Die Materials, Investment and Inlay Wax

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Good morningDie, Inlay Wax And Investment

CONTENTSIntroductionDie materials What is a DieIdeal requisites for die materialsTypes of die materialsProperties of each die materialTechniques of die fabricationCompatibility of die material with impression materials.

CONTENTSInlay waxIdeal requisites of inlay waxCompositionClassificationPropertiesFabrication of wax patternDistortion of wax pattern and prevention

CONTENTSInvestment materialsWhat is an investmentComponents of an investment material

Types of investment materials

Indications of each investment materials

INTRODUCTIONDirect restorations are usually not feasible and advisable in teeth where more than half the tooth structure is lost.In such scenarios the indirect restorations come into playThey provide the requisite resistance and retention form as well support the remaining tooth structure.The indirect restorations are also indicated as treatment options over direct restorations in conditions where the anatomical contours and embrasures are to be modified.INTRODUCTIONThe casting method for fabrication the indirect restorations basically consists of: - Accurate reproduction of the prepared tooth, its relation with adjacent teeth and supporting tissues, Forming a wax pattern,Surrounding it with a suitable investment material, Heating the investment mold to remove the wax Finally casting the molten metal into the mold.

A, Impression. B, Cast. C, Wax pattern on die. D, The pattern is attached with sprue to a rubber crucible former and invested. E, Casting. F, Luted restoration.DIE MATERIALS

INTRODUCTIONDuring fabrication of indirect restorations, accurate reproduction of the tooth, its relation with adjacent teeth and supporting tissues is essential.This is where the role of a die comes into play.DEFINITIONSIt is the positive reproduction of the form of the prepared tooth in any suitable substance. (GPT 7)

The die is a positive reproduction of the prepared tooth and consists of a suitable hard substance of sufficient accuracy usually an improved stone, resin or metal. (Rosenstiel et.al)

DEFINITIONSA reproduction of a prepared tooth made from a gypsum product, epoxy resin, a metal or a refractory material. (Anusavice 11th edition ) The die is a model of the individual prepared tooth on which the margins of the wax patterns are finished. (Schillingburg)

IDEAL REQUIREMENTS OF A DIE MATERIAL

They should accurately reproduce all the details in the impression.They should be dimensionally stable.The die should have a smooth surface and the ability to resist abrasion. They should be compatible with impression materials and there should be no interaction between the surface of the impression and cast or die.


The die should have toughness to resist breakage during fabrication or burnishing of fine edges.Colour of the die should be in contrast to the colour of wax. This helps to facilitate the manipulative procedures that will be carried out.It should be reasonably easy to use without excessive manipulative procedures.


It should be relatively inexpensive.

Should be compatible with separating agent used.

It should be easily wettable by the wax.

TYPES OF DIE MATERIALSA wide variety of materials have been used for the fabrication of the dies. They may be broadly classified as follows:GYPSUM PRODUCTS:Dental stone Type 4, high strength.Dental stone Type 5, high strength high expansion.METALS:Electroformed or electroplated dies.Sprayed metalAmalgam dies

TYPES OF DIE MATERIALSPOLYMERS:Metal filled resins or inorganic filled resins.Polyester resinsEpoxy resins.CEMENTS:Silico phosphateCERAMIC OR REFRACTORY DIE MATERIALS:For direct fabrication of porcelain crowns or preparation of wax patterns.

TYPES OF DIE MATERIALSSelection of the type of die material usually depends on:The impression material used.Purpose for which the cast or die is used.Time factor involved.Materials that will be manipulated on the cast or die.

GYPSUM PRODUCTSAccording to ADA specification No. 25 the gypsum products are classified as follows :-Type I Impression plasterType II Model plaster Type III Dental stoneType IV Dental stone, high strength Type V Dental stone, high strength and high expansion

GYPSUM PRODUCTSPRODUCTION OF GYPSUM PRODUCTS : Plaster and stone products are produced by calcining calcium sulphate dihydrate or gypsum. Gypsum is ground and subjected to temperatures of 1100 to 1200c to drive off part of the water of crystallization and is converted to calcium sulphate hemihydrate [ CaSO4.1/2 H2O ].

1100-1300c CaSO4.2H2O CaSO4.1/2 H2O Calcium sulphate Calcium sulphate Dihydrate hemihydrate

GYPSUM PRODUCTSPRODUCTION OF GYPSUM PRODUCTS : Depending on the method of calcinations, different forms of the hemihydrate can be obtained They are : -hemihydrate hemihydrate -modified hemihydrate

The alpha modified hemihydrate is made by boiling gypsum in a 30% aqueous solution of calcium chloride & magnesium chloride

GYPSUM PRODUCTSSETTING REACTION

(CaSO4)2 . H2O + 3 H2O 2 CaSO4 . 2 H2O + unreacted (CaSO4)2 . 1/2 H2O + heat

Theories regarding the setting reactionColloidal theoryHydration theory Dissolution Precipitation theoryBased on dissolution of plaster and instant recrystalization of gypsum, followed by interlocking of the crystals to form the set solidGYPSUM PRODUCTSManufacture of Type IV Dental Stone and Type V Dental Stone

Types 4 and 5 high-strength dental stones are manufactured with a high-density raw material called densite. This variety is made by boiling gypsum rock in a 30% calcium chloride solution, after which the chloride is washed away with hot water (100 C) and the material is ground to the desired fineness.

TYPE IV STONE WHIP MIX

TYPE V STONE GYPSUM PRODUCTSType IV Dental StoneThese are high strength dental stone.

W: P ratio - 0.22 -0.24Setting time - 12 4 minutesSetting expansion - 0.10%Compressive strength - 5000 psiHardness - 92 RHN

GYPSUM PRODUCTSType V Dental Stone :These are high strength, high expansion dental stones.W: P ratio - 0.18 -0.22Setting time - 12 4 minutesSetting expansion - 0.30%Compressive strength - 7000 psi

GYPSUM PRODUCTSType V Dental StoneThe Type V dental stone gypsum product exhibits a higher compressive strength than does the type IV dental stone. In addition setting expansion has been increased from a maximum of 0.10% to 0.30%. This is because certain newer alloys such as base metal alloys have a greater casting shrinkage than noble metal alloys. Thus higher expansion is required in the stone used for the die to aid in compensating for the alloy solidification shrinkage.

GYPSUM PRODUCTSxAdvantages of Type IV & Type V StonesThey are relatively inexpensiveEasy to useCompatible with all impression material.Good surface detail reproduction (capable of reproducing a 20m-wide line as prescribed by A.D.A specification No.19).Disadvantages of gypsum dies Susceptibility to abrasion during carving of wax pattern.

GYPSUM PRODUCTSGypsum dies are sometimes modified to: To make them more abrasion-resistantTo change the dimensions of the dies.To increase the refractoriness of the dies.To produce a combination of these effects.

GYPSUM PRODUCTSSeveral means are used to increase the abrasion resistance Electoplating, Coating the surface with cyanoacrylate Adding a die hardener to the gypsum

However each of these methods may increase the die dimensions slightly thus reducing accuracy.

GYPSUM PRODUCTSDie hardenerMixing high-strength dental stone with a commercial hardening solution containing colloidal silica (about 30%) improves the surface hardness of the set gypsum.Cyanoacrylate or acrylic resin(polystrene) can be applied to the finish line area of a die to prevent abrasion by waxing instruments during fabrication of the wax pattern.The thickness of the cyanoacrylates at the finish line can range from 1-25m while acrylic lacquers can add 4-10m of thickness.

GYPSUM PRODUCTSSpecial gypsum products ;Since 1991, a plethora of new dental stones have been introduced One type is extremely fast- setting and ready to use in 5min,but it has little working time Another product changes color to help denote when it is ready for use.Most recently another trend is the addition of a small amount of plastic or resin, which reduces brittleness and improves resistance to abrasion during the carving of wax patterns.

GYPSUM PRODUCTSDIE STONE INVESTMENT COMBINATION A commercial gypsum- bonded material called divestment is mixed with a colloidal silica liquid. The die is made from this mix, and the wax pattern is than constructed on it then the entire assembly [die and pattern] is invested in a mixture of divestment and water thereby eliminating the possibility of distortion of the pattern on removal from the die or during the setting of the investment.

GYPSUM PRODUCTSDIE STONE INVESTMENT COMBINATION Divestment is a gypsum- bonded material it is not recommended for high-fusing alloys that are used for metal-ceramic restorations but it is a technique of choice for use with conventional gold alloys especially for extra coronal preparations. Divestment phosphate recommended for high fusing alloys.Type 1: For casting inlays and crownsType 2: For casting complete and partial denture bases

GYPSUM PRODUCTSDie spacer : An agent applied to a die to provide space for the luting agent in the finished castingTypes of die spacers :Resins Model paintColoured nail polish

GYPSUM PRODUCTSMETHODS OF APPLICATION OF DIE SPACERPaint on techniqueSpray techniquePen techniqueIdeal die spacer thickness is 25 micronsSpacers are applied within 0.5mm of the preparation finish line to provide relief for the cement luting agent

ELECTROFORMED DIESElectroplating the dies can be used to overcome the poor abrasion resistance of gypsum The electroplated dies have moderately high strength, adequate hardness and excellent abrasion resistance.Detail reproduction of a line 4m or less is readily attainable on an electroplated die when an nonaqueous elastomeric impression material is used.

ELECTROFORMED DIESAdvantages High strength, Hardness Abrasion resistance.

ELECTROFORMED DIES The first step in the procedure is to treat the surface of the impression material so that it conducts electricity. This process is referred to as METALLIZING. In this process, a thin layer of metal, such as silver is deposited on the surface of the impression material.

Conductive silver lacquer for nonconductive surfaces, e.g. silicone impressions, for copper plating of working models.ELECTROFORMED DIESRequirements for electroplatingThe impression to be coated is made the cathode.Anode is the metal to be deposited either silver or copper Anode and cathode holder.Electrolyte :- the solution through which the electric current is passed. Ammeter :- the current passed is of 10mA / tooth area for 12 hrs. It should not exceed 50mA. Plating tank glass or hard rubber with well fitting cover to prevent evaporation.

ELECTROFORMED DIESCopper platingThe surface of the impression is rendered conductive by coating it with fine particles of copper or graphite. The coated impression is made the cathode (negative electrode) of a plating bath, with an anode (positive electrode) of copper. The electrolyte is an acid solution of copper sulfate (about 250g/l).

ELECTROFORMED DIESCopper platingA current is passed, causing slow dissolution of the anode and movement of copper ions from anode to cathode, thus plating the impressionDental stone is then cast into the plated impression.ELECTROFORMED DIESSilver platingPolysulfide and silicone impression materials can be silver plated by the same general technique except The impression is coated with silver or graphite powder. The anode is silver. The electrolyte is an alkaline solution of silver cyanide An anode of at least twice the size of the area to be plated, should be employed, and the electroplating is carried out approximately 10 hr, using 5 to 10 mA.

ELECTROFORMED DIESProblems in Electroplating Variable degrees of distortion commonly occur and hence the technique must be performed slowly.The time required to produce a cohesive film of metal(typically 8 hrs) is ample for the development of dimensional changes in the impression.Not all the impression materials are suitable for plating because of their low surface energies, silicone impression materials are difficult to electroplate evenly.

ELECTROFORMED DIESProblems in Electroplating Polyether impressions because of their hydrophilic nature imbibe water & become distorted. Therefore they cannot be plated accurately.Poly sulfide polymers can be silver plated but it is difficult to copper plate themDrawback of silver plating is the use of a cyanide solution, which requires special precautions because of its extreme toxicity.

EPOXY RESIN DIESResins are used as die materials to overcome the low strength and abrasion resistance of die stones. Most available resin die material is epoxy resin but polyurethane is also used.The fast setting epoxy hardens rapidly so that dies can be waxed up in half an hour after injecting into the impression

The epoxy resin is in one cartridge and the catalyst is in the other. Forcing the two pastes through the static mixing tip thoroughly mixes the epoxy material, which can be directly injected into a rubber impression.EPOXY RESIN DIESThey cannot be used with water containing agar and alginate impression materials because the water retards the polymerization of the resin and thus are limited to use with rubber impression materials . Working time - 15 minSetting time - 1-12 hrs (depending on product) Shrinkage - 0.03%-0.3% Compressive strength - 9500-14200psi Hardness - 83Rhn

EPOXY RESIN DIESADVANTAGES:-It can be cured at room temperature without expensive or complicated equipment.It is dimensionally stable.Its abrasion resistance is many times greater than gypsum products.High strength.

EPOXY RESIN DIESADVANTAGES:-Suitable for fabrication of precise diesDetail reproduction is better than die stone Hence prostheses fabricated on resin die will fit more tightly than those made of gypsum. Good results are achieved with silicone and polyether

EPOXY RESIN DIESDISADVANTAGES:-It undergoes shrinkage during polymerization. But the amount of shrinkage is approximately equal to the expansion with gypsum.It is more expensive than gypsum.Not compatible with impression materials such as polysulfide and hydrocolloid

POLYURETHANE DIESThese resins compared to epoxy resins were inexpensive and easily manipulated. Long and narrow tooth preparations must be reproduced with high transverse-strength working cast materials to avoid fracturingThe filled polyurethane resins could be indicated for these conditions

SILICOPHOSPHATE CEMENT DIES

This is similar to the filling and cementing material.Advantage :Has greater compressive strength than die stone.Disadvantage:Shrinkage on setting. Loss of water on standing.

AMALGAM DIES

Amalgam may be packed into rigid impression materials such as compound.Advantage: -Produces a hard die -Reproduces fine details and sharp margins. Disadvantages: -Can only be packed into a rigid impression . -Long time to reach a maximum hardness.

METAL SPRAYED DIES

A bismuth-tin alloy, which melts at 138oc, can be sprayed directly on to an impression to form a metal shell, which can then be filled with dental stone. Advantage -A metal coated die can be obtained rapidly from elastomeric impression materials. Disadvantage -The alloy is rather soft, care is needed to prevent abrasion of the die.

CERAMIC DIE MATERIALS:Two ceramic die materials are availableA material for the production of dies on which porcelain restorations are to be fabricated, without the use of a platinum foil matrix. To form the dies heating to over 10000c is necessary. A ceramic material, supplied as a powder and liquid, and mixed to a putty like consistency. After 1 hour the material is removed from the impression and fired at 6000c for 8 minutes to produce a hard strong die.

FLEXIBLE DIE MATERIAL :

Similar to silicone or polyether impression material.Used to make provisional restorations or indirect composite resin inlays .eg; polyvinyl medium viscosity impression material Advantages: -More rapid setting -ease of removal of provisional or inlay Disadvantages: -expensive

Philip Duke et. al in 2000 conducted a study of the physical properties of type IV gypsum, resin containing and epoxy die materials. The results were all gypsum products expanded where as the epoxy resin material contracted during setting. The epoxy resin exhibited much better detail reproduction, abrasion resistance, transverse strength than gypsum materials. In general the epoxy resin exhibited the best properties of the materials studied

Philip Duke et al ;Physical properties of type IV gypsum, resin containing and epoxy die materials JPD April 2000 vol 83, no. 4 p-466-73Jacinthe M et al in 2000 conducted a study on the dimensional accuracy of an epoxy resin die material using two setting methods and concluded that retarding the setting reaction of an epoxy resin die material improved its accuracy. Epoxy resin die materials had a net shrinkage but the gypsum based materials had a net expansion .

Jacinthe M et al in 2000 dimensional accuracy of an epoxy resin die material using two setting methods. JPD March 2000 vol 83 no3 p 301-305

DIE FABRICATIONAvailable methods for die fabrication:A varied number of die systems or methods of fabrication have been proposed, which may be broadly classified as follows:Working cast with a separate die (solved cast with individual die)Working cast with removable dies. Alternate / other die systems

WORKING CAST WITH SEPARATE DIE:

Technique:2 casts are required in this technique of die fabrication:Working cast for full mouth.Sectional cast for die of tooth.These can be obtained by separate impressions or by pouring the same elastomeric impression material twice.If a double pair is utilized, the first cast is used for fabrication of die, because it is the most accurate.


Pours 1 and 2 (individual dies) and 3 (definitivecast). Sufficient bulk for the die handlesPouring an impression. Stone added in the preparation area only. The trimmed dies and definitive cast before articulation.Sectioning the individual diesWORKING CAST WITH SEPARATE DIE:

Advantages:Primary advantage of this technique is its simplicityIt keeps the relationship between the abutments fixed and removable.The working cast requires only limited trimming and no additional armamentarium is required for its fabrication.Since the gingival tissues around the prepared teeth are left intact, they can be used as a guide when contouring the restorations.It also precludes errors caused by the removable die systems due to incomplete seating of the die.


DISADVANTAGES:It may be difficult to transfer complex or fragile wax patterns from cast to die.Seating the pattern on the master cast may be problematic because the second pour of many impression materials is slightly larger than the first, therefore it may be necessary to relieve the stone slightly to seat the pattern before occlusal evaluation.This technique can be used only with elastomeric impression materialsWORKING CAST WITH A REMOVABLE DIEIn a removable die system, the die is an integral part of the master cast and can be lifted from the cast to facilitate access.Precise relocation of the die in the master cast is critical to the success of this system is usually accomplished with brass pins or dowels.

TechniqueSeveral methods can be employed to allow the repositioning of a die in its working cast.Prepour technique: When the dowels are oriented in the impression before it is poured.Post pour techniques: when they are attached to the underside of a cast that has been poured.

WORKING CAST WITH A REMOVABLE DIEPre-pour techniqueAdvantage: Gives more consistent accurate placement of dowel.Disadvantages: It makes pouring more difficult (bubbles may occur). Sticky wax may break loose during vibration of impression. Dowel may settle into the stone.

WORKING CAST WITH A REMOVABLE DIE

Positioning dowel pins before cast pouring with bobby pins and sticky wax or with prefabricated wire tube aid

Dowel pins must be carefully positioned so that the first pour of stone completely covers the knurled head; otherwise, the parts do not separate cleanly. However, the stone should not extend onto the shaft and reduce stability.Post pour techniqueThe dowel will be cemented in holes drilled into flat underside of a cast that has already been poured e.g. by Pindex system


The Pindex system consists of a special drill press and brass dowels and plastic sleeves

The assembly is coated with petrolatum to ensure clean separationPost pour technique


The location of each dowel is marked on the occlusal surface

Light indicates the location of the drill. The cast is held firmly and the lever depressed; this activates the drill, which penetrates into the castThe pins are tried in and cemented in placePost pour technique


The plastic sleeves are positioned.The assembly is placed in the special moldThe second pour of stone is made into the mold. After some stone has been painted between the pins, the first pour is placed into this mixSawing the diesAdvantages:Convenient to use because wax patterns or coping need not be removed from the dies when transferred to working cast, this helps to:Decrease the chances of breakage during transfer of pattern.This is also important for ceramic restorations as the unfired material is quite fragile (up for labial porcelain margins).

WORKING CAST WITH A REMOVABLE DIEAdvantages It eliminates discrepancies between separate die and master cast that may be caused by impression distortion or deformation between pours by cast and die made from separate impressions that are not identical.A removable die also eliminates discrepancies that can occur when the die is coated with a relief agent and the working cast is not, or when they are coated with different thickness

WORKING CAST WITH A REMOVABLE DIEDISADVANTAGESThe principal disadvantage of a removable die system is the risk of an introducing an error in the pattern of the die does not reseat accurately in the working cast.Difficulty may be encountered in sawing the die out of the cast interproximal margins can be easily damaged, particularly if clearance between proximal preparation margins and adjacent tooth is minimal.

WORKING CAST WITH A REMOVABLE DIEALTERNATE / OTHER DIE SYSTEMSDie lock traysThe system involves the use of specially segmented trays. With a single-pour technique, the impression is formed in the usual way.The Di-Lok tray is filled with stone and is inserted into the impression while the stone is still wet. After the die stone has fully set, the locking and curved arms of the tray are removed. The cast can then be removed by tapping the anterior pad of the tray base.

ALTERNATE / OTHER DIE SYSTEMS

Drilling holes for dowel pins as marked.Marking dowel pin locations on clear plateTrimmed impression on alignment fixture.

Inserting dowels in the baseplate. An adhesive is not required.DVA Model SystemALTERNATE / OTHER DIE SYSTEMS

The impression is poured and stone is placed around the dowel pins.The alignment fixture is replaced over poured impressionFinally the Set cast is removed from the baseplate with gentle tapping.

The cast is trimmed and sectioned.DVA Model SystemCOMPATIBILITY OF DIE MATERIAL WITH IMPRESSION MATERIALS

Dental stone Impression compound Zinc oxide eugenol Agar-agar Alginate Impression plaster when used with separator Rubber base material Electroplated Copper Rubber base material Electroplated silver Polysulphide Polyether Addition silicone

Epoxy Resin Polyether, Addition siliconeConclusionA good impression and an accurate die are the first step towards the fabrication of an accurate restoration whether its inlay, onlay or crown. Proper selection of the die material and its manipulation are paramount to achieve accuracy in the die.In conclusion, a wise choice of material, combined with proper handling and meticulous approach to the details of each step of fabrication will bring us that much closer to perfection .

INLAY CASTING WAX

Inlay wax is a specialized dental wax that can be applied to dies to form direct or indirect patterns for the lost wax technique used for casting metal or hot pressing of ceramics.

Generally produced in deep blue, green or purple color.

COMPOSITION OF INLAY WAX Paraffin-60% Carnauba-25% Beeswax-5% Ceresin-10% Natural resins(gum dammer)-less than 1% Organic fillers- added to avoid excessive shrinkage,expansion due to temp change Microcrystalline wax- In minute amount. Candelilla Coloring agents

COMPOSITION OF INLAY WAX

Paraffin wax(mineral wax) 60%

Generally the main ingredient of inlay waxes.Melting range 40-71o C. Likely to flake when trimmed.Does not present smooth, glossy surfaceConsequently other waxes and natural resins are added.

Carnauba wax (plant wax) 25%It is quite hard, melting range 84-910CCombined with paraffin to- Decrease flow at mouth temp increase the hardnessIt contributes to glossiness of the wax surface.


Beeswax (insect wax) 5%Melting range 63-700CBrittle at room temp, becomes plastic at body tempUsed to modify the properties of paraffin waxCOMPOSITION OF INLAY WAX

Natural resin (gum dammar)Less than 1%Added to paraffin to improve its smoothness in moldingIt renders it more resistant to cracking and flakingIncreases the toughness of the wax and enhances the smoothness and lusture of the wax.


Organic Fillers

Normally soft wax shrinks more than hard wax High shrinking wax may cause significant pattern distortion when it solidifies.For this reason organic filler is added , they should be completely miscible with components of inlay wax.They should not leave an undesirable residue after burnout.COMPOSITION OF INLAY WAX

Microcrystalline waxes(mineral)Similar to paraffin waxes.It has a higher melting range 60-910C It is tougher and more flexible than paraffinIt has less volumetric shrinkage during solidification than paraffin.COMPOSITION OF INLAY WAX

Candelilla wax (plant wax)Same qualities as carnauba waxAdded to paraffin to partially or entirely replace carnauba wax.Its melting point is lower, and it is not as hard as carnauba wax.


CLASSIFICATION OF INLAY WAXESAccording to ADA SPECIFICATION 4

TYPE 1 -medium wax used for direct wax pattern

TYPE 2 - soft wax used for indirect wax patternDENTARUM (Germany)Crown and Bridge Inlay Wax

In ropes In bulk

Dark blue-hard, for crowns bridges and inlays.

Light blue-soft, for crowns bridges and inlays

Lilac-stress free, for copings and cervical margins

Red- adhesive, connecting and adhesive wax

Wax Preforms are available (ceratom wax preforms, Dentaurum)It simplifies wax up and shorten finishing procedure. DESIRABLE PROPERTIES OF INLAY CASTING WAX

1.) When softened the wax should be uniform.2) The color should be such that it contrasts with the die material or prepared tooth. 3) There should be no flakiness or similar surface roughening when the wax is bent and molded after softening DESIRABLE PROPERTIES OF INLAY CASTING WAX

4) During carving wax should not pull away with carving instrument or should not chip as it is carved.5) Wax should burn out , forming carbon which is later eliminated by oxidation to volatile gases.6) The wax pattern should be completely rigid and dimensionally stable at all times until it is eliminated .

PROPERTIES OF INLAY CASTING WAXES

FLOW

Type of wax T=300C T=370C T=400C T=400C T=450 T=450C (max) (max) (min) (max) (min) (max) I --- 1.0 --- 20 70 90 II 1.0 --- 50 --- 70 90

The maximum flow permitted for Type I waxes at 37oC (98oF) is 1%. Type I and Type II waxes must have a minimal flow of 70% and a maximum flow of 90% at 45oC (113oF). THERMAL PROPERTIES Thermal conductivity of waxes is low and time is required to both heat them uniformly and to cool them to body or room temperature.Inlay wax thermally expand and contract more per degree of temp change than any other dental material.This is one of the inherent disadvantages of waxes when they are used in the direct technique.A maximum of 0.6% linear change in dimension when heated from 25-37oC (77-99oF) is permitted for Type I wax.

PROPERTIES OF INLAY CASTING WAXESThis property is not significant when wax is used in indirect technique because the pattern is not subjected to a change from mouth to room temp, provided no marked variations in temp occurs after the removal of pattern from die.

93MANIPULATION OF INLAY WAX

Armamentarium for manipulation of inlay wax

Electric waxing instruments. Top to bottom: PKT Nos. 1 to 5.MANIPULATION OF INLAY WAX

Armamentarium for manipulation of inlay wax

B. Left to right, closer view of these threeinstruments.AB

DPT6 Darby Perry trimmer (wax burnisher). A. Top to bottom, No. 2 Ward and Nos. 1/2 and 3 Hollenback..

Always heat the shank of the instrument so that wax flows off its tip.Wax pattern fabricationDirect wax pattern technique

Indirect wax pattern technique

Wax pattern fabricationDirect wax pattern technique

-with matrix band adaptation [closed technique]introduced by Volland

-without matrix band adaptation [open technique]introduced by Sturrock -with copper band adaptation.

Wax pattern fabricationIndication In easy accessible areas. When cavity is small with minimal proximal extension. When cavity walls are flat and line angles are definite.

AdvantagesThe pattern is carved on tooth and not on model.Little laboratory work compared to indirect technique.Time saving.

Direct wax pattern techniqueDisadvantagesGreat skill and patience is required to carve pattern in mouthWhen it is carved in indirect vision it is difficult and fatigueDiscrepancies of the pattern at the gingival margin are difficult to detect until the pattern has been carved and withdrawnIf the casting fails, the patient has to be recalled Direct wax pattern techniqueWith Matrix Band Adaptation [Closed Technique] Isolate the tooth using cotton rolls. Apply matrix band and retainer. Coat the internal surface of band using separating media like vaseline.Soften the inlay wax by heating and moving it over a alcohol flame.Compress the softened inlay wax into the prepared tooth for few minutes with finger pressure. This technique is called compression technique. Direct wax pattern techniqueWith Matrix Band Adaptation [Closed Technique]Remove excess of wax and do the carving. With a hot egg burnisher, contour the occlusal portion of the wax pattern.Now remove the matrix band and retainer carefully without disturbing the wax pattern.Ask the patient to bite in centric occlusion for a few seconds after placing a thin layer of cotton soaked in warm water.Examine the occlusal surface for high points and remove them.Do the occlusal carving.Direct wax pattern techniqueWith Matrix Band Adaptation [Closed Technique]Pass a floss through the contact area while holding the pattern in place.Smoothen the proximal surface of the pattern with fine soft silk.Evaluate and correct all the margins of the pattern.Burnish and remove any excess wax over the axial margins with a warm hollenback waxing instrument.Finally, examine the pattern. There should be a slight excess of wax over the gingival margin.Direct wax pattern techniqueWith Matrix Band Adaptation [Closed Technique]Once the satisfactory wax pattern is formed, attach the sprue former and reservoir to the thickest point of the wax patternRemove the wax pattern from the preparation and examine it for marginal integrity.Direct wax pattern techniqueWithout use of Matrix Band Adaptation [OPEN TECHNIQUE]Here the technique is same except that matrix band is not used during fabrication of wax pattern. In this, after the carving of occlusal portion is done, use dental floss to remove extra wax from the proximal portion and to produce proper contact and contour..Direct wax pattern techniqueINDIRECT WAX PATERNINDICATIONS Large preparations like onlays, full coverage crowns and MOD restoration.Insufficient access and visibility.When minute details like skirts and collars are presentINDIRECT WAX PATERNADVANTAGESEasily made in inaccessible areasLess chair side timeDirect vision is used in pattern fabricationMade on articulated models better occlusal restorationPolishing is done outside the oral cavity If casting failure occurs, patient recall not necessary

INDIRECT WAX PATERNDISADVANTAGES Impression may be distorted.Dentist depends on technician.More laboratory work.Errors in cast can result in inadequate casting. Indirect wax pattern technique

The die is lubricatedProximal surfaces are developed, with correctly located contact areas

The occlusal surfaces are developed with a wax addition technique.margins are reflowed, and the wax pattern is finished DISTORTION OF WAX PATTERN

Wax distortion is probably the most serious problem during forming and removal of the pattern from the mouth or the die.Distortion results from -thermal changes -relaxation of stresses (on cooling) -occluded air -molding -carving -removal and the time and temperature of storage

Waxes tend to return to their original shape after manipulation, this property is known as wax memory.The casting fits best when the pattern is invested immediately after its removal from the die.

Inlay wax softened over bunsen burner and left in room temp water for several hours. It tends to return to its orignal shape, the inner mol were under compression while outer ones under tension. DISTORTION OF WAX PATTERN

One can minimize the incorporation of residual stress by softening a wax uniformly by heating at 500C for at least 15 min before use , also by using warmed carving instruments and a warmed die, by adding wax to the die in small amounts.Greater distortion results at higher storage temperature.Inlay wax pattern if allowed to stand longer than 30 minutes, should be kept in a refrigerator , the distortion will be less as compared to room temperature.

PREVENTION Of DISTORTION OF WAX PATTERN

During spruing distortion can be reduced by use of solid wax sprue or hollow metal sprue filled with sticky wax. If the pattern was stored margins should be re-adapted. Sprue position- Ideal area for the sprue former is the point of greatest bulk in the pattern to avoid distorting thin areas and permit complete flow of the alloy into the mold cavity.It should be attached with the pattern on the master die, provided the pattern can be removed directly in line with its path of withdrawl from the die.PREVENTION Of DISTORTION OF WAX PATTERN

Sprue diameter- same size as the thickest area of the wax pattern , if the pattern is small, the sprue former must also be small.Length should be adjusted , Gypsum bonded investment 6 mm of the open end of ring Phosphate bonded investment 3 - 4mm PREVENTION Of DISTORTION OF WAX PATTERN

Dental Investment MaterialINTRODUCTIONWhen a restoration or appliance is being made by a lost wax process, the wax pattern is embedded in an investment material. The wax is then removed from this mold, and the mold space thus formed will be filled by the molten casting alloy from which the restoration is to be madeDEFINITIONAn 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. Robert G Craig As these materials can withstand high temperatures, they are also known as refractory materials.

IDEAL REQUIREMENTSEasily manipulatedSufficient strength at room temperatureStable at higher temperaturesSufficient expansionIDEAL REQUIREMENTSShould be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting procedure.Should have a smooth surface and fine detail and margins on the casting should be preserved.Investment should break away readily from the surface of the metal and should not have reacted chemically with it.Should be inexpensive.

CLASSIFICATIONBASED ON PROCESSING TEMPERATUREHigh TemperaturePhosphate Bonded InvestmentSilica Bonded InvestmentLow TemperatureGypsum Bonded Investment

BASED ON REFRACTORY MATERIAL Quartz InvestmentCristoballite InvestmentBASED ON TYPE OF BINDER USED

Gypsum bonded investments : According to ADA Specification 2 Uses Shrinkage compensationType I Inlay, Crown Thermal expansion Type II Inlay, Crown Hygroscopic expansionType III R.P.D. Frame workThermal expansion

Phosphate bonded investments

Silicate bonded investments

CLASSIFICATIONGENERAL COMPOSITIONRefractory MaterialMaterial that withstands high temperature without decomposing or disintegrating.Resists heat and force of castingExpands and compensates for casting shrinkageBinder The refractory material alone does not form a coherent solid mass, so some kind of binder is needed. Commonly used binders are ; - Calcium sulfate hemihydrateOthers are Sodium silicate, Ethyl silicate, Ammonium sulfate, Sodium phosphate.

Other Chemical Modifiers Usually a mixture of refractory materials and a binder alone is not enough to produce all desirable properties required of investment.Other chemicals such as sodium chloride, boric acid, potassium sulfate, graphite, copper powder or magnesium oxide are often added in small quantities to modify various physical properties.

GYPSUM BONDED INVESTMENTADA SPECIFICATION NO.2

TYPE-1 Inlays or crowns Thermal expansionTYPE 2 Inlays or crowns Hygroscopic expansionTYPE 3 Fixed partial dentures with gold alloys

COMPOSITION - GYPSUM BONDED INVESTMENTBinder

-Hemihydrate Gives strength Holds ingredients together Provides rigidity

RefractorySilicaQuartz Tridymite Cristobalite Fused QuartzSilica (SiO2) is added to provide a refractory during the heating of investment and to regulate thermal expansion.If proper form of silica is employed in investment, the contraction during heating in an investment can be eliminated and changed to an expansion.

COMPOSITION - GYPSUM BONDED INVESTMENTQuartz and cristobalite are of particular dental interest.When quartz, tridymite or cristobalite is heated, change in crystalline form occurs at transition temperature characteristic of particular form of silica. Low form High form (Alpha quartz) 575C ( Beta quartz) Cristoballite 200C to 270CTridymite 2 inversions at 117C & 163C. Density decreases as alpha form converts to beta form, with resulting increase in linear expansion.

COMPOSITION - GYPSUM BONDED INVESTMENTModifiers

Reducing Agent Carbon Provides a non oxidizing atmosphere

Coloring Agent Powdered copper

Modifying chemicals Boric acid, NaCl Regulates Setting expansion & Setting time Prevents most of the shrinkage when gypsum heated above 300C

COMPOSITION - GYPSUM BONDED INVESTMENTSETTING REACTION - GYPSUM BONDED INVESTMENTSame as Dental stone. CaSO4 H2O + H2O CaSO42H2O + 3900 Cal / gmol. (Ca. Sulfate hemihydrate) (Ca. Sulfate dihydrate)

Sets to form a solid mass which binds the silica particles together.

Reaction is exothermic.

Microstructure of set material shows rod like particles of gypsum intermeshed with large irregular particles of silica refractory.

PROPERTIES- - GYPSUM BONDED INVESTMENT ADA Specification No. 2Setting Time9-18 minutes.This can be altered by addition of K2SO4, NaCl (Increase setting time), Borax and potassium citrate (decrease setting time). ExpansionExpansion aids in enlarging mold. This property of investment is needed for compensation of casting shrinkage of alloy. Expansions are of 3 types : Normal setting expansion. Hygroscopic setting expansionThermal expansion

PROPERTIES- - GYPSUM BONDED INVESTMENT Normal Setting ExpansionSilica & calcinated gypsum results in setting expansion greater than that of gypsum used alone.Silica particles interferes with the intermeshing & inter-locking of crystals as they form.Thus, thrust of crystals is outward during growth, resulting in increased expansion. 0.6% expansion.

PROPERTIES- - GYPSUM BONDED INVESTMENT Factors Affecting Normal Setting ExpansionGreater the gypsum content of the investment, greater the exothermic heat transmitted to the wax pattern and greater the mould expansion.Lower the W/P ratio for the investment, greater the exothermic heat and greater the setting expansion.

PROPERTIES- - GYPSUM BONDED INVESTMENT Factors Affecting Normal Setting ExpansionThinner the walls of the wax pattern, greater the setting expasion of the investment.Softer the wax, greater the setting expansion. If a wax softer than Type B inlay wax is used, the setting expansion may cause a serious distortion of the pattern.

PROPERTIES- - GYPSUM BONDED INVESTMENT Hygroscopic Setting ExpansionThis occurs when the gypsum is allowed to set under or in contact with water. Hygroscopic setting expansion of an investment may be more than six times the normal setting expansion. The ADA specification no.2 for Type II investments requires a minimal hygroscopic setting expansion of 1.2% and a maximal expansion of 2.2%.PROPERTIES- - GYPSUM BONDED INVESTMENT Hygroscopic Setting ExpansionWater Immersion Technique :- the investment mould is placed into water.Water Added Technique :- a measured volume of water is placed on the upper surface of the investment material within the casting ring. PROPERTIES- - GYPSUM BONDED INVESTMENT Factors Affecting Hygroscopic Setting ExpansionComposition:- Increase in silica content increases the hygroscopic setting expansion.W/P ratio:- Higher the W/P ratio of the original investment water mixture, less the hygroscopic setting expansion.Spatulation:- Shorter the mixing time, less the hygroscopic expansionShelf life of investment :- The older the investment, less is its hygroscopic setting expansion

PROPERTIES- - GYPSUM BONDED INVESTMENT 139Factors Affecting Hygroscopic Setting ExpansionConfinement :- Both the normal and hygroscopic setting expansions are confined by opposing forces, such as the walls of the casting ring or the walls of the wax pattern.Temperature of the water bath :- Higher the temperature of the water bath used for immersion, greater the hygroscopic expansionAmount of added water :- An increase in the amount of water added, increases the hygroscopic setting expansion upto a certain point, after which further addition of water does not create any expansion

PROPERTIES- - GYPSUM BONDED INVESTMENT 140Factors Affecting Hygroscopic Setting ExpansionParticle size of silica :- Finer particles of silica produce greater hygroscopic expansionSilica/binder ratio :- If this ratio increases, greater will be the hygroscopic expansion and lesser the strength.PROPERTIES- - GYPSUM BONDED INVESTMENT 141Thermal ExpansionThermal expansion of gypsum is directly related to amount of silica present & to type of silica employed.The thermal expansions of type II investments to be between 0.0 to 0.6% and of Type I investments which rely principally on thermal expansion for compensation, to be between 1.0 to 2.0%.

PROPERTIES- - GYPSUM BONDED INVESTMENT Thermal ExpansionThermal expansion is influenced by particle size of quartz, type of gypsum binder & resultant W/P ratio to provide workable mixInvestments containing cristobalite expand earlier than those containing quartz.Expansion occurs at a lower temperature because of lower inversion temperature of cristobalite compared to quartz.PROPERTIES- - GYPSUM BONDED INVESTMENT StrengthMust be adequate to prevent fracture or chipping of mould during heating and casting of gold alloy.Contraction of investment is fairly constant until it cools below 5500C. Thus, when alloy is still quite hot and weak, investment can resist alloy shrinkage by virtue of strength and constant dimension. Compressive strength 2-4 MPa.

PROPERTIES- - GYPSUM BONDED INVESTMENT Factors Affecting StrengthAlpha hemihydrate ----- strengthModifiers ------- strengthWaterHeating investment to 700oc may increase or decrease strength depending on composition.After investment has cooled to room temparature its strength decreases.

PROPERTIES- - GYPSUM BONDED INVESTMENT PHOSPHATE BONDED INVESTMENT

PHOSPHATE BONDED INVESTMENTMost common type of investment for casting high melting alloys is phosphate bonded investment.

Common BrandsSilikan, Auro bond, Calsite, Deguvest, Eurocent, Nirobond, etc.

COMPOSITIONBinder- 20%Acidic part ammonium diacid phosphateBasic part magnesium oxide

Refractory- 80%Silica (cristobalite or quartz or mixture of two)Provide high temperature thermal shock resistanceProvide thermal expansion at high temperatureModifiers- CarbonActs as reducing agent

MODE OF SUPPLYPowder + liquidPOWDERAmmonium diacid phosphateMagnesium oxideSilicaTraces of carbonLIQUID Colloidal silica

SETTING REACTIONNH4H2PO4 + MgO NH4MgPO4 + H2O OR

NH4H2PO4 + MgO +H2O NH4MgPO4.6 H2O150SETTING AND THERMAL EXPANSIONWhen phosphate bonded investments are mixed with water they exhibit shrinkage between 2000C to 4000C. This contraction is eliminated when colloidal silica replaces water.

Early thermal shrinkage of phosphate bonded investments is because of decomposition of binder and is accompanied by evolution of ammonia.

For gypsum, shrinkage is caused by transformation of calcium sulfate from hexagonal to rhombic configuration.

SETTING AND THERMAL EXPANSIONWorking And Setting TimeWorking Time - 2 minutes SettingTime - 1 hour Warmer the mix, faster it sets. Increased mixing time, faster is the set. Increased L/P ratio, increased is working time.

PROPERTIES

Compressive strength ----- 2.5- 3 MpaSetting expansion ------ 0.4%Hygroscopic setting expansion ------ 0.8%Thermal expansion with water----- 0.8% With special liquid ---1.2%

ADVANTAGEHigh green strengthHigh fired strength less mold cracking and few fins on castingCan withstand temperature up to 1000c for short period of time

DISADVANTAGESTemperature of 1375c ----- mold breakdown and roughen surface of castingSpecial liquid ---- less porous mold -----incomplete castingProduces oxides difficult to remove from castings

SILICATE BONDED INVESTMENTSThese investment materials are being used since 1930 but are slowly loosing there popularity due to complicated and time consuming procedures involved.

COMPOSITIONBinder ---- silica gelRefractory ------ silicaAdditives---- magnesium oxideWetting agent

PROPERTIESCompressive Strength 1.5 MPa Setting Contraction 0 - 0.4% Thermal Expansion 1.5 1.8%, this has only thermal expansion. Can Withstand High Temp 10900C to 11800C AdvantagesHigh permeability, yields sharply defined castingsLow setting expansionSmooth castingsLow burnout strength easy removal of casting & cleaning of oxides from castings

DisadvantagesLimited shelf life of liquidVery expensiveGives off flammable components during processingPotential of cracking during burnout.

InvestmentPrimary useDental plaster or stone Gypsum-bonded materials

Phosphate-bonded materials

Silica-bonded materialsMould for acrylic dentures.Mould for gold casting alloys.Mould for base metal and gold casting alloys, mould for cast ceramics and glasses; mould for soldering.Mould for base metal casting alloys.PROBLEMCAUSESOLUTIONROUGH SURFACEBREAKDOWN OF INVESTMENTDONT OVERHEAT MOULD OR ALLOYAIR BUBBLES ON WAX PATTERNWETTING AGENT/VACCUM INVESTING TECHNIQUEWEAK INVESTMENTAVOID USING EXCESS WATER OR TOO MUCH WETTING AGENT ON WAX FINSCRACKING ON INVESTMENTAVOID HEATING INVESTMENT RAPIDLYSULPHURBREAKDOWN OF INVESTMENTDONT OVERHEATROUNED MARGINSBACK PRESSURE OF AIR DUE TO LOW POROSITY OF MOULDPATTERN 6-8 MM FROM END OF CASTING RINGPROBLEM CAUSED BECAUSE OF INVESTMENTSRECENT ADVANCEMENTSMore recently new investment materials have been marketed for casting commercially pure titanium. These investments are based on alumina, spinels, zirconia and magnesia. The objective in developing these investments has been to minimise the interactions between molten titanium and the investment moulds on casting. In the magnesia-based investment the expansion is controlled using zirconium. As this reaction occurs, investment expands.

INVESTING Two main methods of investing the wax pattern are:1. Hand investing.2. Vacuum investing.

INVESTING

The mix is hand-spatulatedWith the crucible former in place, the ring is attached tothe mixing bowl.The vacuum hose is attachedINVESTING

The bowl is inverted, and the ring is filled under vibrationThe vacuum hose is removed before the mixer is shut offThe filled ring and crucible former are removed from the bowl.

When the investment has set, the skin at the top of the ring is trimmed off.

The rubber crucible former is removed, and any loose particles of investment are blown off. The ring is then placed in the furnace for the recommended burnout schedule.

References: Philips science of dental materials - Anusavice Notes on dental materials - E C CombeRestorative dental materials - Robert G CraigFundamentals of fixed Prosthodontics - Shillingberg. Contemporary fixed Prosthodontics - Stephen F.RosenstielPhilip Duke et al ;Physical properties of type IV gypsum, resin containing and epoxy die materials JPD April 2000 vol 83, no. 4 p-466-73.

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Die Materials, Investment and Inlay Wax