Investments of Dental Materials Lecture PDF

INVESTMENTS OF DENTAL MATERIALS Investment can be described as ceramic material that is suitable for forming a mold in to which a metal or alloy is casted. Or. also can defined as refractory mould material used for embedding the wax pattern and obtating corresponding mould for casting Mate...

INVESTMENTS OF DENTAL MATERIALS Investment can be described as ceramic material that is suitable for forming a mold in to which a metal or alloy is casted. Or. also can defined as refractory mould material used for embedding the wax pattern and obtating corresponding mould for casting Material such as gypsum, Phosphate, and silicate are used for investing  Investing means capturing all the wax pattern in a solid material which capable of maintaining accurate details during burnout of casting of metal alloy. The investment material is available as two component system, Powder and liquid( water)  Ideal Requirement of an investment material It should be reproducing the shape, size, and details record in the wax pattern. Easily manipulated and must readily wet the surface of the wax pattern. Adequate setting time to allow the investing procedure. It should be able to withstand the high temperature during the burnout of the wax and casting of the molten metal.  - Sufficient porosity to allow gases to escape. -controlled expansion during setting to compensate for the casting shrinkage during cooling of the metal. - Smooth surface finish of the inner surface of the mold to have proper finish of the restoration. - It should not react with wax and alloys.  Components of investment material The investment material consist of a mixture of 1-Refractory material 2- Binder material 3-chemical modifiers, Refractory material ; – It is form of silica such as quartz,tridymite, or cristobalite, this is capable of withstanding high temperatures without significant degradation,  Binder material – It is capable of binding the refractory material to form a coherent solid mass, Commonly used binders are Alpha-hemihydrates, ethyl silicate, phosphate. Modifiers - Non oxidizing agent retarders, accelerators, and coloring agent are added to the refractory and binder material to enhance their physical properties ,  Classification of investment material There are three main types of investment materials used in dentistry. I. Gypsum- bonded investment material- these material used for conventional casting of inlay , onlays, crowns, and partial denture made of gold alloys, II. Phosphate-bonded investment material-these are used primarily for alloys that have to be cast at higher temperature then gold based alloys,eg. Metal ceramic prosthesis and for some base metal alloys, III. Ethyl Silicate- bonded investment material- these are used in casting of removal partial denture with base metal alloys, Composition- I. - gypsum bonded investment ( with alloys which melt qt 1200 c or less - porous ); Silica - 60 to 65% Alpha hemihydrates ,( dental stone) -30 to 35 % Chemical modifiers 5%  USE ; For casting of inlays, gold alloys and low fusing alloys. Functions of constituents – - Alpha hemihydrates: Binds and holds the silica particles Permit pouring of the mix in to the mold.  increase strength to mold Contributes to mold expansion( setting) Functions of Silica: - Quartz or cristobalite ; act as refractory during heating. Regulate thermal expansion Increases setting expansion of stone.  Modifiers ; used as coloring matter Reducing agent Modifying chemical –regulate setting expansion and setting time and also prevent shrinkage of gypsum when heat above 100 0c Manipulation ; powder and water mixed manually using a flexible rubber bowl and spatula or in vacuum investment mixing machine.  Setting time- the setting time should not be less than 5 minute and more. The modern inlay investment set initially in 9 to 18 minutes. This powder sufficient time for mixing and investing the pattern. Factors controlling setting time- Manufacturing process mixing time and rate water- powder ratio temperature modifiers  PROPERTISE OF GYPSUM INVESTMENTS Thermal behavior of gypsum – - Quartz when heated at a temperature of 375c converts from a alpha-quartz to beta- quartz Cristobalite - when heated at temperature of 200- and 270 c ,converted from alpha cristobalite to a beta cristobalite EXPANSION OF GYPSUM INVESTMENT Expansion aids in enlarging the mold to compensate for the casting shrinkage of the gold alloys. Three type of expansion 1- Normal setting expansion 2- Hygroscopic setting expansion 3-Thermal expansion  Normal setting expansion A mixture of silica and dental stone in setting expansion which is greater than when another gypsum product is used. Hygroscopic setting -when gypsum products are allowed to set in contact with water,the amount of expansion exhibited is double normal setting expansion.  Thermal expansion - obtained due to adding cristobalite as the temperature rises  Advantages of GBI- -Adequate strength -Adequate porosity -Controlled large setting and thermal expansion -Simple method of manipulation and casting procedure. -Not expensive  Disadvantages of GBI ; Can not used for titanium alloys, GBI powder has short storage time. Too high casting force and careless wax burnout method  PHOSPHATE BONDED INVESTMENT ( with alloys with melting temp higher than 1200 c - porous ) Most widely used investment in dentistry Use of high fusing noble or base metal alloys. For casting of high fusing alloys. Metal ceramic alloys and base metal alloys like nickel- chromium and cobalt- chromium  Composition – Refractory material :quartz or cristobalite or their mixture 80% Function at high temp and gives large setting expansion. it gives strength at room temperature. It is soluble in water and provides phosphate ions. It reacts with silica at high temperatures to increase strengths at casting temperature  Binder: - mixture of basic mgo and acidic sodium phosphate Carbon: it helps produce clean casting and easier removal from the mold. Liquid: - this is form of silica sol, which gives higher thermal expansion.  Manipulation The powder is mixed with a measured amount of liquid using a bowl and spatula. Hand mixing 20 sec.mechanical mixing under vacuum is done for 90 sec. The mixed material is vibrated in the casting ring the material is allowed to bench set for a minimum 30-45 minutes depending the investment.  Factors affecting setting time- Temperature of the mix and environment. Warmer temp. accelerate the setting , cooling the liquid prolong the working time. Increasing the mixing time accelerates the setting time.  Properties – Expansion- expansion are three type wax pattern expansion – during setting allows a significant expansion of the wax pattern Setting expansion-this is around 0.7 to 1% Thermal expansion- around 1 to 1.5%. Strength- these are two type – wet strength it is important for handling the set material and dry strength  Silica Bonded investment material ( high melting temp – non porous) The silica is the binder it derived from ethyl silicate or aqueous dispersion of colloidal silica or sodium silicate. The are less commonly used. Refractory material- powdered are quartz and cristobalite Small amount of magnesium oxide is added to the powder to reduce the ph of silica gel during manipulation  Binder – silica gel in the form of sodium silicate and colloidal silica is the commonly used binder that convert to silica at high temperatures. Investing procedures- wax pattern is fabricated to required dimension , the next step is to invest the wax pattern. This is done following steps. 1- attachment of the sprue former and removal of wax pattern from the master cast.  Attaching to the crucible former and investing of the wax pattern Burnout procedure to remove the wax Casting of molten metal the mold Removal of the cast metal framework from the investment. Finishing and polishing. Thank you