Investment Materials Prepared by Dental Materials Department Yenepoya

Investment Materials Prepared by: Dental Materials Department Yenepoya Dental College, Yenepoya University, Mangalore.

Objectives l l Students can select the type of investments that is compatible with the type of alloys used. Students know why/how something has gone wrong when they work with the investment. (and hopefully…know how to correct it!) 2

Metal Casting l Lost wax technique (Taggart, 1907) A crucible former B sprue C wax pattern D investment material E ring liner G thickness of investment at the top 3

Metal Casting Wax pattern shrinkage Metal shrinkage (~ 1. 5 -2%) Wax pattern expansion Investment expansion • Setting • Hygroscopic • Thermal Investment needs to expand 1. 5 to 2 % before the casting is made to compensate for metal expansion during melting. n 4

“Ideal” Investment l l l Expansion: to compensate the shrinkage of metal Should not decompose at high temperature Strength at various temperatures Porous (to allow the escape of gases) Consistent casting temperatures 5

“Ideal” Investment (cont. ) l l Easy to manipulate Smooth castings Easy to break out Inexpensive 6

Composition l l l Refractory • • Heat resistant silica based materials Thermal expansion Binder • • Gypsum, Phosphate compound, Silica compound To hold refractory material particles Modifiers • • Na. Cl, boric acid, potassium sulfate, graphite, copper, . . etc. reducing agent, accel/retard, increase expansion 7

Types of Investment (classified by binder) l l l Calcium sulfate-bonded or Gypsumbonded Phosphate-bonded Silica-bonded 8

Gypsum-bonded investment

Composition l Binder: a-hemihydrate form of gypsum (25 -45%) • • • l l +H 2 O Ca. SO 4. 2 H 2 O Shringkage when heated Not to be heated above 700 o. C decomposition Si. O 2 a b 1. 5% 1. 2% Refractory: Silica (Si. O 2) • • Quartz, Cristobalite The density decreases as the a form changes to the b form when heated, with a resulting increase in volume increase in the linear expansion. a b Modifiers: • coloring agent, reduction agents e. g. carbon, Cu 10

ADA Specification No. 2 l For gypsum-bonded casting investments (suitable for casting gold alloys) • Type I: For casting inlay-crown • Type II: For casting complete and partial denture 11

Expansion l Setting Expansion l Thermal Expansion • Normal setting expansion • Hygroscopic setting expansion 12

Setting Expansion (normal vs. hygroscopic) l l Stage I: initial mix Stage II: crystals of dihydrate are formed • • l l Lt: water around the particles is reduced by the hydration and the particles are drawn more closely together by the surface tension action of the water Rt: hydrated water is replaced Stage III: crystals grow • • Lt: water is decreased, particles are drawn together Rt: water is replaced, crystals grow freely Stage IV and V 13

Normal Setting Expansion (0. 3 -0. 4%) l Occurs from Ca. SO 4. 1/2 H 2 O Ca. SO 4. 2 H 2 O l A mixture of silica and gypsum hemihydrate results in setting expansion greater than that of the gypsum alone. • l The silica particles probably interfere with the intermeshing and interlocking of the crystals as they form. The thrust of the crystals is outward during growth, and they increase expansion. Can be regulated by retarders and accelerators. 14

l l As the investment sets, it eventually gains sufficient strength to produce a dimensional change in the wax pattern as setting expansion occurs. The softer wax is more easily moved by the expanding investment. select appropriate material for a pattern 16

Hygroscopic Setting Expansion l l l Occurs when the gypsum product is allowed to set under or in contact with water Can be obtained from • • • Hygroscopic setting expansion Water immersion technique Water added Wet ring liner Greater magnitude than the normal setting expansion (> 6 times) Normal setting expansion 17

Factors to Control Normal and Hygroscopic Setting Expansion l l Composition • • Proportional to the Si. O 2 content The finer the particle size of the Si. O 2, the greater the hygroscopic expansion. Water: Powder Ratio • Higher W: P less expansion Spatulation • Insufficient spatulation decrease expansion Shelf life of the investment • Older investment lower expansion 18

l l l Time of Immersion • Delayed immersion decrease expansion • the walls of the investment container, or the walls of a wax pattern Confinement Water bath / Amount of Added Water • • Softens and expands wax pattern, allows maximal hygroscopic expansion The magnitude of the expansion is in direct proportion to the amount of water added during the setting period until a maximum expansion occurs. 19

Linear hygroscopic setting expansion vs. Amount of water added 20

Thermal Expansion (1. 0 -1. 5%) l l l Directly related to the amount of Si. O 2 present and to the type of Si. O 2 employed (quartz vs. crystobalite). The effect of Si. O 2 will balance the contraction of the gypsum during heating. The maximum thermal expansion is attained at a temperature < 700 o. C. Si. O 2 21

Factors to Control Thermal Expansion l l Thinner mixed less thermal expansion The sufficient amount of Si. O 2 to prevent any contraction during heating can weaken the investment. • Chemical modifiers, e. g. , sodium, potassium and lithium chlorides, are added to eliminates the contraction caused by the gypsum and increases the expansion without the presence of an excessive amount of Si. O 2. 22

Phosphate-bonded investment

Composition l l l Filler (refractory) = quartz or cristobalite or both (80%) Binder = magnesium oxide and acid phosphate Liquid = colloidal silica suspension 25

l Setting Reactions: • NH 4 H 2 PO 4 + Mg. O NH 4 Mg. PO 4 +H 2 O • The reaction is not simple and changed on heating. 26

Setting and Thermal Expansion l l Increased expansion and strength is obtained by adding a combination of different silica particle size. (using a colloidal silica solution instead of water) Modified by • • altering the liquid: powder ratio or decreasing the concentration of the special liquid. 27

Working and Setting Time l l Markedly affected by temperature • The warmer the mix, the faster it sets. Increased mixing time and mixing efficiency result in a faster set and a greater rise in temperature. In general, the more efficient the mixing, the better the casting in terms of smoothness and accuracy. Mechanical mixing under vacuum is preferred. 28

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Other Properties l Increasing the special liquid: water ratio used for the mix markedly enhances casting surface smoothness but can lead to oversized extracoronal castings. 30

ADA Specification No. 42 l Specifies two types of phosphate-bonded investments for alloys having a solidus temperature above 1080 o. C • Type I: For inlays, crowns, and other fixed • restorations Type II: For partial dentures and other cast, removable restorations **Can also be used with alloys having casting temp. below 1080 o. C** 31

Soldering Investments

l Are designed to have lower setting and thermal expansions than casting investments • l So the assembled parts do not shift in position during the setting and heating of the investment Do not have as fine a particle size as the casting investment because the smoothness of the mass is less important. 33

Examples of Investment Materials

Summary l l l Ideal investment? Composition of investment Types of investment, properties and material selection Types of investment expansion & factors to control Soldering investment 36

End of Investment Materials