Investment materials crown 2nd year 2024.pptx

Dental Investment materials BY Akhlas Elsarah  INVESTING AND CASTING Are the steps taken after completion of wax pattern immediately :Casting process consist of three main step spruing-1 investing-2 casting-3 a-immediately after the wax pattern is completed a sprue former for...

Dental Investment materials BY Akhlas Elsarah  INVESTING AND CASTING Are the steps taken after completion of wax pattern immediately :Casting process consist of three main step spruing-1 investing-2 casting-3 a-immediately after the wax pattern is completed a sprue former former is attached to wax pattern while the wax pattern is on the die b-the wax pattern is removed from the die and the former attached to crucible former and the casting ring with liner placed around the pattern C-wax pattern invested and investment left to set d-crucible former removed and ring placed in burn out furnace to remove the wax e-alloy molten and cast into the mold  Definition of an investment material An investment can be defined as a ceramic material that is suitable for forming a mold into which a metal or alloy is casted. a – Crucible b – Sprue c – Reservoir d – Wax pattern E – Casting ring F – Ceramic paper liner G – Investment material  Composition of dental investment In general an investment material is consisting of a refractory, a binder and additives are added to improve handling characteristics.  Refractory: A material that resists heat.  Usually Silica (silicon dioxide) SiO2.  It is available in three allotropic forms (polymorphism) – quartz, cristobalite and tridymite – which are all chemically identical but differ slightly in crystalline form.  is added in the investments to:  a. Acts as refractory substance during heating of the investment.  b. To regulate the thermal expansion which is desirable to compensate for the casting shrinkage of the alloy, and shrinkage of the gypsum when it is heated.  Binder: A material that holds the investment together.  Classification of dental investments According to the type of binder used  Gypsum bonded investments  Phosphate bonded investments  Ethyl silicate bonded investments Gypsum bonded investments Gypsum is used as binder & refractory material (silica – quartz- cristobalite) Which responsible for thermal expansion ;It is used for casting of gold alloys because A-it is not chemically stable above 650 B-produce expansion suitable for shrinkage of the gold Phosphate bonded investments Phosphate compound are used as a binder and silica is the refractory material It is used for casting of high melting alloys e.g. base metal alloys :because it is chemically stable up to 1100c-1 it produce thermal expansion higher than gypsum bonded investment-2 which is suitable for compensation of solidification shrinkage of high fusing alloys  Ethyl silicate – bonded investment Application: Base metal alloys. Not used nowday  Expansions of dental investments Every casting under go shrinkage when become solid and lead to shortage of restoration.So to compensate this shrinkage some expansion should allow for the investment :There are three type of expansion of the investment :Setting expansion-1 The water powder ratio can be adjusted to increase or decrease the.amount of setting expansion decrease the amount of water –increase the setting expansion increase the amount of water-decrease the setting expansion.Additional ring liner will increase setting expansion, so produce slightly larger casting :Hygroscopic expansion-2 This type of expansion occurs when water added to setting investment after the ring has filled, usually done by submerging the ring in water.bath at 37c for 1 hour until complete setting.wet ring liner aid in hygroscopic expansion- :Thermal expansion-3 This type of expansion occurs in burn out furnace, and the silica material is responsible for thermal expansion, it advisable to use investment with :minimum setting expansion and higher thermal expansion because to avoid distortion of wax pattern during setting of investment-1.thermal expansion is easier to controlled by increase or decrease time of heat soaking -2  Casting ring  The casting ring holds the investment in place during setting and restricts the expansion of the mold. :Ring-less technique In this technique after complete setting of investment the rubber ring is removed and The investment block is placed in burn-out furnace for wax.elimination.In this technique the investment is allowed unlimited external expansion So the technician Should control the amount of expansion suitable to compensate the solidification shrinkage by control the time of heat.soaking in burn out furnace :Ring technique In this technique ring liner should be used to allow external expansion of investment It placed inside the ring leaving about 2-3mm from the end to allow for.supporting of investment with casting ring  Ring liner  liner is cut to fit the inside diameter of the casting ring with no overlap. Thickness of the liner should not be less than approximately 1mm. Place the liner somewhat short of the ends of the ring 3mm, tends to produce a more uniform expansion, therefore less chance for distortion of the wax pattern & mold.  Most common way to provide investment expansion is by using a liner in the casting ring.Traditionally asbestos was used.  In addition to non asbestos ring liner used are: 1) Aluminum silicate ceramic liner. 2) Cellulose paper liner. The aim of using a resilient liner is to 1.Allow different types of investment expansion (act as a cushion) 2. Facilitate venting during casting procedure. 3. Facilitate the removal of the investment block after casting. if the ring liner is absent, the investment will expand in inward direction towards.4.the mold cavity leading to distortion of the restoration  Investing procedures  Definition: is the process by which the sprued wax pattern is embedded in a material called an investment in other words is the process by which a ceramic materials form a mold into which a metal or alloy is casted.  Preparing the wax pattern for investing  secure wax pattern in its position in the sprue base then cleaning it using pattern cleanser.  The liquid is added to clean dry mixing bowl, then the powder is gradually added to the liquid using care and caution to minimize air entrapment. :Investing techniques I- Hand technique II-Vacuum technique :I-Hand technique the accurate water-powder ratio is mixed under vacuum, a brush is used to paint the wax pattern with the mix, then the ring is filled under vibration.until complete filling :II-Vacuum technique The accurate water-powder ratio is mixed under vacuum and also is investing is done under vacuum machine Brush technique produce better casting because it reduce the air entrapment.and the investment wet the pattern very well :Wetting agent Before investing the wax pattern should be sprayed by wetting agent (Debubblizer) by the use of stable hair brush to allow intimate and complete wetting of wax pattern by investment without any bubbles.The wetting agent should allow to dry completely before investing If any bubbles occur between wax pattern and investment this bubbles will be filled With casting alloy during casting and appear as nodules on the casting surface..  Pour the remaining investment into the casting ring at a slight angle from bottom to the top of the ring.  Wax elimination( Burn out of wax) Wax burn out is done by heating the investment in controlled burn out furnace until all - Traces of wax pattern is vaporized to obtain completely empty mold ready to receive.The molten alloy by casting the ring is placed in the furnace while sprue former facing down to allow complete escape -.Of molten wax :the ring should be maintained at maximum temperature (Heat soak) about 20 minute to - A-allow thermal expansion B-avoid sudden drop in temperature up on removal from furnace which cause incomplete.casting because of rapid solidification of the alloy as enters the mold Once the investment is heated during wax elimination procedure, heating must be continued and casting must be completed. cooling and reheating of the investment can cause casting Inaccuracy because the refractory and binder will not revert to original forms. Inadequate.Expansion and cracking of the investment are typical results