INTRODUCTION TO RAPID PROTOTYPING

Rapid prototyping (RP) is a new manufacturing technique that allows for fast fabrication of computer
models designed with three-dimension (3D) computer aided design (CAD) software. RP is used in a
wide variety of industries, from shoe to car manufacturers. This technique allows for fast realizations
of ideas into functioning prototypes, shortening the design time, leading towards successful final
products.

RP technique comprise of two general types: additive and subtractive, each of which has its own pros
and cons. Subtractive type RP or traditional tooling manufacturing process is a technique in which
material is removed from a solid piece of material until the desired design remains. Examples of this
type of RP includes traditional milling, turning/lathing or drilling to more advanced versions - computer
numerical control (CNC), electric discharge machining (EDM). Additive type RP is the opposite of
subtractive type RP. Instead of removing material, material is added layer upon layer to build up the
desired design such as stereolithography, fused deposition modeling (FDM), and 3D printing.

This tutorial will introduce additive type RP techniques: Selective Laser Sintering (SLS),
Stereolithography Apparatus (SLA), FDM, Inkjet based printing. It will also cover how to properly
prepare 3D CAD models for fabrication with RP techniques.

Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabricate a scale model
of a part or assembly using three-dimensional computer aided design (CAD) data. What is commonly
considered to be the first RP technique, Stereolithography, was developed by 3D Systems of Valencia,
CA, USA. The company was founded in 1986, and since then, a number of different RP techniques
have become available.

Rapid Prototyping has also been referred to as solid free-form manufacturing, computer automated
manufacturing, and layered manufacturing. RP has obvious use as a vehicle for visualization. In
addition, RP models can be used for testing, such as when an air foil shape is put into a wind tunnel.
RP models can be used to create male models for tooling, such as silicone rubber molds and investment
casts. In some cases, the RP part can be the final part, but typically the RP material is not strong or
accurate enough. When the RP material is suitable, highly convoluted shapes (including parts nested
within parts) can be produced because of the nature of RP.
Definition:

Rapid Prototyping is basically a additive manufacturing process used to quickly fabricate a

model of a part using 3-D CAM data. It can be defined as layer by layer fabrication of 3D physical
models directly from CAD.

What is Rapid Prototyping?

Rapid Prototyping is the "process of quickly building and evaluating a series of prototypes" early and
often throughout the design process. Prototypes are usually incomplete examples of what a final
product may look like. Each time a prototype is used, a formative evaluation gathers information for
the next, revised prototype. This cycle continues to refine the product until the final needs and
objectives are met. The following diagram demonstrates the non-linear nature of Rapid Prototyping.

Why Rapid Prototyping?

The reasons of Rapid Prototyping are

To increase effective communication.

To decrease development time.
To decrease costly mistakes.
To minimize sustaining engineering changes.
To extend product lifetime by adding necessary features and eliminating redundant features
early in the design.
Rapid Prototyping decreases development time by allowing corrections to a product to be made early
in the process. By giving engineering, manufacturing, marketing, and purchasing a look at the product
early in the design process, mistakes can be corrected and changes can be made while they are still
inexpensive. The trends in manufacturing industries continue to emphasize the following:

Increasing number of variants of products.

Increasing product complexity.
Decreasing product lifetime before obsolescence.
Decreasing delivery time.
Rapid Prototyping improves product development by enabling better communication in a concurrent
engineering environment.
How does Rapid Prototyping Work?

Rapid Prototyping, also known as 3D printing, is an additive manufacturing technology. The process
begins with taking a virtual design from modeling or computer aided design (CAD) software. The 3D
printing machine reads the data from the CAD drawing and lays down successive layers of liquid,
powder, or sheet material building up the physical model from a series of cross sections. These
layers, which correspond to the virtual cross section from the CAD model, are automatically joined
together to create the final shape.

Rapid Prototyping uses a standard data interface, implemented as the STL file format, to translate from
the CAD software to the 3D prototyping machine. The STL file approximates the shape of a part or
assembly using triangular facets.

Typically, Rapid Prototyping systems can produce 3D models within a few hours. Yet, this can vary
widely, depending on the type of machine being used and the size and number of models being
produced.

NEED FOR THE COMPRESSION IN THE PRODUCT DEVELOPMENT

To increase effective communication

To decrease development time
To decrease costly mistakes
To minimize sustaining engineering changes
To extend product life time by adding necessary features & eliminating redundant features early
in the design.
TRENDS IN MANUFACTURING INDUSTRIES EMPHASIS THE FOLLOWING

Increasing the no of variants of products

Increase in product complexity
Decrease in product lifetime before obsolescence
Decrease in delivery time
Product development by Rapid Prototyping by enabling better communication
HISTORICAL DEVELOPMENT

The development of Rapid Prototyping is closely tied in with the development of applications of
computers in the industry. The declining cost of computers, especially of personal and mini computers,
has changed the way a factory works. The increase in the use of computers has spurred the advancement
in many computer-related areas including Computer-Aided Design (CAD), Computer-Aided
Manufacturing (CAM) and Computer Numerical Control (CNC) machine tools. In particular, the
emergence of RP systems could not have been possible without the existence of CAD. However, from
careful examinations of the numerous RP systems in existence today, it can be easily deduced that other
than CAD, many other technologies and advancements in other fields such as manufacturing systems
and materials have also been crucial in the development of RP systems. Table 1.1 traces the historical
development of relevant technologies related to RP from the estimated date of inception.

Table: 1.1 Historical development of Rapid Prototyping and related technologies

Year of Inception Technology

1770 Mechanization

1946 First Computer

First Numerical Control (NC)

1952
Machine Tool

1960 First commercial Laser

1961 First commercial Robot

1963 First Interactive Graphics System

First commercial Rapid Prototyping

1988
System

First Phase: Manual Prototyping

Prototyping had begun as early as humans began to develop tools to help them live. However,
prototyping as applied to products in what is considered to be the first phase of prototype development
began several centuries ago. In this early phase, prototypes typically are not very sophisticated and
fabrication of prototypes takes on average about four weeks, depending on the level of complexity and
representativeness. The techniques used in making these prototypes tend to be craft-based and are
usually extremely labour intensive.

Second Phase: Soft or Virtual Prototyping

As application of CAD/CAE/CAM become more widespread, the early 1980s saw the evolution of the
second phase of prototyping Soft or Virtual Prototyping. Virtual prototyping takes on a new meaning
as more computer tools become available computer models can now be stressed, tested, analyzed
and modified as if they were physical prototypes. For example, analysis of stress and strain can be
accurately predicted on the product because of the ability to specify exact material attributes and
properties. With such tools on the computer, several iterations of designs can be easily carried out by
changing the parameters of the computer models.

Also, products and as such prototypes tend to become relatively more complex about twice the
complexity as before. Correspondingly, the time required to make the physical model tends to increase
tremendously to about that of 16 weeks as building of physical prototypes is still dependent on craft-
based methods though introduction of better precision machines like CNC machines helps.

Even with the advent of Rapid Prototyping in the third phase, there is still strong support for virtual
prototyping. Lee argues that there are still unavoidable limitations with rapid prototyping. These
include material limitations (either because of expense or through the use of materials dissimilar to that
of the intended part), the inability to perform endless what-if scenarios and the likelihood that little or
no reliable data can be gathered from the rapid prototype to perform finite element analysis (FEA).
Specifically, in the application of kinematic/dynamic analysis, he described a program which can
assign physical properties of many different materials, such as steel, ice, plastic, clay or any custom
material imaginable and perform kinematics and motion analysis as if a working prototype existed.
Despite such strengths of virtual prototyping, there is one inherent weakness that such soft prototypes
cannot be tested for phenomena that is not anticipated or accounted for in the computer program. As
such there is no guarantee that the virtual prototype is really problem free.

Third Phase: Rapid Prototyping

Rapid Prototyping of physical parts, or otherwise known as solid freeform fabrication or desktop
manufacturing or layer manufacturing technology, represents the third phase in the evolution of
prototyping. The invention of this series of rapid prototyping methodologies is described as a

Though the parts (individual components) are relatively three times as complex as parts made in 1970s,
the time required to make such a part now averages only three weeks [9]. Since 1988, more than twenty
different rapid prototyping techniques have emerged.

FUNDAMENTALS OF RAPID PROTOTYPING

a) A model or component is modelled on a Computer-Aided Design/Computer-Aided

Manufacturing (CAD/CAM) system. The model which represents the physical part to be built
 must be represented as closed surfaces which unambiguously define an enclosed volume. This
mean that the data must specify the inside, outside and boundary of the model. This requirement
will become redundant if the modelling technique used is solid modelling. This is by virtue of
the technique used, as a valid solid model will automatically be enclosed volume. This
requirement ensures that all horizontal cross sections that are essential to RP are enclosed curves
to create the solid object.
b)
(Stereolithography) file format which originates from 3D systems. The STL file format
approximates the surfaces of ten model by polygons. Highly curved surfaces must employ many
polygons, which means that STL files for curved parts can be very large. However, these are
some rapid prototyping systems which also accept IGES (Initial Graphics Exchange

c)
the model into cross sections. The cross sections are systematically recreated through the
solidification of either liquids or powders and then combined to form a 3D model. Another
possibility is that the cross sections are already thin, solid laminations and these thin laminations
are glued together with adhesives to form a 3D model. Other similar methods may also be
employed to build the model.
Fundamentally, the development of RP can be seen in four primary areas. The Rapid
Prototyping Wheel depicts these four key aspects of Rapid Prototyping. They are: Input,
Method, Material and Applications.

INPUT

Input refers to the electronic information required to describe the physical object with
3D data. These are two possible starting points- a computer model or a physical model. The
computer model created by a CAD system can be either a surface model or a solid model. On
the other hand, 3D data from the physical model is not at all straight forward. It requires data
acquisition through a method known as reverse engineering. In reverse engineering, a wide
range of equipment can be used, such as CMM (coordinate measuring machine) or a laser

METHOD

While they are currently more than 20 vendors for RP systems, the method employed
by each vendor can be generally classified into the following categories: photo-curing, cutting
and gluing/joining, melting and solidifying/fusing and joining/binding. Photo-curing can be
further divided into categories of single beam, double laser beams and masked lamp.

MATERIAL

The initial state of material can come in either solid, liquid or powder state. In solid
state, it can come in various forms such as pellets, wire or laminates. The current range materials
include paper, nylon, wax, resins, metals and ceramics.

APPLICATION:

Most of the RP parts are finished or touched up before they are used for their intended
applications. Applications can be grouped into 1. Design 2. Engineering, Analysis, and planning
and 3. Tooling and Manufacturing. A wide range of industries can benefit from RP and these
include, but are not limited to, aerospace, automotive, biomedical, consumer, electrical and
electronics products.

Applications of Rapid Prototyping

RAPID TOOLING

Patterns for Sand Casting

Patterns for Investment Casting
Pattern for Injection mouldings
RAPID MANUFACTURING

Short productions run

Custom made parts
On-Demand Manufacturing
Manufacturing of very complex shapes
AEROSPACE & MARINE

Wind tunnel models

Functional prototypes
-Demand-Manufacturing
AUTOMOTIVE RP SERVICES

Needed from concept to production level

Reduced time to market
 Functional testing
Dies & Moulds
BIOMEDICAL APPLICATIONS - I

Prosthetic parts
Use of data from MRI and CT scan to build 3D parts
3D visualization for education and training
BIOMEDICAL APPLICATIONS - II

Customized surgical implants

Mechanical bone replicas
Anthropology
Forensics
ARCHITECTURE

3D visualization of design space

Iterations of shape
Sectioned models
FASHION & JEWELRY

Shoe Design
Jewellery
Pattern for lost wax
Other castings

Classification of Rapid Prototyping Systems

LIQUID-BASED

Stereo lithography (SLA)

Solid Ground Curing (SGA)

SOLID-BASED

Fused deposition modeling (FDM)

Laminated object manufacturing (LOM)

 POWDER-BASED

3D Printing

Selective laser sintering (SLS)

Direct metal laser sintering (DMLS)

Classification of Rapid Prototyping Systems

The professional literature in RP contains different ways of classifying RP processes. However,

one representation based on German standard of production processes classifies RP processes
according to state of aggregation of their original material and is given in figure

Fig 1.1: Classification of Rapid Prototyping Systems

a) Liquid-based
b) Solid-based
c) Powder-based
Liquid-Based

Liquid based RP systems have the initial form of its materials in liquid state. Through a process
commonly known as curing, the liquid is converted into the solid state. The following RP systems fall
into the following category

i) Stereolithography apparatus (SLA)

ii)
iii) reaction Systems (SCS)
iv) -Laser Printer (SOUP)
v) -Darts
vi)
vii) Rapid Prototyping systems for the jewellery Industry
viii)
ix)
x)
xi) Light Sculpting
xii) Aaroflex
xiii) Rapid Freeze
xiv) Two Laser Beams
xv) Microfabrication
Solid-Based

Except for powder, solid-based RP systems are meant to encompass all forms of material in
the solid state. In this context, the solid form can include the shape in the form of a wire, a roll,
laminates and pellets. The following RP systems fall into this definition:

i) turing (LOM)
ii)
iii)
iv) -Jet Modelling Systems (MJM)
v)
vi) Melted Extrusion Modelling (MEM)
and Multi-Functional RPM Systems (M-RPM)
vii) CAM-
viii)
Powder-Based

In a strict sense, powder is by-and-large in the solid state. However, it is intentionally created as a
category outside the solid-based RP systems to mean powder in grain-like form. The following RP
systems fall into the category

i)
ii)
iii) -Dimensional Printing (3DP)
iv) r Engineered Net Shaping (LENS)
v)
vi)
vii)
viii)
ix) Precision Optical Manufacturing;s Direct Metal Deposition (DMD)
x)
xi)
xii)
All the above RP Ssytems employ the Joining/Binding method. The method of joining/binding differs
for the above systems in that some employ a laser while others use a binder/glue to achieve the joining
effect.

RPT Basic Process

Although several rapid prototyping techniques exist, all employ the same basic five-step process. The
steps are:

1. Create a CAD model of the design

2. Convert the CAD model to STL format

3. Slice the STL file into thin cross-sectional layers

4. Construct the model one layer atop another

5. Clean and finish the model

1) CAD Model Creation
First, the object to be built is modelled using a Computer-Aided Design (CAD) software
package. Solid modelers, such as Pro/ENGINEER, tend to represent 3-D objects more accurately than
wire-frame modellers such as AutoCAD, and will therefore yield better results. The designer can use
a pre-existing CAD file or may wish to create one expressly for prototyping purposes. This process is
identical for all of the RP build techniques. The basic process is similar across the different additive
type RP technologies. We will use a ball as an example here. It begins with using a CAD software
such as Solidworks to design a 3D computer model. Figure 1.1 is a golf ball designed in Solidworks.

Fig 1.2: CAD model of a ball

2) Conversion to STL Format

The various CAD packages use a number of different algorithms to represent solid objects. To
establish consistency, the STL (stereolithography, the first RP technique) format has been adopted as
the standard of the rapid prototyping industry. The second step, therefore, is to convert the CAD file
into STL format. This format represents a three-dimensional surface as an assembly of planar triangles,
"like the facets of a cut jewel." 6 The file contains the coordinates of the vertices and the direction of
the outward normal of each triangle. Because STL files use planar elements, they cannot represent
curved surfaces exactly. Increasing the number of triangles improves the approximation, but at the cost
of bigger file size. Large, complicated files require more time to pre-process and build, so the designer
must balance accuracy with manageability to produce a useful STL file. Since the .stl format is
universal, this process is identical for all of the RP build techniques.

This 3D CAD model is next converted into a Stereolithography or Standard Tessellation

Language (STL) file format. The STL file format only describes the surface geometry of a 3D CAD
model. It does not contain any information on the color, texture or material. STL file format can be
saved in either ASCII or binary versions, with the latter as the more compact version. The surface
geometry is described with triangular facets. Each triangle facets uses a set of Cartesian coordinates to
describe its three vertices and the surface normal vector using a right-hand rule for ordering. An
example of how an ASCII STL file format is show in Fig. 1.2.

Figure 1.3: ASCII STL file format

To convert a CAD model to STL in Solidworks, File >Save as> Select

1.4 shows a print screen of the STL export option.
As shown in Fig. 1.4, one can select to export the STL as Binary or ASCII file format in millimetre,
centimetre, meter, inches or feet depending on the unit used in the CAD model.

Figure 1.4: Solidworks STL export option

3) Slice the STL File
In the third step, a pre-processing program prepares the STL file to be built. Several programs
are available, and most allow the user to adjust the size, location and orientation of the model. Build
orientation is important for several reasons. First, properties of rapid prototypes vary from one
coordinate direction to another. For example, prototypes are usually weaker and less accurate in the z
(vertical) direction than in the x-y plane. In addition, part orientation partially determines the amount
of time required to build the model. Placing the shortest dimension in the z direction reduces the
number of layers, thereby shortening build time. The pre-processing software slices the STL model
into a number of layers from 0.01 mm to 0.7 mm thick, depending on the build technique.

The program may also generate an auxiliary structure to support the model during the build. Supports
are useful for delicate features such as overhangs, internal cavities, and thin-walled sections. Each PR
machine manufacturer supplies their own proprietary pre-processing software.

The resolution options allow a user to control the tessellation of non-planar surfaces. There are

and angle tolerances. Lower deviation tolerance sets tighter accuracy to the tessellation whereas
smaller angle deviation sets smaller detail tessellation. The caveat is that tighter tolerances create more
e more finely which causes the file size to be
large.

Figure 1.4 shows a CAD model exported to a coarse resolution STL (114KB), fine resolution STL
(300 KB), and a very fine resolution STL file (1.51 MB). A more complicated design with complicated
features would also result in a large STL file size. Figure 1.5 shows an exaggerated view of how the

depending on how fine the tolerances are set, computation power to export the CAD model and process
the file for fabrication could be an issue. Once the appropriate STL file has been generated, this is then

fabrication.

Figure 1.5: CAD model to a coarse STL, fine STL, and a very fine STL file.
 Figure 1.6: Exaggerated view of different STL tolerances

4) Layer by Layer Construction

The fourth step is the actual construction of the part. Using one of several techniques (described
in the next section) RP machines build one layer at a time from polymers, paper, or powdered metal.
Most machines are fairly autonomous, needing little human intervention.

The different types of additive RP technologies can be categorized into three types: liquid based (SLA
and Inkjet based Printing), solid based (FDM), and powder based (SLS). These are just a few examples
of the different RP technologies in existence. Regardless of the different types of RP technologies, all
of them
to 2D slice layers for fabrication.

5) Clean and Finish

The final step is post-processing. This involves removing the prototype from the machine and
detaching any supports. Some photosensitive materials need to be fully cured before use. Prototypes
may also require minor cleaning and surface treatment. Sanding, sealing, and/or painting the model
will improve its appearance and durability.

Advantages and disadvantages of rapid prototyping

Subtractive type RP is typically limited to simple geometries due to the tooling process where material
is removed. This type of RP also usually takes a longer time but the main advantage is that the end
product is fabricated in the desired material. Additive type RP, on the other hand, can fabricate most
complex geometries in a shorter time and lower cost. However, additive type RP typically includes
extra post fabrication process of cleaning, post curing or finishing.
Advantage of Rapid Prototyping

Fast and inexpensive method of prototyping design ideas

Multiple design iterations
Physical validation of design
Reduced product development time
It encourages and requires active student participation the design process.
Iteration and change are natural consequences of instructional systems development. Clients tend
to change their minds.
Clients don't know their requirements until they see them implemented.
An approved prototype is the equivalent of a paper specification & dash with one exception.
Errors can be detected earlier.
Prototyping can increase creativity through quicker user feedback. (But see below.)
Prototyping accelerates the development cycle.
Disadvantages of Rapid Prototyping

The main disadvantage of prototyping can be summed up in one complaint that is easy to
imagine: it has a tendency to encourage informal design methods which may introduce more
problems than they eliminate.
Resolution not as fine as traditional machining (millimeter to sub-millimeter resolution)
Surface flatness is rough (dependant of material and type of RP)
This failure can be avoided if the following issues are kept in mind: (Tripp and Bichelmeyer)
Prototyping can lead to a design-by-repair philosophy, which is only an excuse for lack of
discipline.
Prototyping does not eliminate the need for front-end analysis. It cannot help if the situation is
not amenable to instructional design.
A prototype cannot substitute completely for a paper analysis.
There may be many instructional design problems which are not addressed by prototyping.
Prototyping may lead to premature commitment to a design if it is not remembered that a design
is only a hypothesis.
When prototyping an instructional package, creeping featurism (the adding of bells and whistles)
may lead to designs that get out of control."