber is extruded through a nozzle. Use of the raw material in
wire form as a consumable piston is a great idea. he nozzle size alone does not decide the layer
thickness and road width. They together depend on speed of head and wire feed speed. Their
relation can be obtained from the principle of conservation of mass. (Analogy: applying
tooth paste on the brush.)

Explicit support structures are required. Therefore, twin heads are used, one for model and the
other for support.

Steps

Starting material is melted and small droplets are shot by a nozzle onto previously formed
layer

Droplets cold weld to surface to form a new layer

Deposition for each layer controlled by a moving x-y nozzle whose path is based on a cross
section of a CAD geometric model that is sliced into layers

Work materials include wax and thermoplastics

Advantages

 Any thermoplastic material can be used as long as the appropriate head is available.
 It does not employ lasers and hence no safety related issues.
 It does not use liquid.
 powder raw materials and hence clean. It can be kept in an office environment as a3D
printer.
 Very easy to remove the support. This is probably the easiest of all RP processes.
 This is the cheapest machine. However, this is also due to their business policy since the
costs of all RP machines are comparable.

Limitations

 As every point of the volume is addressed by a „mechanical device“, it is very slow.

 Not very accurate compared SLA, SGC etc.

6. Electron Beam melting (EBM)

Electron beam melting (EBM) has become a successful approach to PBF (Powder Bed Fusion).
In contrast to laser-based systems, EBM uses a high-energy electron beam to induce fusion
between metal powder particles. This process was developed at Chalmers University of
Technology, Sweden, and was commercialized by Arcam AB, Sweden, in2001.
Laser beams heat the powder when photons are absorbed by powder particles. Electron
beams, however, heat powder by transfer of kinetic energy from incoming electrons into powder
particles. As powder particles absorb electrons they gain an increasingly negative charge.

This has two potentially detrimental effects:

(1) if the repulsive force of neighboring negatively charged particles overcomes the
gravitational and frictional forces holding them in place, there will be a rapid expulsion of
powder particles from the powder bed, creating a powder cloud (which is worse for fine powders
than coarser powders) and

(2) increasing negative charges in the powder particles will tend to repel the incoming
negatively charged electrons, thus creating a more diffuse beam. There are no such
complimentary phenomena with photons. As a result, the conductivity of the powder bed in
EBM must be high enough that powder particles do not become highly negatively charged, and
scan strategies must be used to avoid build-up of regions of negatively charged particles. In
practice, electron beam energy is more diffuse, in part, so as not to build up too great a
negative charge in any one location. As a result, the effective melt pool size increases, creating
a larger heat-affected zone. ConsequentlyThe minimum feature size, median powder particle
size, layer thickness, resolution, and surface finish of an EBM process are typically larger than
for an mLS process.

As mentioned above, in EBM the powder bed must be conductive. Thus, EBM can only be
used to process conductive materials (e.g., metals) whereas, lasers can be used with any material
that absorbs energy at the laser wavelength (e.g., metals, polymers, and ceramics).

Electron beam generation is typically a much more efficient process than laser beam generation.

7. Selective laser Sintering (SLS)

It is developed by University of Texas, Austin.It is marketed by DTM, USA and EOS,
Germany. Raw material is powder. Principle is similar to Powder Metallurgy but for the
absence of compaction. Green part is prepared on the RP machine after partial sintering and
sintering is completed inside another furnace.

Just as SLA, here also laser light is used. When it is scanned on the selected region over a
layer of powder, the particles in that region fuse together. The remaining powder acts as support
as in the case of LOM.

Laser beam is positioned using a small mirror capable of deflecting in two directions.

Therefore, this has very low inertia and hence high speed and accuracy.

The power of the laser decides the layer thickness.

Explicit support structures are not required.

A wide variety of powders can be used.

Steps
When the slicing is done, The working volume is maintained with appropriate
temperature so that laser supplies the energy required to cross the threshold sintering
temperature. An inert environment is created using continuous supply of gas such as Nitrogen.
This is to minimize fire hazards as the fine particles have high activation.
Each slice or layer is realized using the following steps:

The table dips by a layer thickness.

A layer of powder is spread and leveled using a contra-rotating roller.

The beam scans the layer of powder. Thus, the required region is “selectively sintered”.

After all layers are made, the table rises completely revealing a block of cake with the part
inside.

The surrounding powder is soft and it is removed using suitable brushes. This powder is
reusable.

The part is kept in a suitable hot chamber to complete the sintering.

The metallic prototypes require copper impregnation in another furnace to improve their polish
ability.

The part is finished and painted as required.

Advantages

A wide variety of powders can be used.

Fast due to tiny moving mirror parts as in SLA.

Metallic parts can be made.

Suitable for making injection molding tools.

Limitations

Surface finish is less and dictated by the particle size.

Z accuracy is poor due to the absence of milling.

9.Photopolymerization (Stereo lithography (SL)

When a light of appropriate wave length falls on liquid photopolymer, the energy absorbed
causes polymerization. The polymerized photopolymer will be in solid state. Laser light is used.
When it is scanned on the selected region over a layer of liquid polymer, that region become
solid. The remaining liquid can be drained.
Laser beam is positioned using a small mirror capable of deflecting in two directions. Therefore,
this has very low inertia and hence high speed and accuracy. The power of the laser decides the
layer thickness. Explicit support structures are required. This is achieved by modifying the
geometry of the prototype. Typically bristles and thin structures are added.

1.At the start of the process, in which the initial layer is added to the platform

2.After several layers have been added so that the part geometry gradually takes form.

Steps

Support structures are automatically added to the model wherever required.

Slicing is done.

Each slice or layer is realized using the following steps:

The table (called vat) dips and comes up to the required Z level.

A blade wipes off the excess liquid.

The beam scans the liquid layer. For each loop, the border is made and then area filling is
done. Area filling is not in zig-zag pattern but in grids .

After all layers are made, the table rises completely revealing the part.
After the liquid has drained, it is removed from the table and the support structure is carefully
cut off.

The part is kept in a post-cure apparatus where it is kept under UV radiation for an hour or so.
This completes polymerization.

The part is finished and painted as required