Master artefact
An artefact was formed using a series of precision ruby spheres inserted into an aluminium 'jaw'. The distances between the spheres was measured using a highly accurate co-ordinate measuring machine (CMM).
The artefact was designed so that impressions could be taken from it, whilst stone models could also be poured directly onto it.
Impression material performance
A series of impressions was taken from the artefact using different materials. These impressions were then measured using the CMM and the positions of the spheres were compared with those on the artefact. This highlights the distortion of the impression material over the sorts of spans seen in bridge restorations.
Each colour on the plot represents a different material. The errors are random and unpredictable.
Die stone material performance
A series of six gypsum based dies was made directly from the master artefact and allowed to set. Errors in the positions of the spheres were measured and plotted as before.
Expansion is almost isotropic, meaning uniform expansion in all directions.
Conclusions
- The random behaviour of impression materials means that its is impossible to compensate for an impression shrinkage with an expansion in the model. This leads to an even greater spread of errors.
- The best combination can only be found by selecting the materials with the least deformation and the highest accuracy.
This is important for larger restorations such as bridges, where a precise fit must be achieved over a long distance.