The fast-track to product design

Companies who need a one-off part or product concept model made can pursue a number of options. One such option is known as rapid prototyping, the speciality of Silverdale-based company Plastic Design Technologies. The company uses the stereolithography process (SLA) to produce precision and functional parts; master patterns; display models; end use production parts for direct manufacturing; silicon moulding; and polyurethane part production for small run quantities. Company owner Brien O’Brien says Plastic Design Technologies (PDT) is the only company in New Zealand that uses this particular technology. “The SLA rapid prototyping technology is a flexible business tool that can turn a company’s idea into a physical model or master pattern in a relatively short turn-around time. “The technology significantly shortens the product design stage…ideas and concepts can be tested from both an aesthetic and mechanical point of view at relatively low costs, compared with the traditional machining methods. An SLA model optimises time and money at every stage of the production cycle and, as such, is an excellent investment,” he says. O’Brien explains how PDT’s technology works. Rapid prototyping is a relatively new process that enables the fast building of a complex physical object from a 3D CAD model. Dating from the late 1980s, it was originally used to produce models and prototype parts. However, modern sophisticated rapid prototyping techniques and materials have a much wider application. In addition to first off prototypes, rapid prototyping can now also build functional parts and custom fit manufactured objects. It can be used to produce master patterns for later use in spin casting and investment casting, and it is suitable to manufacture small runs of production quality parts. The six rapid prototyping technologies in general use today are selective laser sintering (building objects from thermoplastics and metal powders as base materials); fused deposition modelling (building with thermoplastics and eutectic metals); laminated object manufacturing (building with paper); electron beam melting (building with titanium alloys); 3-D printing (building with ink onto a powder); and SLA (building with photopolymers). Giving it the cutting edge The SLA process was invented in 1986 by American company 3D Systems Inc. It creates a plastic physical object through the action of a 0.2 millimetre laser beam that solidifies an epoxy resin in a predetermined number of layers according to the coordinates contained in a 3D CAD file. The CAD file interfaces with SLA software via STL-format files. The first step in SLA rapid prototyping is to create a 3-D model of the desired object with a CAD programme. This is called the part file. The CAD technician then transfers this file to PDT’s main computer, which operates a preparation software package. The preparation software rotates the part file to optimise build quality and cost characteristics. It also creates a 10mm-thick support structure under the flattest surfaces of the part. The software then “slices” the support and part file into thin horizontal virtual cross-sections or layers, similar to a stack of cards, to create a build file. These cross sections, or layers, will eventually translate the CAD file into a plastic three-dimensional physical object, built in layers and made up of thousands of small triangles. The build file is transferred to the SLA machine, or build machine’s on-board computer. The build file consists of separate layers, each containing the details and 3D coordinates of a “layer” of the object being built. A layer thickness typically ranges from 0.06mm to 0.2mm. The thinner the layer, the smaller the layer steps – and the better the resolution, or quality and surface finish, of the physical end product. The build machine generates a laser beam and directs it to a system of mirrors. The software package that drives the SLA process uses the information in the build file to direct the motion and direction of the mirrors and consequently where the beams will strike the epoxy resin. The epoxy resin is in a 25cm3 resin bath housed in a metal cabinet. As and where the laser beam strikes the resin, it cures the resin in nanoseconds into solid form. The beam only strikes in the surface layer of the resin, where it then traces the pattern of the object as contained in the file. The surface layer thickness of the resin corresponds to the eventual layer thicknesses of the physical object. The first layer is laid down on a special base plate. As a layer is completed, the base plate and its growing object lower into the vat by one layer’s thickness. Subsequent layers are laid down on top of previous ones until the object is complete. Unused liquid runs off back into the bath. After each layer is completed, the resin surface is “wiped” to even out the meniscus. The resin’s self-adhesive and self-fusing properties cause the layers to bond to each other. When the process is complete, the technician takes the plate with the object out of the build machine, breaks off the support structure, and cleans it. If necessary, the object’s surface is sanded down and or polished for a smooth finish. The technician then places it in a post curing apparatus for ultraviolet light post curing, required before the models are ready for use. The objects may be painted. The building time for an SLA rapid-resin prototype ranges from a few hours to days, depending on the object’s size and complexity. More than one object of the same pattern, or several objects of different patterns but preferably similar height, can be built at a time on the same base plate. Fast, flexible, functional O’Brien says experts generally consider stereolithography to provide the greatest accuracy and best surface finish of any rapid prototyping technology. “The advantage of the SLA process is its accuracy and range of resins, and it is eminently suitable for smooth surface finished parts. This method can produce parts for both aesthetic and functional testing and parts with good snap-fit qualities. “Our SLA parts are strong and high-detail, and widely used for conceptual designs, product verification, pattern making, form-fit analysis, and light functional testing.” PDT has a countrywide client base in industries ranging from health care to kitchenware. O’Brien says the company focuses on making functional parts that can be used for electrical and electronic components, lenses, light pipes, plumbing fittings, aluminium joinery, appliance components, latches, handles and other hardware, and most other components used in product design analysis. To this end PDT uses different grades of resin, including polyurethane, which provides parts that are slightly flexible, rigid, or clear. “This allows us to approximate a wide range of thermoplastic polymers,” says O’Brien. A second focus area is pattern work. SLA prototypes can be used to produce silicon moulds. A prime use of silicone moulding is in prototype modelling, and to test the function of the new product or design before the expense of volume manufacture. For some models, this can also become a marketing tool. Silicon moulds can, in addition, provide patterns from which metal objects can be produced through investment casting or spin casting. A third area of focus is making display models. “A well-made and painted model makes it easier for people to visualise a finished product, and is more impressive than a plain graphics programme. This works especially well with houseware products, especially if they need to be tested,” says O’Brien. According to O’Brien recent developments in equipment and materials used in the selective laser sintering and SLA rapid prototyping methods mean they can increasingly be used for rapid manufacturing of product parts such as custom fit healthcare and other specialised manufactured objects. PDT already offers this service via SLA technology to a number of established clients. “The benefits here are speed, fine detail and cost competitiveness,” he says. Another new development is polyurethane part production for small run quantities. O’Brien explains that polyurethane’s chemical properties make it a desirable choice for the designer. Depending on the type of polyurethane, the object will, for example, be flexible or rigid, and “…you can vacuum cast an object with the desired characteristics by choosing the appropriate polyurethane formula.” It has excellent application value for the functional testing of prototypes. A qualified machining fitter and turner, O’Brien holds a NZCE Mechanical Engineering degree from AUT (at the time of his graduation known as ATI). He has worked in the plastics industry many years, from 1975 as company owner. He sold O’Brien Plastic Ltd, an injection moulding company, in the mid-1990s and in August 2001, started up Plastic Design Technologies. When not working, he likes to spend time in the outdoors, fishing or hiking, to recharge his physical, emotional and spiritual batteries. reader enquiries quote:

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