Rapid Prototyping

Dr. Lotfi K. Gaafar 2002

Rapid Prototyping

Group Members KDDS Dissanayake (IN 11- 2016 M 09)HS Amarasekara (IN 11 -2015 M04)

http://www.aucegypt.edu/


Contents Introduction Evolution of prototyping Need of rapid prototyping Rapid prototyping categories Rapid prototyping - steps to prepare control

instructions Rapid prototyping technologies Rapid prototyping application problems with

rapid prototyping



Introduction Prototyping or model making is one of the important

steps to finalize a product design and It helps in conceptualization of a design

The traditional method of fabricating a prototype part is machining, which can require significant lead times

RP is a family of fabrication methods to make engineering prototypes in minimum possible lead times based on CAD models

RP also known as “ Solid freeform fabrication”, “Desktop manufacturing” or ”Layer manufacturing Technology”.



Evolution of Prototyping Manual prototyping By skilled craftsman has been an age-old practice for many centuries

Soft (virtual) prototyping Second phase of prototyping started around mid-1970s, a soft prototype modeled by 3D curves and surfaces could be stressed in virtual environment, simulated and tested with exact material and other properties.

Rapid Prototyping (RP) Third and the latest trend of prototyping, started during early 1980s with the enormous growth in CAD/CAM



Why Rapid Prototyping

Product designers would like to have a physical model of a new part or product design rather than just a computer model or line drawing

Creating a prototype is an integral step in design

A virtual prototype (a computer model of the part design on a CAD system) may not be sufficient for the designer to visualize the part adequately

Using RP to make the prototype, the designer can visually examine and physically feel the part and assess its merits and shortcomings



Rapid Prototyping Categories

Available RP technologies can be divided into two broad categories

Material removal processes Material addition processes



1. Material removal processes

This involves machining, milling and drilling, using a CNC machines

To use CNC, a part program must be prepared from the CAD model

Starting material is often a solid block of wax, which is very easy to machine

Removed material can re-solidified for reuse when the current prototype is no longer needed

Other starting materials ( wood, plastics, or metals) can also be used



2. Material addition processes

Principal is to adding layers of material one at a time to build the solid part from bottom to top

Starting materials(1) Liquid monomers - cured layer by layer into solid

polymers(2) Powders - Aggregated and bonded layer by layer(3) Solid sheets - Laminated to create the solid part.



Steps to Prepare Control Instructions Steps to prepare the control instructions (part program)

in current material addition RP techniques

STEP 1 - Geometric modeling

This consists of modeling the component on a CAD system to define its enclosed volume

Solid modeling is the preferred technique because it provides a complete and unambiguous mathematical representation of the geometry

For rapid prototyping, the important issue is to distinguish the interior (mass) of the part from its exterior, and solid modeling provides for this distinction



STEP 2 - Tessellation of the geometric model

In this step, the CAD model is converted into a format that approximates its surfaces by triangles or polygons, with their vertices arranged to distinguish the object’s interior from its exterior.

The common tessellation format used in rapid prototyping is STL, which has become the de facto standard input format for nearly all RP systems

The term tessellation refers to the laying out or creation of a mosaic, such as one consisting of small colored tiles affixed to a surface for decoration



STEP 3 - Slicing of the model into layers

In this step, the model in STL2 file format is sliced into closely spaced parallel horizontal layers.

These layers are subsequently used by the RP system to construct the physical model.

By convention, the layers are formed in the x-y plane orientation, and the layering procedure occurs in the z-axis direction.

For each layer, a curing path is generated, called the STI file, which is the path that will be followed by the RP system to cure (or otherwise solidify) the layer.



Conversion of a solid model of an object into layers (only one layer is shown)



RAPID PROTOTYPING TECHNOLOGIES

The classification method is based on the form of thestarting material in the RP process

(1) Liquid-based technologies Stereolithography (STL) Solid Ground Curing (SGC) Droplet Deposition Manufacturing (DDM)

(2) Solid-based technologies Laminated-object manufacturing Fused-deposition modeling

(3) Powder-based technologies Selective Laser Sintering (SLS) Three-dimensional printing (3D printing)



Liquid-based Rapid Prototype

technologies



1. Stereolithography (STL) Stereolithography (STL) is a process for fabricating a

solid plastic part out of a photosensitive liquid polymer using a directed laser beam to solidify the polymer

Part fabrication is accomplished as a series of layers, in which one layer is added onto the previous layer to gradually build the desired three dimensional geometry

The Stereolithography apparatus consists of (1) A platform that can be moved vertically inside a vessel containing the photosensitive polymer, and(2) A laser whose beam can be controlled in the x-y direction



Stereolithography

At the start of the process, inwhich the initial layer isadded to the platform

After several layers have been added so that the part geometrygradually takes form



Solid Ground Curing

Like STL, Solid ground curing (SGC) works by curing a photosensitive polymer layer by layer to create a solid model based on CAD geometric data.

Instead of using a scanning laser beam to accomplish the curing of a given layer, the entire layer is exposed to an ultraviolet light source.

The hardening process takes 2 to 3 seconds for each layer.

The starting data in SGC is similar to that used in stereolithography: a CAD geometric model of the part that has been sliced into layers.



Solid ground curing process for each layer

(1) maskpreparation(2) applyingliquid photopolymerLayer(3) mask positioningand exposure of layer,(4) uncured polymerremoved from surface,(5)Wax filling(6) milling forflatness and thickness



Droplet Deposition Manufacturing (DDM)

These systems operate by melting the starting material and shooting small droplets onto a previously formed layer.

The liquid droplets cold weld to the surface to form a new layer.

The deposition of droplets for each new layer is controlled by a moving x-y spray nozzle work head whose path is based on a cross section of a CAD geometric model that has been sliced into layers

After each layer has been applied, the platform supporting the part is lowered a certain distance corresponding to the layer thickness, in reparation for the next layer



Droplet Deposition Manufacturing (DDM)



SOLID-BASED RAPID

PROTOTYPING SYSTEMS



Laminated-Object Manufacturing

Laminated-object manufacturing produces a solid physical model by stacking layers of sheet stock that are each cut to an outline corresponding to the cross-sectional shape of a CAD model that has been sliced into layers.

The layers are bonded one on top of the previous one before cutting.

After cutting, the excess material in the layer remains in place to support the part during building.

Starting material in LOM can be virtually any material in sheet stock form, such as paper, plastic, cellulose, metals, or fiber-reinforced materials.



Laminated-object manufacturing



Fused-Deposition Modeling

FDM is an RP process in which a filament of wax or polymer is extruded onto the existing part surface from a work head to complete each new layer.

The work head is controlled in the x-y plane during each layer and then moves up by a distance equal to one layer in the z-direction.

The extrudate is solidified and cold welded to the cooler part surface in about 0.1 second. The part is fabricated from the base up, using a layer-by-layer procedure similar to other RP systems.



Fused-Deposition Modeling



POWDER-BASED RAPID

PROTOTYPING SYSTEMS



Selective Laser Sintering (SLS) SLS uses a moving laser beam to sinter heat-fusible

powders in areas corresponding to the CAD geometric model one layer at a time to build solid part

After each layer is completed, a new layer of loose powders is spread across the surface using a counter-rotating roller

The powders are preheated to just below their melting point to facilitate bonding and reduce distortion.

Layer by layer, the powders are gradually bonded into a solid mass that forms the three-dimensional part geometry.



Selective Laser Sintering (SLS)



Three-Dimensional Printing

Three-dimensional printing (3DP) builds the part in the usual layer-by-layer fashion using an ink-jet printer to eject an adhesive bonding material onto successive layers of powders.

The binder is deposited in areas corresponding to the cross sections of the solid part, as determined by slicing the CAD geometric model into layers.

The binder holds the powders together to form the solid part, while the un bonded powders remain loose to be removed later.

When the build process is completed, the part is heat treated to strengthen the bonding, followed by removal of the loose powders.



Three-dimensional printing

1) Powder layer is deposited2) Ink-jet printing of areas that will become

the part3) Piston is lowered for next layer



RP Applications

Applications of rapid prototyping can be classified into three categories 1.Design 2.Engineering analysis and planning 3.Tooling and manufacturing



RP Applications: Design

Designers are able to confirm their design by building a real physical model in minimum time using RP

Design benefits : Reduced lead times to produce prototype components Improved ability to visualize part geometry Early detection and reduction of design errors Increased capability to compute mass properties



RP Applications: Engineering Analysis and Planning

Existence of part allows certain engineering analysis and planning activities to be accomplished that would be more difficult without the physical entity

Comparison of different shapes and styles to determine aesthetic appeal

Wind tunnel testing of different streamline shapes Stress analysis of a physical model Fabrication of pre-production parts for process

planning and tool design



RP Applications: Tooling & Manufacturing

Small batches of plastic parts that could not be economically injection molded because of the high mold cost

Parts with complex internal geometries that could not be made using conventional technologies without assembly

One-of-a-kind parts such as bone replacements that must be made to correct size for each user

Used for rapid tool making(RTM) when RP is used to fabricate production tooling



Problems with Rapid Prototyping

1.Part accuracy

Staircase appearance for a sloping part surface due to layering Shrinkage and distortion of RP parts

2. Limited variety of materials in RP

Mechanical performance of the fabricated parts is limited by the materials that must be used in the RP process



Rapid Prototyping Products
