The Term ‘Rapid Prototyping’

The term ‘rapid prototyping’ in manufacturing refers to techniques used for the creation of a scale model of a physical assembly or part with the help of the data obtained via three-dimensional computer-aided design (CAD). Therefore, rapid prototyping is a form of computer-automated manufacturing in which a product is designed and fabricated using visual aids instead of the real materials. This paper examines the various processes used in rapid prototyping. The paper approaches the aspect of rapid prototyping from the manufacturing perspective. The intention of the paper is to examine the individual processes that manufacturers use in rapid prototyping, evaluate advantages and disadvantages of each process and draw conclusions.

Introduction

Techniques that are used for quick creation of a scale model of a physical assembly or part by means of three-dimensional CAD data are commonly termed rapid prototyping. This implies that it is a form of computer-automated manufacturing in which a product is designed and fabricated using visual aids instead of the real materials. In manufacturing technology, rapid prototyping involves creating a product with input from the designing, engineering and marketing departments before really manufacturing it for the mass market. The product, known as a prototype, is created using imaging techniques and in some cases lighter versions of the intended materials. Given its fabricated nature, the product can be easily modified and improved based on the opinions of the marketing and engineering departments without taking too much resources and time. This means that prototyping allows for product design and development without putting the manufacturing company through vigorous procedures such as product recalls and order cancellations. As a manufacturing technology, rapid prototyping comes in various options. There are about six types commonly discussed, and they include Laminated Object Manufacturing (LOM™), Stereolithography (SLA), Solid Ground Curing (SGC), Fused Deposition Modeling (FDM), Selective Laser Sintering (SLS®), and Ink Jet printing techniques. All these rapid prototyping procedures result in the creation of products or parts that are often not exactly the same as the end products but rather a close imitation. In some instances, however, the same procedures are used to manufacture the real product depending on the requirements and expectations related to it. This paper discusses rapid prototyping in manufacturing technology and procedures, citing different types of rapid prototyping that are currently available for manufacturing applications. The paper also discusses various advantages of rapid prototyping in order to establish why the concept is important to manufacturing companies.

Rapid Prototyping in Manufacturing Technology and Procedures

Conventional rapid prototyping uses computer-aided design to generate a data set with which the design can be altered, modified and eventually massively produced (Tseng & Jiao, 2008). This implies that rapid prototyping is mainly a means with which the product can be produced inexpensively during the design phase and presented for evaluation and correction before the company invests in the actual manufacturing process. With this in mind, there are a number of different rapid prototyping procedures that may be used in a manufacturing system depending on the choice and needs of the company in question. These are discussed below in detail.

Stereolithography (SLA)

Stereolithography is a type of rapid prototyping that is already commercially available and is the most commonly used based on its simplicity and cheap cost compared to the other processes. This was the first rapid prototyping procedure, created in the 1980’s, and it involves a vat of photosensitive resin that contains a platform which is movable in the vertical direction (Jarvis, 1999). The vertically movable platform supports the part under construction by a layer with thickness of up to 0.04 inches per layer, and a laser beam is used to trace out the shape of each layer, hardening the photosensitive resin in the process. The uncured resin is then removed and afterwards the resulting product is cured again to fully cure the resin. The resulting model ends up with a surface that is largely composed of stair steps due to the layering nature of the process. However, this is smoothened by the application of sanding techniques to give the model a perfect and cosmetic finish. Altering the orientation of the model affects both the frequency of the stair steps and the amount of time taken to build the model. If the model’s long axis is oriented vertically, the time taken to finish the model will be much longer, but the stair steps generated will be minimal. If the long axis is however oriented horizontally, the time taken to build the model may be shorter, but the resulting stair steps will be relatively magnified. Depending on the purpose of the model, some companies require that they be primed and painted to improve their appearance.

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While the model is being made in the process of ‘fabrication’, some of the extremities of the part may become so weak that support must be added on to prop the model up firmly as required. In these instances, the supports are generated by the same program that has created the slices, and these supports are only used for the fabrication (Tseng & Jiao J 2008). These supports are considered necessary as the weak extremities prevent accurate alignment, and the fabrication process may result in a distorted or poorly aligned model if the supports are not introduced.

Stereolithography is a relatively simple procedure based on a number of facts. First, the process is the most commonly used rapid prototyping process despite the availability of other more recently discovered and thus more advanced procedures. It is inexpensive compared to the other commercially available rapid prototyping processes. The process is also relatively simpler considering that it does not involve any milling or masking steps. In addition, it requires a post-curing step as the power is not enough for a complete curing during the fabrication process.

Considering the outcome of the Stereolithography, the model often comes out brittle with a tacky surface. This is why priming and painting are often recommended for aesthetic purposes. Moreover, the fabrication process often requires the application of supports to prevent distortion and collapse in the models. Another important consideration in Stereolithography is that the ventilation must be adequate considering that the uncured resin has a tendency of being toxic. Generally, it can be said that Stereolithography is the most basic of all rapid prototyping procedures, and thus it remains the most adapted one in the manufacturing industry despite its shortcomings in comparison with the other processes.

Selective Laser Sintering (SLS)

The Selective Laser Sintering (SLS) trademark is registered by DTM Corporation, which is based in Austin, Texas, USA. The SLS process was patented by Carl Deckard in 1989; he was a graduate student at the University of Texas then (Chin, 1998). This process is only a few years younger than the Stereolithography; thus, there are no many significant differences except for the obvious advantages that the former has in terms of the material properties. Selective Laser Sintering allows for use of many various materials, the properties of which are close to those of the known thermoplastics including polycarbonate and glass-filled nylon.

An SLS machine is made up of two powder magazines that are placed on both sides of the fabrication area. The powder is moved over from one magazine by a leveling roller, causing it to cross over the fabrication area to the other one. The laser is then allowed to trace out the required layer. Once this is done, the platform slightly moves down covering a distance that is equal to the formed layer thickness. The leveling roller moves back to its original position awaiting a repeat of the above procedure. This cycle is continued until the model or part being fabricated is complete.

At this point, it is clear that the resulting model would be much better than the one produced by Stereolithography as this process uses polymer powders and not the brittle photosensitive resins. Furthermore, while models or parts generated by Stereolithography can be of the same size as those generated from SLS, the ones from SLS are easier to join if the parts need joining. The brittle nature of the SLA models makes them difficult to join after fabrication. Due to the fact that the SLS process uses powder polymers, the models or parts created often have a powdery surface and thus require sealing for a smooth finish and extra strength.

Among the many advantages that this process has, it is clear that the models or parts fabricated are much stronger and thus the process can be used to make functional parts and not just approximate prototypes. The process allows for a much wider variety of powder polymers implying that the closest approximation of the common engineering plastics can be created. The laser beam effectively fuses these components of the powder polymers and the uncured materials are easy to get rid of as they only require blowing or brushing off. In addition, the SLS® process allows the creation of living hinges; thus, the parts can be easily joined if need be. Considering that the powder is relatively stronger than the photosensitive resin used in SLA, this process does not always require the use of supports. The process is also however simple and has neither the milling nor the masking steps. Consequently, the accuracy of z the surface also suffers as in Stereolithography.

Considering all the advantages that this process has over Stereolithography, it seems unbelievable that it is not as commonly used. The process is considered easier and in some ways more functional than SLA, but it has a few setbacks. First, the powdery surface of the models or parts implies that it is impossible to get a perfect finish unless a sealant or a varnish is applied. The SLA thus remains dominant in cases where detail is important in the model. The SLS® also suffers since the powdery materials tend to be affected by the curing procedures thus impeding dimensional accuracy. The SLA does not have these challenges given that the cured resin is stable and more so with the curing processes.

Laminated Object Manufacturing (LOM)

The LOM involves a paper, metal or plastic sheet that is laminated on one side with an adhesive. For this process, the fabrication of a layer starts with a sheet being stuck to a selected substrate using a heated roller. Once this is done, a laser beam is used to trace out the formed layer outline. The parts of the layer that are not required are then removed by cross-hatching and disposed as waste materials. After this step, the work platform moves out of the working area to allow a new sheet to be placed. The platform then moves back in place, positioning itself a layer’s thickness below its initial position to allow for the creation of a new layer right on top of the first one. The process is then repeated for as many times as is required to complete the model or part in fabrication.

When the process is carried out using a paper sheet, the model has a wooden-like texture upon completion. It is however likely to absorb moisture and thus expand, limiting its dimensional accuracy. To avoid this, the models are coated or sealed with paint to avoid the dimensional instability caused by their absorption of moisture. The procedure is however being improved to allow for the extensive use of stable materials like metal and plastic which will eliminate the need for sealing in a bid to fight of the dimensional instability brought about by moisture absorption.

The LOM is a largely affordable type of rapid prototyping as it involves the use of readily available materials like paper sheets. However, this procedure is not as prevalent as either SLA or SLS. The procedure does not involve any chemical reactions and thus the modes or parts to be made can be of a very large size. Unlike in the SLS or SLA where the processes require chemical reactions with the polymers used, this process only involves adhesive, and the surface area of the model is thus not a challenge. The accuracy of z is also a challenge like in SLS® and SLA since there is no milling or masking step here either. Hypothetically, the LOM™ would be the most popular rapid prototyping given that it has lower cost, is relatively easier and does not limit the size of the parts or models that can be made. The process however does not match either the SLS® or the SLA in terms of popularity of its usage in the manufacturing industry.

Fused Deposition Modeling (FDM)

The process of Fused Deposition Modeling was developed in 1988 by Scott Crump, and it involves the heating of a filament that is made of a thermoplastic polymer. This heat-softened filament is then squeezed out from a tube just like toothpaste and used to form layers on the work platform (Hamblen, Hall, & Furman, 2008). There is a wide variety of FDM machines ranging from the fast concept modelers to the much slower but high-precision varieties. The commonly used materials include standard engineering thermoplastics such as ABS among other materials.

In FDN, the possibility of using standard engineering thermoplastics allows for the fabrication of structurally functional parts and models. Moreover, the process allows for the use of two different building materials for the models provided they are able to be heat-softened and squeezed out of the tube and on to the work platform. This process also requires that the platform be kept at a consistently low temperature to enable rapid cooling of the building materials. The process also does not have a milling step, and thus the accuracy of z is problematic. This means that the plane may suffer as a result on the lacking uniformity on the layer deposition. Currently, FDN is lagging behind both the SLA and SLS, but it may soon catch up as manufacturers continue to appreciate its advantages, especially the ability to fabricate structurally functional models and parts that can be as large as 24 ? 24 ? 20 inches (Hart, 1995).

Solid Ground Curing (SGC)

The Solid Ground Curing process is also commonly referred to as the Solider Process. This is basically a process that uses a photosensitive resin that is then hardened in layers just as is done in the SLA. Unlike the SLA, which is a relatively lower production process, SGC is considered a high-throughput process of production. This is mainly achieved by hardening each layer of photosensitive resin over a large work space, making it faster and more effective (Hamblen, Hall, & Furman, 2008). This also allows for the creation of many parts at a go given the large work space and vertical accuracy provided and maintained by the milling step. The large working space not only enables the creation of multiple parts but also allows for the fabrication of relatively large separate parts of up to 20 ? 20 ? 14 inches. In the non-part areas of each layer, liquid resin is replaced by wax, which provides model support. This implies that no extra support is necessary during the fabrication process.

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This process has a variety of advantages over the other rapid prototyping processes. First, the large working space allows for higher throughput enabling the fabrication of multiple parts or large single parts (Wohlers, 1999). This is further enhanced by the high production speeds resulting from the way through which the resin layer is cured all at once after the masking step. Furthermore, given the thoroughness of this laser exposure, the models or parts created do not need to be post cured as they are already fully hardened. The milling step also ensures an accurate z making the layers reliably flat. Another advantage is that the models do not require extra supports as the wax replacing the resin in the non-parts for each layer provides adequate support. The only considerable disadvantage associated with this process is that it produces a lot of waste. The procedure is currently gaining popularity amongst manufacturers due to the high productivity and efficiency in the models that are fabricated.

Ink Jet Printing Techniques

There are about four Ink Jet techniques that are commercially available for rapid prototyping, but none of them has been established yet. These include Sanders Model Maker™, Z402 Ink Jet System™, Multi-Jet Modeling™, and Three-Dimensional Printing, all of which use the Ink Jet technique of shooting tiny droplets of liquid to solid compounds like wax and other thermoplastics. These techniques are often acknowledged for their accuracy, production speed or usability depending on the specific technique (Chua & Leong, 2003).

Conclusion

Rapid prototyping is considered a breakthrough technology in the manufacturing industry as it allows for the fabrication of models and production of structurally functional parts without going through the rigorous economics of manufacturing. There are various processes that are commercially available, although SLA and SLS remain the dominant ones with SLA taking the lead in the manufacturing industry. The other processes are only struggling to catch up, and they all have numerous advantages that will serve the manufacturing industry well once fully adapted.

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