Overview

Ex.1:  Executive Summary

The key concept of rapid prototyping (RP) was RAPID. RP provides generally an all-in-one-step fabrication of a physical, three dimensional part of arbitrary shape. This was normally obtained directly from a numerical description which was typically a CAD (computer aided design) model.  Rapid prototyping was a very fast growing production or manufacturing techniques.  Some of the benefits of rapid prototyping included:

ü  Reduced lead time to produce prototypes.

 

ü  It improved the ability to visualize the part owing to its physical availability.

 

ü  Rapid prototyping also aids the capability to determine mass properties of components and assemblies.

 

ü  It enabled early detection and if necessary correction of design flaws or errors.

 

ü  It also enables the manufacture of models, parts or components that would not have been possible any other way, e.g. models with varying cavities.[ex.1]

 

Rapid prototyping had proven to be a versatile tool used in the engineering industry and others.  The growth of rapid prototyping had been at an average rate of 58% a year.  Rapid prototyping enabled the manufacture of prototypes and models of engineering products, at later phases of many engineering design process. Because rapid prototyping technology uses CAD (computer aided design) or CAM (computer aided manufacture) packages, these packages in turn increased flexibility, reduced cost, time, reduced difficulty (on complex parts) etc in the design and manufacturing process of models.  Rapid prototyping mainly used additive technologies; it had advantages in many applications when compared to subtractive fabrication methods like milling. Many universities and colleges have purchased and implemented rapid prototype into their curricula which reflects the path of growth RP was enjoying.  RP played an increased role in many engineering sectors like in the automobile, aerospace etc, where it was used in prototyping, manufacturing of parts and assemblies for commercial and military applications.  RP also played an increased part in art where complex geometry was often required.

Some of the applications of rapid prototyping included

Þ    Engineering concept definition

Þ    Form, fit, and function testing

Þ    Styling and ergonomics studies

Þ    Presentations to Clients and consumer evaluation

Þ    Communication of product characteristics; make it easier to explain

Þ    Facilitates meeting schedule and achieving milestones

Þ    Masters for silicone rubber tooling

Þ    Masters for spray metal tooling to be used for injection moulding

Þ    Pattern for investment casting.

Þ    Tooling for injection moulding [ex.2]

The RP technology was applied in manufacturing ever increasingly varied types of models. In its inception in 1988, RP was mostly applied in producing mainly non functional prototypes. Since then, with technological advancements and innovations, RP came to be used in the manufacture of functional models in varied engineering sectors.  As shown in figure.01 below

Figure.01. Showing percentage share of RP use in varied sectors [ex.3]

There were also, as stated above, varied applications RP was employed.  About 36% of the models made with RP were functional.  25% were used for patterns for prototype tooling and metal casting.  Areas of application of RP were expected to increase significant. The figure.02 below shows how RP was applied.[ex.4]

graph.jpg

Figure.02 Showing how RP Models are being used[ex.5]

press32.jpg

Figure.03. Showing number of models sold per year from 1998 to 2003[ex.5]

 

With all the advantages taken into account, RP stood as a promising tool the engineering industry could employ in other to save time and cost.

The future of RP was as promising due to the varied possibilities in application.

Ex.2 References

[ex.1] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp xv

[ex.2] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp 3

[ex.3] Wohler’s Report 2007

[ex.4] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp 5

[ex.5] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp 6 to 8

 

 


1.1 Introduction

The competition in world market for manufactured models, components and products had intensified greatly.  It had become quite crucial for new products to get to the market at the earliest possible time to stand a chance against the competitors.  To bring products to the market swiftly, many of the processes involved in the design, testing, manufacturing and marketing of the products have been squeezed, both in terms of time and material resources.  To achieve this, there was a need to be able to manufacture prototypes faster, given the rise of CAD applications which were already playing an ever increasing role in the design and manufacture of ever more complex shapes.  Many of the shapes were difficult or impossible to manufacture using the additive and subtracting process involved with machining or milling.  Engineers engaged in design and manufacturing of products that were to be competitive were playing an ever changing role due to the emergence of a tool that held great potential. This tool was powerful, sophisticated and very flexible. This tool was Rapid Prototyping (RP).[1.1]

 

Rapid prototyping was defined as the fabrication of a three-dimensional, physical model or component of arbitrary shape from a CAD or CAM package. This was done using a quick and an entirely automated and highly flexible process.  The Wohler’s year 2000 report definition of RP was that it was a special class of machine technology that quickly produces models and prototype parts from 3D data with the aid of additive method in forming the physical model. Rapid prototyping originated from the need to manufacture prototypes as quickly as possible which in turn reduces the time required in the early design phases of many engineering design process. Originally, rapid prototyping objective was the manufacture of prototypes.  However before going further, Rapid prototyping as a term had been used to refer to the unique technological process (RP) used to manufacture prototypes.  However term the prototype had not been fully defined and explained.[1.2]

 

The term prototype refers to the original example of a model or geometry, component, etc that had been or was to be copied or developed. Prototypes as mentioned above had been proven to serve a lot useful purposes like form, fit, and function testing. Prototypes originally was meant to mainly represent an often smaller, less functional and a model to aid the analysis of interest characteristics of the product or model the prototype was meant to represent.[1.3]   There were different types of prototypes. In general, there were three aspects of interest. These were listed below:

 

·         The form of the prototype: these were from the virtual prototype to that of physical one.

·         The prototype implementation:  from the product to its sub-assemblies and components.

·         The degree to which the prototype resembles or similar to the product, model or component.  It varied from a rough similarity to an exact replication of the model.[1.4]

Rapid prototyping was given its name due to the fact that it could prototype parts very rapidly compared to other methods. RP was also referred to as desktop manufacturing, instant manufacturing, and direct CAD manufacturing.  RP can produce prototypes in hours rather than days or weeks in many cases. This was the main selling point of RP. Its flexibility has also played a great role in its rise. However there was a unique trait of RP, which was the manufacturing of models on layer by layer or one layer at a time. RP had other names it was known by that emphasizes the ‘layer by layer’ characteristics. These terms include layered manufacturing, material addition manufacturing, material deposit manufacturing, and material addition manufacturing.[1.5]

Valuable resources require its efficient use. To accomplish this, new tools and approaches were developed and evolved. In the later years before the commercial viability of rapid prototyping, these developments were mainly computer oriented and often involved the computer. Rapid prototyping, tooling and manufacturing (RPTM) was used to refer to the process of rapid prototyping owing to its growing applications in various engineering and other area like the art industry.

 

Rapid prototyping commercially took off in the late 1980s as a viable tool for manufacturing models or prototypes and its growth had never halted.  The advancement and innovation in the science and engineering industry, which occurred after the inception of RP, enabled RP to become an even more versatile and viable tool for manufacturing. The technology spread quickly into rapid tooling and rapid manufacturing.[1.6] However, there were still some hurdles to overcome, like moving RP to rapid manufacturing owing to various reasons. This included the material selection available for RP.   RP had also found an increasing use in the art industry where often complex shapes, which cannot be manufactured using any other tool, were imagined and manufactured using RP.  RP involves the use of several methods in the manufacture of the models, prototypes, components etc. These methods included

 

·         Selective Laser sintering

·         Stereolithography Process

·         Laminated Object Manufacturing

·         Fused Deposition Modelling

·         Solid State 3-dimensional Printing

These above listed methods were discussed in further detail later in this report.

1.2 Stereolithography Process

3D Systems introduced, in the late 1980s, the first commercially viable rapid prototyping process. The first implementation of the commercial stereolithography technique was developed by Charles Hull of 3D Systems. This was achieved by the introduction of 3D Systems SLA 250.  This method created three-dimensional models from liquid photosensitive polymer resins which solidified when exposed to ultraviolet light.[1.7] The model was produce or built upon a platform located below the surface of a vat of photopolymer material. Until the photopolymer was exposed to ultraviolet light, it was relatively a low-viscosity liquid. As a result of the UV light, the photopolymer molecules polymerize, thereby linking together to form a network. The liquid was turned to solid as a result of this process.  Mirrors controlled by a computer were used to point or direct the UV laser beam into the vat.  The 3D model CAD model was sliced into layers with the aid of a software program.  Each layer solidified as a result of the exposure to UV light which was used to trace the layer onto the surface of the vat.  [1.8]

The layer which was solidified was then lowered into the vat in order to allow another layer of liquid to be exposed to the ultraviolet light.  The process explained above was then repeated until the entire solid model was developed from the CAD model.

Figure.1.3 Stereolithography apparatus arrangement.

1.3 Selective Laser Sintering

Selective laser sintering (SLS) process was introduce by DTM in the late 1980s . The technology was patented by Deckard in 1986.  SLS used a laser to sinter a powdered material, which resulted in the formation of the prototype shape.

SLS was rapid prototyping technology in which powders were fused layer by layer using a laser. SLS technology accuracy in producing parts, prototypes or models was quite high.  SLS were used in modelling in engineering, polymer-coated sand for casting applications, polymers and metals.[1.8]

SLS was quite like stereolithography apparatus (SLA). Their main difference was that SLA photocured a polymeric liquid while SLS used the laser to sinter and fuse powder and material selection was wider for SLS.

 

The process involved

 

1.      The spreading of a thin layer of thermoplastic powder by a roller over the surface of a build cylinder and was heated to just under its melting point by by infrared heating panels at the side of the cylinder. ! 

2.      The laser then sintered and fused the desired pattern of the first slice of the model or object in the powder.

3.      The first fused slice then descends one object layer.

4.      While the roller spreads another layer of powder

5.      The process was then repeated until the component or model was completed

During the process of building the model, the unsintered powder also acted as a support for the model.

SLS systems were more complex mechanically than stereolithography and the other technologies.  The surface finish resulting from SLS products was not as good as those of some other technologies; however the material properties were close to intrinsic materials.  The main advantage of selective laser sintering process was its ability to make functional parts or components in its final material. The sintered products or prototypes were porous, therefore, it may have been necessary to infiltrate the product or material with a different material in other to enhance mechanical properties of the sintered product.[1.9] This was especially metal products.

 

 
 Powder Bed,Scanner System,Laser,Fabrication Piston,Powder DeliverySystem
 

 

 

 

 

 

 

 

 


Figure.04 Showing Selective Laser Sintering Diagram

 

 

 

 

1.4 Laminated Object Manufacturing

This method was introduced by Helisys. Refer to figure.1.5.  Laminated object manufacturing (LOM) created solid models from laminated sheets of plastic, paper and metal by a laser. This method involved cutting the outlines of each layer into a sheet of paper rather than hatching the whole part cross section.  The new or next layer was glued onto the sack, the roll of paper advanced after each outline had been cut or created.  The process was repeated until the part was created.   The part was hand finished, trimmed and cured where needed.  LOM was preferred over SLS and SLA processes by companies like that of automobile and aerospace industries that required larger components or parts.

Figure.1.5 the LOM system [1.10]

 

1.5 Solidscape’s 3-Dimensional Printing

This involved basically what the title said; it was the printing of solid part or component from the CAD model to the RP system within a relatively closed system of computer and RP system. [1.11]

 Solidscape was formerly known as the Sanders Prototype Inc...  It was establish in 1994. The company was funded by the Sanders Design Inc. (SDI) which was a research company that was principally owned and managed by the Solidscape’s founder, Royden C. Sanders, in order to research and market its ModelMakerTM rapid prototyping system.

The Solidscape’s machines were based on ink jet technology which was similar to the technology used by Sanders Design Inc., and were installed in over 14 countries around the world.[1.11]

Solidcape’s 3D printing was a rapid prototyping process that was developed at the Massachusetts Institute of Technology [MIT]. It involved the bonding of layers of powder by inkjet to create a part. 

Generally 3D printing was regarded as the low cost segment of the rapid prototyping machine market. System outputs were typically considered suitable for concept and appearance modelling, however lacked the accuracy and precision or other attributes of more costly systems.

 

1.6 Fused Deposition Modelling

The fused deposition modelling (FDM) was a RP technique that was introduced by Stratasys in 1991. Extrusion of a thin stream of melted acrylonitrile-butadiene-styrene (ABS) thermoplastic polymer or wax via an extruder was the process involved in FDM. The extruder head position was controlled by a computer. By moving the extruder head through the volume of the part, the parts were built.  This process built up 3D parts from from the gradual buildup of 2D layers.  Refer to figure.1.6 that showed the extrusion head of the FDM machine.[1.12]

RP_Processinprinter.png

 

Figure.1.6 Fused Deposition Modelling Extrusion Head

 

1.7 References

[1.1] Chua C. K, Leong K. F. And Lim C. S. Rapid Prototyping: principles of Applications, 204-2005, pp. 1.

[1.2] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 2-3

[1.3] Chua C. K, Leong K. F. And Lim C. S. Rapid Prototyping: principles of Applications, 204-2005, pp. 2-3

[1.4] Chua C. K, Leong K. F. And Lim C. S. Rapid Prototyping: principles of Applications, 204-2005, pp. 3-4

[1.5] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 3

[1.6] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 14-16,

Edited by Paulo Jorge Bartolo et al, Virtual And Rapid Manufacturing: Advanced research in Virtual And Rapid Prototyping, Taylor & Francis, 2008, pp 7-8

[1.7] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 17

[1.8] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 14-16

[1.9] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 17

[1.10] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 20

[1.11]  Chua C. K, Leong K. F. And Lim C. S. Rapid Prototyping: principles of Applications, 204-2005, pp. 145

 [1.12] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.19 -20

 

 

 

 

 

 

 


2.0 Rapid Prototyping Process

2.1 Introduction

The main purpose of rapid prototyping was to quickly fabricate complex shaped three dimensional components or model using data from CAD. Rapid prototyping being an additive fabrication process represented a method where a solid CAD model was electronically sectioned into layers of predetermined thickness. The layer thickness was often selected according to how accurate the model to be manufactured was desired to be and these sections also define the shape of the component when put together accordingly.

The shapes of the models or components were affected by CAD files, the STL (stereo lithography) files.  These were system elements.   Also the resolution of problems and repairs of STL files and other file formats.[2.1]

2.2 The Automated Processes Principles

Rapid prototyping was part of an automated fabrication which was a technology that enabled the manufacture of three dimensional parts from digital designs.  There are many advantages of automated fabrication over moulding process and manual fabrication.  Some of these positives of the automated fabrication were from the computer-aided design, precise dimensioning and quick design changes quick makes it more flexible.  There were three major classifications for fabrication processes, automated or manual.  These include ed

1.      Additive

2.      Subtractive

3.      And formative processes.[2.1]

Below were some illustrations of the three classifications

fast.png

Figure.2.1 Three fundamental fabrication processes

Additive process involved a process where the end product was larger than at the start.  This was mainly in terms of volume and mass owing to the addition of material.

Subtractive Process was the opposite of the additive process.  It involved the start with a single block of solid material larger than the final size of the desired object. Material was extracted from the block until the desired shape was attained.

Formative processes involved the use of mechanical forces or restricting forms were employed on a material in order to form it into the desired shape.

Examples of for additive processes used in manufacturing included  most types of rapid prototyping processes like selective laser sintering and stereolithography while subtractive processes included forms of machining processes like CNC or otherwise. They included turning, drilling, milling planning, sawing, grinding, laser cutting, EDM, water jet cutting etc.  Formative processes on the other hand involved processes like plastic injection moulding, Bending, electromagnetic forming and forging. The examples given above were not exhaustive, however, indicative of the range of processes.

There were cases of hybrid machines containing two or more fabrication processes. An example of such machines included the hybrid subtractive, as in blanking and formative processes as in bending and forming which was common in progressive pressworking. [2.2]

2.3 The Process Cycle

Prototypes or models made using rapid prototyping processes that were current and evolving had several features that were common.  Solid or surface CAD model was electronically sectioned into layers of predetermined thickness, as mentioned earlier.  The information of each section was then electronically transmitted to the RP machine layer by layer. The RP machine processes materials only at ‘solid’ areas of the section while the subsequent layers were sequentially processed until the component or model was completed.  The sequential lithographic approach to components manufacturing was what defined RP.

The steps listed below were generally employed in the manufacture of prototypes, models or parts

Þ    Creation of a CAD model of the design

Þ    Conversion of CAD model to STL file formats

Þ    Slicing of STL file into 2D cross-sectional layers.

Þ    Grow the prototype or model

Þ    Post-processing of manufactured models.[2.3]

These step were illustrated in figure.2.2 below

processdiag.png

Figure.2.2. processes involved in rapid prototyping.

The steps were explained in more detail below.

2.3.1 Creation of a CAD model of the design

This was the first step in the manufacture of prototypes or models using rapid prototyping.  The advanced 3D CAD modelling was generally prerequisite in RP processes.  This was usually the most time consuming part of the RP manufacturing process.  RP did require that fully closed, water tight model such that even if water was poured into the volume of the model, the water would not escape.

A solid was defined as a volume completely bounded by surfaces that meant that edges of all surfaces must be coincident with one and only one other surface edge. Solid modelling stored volume information.  CAD solid model did not only capture the complete geometry of an object, it was also able of differentiate between the inside and the outside of the space of that part. Also included in the information that was able to be obtained from the model, were other volume related data.  CAD software packages like AutoCAD, CATIA (Which was used in an exercise in this report), Pro/Engineer, Solid Works etc.

 

 

2.3.2 Conversion of CAD model to STL file formats

The solid model was normally converted to special file format, which was known as stereolithographic (STL) after the solid model was a created and saved.  The file format (STL) originated from 3D Systems which pioneered the STereoLithography process.  To be more precise, the Albert Consulting Group under contract to 3D Systems developed the STL file format which was meant to aid the new revolutionary manufacturing technology, which was known as stereolithography. 

The STL was sufficient to meet the requirements of RP technology that often built monomaterial (single or same material) components of parts.  STL file formats success was impressive and was due to its simplicity and its monopoly.  Its mathematical sufficiency was due to the fact that it described a solid with the use of a boundary representation (B-rep) technique.  STL formats represented the virtual CAD model of the component to be prototyped.  The STL file format approximated the surfaces of model using tiny triangles.  When taken together, these triangular facets described a polyhedral approximation of the objects surface that was a polyhedral approximation of boundary between nonmaterial and material.  For highly curved surface, more triangles must be employed which meant that the STL file for curved surfaces could get quite large. It was not uncommon to find STL files of curved surfaces that were in the megabytes.  The conversion step was generally the shortest and easiest of the steps involved in RP.  Highly complex part or model coupled with extremely low speed or performance workstation. The conversion may take several hours.  Supports were also converted to a separate STL file, where necessary. 

Third party software that allowed for the verification and modifications of model and supports may also be used to create or modify supports as an alternative.  The STL file was basically nothing more than a list of x, y, z coordinate triplets that described a connected set of triangular facets. STL files also included the direction of the normal vector for each triangle that pointed to the outer surface of the model. CAD/CAM software vendors supplied the STL file interface as a norm.

The process of approximating a surface by triangular facets was known as tessellation.  The surface tessellation was performed by the CAD STL file interface  and then outputted the facet information to binary or ASCII STL file format.

STL file output was able to be expressed in binary or ASCII format.  There were three main steps involved in the creation of the STL file

        i.            The Selection of the part to be converted to STL representation.

      ii.            The setting of the various tolerance parameters for the process.

    iii.            Creation of the triangular representation of the geometry into the output file.

The creation of a surface STL files from surface model was very difficult, even though STL files were relatively easier to create from solid models. In order to process surface models, the steps below were to be followed

Þ    The determination of all surface adjacencies

Þ    The triangulation of each surface.

Þ    The ensuring that all edge vertices matched.

Þ    The determination of a normal that pointed the outside of the model for each surface.

Þ    The outputting of triangles and normals to an output file. [2.4]

When creating STL file, below were some interface option that must be addressed;

Table.2.1 Binary against ASCII

Binary

ASCII

Default output type

Referred to as human-readable format

Referred to as machine-readable format

Easily read and understood by humans

More compact and efficient, easier to move

Not very efficient, slower to process,

Through network/transmit

Larger file sizes.

Not easily read or understood by humans

Not recommended if moving files

Without some translation

Through a network.

 

1.      Triangulation tolerance

2.      Adjacency tolerance

3.      Auto-normal generation (on/off)

4.      Normal Display (on/off)

5.      Triangle Display (on/off)

6.      Header Display (text)

Some features of triangulation tolerance were as follows

1.      Determined how smooth the approximation of the surface or solid would be. Like how close the triangles approximate the surface. How close the sides of the triangles that lay along the edges were to the actual edges of the surface.  Usually set to one half desired accuracy of the rapid prototyping process being utilized. Default was set at 0.0025 in., or 0.05 mm.

Some of the features of adjacency tolerance were that it does not affect processing of solids and the default value was, 0.12mm or 0.005 inch.  The system used value to ascertain if two surfaces would be attached to one another. Also the edges whose length was smaller than the adjacency tolerance can cause adjacency problems.

 

 

 

 

Figure.2.3

 

 

There were dramatic effects of decreasing the values of triangle tolerances were illustrated below in figure.2.4

 

Figure.2.4 Effects of reducing the values of triangle tolerances. [2.5]

The effects of adjacency tolerance were shown in figure 2.5.  Some of the features of auto-normal generation were as stated below

Þ    It did not affect processing of solids

Þ    Choose a base surface, check the normal and calculate all others from this surface.

Þ    Default had to be on.

Figure.2.5 Adjacency tolerance.[2.6]

After conversion, the data was transmitted. The transmission was also relatively straightforward.  The purpose of this step was the transfer of STL files from the workstation to the RP system’s computer. Due to the fact that a workstation was a design tool whiles the RP system’s computer was a process or production machine, often the RP systems and the workstation were situated at different locations.  Like a design office for the workstation and usually a shopfloor for the production machine.  The data transmission medium was varied from email, diskette to Local Area Network (LAN). This was often done using an agreed data format.

2.3.3 Slicing of STL File into 2D Cross-Sectional Layers.

The STL file was taken from its 3D model surfaces and converted to many triangles. This step was referred to as slicing. More complex or highly curved surfaces or objects needed more triangles, thus the bigger the file that made up the CAD model as well as support structure for the part to be grown on.  After slicing the model, the file was saved as STL file as had been stated earlier.

2.3.4 Grow the prototype or model

After the file was submitted to the RP computer and the machine runs until the part was complete. The RP machine built one layer at a time from the different materials that range from polymers, papers or powdered metals. Often the machines were autonomous needing little human intervention.  Build time varied from machine to machine depending on size and number of parts to be manufactured.  The growing process often involved non metallic materials.  This reduced the scope of use of RP technology, especially on its extensive use in the manufacture of functional parts.  Rapid tooling was an example of where material selection flexibility was important.

 Manufacturing engineers and scientists had dreamt of using RP process to evolve into rapid manufacturing (RM).  Rapid manufacturing was the use of RP machines in regular production as well. The need for RP to evolve to RM was not only due to material selection issues; speed of growing the parts of solid model also played a major role. 

2.3.5  Post-processing of manufactured models

Post-processing was the final step involved in RP. It consisted of part removal and cleaning and of finishing and posturing.  This were often done manually, therefore prone to danger of damaging a part or component.  The operator therefore for this last process step had a high responsibility for the successful process accomplishment. 

Post-processing tasks were different for different prototyping systems. Below, table 2.2 showed the necessary postprocessing tasks for some selected prototyping machines.

Table 2.2 Postprocessing tasks for varied RP systems. [2.7]

Postprocessing Tasks

Solid-scape

Fused Deposition Modelling

[FDM]

Selective Laser Sintering [SLS]

Laminated Object Manufacturing

[LOM]

Stereo-lithography Apparatus [SLA]

 

1.      Cleaning

ü   

ü   

ü   

ü   

ü   

2.      Postcuring

ü   

 

ü   

 

ü   

3.      Finishing

ü   

ü   

ü   

ü   

ü   

 

Removing the prototype from the prototyping machine and removing excess materials, which included support material which may have remained on the material, was referred to as part removal and cleaning.  Table 2.2 indicated that there was need to clean with SLA machines and FDM machines.  Excess resins that reside in entrapped areas needed to be removed for SLA components.  For selective laser sintering (SLS) parts, excess powder and excess wood-like blocks of paper for LOM part needed to be removed.

Postcuring was a task that was generally only needed for ~SLA and SLS parts.  In the SLA process, laser scanned each layer along the boundary and hatching line only, which resulted in side portions of the layers not being completely solidified.  In order to complete the solidification process and improve the mechanical properties of prototype, postcuring of the prototype was needed.

The postcuring process was carried out in specially designed apparatus which made use of ultraviolet radiation.  The uniformity of postcuring with minimum rise in temperature and maximum component accuracy was determined by the optimizing of the output wavelength of the postcuring apparatus. The size of the component or part often dertermined how long the postcuring process takes. However the time taken to posture the part was normally lesser than the time taken to build the part or component. 

After the part was postcured, the part was then finished.  Finishing involved basic cleaning like sanding or machining, to remove unwanted materials for some simple part applications.  Materials made from wax were to be handled carefully because of their brittle nature.

2.4 Some Problems With STL Files Format

STL file format as the de facto standard file format in the RP industry and also met the needs of the industries that are using RP, also had some short comings or inadequacies.  Some of these inadequacies were due to the very nature of the STL file format because it did not contain topological information or data.  Another short coming was owing to the fact that many CAD vendors used tessellation algorithms that were not robust.  As a result, they tended to create polygonal approximation models that show the following varied issues:

 

1.      Gaps (Holes, Cracks and punctures) Indicated Missing Faces These problems arose because when the solid model forms were replaced by simplified mathematical form, (triangles) on being converted to STL file format; it resulted in undesirable geometric anomalies which may include gaps or holes in boundary surface when the process was not carried out properly. Surfaces of with large curvature were more prone to issues.  This was show in figure.2.7.

2.       

Figure.2.7 Polygonal Approximation resulting from gaps.[2.8]

 

3.      Incorrect Normals:  Surface normals stored in STL file were not the same as those computed from the vertices of the corresponding surfaces.

4.      Inconsistent Normals:  Generally, surface normals point outwards. However, the normals of the some surfaces could flip over, thus becoming inconsistent with the outward orientation of the original surface. This was shown in figure.2.8

Figure.2.8.  Polygonal approximations resulting in inconsistent and incorrect normals.

 

5.      Internal Wall Structure:  In STL file format, geometric algorithms were used for closing gaps.  But faulty geometric algorithms could generate internal walls and structures that could cause discontinuities in the solidification of the material. Refer figure.2.9.

 

Figure.2.9  Polygonal approximations resulting in incorrect intersections.[2.9]

 

6.      Incorrect Intersections:  Sometimes, facets may intersect at locations other than their edges, which resulted in overlapping facets. Refer to figure.2.10.

Figure.2.10 Polygonal approximation resulting in internal wall structures.

7.      Facet Degeneracy: When facets which may not represent a finite area and consequently had no normals, the facets may become degenerated as a result. There were mainly two kinds of facet degeneracies which include geometric degeneracy and topological degeneracy.  Geometrical degeneracy represented when a all vertices of the facet were distinct and all the edges of the facet were collinear. Topological degeneracy on the other hand took place when two or more vertices of facet coincide.  The faults could be discarded since it did not affect the geometry or connectivity of the remaining facets.

8.      Inconsistencies: Two STL files were sometimes combined to create a prototype or part.  If the STL files were created using different tolerance values, it lead to inconsistencies like holes.[2.10]

2.5 Other Translators

Due to the inadequacies of the STL file format, there had been the search for alternative translators. E.g.  IGES, HPGL and tomography (CT) data.

a.       IGES file:  Initial graphics exchange specification was a common format used to exchange graphics information between various CAD systems.  The IGES file format was developed and promoted by the American National Standards Institute (ANSI) in 1981.  IGES file precisely represented the geometry and topological information for the CAD model. It included information about surface modelling, cinstructive solid geometry (CSG) and boundary representation (B-rep).  Boolean operations for solid modelling like union, intersection, and difference were defined in IGES file.

IGES file precisely represented a CAD model by providing entities of points, lines, arcs, NURBS surface, splines, and solid elements. The main advantage of IGES format was its widespread use and comprehensive coverage. But there were various disadvantages of the IGES format as it related to its use as an RP format.  These include, the inclusion of redundant information for rapid prototyping systems, the algorithms for slicing IGES file were more complex than those used for slicing STL file and the support structures that were needed for some RP systems cannot be created using IGES format.

IGES was a very good interface standard for exchanging information between various CAD systems.  It did fall short of meeting the standards for RP systems

 

 

Figure.2.11 Polygonal approximations resulting in geometric degenracy

 

b.      CT data:  This translator was a new format mainly used for medical imaging. CT scan data format had not been standardized. Formats were proprietary and varied from machine to machine.  CT scan generated data as a grid of three dimensional points, where the points had a varying shade of gray indicating density of the body tissue present at that point. Regularly, data from CT scan were used to prototype skull, knee, femur, ankle and other biomedical components on FDM, SLS, SLA and other RP systems. The content of the CT data included raster images of physical objects being imaged. This was used to create or produce models of human temporal bones.

CT scan images made models, were made using CAD systems, direct interfacing and STL interfacing.  One of the uses of CT data included the making of human parts such leg prostheses, which were used by doctors for implants.  The main problem with CT image data included the complexity of its data and the need for a special interpreter to process this data.

 

c.       Hewlett-Packard Graphics Language (HPGL) File: For many years, HPGL had been the standard data format for graphic plotters. Data types were two-dimensional representation of lines, circles, text, splines and others. Commercial card systems had the interface to output HPGL format that was a two-dimensional geometry data format and did not require slicing.  There were two main problems with the HPGL format. The first issue was since HPGL was a 2D data format; the files were not appended, thereby leaving hundreds of small files needing logical names and transformation while the second problem involved all the required support structures which must be generated in the CAD system and sliced in the CAD system. .[2.11]

2.6 References

[2.1] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 34

[2.2] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.34-35

[2.3] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.36-37

[2.4] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp. 37-38,

 Chua C. K, Leong K. F. And Lim C. S. Rapid Prototyping: principles of Applications, 204-2005, pp. 3-4

[2.5] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.40

[2.6] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.41

[2.7] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.43-45,

Edited by Paulo Jorge Bartolo et al, Virtual And Rapid Manufacturing: Advanced research in Virtual And Rapid Prototyping, Taylor & Francis, 2008, pp 7 - 8

[2.8] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.45-46

[2.9] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.47

[2.10] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.48

[2.11] Rafiq Noorani, Ph.D. Rapid Prototyping: principles of Applications, 2005, pp.48-49

 

 

 

 

 

 

 

 

 

3.0 Case Study: Fabricating A Small Friction Turbine

3.1 Introduction

The small friction turbine was relatively a complicated device and included a lot of features that made it very hard or impossible to manufacture any other way.  Rapid prototyping was the tool which enabled the prototype of the friction turbine to be manufactured in more accurate manner and at a shorter time. In this process, not all component parts needed to be manufactured using RP due to the fact that they could be manufactured with the use of other manufacturing methods like machining, casting etc.

Allowances had to be made for expansions or shrinkages.  However, for parts which were to be manufactured using RP, these allowances were not necessary since RP manufactured its parts whole and relatively precisely and accurately without the need to worry about expansion and shrinkage. This was due to the fact that the RP systems made allowances where necessary automatically.  Allowances depended on material to be used in the manufacture of the parts. 

For the parts or components that were manufactured by casting the allowances had to be defined during the use of CAD application in designing the model.  The allowances were carefully defined or sometimes the CAD application was employed to automatically define the allowances.  This was done depending on the shape, material, size and accuracy requirements.

The small friction prototype which was to be manufactured, involved the use of metal as material; specifically stainless steel.  Given that RP systems predominantly involved the use of some form of polymer based material in manufacturing parts or prototypes, the manufacture of the small friction turbine parts required a specific RP technology.  SLS (selective Laser Sintering) was employed in the manufacture of the small friction turbine because of its capability to render parts using metals like stainless steel. However the cost to manufacture the part had to be taken into consideration.

The cost of manufacturing a part using SLS was relatively high, which depended on the size, material, complexity etc of the part to be manufactured.  The component or part had to justify the cost involved. For the parts that were manufactured by casting, the cost was relatively lower compared to that of RP.

Decisions and compromise had to be made on what method to employ in the manufacture of the parts. These depended on the size, complexity, material, time etc.  For relatively simple parts like a square block or rectangular box, casting was considered ideal since it was cheaper and easier to manufacture by casting without compromising the accuracy and precision of the part.

RP costs increased with the level of complexity, accuracy and precision required. Cost was also affected by the kind of postprocessing finishes required.

The small friction turbine had complicated parts that needed to be manufactured by RP.  These parts included the exhaust manifolds and inlet manifold while the other parts were manufactured using casting.  It was also important to note that the casting or other methods could be used to manufacture parts that were elected to be manufactured using RP.  The decision to employ RP was sometimes a balancing act.

fanta.png

(a)

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(b)

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(c)

Figure.3.a.b.c Three views of the Friction turbine

3.2 Design Process

3.2.1 Creating The CAD Model Of The Small Friction Turbine

In order to create the CAD model, the dimensions of the small friction turbine (SFT) was had to be ascertained. In the case of this project, was given. As stated earlier, the CAD creation phase often was the most time consuming phase of the RP process.  The time taken to finalise the design of the SFT, averaged a two hour day for two months.  The design of the CAD model may have taken a shorter period of time, had the knowledge of the CAD application used been fully grasped at the start of the design process.  So multiplying the daily time taken by the number of days in the two months;

Time taken [hours] = 2 * 60 = 120 hours

The above time taken to design the parts was greatly reduced to a quarter when a higher knowledge of the CAD application (Dassaultes systemes CATIA in this case) was achieved.

Below were some of the properties of the components that made up the friction turbine;

 

 

 

 The figures below showed the dimensions used to design the Friction turbine.

Exhaust ManifoldInlet Manifold

Figure.3.1 Section-Side View of Parts of the small friction turbine

The figure.3.1 showed the dimensions for some components of the small friction turbine which included the exhaust manifold, Inlet manifold etc.  

 

 

Figure.3.2 Bottom View of Parts of the small friction turbine

 

 

 

Figure.3.3 Top Section View of Parts of the small friction turbine

 

 

 

 

Figure.3.4 Section View of Exhaust Manifold of the small friction turbine

The figure.3.4 showed the dimensions and wall thickness (1mm) of the exhaust wall.  The weight of the exhaust manifold increased with wall thickness which in turn meant higher cost in material.  However, there were exceptions to this in terms of cost; in the case of the wall thickness of 1mm, the manufacturing cost in using RP was considerably higher due to the higher accuracy or fine tuning involved in the manufacture of such thin wall. 

 

 

 

 

 

 

 

 

 

 

Figure.3.5 Top View of Parts of the small friction turbine

 

 

 

 

 

 

 

 

 

 

 

Figure.3.6 Axial Section View of Parts of the small friction turbine

The figure.3.6 above showed the exhaust holes.

 

 

 

 

Table.3.1 Calculation of Inlet to Exhaust Ratio

Inlet

      

Diameter or Height [mm]

4

      

Rect Length [mm]

10

      

Circle area [mm2]

12.56637

      

Rect Area [mm2]

40

      

Total Area Of Slit [mm2]

52.56

      

Circle area [mm2]

      

Radius

4.090525

      

Area [mm2]

52.56637

      

Exhaust Outlet

       

Branch 1

      

Outlet Square Area [mm2]

144

      

Outlet Circle Radius [mm]

6.770275

Outlet Cirle Area [mm2]

144

    
    

Total Area

(Joined Outlet Cirle Area) [mm2] >

288

Joined Outlet Radius [mm] >

9.57

Branch 2

Outlet Cirle Area [mm2]

144

    

Outlet Square Area [mm2]

144

      

Outlet Circle Radius [mm]

6.770275

      

 

 

Inlet manifold area [mm]

52.5786

exhaust manifold area [mm]

288

ratio Of Inlet to Exhaust

0.18256

 

 

 

 

Below was the quote for the manufacture of the small friction turbine exhaust manifold using SLS.

 

 

Also below was the quote for the manufacture of the small friction turbine exhaust manifold using SLA.

 

 

3.2.2 For the Casting Models

Because some of the parts to be manufactured were by casting and machining, allowances had to be made in order to ensure that the final product was as stated or shown in the above figures. To make allowance for expansion (or shrinkage), the parts dimensions to be manufactured were multiplied by 1.07 (or 7% increase).  Also account had to be taken of the surfaces or areas to be machined. This was done by adding a 3mm thickness to the wall to be machined. Below were dimensions of parts of the friction turbine elected to be manufacture by the use of casting and machining. 

Figure.3.7 Casting Model Section Side view with dimensions

Figure.3.8 Casting Model Front view with dimensions including allowances

  Figure.3.9 Casting Part Model Side Section and Front View with dimension

The manufacture of the parts by the use of Casting was relatively a lot cheaper than the cost of RP parts. This was one of the major advantages of Casting over RP.  Casting also had considerably wide material selection pool.

3.3:  Conclusion

RP had proven to be commercially viable in the manufacture of complex prototypes.  However the cost as shown for the manufacture of the exhaust manifold was relatively high. This was due to the fact that RP with metallic materials were not the norm, therefore needed relatively sophisticated technology like SLS.  This meant that RP sometimes may not be suitable for the manufacture of the exhaust manifold. The cost had to be justified by the value of the part to be manufactured.

On the other hand, casting had been around for a while and relatively considerably cheaper than RP. Though casting did not march the ability of RP in manufacturing complex parts, it was more accessible in terms of finding casting services.

RP had to evolve if it was to become ubiquitous and more versatile.  RP natural evolution tended toward rapid manufacturing in order to meet the challenges as stated above.

 

 

 

4.0  Discussion

RP faced a lot of challenges in becoming commercially viable in the manufacture of complex prototypes, which in turn improved the agility of companies in getting products to the market and had also proven to have a generally shorter lead time in the building of complex prototypes. However RP still had a lot of challenges to overcome. RP needed some technological breakthroughs in order to evolve from RP to rapid manufacturing.  For RP to successfully transform to rapid manufacturing, it had to increase its pool of material that could be used in the RP process.

Particular RP machines handled some certain types of materials. LOM handled only paper rolls. SLA handled only photopolymer resins while FDM handled only thermoplastics.  SLS and 3D printing was better than other RP processes in terms of material flexibility, in the sense that material could be prepared  and mixed in the form of powder. However the material flexibility was nowhere near that of that on conventional manufacturing processes such as casting CNC etc... 

RP processes inherently exhibited anisotropy, owing to the fact that they were made in slices and woven within the layer, their properties were distinctly different in horizontal and vertical directions. The inhomogeneity of RP was used to build Functionally Gradient materials.  This was done by focusing the inhomogeneity in a particular way.

RP suffered from poor life owing to these quality restrictions. A plastic part in regular production used cavity tools could be made in few seconds using injection molding . This could also be achieved for metallic parts using die casting.  The layer by layer method of RP took several hours in RP.  This meant that RP was more often unacceptably slow and costly for Rapid Manufacture [4.1]

RP had been around for two decades and the slicing method had been determined to be inadequate if RP was to evolve to RM.  RM demanded a multi-faceted approach in order to meet its varied needs. Various layer manufacturing processes had been developed, using an increasing range of material.  The quality of parts created had improved in term of size, material durability etc.

 

To overcome the challenges faced by RP in its evolution to RM and direct manufacturing, the steps below had to be met or achieved.

  • Faster processes
  • Use of multiply tools heads:  more heads meant improving speed as in 3D printing
  • Faster motions:  speaks for its self.
  • Material Variety
  • Functionally gradient material build up
  • New slicing mechanisms
  • Multi-axis motions
  • CAPP
  • Support mechanism
  • And conformal LM using Nano-technolgy.[4.2]

4.1 Conclusion

Notwithstanding the challenges faced by RP, its potential could not be denied.  The success of RP processes, even with its higher cost and slower build time, its ability to manufacture complex parts from CAD models gave it unique approach and edge in the design and manufacture of part.  Though the RP machine were relatively for building small parts, there was great potential that RP could evolve to cater for larger components or part. Like RM facilities with capability to manufacture vehicle size solid part. Like aircraft, wing, tail and even fuselage. Since RP was not hampered much by complexity, whole shapes could be built with varied materials. 

4.2 References

 

 

5.0 Appendices

6.0 Glossary

 

3D printing

This refers generally to the low cost segment of the rapid prototyping machine market. The output of these systems is typically considered adequate for concept and appearance modeling, but may lack the accuracy or other attributes of more costly systems. This terminology is used extensively in the Wohlers Report, but others may not draw as fine a distinction.

 

Absolute accuracy

Defined as the difference between an intended final dimension and the actual dimension as determined by a physical measurement of the part. In addition to those for linear dimensions, there are accuracy specifications for such features as hole sizes and flatness.

 

Adaptive slicing

The use of variable layer thickness in an additive fabrication process, generally thinner layers being used where part detail is greatest.

 

Additive fabrication / additive manufacturing

Fabrication of a part by adding materials to a substrate or previously formed portions of a part. The most common additive fabrication methods utilize a layered approach, but other geometries are possible. The term is also used generically as a synonym for rapid prototyping.

 

Advanced Digital Manufacturing [ADM]

3D Systems' trade name for direct manufacturing or direct fabrication. Often used in conjunction with the company's now dormant OptoForm technology.

 

Anisotropic

Refers to the fact that parts may have different physical properties depending on which direction measurements are made, and such differences can also arise if the exact same part is made in a different way. This can happen if the building orientation of the part in the machine is changed, and also from the sequence in which the part's elements are fabricated.

 

 

Computer numerical control [CNC]

Refers to a machine tool which is operated under automatic control, as opposed to manually by an operator.

 

Computer-aided design [CAD]

Also sometimes called computer-aided drafting, is a computer program which implements the functions of geometric design, drafting and documentation.

 

Computer-aided engineering [CAE]

A computer program which automates one or more engineering analysis functions to determine the mechanical, thermal, magnetic or other characteristics or state of a system. CAE programs may use a geometry definition from a CAD program as a starting point, and usually utilize some form of finite element analysis [FEA] as the means to perform the analysis.

 

Computer-aided manufacturing [CAM]

A computer program that generates tool paths or other manufacturing data to fabricate tooling, usually by subtractive means. CAM programs may use a geometry definition from a CAD program as a starting point.

 

Concept model / conceptual model

A part intended primarily for form or appearance study, but which typically cannot be used to either check fit to other parts, or provide functionality of the final part in an application.

 

Conformal cooling

Cooling lines in an injection molding tool that closely follow the geometry of the part to be produced.

 

Desktop manufacturing [DTM]

An early synonym for rapid prototyping, but no longer in current usage. DTM Corp., now incorporated into 3D Systems, was named after this terminology. Use of DTM as a company name became more common usage than the prior technical definition itself.

Direct [fabrication] processes

Generally refers to tooling which is made directly by a rapid prototyping system, as opposed to using the RP part as a pattern in a secondary process.

Direct AIM tooling

3D Systems' trade name for a process of producing injection-mold tooling directly by stereolithography. AIM stands for ACES Injection Molding, where ACES stands for Accurate Clear Epoxy Solid, another 3D trade name.

Direct Composite Manufacturing

3D Systems' trade name for OptoForm technology, a stereolithography process which utilizes paste-like photopolymers to fabricate useable parts.

Direct manufacturing

A synonym for rapid manufacturing. It refers to parts made directly for end-use by an additive rapid prototyping process.

Direct Metal Deposition [DMD]

A rapid laser powder forming process

commercialized by POM Group and based on research done at the University of Michigan.

Directed Metal Deposition System [DMDS]

Optomec's trade name for the LENS ® [Reg. trademark of Sandia National Labs. and Sandia Corp.] process.

Direct Shell Production Casting [DSPC]

Soligen is the exclusive supplier for this specialized version of MIT's three dimensional printing process [3DP]. It is used exclusively for investment casting applications.

Directed light fabrication [DLF]

A laser powder forming rapid prototyping process developed by Los Alamos National Laboratory.

Dots per inch [dpi]

A measure of the resolution of a printer. The number of discrete and distinct printed marks that an instrument is capable of producing in a linear inch. Also sometimes used in RP to describe the ability of an RP system to produce discrete voxels in the X-Y axial directions.

Electron beam melting [EBM]

The Electron Beam Melting [EBM] process from Arcam is a powder-based process having a lot in common with selective laser sintering, but replaces the laser with a scanned electron beam to produce fully-dense metal parts.

Electronic marketplace

A virtual market for buyers and sellers implemented through the Internet or World Wide Web. Also known as a web exchange.

Final machining

A secondary operation in which parts formed by a rapid prototyping method are brought to acceptable final finishes and tolerances typically by subtractive CNC technology.

Finish

A qualitative term for the appearance of a part. For example, technologies based on powders have a sandy or diffuse finish; some inkjet technologies produce a smooth finish due to use of extremely thin layers; sheet-based methods might be considered poorer in finish because stairstepping is more pronounced.

Freeform fabrication [FFF]

A synonym for rapid prototyping. The term is more precise and wider in scope, and somewhat favored by the academic community. One variant is freeform manufacturing [FFM], but a more common one is solid freeform fabrication [SFF].

 

Fused deposition modeling [FDM]

A thermoplastic extrusion-based rapid prototyping technology provided by Stratasys.

 

Fused deposition of ceramics [FDC]

Fused deposition modeling using a composite material of thermoplastic or other binder containing ceramic particles or fibers.

 

Gradient materials

The use of two or more materials in variable and controlled proportions as a function of the geometry of the fabricated part or object. A transition from one material to another may be abrupt or gradual.

 

Green part

A part that has been formed by a rapid prototyping process, but is in a loosely-bonded state. For example, metal or ceramic parts formed by some selective laser sintering systems are in a "green" state when removed from the machine. They are then sintered by a secondary operation to a "brown" state.

 

Indirect [fabrication] processes

Generally refers to tooling which is made by using an RP-generated part as a pattern for a secondary process as opposed to directly fabricating a tool using the RP process itself.

 

Initial Graphic Exchange Specification [IGES]

A standard neutral format for the exchange of 2D and 3D CAD data. STEP is a follow-on to IGES and stands for Standard for the Exchange of Product Model Data.

 

laminated object manufacturing [LOM]

Helisys, now defunct and succeeded by Cubic Technologies, was the first producer but also several other manufacturers provide this technology. Layers of paper or other materials are cut and bonded to form a part.

 

Laser Additive Manufacturing [LAM]

A laser powder forming rapid prototyping process developed by AeroMet Corporation. It was mainly aimed at producing large parts from reactive materials such as titanium for aerospace applications, but the company became inactive in Oct., 2005.

 

Laser Engineered Net-Shaping [TM] [LENS ®]

A rapid prototyping process which deposits metal powder into a pool of molten metal or other build material formed by a focused laser beam. There are several variants either commercially available or under development. LENS ® was developed by Sandia National Laboratories and commercialized by Optomec. It can also be used for repairing and modifying existing parts and tools. [LENS ® and Laser Engineered Net-Shaping [TM] are registered trademarks of Sandia National Labs. and Sandia Corp.]

 

Laser sintering [LS]

See selective laser sintering [SLS].

 

Liquid metal jet printing [LMJP]

Similar to inkjet printing where individual molten droplets are controlled and printed to specific locations.

 

Mass customization

A process whereby small lots of individualized parts or products are produced. The opposite of mass production whereby large numbers of identical parts or products are produced.

 

Mesoscopic Integrated Conformal Electronics [MICE]

This DARPA program is aimed at simplifying the manufacture of electronic devices and systems, and providing greater flexibility than is possible using existing technologies. In size, mesoscale devices fall between integrated circuits and surface-mount components.

 

Minimum feature size

Refers to the smallest detail of a part that can faithfully be reproduced. Mathematical definitions are usually based on a minimum curvature as a limit, but anecdotal values based on experience are more commonly utilized.

 

Model Maker

This is an inkjet RP method produced by Solidscape [formerly Sanders Prototypes], and the related company, Sanders International. It produces the highest accuracy and resolution of all RP methods, but is slow and has limited material choices.

 

MultiJet Modeling

This is an inkjet RP method produced by 3D Systems, Inc. It uses a wide area head and is most often used for generating quick concept models. The materials available are wax-like plastics and accuracy is lower than that available from stereolithography.

Paper Lamination Technology [PLT]

A variant on laminated object manufacturing RP technology from Kira Corp. of Japan.

 

Pattern

An object or part which possesses the mechanical geometry of a final object or part, but which may not possess the desired mechanical, thermal or other attributes of the final parts. Patterns are used in secondary processes to form tools to make parts for end-uses.

 

Photocuring

See photopolymerization below.

 

Photopolymer / photopolymerization

Material systems which change from a liquid to a solid state upon application of light [actinic] radiation. Light sources can be a laser or lamp, but related radiation-curable materials may be made solid by application of microwave or heat-based radiation sources. Photopolymers are typically complex mixtures of compounds rather than consisting of a single component.

 

Post processing

Secondary operations necessary to turn an additively fabricated-part into a useable one. Such operations may include cleaning, removal of supports or unused powder, post-curing and surface finishing operations such as sanding. Also see secondary operations.

 

 

ProMetal

An application of MIT's Three Dimensional Printing Process to the fabrication of injection molds. Steel powder layers are bonded by photopolymer selectively applied by a wide area inkjet head.

 

QuickCast

3D Systems' trade name for a stereolithography build style used to make investment casting patterns.

 

Rapid manufacturing

Refers to the process of fabricating parts directly for end-use from a rapid prototyping machine. A synonym is direct manufacturing.

 

Rapid prototyping

Computer-controlled additive fabrication. Commonly used synonyms for RP are: 3-Dimensional Printing, additive fabrication, freeform fabrication, solid freeform fabrication, stereolithography. Note that most of these synonyms are imprecise.

 

Rapid tooling

Most often refers to the process of fabricating tools from a rapid prototyping process. Rapid tooling may utilize direct or indirect methods: In direct methods, the part fabricated by the RP machine itself is used as the tool. In indirect methods, the part fabricated by the RP machine is used as a pattern in a secondary process. The resulting part from the secondary process is then used as the tool. In recent years, the term rapid tooling has been borrowed by practitioners of industry-standard methods such as subtractive CNC to refer to the ability to streamline these processes to compete with additive technologies.

 

Resolution

Refers to the minimum increment in dimensions that a system achieve. It's one of the main determining factors for finish, appearance and accuracy, but certainly not the only one.

 

Reverse engineering

The process of measuring an existing part to create a geometric CAD data definition of the part. In common non-technical usage, reverse engineering may also refer to measuring or analyzing a part or a product for the purpose of copying it.

 

Secondary operations

Manual or machine-based operations which must be carried out on a part fabricated by a rapid prototyping system before use. Secondary operations may include, post curing, support-removal, sanding, machining, etc.

 

Secondary process

Any one of a large number of processes such as rubber molding. Sprayform, EcoTool, etc., that utilize a rapid prototyping-fabricated part as pattern to create a final tool or part.

 

Selective laser melting

A process similar to selective laser sintering, but which fully melts metal or ceramic powders to directly form fully-dense parts.

 

Selective laser sintering [SLS]

A rapid prototyping technology in which powders are fused layerwise by a laser. The technology produces accurate parts and models in engineering polymers, metals and polymer-coated sand for casting applications. Speed is similar to stereolithography, but material selection is wider.

 

Solid freeform fabrication [SFF]

A synonym for rapid prototyping. The term is more precise and wider in scope, and somewhat favored by the academic community. A variant is freeform fabrication [FFF].

 

Solid ground curing [SGC]

This photopolymer-based technology was provided by Cubital. The company has been dissolved, but the process may still be available from a very few service bureaus. A xerographically-generated mask is used to cure an entire layer of photopolymer at one time. It offers good accuracy coupled with high throughput, but is considered quite expensive.

 

Stair stepping

A type of inaccuracy, as well as a visual appearance artifact It refers to the stepped appearance of the edges of a part, a consequence of additive fabricating a part in layers of necessarily finite thickness.

 

STEP

A follow-on to the IGES neutral file exchange format. The acronym stands for Standard for the Exchange of Product Model Data.

 

Stereolithography [SL] [SLA]

A rapid prototyping process that fabricates a part layerwise by hardening a photopolymer with a guided laser beam. Stereolithography is frequently used as a general term for "rapid prototyping," but this is neither precise nor correct.

 

STL

A file format used in RP to define the geometry of the part to be made. STL files are created by CAD programs by translating their native or neutral files into the STL format. The STL file defines the coordinates of numerous triangular facets that approximate the shape of an object or part.

 

Subtractive machining

The fabrication of a part by removing material from a stock shape of material. The stock shape may be a prismatic solid, cylinder, plate, etc. The removal of material may by cutting, turning, electro-discharge or other means. Common machinery such as millers, lathes and drills are subtractive tools.

 

Support structure

Many rapid prototyping machines need a means to hold in place unsupported geometries during fabrication, such as the top of a part in the shape of the letter "T." These supports are usually calculated and added to the part by the system's software and may be formed of the same material as the part, or from a different material entirely. Support structures are either mechanically removed or dissolved away in secondary operations before the part can be used.

 

Virtual prototyping

Computer-based prototyping without recourse to a physical part or object.

 

Voxel

The three dimensional equivalent of a pixel. A pixel is a "picture element," and a voxel is a "volume element." A voxel may also be defined as the minimum volume that a rapid prototyping system can fabricate.

 

Web exchange

A virtual market for buyers and sellers implemented through the Internet or World Wide Web. Also known as an electronic marketplace.

 

 

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