Mould-making gets help from smart software

Dave Wilson looks at how software has helped re-establish the nickel carbonyl vapour deposition process as a viable entity

The process of making moulds has traditionally been either to machine the mould or to cast it. In the casting process, heated material in a liquid state is poured around a pattern. The material cools into a solid state and the mould is then removed from the pattern. Unfortunately, both these techniques are time-consuming and require large amounts of material.

Mirotech, a Canadian company has developed a technology for forming pure nickel moulds and shapes that have very high resolution surface replication, excellent wall thickness uniformity and low residual stress levels. These moulds have already been used for various tooling applications in the plastics, automotive, defence and aerospace industries.

Mirotech uses a technology called nickel vapour deposition to create moulds. Nickel Carbonyl Vapour Deposition (NVD) is claimed to be less expensive than traditional tooling and replicates a pattern with very good surface definition. Although there are other methods for creating moulds, such as autocatalytic and electrochemical deposition methods, the autocatalytic method is not precise enough for toolmaking and the electrochemical method is expensive and presents some environmental hazards which manufacturers must overcome.

Deposition rates 20 times faster than other technologies are claimed for the process, which has been used to produce a variety of parts including body panels and a petrol cap for the US Dodge Viper, an exhaust gas manifold for Chrysler, wing forms for Boeing and optical reflectors for 3M.

How it works

NVD is a metal forming process based on the growth of a metal from gaseous vapours. The NVD process uses nickel carbonyl, which breaks down into solid metal at a high rate of decomposition. By heating a mandrel or substrate and having the gas flow over the mandrel in a sealed chamber, an exact nickel duplicate of the mandrel is obtained.

Extreme fidelity of the surface replication is a direct result of the way the process builds the nickel lattice atom by atom. No intermediate electroconductive layer is required for the NVD process to occur.

Deposition rates between 0.004in/h and 0.04in/h can be obtained on mandrel surfaces held between 110 degrees C and 190 degrees C. The process’ high deposition rate make it very appealing to complex net shape manufacturing.

The size of the nickel dendritic crystalline structure can be suitably controlled by changing the operating parameters of the process and by the doping process. The doping is achieved by the addition of certain gaseous species in the main gas stream at the appropriate process location. The net effect is that the mechanical properties of the nickel deposit can be varied during the process in a gradual fashion ensuring no delamination in the layered material.

The NVD process is a thermally activated chemical vapour deposition process. The substrate or mandrel temperature is one of the critical parameters. It influences both the deposition rate and accurately controls the residual stress level in the deposited nickel. This feature sets the process apart from the traditional electrolytic type of deposition in terms of repeatability and quality of deposit material properties.

New software based tools are extensively used in the analysis of the mandrel surface temperatures and in particular, transient phenomena associated with the process start up and shut down.

In order to accommodate the requirements of the NVD technology, a US patent was issued in November 1995 for the development of a composite mandrel. Initially, an aluminium machined mandrel was used in some applications and was quite successful. However, for large tools and for tools where the starting point is a model, the aluminium mandrel was not commercially viable.

One of the most important features of the NVD composite mandrel is the fact that CNC machining can be eliminated. In some markets, like the automotive aftermarket, the tooling expense is critical. The NVD composite mould can be made from the supplied plastic part in shorter time and without the traditional CNC machining process.

Software is the key

In the development of its new mandrel, Mirotech used Algor software as a tool to ensure some specific design parameters were met. The material itself had to be chemically inert and thermally stable at all temperatures of the process. In addition, the coefficient of thermal expansion was required to match closely with that of nickel. Furthermore, the thermal conductivity of the mandrel had to be high enough to facilitate the even heating of the surface.

Engineers at Mirotech used Algor’s stress analysis software to learn if the mandrel could withstand any deformation that could occur in the NVD process. The analysis indicated any areas of weakness in the design so that engineers could strengthen the mandrel by modifying the design to minimise deformation.

Steady state heat transfer analysis was also used to give a temperature profile of the mandrel during operation. The analyses showed areas of uneven heat in the mandrel, especially the areas at its surface, where temperature is most important. Based on this information, the engineers could move the heat generating elements in the mandrel to different locations for a more evenly heated mandrel.

Similar to the steady-state analysis, a transient heat transfer analysis yielded a temperature map of the mandrel as a function of time. This analysis showed how the mandrel would react while it is heating to the temperature range of operation. If the mandrel heats up too quickly or too slowly it may weaken or crack, making it unsuitable for use in NVD.

There are several benefits of using the NVD process for making moulds. First, nickel forming on the mandrel during the NVD process does not damage the mandrel, thus making it possible to create multiple moulds from the same material. And, because nickel moulds are so precise, they can be used on designs with small external angles and even for optical applications. Nickel moulds can be repaired by TIG welding or brazing and the moulds themselves can be altered, if desired, during the NVD process.

At the present time, Mirotech claims to be the only company in the world engaged in the development of NVD for manufacturing nickel shapes and moulds. But last year, the company concluded a technology transfer agreement with an electroformed tooling company, Galvanoform in Lahr in Germany, that will bring the NVD process closer to design engineers working in Europe.

{{MirotechTel: Canada +1 (416) 747-8681Enter 560}}

{{GalvanoformTel: Germany +49 782 1947 101Enter 561}}