rapid prototyping.doc

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Dr.Mahalingam College of Engineering andTechnology Pollachi-642003

Department of Mechanical EngineeringRapid Prototyping

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, Stereo lithography, 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 airfoil 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.

The reasons of Rapid Prototyping are

1. To increase effective communication.

2. To decrease development time.

3. To decrease costly mistakes.

4. To minimize sustaining engineering changes.

5. 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:

1. Increasing number of variants of products.

2. Increasing product complexity.

3. Decreasing product lifetime before obsolescence

4. Decreasing delivery time.

Rapid Prototyping improves product development by enabling better communication in a

concurrent engineering environment

C.Selva Senthil Prabhu and M.Giridharadayalan AP Mech Page 1


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Department of Mechanical EngineeringThe basic methodology for all current rapid prototyping techniques can be summarized as

follows:

1. A CAD model is constructed, then converted to STL format. The resolution can be set to

minimize stair stepping

2. The RP machine processes the .STL file by creating sliced layers of the model.

3. The first layer of the physical model is created. The model is then lowered by the

thickness of the next layer, and the process is repeated until completion of the model.

4. The model and any supports are removed. The surface of the model is then finished and

cleaned.

Rapid Tooling

The term Rapid Tooling (RT) is typically used to describe a process which either uses

a Rapid Prototyping (RP) model as a pattern to create a mold quickly or uses the Rapid

Prototyping process directly to fabricate a tool for a limited volume of prototypes. RT is

distinguished from conventional tooling in that:

a. Tooling time is much shorter than for a conventional tool. Typically, time to first

articles is below one-fifth that of conventional tooling.

b. Tooling cost is much less than for a conventional tool. Cost can be below five

percent of conventional tooling cost.

c. Tool life is considerably less than for a conventional tool.

d. Tolerances are wider than for a conventional tool.

In addition to Silicone (SRM), we present Composite Molding andDirect AIM (ACES

Injection Molding). The field of RT is expanding rapidly and information on many of the new

methodologies is still changing.

Selective Laser Sintering

Selective Laser Sintering (SLS, registered trademark by DTM of Austin, Texas, USA) is a

process that was patented in 1989 by Carl Deckard, a University of Texas graduate student. Its

chief advantages overStereo lithography (SLA) revolve around material properties. Many

varying materials are possible and these materials can approximate the properties of

thermoplastics such as polycarbonate, nylon, or glass-filled nylon. As the figure below shows,

an SLS machine consists of two powder magazines on either side of the work area. The

leveling roller moves powder over from one magazine, crossing over the work area to the other

magazine. The laser then traces out the layer. The work platform moves down by the thickness

http://www.efunda.com/processes/rapid_prototyping/intro.cfmhttp://www.efunda.com/processes/rapid_prototyping/rt_composite_molding.cfmhttp://www.efunda.com/processes/rapid_prototyping/rt_aim_aces.cfmhttp://www.efunda.com/processes/rapid_prototyping/rt_aim_aces.cfmhttp://www.efunda.com/processes/rapid_prototyping/rt_aim_aces.cfmhttp://www.efunda.com/processes/rapid_prototyping/sla.cfmhttp://www.efunda.com/processes/rapid_prototyping/rt_composite_molding.cfmhttp://www.efunda.com/processes/rapid_prototyping/rt_aim_aces.cfmhttp://www.efunda.com/processes/rapid_prototyping/rt_aim_aces.cfmhttp://www.efunda.com/processes/rapid_prototyping/sla.cfmhttp://www.efunda.com/processes/rapid_prototyping/intro.cfm


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Department of Mechanical Engineeringof one layer and the roller then moves in the opposite direction. The process repeats until the

part is complete

Fused Deposition Modeling

Stratasys of Eden Prairie, MN makes Fused Deposition Modeling (FDM) machines. The

FDM process was developed by Scott Crump in 1988. The fundamental process involves

heating a filament of thermoplastic polymer and squeezing it out like toothpaste from a tube to

form the RP layers. The machines range from fast concept modelers to slower, high-precision

machines. The materials include polyester, ABS, elastomers, and investment casting wax. The

overall arrangement is illustrated below:

CNC machining as an RP technology

The application of CNC milling as an RP technology still is new, and has been made

possible by a number of new developments. As these are not yet so well known, we will briefly



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Department of Mechanical Engineeringdescribe some developments in CNC machining that have made it a competitive technology for

RP.The basics of machining are very straightforward: cutting off small chips. Traditionally the

cutting tool was moved by manual control (rotating a wheel), after World War II new

developments made Numerical Control (punched tape) possible, and now for many years CNC

(Computerized Numerical Control) has become mature a technology. CNC milling has been

used for prototype building in the past: involving large, heavy and expensive machines,

powerful, though very complicated, CAM software, and skilled CAM operators. Surely not an

automatic process, so not to be called Rapid Prototyping.

Examples of low-cost 3D CNC milling machines, perfectly suited for RP applications.

(Sources Delft Spline Systems - left, and Minitech - right).

Since that time things have changed: both on the hardware side and on the software

side. New hardware developments resulted in small, light and very inexpensive CNC milling

machines. Nowadays prices for a 3D CNC milling machine even start below USD 1,000 ! Such

a machine (obviously with very limited

capabilities) is within the reach of any product designer. Many manufacturers now offer a

large variety of light CNC machines, including a fit for almost any application (figure 2). The

heavier industrial machines can of course also be used for RP purposes, offering advantagessuch as larger size, speed, stability, power, etc. You can find machine prices ranging between

USD 1,000 and 1,000,000 - for any specific application a fit can be found.

A second important development in machining hardware is High Speed (HS) milling.

Here the advantage is not in price but in speed. Using a very high spindle speed being ca

40,000 to 80,000 rounds per minute (rpm), the cutter can move much faster than on traditional



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Department of Mechanical Engineeringgeometry can be machined (within the tolerances to be set for cusp height and chordal

deviation). High-end CAM software may also offer options like detection and removal of rest

material at sharp inner corners, optimizes start- and end-procedures for operations, and the use

of machining features. Support of 2.5 D machining, which creates a model by combining a

number of 2D contours, each at a constant Z-level. Options like drilling holes on certain

positions belong to this category as well. This way of machining is very standard for mechanical

applications.

Number of axes supported. The basic CNC machine uses 3 controlled axes: X, Y and Z.

More elaborate machines may be equipped with a fourth axis (type 'barbecue' or rotation table),

or with axes where the tool can be rotated to approach the geometry from different directions.

Capability to optimize the tool paths for HS milling, by removing all angles (all subsequent

movements connected at a continuous tangent). Combining these new developments of a light

CNC milling machine and an easy-to use CAM software package results in a Rapid Prototyping

system that offers a number of special characteristics, and as we will show, supplements the

LMT based RP systems.


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