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Contents
Introduction
RP Techniques
The StL Format
An Example
Acknowledgements
Further Reading

R e a l 2 V i r t u a l 3 D A n d R a p i d P r o t o t y p i n g

Introduction

The term Rapid Prototyping (RP) refers to a group of techniques used to automatically fabricate scale models of a part or assembly from 3D computer aided design (CAD) data. Stereolithography is the earliest of many such techniques, and the data format it employs, StL, has become the standard for rapid prototyping systems in general.

Rapid Prototyping is of interest to users of Real2Virtual3D in two main ways.

Duplicating real objects for which no computer model exists.
Where as Real2Virtual3D creates 3D digital models of real objects, Rapid Prototyping is the reverse process, that of automatically creating real objects from digital data. Together Real2Virtual3D and Rapid Prototyping provide the ability to copy three-dimensional physical objects in a way similar to photo-copying or faxing documents.
Creating prototypes directly from two dimensional drawings.
Given a series of two-dimensional drawings describing an object from a number of viewing directions, a finished three-dimensional prototype can be generated directly, bypassing the need to design the exact three-dimensional data as is usual in CAD systems.

In this article we give an introduction to Rapid Prototyping, and briefly describe the various RP techniques. We describe the standard format for Rapid Prototyping, StL, and show how Real2Virtual3D can be used to create StL data from drawings. The results of RP using the Selective Laser Sintering (SLS) technique are given for a worked example. Finally, references for further reading are provided.

RP Techniques

The original RP technique was Stereolithography (SLA), developed by 3D Systems of Valencia, California, USA. The Stereolithography process builds up the product in a series of horizontal layers, made of cured photosensitive resin. The liquid resin is contained in a tank in which a platform initially sits with it's horizontal surface just below the resin level. A laser illuminates the resin in those areas that are required to be solid for that layer, this cures the resin making it hard. The platform then drops (usually 1mm or 0.004") and the next layer is added. This continues until the complete part is finished. Uncured resin is washed away and the product undergoes a final curing.

The data required to drive the process is obtained from an StL description of the part. This is pre-processed by slicing it into 1mm thick horizontal layers. Each slice is then a two-dimensional representation of the solid regions of the part on that layer.

Many later techniques use very similar concepts, the product being built out of layers of a material in fluid or powder form which is then cured by interaction with a laser. Consequently StL has become the standard format for a wide range of RP processes.

Selected Laser Sintering (SLS) extends RP for use with a wider range of materials than Stereolithography, with layers of polymide powder being melted ( or sintered) by a high power laser. The process proceeds with a layer of powder being deposited on the surface of a platform, this is sintered to create the solid part for that layer and the uncured powder is brushed or blown away. The platform is lowered and the next layer of powder deposited and sintered. Unlike Stereolithography, no post curing is required. The finished product is usually stronger than in Stereolithography due to the stronger polymers which can be used.

Whilst SLA and SLS are the most prevalent of RP techniques, several other methods are becoming more common. Please see the Further Reading section for more sources of information on these techniques.

Fused Deposition Modeling (FDM)
Solid Ground Modeling (SGM)
Ink-Jet techniques

The Stl Format

The stereolithography format is an ASCII or binary file with the extension .stl or .StL. Originally developed for the stereolithography process, it has now become the standard input for most Rapid Prototyping machines.

An StL file is a triangular representation of a three-dimensional closed volume. The surface of the volume is tessellated into a series of small triangles (facets). Each facet is described by a surface normal and three points representing the vertices (corners) of the triangle. These data are used by a slicing algorithm to determine the cross sections of the 3-dimensional shape to build.

The quantity and size of these triangles dictate how accurately the surface mesh represents the actual object. As the number of triangles used to represent a smooth surface increases, the error between the model and the actual surface is reduced. Whereas non-smooth objects may be represented exactly (such as a cube for example ), smooth objects can only be approximated. However since there is no limit to the detail of the representation, this drawback can be removed by reducing the representation error below that of the fabrication machine employed to create the product.

Real2Virtual supports the StL file format. Any model created with Real2Virtual3D can be saved as an StL file, and viewed using Real2Virtual Viewer. In the next section we will see a worked example in which StL created directly from drawings is used to make a prototype using Selective Laser Sintering.

An example

In this section we show the steps taken to create a finished prototype from a set of two-dimensional drawings. The drawings are based on the following three images of the plans for a model aircraft, seen in front, plan and side elevations.

The outline (shown in red) of the aircraft in each of the elevations is drawn using Modeller's drawing tools. Rather than copy the design exactly we used the images as a basis for a simpler model of the main body of the aircraft without the landing-gear or armaments.

Real2Virtual3D automatically converts the two-dimensional drawings into a three-dimensional model, which can then be exported as StL data. The resulting StL model can be viewed using Real2Virtual Viewer. The model is shown below left.

The StL file was then used as input in the Selective Laser Sintering process, to produce the object shown in the photograph below right.

Acknowledgements

Real2Virtual would like to thank Phoenix Analysis and Design Technologies Inc (PADT) of Tempe Arizona USA ( www.padtinc.com ), and in particular Bob Baker for producing the prototype in our example. We would also like to acknowledge Bob Baker for so generously sharing his expert knowledge of rapid prototyping.