The setting expansion is an important property for a phosphate-bonded investment material. This research was undertaken to investigate a test that might be suitable for its measurement when used in a Standard. In the 'Casting-Ring Test', the investment sample is contained in a steel ring and expands to displace a precisely positioned pin. Variables with the potential to alter routine reproduction of the value were investigated. The vacuum-mixer model is a production laboratory variable that must not be ignored and for this reason, experiments were repeated using a different vacuum-mixer located at a second test site. Restraint by the rigid ring material increased expansion, while force on the pin reduced it. Expansion was specific to the lining selected. Increased environmental temperature decreased the final value. Expansion was still taking place at a time at which its value might be measured. However, when these factors are set, the reproducibility of values for setting expansion was good at both test sites (coefficient of variation 14%, at most). The results revealed that with the control that is available reliable routine measurement is possible in a Standard test. The inter-laboratory variable, vacuum-mixer model, produced significant differences and it should be the subject of further investigation.

No abstract

The Casting-Ring Test has the potential to fulfil the requirement for a reliable 'Standard' test to measure the setting expansion of phosphate-bonded investment materials. The purpose of this study was to investigate the reproducibility of the measurements and the value that is produced at 10 test sites. The Casting-Ring Test apparatus, three phosphate-bonded investment products and one ring liner product were sent to the 10 participants, together with a detailed test protocol. Participants were asked to use their normal vacuum mixers. Reproducibility at individual sites was good. However, there were significant differences between test sites for the mean setting expansions, attributed to the range of vacuum mixers employed. From these results, the Casting-Ring Test appears to satisfy the requirements for adoption as an ISO standard test. The effect produced by different vacuum-mixer models would be manifest in any test. Not only does this have implications when determining the value of setting expansion for product information, but such a range of setting expansions will also be reproduced during production of moulds in commercial dental laboratory practice. However, for any specific product and vacuum-mixer combination, the setting expansion will be consistent and will not vary from mould to mould.