INDIAN INSTITUTE OF TECHNOLOGY DELHI

“INVESMENT CASTING”

Submitted to: Submitted by: Prof. MP Gururajan Emrati Kumari (2008AMD3240)

INTRODUCTION

Investment Casting:-

• One of the oldest known metal-forming Technique.

• Today high-technology waxes, refractory material and specialist alloys used.

• Casting allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys.

• Utilized it when complex detail, undercut or non-machinable features and accurate parts are required.

• It requires best significant lead time and is best suited for low volume production rate.

Manufacturing Process

Step I tooling

: • Investment Casting is to create a die and form of part to the required specifications. • The die is used to inject paraffin wax, which is used to create the complex shapes required manufacture component or part. • Die is attached to a central sprue. Multiple die may be attached to same sprue. The sprue is a channel which distribute the molten wax into each of the die or forms.

• Tool is constructed from Aluminum or hot die steel material.

• For most applications single/multi cavity dies are used for high production quantities.

Step II Shell Building:-

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• The wax assembly is now dipped into a ceramic slurry, to obtain the shell thickness desired. • A layer of fine sand (zircon) is added on top of each ceramic layer. • This process repeated until the desired shell or casting form is created.

Step III :-

• After the shell/casting form is created, the wax is removed. • This is where the term

Lost-wax process

comes form. • This leaves an impression or casting form which will create the desired investment casting part, which will be filled with the desired metal or material. • Before beginning the casting, shell must be heated up in a furnace so they do not fail due to differential temperature introduced poring in the molten metal or material.

Step IV :-

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• The casting metal or material is poured into the pre-heated ceramic shell.

• The casting material fills each part and feature of the investment casting shell. • The individual cast part will be removed after the mold cools and the shell is removed. • The shell is typically removed by water blasting, through alternative methods are available. • They are still attached to the sprue assembly or each other.

• Individual parts are removed by cold-break (dipping in liquid nitrogen and breaking the parts off with hammer and chisel) or with alternative manufacturing machine tools.

Step V :-

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• The rough cast parts are now separated with each other and the sprue.

• First the gate or the place where parts was connected to the sprue, must be removed.

Step VI :-

• Investment Casting yield exceedingly fine quality products made off all type of metals. • It has special application in fabrication very high temperature metal, specially those which can not cast in metal or plaster molds and those which are difficult to machine or work.

Type of material for which the process is used

• A pattern of the component to be cast is produced by injection-molding special waxes into a metal die. • Pre-formed ceramic cores can be included in the wax pattern as it is molded, which can create intricate hollows within the finished casting. • As many as several hundred patterns may be assembled into a tree around a wax runner system (riser and sprue). Once a tree has been assembled, a pour cup is attached.

Shell for cast turbocharger rotor

• The completed tree is dipped or inverted by hand or via robotic control into a ceramic slurry of ethyl silicate, colloidal silica.

or by hand.

shell.

refractory material.

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• During primary coat the sand will be typically be a zircon-based, as zirconium is less likely to react with the molten metal when poured into the • The stuccoed tree is then allowed to dry before re-dipping in slurry and applying secondary coats of mullite, molochite, chamotte or fused silica • This process is repeated until the shell is thick enough to withstand te mechanical shock of receiving the molten metal. Dry times range from 24 to 48 hours, and total production from two days to one week.

View of the ceramic impression in a turbocharger shell

• After the shell has been constructed, the wax is removed in an autoclave or

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• Most shell failures occur at this point, as the fragile stuccoed shell is subjected to extremes of temp. and in an autoclave pressure.

• The shell is fired at temp. of around 1100 C to induce chemical and physical changes in the set refractory materials forming a ceramic shell.

• This leave a ceramic impression of the part to be cast.

• Most foundries remove the shells from the furnace while still hot and pour the molten metal into the ceramic shell.

• Various methods of pouring the molten metal include vacuum casting, anti gravity casting, tilt casting, gravity pouring, pressure assisted pouring, centrifugal casting.

• After the molten metal cools, the shell is removed. This is generally done with water jets, vibration, grit blasting or chemical dissolution.

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dipping them in liquid nitrogen and breaking them off with a hammer and chisel.

• Many cast parts require grinding of the gate and runner bar attachments.

• Because molten metal cools slowly, it does not finish as hard as some forging and machining processes.

• Cast parts often are subsequently hardened by heat treatment and surface hardening.

• The parts are inspected by eye or in special cases by X-ray at the foundry or by specialty firms.

Completed turbocharger rotor

A property of the material is affect by the process: Thermophysical properties of wax

: • The linear deformation of the sample, which was measured as a function of temperature, was converted to thermal expansion and density data.

• At temperatures below 55 °C, the wax behaved like a hard paste or solid. • As the wax was heated above 60 °C, the wax lost its strength, became very soft, and flowed out into the surrounding silica powder. The density at 110 °C was obtained using a hydrometer.

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Advantages:-

Investment casting is a special propose manufacturing process, use in

jewelry.

Through investment casting we can produce complex parts. Example turbine blades, aerospace, power generation, complete aircraft door frames.

It gives extremely good surface finish 80 to 120 micron inches.

It is also widely used by firearms manufacturers to fabricate firearm receivers, triggers, hammers, and other precision parts at low cost.

Greate design freedom.

Feasibility of replacing two or more fabricated sub-assembling by a single piece casting.

Lighter, Stronger components may be designed, without a mismatch giving an improved aesthetic appearance.

The process permits a high level of consistency, batch to batch.

• • • • •

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wax process close dimensional tolerance of ± 0.13mm to 0.25mm.

With the ability to produce special alloys to customers' requirements, the choice of metallurgical specifications is virtually unlimited.

The light stressed wax impression dies have a very long life and are not costly considering design complexity.

If required, minor design modification may be made economically without going for few tooling.

A high level of metallurgical integrity and Strength can be achieved in the lost wax process.

Disadvantages:-

• • • • • Investment casting cost is high.

Investment casting requires specialized equipment.

In this costly refractories and binders.

Many operations to make a mold so it is time taking process around two to one week.

Some time leave occasional minute defects like macro and micro defects, includng shrinkage porosity, grain size and the extent of surface (100 microm) of the casting solidifies after the bulk.

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