Product development cycles have reduced significantly with the increasing use of Computer Aided Design (CAD). Solid modelling used with techniques such as finite element analysis and mechanical simulation has significantly reduced the cost and time scale of product development in a wide range of engineering sectors.
A three-dimensional rendered computer image is an excellent method of showing the details of the finished item. However the demand for actual solid product prototypes is increasing.
Uses for Prototypes
Prototypes have a range of uses:
• Product visualisation
• Marketing tools
• Assessment of form and fit
• Functional testing
• Proving production processes
• Short run production cycles
Advantages of Rapid Prototyping
The traditional prototyping routes are costly, inflexible and add considerable cost to the product development cycle. However, the application of Rapid Prototyping techniques and Computer Aided Manufacturing (CAM) processes such as CNC machining has shown the potential to improve the process.
Commercial Rapid Prototyping
Commercial Rapid Prototyping systems produce three-dimensional models by adding small amounts of raw materials selectively to grow the final part.
Rapid Tooling Processes
Rapid Tooling processes use CAD/CAM techniques to make tools and moulds for product assessment, short production runs and full production. The development cycles are considerably shortened and more economic.
Types of Rapid Tooling Processes
Rapid tooling approaches fall into two categories:
• Direct routes, which use the CAD file to produce the tool in the final material.
• Indirect routes, which use a rapid prototyping model as master pattern or case to make the moulds by established routes.
Guided by the CAD file, selective laser sintering (SLS) machines can form green parts from polymer coated metal powders by fusing the polymer coat. The parts are then heated to remove the binder and sintered to produce a porous metal part, which is infiltrated with liquid copper or bronze to make a useable tool.
Tools can be made in a range of materials, including steel and copper polyamide. Manufacturers claim that steel moulds can be used for plastic injection moulding with lives in excess of 100,000 parts, and pressure die-casting capable of making several hundred parts in magnesium, zinc or aluminium.
Copper polyamide is a metal-plastic composite, which eliminates the need for the intermediate sintering stage and can reduce the mould production time to one day. The mould can then be used to produce several hundred parts in common plastics such as polyethylene, polypropylene and ABS at cycle times similar to those of production values.
Porous metal moulds can be made by a CAD/CAM route for use in ceramic processes such as pressure casting. The combination of these two techniques reduces the product development cycle by 75% and can be used for both product approval and manufacture.
Plaster moulds can be made directly by CNC machining of plaster bodies. The resulting mould is suitable for bench casting prototypes for product approval. Three-dimensional printing can also be used to grow moulds from plaster powders, however the finish of the mould surface is poor and the life of the mould is limited by its low strength.
Three-dimensional printing uses a CAD file to define the cavity and produce a shell for pressure casting. The process uses alumina powder and provides the ability to build shells for investment casting without waxes or tooling.
Sand casting moulds can be built using phenolic-coated foundry sand as the fusible base material. Sand moulds can be built directly with integral cores, eliminating the need for sand patterns and core boxes. The system is limited to the envelope of the sintering machine, typically, 700mm x 400mm x 400mm, but has been used to produce prototype automotive castings.
Patterns for Moulds and Tools
Rapid prototyping models can be used as the master models for several applications, including:
• Silicone tooling to make wax models for investment casting or cast plastic parts
• Porous plastic moulds for pressure casting ceramics
• Case moulds for bench casting
• Patterns for sand casting metals
• Wax rapid prototyping models for lost wax investment casting
Models have been coated with metal and then reinforced with epoxy resin to produce moulds for short production runs of injection moulded parts. Coating techniques include spraying, electrodeposition and vapour deposition. Nickel is the most commonly used metal.