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RP - no longer a futuristic concept, but a current and practical reality for toolmakers

Getting mould tools direct from CAD files, with virtually no intermediate handling, was once an impossible dream, but this dream of RP has become a reality in recent years – and not just for high tech blue chip companies. This technology is not yet in widespread use among toolmakers – one of the main obstacles to this is the fact that the whole RP route is still a relatively unknown technology – but with the establishment of sites that deal specifically with demonstrating the benefits it could bring to the design offices and toolrooms of small and medium size enterprises (SMEs), this situation is starting to change.

These sites include the Product Innovation Development Centre (PIDC) in the Department of Industrial Studies at Liverpool University (Tel: 0151 794 2000) and the Innovative Manufacturing Centre (MC) at Nottingham University (Tel: 0115 925 6142); there are also facilities at the universities of Warwick and Newcastle. All these sites can generally take a job right through from concept to tooling, with the aim of transferring this technology into industry.

Katie Stewart at the PIDC points out that RP is no longer just about shaving time off the normal 12 week lead time; rapid tooling and rapid manufacturing are increasingly coming to mean cheaper tooling. Throughout industry, batch sizes are becoming ever smaller, as a result of the fast response required to ’fashion’ or product updates. At the same time, manufacturers are pushing more responsibility for tooling development onto sub-contractors, as part of long-term partnership agreements. The effect is to increase the demand for cheaper tooling and pressure sub-contractors into accommodating runs as short as a few hundred. Fortunately, specialist centres – often backed by technology transfer grants – are making it easier for small firms to experiment with RP (several RP processes including laminated object modelling (LOM) for the subsequent production of basic sand cast parts, and stereolithography (SLA) for more complex geometry's).

While all processes associated with RP require the use of CAD data to produce the pattern, non CAD operations are no longer excluded from the application of these techniques, since the specialist centres have been specifically set up to take a project from the drawing or idea stage through to finished part.

Only a small proportion of toolmakers who could benefit from RP actually realise it. In fact, Richard Hague at the Nottingham IMC estimates that perhaps just 10% of those toolmakers for whom these techniques are applicable are aware of what it can do. Mr Hague also sees this technology developing into the distinct areas of office based models, which rely on extruding a low-melting-point thermoplastic to produce simple, low-cost but accurate models, and workshop based machines, which exploit the more technically advanced end of the process.

One of the cheaper RP processes – that of fused deposition modelling (FDM) – plots a series of lines of extruded thermoplastic material to produce a block of ABS material. In fact, the Nottingham centre took delivery of an Actua machine from 3D Systems Inc Ltd, Hemel Hempstead (Tel: 01442 66699) – the first of its type in the UK – which utilises ink-jet technology to make a thermoplastic pattern. The machine currently produces concept models with a material that has a slightly soapy feel, but Mr. Hague is confident that new materials are being developed and that the process will prove useful for a range of toolmaking applications.

At present, rapid manufacturing generally means using some kind of secondary processing to create tooling from the rapid prototype. This could be by creating rubber moulds from an SLA master, or by using investment casting techniques or a form of powder casting technology to make the tooling.

For example, Keltool – a proprietary process available from 3D Systems – is fast being accepted as a quick and economic way of providing steel tool moulds (equivalent to an A6 tool steel) that are suitable for producing plastic injection moulded parts in production runs from as small as 100 up to millions. The process is based on combining the speed and accuracy of producing an SLA model master with the production of sintered-metal mould cores and cavity inserts. In the production environment, the time required to develop an SLA pattern to a master Keltool part can be as little as two weeks, with CAD model to the first injection part being practical in less than four weeks. 3D Systems, which recommends this particular process for complex geometry's, says it can halve the cost of traditional tooling and, in certain applications, take just 30% of the time.

Other methods of producing tooling directly from the rapid prototype – such as metal spraying – are also being developed, and this seems to have potential in the production of less complex parts, although it is inevitable that some detail will be lost.

CAD/CAM developments

We still like to deal with models we can understand rather than CAD data and wire frames. Indeed, one of the attractions of rapid prototyping is having a solid model that can be examined. The availability of RP models has also put an increased emphasis on reverse engineering, as these models can be re-machined and modified, then re-digitised to generate the CAD surface again.

This need for easy visualisation also explains why currently one of the most popular CAD developments is the production of realistic component images; this can be used to achieve faster customer approval for a project, thereby contributing to shorter lead times. In fact, a system installed at the Gwynedd based toolmaker Bico demonstrates how RP, CNC machining and rendered imaging can all work together.

Bico has installed an AutoCAD system from Autodesk Ltd, Guildford (Tel: 01483 303322), featuring the Autosurf and Designer packages. It uses AutoCAD to draft and generate engineering drawings for design and build projects, and Autosurf for prototyping and design visualisation (the package is used to generate a surface model and rendered images, screen shots of which are sent to the clients). Autosurf also generates the geometry required for RP, and models the injection mould tool geometry for transfer to the company’s MasterCAM package from 4D Engineering Ltd, Swindon (Tel: 01793 861050). In addition, an Iges translator is used to translate the Autosurf models into a format that can be used by MasterCAM; the translator is also used for translating model data supplied direct from customers.

Other developments in CAD/CAM, which recognise the requirements of practical toolmaking, are designed to accommodate the fact that component information is frequently incomplete or incorrect. In fact, errors seem to be very much part of the CAD process, as it has been suggested that well over half of all CAD files contain incorrect data which, without some sort of remedial action, could cause problems. Systems manufacturers are recognising this fact by equipping their products with a geometry checking capability.

The spread of CAD systems also has implications for conventional toolmaking. Where a part exists as a digital file that can be transferred down a telephone line, the appropriate tool can be manufactured by conventional means anywhere in the world. Indeed, some companies are already buying tooling on this basis, from wherever it is cheapest.

LPM Consultants is one company that uses toolmaking facilities world-wide in the specialist area of mobile phones, as this product is subject to the whims of fashion and is therefore produced in relatively short runs before the part is changed. According to LPM’s John Penn, he can be in possession of completed tooling just six weeks after sending a 3D CAD file to Taiwan. His experience, together with the development of rapid manufacturing technology, demonstrates that CAD/CAM is opening up new ways for sourcing tooling. There will always be parts, particularly the simpler ones, for which small run tooling will be more economic to produce using conventional machining; but the on going development of RP and easier access to bureau's and specialist advice mean that small and medium size companies should be looking carefully at what these new techniques can do for them.

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Technology Clubs for RP

It was a Department of Trade and Industry study in the early 1990s that identified RP as one of the technologies that merited Government support, to develop it as an R&D tool for manufacturers. This was followed by the formation of specialist units in university technical departments, dedicated to furthering these technologies, and transferring them from the laboratory to general industrial use. Some of these units have sought to satisfy these aims by providing RP facilities, together with instruction and training for specific companies, by means of a ’club’ arrangement. For example, Nottingham University, a leading centre in the application of this technology to injection moulding, operates a club comprising some 20 or 30 members, each of which is allocated a number of development hours according to its annual subscription.

Members are encouraged to participate directly in the development process, so that they familiarise themselves with the technology, are in a position to set up their own RP, and know what to expect when using commercial RP bureau's. Furthermore, to ensure that as many companies as possib1e benefit from this technology, club membership is changed every year.

The Nottingham centre, which is now independent of Government backing, provides an integrated service that combines 3D CAD modelling, stereolithography prototyping, and the replication of prototypes that are destined for injection moulding. Of several CAD programs available, Unigraphics and Intergraph are the most frequently used for modelling in conjunction with the centre’s stereolithography unit – an SLA250 supplied by 3D Systems. Because the stereolithography process is still somewhat expensive, when facsimiles are required for trials and evaluation, a stereolithography master is replicated by vacuum casting or by injection moulding in sprayed metal tooling, using vacuum casting and metal spraying systems supplied by MCP Equipment of Stone (Tel: 01785 815651)

Vacuum casting is employed for smaller runs and when there is no need for replication in the final material, while injection moulding is applied to larger quantities and to replicas required in the ’as finished’ plastic. The first step is to convert information from 2D data to a format that is ’understood’ by the SLA250 (Unigraphics being preferred for solids modelling and Intergraph for surface modelling).

The SLA250 forms the prototype by solidifying successive layers of a special polymer, using laser energy controlled in accordance with data contained in the CAD file. When the model is complete, it is cleaned, and then cured by ultra violet radiation.

A model that is to be replicated by vacuum casting is sprayed with an acrylic or cellulose paint, to inhibit any interaction between the polymer and the silicone rubber mould. The vacuum casting process, which is capable of producing accurately detailed facsimiles that are suitable for aesthetic, visual and physical assessment, uses an MCP C003MC computerised system, which accepts moulds up to 580 x 600 x 615mm, and can deliver a maximum ’shot’ of 2,000gm.

Using the model as the master, a split casting mould is first made in silicone rubber. A two part polyurethane resin is then mixed automatically and cast into the mould under microprocessor control.

With both mixing and casting taking place in a controlled vacuum, entrapped air is completely withdrawn and the mix forced into every part of the mould (if required, coloured dies and pigments can be added to the casting resin). Once removed from the mould, the replica is stoved to improve its mechanical properties.

For replication by injection moulding, the Nottingham centre uses metal spraying equipment. Most of its mould making requirements are met using alloys with melting points between 200 and 400’C, although the MCP equipment used is capable of liquefying metals with melting points up to 2,500’C. The SLA model, which is used as the master, has molten alloy sprayed onto its surface in progressive layers, as part of a two stage operation.

The fluidity of the alloy enables moulds of all types and complexities to be produced with significant accuracy and a good reproduction of detail. As a result, a mould that could take weeks by conventional toolmaking can, via metal spraying, be ready in a few days.

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Thin Layer Technology

Prototyping company ARRK Europe Ltd, London (Tel: 0181 961 6366), has developed a thin layer technology which removes many of the inaccuracies inherent in mechanical finishing. It claims to be the first company in the world to achieve 0.05mm layer build accuracy.

ARRK’s development team used a combination of subtle mechanical alterations to the set up of its four SLA rapid prototyping machines and proprietary techniques to over come de-wetting. It also called upon its knowledge of resins.

Using this new technology, the company is now able to produce extremely accurate high-quality master parts and tooling. With 0.05mm layers, the build is more precise and all but eliminates the stair-stepping effect. In addition, the need for finishing is dramatically reduced. As a result, the process is ideal for switches, electronic components or any finely detailed part.

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