The development of thermal spray tooling
Thermal spray tooling is one of a number of technologies which have been developed to satisfy the need for low cost tooling, which is used when prototype components are required in the correct engineering material. In its current state, the technology has a number of fundamental shortcomings. The aim of this portfolio was to address these shortcomings, via a combination of experimental work and technology demonstrators - these are summarised as follows: • An experimental programme aimed to quantify many of the problems associated with thermal spray tooling. A wide variety of tests on thermal spray surfaces was carried out, in order to compare their performance with other 'low cost' tooling techniques. For the first time, tooling shells were produced using the High Velocity Oxy-Fuel (HVOF) technique. This system is capable of producing extremely high quality tool shells, and the technique developed involves the novel use of castable ceramic patterns - the first time a releasable pattern has been developed for this spray system. In addition, it was established that 'hybrid' tooling shells could be produced - these were formed using a combination of arc spray and HVOF layers. The work proved that these hybrid shells could provide substantial performance benefits in terms of wear performance and vacuum integrity, when compared to conventional arc sprayed shells - this benefit was also achieved without significant cost penalty. The programme also investigated the effect of thermal cycling on thermal spray samples - it was shown that repeated cycling at high temperature had an adverse effect on both arc sprayed and HVOF samples - the extent of this effect was very much dependent on the material. • The portfolio includes a technology demonstrator programme, which was carried out for Rover Group to show the potential of thermal spray tooling. The programme entailed the manufacture of a suite of 5 tools for compression moulding of Glass Mat Thermoplastic(GMT). The actual route used for the production of the tooling suite involved many unique features, which had not previously been utilised for thermal spray tooling production. One of the tools is the largest ever produced for compression moulding using thermal spraying, being approximately 4m2, and weighing in excess of 3 tonnes. Due to the compressive stresses involved in the moulding process, conventional resin backing systems were unsuitable for this tooling. It was therefore necessary to use a Chemically Bonded Ceramic (CBC) material, with an exceptionally high compressive strength. However, this material does not adhere to thermal spray surfaces, and it was therefore important to develop a novel fixing method at the interface of the materials. Further to this, in certain cases the use of thermal spraying was precluded by the component geometry - in these cases it was necessary to use the CBC material as the direct tool face. This was the first time that CBC tooling had been used for compression moulding GMT, and it was therefore necessary to develop new post-treatments for this inherently porous material. The moulding operation then entailed the development of specific techniques and conditions for this prototype tooling, which would not generally be used in production - normal moulding conditions for 'production' tooling were therefore inappropriate. Further work will entail materials development, the introduction of automation and development of design rules, specifically aimed at the production of large tooling for the aerospace and automotive sectors - this will be carried out via a successful project submission under the Innovative Manufacturing Initiative (IMI).