The effect of casting ring liners on the potential expansion of a gypsum-bonded investment.

Cellulose paper (used wet) and ceramic paper (used dry) are replacing asbestos paper as cushioning ring liners in dental casting. A study was made of the effects of all three of these materials on the setting and subsequent thermal expansion of a gypsum-bonded cristobalite casting investment (W/P = 0.40). Thermal expansion measurements were made on the same specimens that were produced during the setting expansion tests. Control specimens setting against a smooth dry surface showed a total expansion of 1.7%. Specimens setting against dry ceramic liners had similar total expansions, in the range 1.6 to 1.7%. Specimens setting against either of the wet lining materials showed an increased total expansion (in the range 2.2 to 2.3%), by virtue mainly of a large increase in setting expansion. In order for reproducible setting expansion results to be obtained with wet liners, it was necessary to control the amount of absorbed water carefully. Dry asbestos and dry cellulose liners gave higher expansions than pre-wetted ones, since they abstracted water from the mix, reducing its effective W/P ratio (giving a thicker mix), and then functioned as wet liners. These results suggest that, at least as far as potential investment expansion is concerned, wet cellulose liners have an effect similar to that of the traditional wet asbestos liners. Dry ceramic liners give a much lower investment expansion, and when these liners are used, an investment with an increased measured expansion could be an advantage.