1. Rapid Prototyping Dr. Lotfi K. Gaafar The American University in Cairo
Department of Mechanical Engineering
(202)

2. Introduction
Rapid Prototyping (RP) techniques are methods that allow designers to produce physical prototypes quickly.
It consists of various manufacturing processes by which a solid physical model of part is made directly from 3D CAD model data without any special tooling.
The first commercial rapid prototyping process was brought on the market in 1987.
Nowadays, more than 30 different processes (not all commercialized) with high accuracy and a large choice of materials exist.
These processes are classified in different ways: by materials used, by energy used, by lighting of photopolymers, or by typical application range.

3. The Rapid Prototyping Technique
In the Rapid Prototyping process the 3D CAD data is sliced into thin cross sectional planes by a computer.
The cross sections are sent from the computer to the rapid prototyping machine which build the part layer by layer.
The first layer geometry is defined by the shape of the first cross sectional plane generated by the computer.
It is bonded to a starting base and additional layers are bonded on the top of the first shaped according to their respective cross sectional planes.
This process is repeated until the prototype is complete.

4. Rapid Prototyping Technique
Process Flow
3D Solid
modeling
Data
preparation
Part Building
Pass
The rapid prototyping process starts by 3D modeling using CAD system. Then the computer divides the model into slices of nearly 0.05 inch thickness.
The computer sends this data to the rapid prototyping machine which builds the part.
If this part or design is not accepted, the design is modified and another part is built.
Reject
Redesign

5. Prototyping- What is it ?
. Physical Model of the product
. Degrees of Prototyping
. Full Complete scale Model - functional model
. Scaled Model - functional/ simulated material
. Geometrical configuration
. Partial ….

6. Prototyping- Why? Visualization Design Change (iterations)
Free Form Prototyping (complex object fabrication/ visualization)
Testing Fit/ Packaging
Cost, Time, and resource estimation
Process Planning
First to Market -- Critical for today’s industry
Rapid production (concurrent activities)
JIT concept (0 Inventory)
Rapid tooling / no tooling -- trend in technology

7. Prototyping- Why? Design for manufacturability Design for assembly
Design verification
Design for manufacturability
Design for assembly
Design for maintainability
Design for reliability
Design for Quality
Design Parameters (Tolerances/ allowances)
Concurrent Engineering
Tooling
. Reverse Engineering
. Die fabrication
. Tool Path generation
Limited Production

8. Classification of Prototyping Technology
Subtractive Processes (Material Removal)
Ex : Milling, turning, grinding,-- machining centers .., when used for prototype production
Degree of automation vary
Additive (Material Build-up)
Ex : Stereolithography
Degree of sophistication vary
Formative (Sculpture)
Ex : Forging, Casting, ..
When used for Prototyping, it is usually manual

9. Sophistication of Prototyping Technology
Such Technology is known by different terms, such as :
Desktop Manufacturing
Rapid Prototyping
Tool-less Manufacturing
3-D printing
Free form Fabrication (F3)

10. Sophistication of Prototyping Technology
Fabrication process :
The process must take a material in some shapeless form, and turn out solid objects with definite shape
Degree of Automation :
High degree of automation. Since Prototyping is a stage in a cycle, it is expected that the technology will enable “automated chaining” to the before and after links in the cycle.
Ability to build complex objects
The more complex the build object, the more sophistication in the technology.

11. Sophistication of Prototyping Technology
Tooling (no Tooling): Less tools is better
One shot operations: No assembly of parts, ..etc.
Time: The less time the better it is
The closeness to serve the purpose of the prototype: Accurate representation of the design
Flexible: Modifications, addition of parameters, scaling
Equipment: size, weight, maintenance..etc
Economical: Both equipment and operating costs
Clean, safe operation
User friendly

12. Rapid Prototyping Processes
SLS --- Selective Laser Sintering
SLA --- Stereolithography
LOM --- Laminated Object Manufacturing
FDM --- Fused Deposition Modeling
Others

13. Rapid prototyping Processes- SLS
Selective Laser Sintering
Selective Laser Sintering
The process operates on the layer-by-layer principle. At the beginning a very thin layer of heat fusible powder is deposited in the working space container. The CO2-laser sinters the powders.
The sintering process uses the laser to raise the temperature of the powder to a point of fusing without actually melting it. As the process is repeated, layers of powder are deposited and sintered until the object is complete.
The powder is transferred from the powder cartridge feeding system to the part cylinder (the working space container) via a counter rolling cylinder, a scraper blade or a slot feeder. In the not sintered areas, powder remains loose and serves as natural support for the next layer of powder and object under fabrication. No additional support structure is required.
An SLS system contains also an atmosphere control unit that houses the equipment to filter gas recirculated from the process chamber. It also maintains a set temperature on the air flowing into the process chamber.

14. Rapid prototyping Processes- SLS
Application Range
Visual Representation models
Functional and tough prototypes
cast metal parts
Advantages
Flexibility of materials used
PVC, Nylon, Sand for building sand casting cores, metal and investment casting wax.
No need to create a structure to support the part
Parts do not require any post curing except when ceramic is used.
Disadvantages
During solidification, additional powder may be hardened at the border line.
The roughness is most visible when parts contain sloping (stepped) surfaces.

15. Rapid prototyping Processes- SL
Stereolithography
Stereolethography
SLA system consists of four main components: the slice computer, the control computer, the process chamber and the laser unit.
The slice computer reads the triangulated CAD model and cuts it into thin slices according to process parameters. The input for the slice computer is usually a file generated on a CAD workstation (.STL file). Afterwards, the control computer reads the file (.SLI) provided by the slice computer and allows moving and rotating the parts of the machine (elevator, sweeper, mirrors, etc.) during the manufacturing time.
The process chamber is the “heart” of the system: initially, the elevator is located at a distance from the surface of the liquid equal to the thickness of the first layer. The laser beam will then scan the surface following the contours of the slice. The liquid is a photopolymeric fluid which, when exposed to the UV laser beam, solidifies by low energy absorption. When the laser beam has completely “written” the first layer, the elevator is moved downwards and the following layers are produced like the first.
Finally, the part is removed from the vat and completely cured in a special UV post cure apparatus. Because the part is built in a liquid environment and the interior of the part contains liquid, it is necessary to add support structures. They are used to hold the parts in place while the layers are being built and to maintain the structural integrity of the part.
The support structures attach the part to the elevator platform (a perforated steel plate) and have to be removed when the part is completely manufactured.

16. Rapid Prototyping Resin
Basic Polymer Chemistry
SL Resin : It is a liquid photocurable resin
Characteristics
Fully 100% reactive component
Energy efficient requiring 50 to 100 times less energy than thermally cured coatings
Polymerization : It is the process of linking small molecules (monomers) into larger molecules (polymers) comprised of many monomer units.
As polymerization occurs (chemical reaction) many properties changes, shear strength increase, density increased as resin changes from liquid to solid (shrinkage)
Polymerization occurs in SL through the exposure of liquid resin to laser. The layer thickness to be polymerized is given by the amount of liquid which has been recoated onto the part, and any excess laser radiation that penetrates this layer acts to slightly increase the curing of the previous layers.
The important properties for selecting the resin has to do with posture shrinkage and the resulting posture distortions.

17. Desirable features of SL resin
Improved Impact resistance (less brittleness)
Greater Flexibility
Improved photospeed
Increased Strength
Better overall part accuracy
Electrical conductivity
High temperature resistance
Solvent resistance or vice versa

18. Some measures to reduce distortions
Use high exposure and slow scan speed such that polymerization is essentially complete under the laser spot.
Use resin with a faster rate of polymerization
Decrease laser power to decrease scan speed for a given exposure.
Use low-shrinkage resin
Increase layer thickness to increase the strength

19. Rapid prototyping Processes- SL
Application Range
Parts used for functional tests
Manufacturing of medical models
Form –fit functions for assembly tests
Advantages
Possibility of manufacturing parts which are impossible to be produced conventionally in a single process
Can be fully atomized and no supervision is required.
High Resolution
No geometric limitations
Disadvantages
Necessity to have a support structure
Require labor for post processing and cleaning

20. Rapid prototyping Processes- LOM
Laminated Object Manufacturing
Laminated Object Manufacturing
The computer that runs the system is capable of slicing a 3-D solid model into thin two-dimensional cross sections. The thickness of each cross-section is equal to the thickness of the material used in the process.
The mechanical part of the system contains an unwinding and rewinding roll connected by a ribbon of sheet material, routed through several idler rollers. These rolls store and supply the material. The laminated part is grown on a platform capable of a vertical incremental movement under the action of a stepping motor. Above the platform there is a heated roller, capable of heating and compressing the ribbon on the stack of laminations on the platform. As a result of a single reciprocal motion of the heated roller the ribbon material is bounded to the top of the stack. An x-y positioning table carries two mirrors that reflect a beam from CO2 laser and a lens that focuses the beam on the upper surface of the laminated stack in order to cut the very top layer.
Scrap pieces remain on the platform as the part is being built. They are diced by the laser beam into cross hatched squares and serve as a support structure for the part.
The product comes out of the machine as a rectangular block containing the part and the cubes due to a ''cross hatch'' cut by the laser are separated easily from the part. The LOM parts have the look and the feel of wood.

21. Rapid prototyping Processes- LOM
Application Range
Visual Representation models
Large Bulky models as sand casting patterns
Advantages
Variety of organic and inorganic materials can be used
Paper, plastic, ceramic, composite
Process is faster than other processes
No internal stress and undesirable deformations
LOM can deal with discontinuities, where objects are not closed completely
Disadvantages
The stability of the object is bonded by the strength of the glued layers.
Parts with thin walls in the z direction can not be made using LOM
Hollow parts can not be built using LOM

22. Rapid prototyping Processes- FDM Fused Deposition Modeling
The FDM system consists of the main 3-D Modeler unit, a slicing software and a workstation.
The process starts with the creation of a part with a CAD system as a solid or surface model. The model is then converted into an .STL file and send to the FDM slicing software. There, the .STL file is sliced into thin cross sections of a desired resolution, creating an .SLC file. Supports are created if required by the geometry and sliced as well. The sliced model and supports are converted into an .SML file that contains actual instruction codes for the FDM machine.
The FDM machine follows the principle of a three axis NC-machine tool. A nozzle, controlled by a computer along three axes, guides the specific material that is melted by heating. The material leaves the nozzle in a liquid form, which hardens immediately at the temperature of the environment. For this reason, it is fundamental for the FDM process that the temperature of the liquid modeling material is balanced just above the solidification point. A spool of modeling filament with a diameter of 1.27 mm feeds the FDM head, it can be changed to a different material in less than 1 minute. Within the building of the desired object the material is extruded and then deposited in ultra thin layers from the lightweight FDM machine layer-by-layer.

23. Rapid prototyping Processes- FDM

24. Rapid prototyping Processes- FDM
Application Range
Conceptual modeling
Fit, form applications and models for further manufacturing procedures
Investment casting and injection molding
Advantages
Quick and cheap generation of models
There is no worry of exposure to toxic chemicals, lasers or a liquid chemical bath.
Disadvantages
Restricted accuracy due to the shape of material used, wire is 1.27 mm diameter.

25. Rapid prototyping Processes
Other Processes
Ballistic Particle Manufacturing (BPM)
This process uses a 3D solid model data to direct streams of material at a target.
3D Printing
It creates parts by layered printing process. The layers are produced by adding a layer of powder to the top of a piston and cylinder containing a powder bed and the part is being fabricated.
Model Maker
It uses ink jet printer technology with 2 heads. One deposits building material, and the other deposits supporting wax.

26. Rapid Prototyping Products