• The lost wax casting process developed by the ancient Egyptians some 3500 years ago.

• fashioned a core in the general shape of the piece, but smaller than the desired final dimensions, and coated it with wax to establish the size.

• The wax proved to be an easy material to form, and intricate designs and shapes could be created by the craftsman.

• On the wax surface, he carefully plastered several layers of clay and devised a means of holding the resulting components together.

• He then baked the mold in a kiln, so that the clay hardened and the wax melted and drained out to form a cavity.

• At last, he poured molten bronze into the cavity and, after the casting had solidified and cooled, broke away the mold to recover the part.

• Considering the education and experience of this early pottery maker and the tools he had to work with, development of the lost wax casting process demonstrated great innovation and insight.

• In investment casting, a pattern made of wax - coated with a refractory material - make the mold- wax is melted - molten metal.

• The term investment comes from word invest,invest-to cover completely then coating of the refractory material around the wax pattern.

• It is a precision casting process• because it is capable of making castings of high accuracy

and intricate detail. • is also known as the lost-wax process,• because the wax pattern is lost from the mold prior to

casting.

• Since the wax pattern is melted off after the refractory mold is made, a separate pattern must be made for every casting.

• Pattern production is usually accomplished by a molding operation

• Pouring or injecting the hot wax into a master die -designed with proper allowances - for shrinkage of both wax and subsequent metal casting.

• Complex part geometry - several separate wax pieces joined to make the pattern.

• In high production operations- several patterns are attached to a sprue,- made of wax, to form a pattern tree- this geometry will be cast out of metal.

Schematic illustration of investment casting (lost-wax) process. Castings by this method can be made with very fine detail and from a variety of metals.

• Coating with refractory accomplished dipping the pattern tree into a slurry of very fine grained silica

or refractory (almost in powder form) + plaster (bond the mold into shape)

• The small grain size of the refractory material - provides a smooth surface - captures the intricate details of the wax pattern.

After this initial coating has dried, the pattern is coated repeatedly to increase itsthickness for better strength• The final mold is accomplished by

- repeatedly dipping the tree into the refractory slurry or

- by gently packing the refractory around the tree in a container. • The mold is allowed to air dry for about 8 hours to harden the binder.• The one-piece mold is dried in air and heated to a temperature of 90° to 175°C.• It is held in an inverted position for a few hours to melt out the wax.

• The mold is then fired to 650° to 105 0°C for 4 hours to drive off the water of crystallization (chemically combined water) andWax patterns require careful handling because they are not strong enoughunlike plastic patterns, wax can be recovered and reused.

Advantages and disadvantages of investment casting include: (1) parts of great complexity and intricacy can be cast(2) close dimensional control—tolerances of 0.075 mm are possible(3) good surface finish is possible(4) the wax can usually be recovered for reuse(5) additional machining is not normally required—this is a net shape process.

• Parts up to 1.5 m in diameter and weighing as much as 1140 kg have been cast successfully by this process.

• many steps are involved - relatively expensive process. • Used normally small in size, parts with complex geometries• All types of metals, including steels, stainless steels, and other high

temperature alloys, can be investment cast.• Examples of parts include complex machinery parts, blades, and other

components for turbine engines, jewelry, and dental fixtures.

IN INVESTMENT CASTING, a ceramic slurry is applied around a disposable pattern, usually wax, and allowed to harden to form a disposable casting mold.

• The term disposable means that the pattern is destroyed during its removal from the mold and that the mold is destroyed to recover the casting.

• There are two distinct processes for making investment casting molds solid investment (solid mold) process and the ceramic shell process.

• The ceramic shell process has become the predominant technique for engineering applications, displacing the solid investment process .

• By 1985, fewer than 20% of non-airfoil investment castings and practically no airfoil castings (the largest single application of investment casting) were being made by the solid Investment process

• 2008 the solid investment process is primarily used to produce dental, jewelry castings and has only a small role in engineering applications, mostly for nonferrous alloys.

Pattern Materials• grouped into waxes and plastics.• Waxes are more commonly used• plastic patterns much less • foamed polystyrene patterns are frequently used in conjunction with relatively thin

ceramic shell moldsWaxes• preferred base material for most investment casting patterns, • Waxes are usually modified to improve their properties through the addition of

such materials as resins, plastics, fillers, plasticizers, antioxidants• The most widely used waxes for patterns are paraffins and microcrystalline waxes. • These two are often used in combination because their properties tend to be

complementary.• Paraffin waxes are available in closely controlled grades with melting points varying

by 2.8 C increments; melting points ranging from 52 to 68 C are the most common.• low cost of waxes, ready availability, convenient choice of grades, high lubricity,

and low melt viscosity, accounts for their wide use. • Paraffins, Ozocerite, Fisher-Tropsch waxes, Polyethylene waxes, Candelilla is an

imported vegetable wax, Carnauba is another imported vegetable wax, Beeswax is a natural insect wax,

Additives.

there are two important areas in which plain waxes are deficient:Strength and rigidity (where very fragile patterns need to be made)Dimensional control (surface cavitation resulting from solidification shrinkage when pattern injection)

• Improvements can be made in these areas with non waxy additives.• by the addition of high molecular- weight plastics such as polyethylene, ethyl cellulose, nylon,

ethylene vinyl acetate, and ethylene vinyl acrylate. • These materials are highly viscous at wax-working temperatures, which tends to limit the

amounts that can be used.• Polyethylene is widely used because it is economical and compatible with a wide spectrum of

waxes.• Ethyl cellulose has had some use, but it is much more limited in terms of compatibility and is

more expensive. • Polystyrene is rarely used because of its incompatibility with the commonly used waxes. • Ethylene vinyl acetate and ethylene vinyl acrylate are newer materials that are finding

increased application. • Nylon has had only small use.• Solidification shrinkage, which causes surface cavitation, is reduced somewhat by the plastics

mentioned previously. • The effect, however, is limited by the low amounts that can be used before viscosity becomes

excessive.• Greater effects can be obtained by adding resins and fillers. Resins are used in most present-day

Wax Selectionalong with the properties or considerations appropriate to each:• Injection: Softening point, freezing range, rheological properties, ability to duplicate

detail, surface, and setup time• Removal, handling, and assembly: Lubricity, strength, hardness, rigidity, impact

resistance, stability, and weldability• Dimensional control: Thermal expansion/shrinkage, solidification shrinkage,

cavitation tendency, distortion, and stability• Mold making: Strength, wettability, and resistance to binders and solvents• Mold dewaxing and burnout: Softening point, viscosity, thermal expansion, thermal

diffusivity, and ash content• Miscellaneous: Cost, availability, ease of recycling, toxicity, and environmental factorsPlastics• Next to wax, plastic is the most widely used pattern material. • A general-purpose grade of polystyrene is usually used.• The principal advantages of polystyrene and other plastics are their ability to be

molded at high production rates on automatic equipment and their resistance to handling damage even in extremely thin sections.

• polystyrene is very economical and very stable, so patterns can be stored indefinitely without deterioration.

• Most wax patterns deteriorate with age and eventually must be discarded.• A disadvantage that limits the use of polystyrene is its tendency to cause mold

cracking during pattern removal, • Other common plastics, such as polyethylene, nylon, ethyl cellulose, and

cellulose acetate, are similar in this regard. • tooling and injection equipment for polystyrene is more expensive than for wax• most important application for polystyrene is for small, delicate airfoils. • Patterns for such castings are incorporated into composite wax/plastic

assemblies for integral rotor and nozzle patterns. • These airfoils are extremely thin and delicate;• they would be too fragile if molded in wax because even a single chip or crack

will cause an expensive casting to be rejected.• Polystyrene solves this problem, and the use of wax for the rest of the

assembly makes it feasible to process such large patterns without excessive mold cracking.

• The other important use for polystyrene is for small patterns running in large quantities.

• However, this application has declined as ceramic shell molds have replaced solid investment molds.

Patternmaking• Patterns for investment casting are made by injecting the pattern material into

metal molds of the desired shape. • Small quantities of patterns can also be produced by machining Injection of Wax Patterns

Wax patterns are generally injected at relatively low temperatures and pressures in split dies using equipment specifically designed for this purpose. Patterns can be injected in the liquid, slushy, pastelike, or solid condition. Injection in the solid condition is often referred to as wax extrusion. Temperatures 43 to 77 C and pressures 275 kPa to 10.3 Mpa Liquid waxes are injected at higher temperatures and lower pressures; solid waxes, at lower temperatures and higher pressures, It can be as simple as a pneumatic unit with a closed, heated reservoir tank that is equipped with a thermostat, pressure regulator, heated valve, and nozzle and is connected to the shop air line for pressurization.complexity, or dimensional requirements are made on hydraulic machines. These machines provide higher pressures for improved injections as well as high clamping pressures to accommodate large dies and high injection pressures. They can also be operated with very low pressures, as is often necessary when injecting around thin ceramic cores

Ceramic Shell Mold Manufacture• ceramic shell investment casting and ceramic cores for investment casting. • Investment shell molds are made by applying a series of ceramic coatings to the pattern clusters.• Each coating consists of a fine ceramic layer with coarse ceramic particles embedded in its outer

surface. • A cluster is first dipped into a ceramic slurry bath. • The cluster is then withdrawn from the slurry and manipulated to drain off excess slurry and to

produce a uniform layer.• The wet layer is immediately stuccoes with relatively coarse ceramic particles either by

immersing it into a fluidized bed of the particles or by sprinkling the particles on it from above.• The fine ceramic layer forms the inner face of the mold and reproduces every detail of the

pattern, including its smooth surface. • It also contains the bonding agent, which provides strength to the structure. • The coarse stucco particles serve to arrest further runoff of the slurry, help to prevent it from

cracking or pulling away, provide keying (bonding) between individual coating layers, and build up thickness faster.

• Each coating is allowed to harden or set before the next one is applied. • This is accomplished by drying, chemical gelling, or a combination of these. • The operations of coating, stuccoing, and hardening are repeated a number of times until the

required mold thickness is achieved. • The final coat is usually left unstuccoed in order to avoid the occurrence of loose• particles on the mold surface. • This final, unstuccoed layer is sometimes referred to as a seal coat.

Mold Refractories• The most common refractories for ceramic shell molds are siliceous, for example, silica itself, zircon,

and various aluminum silicates composed of mullite and (usually) free silica.• These three types in various combinations are used for most applications.• Alumina has had some use for super alloy casting, and this application has increased with the

growth of directional solidification processes.• Alumina is generally considered too expensive and unnecessary for commercial hardware casting. • Silica, zircon, aluminum silicates, and alumina find use for both slurry refractories and stuccos. • Other refractories, such as graphite, zirconia (ZrO2), and yttria (Y2O3), have been suggested for use

with reactive alloys. Silica

generally used in the form of silica glass (fused silica), which is made by melting natural quartz sand and then solidifying it to form a glass. It is crushed and screened to produce stucco particles, and it is ground to a powder for use in slurries. Its extremely low coefficient of thermal expansion imparts thermal shock resistance to molds, and its ready solubility in molten caustic and caustic solutions provides a means of removing shell material chemically from areas of castings that are difficult to clean by other methods.

• Silica is also used as naturally occurring quartz. • This is the least expensive material used to any extent.• its utility is limited by its high coefficient of thermal expansion and by the high, abrupt expansion at

573 C accompanying its a-to-b-phase transition. • As a result, shells containing quartz must be fired slowly, a practice most foundries find

inconvenient.

• Binder• The commonly used binders are

siliceous and include colloidal silica, hydrolyzed ethyl silicate, Sodium silicate.

Hybrid binders have also been developed, Alumina or zirconia binders are used for some processes.

Slurry Formulation• The actual percentage composition of ceramic shell slurries

depends on the particular refractory powder, type and concentration of binder, and desired slurry viscosity.

• Composition generally falls in the following broad range by weight:Binder solids 5–10%Liquid (from binder or added) 15–30 %Refractory powder 60–80%