INTRODUCTION : The principal laboratory technique of making metal inlays, onlays, crowns and bridges, is based on casting practice. This application of casting practice is one of the major advances in restorative dentistry. This is mainly based on “Lost wax technique”. This process of casting involve some basic steps 1. Preparation of a wax pattern . 2. Preparation of mold - It is done by pouring the mixed investment material around the wax pattern and allow it to set Burn out: - wax is eliminated from the investment by boiling (or) burning it in oven. 3. Then casting is done by melting the alloy and forcing the molten metal into the mold cavity.
History : This meticulous procedure of casting was used by various craftsmen to produce jewellary and ornaments. Its history can be traced back around 3000 B.C. But origin of lost wax technique, when viewed history makes its presence in the writings of theophilus (11th Century). 11th Century Theophilus Described lost wax technique, which was a common practice prevailed in 11th century. 1558 Benvenuto Cellini claimed to have attempted, use of wax and clay for preparation of castings. 1884 Aguilhon de saran used 24K gold to form Inlay 1887 J. R. Knapp invented Blowpipe. 1897 Phillibrook Described a method of casting metal filling. 1907 Taggart Deviced a practically useful casting machine. Various studies conducted on the properties of investment materials and casting alloys have led to a path for better, practical and useful processing methods.
DEFINITIONS : 1. Investing : The process of covering, enveloping, wholly (or) in part an object such as denture tooth, wax form, crown, etc with a suitable material before processing, soldering, casting. 2. Dental casting Investment: Material consisting principally of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperature) (or) phosphates and silica (for use in higher casting temperatures). 3. Refractory : (adj) - Difficult to fuse/corrode, capable of enduring high temperatures 4. Refractory investment : An investment that can withstand high temperature using a soldering/casting. 5. Allotropic phase : Phases of similar composition but different crystallographic structures, with different properties. 6. Casting Noun : Something that has been cast in a mold, an object formed by the solidification of a fluid that has been poured/ injected into a mold. Verb : The act of forming an object in a mold
Ideal requirements of an investment material: 1. The powder should have a fine particle size to ensure a smooth surface on the casting. 2. The mixed unset material should have a smooth consistency. 3. It should be easy to manipulate – easy to mix and also harden within a relatively short time. 4. It - Should have sufficient strength at room temperature. - Should exhibit sufficient strength at high temperature. - Inner surface of the mold should not break at a high temperature. - Should exhibit sufficient strength, to withstand the force of molten alloy entering the mold. 5. Inner surface of mold should be smooth. 6. At higher temperatures. - It should be stable without any decomposition of investment. - Should show sufficient expansion enough to compensate for shrinkage of wax pattern and solidification of molten metal. 7. The material should be sufficiently porous enough to permit escape of air/ other gases from the mold cavity during casting of molten metal. 8. It should show ease of divestment. - It should not react with metal. - It should easily break away from the surface of casting. 9. It should be economical No single material is known that can fulfill all the ideal requirements. So various ingredients/ modifiers are added to get the desired properties. Classification of Dental Investment I. Based on processing temperatures A. High temperature casting investments - Phosphate bonded investments - Silicate bonded investments B. Low temperature casting investments - Gypsum bonded investments (for low temperature gold alloy) Inlay, onlay - Type II & III gypsum products - Used for processing dentures - Reversible hydrocolloids - For processing a fluid resin of complete denture - Layered silicon rubber - For processing of complete denture
II. Depending on the type of refractory used A. Silica - Quartz investment - Cristoballite investment B. Magnesium oxide C. Zirconia based investments .
III. Based on the type of binder used A. Gypsum bonded investments : According to ADA. Sp. 2 Uses Shrinkage compensation Type I - Inlay Cowns - Purely thermal expansion Type II - Inlay & Cowns - Purely Hygroscopic expansion Type III - R. P. D. frame work
B. Phosphate bonded Investments C. Silicate bonded investments D. Calcia bonded investments
Basic Composition of refractory investments : Refractory investment usually contain 3 distinct type of material
A. Refractory Materials : The most commonly used refractory material is silicon dioxide in form quartz, crystoballite. Function - Resist the heat and forces of casting. - To expand and compensate for casting shrinkage.
B. Binder : commonly used binders are - - calcium sulfate hemihydrate - Others are – Sodium silicate, ethyl silicate, ammonium sulfate, sodium phosphate.
C. Other chemical modifiers i) Reducing agents – Ex : Charcoal ii) Sodium chloride, boric acid – to shrinkage, during dehydration of calcium sulfate dihydrate. iii) Potassium sulfate (accelerator) iv) Copper powder(reducing agent)/magnesium oxide
Physical and chemical changes occurring in refractory material with change in temperature: - The most commonly used refractory material is silica (SiO2). The study of silicate structure is of utmost importance in dental investments Silicon is a group IV element in the periodic table showing a valency of 4 electrons. It exists abundantly in nature as silica. The basic unit of silica is the (SiO4)4- tetrahedron (fig1). The bond is formed by the sharing of electrons between oxygen and silicon. The silica tetrahedron exhibits different structures based on atomic arrangement.
a. Double tetrahedron (fig 2). Two tetrahedron units share an oxygen corner. This structure may be as single/double chains.
b. Sheet Structures (fig3). Tetrahedral units are built up as a planar structure in 2 dimensions called as sheath, there is a lack of direct interatomic bond in 3rd dimension and show only weak secondary bonds, which accounts for their mechanical properties. Talk – Lubrication Mica – cleavage characteristic
c. Frame work structure fig4). If all 4 O2 ions of tetrahedral structure share the silicon ions, then O/Si would be reduced to Sio2, which is the stoichiometric formula for quartz. It is densest hexagonal crystalline type. The frame work structure generated by sharing of the corner oxygen ions is cristobalite (Fig-5) which shows diamond cubic structure. Tridymite (fig 6) shows Hexagonal modification of diamond cubic structure
Effect of Temperature on Silicon Silica shows 2 changes when heated a. Transformations • Reconstructive transformation. • Displacive transformation. b. Expansion observed due to these transformation. I. Transformations • Reconstructive transformation : It is a slow process of rearrangement of atoms, after their bonds are broken at higher temperatures. This is a reversible transformation. During this transformation the density of the material decreases and finally loose its crystalline structure to form amorphous fused quartz. They need to be rapidly cooled to retain its structure at room temperature. • Displasive transformations : This is fast reversible transformation occurring due to slight rotation of Si-O tetrahedron occurring without breaking Si-O bonds(fig 7). As the rotation involves a straightening of bonds, the transformation involves decrease in density and expansion of the whole mass. The temperature at which these transformation occurs is called the inversion temperature. Changes of quartz seen with temperature may be seen as a flow chart. Inversion Temperature - Form - Form (Low form stable at room temperature) (High form stable at high temperature)
Preparation of Cristobalite : a. By reconstructive transformation of quartz – Tridymite, cristobalite It is a slow process, and is expensive due to need of high processing temperature. b. Potch opal (non precious hydrous silica scrap material mined in opal fields – it is amorphous) – this technique gives cheaper crystobalite potch opal 1100 to 11500c cristoballite 16-24 hrs
II. EXPANSION:- (fig 8).When heated silica shows transformation at inversion temperature
REFRACTORY INVERSION TEMPERATURE EXPANSION quartz 5750 c 1.4% cristoballite 200-2700c 1.6% trydimite it shows 2 inversion temperatures <1% a)1170c b )1630c fused quartz it shows no inversion temperature ,below its fusion point
Investments for low casting temperature : A) Hydrocolloids : Fluid resin technique (given by Shepard and Winkler 1967). Makes use of hydrocolloid as investment material. The waxed-up denture is sealed and positioned in a specially designed flask. Which is then filled with reversible hydrocolloid investment medium. After gelation of hydrocolloid, the cast with the attached waxed up denture is removed then vents and sprues are cut from outside the flask into the mold space. The wax and base plate are eliminated, then the teeth and the cast are replaced in the vented flask. Fluid resin is mixed and poured into the mold via the sprue openings (28 ml powder of 13 ml of liquid). Then filled flask is held in a pressurized (0.14 mpa) chamber at room temperature until the resin cures completely . The set resin is removed and trimmed. Advantages : 1) Better tissue fit – JADA 119 : 271-276:1989. 2) Fewer open bites 3) Less fracture of porcelain teeth during deflasking 4) Decreased material cost 5) Simple processing method. Disadvantages : 1) Air Inclusions 2) Shifting of teeth 3) Decrease in occulsal vertical dimension JADA 83:848-851:1971 4) Occulsal imbalances 5) Incomplete flow of denture base material 6) Technique sensitive Sheldon winker, Harold F. marris, T. Sudjit, James H. Shorr ((1971) JADA, Vol 83: 848-85,1971 “change in vertical dimension of processed dentures using reversible and irreversible hydrocolloids”. Investing medium premolar cusps molar cusps Agar – Agar 0.0 0.19 0.19 0.18 Alginate 0.19 0.15 0.38 0.19
They concluded that reversible hydrocolliods exhibit superiority over irreversible hydrocolliods to be used as investment for processing dentures.
B) Gypsum Investment : They are used in investment of conventional heat and cold cure dentures, and also in fluid resin denture preparation [J. prosth dent – 30(3), 339-346, 1973 by Daniel H. Getti and S. Howenal payne] Basic reaction of gypsum products Dehydration (Endothermic) Ca So4 2H2O Ca So4 ½ H2O + 1 ½ H2O (Endothermic) Hydration
Gypsum products used for processing dentures are – Type II, III plaster Type II – Dental plaster –( - Hemihydrate of calcium sulfate)crystal are large, irregular and porus. It is prepared In open kettle at 1200c. W:P ratio – 0.45 – 0.50 Properties – Set by exothermic hydration reaction Setting accelerated by - Heat - Less than 450 Nuclei – Dihydrate particles. Chemicals-K2SO4,NaCl Setting retarded by - Heat - above 500 Chemicals– Borax, Potassium citrate Setting Expansion - 0.3 - 0.4% Setting time = 12 4 min Type III dental stone (Dental stone /hydrocal):- Hemihydrate of calcium sulfate Gypsum 120-130 - hemihydrate Autoclave under steam Common brand : Duralit, Hydrocal, Kaffir D. Manipulation – W:P ratio - 0.28 – 0.30 Properties – Set by exothermic hydration reaction Setting expansion - 0.1 – 0.2 % Setting time - 12 4 min Type II plaster is commonly used investment material for processing complete dentures. Investing is done by 2 pour (or) 3 pour technique. Type III Stone is used in cases of dentures which need high precission and where characterization of dentures is done.
C) Layered Silicon rubber :This was given by Macroft K. R, Tencate I. L and Hurst W. W. in – 1961. This technique showed a good accuracy in reproducing vertical dimension of denture. Technique 1st pour - is done with artificial stone. 2nd pour - thin layer of silica rubber is coated on teeth. 3rd pour - Investment completed with artificial stone. “A comparative study of changes in vertical dimension of occlusion using different investing mediums” was done by Bennie et al by recording the mean opening at incisal guidance pin (inch) (J. prosthet. dent – 49 :568-571, 1983)
Materials Mean Opening (inches) Artificial stone and silicone Rubber - 0.016 Artificial stone - 0.025 Flasking stone - 0.030 Plaster of Paris + artificial stone - 0.034 Plaster of Paris - 0.040 They concluded that artificial stone with a layer of silicone rubber, accurately reproduced the vertical dimension of occlusion during processing than other investing mediums used in investigation.
D. GYPSUM BONDED INVESTEMENTS ADA.SP.2 for casting investments for dental gold alloys Type I - For Inlay and crown – use mainly thermal expansion Type II - For Inlay and crown – use mainly hygroscopic expansion Type III - For RPD framework Common brands –Baker’s Sterling, begocast, crisobalite ,Inlay-vest, luster cast. Composition : 1. Calcium sulfate hemihydrate ( - hermihydrate (25-45%)) 2. Silica - 60-25% - quartz/ cristobalite 3. Chemical modifiers (2-3%) Carbon/ copper powder( reducing agents).
1. Gypsum - - hemihydrate - They are used with low temperature casting gold alloy, with melting range below 10000c. Gypsum after heating undergoes dehydration with shrinkage and fracture of mold. 130-2000c 200-10000c (CaSo4)2 H2O CaSo4 CaSo4 Gypsum Hexagonal anhydrate orthorhombic Anhydrate Above 12000 c CaSo4 + Sio2 CaSio3 + So3/So2 Liberated So3:-1) Highly Corrosive- cause contamination of casting. 2) Acts as a source of porosity. 3) Induce a back – pressure When thermal expansion curves of 3 common forms of gypsum are observed – all show a common form of changes (fig. 9) a) 200-4000c Shrinkage due to dehydration . b) 400-7000c a slight expansion . c) 700 and above – shrinkage due to decomposition of investment so gypsum bonded investments should never be heated above 7000c. Investments containing carbon should not be heated more than 6500c.A minimum of 25% of gypsum is needed for adequate strength of investment.
2. Silica:- exhibits four basic allotropic forms. A. Quartz. B. Cristoballite. C. Tridymite. D. Fused quartz. Most commonly used forms, in investment as fillers are cristoballite, quartz, combination of both. They function as refractories at high temperature, show expansion and also counter shrinkage of gypsum. 3. Modifiers : a) Reducing agents - carbon, powdered copper provide non-oxidizing atmosphere in mold. b) Balancing agents - Boric acid and Nacl – regulate - Setting time - Setting expansion - Shrinkage above 3000c Setting reaction of gypsum bonded investments : CaSo4 ½ H2O + 1½ H2O CaSo4, 2H2o + 3900 Cal/g mol. The microstructure of set material shows, rod-like particles of gypsum intermeshed with large irregular particles of silica refractory. Setting time : It is dependent on the gypsum content and upon the type of gypsum employed. Initial setting time - 8-15 min Final setting time - 12-25 min. This can be altered by addition of K2So4, Nacl, ( setting time), Borax and Potassium Citrate ( setting time) ADA Sp 2 - States that setting time should not be less than 5 min and nor more than 25 min. Properties of gypsum bonded investment 1. Expansion 2. Contraction 3. Strength 4. Other consideration Expansion : This property of investment is needed for compensation of casting shrinkage of alloy. The expansion occurs because of:-
Normal setting expansion Hygroscopic setting expansion Thermal expansion. A. Normal setting expansion : occurs with normal setting reaction in open air. Normal Setting expansion of :- gypsum bonded investment > gypsum Type I Investment – 0.6% stone –0.1 –0.2 % Modern Investment – 0.4% This difference is because of interference of silica with the growing spherulites. Effect of wax pattern on normal setting expansion : The setting reaction of gypsum bonded investment is exothermic in nature (3900 cal/g mol). The heat causes expansion of wax pattern leading to expansion of the mold. The amount of heat liberated depends on The ratio of gypsum - Gypsum heat W:P ratio - lower W:P ratio heat
B. Hygroscopic setting expansion : Fig (9)is the leniar expansion of the investment which occurs, if the investment is in contact with water from any source during the setting process. Contact with water can be achieved by placing the casting ring pattern in a water bath/using a wet casting ring liner. Both setting and hygroscopic expansion occur simultaneously. Hygroscopic expansion may occur because of Gypsum Refractory
Gypsum : water acts on gypsum its a)Lower the surface tension between growing crystal and allow them to grow without change in position b)Permits additional crystal growth c)Provides sufficient amount of water for complete growth of crystal. Refractory :- The water physically separates the fine particles of silica by capillary action leading to expansion of mass .This is reversible in case of absence of binder. But if binder is present and sets, the expansion is retained. The expansion is affected by : - Amount of silica - amount of silica -expansion W:P ratio - W:P ratio- expansion Time of insertion – investment immersed in water after initial setting cause decreased expansion.
The hygroscopic setting expansion of gypsum bonded investment(5%) 6 times of setting expansion (0.4-0.6%) ADA Sp. 2. For type II investment Min - 1.2% Max - 2.2%
Factors affecting Hygroscopic expansion 1.Composition : HSE is directly proportional to the amount of silica present, fineness of powder. HSE Greater in hemihydrate than hemihydrate. A Minimum Min 15% binder is needed to prevent drying shrinkage. 2.Water :powder ratio:- water :power ratio -- hygroscopic expansion 3.Spatulation- Mixing time HSE 4.Shelf life of investment – old investment HSE 5.Time of investment – Immersion of setting investment done : Before initial setting HSE After initial setting HSE 6. Effect of confinement : confinement is shown by walls of container/ investment ring ,wax pattern 7.Amount of water added : Till maximum expansion occurs increased amount of water added causes increased HSE (Fig-10)
c) Thermal expansion : is directly related to the amount and type of silica used. Amount of silica used • 60% of silica do not eliminate initial gypsum contraction • 75% of silica contraction of gypsum entirely eliminated Type of silica used (Fig – 11, 12) • Crystoballite 1.6% at 2500c • Quartz 1.4% at 5750c Crystoballite produces adequate mold expansion at lower temperature. When low temperature casting alloys are used. The amount of thermal expansion observed are :- • Type I alloy 1 - 1.6% • Type II alloy 0 - 0.6% Factors affecting thermal expansion : 1. W:P ratio- thermal expansion depends on the amount of solids present. W:P ratio Thermal expansion (Fig – 13) 2. Effect of chemical modifiers Sodium, Potassium and lithium chloride eliminates contraction of gypsum. increases expansion . Boric Acid expansion hardness of set investment heat causes its disintegration, which roughens the casting.
Thermal contraction : After thermal expansion if the investment is allowed to coal. The cooling path will follow the expansion curve. The investment contract to less than its original dimension due to shrinkage of gypsum when it is first heated. The process of heating and cooling cause internal cracks (fig - 4)
Strength : Adequate strength of investment is needed to prevent fracture/chipping of the mold and to resist contraction of the investment.After burn-out of the mold, the strength should be no greater than that required to resist the impact of the metal entering the mold. Compressive strength increases depending on amount of gypsum binder present . According to A.D.A. sp 2,the compressive strength of investment should not be less than 2.5 Mpa ( 2 hrs after setting). Increased strength is needed for investing large complicated casting. Factors affecting strength 1. Increased W:P ratio decreased compressive strength. 2. Heating of investment to 700 o C – may (or) strength by 65%. 3. Greatest reduction in strength is found in investments with NaCl. 4. Repeated heating & cooling cause internal cracks which decreased the strength. Other considerations a) Fineness fineness of the investment affects setting time ,surface roughness of the casting. Finer the silica particle – greater the hygroscopic expansion. b) Porosity :- During casting, the molten metal is forced into the mold under pressure. As the molten metal enters the mold, the air must be forced out ahead of it. If air does not escape a back-pressure builds up which prevents complete filling of gold alloy into the mold. The common method of venting the mold is through the pores of investment. The amount of porosity depends on : Less the amount of hemihydrate porosity. uniformity of particle size porosity. (a mixture of coarse & fine particles porosity)
c. Technical consideration: They are to be stored in airtight & moisture proof containers. Purchase investments in relatively small quantities due to increased storage time & effect of gravity segregation of components occur based on individual specific gravity. Which affects setting time and expansion. Investment powder should be weighed and water should be measured during proportioning for mixing.
Modified types of gypsum-bonded investments :- A)Hygroscopic – thermal inlay casting investment: It can be used as hygroscopic /thermal expanding Investment For hygroscopic expansion setting is done using & normal water immersion technique. Later heated to 482OC. For thermal expansion no water-immersion is done. Once it is set it is heated to 640OC. B) Gypsum-based investments containing Niobium carbide. (Nbc) (Abstract from shika-Zairyo-kikai, 1990 Jul:9(4): 617-22) Experimental gypsum- bonded investments containing 0.5 – 5.0 wt% of NbC was prepared by mechanically mixing each powder. Investigation on - setting & thermal expansion. - compressive strength. - casting accuracy of Ni-Cr alloy was done.
Effect of heat:- It showed oxidation 300 – 600OC 2 Nbc + 4½ O2 Nb2O5 + 2 CO2 volume of ½ Nb2O5 is 4 times of Nbc Experimental investment of 70 Wt% cristobalite & 30 wt% gypsum with varying wts of Nbc amount of thermal expansion. 2.0 wt % 7 % 3.0 wt % 10 % 5.0 wt 15 % - The investment containing 2.0 wt of Nbc showed, nearly the same casting accuracy for Ni-Cr alloy for metal ceramic restoration as the commercial phosphate – bonded investment. a) Rapid burn out gypsum bonded Investments : (abstract from dental mater journal, 1994 : 13(2), 240-250 ) They decrease the time needed for dental casting procedure. Mixing and setting time totally is 30 min .After setting immediately heated to 7000c for 30 min .No cracks are found, but very rarely small casting fins are observed. It shows increased setting expansion and high accuracy of fit of casting But no change in thermal expansion.
Variables affecting casting accuracy of quick heat casting investment :- (Abstract from Kokubyo – Gakkai – Zasshi 1994 Ju 61(2), 242(a)) Study was done of evaluate a) Effect of thickness of casting ring liner on casting accuracy. b) Effect of waiting time before placing mold into 7000c furnace on casting accuracy. Results : Increased Dimensions of cast crown were found with:- - Increased Thickness of ring liner - Increased waiting time before placing into furnace
Applications of gypsum bonded investment. 1) Investing of wax patterns to cast for Type I – IV dental gold alloys for crown and bridge, post and core systems etc. 2) In Investment soldering to stabilize components of silver and its alloys.
INVESTMENTS USED FOR HIGH TEMPERATURE CASTING PROCEDURE
The alloys used preparation for partial denture framework and porcelain fused to metal are usually base metals with high melting temperature above 10000c. So, gypsum bonded investments cannot be used, because it disintegrates at such high temperature.
Phosphate – bonded Investments : There are 2 types of phosphate bonded investments. Type I – Inlay, crown and restoration. Type II – R. P. D frame work Common brands : Aurobond, calsite, cera fine, Deguvest, DVP, Eurocast, Nicrobond, Roma exalct etc. Composition : 1. Binder (20%)- consists of 2 components which react at room temperature to form a phosphate binder Magnesium. Ammonium phosphate . (NH4 Mg Po4. 6H2O)n Acid part - Diacid phosphate – NH4 H2Po4 Basic part- Magnesium Oxide-MgO 2. Refractory - Cristoballite/ quartz/ combination of both. They function as refractory and produce thermal expansion at high temperatures. Colliodal silica suspensions are also used to obtain greater amount of expansion. 3. Modifiers - carbon at as reducing agent used to produce clean castings used and facilitate dig-out of castings. It is used for high - temperature gold casting alloys. But with silver palladium alloy, carbon embrittles the alloy . Palladium alloy do not react till 15040c. But above this they tend to react with carbon.
Mode of supply : 1) Powder contain NH4 H2 Po4, Mgo, Silica, traces of carbon. This powder may be mixed with water to form the investment. 2) Special liquid contain colloidal silica. This liquid shows increased Setting expansion, produce significant amount of hygroscopic expansion (as with pure water the amount of hygroscopic expansion is less), and increases its strength.
The product formed is predominantly colloidal multimolecular (NH4 Mg Po4 6H2O) which coagulates arround excess Mgo and fillers. After the initial setting reaction. The set colloid undergoes various changes during the thermal expansion stage. Setting Reaction:- Mgo + NH4 H2 Po4 + H2O Room temperature (NH4 Mg Po4 6H2O)n MgO colloidal type particles NH4 H2 Po4 5H2O
(Prolonged setting at room temp/ dehydration at 500c) (NH4 Mg Po4. 6 H2O)n
Setting and thermal expansion : Theoretically, the setting reaction should show shrinkage, but when colloidal silica is used there is slight expansion. (Fig 15).Thermal expansion when with water have a shrinkage at 200-4000c. This can be eliminated by adding colloidal silica. This shrinkage at 200-4000c is due to decomposition of binder magnesium ammonium phosphate with evolution of ammonia, which can be detected by odor but this shrinkage is marked by expansion of cristoballite (fig 16). working time and setting time : working time – 2 min. setting time – 1 hour Factors affecting and setting time 1) temperature fast set : (as the setting reaction itself is exothermic) 2) mixing time fast set (generally mechanical mixing under vacuum is preferred 3) L : p ratio working time. Miscellaneous properties : When used with gold alloy to increase the smoothness of the mold formed , increased special liquid :water ratio is used to mix . Compressive strength Type I 2.5 mpa/ min Type II 10 mpa/ min Setting expansion 0.4% Hygroscopic expansion 0.6 – 0.8% Thermal expansion with water 0.8% with special liquid 1.2% Modulus of rupture 0.1 – 0.5 mpa Advantages : 1. High green strength 2. High fired strength less mold cracking and few fins on casting. 3. They can withstand temperatures up to 10000c for short periods of time. Disadvantages : 1. At temperatures greater than 1,375 c cause mold breakdown and roughen the surface of casting. 2. Due to high strength divesting is defect. 3. To increase expansion with use of special liquid cause less porous mold incomplete casting. 4. High tendency for reaction with non-precious alloy producing oxides which is difficult to remove from castings.
Surfactant containing phosphate bonded investment : Mueller, et al 1986. (JPD-54: 367: 1985). Addition of surfactants to phosphate bonded investment can increase the hygroscopic setting expansion . The surfactant also makes the unset investment more viscous and reduces the compressive strength.
Ethyl-silicate bonded investments : This investment material are being used since 1930. But now it is loosing popularity due to complicated and time consuming procedures involved. Brand name - Nobilium rapid set (low, iron, & sodium investment) - Saddle lock (a ferruginous investment) - Howmet vary rapid (intermediate iron and sodium) Composition : A. Binder silica acid gel B. Refractory silica (cristobalite) C. Additive magnesium oxide - Make it alkaline, Strengthen then the gel D. Wetting agent To reduce accumulation of air bubbles on surface of wax pattern.
Various methods of producing the silica acid gel binder 1. Sodium silicate +H+ (acid) silicic acid gel 2. Aq. colloidal silica + NH4 cl Silicic acid gel 3. Using ethyl silicate : 3 stages Stage I hydrolysis
Hcl ,C2H5OH Si(OC2H5)4 + 4 H2O Si (OH)4 + 4C2H5OH (Ethyl alcohol) (Ethyl silicate ) (sol of poly (silicic acid)) Stage II Gelation n Si(OH)4 + MgO MgO [Si(OH)]n This sol is mixed with cristoballite/ quartz, then gelation is completed done under alkaline condition by adding magnesium oxide. There is slight shrinkage at this stage. Stage III drying (<1680c) On heating loss of alcohol and water cause formation of a mold of silica particles tightly packed with considerable shrinkage. The shrinkage found during drying (stage-3) is called green shrinkage. This method is time consuming so another method is used . Simultaneous Hydrolysis 4. Ethyl silicate + piperidine silica gel (Amine) Gelation Mode of supply : 1. Powder - Refractory particles of silica and glass - Calcined Mgo - Other refractory oxides 2. Liquid - Single liquid of stabilized alcohol solution of silica gel (or) - With 2 liquids to form the above (ethyl silicate, denatured ethyl alcohol) (or) - With 3 Components stabilized colloidal silica ethyl silica denatured ethyl alcohol
Properties : - Compressive strength 1.5 mpa/min - Setting contraction 0-0.4% - Thermal expansion –1.5-1.8% - This investment can withstand higher temperature 1,094 – 1,1170c - Porosity The particles in the set material are very closely packed leading to low porosity. Air, space/ vents must be left in investment to permit escape of air from the mold.
Advantages : 1. High permeability yields sharply defined castings. 2. Low setting expansion (contraction) reduces refractory partial denture models that may be articulated against stone model. 3. A nearly flat expansion Vs temperature curve at the temperature (1,1500c) and low expansion Vs time effect expansion. 4. The investment is more refractory form smooth castings. 5. Low burnout strength results in easy removal of castings and cleaning of oxides from the castings.
Disadvantages : 1. Limited shelf life of liquid. 2. Must wait for substantial period of time, prior to using freshly mixed liquid. 3. Potential of cracking exists during burnout, owing to high thermal expansion. 4. Very expensive. 5. Give off flammable components during setting.
Manipulation : Powder + hydrolyzed ethyl silicate liquid mixed quickly and vibrated into a mold allow heavier particles to settle quickly while the excess liquid and some of the fine particles rise to top. Within 1½ hrs the accelerator (NH4cl) hardens the settled part. The excess on top is poured off and there by decreasing the L:P ratio at the bottom.
Uses : 1. They are mainly used for casting Co-Cr R. P. D. framework. 2. Accurate casting of nickel –based alloys
INVESTMENT USED FOR CASTING TITANIUM BASED ALLOYS Titanium based alloys provide advantages of biocompatability, corrosion resistance, due to oxide layer Tio2 light weight (low density 4.5g/cm3) and less expensive.
Properties : 1. High melting points 16680c 2. Casting shrinkage of titanium 1.8 – 2% 3. Reactivity molten titanium is highly reactive with O2 and is capable of reducing oxides, so it is casted in insert gas Argon
Various Investments used for casting titanium as : Based on binder 1. Mgo, phosphate – various refractories used are Al2O3, Zro2, Sio2, ZrSio4, LiAlSi2O6 2. Ethyl silicate various refractories Mgo, Al2O3, Sio2, Zr (Cause oxidation which cause expensive) 3. Aluminum cement various refractories Mgo, Zr, Al2O3, 4. Calcia bonded investments 5. Zirconia – Phosphoric acid Investment
Titanium reactivity with refractories : Shika zairyo – Kikai 1989 8(1), 83-96. The reactivity of molten titanium was tested with various refractories. It was reported that, the most reactive refractories are Sio2 (which form about 250m silica rich interface region) >Al2O3 (which form 100m thick region) Mgo, Cao, Zro2, exhibited very little reactivity with less than 1m thick layer of interface region formed.
This is a the recently developed investment for casting titanium inlay, crown and bridge. Binder calcia Refractory Zirconia Setting characteristics Calcia oxide (Cao) slurry mixed with water hardens and expand by hydration hydration Cao + H2O (Ca (OH)2 Open air This reaction in open air results in larger value of setting expansion which continues for several days. To control this extreme situation, it was allowed to set in presence of Co2 gas. Thus setting expansion values were controlled. Cao + H2O Ca(Co3) + Ca(oH)2
Shika zairyo kikai 1990 9(5) 734 – 40 The calcia bonded investment was studied to find the effect of calcia concentration on various properties. It was concluded that 10 mol% of calcia was the optimum concentration. At this concentration the investment shows a change from contraction to expansion during setting reaction and thermal treatment. They are dispensed in powder and liquid [Cao and mixing liquid. There are 2 types of Cao and mixing liquid. 1. Saturation type – (total expansion 2 3%) 2. Delayed expansion type
Properties : 1. Total thermal and setting expansion found was 1.5 - 2.5% 2. The maximum thermal expansion is found at 900 - 1, 2000c
Steps of investing : 1. Coating of wax pattern with zirconia slurry and drying 2. Sintering 3. Dewaxing by thermal shock4 steps a. Hot air drying (500c, 5 min) b. Heat stock (9000c, 3 sec) c. Redrying (2200c, 3 min) d. Dewaxing (5500c, 3 min)
MODIFIED PHOSPHATE BONDED INVESTMENTS (Takahasi 1993) They usually contain phosphate binder and Mgo /quartz refratories. 1. Magnesia bonded investment - Phosphate binder - Alumina /Mgo refractories good heat resistance Low thermal expansion 2. Phosphate bonded investment : - Phosphate binder - Mgo + Al2o3 Mgo -Al2o5 (spinel highly refractory. ) Show large expansion due to spinel reaction .
3. Spondumen (H2o – Al2o5 – Sio2(Okuda et al 1991) Expand irreversible on heating at 900 – 11000c
4. Aluminous cement : (Togaya 1985) (Cao – Al2o5) + mgo (refractory) + 5% zirconia Burnout(o) Zr Sio4 Zr Expansion DIVESTMENTS : (By whipmix corporation) It is a combination of Die stone and gypsum bonded investment material. The powder is mixed with colloidal silica properties Setting expansion – 0.9% Thermal expansion 0.6% (at 9770c) 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.
BRAZING INVESTMENT : ADA Sp No 93 Type I - Gypsum bonded dental brazing investment Type II - Phosphate bonded. Steps 1) Broken parts are Stabilize by sticky wax 2) The broken parts are then embedded in investment with portion to be solder is left exposed and free of investment. They should have low setting and thermal expansion. Particle size is usually not fine. They possess usually a compressive strength of 2-10 Mpa.
1. William J. O’ Brien, Dental materials and their selection, 249-258. 2. Robert G. Craig, Restorative dental material Pg No 404-406. 3. John F Mc cabe, applied dental material 41-46 4. C. Coombe, Notes on dental material 318-324 5. Kenneth J Anusavice, Science of dental materials 471 6. E.H. Greener, material science in dentistry 219-223 7. Bernond G. N. Smith, The clinical handling of dental material 183-184 8. Malvin, Dentistry an illustrated history. 9. JPD 1999 journal 48-169 10. Jada 1989, 119, 271-276, 1971 83 848 - 851 11. Bennie, JPD - 1983 - 49 - 568-571 12. Dental Material journal 1994,13(2) 240-280 13. Australian dental journal 1981-26(6) 382 14. Dental mater 1984, 5(1), 45-50 15. Shika - Zairyo Kikai– 1981, 8(1) 97-102 16. Shika - Zairyo Kikia 1989 8(1), 83-96 17. Shika - Zairyo Kikia 1989 8(4), 559-566 18. Shika - Zairyo Kikia 1990 9(5), 734-743 19. Shika - Zairyo Kikia 1990 8(2), 336-355
_________________ kumar niwlikar
Page 1 of 1
Who is online
Users browsing this forum: No registered users and 0 guests
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot post attachments in this forum