Product developers need to reduce the time to market, but this should not be at the expense of product quality. Paul Stevens reports on three processes for producing aluminium, zinc and magnesium parts either for prototyping or, in some cases, production.
When competition is fierce, the consequences of bringing a product to market late can be severe, with significant loss of market share and future profits. It is no surprise, therefore, to see the expanding use not just of rapid prototypes for investigating the form and fit of components but also components that are fully functional. Such rapid prototypes can provide high-strength, dimensionally accurate parts with all the features, operating capabilities, appearance and performance of the finished part. Because they can be delivered quickly and affordably, these rapid prototypes offer designers the means to compress development times and be more confident that the product will perform as intended for its required lifetime.
Rapid functional prototyping can, in certain circumstances, be extended so that the same tool that provided the initial prototype can also be used for small batches of pre-production parts or even, where low volumes are concerned, the initial production run. Indeed, in some cases the one tool may even be used for the entire production run.
This step enables companies to move from design to initial production much more quickly, thereby reducing the time required to progress from design sign-off to getting products to market. Depending on the process chosen, functional prototyping can cost-effectively deliver as few as 50 parts or as many as 200000 to give the company adequate time to get tooling in place for full-scale manufacture.
Danish company Temponik specialises in providing fast, functional prototypes cast in aluminium, magnesium and zinc using three processes: Tempoform, Tempocast and Temptool.
Tempoform is Temponik's investment casting technology. Requiring no production tools, it is a quick and low-cost route to producing functional prototypes, with parts delivered in 10-12 working days - and sometimes faster, with a turnaround of three to five days possible (Fig. 1).
Instead of conventional tooling, a 3D file is converted into two negative moulds, produced on a 3D printer. After baking, the moulds are assembled and the molten aluminium, magnesium or zinc is poured into the cavity. There are no special design rules to consider, and parts can be produced with complete design freedom. Parts can be designed without draft angles, and the parts do not need a mould split line. The Tempoform process is cost-effective for parts in sizes from 10 x 10 x 10 mm up to around 340 x 340 x 600 mm, with the limit being the maximum mould size that can be created with the 3D printer. For each casting, a new set of moulds is required, making the process ideally suited to producing one to five prototypes, but Temponik says the process could be used to produce up to 800 parts cost-effectively.
Tempocast is Temponik's plaster casting process for producing prototypes in a quality said to be almost identical to die casting in terms of tolerances and surface finish (Fig. 2). Delivery of prototypes is quoted as within two to four weeks, depending on size and complexity. However, Temponik says it has been able to produce parts within just five working days.
Tooling begins with the production of plastic mould halves using high-precision CNC machining. These mould halves are then converted into a set of positive rubber moulds that serve as the production tool. Using this soft rubber tool, an almost unlimited number of plaster moulds can be made (Fig. 3).
For the casting process, the negative mould halves are joined and the molten metal is poured into the cavity. Adding a number of cores makes it possible to produce even the most complex of parts. After casting, the parts are cleaned and post treatments can be applied. Designers need to be aware that some design rules need to be followed. Typically, in order to remove the dried plaster moulds from the soft rubber tooling, the mould needs to have draft angles. Furthermore, the part needs to have a mould split. But these are the no different from the design rules in standard pressure die casting. There is some design freedom, though, in that the same tooling can be used for aluminium, magnesium or zinc parts.
Along with providing quick delivery of high-quality functional prototypes, the Tempocast process is suitable for small-batch and pilot production runs in quantities from 50 up to 1500 parts, with part sizes ranging from 10 x 10 x 10 mm up to 1500 x 1500 x 1000mm.
Each method therefore has its own advantages and disadvantages. Primarily they are differentiated by tooling costs and delivery times. Beyond that, if a project is balancing near the break-even between the two processes for price as well as delivery times, or if a certain project has a particular demand, then other factors come into play. It is also important to be aware of the features that can or cannot be achieved using either process, such as surface roughness or complexity.
A recent addition to Temponik's suite of processes is Tempotool, a casting process that gives prototypes of die cast quality within four to six weeks and that can subsequently deliver low-volume production quantities over a year or more. Temponik says the set-up costs are considerably lower than the costs for pressure die casting. Using a special type of graphite as the moulding material reduces both the tooling costs and the delivery time in comparison with traditional aluminium and steel tooling materials. The process is suitable for production in batches of 300 to 2000 parts.
For many applications, designers looking for a fast functional prototype will be considering one Temponik process over another. Often, though, combinations of these processes will be employed at different stages in a part's life cycle. To illustrate this, Temponik points to work it undertook as part of a development project for a new navigation system, carried out in conjunction with a large American industrial design company for an automotive end customer. Temponik was brought in fairly late in the design phase, when a prototype was needed to test the design prior to production. Up to that point the design agency had relied on prototypes machined form solid, but now required a cast part to test the housing in relation to other components. Tests for strength, heat absorption, flexibility and other functions were all necessary before the design could be finalised. A set of Tempoform casts was therefore produced to enable these tests to be carried out.
Four months later, the tests were completed and the design finalised. Now the design agency's priority was to get products to the customer for feedback. A small series was produced using the Tempocast process, allowing feedback to be received before committing to pressure die cast tooling for the product launch. Finally, when the design had been signed off, Temponik was able to produce the pilot series until the die casting tools were ready for volume manufacture.
