Dental Waxes, Casting Investments, and Casting Procedures
Baseplate wax—Dental wax provided in sheet form to establish the initial arch form in the construction of complete dentures. This product typically contains approximately 75% paraffin or ceresin wax, beeswax other waxes, and resins.
Bite wax—A wax form used to record the occlusal surfaces of teeth as an aid in establishing maxillo-mandibular relationships.
Boxing wax—A wax sheet form used as a border at the perimeter of an impression to provide an enclosed boundary for the base of the cast to be made from a poured material such as gypsum or resin.
Corrective wax (dental impression wax)—A thermoplastic wax that is used to make a type of dental impression.
Dental wax—(1) A low-molecular-weight ester of fatty acids derived from natural or synthetic components, such as petroleum derivatives, that soften to a plastic state at a relatively low temperature. (2) A mixture of two or more waxes and additives used as an aid for the production of gypsum casts, production of nonmetallic denture bases, registering of jaw relations, and laboratory work.
Direct wax technique—A process whereby a wax pattern is prepared in the mouth directly on prepared teeth.
Elastic memory—Tendency of a solid wax form to partially return to its original shape when it is stored at a higher temperature than that to which it was cooled.
Flow—Relative ability of wax to plastically deform when it is heated slightly above body temperature.
Hygroscopic expansion—Amount of setting expansion that occurs when a gypsum-bonded casting investment is immersed in water, which is usually heated to approximately 38 °C. (See Chapter 9 for more information on this process.)
Inlay wax—A specialized dental wax that can be applied to dies to form direct or indirect patterns for the lost-wax technique, which is used for the casting of metals or hot pressing of ceramics.
History of Dental Wax
Wax has been a valuable commodity for over 2000 years. In ancient times beeswax was used for softening skin, binding together reeds used for flutes, coating and preserving valuable objects, candle production, and making sculptures and statues of highly regarded public figures. Beeswax was derived from secretions that bees use to build honeycombs. Although beeswax is still used today, modern waxes, such as those used to preserve furniture and automobile surfaces and those designed for dental procedures, are made from natural plant and animal sources; some types are derived synthetically from petroleum products and distillates. Synthetic waxes are typically composed of hydrogen, carbon, oxygen, and chlorine. Synthetic waxes are more uniform than natural waxes in their organic structure and more homogeneous in composition.
Carnauba is one of the hardest and most durable waxes. It is derived from the fronds of carnauba palm trees and is one of the main components of dental inlay wax. Candelilla wax, a major component of some dental waxes, is obtained from plants growing in Costa Rica, Guatemala, Mexico, Nicaragua, Panama, and the southwestern United States. In comparison to plant-derived carnauba and candelilla waxes, animal-derived beeswax, smd mineral-derived paraffin and ceresin waxes, other dental waxes are produced from components of fats, gums, oils, and resins.
Types of Inlay Waxes
The wide variety of dental waxes can be classified into two groups, those used primarily in the clinic and those used in commercial dental laboratories. Clinical products include bite registration wax, disclosing wax (also known as pressure-indicating paste), utility waxes for altering and adapting impression trays, and low-melting type I inlay waxes used in the mouth for direct-waxing processes for pattern production. Laboratory products include boxing wax, baseplate wax, sticky wax, beading wax, utility wax, and hard, medium, and soft type II inlay-type waxes for making patterns on patients’ models using the indirect wax technique. Type II waxes are required for the lost-wax processing of cast prostheses and frameworks. Each of these waxes has a melting range over which the temperature must be adjusted by means of a burner flame to control the flow properties for each specific application.
Dental waxes can also be classified in one of three types, pattern wax (inlay, casting, and baseplate types), processing wax (boxing, utility, and sticky types), and impression wax (bite registration and correction types). Casting wax is used for partial denture frameworks and other metal frameworks. One of the correction types includes waxes for repairing ceramic margin defects on all-ceramic inlays and crowns.
Inlay waxes are used to prepare patterns. These patterns are reproduced in gold via a casting process and in ceramic by hot-isostatic-pressing procedures. Inlay wax is sometimes referred to as casting wax, although other types of pattern waxes also fall into this category. Inlay wax must exhibit excellent adaptability to model or die surfaces, and it must be free from distortion, flaking, or chipping during the preparation of patterns. It must also be able to disintegrate, volatilize, and be eliminated completely from an investment mold during the burnout or wax elimination procedure. For direct wax techniques type I inlay wax must soften at a temperature that is not hazardous to the pulp tissue, and it must harden at a temperature above mouth temperature.
The colors of inlay waxes should contrast with the hues of teeth and dies. Dental waxes are supplied in a variety of colors including blue, green, yellow, red, and ivory. The colors are useful to provide a suitable contrast against a die that is an accurate replica of a prepared tooth or dental arch form. Ivory-colored wax is useful for aesthetic case presentations to patients. If applied as a veneer in a sufficient thickness, its opacity must be sufficient to mask colored die stones.
Inlay waxes may be softened over a flame or in water at 54 °C to 60 °C to enable their flow in the liquid state and their adaptation to the prepared tooth or die. These waxes are designed to maintain uniform workability over a wide temperature range and to facilitate accurate adaptation to the tooth or die under pressure. Additive layers and corrections may be applied to produce a relatively homogeneous pattern. These fused layers can be carved easily without chipping or flaking. A regular or soft type of wax is typically used for indirect work at room temperature or in cool weather. A harder or medium type with a low flow property is indicated for use in warmer climates.
The first procedure in the casting of an inlay or crown for the lost-wax process is the preparation of a dental wax pattern. The cavity is prepared in the tooth and the pattern is carved directly on a die that is a reproduction of the prepared tooth and dental tissues (indirect technique). The direct technique for producing wax inlay patterns within prepared teeth is rarely used because of the wax’s sensitivity to changes in pressure, temperature, and heating and cooling rates during manipulation. Because the thermal expansion coefficient of wax is extremely high compared with the values for other dental materials, a wax pattern made in the mouth (direct technique) will shrink appreciably as it is cooled to room temperature. A pattern made by the indirect method may not shrink as much, although the amount depends on whether or not the pattern is allowed to reach room temperature before it is removed from the die. Dipping waxes are used occasionally to facilitate the wax pattern preparation process. This wax is kept molten to provide a station for mass production of patterns.
Type I is a medium wax employed in direct techniques and type II is a soft wax used in the indirect techniques. No matter how a pattern is prepared, it should be an accurate reproduction of the missing tooth structure or part of a prosthesis that is adapted to soft tissues. The wax pattern forms the outline of the mold into which an alloy is cast or a ceramic is hot-isostatically pressed. Consequently, the resulting appliance, device, prosthesis, or framework can be no more accurate than the wax pattern regardless of the care observed in subsequent procedures. Therefore, the pattern should be well adapted to the prepared cavity or replica cavity and properly carved without any significant distortion. Before the adaptation of the wax pattern within a tooth or a die, a separating medium must be used to ensure the complete separation of the wax pattern without distortion. After the pattern is removed from the prepared cavity, it is encased in a gypsum- or phosphate-based material or other type of refractory material known as an investment. This process is called investing the pattern.
After investing anatomically accurate wax or resin patterns for inlays, onlays, crowns, bridges, and frameworks for removable partial dentures, the invested material must be eliminated completely before molten metal is cast or core ceramic is hot-pressed into the mold cavity. Wax patterns are used in the production of several types of complex removable devices or prostheses in addition to single-tooth restorations. However, this chapter is limited primarily to the construction of prostheses employed in operative dentistry and fixed prosthodontics.
Composition of Dental Waxes
The primary components of dental waxes are derived from synthetic waxes and natural waxes (hydrocarbons of the paraffin and the microcrystalline groups, carnauba wax, candelilla wax, and resins). As stated earlier, natural waxes are derived from mineral, vegetable, and animal origins. Synthetic waxes are chemically synthesized from natural wax molecules. Most synthetic waxes are more homogeneous than pure natural waxes. Coloring agents are added for contrast of wax patterns against tooth, die, and model surfaces or to provide an ivory-colored or other natural tooth color as demonstration models used for educating patients about treatment options. Some formulations contain a compatible filler to control expansion and shrinkage of the wax product.
Most dental waxes contain 40% to 60% paraffin by weight, which is derived from high-boiling fractions of petroleum. They are composed mainly of a complex mixture of hydrocarbons of the methane series together with minor amounts of amorphous and microcrystalline phases. The wax can be obtained in a wide range of melting or softening temperatures depending on the molecular weight and distribution of the constituents. The melting range can be determined by a temperature versus-time cooling curve, as shown in Figure 10-1, for a paraffin-based inlay wax. The temperature-time relationship during cooling indicates the successive solidification of progressively lower-molecular-weight fractions. This condition promotes moldability of the wax below its melting temperature. Paraffin that is used for type I waxes has a higher melting point than the paraffin used for type II waxes. Paraffin wax is likely to flake when it is trimmed, and it does not produce a smooth, glossy surface, which is a desirable requisite for an inlay wax. Thus, other waxes and natural resins must be added as modifying agents.
Gum dammar, or dammar resin, is a natural resin. It is added to the paraffin to improve the smoothness in molding and to render it more resistant to cracking and flaking. It also increases the toughness of the wax and enhances the smoothness and luster of the surface.
Carnauba wax occurs as a fine powder on the leaves of certain tropical palms. This wax is very hard, and it has a relatively high melting point and it has an agreeable odor. It is combined with the paraffin to decrease flow at mouth temperature. Carnauba wax contributes greater glossiness to the wax surface than dammar resin.
Candelilla wax can also be added partially or entirely to replace carnauba wax. Candelilla wax provides the same general qualities as carnauba wax but its melting point is lower and it is not as hard as carnauba wax. Ceresin (see below) may replace part of the paraffin to modify the toughness and carving characteristics of the wax.
Ceresin is typically a white wax extracted from ozokerite, a waxy mineral mixture of hydrocarbons that is colorless or white when pure, but it has a somewhat unpleasant odor.
Carnauba wax is often replaced in part by certain synthetic waxes that are compatible with paraffin wax. At least two waxes of this type can be used. One is a complex nitrogen derivative of the higher fatty acids and the other contains esters of acids derived from montan wax, a derivative hard wax that is obtained by solvent extraction of certain types of lignite or brown coal. Approximately a third of all montan wax produced is used in automobile polishing pastes.
For an impression compound, a synthetic wax is preferable to a natural wax because it has greater uniformity. Because of the high melting point of the synthetic waxes, more paraffin can be incorporated to improve the general working qualities of the product.
Desirable Properties of Wax
Control of the properties of dental wax is accomplished by a combination of factors. For example, certain wax formulations may be based on the amount of carnauba wax, the desired melting range of the hydrocarbon wax, and the addition of resin to achieve desirable properties.
Depending on the specific application of a given wax, the melting range, viscosity, adaptability, flow, elastic recovery, carvability, and burnout properties of these materials control the quality and reproducibility of the final prostheses and restorations. The most important properties of inlay waxes are as follows:
1. The wax should be uniform when softened. It should be compounded with ingredients that blend with each other so that there are no granules on the surface and no hard spots within the surface when the wax is softened.
2. The color should contrast with die materials or prepared teeth. Since it is necessary to carve the wax margins against the die surface, the wax must exhibit a definite contrast in color and sufficient opacity in thin layers to facilitate proper finishing of the margins.
3. The wax should not fragment into flakes or similar surface particles when it is molded after softening. Such flakiness is likely to be present in paraffin wax, so modifiers must be added to minimize this effect.
4. Once the wax pattern has solidified, it is necessary to carve the original tooth anatomy and the margins so that the pattern conforms precisely to the surface of the die. The latter procedure sometimes requires that the wax be carved to a very thin layer. The wax must not be pulled away by the carving instrument or chip as it is carved or such precision cannot be achieved.
5. For lost-wax casting of metals, an investment mold is formed around a wax pattern. After the mold containing the wax pattern has been formed, the wax must be eliminated from the mold. Elimination of the sprued wax pattern is usually accomplished by heating the mold to melt and ignite the wax. If the wax leaves a residue or an impervious coating on the walls of the mold, the cast metal inlay may be adversely affected. Consequently, the wax should burn out completely by oxidizing residual carbon to volatile gases. Ideally, when wax melts and is vaporized at 500 °C, it should not leave a solid residue that amounts to more than 0.10% of the original weight of the specimen.
Expansion and shrinkage of casting wax are extremely sensitive to temperature. Normally soft wax shrinks more than hard wax. High-shrinkage wax may cause significant pattern distortion when it solidifies. It is necessary to avoid excessive shrinkage and expansion caused by a temperature change. For this reason, organic filler is added to certain wax formulations. Such fillers should be completely miscible with the components of the inlay wax during manufacture, and they should not leave an undesirable residue after burnout.
Flow of Dental Wax
One of the desirable properties of type I inlay wax is that it should exhibit a marked plasticity or flow at a temperature slightly above that of the mouth. The temperatures at which the wax is plastic are indicated by the time-temperature cooling curve for a typical type I wax, as shown in Figure 10-1. The wax begins to harden slowly below 65 °C and becomes solid below approximately 48 °C; below which it cools rapidly at a constant rate.
Different types of casting waxes exhibit characteristic flow curves as a function of temperature. Each wax exhibits a sharp transition temperature at which it loses its plasticity. Soft wax exhibits a transition point at a lower temperature than hard wax. Inlay waxes do not solidify with a space lattice, as does a metal. Instead, the structure likely exhibits a combination of crystalline and amorphous structures, with limited short-range order of the molecules. The wax lacks rigidity and may flow under applied pressure even at room temperature.
Requirements for the flow properties of inlay waxes at specific temperatures are summarized in Table 10-1. The flow is measured by subjecting cylindrical specimens to a designated load at the stated temperature and measuring the percentage of reduction in length. The maximum flow permitted for type I waxes at 37 °C is 1%. Their low flow at this temperature permits carving and removal of the pattern from the prepared cavity at oral temperature without distortion. In addition, both type I and type II waxes at 45 °C must have a minimal flow of 70% and a maximum flow of 90%. At this temperature, the wax is inserted into the prepared cavity. If the wax does not have sufficient plasticity, it will not flow into all of the areas in the preparation and reproduce the details that were established by the invested wax pattern.
Requirements for Inlay Wax Flow (%)
|Type of Wax||T = 30 °C
|T = 37 °C
|T = 40 °C
|T = 40 °C
|T = 45 °C
|T = 45 °C
Thermal Properties of Dental Waxes
Inlay waxes are softened with heat, forced into the prepared tooth cavity in either the tooth or the die, and cooled. The thermal conductivity of the waxes is low (e.g., kparaffin = 0.25 W/mK), and sufficient time must be allowed both to heat them uniformly throughout and to cool them to body or room temperature.
Another thermal characteristic of inlay waxes is their high coefficient of thermal expansion. As shown in Figure 10-2, the wax may expand as much as 0.7% with an increase in temperature of 20 °C or contract as much as 0.35% when it is cooled from 37 °C to 25 °C. The average linear thermal expansion coefficient over this temperature range is 350 × 10−6/K, with values ranging from 217 to 512 × 10−6/K.
A comparison of the thermal expansion coefficients of dental materials given in the chapters on physical properties and ceramics indicates that inlay wax expands and contracts thermally more per degree temperature change than any other dental material. This property is less significant when the wax is used in the indirect technique because the pattern is not subjected to a change from mouth to room temperature.
The thermal dimensional change may be affected by the previous treatment of the wax. Curve A in Figure 10-2 represents the thermal expansion of inlay wax as a function of temperature. The expansion rate increases abruptly above approximately 35 °C. The temperature at which a change in rate occurs is known as the glass transition temperature. Some constituents of the wax probably change in their crystalline form at this temperature, and the wax is more plastic at higher temperatures. Not all waxes exhibit transition temperatures. The transition point shown in Figure 10-2 appears to be characteristic of an inlay wax with a high paraffin wax content. If the wax is allowed to cool without being placed under pressure, the transition temperature region is not as pronounced when it is reheated, nor is there much change in the thermal expansion coefficient, as shown in curve B of Figure 10-2.
Manipulation of Inlay Wax
The higher flow of softer waxes produces larger cast metal or hot-isostatically-pressed (HIP) ceramic prostheses than harder waxes because the soft waxes expand more as the investment heats up during setting and they offer less resistance to the expanding investment during setting. Waxes oxidize on heating, and on prolonged heating some waxes evaporate, so that the storage container for melted wax will be coated by gummy deposits. Therefore, care should be exercised to use the lowest temperature possible and to clean the wax pot and replace the wax periodically.
To manipulate inlay wax, dry heat is preferred to the use of a water bath. The latter can result in the inclusion of droplets of water, which can splatter on flaming, smear the wax surface during polishing, and distort the pattern during temperature changes. To avoid distortion during removal of the pattern, it should be penetrated slightly with an explorer point and carefully removed from the cavity. A mesial-occlusal-distal (MOD) pattern can best be removed by inserting a staple-like wire within the surface so that each prong is positioned above the proximal wall areas of the pattern. It should be removed with dental floss looped through the staple and withdrawn in a direction parallel to the axial walls and with minimum distortion. After removal, touching the pattern with the fingers should be avoided as much as possible to prevent any temperature changes and distortion.
To fabricate indirect patterns, the die should be lubricated, preferably with a lubricant containing a wetting agent. Any excess must be avoided because it will prevent intimate adaptation to the die. The melted wax may be added in layers with a spatula or a waxing instrument. The prepared cavity should be overfilled, and the wax then carved to the proper contour. When the margins are being carved, care should be taken to avoid abrading any surface of the stone die. A silk or other fine cloth may be used for a final polishing of the pattern, rubbing toward the margins. Theoretically, applying pressure is undesirable, as shown by change in shape of the the stick of wax in Figure 10-3. However, some clinicians prefer to apply finger pressure as the wax is cooling to help fill the cavity and prevent distortion during cooling. The fingers also accelerate the cooling rate. Although temperature changes should be avoided, some technicians prefer to repeatedly remelt small areas along the margins and examine them under a low-power microscope. Regardless of the method chosen, the most practical method for avoiding any possible delayed distortion is to invest the pattern immediately after removal from the mouth or die, as noted earlier. Once the investment hardens (sets), no distortion of the pattern will occur.
Distortion of wax patterns is the most serious problem one can experience in forming and removing the pattern from a tooth or die. Distortion of a wax pattern results from occluded air in the pattern, physical deformation (during molding, carving, or removal), release of stresses “trapped” during previous cooling, excessive storage time, and extreme temperature changes during storage.
A newly made wax pattern tends to change its shape and size over time. Upon cooling it contracts and, after attaining equilibrium, reaches a state of dimensional stability. It is important that the wax pattern be retained on the die for several hours to avoid distortion and ensure that equilibrium conditions are established.
Like other thermoplastics, waxes tend to return partially to their original shape after manipulation. This is known as elastic memory. To demonstrate this effect, a stick of inlay wax can be softened over a Bunsen burner, bent into a horseshoe shape, and chilled in this position. If it is then floated in room-temperature water for a number of hours, the horseshoe will open, as shown in Figure 10-3, A and B. This is more critical for inlay waxes than for other impression materials because the metal or hot-pressed ceramic restorations made from the wax must fit onto unyielding hard tooth tissue.
The elastic memory of waxes is further illustrated during measurement of the thermal expansion of a wax held under pressure during cooling. The expansion increases above the glass transition temperature more than when it is cooled without pressure, as shown in curve B of Figure 10-2. Again, this illustrates the tendency of wax to return to its normal undisturbed state. In recalling Figure 10-3, A, when the wax is bent into a horseshoe, the inner molecules are under compression and the outer ones are in tension. Once the stresses are gradually relieved at room temperature, the wax tends to recover its elastic strain.
The components of a casting ring with a wax pattern surrounded by casting investment are shown in Figure 10-4. A sprued wax pattern is shown in the center of the investment ring. Examples of properly sprued wax patterns for a single crown and for a three-unit bridge are shown in Figure 10-5. Note the spherical reservoir that is attached to the single crown pattern. The purpose of the reservoir is to maintain a molten pool of metal to ensure complete filling of the crown area of the mold cavity. For the bridge pattern on the right side of Figure 10-5, a runner bar is used as a reservoir. Additional details are presented later in this chapter.
Storage of a wax pattern for too long can lead to a distortion of its form because of stress relaxation effects. A casting will fit most accurately when the pattern is invested immediately after its removal from the preparation.
A pattern made of hard wax is less sensitive to temperature conditions than one made of soft wax. The exothermic heat generated during the setting of an investment affects the pattern selectively. A soft wax pattern may result in a slightly larger and relatively rougher casting than a hard wax pattern. This tendency of softer inlay waxes to expand during setting in a hygroscopic bath at 37.8 °C (100 °F) may contribute to the phenomenon of hygroscopic expansion, described further in Chapter 9.
Other types of waxes are employed for different purposes than those described for the inlay waxes. The composition of each type is adjusted for the particular requirements. One of the most common is baseplate wax.
Baseplate wax is used to establish the initial arch form in the construction of complete dentures. Supplied in 1- to 2-mm-thick red or pink sheets, the wax is approximately 75% paraffin or ceresin with additions of beeswax and other resins or waxes. The harder the wax, the less the flow at a given temperature. The difference in flow of the three types may be advantageous for a particular application. Type I, a soft wax, is used for building veneers. Type II, a medium wax, is designed for patterns to be placed in the mouth in normal climatic conditions. Type III, a hard wax, is used for trial fitting in the mouth in tropical climates. Because residual stress is present within the wax from contouring and manipulating the wax, the finished denture pattern should be flasked as soon as possible after completion of all adjustments and manipulations.
The impression waxes, also referred to as bite waxes or corrective waxes, tend to distort if they are withdrawn from undercut areas. Thus, they are limited to use in edentulous sites of the mouth or in occlusal surface areas. Although corrective waxes are relatively soft at mouth temperature, they have sufficient body to register the detail of soft tissues, and they are rigid at room temperature.
Other types of dental waxes include sticky wax, an orange-colored stick wax, which is tacky when melted but firm and brittle when cooled. Sticky waxes are used to temporarily fasten gypsum model components, join and temporarily stabilize the components of a bridge before soldering, or attach pieces of a broken denture prior to a repair.
Boxing wax is another useful material for enclosing an impression before the plaster or stone cast is poured. Typically provided in pink-colored flat sheets, this wax is relatively soft and pliable and can easily be pressed to the desired contour around the perimeter of an impression and self-sealed at the overlapped area with firm pressure.
Carving wax and presentation wax are used for demonstration purposes. Such waxes contain synthetic and polymeric materials with additives such as fillers and coloring agents.
The most common method used to form metal inlays, onlays, crowns, bridges, and other metal frameworks is to cast molten alloys by centrifugal force, under pressure, or under vacuum and pressure into a mold cavity. The material used for the mold must be sufficiently refractory and thermally stable that it can withstand exposure to the high temperatures of molten metal as the metal solidifies and cools to room temperature. In addition, the mold or investment material must not interact chemically with the metal surface, and it must be easy to remove from the metal casting.
The mold cavity is produced by eliminating a wax or resin pattern by heating the mold to a specific temperature and for a specific time. This is called the burnout process. To provide a pathway to the mold cavity for molten metal, the wax or resin pattern must have one or more cylindrical wax segments attached at the desired point(s) of metal entry; this arrangement is termed a sprued wax pattern. A sprue is the channel in a refractory investment mold through which molten metal flows. After the wax pattern has been made, either directly on a prepared tooth or on a replica die of the tooth, a sprue former base is attached to the sprued wax pattern, an investment ring is pressed into the sprue former base, and an investment slurry is vibrated into the ring to embed the wax pattern in the investment. Examples of sprued wax patterns on a sprue former base are shown in Figure 10-5. The investment material is mixed in the same manner as plaster or dental stone, poured around the pattern, and allowed to set. After the investment hardens, the sprue-former base is removed. The molten metal is then forced through the sprue or ingate created by the sprue former base into the mold cavity left by the wax.
The remainder of this chapter deals with refractory investments and casting methods used for the fabrication of small dental crown and bridge prostheses either by casting metal or by hot-pressing ceramic. Generally two types of investments—gypsum-bonded and phosphate-bonded—are employed, depending on the melting range of the alloy to be cast. The gypsum-based materials represent the type traditionally used for conventional casting of gold alloy inlays, onlays, crowns, and larger fixed dental prostheses (FDPs). Phosphate-based investments are designed primarily for alloys used to produce copings or frameworks for metal-ceramic prostheses (Chapter 18) and some base metal alloys. It can also be used for pressable ceramics. A third type is the ethyl silicate–bonded investment, which is used principally for the casting of removable partial dentures made from base metals (cobalt-based and nickel-based alloys). Commercially pure titanium and titanium alloys require a special investment as well as a controlled atmosphere to achieve satisfactory castings.
The type of investment used depends on whether the appliance to be fabricated is fixed or removable and on the method of obtaining the expansion required to compensate for the contraction of the molten alloy during solidification. Type I investments are those employed for the casting of inlays or crowns when the compensation for alloy casting shrinkage is accomplished principally by thermal expansion of the investment. Type II investments are also used for casting inlays, onlays, or crowns, but the major mode of compensation for alloy shrinkage during solidification is by hygroscopic expansion achieved by immersing the invested ring in a warm water bath. Burnout of the investment is performed at a lower temperature than that used for the high-heat burnout technique. Type III investments are used rarely in the construction of partial dentures because they are designed for casting gold alloys. This chapter focuses primarily on type I and type II investments.
The ingredients of dental inlay investments employed with conventional gold casting alloys are α-hemihydrate of gypsum, and quartz, or cristobalite, which are forms of silica. Most investments contain the α-hemihydrate of gypsum because of its greater strength. This gypsum product serves as a binder for the other ingredients and to provide rigidity. The strength of the investment is dependent on the amount of binder used. The investment powder may contain 25% to 45% of calcium sulfate hemihydrate. The remainder consists of silica allotropes and controlling chemicals.
The α-hemihydrate form of gypsum is generally the binder for investments used in casting gold-containing alloys with melting ranges below 1000 °C. When this material is heated at temperatures sufficiently high to completely dehydrate the investment and to ensure complete castings, it shrinks considerably and occasionally fractures.
The thermal expansion curves for the three common forms of gypsum products are shown in Figure 10-6. All forms shrink considerably after dehydration between 200 °C and 400 °C. A slight expansion takes place between 400 °C and approximately 700 °C, and a large contraction then occurs. This latter shrinkage is most likely caused by decomposition and the release of sulfur dioxide. This decomposition not only causes shrinkage but also contaminates the castings with the sulfides of the nonnoble alloying elements, such as silver and copper. Thus, it is imperative that gypsum investments not be heated above 700 °C. However, for gypsum products containing carbon, the maximum temperature is 650 °C. In this way proper fit and uncontaminated alloys are obtained.
Usually castings made from pure gypsum (CaSO4•2H2O) molds are extremely undersized. The α-hemihydrate product, which requires less mixing water and shrinks less, is the optimal choice as a binder.
The wax pattern is usually eliminated from the mold by heat. During heating, the investment is expected to expand thermally to compensate partially or totally for the casting shrinkage of the solidifying alloy. As shown in Figure 10-5, gypsum shrinks considerably when it is heated. If the proper forms of silica are employed in the investment, this contraction during heating can be eliminated and changed to an expansion. Silica exists in at least four allotropic forms: quartz, tridymite, cristobalite, and fused quartz. Quartz and cristobalite forms are of particular dental interest.
When quartz, tridymite, or cristobalite is heated, a change in crystalline form occurs at a transition temperature characteristic of the particular form of silica. For example, when quartz is heated, it inverts (transforms) reversibly from a “low” room-temperature crystal form, known as α quartz, to a “high” form, called β quartz, at a temperature of 573 °C. This α to β phase transformation is called an inversion, and it is accompanied by a linear expansion of 0.45%. In a similar manner, cristobalite undergoes an analogous transition between 200 °C and 270 °C from “low” (α cristobalite) to “high” (β cristobalite). Two inversions of tridymite occur at 117 °C and 163 °C, respectively. The β-allotropic forms are stable only above the transition temperature noted, and an inversion to the lower α form occurs on cooling in each case. In powdered form, the inversions occur over a range of temperature rather than instantaneously at a specific temperature.
The density decreases as the α form changes to the β form, with a resulting increase in volume that occurs by a rapid increase in the linear expansion as indicated in Figure 10-7. Consequently, the shrinkage of gypsum shown in Figure 10-6 can be counterbalanced by the inclusion of one or more of the crystalline silicas. Fused quartz is amorphous and glasslike in character, and it exhibits no inversion at any temperature below its fusion point. It has an extremely low linear coefficient of thermal expansion and is of little use in dental investments. Quartz, cristobalite, or a combination of the two forms may be used in a dental investment. Both are available in the pure form. Tridymite is no longer an expected impurity in cristobalite. On the basis of the type of silica principally employed, dental investments are often classified as quartz or cristobalite investments.
In addition to silica, certain modifying agents, coloring matter, and reducing agents, such as carbon and powdered copper, are present. The reducing agents are used in some investments to provide a nonoxidizing atmosphere in the mold when a gold alloy is cast.
Unlike the dental stones, a setting expansion is usually desirable to assist in compensating for the contraction of the alloy. Some of the added modifiers—such as alkali-earth and transition-metal chlorides, boric acid, and sodium chloride—not only regulate the setting expansion and the setting time but also prevent most of the shrinkage of gypsum when it is heated above 300 °C. In some instances, the modifiers are needed to regulate the setting time and setting expansion, as described for the dental stones. The microstructure of a set gypsum-bonded investment can be seen in Figure 10-8.
The setting time of an investment can be measured in the same manner as plaster. Furthermore, it can be controlled in the same manner. The setting time for dental inlay casting investment should not be less than 5 or more than 25 minutes. Usually the modern inlay investments set initially in 9 to 18 minutes. Sufficient time should be allowed for mixing and investing the pattern before the investment sets.
Normal Setting Expansion
A mixture of silica and calcinated gypsum powder (calcium sulfate hemihydrate, CaSO4•H2O) results in setting expansion greater than that of the gypsum product 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 they increase expansion. Generally the resulting setting expansion in such a case is high. Type I investments should exhibit a maximum setting expansion in air of 0.6%. The purpose of the setting expansion is to aid in enlarging the mold to compensate partially for the casting shrinkage of the alloy. Typically, the setting expansion of these investments is approximately 0.4%. This expansion is controlled by retarders and accelerators.
Variables other than the exothermic heat of reaction also influence the effective setting expansion. As the investment sets and setting expansion occurs, it eventually gains sufficient strength to produce a dimensional change in the wax pattern and mold cavity. The inner core of the investment adjacent to a mesial-occlusal-distal (MOD) wax pattern can actually force the proximal walls outward to a certain extent. If the pattern has a thin wall, the effective setting expansion is somewhat greater than for a pattern with thicker walls because the investment can move the thinner wall more readily. Also, the softer the wax, the greater is the effective setting expansion, because the softer wax is more readily moved by the expanding investment. If a wax softer than a type II inlay wax is used, the setting expansion may cause an excessive distortion of the pattern.
Hygroscopic Setting Expansion
The theory of hygroscopic setting expansion was previously described in connection with the setting of dental plaster and stone. Hygroscopic setting expansion, which is greater in magnitude than normal setting expansion, differs from normal setting expansion in that it occurs when the gypsum product is allowed to set when placed in contact with heated water.
Hygroscopic setting expansion was first discovered in connection with an investigation of the dimensional changes of a dental investment during setting. As illustrated in Figure 10-9, the hygroscopic setting expansion may be six or more times greater than the normal setting expansion of a dental investment. In fact, it may be as high as 5 linear percent. The hygroscopic setting expansion is one of the methods for expanding the casting mold to compensate for the casting shrinkage of gold alloys.
Commercial investments exhibit different amounts of hygroscopic expansion. Although all investments appear to be subject to hygroscopic expansion, the expansion in some instances is not as great as in others. For this reason, certain investments are specially formulated to provide a substantial hygroscopic expansion when the investment is permitted to set in contact with water. Type II investments should exhibit a minimum setting expansion in water of 1.2%. The maximum expansion permitted is 2.2%. As discussed in the following sections, a number of factors are important in the control of hygroscopic expansion.
Variables That Affect Hygroscopic Expansion
The magnitude of the hygroscopic setting expansion of a dental investment is generally proportional to the silica content of the investment, other factors being equal. The finer the particle size of the silica, the greater is the hygroscopic expansion. In general, α-hemihydrate is apt to produce a greater hygroscopic expansion in the presence of silica than is the β-hemihydrate, particularly when the expansion is unrestricted.
A dental investment should have enough hemihydrate binder with the silica to provide sufficient strength after hygroscopic expansion. Otherwise shrinkage occurs during the subsequent drying of the set investment. At least 15% of binder is necessary to prevent drying shrinkage.
With most investments, as the mixing time is reduced, the hygroscopic expansion is decreased. This factor is also important in the control of the effective setting expansion.
The older the investment, the lower is its hygroscopic expansion. Consequently the amount of investment purchased at one time should be limited. The greatest amount of hygroscopic setting expansion is observed if the immersion takes place before the initial set. The longer the immersion of the investment in the water bath is delayed beyond the time of the initial set of the investment, the lower is the hygroscopic expansion.
Both the normal and hygroscopic setting expansions are confined by opposing forces, such as those exerted by the walls of the container in which the investment is poured or by the walls of the wax pattern. However, the confining effect on hygroscopic expansion is more pronounced than the similar effect on the normal setting expansion. Therefore, the effective hygroscopic setting expansion is likely to be less relative to the expected expansion compared with the normal setting expansion.
When the dimensional change in the wax pattern itself is measured after investing, the increase in the effective setting expansion during immersion of investment in a 37.7 °C (100 °F) water bath is apparently not only the result of hygroscopic expansion. Rather, it may be caused mainly by heating and expanding the wax pattern and softening the pattern at the water temperature, permitting an increase in effective setting expansion. The latter results from a combination of thermal expansion of the wax pattern plus the softened condition of the wax, reducing its confining effect on the expansion of the setting investment. This is substantiated by the fact that immersion in water at room temperature (rather than 37.7 °C) reduces the effective expansion.
The magnitude of the hygroscopic setting expansion can be controlled by the amount of water added to the setting investment. It has been proved that the magnitude of the hygroscopic expansion is proportional to the amount of water added during the setting period until maximal expansion occurs. No further expansion is then evident regardless of the amount of water added.
The effects of some of the factors previously discussed (W/P ratio, mixing, and shelf life) on the maximal hygroscopic setting expansion are illustrated in Figure 10-9 relative to the amount of water added. As shown in Figure 10-9, the magnitude of the hygroscopic setting expansion below the maximal expansion value is dependent only on the amount of water added and independent of the W/P ratio, the amount of mixing, and the age or shelf life of the investment. This finding is the basis for the mold expansion technique.
Hygroscopic setting expansion is a continuation of ordinary setting expansion because the immersion water replaces the water of hydration, thus preventing confinement of the growing crystals by the surface tension of the excess water. Because of the dilution effect of the quartz particles, the hygroscopic setting expansion in these investments is greater than that of the gypsum binder when used alone. This effect is the same as previously described for normal setting expansion. This phenomenon is purely physical. The hemihydrate binder is not necessary for hygroscopic expansion because investments with other binders exhibit a similar expansion when they are allowed to set under water. Expansion can be detected when water is poured into a vessel containing only small smooth quartz particles. The water is drawn between the particles by capillary action, thereby causing the particles to separate, creating an expansion. The effect is not permanent after the water is evaporated unless a binder is present.
The greater the amount of the silica or the inert filler, the more rapidly the added water can diffuse through the setting material and the greater the expansion. The W/P ratio affects the hygroscopic expansion for the same reason that it affects the normal setting expansion. Once setting starts, the later that water is added to the investment, the less the hygroscopic setting expansion will be, because part of the crystallization has already started in a “normal” way. Some of the crystals have intermeshed, inhibiting further crystal growth when the water is added.
Thermal Expansion of Gypsum-Bonded Investments
To achieve sufficient expansion of gypsum-bonded investment, the silica must be increased to counterbalance the contraction of the gypsum during heating. However, when the quartz content of the investment is increased to 60%, with the balance being the calcium sulfate hemihydrate binder, the initial contraction of the gypsum is not eliminated. The contraction of the gypsum is entirely balanced when the quartz content is increased to 75% (Figure 10-10). If a sufficient amount of setting expansion had been present, a casting made at 700 °C would probably have fit the die reasonably well. The thermal expansion curves of quartz investments are influenced by the particle size of the quartz, the type of gypsum binder, and the resultant W/P ratio necessary to provide a workable mix.
The effect of cristobalite compared with that of quartz is demonstrated in Figure 10-11. Because of the much greater expansion that occurs during the inversion of cristobalite, the normal contraction of the gypsum during heating is readily eliminated. Furthermore, the expansion occurs at a lower temperature because of the lower inversion temperature of the cristobalite in comparison with that of quartz. A reasonably good fit of the castings is obtained when a gold alloy is cast into the mold at temperatures of 500 °C and higher. The thermal expansion curves of an investment provide some idea of the form of the silica that is present. As can be seen from Figures 10-11 and 10-12, the investments containing cristobalite expand earlier and to a greater extent than those containing quartz. Some of the modern investments are likely to contain both quartz and cristobalite.
The desired magnitude of the thermal expansion of a dental investment depends on its use. If the hygroscopic expansion is to be used to compensate for the contraction of the gold alloy, as for the type II investments, thermal expansion should be between 0% and 0.6% at 500 °C. However, for type I investments, which rely principally on thermal expansion for compensation, the thermal expansion should not be less than 1% or greater than 1.6%.
The magnitude of thermal expansion is related to the amount of solids present. Therefore, it is apparent that the more water used in mixing the investment, the less is the thermal expansion that is produced during subsequent heating. This effect is demonstrated by the curves shown in Figure 10-13. Although the variations in the W/P ratios shown are rather extreme, the curves indicate that it is imperative to measure the water and powder accurately if the proper compensation is to be achieved.
Effect of Chemical Modifiers
A disadvantage of an investment that contains sufficient silica to prevent any contraction during heating is that the weakening effect of the silica in such quantities is likely to be too great. The addition of small amounts of sodium, potassium, or lithium chlorides to the investment eliminates the contraction caused by the gypsum and increases the expansion without the need for an excessive amount of silica.
Boric acid has a similar effect. It also hardens the set investment. However, it apparently disintegrates during the heating of the investment and a roughened surface on the casting may result. Silicas do not prevent gypsum shrinkage but counterbalance it, whereas chlorides actually reduce gypsum shrinkage below temperatures of approximately 700 °C.
When an investment is cooled from 700 °C, its contraction curve follows the expansion curve during the inversion of the β quartz or β cristobalite to its stable α form at room temperature. Actually, the investment contracts to less than its original dimension. This contraction below the original dimension is unrelated to any property of the silica; it occurs because of the shrinkage of gypsum when it is first heated.
As the investment is reheated, it expands thermally to the same peak value reached when it was first heated. However, in practice the investment should not be heated a second time because internal cracks can develop.
The fracture resistance of the investment must be adequate to prevent cracking, bulk fracture, or chipping of the mold during heating and casting of gold alloys. Although a certain minimal strength is necessary to prevent fracture of the investment mold during casting, the compressive strength should not be unduly high. It has been found that all castings for the standardized MOD die used by the National Institute of Standards and Technology showed a constant pattern of distortion. The distortion apparently results from a directional restraint by the investment to the thermal contraction of the alloy casting as it cools to room temperature.
The strength of the investment is affected by the W/P ratio in the same manner as any other gypsum product; the more water that is employed in mixing, the lower is the compressive strength. Heating the investment to 700 °C may increase or decrease the strength as much as 65%, depending on the composition. The greatest reduction in strength on heating is found in investments containing sodium chloride. After the investment has cooled to room temperature, its strength decreases considerably, presumably because of fine cracks that form during cooling.
Although the total thermal contraction of the investment is similar to that of gold alloys from the casting temperature to room temperature, the contraction of the investment is fairly constant until it cools to below 550 °C. Thus, when the alloy is still hot and weak, the investment can resist alloy shrinkage by virtue of its strength and constant dimensions. This can cause distortion and even fracture of the casting if the hot strength of the alloy is low. Although this is rarely a factor with gypsum-bonded investments, it can be important with other types of investments.
The strength of an investment is usually measured under compressive stress. The compressive strength is increased according to the amount and the type of the gypsum binder present. For example, the use of α-hemihydrate instead of plaster definitely increases the compressive strength of the investment. The use of chemical modifiers increases strength because more of the binder can be used without a marked reduction in thermal expansion.
The compressive strength for the inlay investments should not be less than 2.4 MPa when tested 2 hours after setting. Any investment that meets this requirement should have adequate strength for casting of an inlay. However, when larger, complicated castings are made, greater strength is necessary, as required for type III partial denture investments.
Fineness of Gypsum Investment
The fineness of the investment may affect its setting time, the surface roughness of the casting, and other properties. A fine silica results in a higher hygroscopic expansion than does a coarser silica. A fine particle size is preferable to a coarse one because the finer the investment, the smaller the surface irregularities on the casting.