CENTRE FOR RAPID PROTOTYPING & MANUFACTURING

INTRODUCTION The essential difference between the new and conventional prototyping techniques is that the new processes build up the part layer by layer, whereas conventional techniques all involve the removal of material from a solid block to make either a direct model of the part required, or its inverse shape as a mould or die.

All new rapid prototyping techniques begin with a completely defined CAD model (solid model) of the part to be made. The CAD data are then translated into STL faceted representation of the exterior surface of the part. Although a solid model is preferred, some surface modelers can generate the data in height. This information on the shape and dimensions of each layer is then fed into the prototyping system. The systems differ in the way the component is built up by layer.

INITIATIVES AT THE CENTRAL UNIVERSITY OF TECHNOLOGY, FREE STATE

CRPM (Centre for Rapid Prototyping and Manufacturing) adopted a concurrent engineering approach to accelerate new product development, based on CAD, reverse engineering, rapid prototyping ( SLA , SLS and Sanders 3D printing), soft tooling and spin casting.

Through collaboration with a number of private companies these offerings were broadened to include industrial design, finite element modeling, process modeling, rapid tooling and investment casting. Once concepts are proven, the master files can be used directly to produce tooling or patterns for EDM.

RAPID PROTOTYPING, TOOLING AND MANUFACTURING AT THE CENTRAL UNIVERSITY OF TECHNOLOGY, FREE STATE

Rapid Prototyping at the Central University of Technology, Free State

Apart from conventional methods such as high-speed milling, the following rp processes are available at the Central University of Technology :

STEREOLITHOGRAPHY APPARATUS ( SLA )

This process is one of the pioneers of rapid prototyping and was introduced by 3D-Systems, Valencia , California . It uses a software-controlled, laser-generated beam of ultraviolet light, focused on the surface of a vat of photosensitive resin. Cross-sections (layers) of the CAD model, which range in size from about 0,15 mm, are constructed one at a time on the surface of the resin.

To construct the layer, the laser traces the perimeter of the layer and then cross hatches the areas that are to be solid. A thin line of fluid is solidified as the laser moves along the surface. The depth of the layer is controlled by the speed of movement: the faster it moves, the thinner the layer it cures. The cured layer is lowered beneath the surface at exactly one-layer thickness. Fresh liquid polymer flows in, to cover the newly solidified layer. Once the surface of the vat is perfectly level, construction of the next layer begins. The depth of cure is slightly greater than the layer thickness so that the new layer will be solidly attached the one below it. The process is repeated, layer by layer, until the three-dimensional hard plastic object is completely constructed, from the bottom up. The model is then post-cured to finish it. The SLA 250 allows dimensions of 245 mm x 245 mm x 245 mm on one platform (one building cycle). However, through the use of MAGICS, models can be cut for smooth assembly, hereby enabling any build volume.

SELECTIVE LASER SINTERING (SLS)

While the majority of other rapid prototyping systems are limited to a single class or limited range of materials, selective laser sintering (SLS) offers a unique, versatile process that can be used for a broad range of applications., including functional prototyping, flexible/functional prototyping, investment casting, sand casting, hard tooling and soft tooling. As the traditional rapid prototyping industry makes the transition to rapid manufacturing – the production of multiple prototypes in actual manufacturing materials – SLS emerges as the premiere technology within the industry, with solutions that support the entire product development and manufacturing process. The SLS technology works by creating – layer by layer – three-dimensional objects from powdered materials, using heat generated by a C02 laser within the system CAD files are sliced and drawn, one cross-section at a time, by applying the laser beam to a thin layer of powder. The laser beam fuses the powder particles to form a solid mass that closely matches the CAD design. As each layer is drawn, the prototypes take shape within the system.

Due to the limited range of materials available for other rapid prototyping systems, their ability to adapt to new applications follows suit, locking customers into a narrow range of applications. However the SLS process provides virtually unlimited materials and application flexibility. The system works with a variety of powered materials, which currently includes:

ProtoForm TM Composite; Fine Nylon; Standard Nylon; Polycarbonate; TrueForm TM PM Polymer; ST100 TM Metal; SandForm TM Zr; Somos 201 Polymer

As a result of the variety of powdered materials available, and the differences in characteristics of these materials, the Sinterstation System offer the greatest flexibility of any rapid prototyping or rapid manufacturing system. The SLS machine got a build envelope of 320 mm x 320 mm x 620 mm.

Material flexibility allows the provision of diverse solutions to support the product development and manufacturing process. When different applications are required – or when it is found that the application needs are changing – SLS offers several solutions and/or applications, including:

FUNCTIONAL PROTOTYPING:

Nylon sintering material offers high durability, along with heat and chemical resistance, enabling the formation of strong, testable prototypes. Inter alia, Composite (glass-filled) Nylon, the industry's first composite material for rapid prototyping, is available. This should offer a very good solution for functional prototypes – prototypes that can withstand extreme stress and temperature variations.

FLEXIBLE FUNCTIONAL PROTOTYPING:

Another unique feature of the SLS process is that it can create “rubber-like” prototypes. Initial interest in this material came from automotive, medical, sports equipment and toy markets, where it can be used to quickly prototype a wide variety of parts ranging from door mouldings, hose assemblies, boot covers, pliable keyboards, seals and shoe soles. A positive characteristic of this flexible polymer is high resistance to elevated temperatures and harsh chemicals such as gasoline and automotive coolants.

INVESTMENT CASTING:

Prototypes from the SLS process are excellent for investment casting patterns. The material can handle complex structures and geometries, sharp edges and fine detail; it needs no specialized casting procedures, and vaporizes with minimal residue. The material also requires less post-processing than patterns made with liquid resin.

SAND CASTING

Cores for sand casting have always required tooling to be created, taking as long as six months to procure. With the SLS, cores and moulds can be in hand in as little as two days. One or more cores can be created directly in the build chamber of the SLS system, then removed and cured in a conventional sand-casting oven.

HARD TOOLING

The revolutionary ST100 process uses a powdered metal material that allows the creation of core and cavity sets for plastic injection moulding. Complex prototype tooling can now quickly be produced inhouse using the SLS process, in a fraction of the time of other methods. Multiple quantities of “true” prototypes can be created, using the actual manufacturing process (plastic injection moulding) and the actual. manufacturing materials. The ST100 ushers in a new era of rapid manufacturing and represents a revolutionary development within the rapid prototyping and rapid manufacturing industry.

SOFT TOOLING

Patterns made from polycarbonate and nylon materials with SLS are extremely durable and can be re-used many times to ensure cost effectiveness. Soft tooling patterns made from SLS materials also exhibit dimensional stability and reduced finishing time for maximum efficiency.

SANDERS 3D PRINTING

The Sanders Modelmaker II system uses a patented inkjet technology to build up models, layer upon layer, on a Z –axis build table. Build layers as fine as 0.013 mm enables the system to build models, prototypes and patterns with exceptionally fine detail and smooth surface finishes, even on rounded contours. The table is precisely positioned (within0.003 mm) after each build layer, and a flatbed milling subsystem mills off excess vertical height. In addition to creating a known surface reference for the next layer to build on, the milling technique enables any level of surface depth cutting within the 0.003 mm limit.

Multiple print heads, mounted over the table on a precision X/Y drive carriage, deposit tiny droplets (0.075 mm in diameter) according to a drop-by-demand method, to deposit material digitally controlled to within 0.006 mm.

One jet deposits droplets of green thermoplastic material, to build the actual pattern. A second jet deposits droplets of red wax that supports all pattern overhangs and cavities during the build process. The materials, which are ejected from the heads as hot liquids, solidify to align trace of 0.1 mm high by 0.06 mm high, enabling free-standing walls as narrow as 0.1 mm to be constructed.

When the model is completed, it is immersed in a solvent bath that completely dissolves the wax support. Within minutes of the supports dissolving, the model is ready for visual inspection, painting, and investment casing or moulding in RTV (room temperature vulcanizing). In the casting process, these patterns exhibit a very low coefficient of thermal expansion, preventing casting-shell raptures.

For most parts, the models are accurate enough to be used directly as patterns for investments casting or injection moulding tooling.

CURRENT DEVELOMPENTS USING RP MODELS

RP has already moved beyond the primary objective of producing parts for design verification, or even functional prototypes. Using RP masters for investment or sand casting or silicone rubber moulds already reduces time in the product development cycle. Future application includes the direct production of cast tooling. Al of the above RP models can be used in different ways for design verification, form fit and function and ways to produce more parts for marketing or intermediate parts while the final tooling is manufactured.

RP AND CONCURRENT ENGINEERING

Concurrent engineering is the buzzword in product development into the new millennium, and involves the use of a single master file, from design through to manufacturing and quality control. Following these principles, CRPM facilitates the concurrent engineering process based on the strength of the information technology and file transfer used to drive the various interconnected systems. The technique facilitates co-ordination of the reverse engineering, structural and process analysis (FEM), rapid prototyping metrology, quality systems and project management, in order to get the product down the development track more rapidly and shorten the time-to-market.

THE CONCLUSION

Globalisation of markets and increasing international competition forces many companies to rethink their competition strategies. As a result they realize that a focus on time-to-market quality alone is no longer sufficient to survive. Only those companies that are able to satisfy the complete set of customer requirements – time, quality and costs – will win the race. Beyond these basic goals there is also a need to attract the customers' attention and money by means of new and innovative products – a task which calls for extremely efficient and flexible product development.

In the past a series of technologies, e.g. CAD, CAM and NC manufacturing, was identified to solve these problems. Rapid prototyping represents the latest trends in manufacturing technology. However, all these techniques reveal only a technological view on how product development can meet the tremendous challenge in the future. In fact, it is not the use of a single technology that provides better products faster for the market, but rather the integration of a large number of them. Therefore, aspects of information processing, cost, quality and time management, teamwork, organistional issues, and many more enabling technologies like data highways, multi-media or distributed databases, have to be taken into account as well.

Global changes in patterns of product consumption and the increasing demand for new improved and innovative products are challenging traditional concepts of manufacturing. In a world of global competition, speed is a strategic weapon. Introducing new products and refining existing products in today's competitive environment requires shorter development cycles. Rapid product development (and rapid tooling to enable rapid manufacturing) is a fundamental challenge in today's global marketplace.

RP is here to stay, and is positively influencing the entire concept of product development. The only decision a company has to make, is how soon RP is going to influence its competitiveness.