INTRODUCTIOn The principal laboratory method used to form metal inlays, onlays, crowns and bridges is to cast molten alloys by centrifugal force, under pressure, or under vacuum and pressure, into a mold cavity. The mold cavity is produced by elimination of wax or resin pattern by a burnout process. There are many varieties of waxes, die materials, investment materials and procedures that are used in dentistry. Each has particular properties depending on the purpose for which it is used. The basic constituents employed in each preparation are essentially similar, their exact proportioning is important in determining their final properties.
WAXES VARIETY OF WAXES Waxes occur in many forms from animal, insect, plant and mineral sources. Synthetic waxes are also used for making dental waxes. COMPONENTS OF DENTAL WAXES Natural Waxes Mineral:
Microcrystalline Aerosol, OT
1. Stearic acid
Oils: Turpentine Colour
3. Sandarac 4. Mastic 5. Shellac
Insect : Animal: Beeswax Spermaceti Synthetic Resins l. Elvax 2. Polyethylene 3. Polystyrene CHEMICAL NATURE OF WAXES The two principal groups of organic compounds contained in waxes are: 1. Hydrocarbons and
Some waxes in addition contain free alcohol and acids. Common vegetable and plant waxes consist of Ester - From union of higher fatty acids with higher aliphatic alcohol with elimination of water. Alcohol + Fatty Acid
Ester + Water.
In addition waxes contain fatty acids, alcohol and saturated hydrocarbons. Paraffin & Macrocrystalline Waxes: Are distillation products of petroleum. They are both hydrocarbons. Paraffin (melts 40-70째) and tends to be brittle. Macrocrystalline (6090째C) and is more flexible and tougher. Vegetable Source: Carnauba wax and candellia wax. These are natural esters and are comparatively tough substances. Natural Gum and Dammer Resin: Impart adhesive properties making the resultant wax tougher. Bees Wax: It's addition reduces brittleness. 2
The natural waxes are not consistent in their composition, and thus their properties. To overcome this, synthetic waxes have been increasingly used. Synthetic Waxes: These are carefully prepared under controlled conditions to give standardized reliable results. They may be based on polyethelene nitrogenous derivatives of fatty acids of by modification of natural substances like petroleum. Most of modern dental waxes are therefore made up of a combination of natural & synthetic products. Waxes have a number of important properties in relation to their dental use. Different uses require different properties. Waxes for production of inlay pattern probably require most careful balance. CLASSIFICATION OF DENTAL WAXES PATTERN
b. Ready shapes c. Wax-up Base Plate CHARACTERISTIC PROPERTIES OF WAXES Some of the most useful and important properties of waxes are : 1. Melting range 2. Thermal expansion 3. Mechanical properties 4. Flow 5. Residual stresses 3
6. Ductility. Melting range: i Waxes have melting ranges rather than melting points. ii. Mixing of waxes may change their melting range. Thermal Expansion: Waxes expand when subjected to rise in temperature and contract as the temperature is decreased. Dental waxes and their components have the largest coefficient of thermal expansion as compared to any material and in restorative dentistry. Since the coefficient of thermal expansion of inlay wax is so great, temperature changes in wax patterns after carving and removal from the mouth, before the investing procedures are the factors which contribute to the inaccuracy of the finished restoration. Mechanical Properties: The elastic modulus, proportional limit, and compressive Strength of waxes are low compared to other dental materials. These properties are strongly dependent on the temperature. Flow: Flow is an important property, particularly in inlay. Flow is dependent on the temperature of the wax, force bringing about the deformation, and the time the force is applied. Flow increases as the melting point of the wax is approached. The inlay wax for direct wax pattern must have high flow, a few degrees above mouth temperature and should be tolerated by the patient at that temperature. At mouth temperature it should have no flow to minimize the distortion of the pattern when removed from the mouth. Residual Stress: Regardless of the method used to prepare wax pattern, residual stresses exist in the completed wax [pattern. When the specimen is held under compression during cooling, the atoms and molecules are forced closer together than when they are under no external stress. After the specimen is cooled on room temperature and the load is removed, the motion of the molecules is restricted, and this restriction results in residual stress in the specimen. When the specimen is heated, release of the residual stress is 4
added to the normal thermal expansion, and the total expansion is greater than normal. When the specimen is cooled while under tensile stress and the expansion as a result of heating is measured, the release of the residual tensile stress results in a dimensional change that is opposite to thermal expansion. If a sufficiently large amount of residual stress is introduced, the overall dimensional change on heating results in a contraction of the specimen. Ductility: Like flow, the ductility increases as the temperature of the wax is increased. In general, waxes with low melting temperatures have greater ductility than those with higher melting temperature.
DENTAL WAXES Pattern Waxes: Are used to form the general predetermined size and contour of an artificial dental restoration, which is to be constructed of a more durable, material such as cast gold alloys, cobalt-chromium-nickel alloys, or polymethyl methacrylate resin. All pattern waxes have two major qualities, thermal change in dimension and tendency to warp or distort on standing, which cause serious problems in their use. INLAY CASTING WAX Restorations such as inlays, crowns and bridges are formed by gold casting process that uses the lost wax pattern technique. If the pattern is made in tooth itself, it is said to be prepared by direct technique. If it is prepared on a die, the procedure is called the indirect technique. IDEAL REQUIREMENTS 1.
When softened, the wax should be uniform. There should be no graininess or hard spots in the plastic material.
The colour should be such that it will contrast with die materials. A definite contrast in colour facilitates proper finishing of margins.
There should be no flakiness or roughening of the surface when the wax is moulded after softening
The wax should not pull with the carving instrument or chip as it is carved.
After the mould has been formed, the wax should vaporize at 500째 C and have no residue.
The wax pattern-should be completely rigid and dimensionally stable at all times until it is eliminated
It should be sufficiently plastic at a temperature slightly above mouth temperature and should becom; rigid when cooled to mouth temperature.
It should have a flow of not less than 70% at 45째 c and not more than 1% at 37째 c. CLASSIFICATION A.D.A. specifiction No.4 divides dental inlay casting wax into 2 types Type-I Type-II COMPOSITION A medium wax, employed in direct tehnic A soft wax used for indirect technic for inlays and crowns. The essential ingredients of a succesoful inlay wax axe. Paraffin wax Gum dammar Carnauba wax Colouring agents. Paraffin Wax: It is the main ingredient usually 40 to 60%. It is used to establish the melting point. Different varieties, with a different melting point can be produced. Paraffin wax flakes when trimmed and does not give a smooth surface, so other waxes are added as modifying agents. Gum Dammar: Or Dammar resin which is a natural derivative from pine tree improves the smoothness in moulding and makes it more resistant to cracking and flaking. It also increases toughness of the wax and enhances the luster of the surface. Carnauba Wax: Occurs as a fine powder on the leaves of certain tropical palms. This wax is quite hard, it has a high melting point. It is combined with paraffin to decrease the flow at mouth temperature. It has an aggreeable odour and gives glossiness to the wax surface. Candelilla Wax: Can also be added to replace carnauba wax. It contributes the same qualities as carnauba wax, but its melting is lower and is not as hard as carnauba wax. 6
In modern inlay waxes, the carnauba wax is often placed partly by certain synthetic waxes that are compatible with the paraffin wax. Because of the high melting point of synthetic waxes, more paraffin can be incorporated and the oral working are improved. PROPERTIES OF INLAY WAX Flow: Type I inlay wax exhibits a marked plasticity or flow at a temperature slightly above mouth temperature. The wax begins to harden at approx. 50° C, and it is solid below approx. 40° C when it cools at a constant rate. Inlay waxes do not solidify with space lattice. The structure is more likely to be a combination of crystalline and amorphous materials with a limited ordering of molecules. The wax lacks rigidity and may even flow under stress even at room temperature. The maximum flow for type I waxes at 37° C is 1%. The low flow at this temperature allows carving and removal of the pattern from the cavity at mouth temperature without distortion. Both type-I and type-II waxes must have a minimal flow of 70% at 45° C and a maximum of 90%. It is at this temperature the wax is inserted into the prepared cavity. If the wax does not have sufficient plasticity, it does not flow into all the details of the prepared cavity. Thermal Properties: a.
Thermal conductivity : Inlay waxes are softened with heat, forced into the prepared cavity in the tooth or die and cooled. The thermal conductivity of waxes is low and time is required to heat the wax uniformly and to cool them to body or room temperature.
Co-efficient of thermal expansion: Inlay wax has a high coefficient of thermal expansion. The wax may have a linear expansion of 0.7% with increase in temperature of 20° c. It's thermal changes are higher than any other dental materials.
This property is less significant in indirect technic as it is not subjected to a change from mouth to room temperature. If the wax is allowed to cool under pressure its thermal properties are changed. When reheated, the linear coefficient of thermal expansion is increased. Factors such as temperature of die and method used for application of pressure to wax as it solidifies also influence the coefficient of thermal expansion. Wax Distortion: 7
Distortion is the most serious problem in inlay wax. Distortion is due to release of stresses inherent in the pattern caused due to : 1. Contraction on cooling 2. Occluded gas bubbles 3 Change of shape of the wax during moulding 4. From manipulation variables like carving, pooling, and removal. Thus the amount of residual stress are dependent on ; 1. The method of forming the pattern 2. Its handling and 3. Time and temperature of storage of the wax pattern. Causes of Distortion: Distortion is due to any method of manipulation that creates inhomogenity of wax involving the inter molecular distance. Factors causing distortion under control of the operator cannot be totally eliminated. Distortion of the wax can occur: 1.
If wax is not at uniform temperature when inserted in the cavity, some parts of the wax pattern may thermally contract more than others when stresses are introduced.
If the wax is not held under unifom pressure during cooling.
If wax is melted and added in an area of deficiency, the added wax will introduce stresses during cooling.
During carving some molecules of wax will be disturbed and result.
If the technic used where minimum carving and change in temperature and similar factors are minimized, there will be less relaxation and warpage of the wax pattern.
In direct technic, distortion is not as great if relaxation of stresses may take place away from the die in marginal areas or on surface. If this happens the wax should be readapted. Some relaxation distortion of pattern occurs regardless of the method used. It can only be reduced to a point that it is not of clinical importance. MANIPULATION OF INLAY WAX Inlay casting wax should exhibit high flow at the time of insertion to the cavity so that it reduces the internal details of the cavity. The wax is softened uniformly to avoid distortion due stresses induced which are released later. The wax is softened with dry heat over a flame taking care not to volatilize the wax. Hold the stick of wax over the visible flame and rotate it rapidly until it becomes plastic. The wax is heated and shaped approximately to the form of the prepared cavity. The pattern should be formed at the minimum temperature required to give adequate plasticity. A.D.A. specification No.4 gives adequate plasticity in a temperature range which is tolerated :he pulp. After the wax is introduced (inserted) into the cavity, it is held under pressure while it solidifies, wax should be allowed to cool gradually to mouth temperature in order to minimize internal stresses. Cooling rapidly by application of cold water results in differential contraction and development of internal stresses. Localized reheating of wax with warm carving instruments has a similar effect and more distortion may occur. A cold carving instrument should be used for direct wax pattern. With aw the wax pattern carefully in the long axis of the preparation. The pattern should be touched as little as possible with the hands to avoid temperature changes, invest the pattern as soon as possible to minimize distortion. Indirect pattern is prepared over a lubricated die. If molten wax is poured, very little residual stresses r. In case of full crowns, the die can be dipped repeatedly into liquid wax. The wax is allowed to cool, carved and removed from the die. The wax pattern is invested as soon as possible to minimize errors. Uniform softening can also be achieved by immersing the wax in warm water. However, this technic is not recommended as the soluble constituents maybe leached 9
out and the properties of wax will change. Water will incorporated in the wax which causes splattering upon the flame, interfere with the softening of t wax surface and distortion of the pattern on thermal changes. CASTING WAX The patterns for the metallic framework of removable partial dentures and other similar structures is fabricated from the casting waxes. The casting waxes are highly ductile. Mode of Supply: It is available in the form of i. Sheets 0.40 and 0.32 mm thickness ii. Readymade shapes -
Round rods (10cm long)
Half round rods
Half pear shaped rods,
iii. Bulk. PROPERTIES: These waxes are tacky and highly ductile. They should adapt and adhere easily to the refractory cast. They should burn out without leaving any residue. BASE PLATE WAX Applications: a) To make occlusion rims, which is used on the baseplate tray to establish the vertical dimension, the plane of occlusion, and the initial arch form in the technic for complete denture construction. This wax may also be used to form all or a portion of the tray itself. b) To produce the desired contour of the denture after teeth are set in position. c) To make patterns for orthodontic appliances and prostheses other than complete dentures which are to be constructed of plastics. d) To check various articulating relations in the mouth and to transfer them to mechanical articulators. Classification: 10
ADA specification No. 24 Type I Soft
- for building veneers
Type II Medium
- tried in mouths in normal climatic conditions.
Type III Hard
- for trial in mouths
Mode of supply: Available in the form of sheets of pink or red colour. Composition: Ceresin
Natural or synthetic
PROCESSING WAXES These are those waxes used mainly as accessory aids in the construction of a variety of restorations and appliances either clinically or in the laboratory; eg. boxing wax, beading wax, utility wax and sticky wax. BOXING WAX It is used to build up vertical walls around the impression, to produce the desired size and form of the base of the cast, and to preserve certain landmarks of the impression. Mode of supply - Boxing wax as sheets, Beading wax as ropes. 1. It preserves the extensions. 2. It controls the thickness of the borders. 3. It controls the form and thickness of the base of the cast. 4. It conserves the artificial stone. The impression is placed on a bench and soft wax is used so that the ridge portion is approximately parallel to the bench top. 11
Beading wax is adapted around the periphery. This wax should be approximately 4mm wide and 3 -4mm below the borders of the impression. The height is adjusted until a boxing wax strip extends approximately 13mm above the highest point on the impression. Artificial teeth are attached with this wax on mounts. Properties: They are pliable so it can be adapted easily. Its tackyness allows it to stick to the impression. UTILITY WAX It consists mainly of bees wax, petrolatum, and other soft waxes in varying proportions. Supplied in the form of sticks and sheets. It is used for giving a more desirable contour to a perforated tray for use with the hydrocolloids; to build up flange of tray and raise palatal portions of tray posteriorly and deep palate. As it is pliable it can be moulded at room temperature and as it is adhesive it can stick to the tray. STICKY WAX It consists mainly of yellow beeswax, rosin, and natural resins such as gum dammar. It is sticky when melted and adheres closely to the surfaces upon which it is applied. At room temperature, it is firm, free from tackiness, and brittle. It is used for joining metal parts before soldering and for joining fragments of broken dentures before repair procedure. IMPRESSION WAXES These are used to record non-undercut edentulous portions of the mouth, and are generally used in combination with other impression materials such as polysulfide rubber, ZOE, or dental impression compound; eg. corrective impression wax, bite registration wax. CORRECTIVE IMPRESSION WAX It is used as a wax veneer over an original impression to contact and register the detail of the soft tissues. They consist of paraffin, ceresin and beeswax. The flow at 37째 C is 100%. These waxes are subjected to distortion during removal from the mouth. Application: Functional impression of distal extension partial dentures. To record posterior palatal seal in dentures. Functional impression for obturators.
BITE REGISTRATION WAX It is used to articulate accurately certain model of opposing quadrants. Bite waxes consist of bees wax or hydrocarbon waxes such as paraffin or ceresin. The flow at 37째 C ranges from 2.5 % to 22%. Mode of supply - as u-shaped rods. The bite registration wax is interposed between teeth and the patient is asked to close in the position for jaw relation recording. The indentations thus formed on the wax are used to place the casts in position and then transfer to the articulator.
DENTAL CASTING INVESTMENT MATERIALS DEFINITION An investment can be described as a ceramic material which is suitable for forming a mold into which a metal or alloy is appropriately cast. The procedure for forming the mold is described as "investing" (wax pattern). REQUIREMENTS OF AN INVESTMENT MATERIAL All investment materials must contain. a)
Refractory substance : A material that will withstand high temperatures without decomposing or disintegrating.
A binder : A material which will set and bind together the particles of refractory substance. In addition, the following properties are desirable.
The mold must expand to compensate for the casting shrinkage, on cooling of the alloy
The powder should have a fine particle size to give a smooth surface on the casting.
The investment should be easily manipulated. It should have a suitable setting time.
The material should have a smooth consistency when mixed.
The set material should be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting procedure. 13
On being heated to higher temperatures, the investment must not decompose to give off gases that may corrode the surface of the alloy.
The investment must have adequate strength at room temperature to permit ease in handling, and enough strength at higher temperatures to withstand the impact force of the molten metal.
Casting temperatures should not be critical.
After the casting is completed, the investment should break away readily from the surface of the metal and should not react chemically with it.
The investment should have enough expansion to compensate for shrinkage of the wax pattern and the metal that takes place during the casting procedure.
m) The material should be economical. In general, the investment materials contains : i)
TYPES OF INVESTMENT MATERIALS There are three types of investment materials. They all contain silica as the refractory constituent. The type of binder used is different. 1.
Gypsum bonded investments: They are used for casting gold alloys. They can withstand temperature upto 700째 C.
Phosphate bonded investments: They are used for casting cobalt-chromium alloys. They can withstand high temperatures.
Silica bonded investments: They are an alternative to the phosphate bonded investments, for high temperature casting. They are principally used in the casting of base metal alloy partial dentures.
GYPSUM BONDED INVESTMENTS Classification: According A.D.A. specification No.2 for casting investments for dental gold alloys, there are 3 types. Type - I: Are those used for the casting of inlays and crowns and major compensation of casting shrinkage is accomplished principally by thermal expansion of the investment. Type-II: Are used for casting of inalys or crowns and major mode of compensation is by hygroscopic expansion of the investment. Type-III: Are utilized for construction of partial dentures with gold alloys. COMPOSITION 1. Refractory Material: They are form of silica (silicon dioxide), such as quartz, tridymite, or cristobalite, or a mixture of these. They serve 2 functions: 1. Provides refractory material during heating. 2. Regulates the thermal expansion. 2. Binder: Since the refractory materials alone do not form a coherent solid mass, some kind of binder is needed. The common binder used for dental casting gold alloys is dental stone i.e., alpha-calcium sulfate hemihydrate. The investments used for casting cobalt chromium alloys utilize sodium silicate, ethyl silicate, ammonium sulphate, sodium phosphate. 3. Other Chemicals: Usually a mixture of refractory materials and a binder alone is not enough to produce all the desirable properties of an investment. Other chemicals such as sodium chloride, boric acid, potassium sulfate, grafite, copper powder or magnesium oxide often are added in small quantities to modify the physical properties. Composition: 1. Silica
60 to 65%
2. Alpha hemihydrate
OR Dental stone
30 to 35%
3. Chemical modifiers
FUNCTION OF EACH CONSTITUENT 1.
Gypsum: Alpha-hemihydrate: a)
It acts as a binder for silica in casting gold alloys with a melting range below 1000째 C.
It imparts strength to the mold.
Contributes to mold expansion by setting expansion.
When gypsum is heated to high temperature, it shrinks and fractures. At 700째 C, it shows slight expansion and then great amount of contraction. The shrinkage is due to decomposition and release of sulfur dioxide, it contaminates the casting with the sulfides of silver and copper. So the gypsum bonded investments should not be heated above 700째 C. 2.
Silica: Quartz or Cristobalite:
a. Acts as a refractory during heating. b. Regulates thermal expansion. c. Provides mold expansion. The dental stone shrinks when heated. With addition of proper form of silica in the investment, such a contraction is eliminated and changed to an expansion during heating. Silica: Exists in at least four allotropic forms : 1. Quartz 2. Tridymite 3. Cristobalite 4. Fused quartz. Quartz or cristobalite or a combination of the two is used. 16
When heated quartz or cristobalite a change in crystalline form occurs at a transition temperature, characteristic of the particular form of silica. Quartz when heated, inverts from a 'low' form know as alpha-quartz to a 'high' form called as beta-quartz, at a temperature of 575째 C. Cristobalite similarly when heated, inverts from 'low' or alpha-cristobalite to 'high' or beta-cristobalite at a temperature between 200째 C and 270째 C. The beta-allotropic forms are stable only above the transition temperature. An inversion to the low or alpha-form occurs upon cooling in each case. In powdered form, the inversion occur over a range of temperature rather than instantaneously. The density decreases as alpha-form changes to beta form, with a resulting increase in volume and a rapid increase in linear expansion takes place. Depending on the type of silica used, the dental investments are classified as quartz investments or cirstobalite investments. 3. Modifiers: Modifiers used are : -
Reducing agents: They reduce any oxides formed on the metal by providing a nonoxidizing atmosphere in the mold when the mold alloy enters. Eg. Carbon or Copper powder. Modifying chemicals: They regulate setting expansion and setting time and also prevent shrinkage of gypsum when heated above 300" C. Eg. Boric acid and Sodium chlroide. SETTING TIME According to A.D. A. specification No.2 for inlay casting investments, the setting time should not be less than 5 minutes and not more than 25 minutes. 17
The modern inlay investments set initially in 9 to 18 minutes. Sufficient time should be allowed for mixing and investing the pattern. The setting time is measured in the same manner as for plaster. Factors controlling setting time: a. Manufacturing process. b. Mixing time and rate. c. Water/Powder ratio. d. Temperature. e. Modifiers - Accelerators and retarders. SETTING EXPANSION The purpose of the setting expansion is to aid in enlarging the mold to compensate partially for the casting shrinkage of the gold. Setting expansions are of 3 types: i
Normal setting expansion,
Hygroscopic setting expansion,
Normal Setting Expansion:
A mixture of silica and gypsum hemihydrate results in setting greater than that of the gypsum product when it is used alone. The silica particles probably interfere with the intermeshing and interlocking of the crystals as they form. Thus, the thrust of the crystals is outward during growth and therefore more effective in the production of an expansion. A.D. A. Sp. No.2 for Type-I investment permits a maximum setting expansion in air of 0.5%. Modern investments show setting expansion of 0.4%. It can be regulated by retarders and accelerators. ii)
Hygroscopic Setting Expansion: 18
When gypsum products are allowed to set in contact with water, the amount of expansion exhibited is much greater than normal setting expansion. The increased amount of expansion is because, water helps the outward growth of crystals. This additional expansion is known as hygroscopic setting expansion. The investment should be immersed in water before initial set is complete. A.D. A. Sp.No.2 for Type II investments requires a minimal 1.2% and maximum 2.2% expansion. Factors affecting hygroscopic setting expansion: 1.
Effect of composition: The finer the particle size of the silica, the greater is the hygroscopic expansion. The alpha-hemihydrate produces a greater expansion than beta-hemihydrate in the presence of silica. Higher the silica content greater is the expansion.
Effect of the W:P ratio: The higher the W/P ratio of the original investment water mixture, the less is the hygroscopic setting expansion.
Effect of Temperature: Higher the temperature of immersion water, less is the surface tension and hence greater is the expansion.
Effect of time of immersion: Immersion before the initial set causes greater expansion.
Effect of Spatulation: The shorter the mixing time, the less is the hygroscopic expansion.
Effect of Shelf Life of the Investment: The older the investment, the less is the hygroscopic expansion.
Effect of Confinement: The confining effect by opposing forces, such as the walls of the container in which the investment is placed on the wall of a wax pattern, is much more pronounced on the hygroscopic expansion than the similar effect on the normal setting expansion. The effective hygroscopic setting expansion is, therefore, likely to be less in proportion to the expected expansion than is the normal setting expansion.
Effect of the Amount of Added Water: More amount of water added during the setting period, more is the expansion (till maximum expansion only).
iii) Thermal Expansion: 19
The thermal expansion of the gypsum-bonded investment is directly related to the amount of silica present and the type of silica employed. Type-I investments should have a thermal expansion of not less than 1.0% nor greater than 1.6%. The desirable amount of thermal expansion depends on whether thermal expansion will compensate the casting shrinkage or it will be compensated by hygroscopic setting expansion. If hygroscopic expansion technique is used, then the thermal expansion of 0.5% to 0.6% is sufficient. But, if the compensation is by thermal expansion together with normal setting expansion, then the thermal expansion should be 1.0% to 2.0%. The maximum thermal expansion should be achieved or attained at a temperature not greater than 700째C. The gold alloys get contaminated at a mold temperature of higher than 700째 C. Effect of the W:P ratio: More the water used for mixing, less will be the thermal expansion Effect of Chemical Modifiers: Addition of small amount of sodium chloride, potassium chloride and lithium chloride increases the thermal expansion and eliminates the contraction caused by gypsum. STRENGTH: According to A.D.A. specification No.2, the compressive strength for inlay investments should not be less than 2.5 MPa (350 psi) when tested 2 hours after setting. Use of alpha-hemihydrate increases compressive strength (than beta-hymihydrate). Use of chemical modifiers increases the strength. More water used during mixing, less is the strength. Heating investment to 700째 C may increase or decrease strength as much as 65% depending on the composition. The greatest reduction in strength upon heating is found in investments containing sodium chloride. After the investment has cooled to room temperature, its strength decreases considerably, because of fine cracks that form during cooling. POROSITY The more gypsum crystals present in the set investment, the less is its porosity. Less the hemihydrate content and greater the amount of gauging water, the more is its porosity. A mixture of coarse and fine particles exhibits less porosity than an investment composed of a uniform particle size. 20
FINENESS A fine particle-size is preferable to a coarse one. The finer the investment, the smaller will be the surface iregularities on the casting. STORAGE The investments should be stored in air-tight and moisture-proof containers. Purchase the investment in relatively small quantities. HYGROSCOPIC -THERMAL INLAY CASTING INVESTMENT A new inlay casting investment that can be used as a hygroscopic or thermal type has become available. The investment contains a blend of quartz and cristobalite as the refractory. When used in the hygroscopic casting technic, the investment is heated to 482° C (900° F) after setting in accordance with the normal water immersion technic. When used in the thermal casting technic, the investment is not immersed in water but after setting is heated to 649° C (1200° F) so that the appropriate expansion is achieved. INVESTMENTS FOR CASTING HIGH MELTING ALLOYS The alloys used for metal ceramic restorations and cobalt chromium alloys for casting partial dentures, have high melting temperatures. They are cast in moulds at 850° C to 1100° C. At high temperatures, the gypsum bonded investments disintegrates. Hence investments disintegrates. Hence investments which can withstand high temperatures are required. The binder used is different. The investment used for this purpose are : 1. Phosphate bonded investments. 2. Silica bonded investments. 1. PHOSPHATE BONDED INVESTMENTS This type of investment consists of three different types of components, each of which is responsible for certain characteristic properties. The components are: i. Ammonium diacid phosphate : NH4 H2 PO4 21
a. It gives strength at room temperature. b. It is soluble in water and provides phosphate ions. c. It reacts with silica at high temperature to increase strength at casting temperature, ii. Silica: Refractory iii. Magnesium oxide : Reacts with phosphate ions. NH4 H2 PO4 + MgO -------ďƒ NH4 MgPO4 + H2O At room temperature ammonium diacid phosphate reacts with magnesium oxide to give the investment green strength, or room temperature strength. The ammonium diacid phosphate is used in a greater amount than is necessary for this reaction, so that the additional amount can react with silica at an elevated temperature. At higher temperature there is probably a superficial reaction between P 2O5 and SiO2 to form silicophosphate. which increases the strength of investment at higher temperature. The phosphate bonded investments are mixed with a special liquid supplied by the manufacturer. This liquid is a form of silica sol in water, which gives higher thermal expansion. SILICA BONDED INVESTMENTS The silica is the binder which may derive it from ethyl silicate or aqueous dispersion of colloidal silica or sodium silicate. One such investment consists of silica refractory, which 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 solution of silicic acid and ethyl alcohol. HCL Si(OC2H5) + 4H2O
Si(OH)4 + 4C2H5OH
Ethyl silicate has the disadvantage of containing inflammable components which are required for manufacture. Therefore, sodium silicate and colloidal silica are more common binders used.
These investments are supplied with two bottles of special liquid to be mixed with the investments. One bottle contains diluted water-soluble silicate solution such as sodium silicate. The other bottle usually contains diluted acid solution such as a solution of hydrochloric acid. The content of each bottle can be stored indefinitely. Before use the equal volume of each bottle is mixed so that hydrolysis can take place and freshly prepared silicic acid is formed. The powder/liquid ratio is used according to manufacturers instruction. SOLDERING INVESTMENT In the process of assembling the parts of a restoration by soldering, such as clasps on a removable partial denture, it is necessary to surround the parts with a suitable ceramic or investment parts are temporarily held together with sticky wax until they are surrounded with the appropriate investment material, after which the wax is softened and removed. The portion to be soldered is left exposed and free from investment to permit removal of the wax and effective heating previous to being joined with solder. The investment for soldering purposes is similar to casting investments containing quartz and a calcium sulfate hemihydrate binder. Soldering investments are designed to have lower setting and thermal expansions than casting investments, a feature that is desirable so that the assembled part; do not shift in dimension or position during the setting and heating of the investment. Soldering investments often are made of ingredients that do not have as fine a particle size as the casting investment, since\ the smoothness of the mass is less important. Relatively little information is available in the dental literature on the properties of soldering investments.
MODEL, CAST AND DIE MATERIALS Dental impressions are poured to prepare â€“ Models: Used primarily for observation, diagnosis. Casts : A working model, master cast. It is the positive replica on which restorations or appliances are fabricated. Dies : A working replica of single tooth or teeth on which inlays, crowns and bridges are made. Types of cast and die materials: 1. Gypsum
Dental stone, High strength - Type IV 23
Dental stone, High strength, High Expansion - Type V Dental stone + lignosulphonates. (This wetting agent reduces the water requirement of a stone and thus enables the production of a harder, stronger and more dense set gypsum). Dental plaster. 2. Metals Electroformed/Electroplated. Sprayed metals. Amalgam. 3. Polymers Metal-filled resins or inorganic filled resins. Epoxy. 4. Cements Silicophosphate or polyacrylic acid bonded cements. 5. Ceramic or For direct baking of porcelain crown or preparation Refractory
of wax patterns for casting.
die materials Ideal Properties of die materials: 1.
It should be dimensionally accurate.
It should have high abrasion resistance, should possess good strength and have a smooth surface. :
Toughness-To allow burnishing of foil and resist breakage.
Ability to reproduce all fine details in the impression. 24
Compatibility with all impression materials.
Colour contrast with wax, porcelain and alloys.
Easy and quick manipulation and rapid fabrication (setting time).
Non-injurious to health by touch or inhalation.
Economical - low cost.
Recommended die materials: Stone dies Type IV and Type V Electroformed dies. Epoxy resins, refractory or ceramic materials. Disadvantages of die materials which are least suitable: 1.
Polymers: Shrink during polymerisation, so they produce an undersized die.
Cements : All cements shrink slightly and exhibit brittleness and have a tendency to crack due to dehydration.
Metal - sprayed dies: The bismuth - tin alloy is rather soft; care is needed to prevent abrasion of the die.
1) Improved dental stone or Die Stone: The most accurate and commonly used die materials are still alpha-hemihydrate Type IV and Type V gypsum products. Type IV gypsum products have cuboidal-shaped particles and the reduced surface area produce the required properties of strength, hardness and minimal setting expansion. The most recent gypsum product, having an even higher compressive strength than the Type IV is Dental Stone, High strength, High expansion - Type V. The setting expansion has been increased from 0.01 to 0.30%. This higher setting expansion is required in the stone used for the die to aid in compensation for the base metal alloy solidification shrinkage. Advantages: 25
Excellent working time.
Compatible with impression materials.
Have smooth, hard surface.
Can be easily trimmed.
Have good colour contrast.
Edges and occlusal surface may be rubbed off by repeated contact.
2) Electro formed/Electroplated Casts and Dies: Electro deposition of copper or silver on the impression gives a hard metallic surface to the cast. Advantages: 1.
Hard, abrasion resistant.
Imparts a smooth surface to the wax pattern in contact.
Not very expensive.
Better marginal definition.
Does not absorb oil or water.
Prevents cuspal wear due to repeated contact with opposing cast.
Difficult to trim.
Silver bath. - health hazard.
Not compatible with all impression materials.
Colour contrast not as good as die stone.
Adaptation of wax not as good as to the alpha-hemihydrate. Pattern tends to lift from margins
Electro forming of impression for dies and casts: It is also known as electroplating and electrodeposition. Electroforming is a process by which a metal surface is deposited on the impression i.e. a metal is deposited on a nonmetal which is a non-conductor of electricity. Thus a cast is formed with a metal surface. First the impression surface is made conductive by metalizing it and then metal is deposited over the impression surface. Metalizing: The impression surface is made conductive so that it can conduct electric current. Thin layer of metal is laid down, on the surface of the impression. After metalizing subsequently it is electroplated. The metalizing agents are: 1. Bronzing powder suspended in almond oil. 2. Aqueous suspension of silver powder. 3. Powdered graphite. The metalizing agents are burnished on the impression with a camel's hair brush. Metals used for electroforming are: 1. Copper. 27
2. Silver. Plating can be done for : A) Individual tooth impression. B) Full arch impression. Plating is done on
Compound impression. Polysulphide impression. Silicone impression. Other impression materials show dimensional inaccuracies when plated. Requirements for electroplating: 1. The impression to be coated is made the cathode. 2. Anode is the metal to be deposited, copper or silver. 3. Anode holder. Cathode holder. 4. Electrolyte is the solution through which the electric current is passed. ions are deposited from the anode to the cathode.
- Silver cyanide or copper sulphate. 5. Ammeter -Registers the current in milliamperes 0 to 500 ma. The current passed is of 10 milliamps per tooth area, for 12 hours. 6. Plating tank - Glass or hard rubber with well fitting cover to prevent evaporation. 7. In 10 to 12 hours, deposition of 0.5 mm takes place. Temperature : 77째 to 80째 F (room temperature). With a lower current - deposit is slow but hard. With a higher current - deposit is fast but soft. Compound impression is copper plated. Polysulphide rubber impression is silverplated. 28
1. Wash and dry the impression. 2. Attach the impression to the cathode holder. An insulated wire is used for the purpose. 3. The impression material is a non-conductor of electricity. It is made conductive by applying a metalizing solution. Colloidal graphite
- For copper plating.
- For silver plating, with a brush. The excess powder is blown off.
4. The surface of the copper ring or impression tray is covered by wax 2mm. beyond the margin of the impression, so the ring is not plated. 5. With a dropper, the impression is filled with the electrolyte. Silver cyanide for silver - plating.
Copper sulphate - for copper -plating. Avoid air - bubbles. 6. The electrode is attached to the cathode and the impression is immersed in the electrolyte bath. The distance between the cathode (impression) and anode (metal) should be atleast 4". 7. The electric current is turned on. It should not exceed 50 milliamperes. The current is adjusted to 10 mm per tooth. If high current is used the surface will be granular, rough and weak. With low currents the deposit is smooth and hard. 8. Run the current for 12 to 15 hours, to get a deposit of 0.5 mm. 9. The current is disconnected. The impression is washed. The die is completed by pouring resin or dental stone to support the surface of the cast and to form the base. Then the copper ring is gently warmed over the flame & removed from the die. The die is trimmed.
Composition of Electroplating bath: Copper Copper sulphate crystals
Sulfuric acid concentrate
Silver Silver cyanide
3) Epoxy Resin die materials: Are compatible with -
Polysulphide Silicone impression materials.
Advantages : Tougher and more abrasion resistant then die stone. Disadvantage : Not as accurate and dimensionally stable, as it shrinks (0.1%). It may take 24 hrs. for setting
Available as : 2 components - Resin and hardner. Die stone Investment combination. (Divestment) This is a combination, of die material and investing medium. A gypsum bonded material called Divestment is mixed with a colloidal silica liquid. A die is prepared from the mix and a wax pattern is constructed on it. Then the die with the wax pattern is invested in Divestment. The possibility of distortion of wax pattern during removal from the die or during setting of the investment, is minimised. The setting expansion of divestment is 0.9% and thermal expansion 0.6%, when heated to 677째 C. As it is a gypsum bonded material it is not recommended for high fusing alloys like those used in metal-ceramic restorations. It is a highly accurate technique for conventional gold alloys especially for extra-coronal preparations. Divestment phosphate or DVP is a phosphate - bonded investment that is used in the same manner as Divestment and is suitable for use with high fusing alloys.
CASTING PROCEDURES FOR DENTAL ALLOYS STEPS IN CASTING 1) Tooth Preparation: Prepare the tooth or teeth to receive a cast restoration. 2) Impression: Make an impression of the prepared tooth. 3) Die Preparation: A die is prepared from die stone (with expansion of 0.01%) or the impression is electroformed. A die spacer is coated or painted over the die. 4) Wax Pattern: Make a wax pattern from type II inlay casting wax, with all precautions to avoid distortion. 5) Sprue Former: Is made of wax, plastic, or metal. Thickness is in proportion to the wax pattern. A reservoir it; attached to the sprue or the attachment of sprue to wax pattern is flared. Adjust the length of the sprue to 3/8" to 1/2", so that the wax pattern will be approximately 1/4" from the top of the ring. The purpose of spruing the wax pattern is fourfold : a)
To form a mount for the wax pattern. 31
To create a channel for elimination of wax during burnout.
To form a channel for the ingress of molten alloy during casting.
To compensate for alloy shrinkage during solidification.
6) Casting Ring Liner: A ring liner is placed on the inner side of the casting ring. It should be short at one end. The types of nonabestos ring liners used are : a)
Fibrous ceramic aluminous silicate.
Ceramic - cellulose combination.
(Asbestos has been discontinued^ its fibres when inhaled are carcinogenic). Functions:
Allows for mould expansion.
When the ring is transfered from the furnace to the casting machines, it reduces loss of heat as it is a thermal insulator.
Permits easy separation of the investment from the ring after the casting is over.
Investing: Apply wetting agent on the wax pattern. Seat the casting ring into the crucible former taking care that there is uniform space around the wax pattern. The pattern should be located near the centre of the ring.
Mix the investment as per the recommended W/P ratio and vibrate. Take some investment on the brush and apply on the wax pattern pushing it forwards from an edge to the deeper parts of the pattern to avoid air bubbles. The ring is reseated on the crucible former and placed on the vibrator and gradually filled with the remaining investment mix. Allow it to set for 1 hour. 8)
Wax Elimination and Heating (Burnout): The purpose of burnout is the elimination of wax from the mould cavity and acheiving thermal expansion. 32
Separate the crucible from the ring. If metallic sprue former is used, it should be removed before burnout. Plastic sprue requires double burnout. Burnout is started when the mould is wet. Store in humidator if burnout is to be delayed. Heating should be gradual. Rapid heating causes formation of steam which causes the walls of the mould cavity to flake. In extreme cases an explosion may occur. Fast or too rapid heating causes cracks in the investment due to uneven expansion. For burnout the ring is heated gradually to 400째C in 20 minutes and maintain it for 30 minutes. In the next 30 minutes, raise the temperature to 700째C and maintain it for 30 minutes. The casting should be completed as soon as the ring is ready. If the ring cools before casting, the investment contracts. 9) CASTING CASTING MACHINES All casting machines can be divided into two general types, which force the metal into the mould They are: 1) Centrifugal force type, and 2) Air pressure type. Numerous modifications and variations of these principles are employed in different instruments. A variety of centifugal machines are available. -
It is spring driven or motor driven.
Alloy is fused by electric resistance or induction furnace or on a refractory tray by a blowtorch.
The main advantage of the centrifugal machines is the simplicity of design and operation, with the opportunity to cast both large and small castings on the same machine. In air pressure type of machine, either compressed air or gases like carbon dioxide or nitrogen, can be used to force the molten metal into the mould. This type of machine is satisfactory for making small castings.
Casting machines (both centrifugal and gas pressure type) with an attached vaccum system are also available. FUSION OF NOBLE METAL ALLOY The various modes for melting alloys may be grouped into two categories : 1) By blow torch and 2) By electrical resistance or induction. Blow Torch: The fuel used is a combination of: 1) Natural or artificial gas and air, or 2) Oxygen and acetylene gas (high fusion alloys). The flame has Four Zones: 1)
Mixing Zone: Air and gas are mixed here. No heat is present. It is dark in colour. 2) Combustion Zone: This surrounds the inner zone. It is green in colour. It is a zone of partial combustion and has an oxidising nature. 3) Reducing Zone: It is blue in colour. It is just beyond the green zone. It is the hottest part of the flame. This zone is used for fusion of casting alloy. 4) Oxidizing Zone: It is the outermost zone in which final combustion between the gas and surrounding air occurs. This zone is not used for fusion. The air and gas mixture is adjusted to get a reducing flame, which is used to melt the alloy, because it does not contaminate the alloy and is the hottest part of the flame. When the alloy is molten it has a mirror like - appearance, like a ball of mercury. When the alloy is molten, the casting ring is shifted to the casting machine which is already wound 3 to 4 turns. Place the ring in the casting cradle so that the sprue hole adjoins the crucible. Slide the crucible against the ring to avoid spilling of molten metal. Sprinkle the flux over the metal to reduce the oxides and increase its fluidity for casting. Hold the casting arm so that the stop pin drops away. Release the arm and allow it rotate till it comes to rest. This creates centrifugal force which casts the metal into the mould cavity. The ring is allowed to cool for 10 minutes till the glow of the metal disappears. Quenching: The ring is then immersed in a container of water. This leaves the casting metal in an annealed (softened) condition and also helps to fragment the investment. 34
Recovery of Casting: The investment is removed and the casting recovered. Sand Blasting: The casting is held in a sandblasting machine to clean the investment from its surface. Pickling: The surface oxides from the casting are removed by pickling in 50% hydrochloric acid. HC1 is heated, but not boiled with the casting in it. Polishing: Minimum polishing is required if all the procedures from the wax pattern to casting are followed meticulously.