Investment material

Dental casting investment: A material consisting primarily 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 8) 

1. INVESTMENT MATERIAL Presented by- Dr. Nakul Patidar

2. INTRODUCTION Casting procedure :- It is a principal laboratory method used to form metal inlays, onlays , crowns and bridges by casting molten alloys, under pressure, or under vacuum with the help of centrifugal force .

3. It involves:- ( 1) A wax pattern of the object to be reproduced (2) A suitable mold material, known as investment, which is placed around the pattern and permitted to harden (3) Suitable furnaces for burning out the wax patterns and heating the investment mold (4) Proper facilities to melt and cast the alloy.

4. DEFINATION An investment can be described as a ceramic material that is suitable for forming a mold into which a metal or alloy is cast. The operation of forming the mold is described as investing. (CRAIG’S 13 edition)

5. DEFINATION Dental casting investment: A material consisting primarily 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 8) 

6. Ideal requirements Easily manipulated Should not separate when mixed. Smooth surface  A ppropriate setting time Sufficient strength during casting Sufficient expansion: Porosity: Ease of divestment Inexpensive

7. In general investment material contain:- Refractory material :- Silica based regulate thermal expansion Quartz investment • Cristobalite investment • Tridymite investment • Mixture of quartz, cristobalite and tridymite • Mixture of alumina, magnesia, zirconia Binder :- Gypsum( α- calcium sulfate hemihydrate), Phosphate, Ethyl-Silicate To hold or bind the refractory material particles Modifier :- Modifiers NaCl , boric acid, potassium sulfate, graphite, copper powder,.. etc .

8. CLASSIFICATION Based on the binder used: 3 types Rosensteil   Types Uses 1. Gypsum Bonded Investments, Or Calcium Sulphate Bonded Investments Gold alloy inlay, onlay,FPD 2. Phosphate Bonded Investments Base metal alloy & Metal ceramic alloy 3. Ethyl Silicate Bonded Investments, Or Silica Bonded Investments Co-Ni alloy

9. CLASSIFICATION II. On the basis of the type of silica principally employed, dental investments are often classified as Quartz investments Cristobalite investments Phillips 10th ed

10. Gypsum Bonded Investments, or CALCIUM SULPHATE BONDED INVESTMENTS ADA spec No. 2 Type I investments(Thermal expn .) – INLAY AND CROWN Type II investments(Hygroscopic expn .)- INLAY, ONLAY AND CROWN Type III investments- CAST PARTIAL DENTURE WITH GOLD ALLOY

11. GYPSUM INVESTMENT - COMPOSITION Gypsum: Binder+ strength+ setting expansion (25-35%) Silica : regulate the thermal expansion . (65-75%) Modifiers (2-3%) Reducing agents – provide non oxidizing enviroment Coloring matter Boric acid and sodium chloride - Regulate setting expansion, setting time, & prevent shrinkage of gypsum when heated above 300 C .

12. Advantages: Adequate strength Adequate porosity Controlled large setting and thermal expansion Simple methods of manipulation and casting procedure Not very expensive.

13. Disadvantages: Sulphur dioxide gas produced ; during heating causes ; tarnishing or discoloration of gold alloys. Hence this is not suitable for casting of high fusing Noble and High Noble alloys. Thus phosphate-bonded investment material is preferred.

14. Setting Time According to ANSI/ADA specification No . 2 for dental inlay casting investment 5-25 min

15. Setting reaction Gypsum bonded investment (GBI) shows shrinkage when heated above 700o C and is most likely due to decomposition and release of sulfur gases. CaSO4 ---------> SO2 + SO3 (above 700o C ) This decomposition not only causes shrinkage but also contaminates the castings with the sulfides of the non- noble alloying elements. • Hence gypsum investments should not be heated above 700o C.

16. Effect of Temperature on Silicon Dioxide Refractories Each of the polymorphic forms of silica—quartz, tridymite, and cristobalite—expands when heated, but the percentage of expansion varies from one type to another. Pure Cristobalite expands to 1.6% at 200°C- 270° C Quartz expands about 1.4% at 573°C Tridymite expands to less than 1% at 600°C. The a m ount of expa n sion i s high e st f or cri s t ob a l i te and lowest for tridymite.

17. Effect of Temperature on Calcium Sulfate Binders Up to about 105° C, ordinary thermal expansion occurs. Above 105° C, the calcium sulfate dihydrate is converted to anhydrous calcium sulfate by loosing water . Dehydration of the dihydrate and a phase change of the calcium sulfate anhydrite cause a contraction. However at elevated temperatures, α-forms of silica present in the investment are converted to the β-forms, which causes some expansion and compensates for the contraction of calcium sulfate.

18. Factors controlling setting time:- Setting time decrease:- A. C alcination is incomplete and considerable gypsum is left B. M ixing time and rate is more C. W ater/powder ratio is more D. T emperature increases E. Sodium chloride 2% F . Sodium sulfate at 3.4%

19. Factors controlling setting time:- Setting time increase A. Sodium chloride > 2% B. Sodium sulfate at > 3.4 % C. G lue , gelatin , and some gums D. Citrate , acetate, borate

20. SETTING EXPANSION Normal or linear setting expansion Hygroscopic expansion Thermal expansion

21. Normal or linear setting expansion Setting surrounded by air Compensate casting shrinkage Occur btw 0.4-0.6 % Increase expansion Gypsum Content = more Water powder ration = less

22. HYGROSCOPIC EXPANSION Occur when it set under water 6 time more than normal 1.2-2.2%

23. Greater in magnitude than the normal setting expansion ( > 6 times).

24. FACTORS AFFECTING HYGROSCOPIC EXPANSION Propotional to silica content Finer particle – more expansion α particle produce more expansion As water added during setting – more expansion Increase W/P ratio- Less expansion Reduced mixing time- less expansion Older investment- less expansion

25. THERMAL EXPANSION Directly related to amount and type of silica 1.0-1.6% Cristobalite more expansion > Quartz It is achevied upto 700 C. Type I investment rely upon this

26. Factors to Control Thermal Expansion More silica more expansion More water less expansion On cooling little contration

27. STRENGTH 2-4 Mpa Factor affecting strength 1. W/P ratio 2. Heating >700 C 3. On cooling

28. Other gypsum investment considerations Fineness : Finer particle size- smooth porosity More gypsum crystal- more porosity Storage- must be stored in airtight and moisture- proof containers .

29. Phosphate Bonded Investments Metal ceramic alloy invested High temperature investments Need to be heated to casting temperatures of about 1200°C to 1300°C . High-gold and palladium-based alloys.

30. PHOSHPHATE INVESTMENT - COMPOSITION Refractory filler; 80% silica to provide high temperature thermal shock resistance (refractoriness) and a high thermal expansion. Binder- <20% Crystoballite , quartz ( MgO + Monoammonium phosphate) Carbon; clean and devesting the casting

31. Setting Reaction

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

33. Modifiers : Carbon is often added - to produce clean casting and facilitate divesting. Palladium reacts with carbon even at temp 1500°C, so carbon free phosphate bonded investment began to be used for higher temp.

34. Setting and Thermal Expansion When PBI are mixed with water- initial shrinkage is seen same that of GBI within the temperature range of (200 ˚ C to 400 ° C) . The early thermal shrinkage of phosphate investments is associated with the decomposition of the binder, magnesium ammonium phosphate, and is accompanied by evolution of ammonia, which is readily apparent by its odour .

35. This contraction is practically eliminated when a colloidal silica solution is used for mixing phosphate bonded investment instead of water. 61

36. E xpan s i on and st r ength of these investment can be modified by:- altering the liquid:powder ratio or Increasing the concentration of the special liquid. But if more liquid is used it causes dense nonporous investment which can effect casting. Influence of liquid concentration on the setting and thermal expansion

37. Working and Setting Time 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. 63

38. Other Properties Detailed reproduction and surface smoothness of phosphate-bonded investment is inferior than gypsum- bonded investment.

39. ADVANTAGE-PBI Higher strength High fired strength – less mold cracking and few fins on casting Can withstand temperature up to 1000°C for short period of time (useful or performing metal-joining operations ). They can also provide setting and thermal expansions high enough to compensate for the thermal contraction of cast-metal prostheses or porcelain veneers during cooling.

40. DISADVANTAGE- PBI When used with higher-melting alloys (casting temp. higher than 1375°C) - results in mold breakdown and roughen surface of castings. Difficult to divest (removal of casting from the investment) When higher expansion is required, more of the silica Liquid is used with the result that a more dense and less porous mold is produced. This can result in incomplete castings if a release for trapped gases is not provided.

41. ETHYL SILICATED BONDED INVESTMENT

42. Second type of investment used in dentistry. Used since the early 1930’s High fusing base metal partial denture alloys Cobalt chromium or nickel titanium alloy Disadv - more complicated and time consuming Ethyl alchol - flammable content

43. COMPOSITION Refractory material – Silica Binder –Silica gel or ethyl silicate Modifier – Magnesium oxide (strengthen the gel) Ammonium chloride - accelerator

44. BINDER Binder silica gel reverts to silica ( cristobalite ) on heating. Methods to obtain silica or silicic acid gel binders :- 1)By adding an acid or an acid salt to sodium silicate. 2)By the adding an accelerator, such as ammonium chloride to an aqueous suspension of colloidal silica. 3)By hydrolyzing ethyl silicate in the presence of HCl , ethyl alcohol , and water.

45. SETTING REACTION During drying at temp below 168°C , soft gel loses alcohol and water to form a concentrated, hard gel which lead to volumetric contraction (GREEN SHRINKAGE) which ultimately reduces the size of the mold. This shrinkage occurs in addition to setting shrinkage.

46. Powder consists of refractory particles of silicas and glasses along with magnesium oxide and some other refractory oxides in minor amounts. Liquid contains stabilized alcohol solution of silica gel A. 1 bottle contains diluted water-soluble silicate solution. B. 2 bottle contains a properly diluted solution of hydrochloric acid .

47. ADVANTAGE It offers the ability to cast high temp Co-Cr and Nickel – Cr alloys , and attain good surface finishes, low distortion, and high thermal expansion (good fit). High permeability(gel form), yields sharply defined castings Low fired strength – easy removal of casting.

48. Disadvantage It gives off Inflammable components during processing. Expensive. Strength is low so precautions should be taken. More time consuming and complicated procedures involved. Manipulation is very technique sensitive . Satish Chandra

49. Divestment (whip-mix, Louisville, KY) It is a combination of Die stone and gypsum bonded investment material . The powder is mixed with colloidal silica. 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 & water. This combination used to compensate distortion of wax pattern of long span bridges or RPD frameworks during removal from die . Special GBI or PBI investment materials are used.

50. PROPERTIES Setting expansion - 0.9% Thermal expansion - 0.6% (at 977°c) Advantages The wax pattern and die are invested simultaneously with out removal of pattern. Useful with gold alloys DIVESTMENT PHOSPHATE(DVP) •Similar to divestment, but used for casting post and core, crowns of base metal alloys without any need of removal of wax pattern.

51. Different type of wax pattern Pattern Processing Impression Inlay Casting Base plate Ivorian Beading & Boxing Utility Sticky Block out Carving Corrective Bite registration

52. Rise in temperature- Wax expands Lower temperature- contract Dental wax has largest coefficient of thermal expansion A wax pattern prepared directly in a patient's mouth will shrink about 0.4% when cooled from oral temperature. In the indirect method of preparing the wax pattern on a die , the wax shrinkage is about 0.2%.

53. Metals- Casting Shrinkage Gold alloy – Alloy shrink from liquid to solid state 1.25-1.65% Base metal alloy – (Nickel based ) Higher shrinkage than gold alloy -2.0%

54. SETTING EXPANSION Gypsum bonded Phosphate bonded Ethyl silicate Normal-0.4-0.6% Normal-0.9% Thermal-1.0-1.6% Thermal-0.8% Thermal-0.6% Hygroscopic expansion-1.2-2.2%

55. Setting reaction

56. Gypsum bonded investment 56 α- hemihydrate form of gypsum act as a binder to hold the other ingredients together and to provide rigidity. When this material is heated to the temperatures required for complete deydration and sufficiently high to ensure complete castings, it shrinks considerably and occassionally fractures. All forms shrink considerably after dehydration between 200oC and 400oC. • A slight expansion takes place between 400oC and 700oC . • And a large contraction then occurs. • The shrinkage on heating is due to the dehydration of the set gypsum in two stages . 2CaSO4 · 2H2O --------------------  ( CaSO4 )2 H2O + 3H2O (CaSO4 )2 · H2O ------------------  2CaSO4 + H2O • Shrinkage is due to the transformation of calcium sulphate from the hexagonal to the orthorhombic configuration.

57. Phosphate bonded investment This contraction is practically eliminated when a colloidal silica solution replaces the water . The early thermal shrinkage of phosphate investments is associated with the decomposition of the binder, magnesium ammonium phosphate. 2(Mg·NH 4 ·PO 4 ·6H 2 O) → Mg 2 ·P 2 O 7 + 2NH 3 + 13H 2 O At a higher temperature some of the remaining phosphate reacts with silica forming complex silicophosphates . These cause a significant increase in the strength of the material at the casting temperature. 57

58. THE SETTING REACTION - Stage 1 – Hydrolysis : The silica is bonded by the hydrolysis of ethyl silicate in the presence of hydrochloric acid. The product of the hydrolysis is the formation of a colloidal sol of polysilicic acid and ethyl alcohol. (C 2 H 5 O) 4 Si + 4H 2 O HCl Si(OH) 4 + 4C 2 H 5 OH ETHYL SILICATE SETTING REACTION- 58

59. ETHYL SILICATE SETTING REACTION- Stage 2 – Gelation : The solution is then mixed with the quartz or cristobalite to which is added a small amount of finely powdered magnesium oxide to render the mixture alkaline. A coherent gel of polysilicic acid then forms accompanied by a shrinkage. 59

60. Stage 3 – Drying : The soft gel is dried at a temperature below 168°C. During the drying process, the gel loses alcohol and water to form a concentrated, hard gel. A volumetric contraction accompanies the drying, which reduces the size of the mold. This contraction is known as “green shrinkage”, and it occurs in addition to the setting shrinkage. ETHYL SILICATE SETTING REACTION- 60

61. PROBLEM CAUSE SOLUTION Rough surface Don’t overheat mold or alloy Breakdown of investment Weak surface of investment Avoid use of high water/powder ratio of investment. Avoid dilution of the investment material from application of too much wetting agent F i ns Cracking on investment Avoid too rapid heating investment. Casting defects because of investment material :

62. PROBLEM C A USE METHOD OF AVOIDING FAULT Casting too large Excessive mold expansion • • Use correct t e m p e r a t u r e Use correct type of investment material Casting too small Too little mold expansion Heat the mold sufficiently.

63. PROBLEM C A U S E Irregular voids Inclusion of particles of investment material Rounded margins; Regular large voids Back-pressure effect; air unable to escape from mould METHOD OF AVOIDING FAULT Heat mold upside down so that particles fall out of the mold Use sufficient casting force Use investment of adequate permeability Avoid presence of residue of wax in mould Place pattern no more than 6-8mm away from end of casting ring.

64. Newer investments have been aimed at the casting of titanium and titanium based alloys. This is because molten titanium is highly reactive with oxygen and is capable of reducing some of the oxides commonly found in investment. Titanium can react with residual oxygen, nitrogen and carbon from investment. These elements harden and embrittles titanium in the solid state. Recent advances:

65. As a result, either modifications of existing refractory formulations and binder or new refractory formulations and binder system are required. A variety of investment formulations for the casting of titanium have been developed over the past several years. These investments might be classified as PBI and Cemented (magnesia bonded by an aluminous cement, which contains a mass fraction of 5% zirconia) In these investments many kinds of refractories such as silica, alumina, magnesia and zirconia have been used.

66. T itave s t , an alu m in i a - m agnesia - s y ste m . M ixed w i t h a special liquid the material becomes a spinel structure.

67. Re f er e nces Text book dental material Craig’s Restorative Dental Materials 13 th edition by Ronald L. Sakaguchi and John M. Powers. Philip’s science of dental materials, 11 th edition by Kenneth J. Anusavice .