Metal castingShell & investment method

by

Aruna c p

Rapid vacuum investment casting

• The Rapid vacuum investment casting is one of the recenttechnologies in the Rapid Prototyping (RP) field for producingmetallic components

• It is a combination of Rapid prototyping and Vacuum castingtechnologies.

• Here the master patterns are made using rapid prototypingtechniques followed by investment casting process.

• Although this technology has been subjected to numerousimprovements to enhance the mould filling, casting accuracy,casting quality and alloys range, some intrinsic constraints of theprocess limits it’s application

Steps involved in Rapid Vacuum Investment

Castings

3D CAD model is the Input for this process .This needs to be provided by the customer in STL, IGES or STEP format.

Step 1:Producing Rapid Prototype Master using SLA or CNC machining.

Step 2:Preparation of Silicon Rubber Mould (SRM) using the RP Master pattern in Vacuum

• Step 3:Produce wax pattern (replica of the master pattern) in the SRM.

• Step 4:Construction of wax pattern tree by adding sprue, runners and gates. The wax tree is placed in a casting flask

Step 5: Investment pouring

• Wax pattern is dipped in pre coat slurry Pre coat slurry - fine grained,

heat resistant, costly• The slurry coated pattern is

covered with fine ceramic (alumina of silica) powder.

• Drying of 8 to 10 hours in room temperature (slow cooling is done else cracking may occur)

• After drying secondary coating is done by dipping in secondary slurry and coating with silica powder.

• The secondary coating Is done until the shells attain optimum thickness, usually 7-8 times by allowing 8 to 10 hours in between each coating for drying.

Step 6: Burn out in furnace

• After the coating, dewaxing is done by steam heating the shell for 10 minutes at 10 bar pressure, 175°C

• After dewaxing shell is kept in oven at 850-900°C for 2-3 hours for complete burnout of the wax.

• Shell is ready

Step 7:The flask with the ceramiccavity is placed in the castingmachine.

Step 8: Metal pouring

• Raw alluminium metal is meltedin burner at 680°C.

• Nitrogen gas (inert gas) is passedand stirred using degassingmachine, to make slag andimpurities to float.

• The shell and the filter ispreheated to 400°C

• Liquid metal is transferred tographite crucible, the floatingimpurities are removed.

• Then the liquid metal is poured to shell.

• Allowed it to cool for 30 minutes at room temperature

Step 9:Flask is removed . The ceramic mould is broken. The metal tree will be fettled and cleaned to get the final castings. Shell at complex places are removed by dry sand blasting.

• Step 10:Heat treatment

• Step 11:Machining

Investment casting using Crystalline silica powder

• Mould making

• Wax pattern is fitted in the cylindrical casing.

• Investment powder and chilled water (at the ratio 100:28 by weight) are stirred in vacuum for 4 minutes.

• The mixture poured to the casing in vacuum in 6 minutes.

• Vacuum condition is maintained to eliminate the air bubbles.

• Investment mould is air cooled for 2 hours.

• Then the mould is kept in heating chamber for 3 days.

• In first 2 days the temperature is increased slowly to 750°C, for hardening of the mould and dewaxing.

• On 3rd day temperature of mould is decreased to 250°C.

• Metal casting

• Measure Al ingot weight based on weight f wax pattern.

• Preheat ingots to 100 to 120°C for 15 minutes and loaded in vacuum casting machine.

• Flask is taken out of the heating chamber and flask inside temperature is checked. (it should be around 250°C)

• melt the ingots up to 700°C at 0.9 bar vacuum

• Open the top chamber, clean the slug

• Load the flask in bottom chamber

• Supply nitrogen gas to top chamber.

• Vacuum is created in bottom chamber.

• metal is poured to the flask at vacuum for 6 to 10 minutes.

• Unload the flask and air cooled for 2 hours

• Then the flask is immersed in water to loose the hardness of the mould.

• The metal part is removed by breaking the mould.

• Then the remaining powder is removed with help of pressurized water.

• Unload the flask and air cooled for 2 hours

• Then the flask is immersed in water to loose the hardness of the mould.

metal finishing

• Supports and gates are cut, and polishing is done with grinding wheels.

• Emery is done for surface polishing.

• Wet sand blasting is done for the final finishing of the products.