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Essay: Comparative Study of Rapid Prototyping Technologies

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1. Introduction

Prototype is an important part in the product development process. According to [1], a prototype is an approximation of a product or components in some form for a definite purpose in its implementation. The process of realising prototypes is called as prototyping. The prototyping process can be in the form of execution of computer program to the actual functional prototype. Three aspects are very important for any prototypes which are: (i) implementation of prototype, (ii) the form of the prototype and (iii) the degree of approximation. In order to make prototypes rapidly with more accuracy and to meet the market demands as quickly as possible, Rapid Prototyping (RP) technology was developed in 1970’s. This RP technology is now very popular in all fields of engineering and science; therefore it became a novel technology, known as 3D printing or additive manufacturing. This report is aimed to discuss about the background of RP technologies, different types of apparatus available, suitable materials and its advantages etc.

2. Research About Rapid Prototyping Technology

Rapid Prototyping (RP) is a modern manufacturing technology which serves as a tool for quick and easy manufacturing of customised complex shaped parts from 3D CAD models. RP plays an important role in the product development process due to its decreased time to market, highly customised and low quantity parts. RP technologies are also called as solid freeform fabrication or desktop manufacturing or layer manufacturing [1]. RP is a technology builds solid objects one layer at a time and high quality prototypes can be produced overnight. RP techniques are capable of performing additive processes rather than subtraction or the traditional material removal process under computer control with a limited human interaction [2-5]. Since RP is a computer depended technology, any change in the design could be easily corrected at the early product development process that will eliminate more expensive corrections in the component design and assembly. In overall, RP technologies are called as Additive Manufacturing (AM) [2-5]. RP technology has a numerous applications includes automotive, aerospace, defence, textiles, electronics, medical implants, biomedical and toy manufacturing industries [5]. RP technology is also termed as automated fabrication technique that is made up of different processes (see Figure 1): (a) Stereo-Lithography (SL), (b) Selective Laser Sintering (SLS), (c) Fusion Deposition Moulding (FDM), (d) Laminated Object Manufacturing, (LOM), (e) Direct Shell Production (DSP) and (f) 3D Printing. A short brief about these RP techniques are discussed below.

Figure 1. Current RP Technologies [2]

2.1. Stereolithography Apparatus (SLA)

Stereolithography (SL) aka Stereolithography Apparatus (SLA is a form of 3D printing rapid prototyping technology used for creating models, prototypes, patterns, and production parts in a layer by layer fashion using photo-polymerization process by which light causes chains of molecules to link and forming polymers. SLA is also known as optical fabrication, photo-solidification or resin printing. Research was conducted during the 1970s, but the term stereolithography was invented by Charles W. Hulland the process was patented in 1986. Company namely 3D Systems Inc was set up in order to commercialize his patent. The first and foremost technology developed for rapid prototyping was stereolithography; it is still (even today) standing out in the market as a modern or advanced RP technologies [6-9]. An example of a SLA process is as shown in Figure 2, the steps are as follows:
1. Machine accepts part as a .stl file and slices the file into thin layers.
2. The part is developed in a vat of resin. A layer of photo-curable resin sits above Z-stage elevator shown in diagram
3. A laser beam scans the surface of this resin, drawing the bottom layer of the part
4. As soon as one layer is completed, the platform moves lower down into the vat of resin; then fresh resin washes over the part and the layer proceeds to build the next layer
5. When all layers are completed; the final component/part is cleaned and post-cured.

Figure 2. An example of a SLA process

2.2. Selective Laser Sintering (SLS)

SLS is one of the most popular RP techniques suitable for making strong metallic parts directly from metal powders. In SLS, the metal powders are melted and solidified in a microscopic zone to form the defined shape of a product [10]. SLS technology was developed at the University of Texas at Austin by Carl Deckard and Joesph Beaman [11, 12]. This process was commercialized by two companies namely DTM Corporation and EOS GmbH Electro-Optical Systems. SLS uses a high powered Co2 laser beam to fuse the small particles of powdered material to create 3D parts. Laser selectively fuses the powdered material by scanning the cross sections of the surface of a powder bed and the model is developed by one layer at a time from the supplied CAD file. First, the 2D slice data of the object is fed into the SLS machine that directs the exposure path of the laser. The laser beam traces the path on the powder surface and heats it up to the sintering temperature to bond the powder on the scanned path. After the first layer fuses, the build tray moves downward and a new layer of the powder is deposited and sintered and the process is repeated until the object is complete. SLS is suitable for manufacturing of complex geometries [11, 12]. A wide range of materials can be manufactured using SLS technique which includes wax, cermet, ceramics, nylon/glass composites, metal-powder powders, metals, alloys, steels, polymers, nylon and carbonate [12, 13, 14]. Example of a SLS machine is shown in Figure 3 and the steps are as follows:

1. Input the .stl file
2. A layer of heat on a fusible powder is deposited across the part build chamber
3. Initial cross-section of the object is drawn on the layer of powder by a heat generating CO2 laser.
4. An additional layer of powder will be deposited via a roller mechanism on top of the previously scanned layer.
5. The process is repeated for each layer fusing to the layer below it.
6. Success layers of powder will be deposited and the process is repeated until the part is complete.
7. The final part will be removed from the build chamber and the loose powder is removed and reused.

Figure 3. An example of a SLS machine

2.3. Fused Deposition Modelling (FDM)

Fusion deposition modeling is a process of additive manufacturing technology. This process involves using a work head to melt a thermoplastic wire or filament and to extrude the molten thermoplastic through a small nozzle to deposit material along a preplanned path using computer control system. FDM is used as rapid prototyping technology for many years for manufacturing of variety of patterns, mold and tooling. FDM is manufactured by a company called Stratasys and this machine offers functional prototypes with ABS, polycarbonate materials. These thermoplastics are extruded as a semi-molten filament and the filament is deposited on a layer-by-layer basis to construct a prototype directly from 3D CAD data. It is important to note that FDM provides accuracies that are equal or much better than SLA and Polyjet, SLS [15-19]. An example of FDM process is as shown in Figure 4. The following steps are involved in the FDM process:

1. Receive the stl file and slices the stl file
2. The FDM machine starts to build the part by extruding a semi-molten filament through a heated nozzle onto a platform
3. When one layer is completed, the platform lowers by one layer thickness and the process continues
4. The final part will be easily removed from the platform, supports are removed and the part is ready.

Figure 4. An example of a FDM process

2.4. Laminated Object Manufacturing (LOM)

LOM is not a new technique and is commercialized in 1991 by Helisys, Inc. Paper coated with adhesive was used as a basic material which was cut by a laser beam into the desired shape. The paper sheets were then stacked and joined by thermal activated gluing in an automated procedure. The LOM produced parts have high accuracy. An example of LOM process is shown in Figure 5 and the steps are as follows:

1. CAD data goes into the LOM process controller and a cross sectional slice is created by the LOM software.
2. The laser cuts the cross sectional outline in the top layer.
3. A new layer will be bonded to the previously cut layer; a new cross section will be created and cut as before. Once all layers have been laminated and cut, excess material is removed to expose the finished model.

Figure 5. An example of LOM process

3. Description of Operating Principles

Assignment 1 asked to discuss the operating principles of two additive manufacturing processes, SLA and FDM processes are selected for the stated purpose and discussed in the following sections.

1.1. Stereolithography Apparatus (SLA)

As discussed in section 2.1, SLA uses photo-polymerization of resin by the energy delivered through an ultraviolet laser beam. The detail of the polymerization process is given in the following section.

Polymerization process:

Plastics are made up of long carbon chains. Resin is a plastic composed of shorter carbon chains from one carbon to a few thousand carbons. The chains join together to form much longer and stiffer chains when the resin is exposed to UV light; the result of this process will produce a solid part. The polymerization process is shown in Figure 6. Monomer and oligomer chains in the resin are haveing active groups. The resin is exposed to Ultra-Violet (UV) light, the photoinitiator molecule breaks down into two parts. The bond will hold it together to become two very reactive radicals. These molecules then transfer the reactive radicals into the active groups on the monomers and oligomer chains. It start to react with other active groups to forming longer chains. As the chains get longer and create cross-links, the resin begins to solidify. The entire process, from liquid to highly polymerized solid state, takes place in a matter of milliseconds [7].

Figure 6: The Polymerization Process

A schematic diagram of the SLA process is as shown in Figure 7. SLA uses CAD (Computer Aided Design) files. CAD files are digitalized representations of an object. CAD is used by engineers and manufacturers to turn ideas into computerized models which can be digitally tested, improved and 3D printed. CAD files must be translated into a Standard Tessellation Language (STL) file type. The STL file is imported into the Computer Aided Manufacturing (CAM) of each process and is then sliced into thin layers. Each layer represent as one step of the process in the STL machine. CAM system also produces the paths and manufacturing parameters according to the material and machine that will be used for making the prototype. For better understanding of computer based software suitable for rapid prototyping technologies are as follows:

1. CAD – is a technology used for creation and modification of design. Geometric modelling is well advanced to handle highly complex and sophisticated geometry and assembly of parts. CAD systems are interactive to other areas such as planning, scheduling and manufacturing.

2. CAM – is a technology used for making plan, manage and control manufacturing operations.

According to Figure 7, the ultraviolet laser beam is passed through galvanometric mirrors that will scan the vat surface. The vat surface will carry a liquid photo-sensitive resin. The laser radiation activates the photo-polymerization process in which the resin hardens and forming a solid layer of the object. After forming a layer, the platform allows the liquid resin spreads over to form the solid layer. When the resin is viscous, a blade passes through the surface of the liquid resin providing a gap in between the solid layer and the blade. The scanner starts to solidify a new layer attaching the new layer to the previously made layer. Layer-by-layer the object is manufactured by scanning selectively the laser beam over the resin surface [3]. In short, SLA process will be carried out in three different stages [3, 4]:

1. Pre-Processing: SLA starts with a model created via a CAD software. The design will be exported in a readable file (.stl file).

2. Production: A UV laser beam will scans the surface of the resin for curing of the material and develop the part from bottom-up.

3. Support Removal: Material can be easily removed suing hand. The remaining liquid will be washed and the supports will be removed.



Figure 7. Schematic diagram of stereolithography process

3.2. Fused Deposition Modelling (FDM)

The 3D printers that work on FDM technology consist of the printer platform, a nozzle (also called as printer head) and the raw material in the form of a filament as shown in Figure 8.

Figure 8. Typical FDM process

Printer Platform: The platform or the bed is made up of metals, ceramics and hard plastics and each adjacent layers will be deposited on top of this the printer bed.

Extrusion Nozzle: The extrusion nozzle of the 3D-FDM printers is attached to a mechanical chassis which uses belt and / or lead screw systems to move it. The entire extrusion assembly is allowed to move in X, Y and Z dimensions by a motorized system. A fourth motor called as the stepper motor is used to advance the thermoplastic material into the nozzle. All the movements of the head and the raw material are controlled by a computer.

Material: The material is typically production grade thermoplastics, though sometimes metal is used as well. The thermoplastic material is capable of being repeatedly melted when exposed to heat and re-solidified when the heat is withdrawn. The thermoplastic filament or metal wire is wound as a coil on a mounted spool. It is then fed through the printer nozzle. The better class of 3D FDM printers allows the temperature of the nozzle to be maintained just close to the glass transition temperature of the material being extruded. This allows the material to be extruded in a semi-liquid state, but return to solid state immediately. This results in a better dimensional accuracy.

In principle, any thermoplastic can be used as raw material for FDM printers. Commercially, a few of the popular choices of raw material include nylon, Acrylonitrile Butadiene Styrene (ABS) and its variations, polycarbonates, ply-lactic acid, polystyrene and thermoplastic urethane. MED610, a raw material that Stratasys provides is bio-compatible. Their ULTEM material too is certified by the aerospace industry.

FDM Printing Process: When the FDM printer begins printing, the raw material is extruded as a thin filament through the heated nozzle. It is deposited at the bottom of the printer platform, where it solidifies. The next layer that is extruded fuses with the layer below, building the object from the bottom up layer by layer. Most FDM printers first print the outer edges, the interior edges next and lastly the interior of the layer as either a solid layer or as a fill in matrix.

In some objects / models, there are fragile ‘overhangs’ that will droop unless they are given some support. FDM printers incorporate a mechanism whereby these support structures (called struts) are printed along with the object. They are later removed once the build is complete. These struts are usually of the same material as the object. Some printers have a second extruder to specifically deposit soluble thermoplastic struts when there is a need to prevent the overhangs from drooping. These struts may be of a different composition than the thermoplastic used for the 3D model. They are later dissolved by an appropriate solvent.

4. Comparison Between SLA and FDM Technologies

It is asked in the third part of the assignment to compare between two different RP technologies (selected from Q2) in relation to materials, processes, cost etc. In order to answer to this, a comparison between SLA and FDM processes are discussed in this section.

Table 1: Comparison between SLA and FDM Technologies [1-20]

Comparison Parameters SLA FDM

Process Invented in 1980’s, works by curing resin with light. The light solidifies a liquid resin via photo-polymerization process and builds objects layer by layer. 3D model file is imported into a program called slicer (Cura, Slic3r and Simplify3D). The program will slice the object into single layers and create gcode. The printer then receives the gcode accordingly heats and melts the filament that’s forced through the nozzle. The object is built layer by layer with each successive layer fusing on top of the one below until the 3D object is complete

Ability to create more detailed and accurate designs.

Layers are chemically bonded Layers are mechanically bonded, so less accuracy

Advantages Great value

High accuracy

Smooth surface finish

Range of functional applications Fast

Low cost system and materials

Limitations Limited build volume

Sensitive to long exposure to UV light Low accuracy

Low details

Limited design capability

Applications Functional prototyping

Dental applications

Jewelry prototyping and casting

Model making Low cost rapid prototyping

Cost Higher than FDM – Medium priced printer, but resin will be expensive Most affordable – Inexpensive printers and materials

Materials used Photosensitive resins ABS – Acrylonitrile butadiene Styrene

PLA – Polyactic Acid

Nylon – Polyamide and

Blend of plastic, wood and carbon

Filament diameters N/A 1.75 or 2.85mm

Material colors Limited availability both material and colours

Layer thickness 0.05 to 0.015 mm 0.5 to 0.127 mm

Minimum wall thickness 5 mm 1 mm

Surface texture Smooth Rough

Mechanically Strong and brittle Variable (strong or flexible)

Mechanical Failure Almost no deformation until sudden fracture Gradual deformation until failure

Abrasion resistance Variable Variable

Post-process Polishing and painting Polishing, painting, sealing, smoothing

5. Selection of RP process

The selected method of manufacturing of the part given in Figure 9 would be Stereolithography or Fused Deposition Modeling. However, the best recommendation would be FDM because it will be faster and cheaper than SLA. FDM machines are capable of manufacturing for the size of 450mm x 200mm x 80mm. FDM produces with least quality compared to SLA, but the cover does not require high precision assemblies, therefore the FDM quality can be accepted for this particular application.

Figure 9. Model of product cover to be prototyped

6. Summary:

In this report (as a part of assignment 1), the definition of RP technologies and its novelty, different types of additive manufacturing processes available, typical operating conditions of two different RP technologies such as SLA and FDM are discussed in detail. Various factors including process, materials, cost etc are compared between the selected RP technologies. Finally, the FDM process is recommended for manufacturing of the given component (i.e. cover).


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