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

Our plain-English summary of The effect of casting ring liners on the potential expansion of a gypsum-bonded investment, by R. Earnshaw (Journal of Dental Research, 1988).

The study

• Author: R. Earnshaw
• Journal: Journal of Dental Research
• Published: 1988;67(11):1366-70
• Cited by: 22 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

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 (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.

In plain English

This University of Sydney laboratory study tested how casting ring liner materials affect the potential expansion of a gypsum-bonded investment, the mould material for dental castings. Specimens were set in a lined laboratory trough, not in actual casting rings. It did not study health effects; its remarks on asbestos health risks cite other publications. Cristobalite Inlay investment was set against asbestos paper, cellulose paper (wet or dry) and five dry ceramic liners, with wax as a control. Setting and thermal expansion were measured, with at least five tests per condition. Control specimens expanded 1.68% in total. Wet asbestos liners raised it to 2.23% (minimum water) and 2.32% (maximum water). Dry asbestos gave 2.46%. Pooled results for pre-wetted asbestos (2.27%) and pre-wetted cellulose (2.20%) were not significantly different. Dry ceramic liners gave totals of 1.60 to 1.74%. The author concluded that wet cellulose liners affect potential expansion much as traditional wet asbestos liners do. With ceramic liners, the author added, an investment with higher potential expansion, when measured under dry conditions, would be desirable. The author noted that liner compressibility was not studied.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory materials experiment (dental materials testing) in which investment specimens were set in a liner-lined triangular trough; no people or health outcomes were studied
• Who was studied: Specimens of Cristobalite Inlay investment (Sybron/Kerr), mixed at a water/powder ratio of 0.40, with at least five tests per condition; Department of Prosthetic Dentistry, University of Sydney, Australia
• Exposure: Casting ring liner materials. Asbestos paper (Strip Asbestos, Sybron/Kerr; five asbestos materials in preliminary tests; asbestos fiber type not stated) and cellulose paper were each used with minimum water, maximum water and dry. Five ceramic liners were used dry. Dental modelling wax served as the dry control.
• Main results:
  • In preliminary tests, all specimens set against the five wet asbestos liners had much greater setting expansion than the controls, with considerable variation between asbestos products
  • Asbestos liner with minimum water: setting expansion rose to 1.11% and thermal expansion fell to 1.12%. Total expansion was 2.23%, compared with 1.68% for dry control specimens, a difference reported as highly significant (p less than 0.0005).
  • Asbestos liner with maximum water: total expansion rose further, to 2.32%
  • Asbestos liner used dry: total expansion was 2.46 plus or minus 0.11%
  • Pooled results for pre-wetted asbestos (2.27%) and pre-wetted cellulose (2.20%) were not significantly different
  • Dry ceramic liners: mean total expansions ranged from 1.60% (Kaoliner) to 1.74% (Triton Kaowool)
  • Grand mean total expansion was 2.23 plus or minus 0.08% for all wet liners and 1.69 plus or minus 0.06% for all ceramic liners, a difference reported as highly significant (p less than 0.0005)
• The authors' conclusion: The author concluded that, as far as potential investment expansion is concerned, wet cellulose liners have an effect similar to traditional wet asbestos liners. With ceramic liners, an investment with increased potential expansion, when measured under dry conditions, would be desirable.
• Limitations the authors state:
  • The compressibility of ring liners, which may affect actual mould expansion in practice, was not studied
  • Laboratory measurements show potential expansion, not necessarily the mould expansion that occurs around a wax pattern in a lined ring

Related studies

• The fit of gold-alloy full-crown castings made with pre-wetted casting ring liners — E.F. Morey, R. Earnshaw (1992)
• Distortion of a MOD pattern caused by the setting and hygroscopic expansion of the investment in the casting ring. II. Influence of W/P ratio, pattern position in the ring, and the condition of the asbestos lining — H. Ohno, O. Miyakawa, et al. (1982)
• Dimensional accuracy of small gold alloy castings. Part 4. The casting ring and ring liners — E.F. Morey (1992)
• An investigation of the potential biohazards associated with asbestos-substitute casting ring liners — G.J. Tulachka, W.A. Brantley, et al. (1987)
• Full-crown castings made with water-saturated and dry casting ring liners — F. Keyf, G. Erman (2000)
• Release of asbestos fibers during casting ring liner manipulation — D.R. Davis (1995)

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