Investment materials .pptx

Investment materials

1. Investment materials Dr Lekshmy AR 1 st year MDS

2. Contents Definitions Requirements of investment materials Gypsum-bonded investment Phosphate-bonded investment Silicate-bonded investment Implication of material science during casting Other investment materials Summary References

3. I nvesting : T he process of covering or enveloping, wholly or in part, an object such as a denture, tooth, wax form, crown, etc., with a suitable investment material before processing, soldering, or casting (GPT 9) Definitions Dental casting investment : A material consisting principally of an allotrope of silica and a bonding Agent; the bonding substance may be gypsum (for use in lower Casting temperatures) or phosphates and silica (for use in higher Casting temperatures ) (GPT 9)

4. R equirements of an investment material Must reproduce precisely the details form of the wax pattern Must provide sufficient strength to withstand the heat of burnout and the impact and the pressure of cast molten alloy Must maintain the integrity at higher temperatures and should not decompose to give off gases Must expand sufficiently to compensate for the solidification shrinkage of the alloy Easily manipulated and setting time should be less Should be inexpensive Biological safety Fundamentals Fundamentals of fixed prosthodontics, 3 rd edi.Herbert T. Shillingburg

5. Composition An investment is a mixture of three distinct types of materials Refractory material Binder material A dditives .

6. Refractory material It is usually a form of silicon dioxide, such as quartz, tridymite, or cristobalite, or a mixture of these. Binder material As the refractory material alone do not form a coherent solid mass, some kind of binder is needed.

7. Common binder used are : α-calcium sulfate hemihydrate Phosphate E thyl silicate

8. Other chemicals S odium chloride, boric acid, potassium sulfate, graphite, copper powder, or magnesium oxide are added in small quantities.

9. Classification Types Uses 1 Gypsum bonded investment Gold alloy inlays, onlays, FPD 2 Phosphate bonded investment Base metal alloys FPD & metal ceramic alloys 3 Silica bonded investment Co-Ni alloys, base metal alloy partial dentures.

10. Gypsum bonded investment material They are the mold materials used in the casting of dental gold alloys with temperature below 700 C. Used for conventional casting of gold alloy inlays, onlays, crowns and fpd.

11. ADA specification-2; divided them further into Type I- for casting inlays/crowns. Mode of expansion: thermal Type II-for casting inlays/crowns Mode of expansion : hygroscopic Type III-for partial dentures with gold alloys

12. Composition Refractory Crystalline polymorphs of silica (quartz or cristobalite) 55-75% Silica is added to provide a refractory component during the heating of the investment and to regulate the thermal expansion.

13. Binder Alpha hemi hydrate form of gypsum(25-45%) Strength of investment depends on amount of binder present.

14. Additives - (4-7%) Used are Reducing agents Modifying chemicals Coloring matter Reducing agents : they reduce any metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the alloy enters mold. Eg– Copper

15. Modifying chemicals : They regulate setting expansion and setting time and also prevent shrinkage of gypsum when heated above 200ºC . Eg– Boric acid, Soluble salts of alkali or alkaline earth metals

16. Setting reaction Theories to explain setting reaction 1.Colloidal theory 2.Hydration theory 3.Dissolution –precipitation theory Among these dissolution-precipitation theory most accepted

17. Dissolution of calcium sulfate hemihydrate Formation of saturated solution of calcium sulfate Aggregation of calcium sulfate dihydrate Precipitation of dihydrate crystals The crystallization of calcium sulfate dihydrate

18. Properties Setting time According to ANSI/ADA Specification No. 2 for dental inlay casting investment, the setting time should not be shorter than 5 min or longer than 25 min. Usually, the modern inlay investments set initially in 9 to 18 min. Should allow sufficient time for mixing and investing the pattern.

19. Factors controlling setting time Mixing time Water powder ratio Temperature Modifiers

20. 2.Setting expansion Normal setting expansion: the expansion takes place when the material sets in air Hygroscopic setting expansion: when setting occurs under water. Thermal expansion: when it is affected by heat Purpose: enlarging the mold to compensate for the casting shrinkage of the gold alloy

21. Normal Setting Expansion Liner dimensional change as the investment sets A mixture of silica and hemihydrate gypsum results setting expansion greater than that of gypsum product used alone Si lica particles interfere with the intermeshing and interlocking of crystals resulting in outward thrust of crystals resulting in expansion Maximum setting expansion – 0.5%

22. b) Hygroscopic setting expansion Greater in magnitude than NSE. The gypsum product is allowed to set in contact with water for mold expansion. Curve A-NSE; Curve B-Hygroscopic expansion Water is added 5 min after beginning of the mixing

23. The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment The increased amount of expansion is because the water helps the outward growth of crystals The investment should be immersed in water before the initial set is complete. ADA sp no 2 for such type 2 investments require minimum setting expansion in water of 1.2% and maximum 2.2%.

24. Factors affecting expansion Composition : more finer silica particles -more HSE. α hemihydrate greater HSE than β hemihydrate. W:P ratio -lower W:P ratio-more HSE Spatulation : more mixing time- more HSE. Time of immersion : immerse in water before initial set more expansion than immersion is delayed beyond the time of initial set.

25. Confinement of the investment by the walls of the container or the wax pattern reduces HSE. Added Water : HSE directly related to amount of water added during the setting period until a maximum expansion occurs. Shelf life : fresher investment-more HSE

26. c) Thermal setting expansion The thermal expansion is directly related to the amount and type of silica present. Contraction of gypsum is balanced when quartz content increases to 75%. Type 1 investments should have thermal expansion of not less than1% and not greater than 1.6%.

27. Type II investments should have thermal expansion of between 0% to 0.6% at 500°C. Maximum thermal expansion should be attained at temperature not higher than 700 degree centigrade Factors affecting thermal expansion W/P ratio-more water less expansion Related to the amount and type of silica used Chemical modifiers : NaCl, KCl, LiCl increases TE

28. 3.Setting contraction When an investment is cooled from 700°C, it contracts but less than its original dimension. On reheating it expand thermally to the same maximum reached before. Precaution – Investment should not be heated a second time because internal cracks may develop.

29. 4.Compressive strength According to ADA sp no 2 the compressive strength should not be less than 2.4mpa when tested 2 hr after setting 5.Fineness Surface roughness of the casting and setting time depend upon the fineness of the material. Fine Silica- more hygroscopic expansion.

30. 6.Porosity The material should be porous to allow escape of air from mold space while casting. T he common method of venting the mold is through the pores of the investment.

31. Storage Should be stored in airtight and moisture proof containers. Purchase in small quantities .

32. Phosphate bonded Investment material Most palladium and base metal alloys used for partial dentures and porcelain fused to metal restorations have high melting temperatures. They should be cast at a mold temperature higher than 700ºc.

33. To withstand these high temperatures ,molds require different types of binders such as phosphate compounds. They are used in construction of high melting temperature dental alloys. Soldering and porcelain veneering

34. Types ADA Sp No.42 Type 1 For casting of inlays crowns and other fixed restorations Type 2 For casting of removable partial dentures

35. Composition Refractory materials ( Approximately 80%) Silica in quartz , cristobalite or a mixture of two . Purpose To provide high temperature thermal shock resistance To provide high thermal expansion.

36. Binder (<20%) Magnesium oxide (base) and a phosphate (acid) Originally phosphoric acid was used but mono ammonium phosphate has replaced it. It can be incorporated in powder form

37. Modifiers Carbon is often added -clean casting. F acilitates easy divesting of casting and mold. Generally added when casting alloy is gold. Not used with Ag-palladium alloys or base metal alloys as; palladium reacts with carbon at temperatures above1504 C carbon embrittles the alloys.

38. It is available as two component systems P owder which contains refractory materials, binders and modifiers. Aqueous solution stabilized with colloidal silica Colloidal silica suspension facilitate greater expansion of the investment which can compensate the greater casting shrinkage of alloys used in Metal Ceramic & newer gold alloys.

39. Setting reaction The chemical reaction that causes the investment to set and harden is NH 4 H 2 PO 4 +MgO+5H 2 O→NH 4 MgPO 4 +6H 2 O The product formed is predominantly colloidal multimolecular (NH 4 MgPO 4 ·6H 2 O) n aggregate around excess MgO and fillers.

40. Properties Compressive strength Type 1- 2.5 Mpa Type 2- 3 Mpa Thermal expansion 0.8% when 50:50 mixture of liquid and water

41. Working & setting time Temperature Warmer the mix faster it sets The setting reaction also liberate the heat and accelerates rate of setting Mixing time I ncreased mixing time and mixing efficiency result in faster set.

42. Surface Quality of cast metal Detail reproduction and surface smoothness of a metal-ceramic gold alloy restoration cast in a phosphate bonded investment were considered inferior to those with gypsum-bonded investment. Due to improvement in technique and in composition of material Phosphate bonded investments now approach the surface quality or fineness of the gypsum investments,

43. Advantages They have high fired strength. They also provide high setting and thermal expansion enough to compensate cast metal shrinkage. 3They can withstand temp more than 700°C

44. Disadvantage R ougher surfaces on casting. Difficult in divesting. Less porous than gypsum bonded investment .

45. Modifications Phosphate bonded refractory cast for RPD Investment for ceramics Eg: Polyvest and VHT- whipmix Fine grained Working time 4-5 min Withstand repeated firing up to 1200 c Used with high expanding porcelains.

46. Casting Investing procedure-common for both gypsum and Phosphate investment Mixing by- 1.Hand mixing 2.Vacuum mixing. ( In vacuum mixing-removes air bubbles, texture somewhat smoother and better details reproduction)

47. Excessive vibration should be avoided as- Causes solids in the investment to settle and may lead to free water accumulation adjacent to wax pattern Surface roughness May dislodge small pattern produce a miscast

48. 3. Wax elimination and Heating after investment set, approximately-1hr,it is ready for burnout Burnout temperature Gypsum investment 500 C for hygroscopic technique 700 C for thermal expansion technique Phosphate bonded investment Range from 750 C to 1030 C depending on the type of alloy

49. The temperature setting is more critical with gypsum-bonded investments than for the phosphate type because the gypsum investments are more prone to investment decomposition. As temperature rises above 700 C ,calcium sulfate reduce as following reaction- CaSO 4 + 4C CaS + 4CO 3CaSO 4 + CaS 4CaO+ 4SO 2

50. Heating in burnout furnace For gypsum bonded investment The molds are usually placed in a furnace at room temperature, slowly heated to 650 °C to 700 °C in 60 minutes, and held for 15 to 30 minutes at the upper temperature. For phosphate bonded investment The heating rate is usually slow to 315 °C and is quite rapid thereafter, reaching completion after a hold at the upper temperature for 30 minutes.

51. Ethyl silicate bonded Investments Though losing popularity because of the more complicated and time consuming procedures, still used in the construction of high fusing base metal partial denture alloy.

52. Composition Refractory material – Silica Binder –Silica gel that reverts to silica (cristobalite) on heating. Modifier – Magnesium oxide (strengthen the gel) Ammonium chloride - accelerator

53. Binder Ethyl silicate Colloidal silicic acid is formed by hydrolysing ethyl silicate in presence of HCl, ethyl alcohol& water. Si(OC 2 H 5 )+4H 2 O Si(OH) 4 +4C 2 H 5 OH

54. It is then mixed with quartz or cristobalite and magnesium oxide (alkaline). Coherent gel of polysilicic acid formed(accompanied by a setting shrinkage) ,dried at temp 168°C, loses alcohol & water to form concentrated hard gel . Volumetric contraction during drying is known as green shrinkage. As the gelation process is slow and time consuming , amines are added to ethyl silicate- hydrolysis and gelation occurs simultaneously.

55. It is supplied as a powder and liquid Powder consists of refractory particles of silica and glasses along with the magnesium oxide and some other refractory oxides in minor amounts Liquid contains stabilized alcohol solution of silica gel

56. Manipulation The powder is added to hydrolysed ethyl silicate liquid, mixed quickly and vibrated into a mold , that has an extra collar to increase the height. The mold is placed on a vibrator that has a tamping action This allows the heavier particles to settle while the excess liquid and some of the finer particles rise to the top .

57. In about 30 minutes the accelerator in the powder hardens the settled part, and the excess is poured off (to avoid crack formation). The liquid powder ratio in settled part is greatly reduced and the setting shrinkage is reduced to 0.1%

58. Can be used for higher temp castings (1090°C-1180°C) and compactible with higher fusing alloys. Care should be taken while handling and burnout as inflammable alcohol is given off.

59. Disadvantage Non porous material. Complicated manipulation Cannot be used for titanium and its alloys as silica can oxidize titanium or its alloys easily.

60. Other investment materials Soldering investment Divestment materials Investment material for titanium alloys  Investments for all ceramic restoration

61. Soldering investment ANSI/ADA Specification No. 93 (IS0 11244) for dental brazing investments defines two types of investment: Type 1: Gypsum-bonded dental brazing investments Type 2: Phosphate-bonded dental brazing investments

62. Soldering investment differ from casting investment by lower setting and thermal expansion most often ingredients do not have as fine particle as casting investment Uses Type 1- for soldering of low melting alloys Type 2 - for soldering of high melting alloys

63. Divestment Die stone and investment combination. These mixed with colloidal silica liquid The die is made from this mix and the wax pattern is then constructed on it. The whole complex is then invested in a mixture of divestment and water This combination used to compensate distortion of wax pattern of long span bridges or RPD frameworks during removal from die.

64. Newer investments for casting titanium based alloys Problems with conventional investment materials- Molten titanium is highly reactive with the oxygen and is capable of reducing some of the oxides commonly found in the investment. Titanium can also dissolve residual oxygen, nitrogen, and carbon from the investments. These elements can also embrittle titanium in the solid state.

65. Objective for a titanium investment To reduce breakdown of the investment To reduce contamination of the titanium – refractory materials that are less easily reduced by titanium should be used

66. Investments used for casting Titanium alloy Silica based investment Magnesia based investment Zirconia based investment Alumina based investment Resin based calcium investment

67. Conclusion Due to increase in the use of the higher melting alloys, the use of phosphate bonded investment materials increased. Gypsum bonded investment cannot withstand temperature higher than 700 C and it can be used only with conventional gold alloys. The complex procedures limited the use of silicate bonded investments. Due to highly reactive of titanium with the oxygen in the mold cavity , newer investment materials are developed.

68. Gypsum bonded investment (700 C) Phosphate bonded investment ( 700-1030 C ) Silica bonded investments (1090-1180 C) Mold for gold casting alloys Mold for base metal and gold casting alloys, cast ceramic and glasses Mold for base metal casting alloys

69. References- Phillips Science of Dental material, 11th edition Restorative Dental materials, Robert G. Craig.11th edition Fundamentals of fixed prosthodontics,3 rd edi . Herbert T. Shillingburg