This step refers to heating the This step refers to heating the invested wax pattern in a invested wax pattern in a thermostatically controlled furnace thermostatically controlled furnace until all traces of wax are vaporized.until all traces of wax are vaporized.

DefinitionDefinitionDefinitionDefinition

1)1) Wax elimination.Wax elimination.

2)2) Allow thermal expansion.Allow thermal expansion.

3)3) Prepare the mold for the molten Prepare the mold for the molten metal.metal.

AimsAimsAimsAims

Burnout techniquesBurnout techniques

High heat techniqueHigh heat technique

High heat techniqueHigh heat technique

This technique is employed if thermal This technique is employed if thermal expansion was utilized alone to compensate expansion was utilized alone to compensate the shrinkage.the shrinkage.

Investing utilized in this technique require Investing utilized in this technique require metal rings.metal rings.

The heating rate should be slow, gradual and The heating rate should be slow, gradual and is continued until a temperature of is continued until a temperature of 650ºC(1200ºF)is reached.650ºC(1200ºF)is reached.

This ring is placed in the furnace at the room This ring is placed in the furnace at the room temperature.temperature.

Low heat techniqueLow heat technique

In this technique hygroscopic expansion In this technique hygroscopic expansion provides the needed compensation expansion provides the needed compensation expansion plus the additional thermal expansion(0.55%)plus the additional thermal expansion(0.55%)

Investing is completed in rubber rings Investing is completed in rubber rings allowing the unrestricted hygroscopic allowing the unrestricted hygroscopic expansion.expansion.

The ring can be placed in an oven preheated The ring can be placed in an oven preheated to 480ºC (900ºF)to 480ºC (900ºF)

In either technique the temperature can be In either technique the temperature can be brought to 200-300ºC and hold at this brought to 200-300ºC and hold at this temperature for 30min.,by this time most of temperature for 30min.,by this time most of the wax is eliminated.the wax is eliminated.

Once the investment is heated during the Once the investment is heated during the wax-elimination procedure, heating must wax-elimination procedure, heating must be continued, and casting must be be continued, and casting must be completed. completed.

CoolingCooling and reheating of the investment and reheating of the investment can cause casting inaccuracy because can cause casting inaccuracy because the refractory and binder will not revert the refractory and binder will not revert to their original forms to their original forms (hysteresis)(hysteresis). . Inadequate expansion and cracking Inadequate expansion and cracking of the of the investment are typical results.investment are typical results.

Two burnout furnacesTwo burnout furnaces can be setcan be set at at 200°C and 650°C or 480°C, or a 200°C and 650°C or 480°C, or a programmable two-stage furnace can serve programmable two-stage furnace can serve equally well. However, the investment equally well. However, the investment should not be overheated or kept at should not be overheated or kept at temperature too long. temperature too long.

Gypsum-bonded investments are not stable Gypsum-bonded investments are not stable above 650°C (1200°F). Also, some carbon above 650°C (1200°F). Also, some carbon in carbon-containing investments burns off, in carbon-containing investments burns off, causing increased surface roughness of the causing increased surface roughness of the casting.casting.

Burnout furnaces

Burnout RequirementsBurnout Requirements Begin burnout while the mold still wet Begin burnout while the mold still wet as Has H22O trapped in the pores of the O trapped in the pores of the investment reduces the absorption of investment reduces the absorption of waxwax

All HAll H22O in the investment must be O in the investment must be driven off during wax elimination driven off during wax elimination

The temp. must be sufficiently high & The temp. must be sufficiently high & maintainedmaintained long enoughlong enough ((heat soakheat soak))

Heat rate should be standardized as Heat rate should be standardized as the part of the investing & casting the part of the investing & casting protocol for accuracy of castingprotocol for accuracy of casting

Temperature SoakTemperature Soak The temperature to which the casting ring is The temperature to which the casting ring is

heated must be sufficiently high (according heated must be sufficiently high (according to the technique used) and should be to the technique used) and should be maintained long enough (45min.)to avoid maintained long enough (45min.)to avoid sudden drop in temperature upon removal of sudden drop in temperature upon removal of the ring from the burn-out furnace. This drop the ring from the burn-out furnace. This drop in the temperature lead to incomplete in the temperature lead to incomplete casting as a result of rapid solidification of casting as a result of rapid solidification of the alloy as it enters the mold cavity. the alloy as it enters the mold cavity. However over heating should be avoided as it However over heating should be avoided as it results in decomposition of the investment.results in decomposition of the investment.

Burnout TemperatureBurnout Temperature

Gypsum- bonded investment : Gypsum- bonded investment : hygroscopic hygroscopic ((low heat-900°Flow heat-900°F)) thermal expansionthermal expansion ((high heat-high heat-

1200°F1200°F))

Phosphate-bonded investment :Phosphate-bonded investment : 1300F-1600F based on the type of 1300F-1600F based on the type of

casting alloy casting alloy

TechniqueTechnique Allow invest. To set & then remove rubber Allow invest. To set & then remove rubber

crucible & metal sprue.crucible & metal sprue. Remove smooth skin that forms on the Remove smooth skin that forms on the

ring with phosphate bonded invest. ring with phosphate bonded invest. [ because this layer occlude the porosity [ because this layer occlude the porosity in the investment which allow air to in the investment which allow air to escape ]escape ]

Increase temperature in furnace up to Increase temperature in furnace up to 400°F for 30 min. while ring inverted 400°F for 30 min. while ring inverted inside the furnace.inside the furnace.

Increase heat to the final [ 900 or 1200°F Increase heat to the final [ 900 or 1200°F ] for 45 min.] for 45 min.

BurnoutBurnout Step- by stepStep- by step

Allow the investment to set & then Allow the investment to set & then remove the rubber crucible & metal sprueremove the rubber crucible & metal sprue

Smooth skin that forms on the ring Smooth skin that forms on the ring with phosphate-bonded investment with phosphate-bonded investment should be removed and any loose should be removed and any loose particles of investment should be particles of investment should be blown off with compressed air.blown off with compressed air.

Reexamine the ring for any residual Reexamine the ring for any residual particles and then place it with the sprue particles and then place it with the sprue facing down in the furnace on a ribbed facing down in the furnace on a ribbed tray. The tray allows the molten wax to tray. The tray allows the molten wax to flow out freely. with the ring inside, bring flow out freely. with the ring inside, bring the furnace tothe furnace to 400°F400°F & hold this temp. & hold this temp. forfor 30 min.30 min.

Ring inverted into the furnaceRing inverted into the furnace

Why invert the ring inside the Why invert the ring inside the furnacefurnace??

Permit molten wax to drain out along the Permit molten wax to drain out along the gravity.gravity.

Prevent small fragment of invest. that may Prevent small fragment of invest. that may dislodged.dislodged.

OO22 contact wax ensure complete elimination. contact wax ensure complete elimination.

Increase the heat to the final Increase the heat to the final burnout tempburnout temp.. ((1200°F or 900°F1200°F or 900°F))& & hold for hold for 45 min.45 min.although a large casting ring although a large casting ring usuallyusuallyrequires increased heating time.requires increased heating time.

The rate at which the investment is The rate at which the investment is heated, is a factor in producing a heated, is a factor in producing a smooth surface casting. smooth surface casting.

Rapid heatingRapid heating will result in will result in evaporation of the water of the evaporation of the water of the investment forming steam that investment forming steam that cause cracking with a rough cause cracking with a rough surface and fins.surface and fins.

Transfer the casting ring 2 casting Transfer the casting ring 2 casting machinemachine

Common ErrorsCommon ErrorsToo rapid burnout :Too rapid burnout : cracks in the investmentcracks in the investment fins or feathersfins or feathers voidsvoids

Heating the mold too hot :Heating the mold too hot : dark castingsdark castings

Incomplete wax elimination :Incomplete wax elimination : dark castingsdark castings

Sudden drop in temp. :Sudden drop in temp. : Incomplete castingIncomplete casting

Hysteresis :Hysteresis : Cooling & reheating investment can cause casting Cooling & reheating investment can cause casting

inaccuracyinaccuracy

Time allowable for Time allowable for castingcasting

No more than 30 sec.-1 min. should be No more than 30 sec.-1 min. should be allowed to elapse between the time the allowed to elapse between the time the ring is removed from the oven & the ring is removed from the oven & the molten alloy is centrifuged into the mold.molten alloy is centrifuged into the mold.

When transferring the ring to the When transferring the ring to the casting machine check the sprue casting machine check the sprue channel in shaded light, it should be channel in shaded light, it should be cherry red color.cherry red color.

It is best to start the burn out while the It is best to start the burn out while the mold is still wet. The water in the pores mold is still wet. The water in the pores of the investment reduces the of the investment reduces the absorption of wax ,and as the water absorption of wax ,and as the water boils, it tend to flush the wax out. So, if boils, it tend to flush the wax out. So, if the mold is to be stored overnight, keep the mold is to be stored overnight, keep it in a humidor and soak it in water for it in a humidor and soak it in water for few minutes prior to heating.few minutes prior to heating.

Burn out could be started at any time after Burn out could be started at any time after setting of the investment. However once it setting of the investment. However once it is started, casting should be completed is started, casting should be completed immediately. So, the burnout oven and the immediately. So, the burnout oven and the casting machine must be conveniently casting machine must be conveniently located relative to one another to avoid any located relative to one another to avoid any time loss, that will result in metal time loss, that will result in metal contraction.contraction.

Cooling of the ring and reheating will result Cooling of the ring and reheating will result in marked loss of the investment strength in marked loss of the investment strength which is critical during casting.which is critical during casting.