Why is a metal stock tray preferred for an RPD master impression?

A metal stock tray is preferred for an RPD master impression primarily because it provides good dimensional stability and accuracy. This stability is crucial in creating a precise negative imprint of the oral structures, which is essential for the accurate fitting of the removable partial denture.

Metal trays are constructed from materials that do not easily warp or change shape, ensuring that the impression maintains its intended dimensions during the impression-taking process and while the material sets. This characteristic is critical because any distortion could lead to inaccuracies in the final denture, impacting comfort, fit, and function.

Additionally, the rigidity of a metal stock tray supports the impression material properly, allowing for even distribution of pressure while taking the impression. This prevents any potential displacement or deformation that might occur with more flexible materials.

Other options like cost-effectiveness, flexibility, and ease of cleaning do not address the fundamental qualities required for an accurate master impression. While those factors may be beneficial, they do not provide the same level of precision necessary for the successful fabrication of an RPD.