The Effect of Casting Ring Liners on the Potential Expansion of a Gypsum-bonded Investment

Abstract

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 (WIP = 0.40). Thennal 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. 7o. 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 ofa 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 WIP 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.J Dent Res 67 (11):1366(11): -1370(11): , November, 1988 Introduction.In most dental casting techniques, the investment sets, and is then heated, in a heat-resisting alloy casting ring. The need for a soft ring liner, to reduce the restriction to investment expansion, was first suggested by Souder, who recommended asbestos paper for the purpose (Hollrnback and Rhoads, 1960;Hollenback, 1962Hollenback, , 1964. Since dry asbestos readily absorbs water, the ring liner was pre-wetted to prevent its absorbing water from the unset investment mix. The technique of lining the casting ring with wet asbestos was described by Taylor et al. (1930). From then until comparatively recent times, the use of a wet asbestos liner was a standard procedure.It was soon recognized that the wet ring liner produced an increased setting expansion of the investment, by virtue of socalled "hygroscopic expansion". Scheu (1935) reported that an investment mix setting in contact with wet asbestos paper had its setting expansion raised from 0.25 to over 1%.He found that dry asbestos paper also increased the setting expansion, since it was rapidly wetted by absorption of water from the unset investment mix. This enhanced setting expansion has generally been accepted, though some workers, preferring a lower setting expansion, have advocated the use of a dry, waterproofed asbestos liner (Fusayama, 1959a(Fusayama, , 1959bHanari, 1967), or have recommended dispensing altogether with the rigid metallic ring and its liner (Finger and Jorgensen, 1980).Received for p...

The effect of casting ring liner length and prewetting on the marginal adaptation and dimensional accuracy of full crown castings

“…The finding from this research reconfirmed the observation of Earnshaw et al .,[27] who reported that the investment setting in dry condition improves better marginal adaptation and less distortion.…”

Accuracy of Ringless Casting and Accelerated Wax‐Elimination Technique: A Comparative In Vitro Study

“…The above problems can be partially overcome with the use of a ring liner . The liner provides the needed resilience against the rigid metal ring and permits setting and thermal expansion of investment, allowing mold cavity enlargement . However, expansion provided by the liner still remains anisotropic in nature, with the axial component being greater than the lateral/radial component .…”

“…3,22 The liner provides the needed resilience against the rigid metal ring and permits setting and thermal expansion of investment, allowing mold cavity enlargement. 6,21,[25][26][27][28] However, expansion provided by the liner still remains anisotropic in nature, with the axial component being greater than the lateral/radial component. 5,28,29 The thermal expansion of the metal casting ring also undergoes expansion during heating and may additionally account for the increased marginal gaps obtained in the metal ring casting groups (IIA and IIB) in the present study.…”

“…6,21,[25][26][27][28] However, expansion provided by the liner still remains anisotropic in nature, with the axial component being greater than the lateral/radial component. 5,28,29 The thermal expansion of the metal casting ring also undergoes expansion during heating and may additionally account for the increased marginal gaps obtained in the metal ring casting groups (IIA and IIB) in the present study. 6,7,21,23,30 In the ringless technique, a soft plasticized PVC or elastomeric ring is used to support and confine the investment material until it attains sufficient green strength.…”

The Fit of Gold-alloy Full-crown Castings Made with Pre-wetted Casting Ring Liners

“…Investment expansion.-Laboratory measurements of the investment's potential expansion were made on specimens which set against each of the pre-wetted lining materials; measurements were made by continuous readings on the one specimen during setting and then duringheatingto 650'C (Earnshaw, 1988). Control measurements were also made on specimens setting without exposure to additional water (trough lined with PTFE).…”