Manufacturing Process

Pattern

Definition Role in Casting Types Pattern Allowances

Job, pattern , mould

Definition

Pattern is the replica of the final object to be made. The mold cavity is made with the help of pattern.

Role in Casting

Steps in Making Sand Castings There are six basic steps in making

sand castings: Patternmaking Core making Molding Melting and pouring Cleaning and ejecting

Functions of Pattern

1.     A pattern prepares a mold cavity for the purpose of making a casting.

2.     A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.

3.     Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern.

4.     Patterns properly made and having finished and smooth surfaces reduce casting defects.

5.     A properly constructed pattern minimizes the overall cost of the castings.

Characteristics of patternWays in which pattern differs from an actual component:: It carries an additional allowance to compensate for metal shrinkage It carries an additional allowance over those portions which are to be machined or finished otherwise It carries an additional draft to enable its easy removal. It carries additional projection for cores.

Important consideration for pattern making 1.Surface finish of casting to be produced. 2. No of casting desired from same pattern 3. Facility and ease of removal. 4. Method of withdrawal from mould.

Requirement of pattern maker Thorough knowledge of working drawing High skill of workmanship Fully conversant with techniques and process of moulding and casting Knowledge of properties of metals, non metals and alloys

Pattern Material

Patterns may be constructed from the following materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins. To be suitable for use, the pattern material should be:

1.     Easily worked, shaped and joined2.     Light in weight3.     Strong, hard and durable4.     Resistant to wear and abrasion 5.     Resistant to corrosion, and to chemical reactions6.     Dimensionally stable and unaffected by variations in temperature and

humidity7.     Available at low cost The usual pattern materials are wood, metal, and plastics. The most

commonly used pattern material is wood, since it is readily available and of low weight. Also, it can be easily shaped and is relatively cheap. The main disadvantage of wood is its absorption of moisture, which can cause distortion and dimensional changes. Hence, proper seasoning and upkeep of wood is almost a pre-requisite for large-scale use of wood as a pattern material

Pattern Material

The selection of pattern material consists of the following factors:

Service requirement Type of production of casting and type of moulding process Possibility of design changes Number of casting to be produced

Requirement of good pattern material – easily worked, shaped, joined Light weight Strong, hard, durable Dimensionally stable Easily available at low cost Repairable and reusable Able to take good surface finish

Pattern material usedWoodMetal – cast iron, brass, aluminum, white metalPlasticRubber, plasters, waxes

Types of pattern

Types of patterns used in sand casting:(a) solid pattern(b) split pattern(c) match-plate pattern(d) cope and drag pattern

(e) Gated pattern(f) Skeleton pattern(g) Segmental pattern(h) Built up pattern(i) Boxed up pattern

Types of pattern Solid pattern – Pattern made without joints, partings or any loose

piece in its construction is called a single piece or solid pattern. These are cheaper .

Split pattern – made of two parts, and held in their proper position by means of dowel –pins fastened in one piece and fitting holes bored in another

Match-plate pattern – when split pattern are mounted with one half on one side of the plate and other half directly opposite on the other side of the plate, the pattern is called match plate pattern and a single or no of pattern are mounted on match plate.

Cope and drag pattern – to facilitate handling in large casting, the cope and drag patterns used are made in halves, split on convenient joint lines and separately mounted on individual plates or boards.

Gated pattern – In mass production , number of castings are produced in a single multi cavity mould by joining a group of patterns by gate and runners and gates ensure proper flow of material into the mould.

Loose piece pattern – Produced as assembly of loose component .these are needed when the parts cannot be removed as one piece.

Types of pattern (g) Skeleton pattern – When the size of casting is large

but easy to shape and only a few members are to be made , it is uneconomical to make a large solid pattern of that size. In such case a pattern consisting of wooden strip and frame is made .

(h) Sweep pattern – sweeps can be advantageously used for preparing a mould of large symmetrical casting , particularly of circular cross section .The full equipment consists of a base, suitably placed in the sand mass, a vertical spindle and wooden template, called sweep. The outer end of sweep carries the counter corresponding to the shape of desired casting and is rotated about the spindle to form the cavity

(I) Segmental pattern – it is used to prepare moulds of large circular casting and is produced in segments.

Single Piece Pattern

The one piece or single pattern is the most inexpensive of all types of patterns. This type of pattern is used only in cases where the job is very simple and does not create any withdrawal problems. It is also used for application in very small-scale production or in prototype development. This type of pattern is expected to be entirely in the drag and one of the surface is expected to be flat which is used as the parting plane. A gating system is made in the mold by cutting sand with the help of sand tools. If no such flat surface exists, the molding becomes complicated. A typical one-piece pattern is shown in Figure.

Single Piece Pattern

Split or Two Piece Pattern

Split or two piece pattern is most widely used type of pattern for intricate castings. It is split along the parting surface, the position of which is determined by the shape of the casting. One half of the pattern is molded in drag and the other half in cope. The two halves of the pattern must be aligned properly by making use of the dowel pins, which are fitted, to the cope half of the pattern. These dowel pins match with the precisely made holes in the drag half of the pattern. A typical split pattern of a cast iron wheel Figure 7 (a) is shown in Figure 7 (b).

Split or Two Piece Pattern

Match Plate Pattern Here the cope & drag patterns along with the gating and

the risering are mounted on a single matching metal or wooden plate or wooden plate on either side.

On one side of the match plate, the cope flask is prepared and on the other, the drag flask. After molding, when the match plate is removed, a complete mold with gating is obtained by joining the cope & drag together.

Match Plate Pattern

Cope and drag pattern

to facilitate handling in large casting, the cope and drag patterns used are made in halves, split on convenient joint lines and separately mounted on individual plates or boards.

Cope and drag pattern

When the size of casting is large but easy to shape and only a few members are to be made , it is uneconomical to make a large solid pattern of that size. In such case a pattern consisting of wooden strip and frame is made

SKELETON PATTERN

SKELETON PATTERN

Pattern Allowances The dimensions of the pattern are different from the final

dimensions of the casting required. This dimension difference is known as Allowance.

Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting. Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with the particular specification can be made. The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns and core boxes are as follows:

1.     Shrinkage or contraction allowance2.     Draft or taper allowance3.     Machining or finish allowance4.     Distortion or camber allowance5.     Rapping allowance

Shrinkage Allowance Shrinkage or Contraction Allowance ( click on Table 1 to view various

rate of contraction of various materials)All most all cast metals shrink or contract volumetrically on cooling. The metal

shrinkage is of two types:      i.        Liquid Shrinkage: it refers to the reduction in volume when the metal

changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold.

     ii.        Solid Shrinkage: it refers to the reduction in volume caused when metal loses temperature in solid state. To account for shrinkage allowance is provided on the patterns.

The rate of contraction with temperature is dependent on the material. For example steel contracts to a higher degree compared to aluminum. To compensate the solid shrinkage, a shrink rule must be used in laying out the measurements for the pattern. A shrink rule for cast iron is 1/8 inch longer per foot than a standard rule. If a gear blank of 4 inch in diameter was planned to produce out of cast iron, the shrink rule in measuring it 4 inch would actually measure 4 -1/24 inch, thus compensating for the shrinkage. The various rate of contraction of various materials are given in Table 1.

Table 1 : Rate of Contraction of Various Metals

MaterialDimension Shrinkage allowance (inch/ft)

Grey Cast Iron Up to 2 feet2 feet to 4 feetover 4 feet

0.1250.1050.083

Cast Steel Up to 2 feet2 feet to 6 feetover 6 feet

0.2510.1910.155

Aluminum Up to 4 feet4 feet to 6 feetover 6 feet

0.1550.1430.125

Magnesium Up to 4 feetOver 4 feet

0.1730.155

Exercise 1

The casting shown is to be made in cast iron using a wooden pattern. Assuming only shrinkage allowance, calculate the dimension of the pattern. All Dimensions are in Inches

Solution 1

The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet (as per  Table 1)

For dimension 18 inch, allowance = 18 X 0.125 / 12 = 0.1875 inch »  0.2 inch

For dimension 14 inch, allowance = 14 X 0.125 / 12 = 0.146 inch »  0.15 inch

For dimension 8 inch, allowance   =  8 X 0.125 / 12 = 0.0833 inch »  0. 09 inch

For dimension 6 inch, allowance   =   6 X 0.125 / 12 = 0.0625 inch »  0. 07 inch

The pattern drawing with required dimension is shown below:

Draft or Taper Allowance

By draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder. Figure 3 (a) shows a pattern having no draft allowance being removed from the pattern. In this case, till the pattern is completely lifted out, its sides will remain in contact with the walls of the mold, thus tending to break it. Figure 3 (b) is an illustration of a pattern having proper draft allowance. Here, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity.

Before After

Draft allowance varies with the complexity of the sand job. But in general inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material. Table 2 provides a general guide lines for the draft allowance.

Table 2 : Draft Allowances of Various Metals

Machining or Finish Allowance

The finish and accuracy achieved in sand casting are generally poor and therefore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Machining or finish allowances are therefore added in the pattern dimension. The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the casting orientation; the metal; and the degree of accuracy and finish required. The machining allowances recommended for different metal is given in Table 3.

Exercise 2The casting shown is to be made in cast iron using a wooden pattern. Assuming only machining allowance, calculate the dimension of the pattern. All Dimensions are in Inches

Solution 2

The machining allowance for cast iron for size, up to 12 inch is o.12 inch and from 12 inch to 20 inch is 0.20 inch ( (Table 3)

For dimension 18 inch, allowance = 0.20 inch For dimension 14 inch, allowance = 0.20 inch For dimension 8 inch, allowance   = 0.12 inch For dimension 6 inch, allowance   = 0.12 inch The pattern drawing with required dimension is shown

in Figure below

Distortion or Camber Allowance

Sometimes castings get distorted, during solidification, due to their typical shape. For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end causing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical ( (Figure 4).

The distortion in casting may occur due to internal stresses. These internal stresses are caused on account of unequal cooling of different section of the casting and hindered contraction. Measure taken to prevent the distortion in casting include:

i.           Modification of casting designii.          Providing sufficient machining allowance to cover the distortion affectiii.         Providing suitable allowance on the pattern, called camber or distortion allowance (inverse reflection)

Rapping Allowance

Before the withdrawal from the sand mold, the pattern is rapped all around the vertical faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the final casting made, it is desirable that the original pattern dimension should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. It is a negative allowance and is to be applied only to those dimensions that are parallel to the parting plane.

• Strength - to maintain in shape and resist erosion • Permeability - to allow hot air and gases to pass through voids in sand • Thermal stability - to resist cracking on contact with molten metal • Collapsibility - ability to give way and allow casting to shrink without

cracking the casting. Also automatically collapse the sand mould after solidification of casting.

• Reusability - can sand from broken mold be reused to make other moluds?

Refractoriness- ability of molding sand to withstand high temperature Flowability – ability to flow over and around the pattern and all portions of

moulding mask during ramming Cohesiveness – ability of sand particles to stick together and retain a given

shape of mould Adhessiveness – property by which sand particles stick to other body like

sides of moulding base . Chemical resistivity – property due to which sand does not react

chemically with molten metal.

MOULDING SAND AND ITS DESIRABLE PROPERTIES

• Green-sand moulds - mixture of sand, clay, and water." Green" means mold contains moisture at time of pouring

• Dry-sand mould - organic binders rather than clay and mold is baked to improve strength

Facing sand –posses high strength and refractory Loam sand – contains clay up to 50%, and sand mixed with water.

It has adhesive property so to hold on vertical surface of mould. Backing sand – Parting sand – sprinkled on pattern to avoid sand of one flask stick to sand of other flask Core sand – used to make core and has high silica content System sand – used to fill whole flask in machine moulding and has high strength, permeability and refractoriness.

TYPES OF MOULDING SAND

MOULDING TOOLS(1) Vent wire

(2) Pattern lifter.

(3) Joint trowel and (4) heart trowel

(5) Gate cutter and pattern lifter.

6) Slick and oval spoon

(7) (8) Sand lifters and slicks.

(9) Yankee heel lifter and flat slick.

(10) Flange and bead slick.

(11) Corner slick.

(12) Edge slick.

(13) Bound corner slick.

(14) Pipe slick.

(15) Button slick. (16.) Oval Slick.

(17) Hand rammer

(18) Spirit level

Sand casting

Sand casting is a method involving

pouring a molten metal into a sand

mold.

Sand casting A sand casting or a sand molded casting is a

cast part produced by forming a mold from a sand mixture and pouring molten liquid metal into the cavity in the mold. The mold is then cooled until the metal has solidified. In the last stage the casting is separated from the mold. There are six steps in this process:

Place a pattern in sand to create a mold. Incorporate a gating system. Remove the pattern. Fill the mold cavity with molten metal. Allow the metal to cool. Break away the sand mold and remove the casting.

Sand casting

Steps of sand casting

Sand moulding

Avdantages and disadvantages

Advantages of sand casting• Low cost of mold materials and equipment.• Large casting dimensions may be obtained.• Wide variety of metals and alloys (ferrous and non-ferrous) may be cast (including high melting point metals).

Disadvantages of sand casting

• Rough surface.• Poor dimensional accuracy.• High machining tolerances.• Coarse Grain structure.• Limited wall thickness: not higher than 0.1”-0.2” (2.5-5 mm). to top

Core and Core Prints

Castings are often required to have holes, recesses, etc. of various sizes and shapes. These impressions can be obtained by using cores. So where coring is required, provision should be made to support the core inside the mold cavity. Core prints are used to serve this purpose. The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the mold. The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation. Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity. A typical job, its pattern and the mold cavity with core and core print is shown.

Core and Core Prints

Core and Core Prints

Vertical cores Horizontal cores

Gating system

A mould cavity must be filled with clean metal in a controlled manner to

ensure smooth, uniform and complete filling, for the casting to be free of discontinuities, solid inclusions and voids. This can be achieved by a well-designed gating system

Gating systems can be classified depending on the orientation of the parting plane (which contains the sprue, runner and ingates), as horizontal or vertical. Depending on the position of the ingate(s), gating systems can be classified as top, parting and bottom.

Horizontal gating systems are suitable for flat castings filled under gravity. They are widely used in sand casting of ferrous metals, as well as gravity diecasting of non-ferrous metals.

Vertical gating systems are suitable for tall castings. They are employed in high-pressure sand mould, shell mould and diecasting processes, where the parting plane is vertical.

Top gating systems, in which hot molten metal enters at the top of the casting, promote directional solidification from bottom to top of the casting. These are however, suitable only for flat castings to limit the damage to metal as well as the mould by free fall of the molten metal during initial filling.

Gating system

Gating system Bottom gating systems have the opposite

characteristics: the metal enters at the bottom of the casting and gradually fills up the mould with minimal disturbances. It is recommended for tall castings, where free fall of molten metal (from top or parting gates) has to be avoided.

Middle or side or parting gating systems combine the characteristics of top and bottom gating systems. If the gating channels are at the parting plane, they are also easier to produce and modify if necessary, during trial runs.

Gating system

Casting defects• There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product• The defects can be classified as follows: – Defects common to -all casting processes – Defects related to sand casting process

Casting defects

Cold ShotMetal splatters during pouring and solid globule

form and become entrapped in casting

Shrinkage CavityDepression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze

Sand BlowBalloon-shaped gas cavity caused by release of mold gases during pouring

Pin HolesFormation of many small gas cavities at orslightly below surface of casting

Casting defects

PenetrationWhen fluidity of liquid metal is high, it may penetrate into sand mold or sand core, causing casting surface to consist of a mixture of sand grains and metal

Mold ShiftA step in cast product at parting line caused by sidewise relative displacement of cope and drag

MisrunA casting that has solidified before completely filling mould cavity

Cold ShutTwo portions of metal flow together but there is a lack of fusion due to premature freezing

Permanent mould casting Permanent mold casting is a metal casting process

that shares similarities to both sand casting and die casting. As in sand casting, molten metal is poured into a mold which is clamped shut until the material cools and solidifies into the desired part shape. However, sand casting uses an expendable mold which is destroyed after each cycle. Permanent mold casting, like die casting, uses a metal mold (die) that is typically made from steel or cast iron and can be reused for several thousand cycles. Because the molten metal is poured into the die and not forcibly injected, permanent mold casting is often referred to as gravity die casting.

Permanent mould casting steps Mold preparation - First, the mold is pre-heated to around 300-

500°F (150-260°C) to allow better metal flow and reduce defects. Then, a ceramic coating is applied to the mold cavity surfaces to facilitate part removal and increase the mold lifetime.

Mold assembly - The mold consists of at least two parts - the two mold halves and any cores used to form complex features. Such cores are typically made from iron or steel, but expendable sand cores are sometimes used. In this step, the cores are inserted and the mold halves are clamped together.

Pouring - The molten metal is poured at a slow rate from a ladle into the mold through a sprue at the top of the mold. The metal flows through a runner system and enters the mold cavity.

Cooling - The molten metal is allowed to cool and solidify in the mold.

Mold opening - After the metal has solidified, the two mold halves are opened and the casting is removed.

Trimming - During cooling, the metal in the runner system and sprue solidify attached to the casting. This excess material is now cut away.

Permanent mould casting

Permanent mould casting

Using these basic steps, other variations on permanent mold casting have been developed to accommodate specific applications. Examples of these variations include the following:

• Slush Casting - As in permanent mold casting, the molten metal is poured into the mold and begins to solidify at the cavity surface. When the amount of solidified material is equal to the desired wall thickness, the remaining slush (material that has yet to completely solidify) is poured out of the mold. As a result, slush casting is used to produce hollow parts without the use of cores.

• Low Pressure Permanent Mold Casting - Instead of being poured, the molten metal is forced into the mold by low pressure air (< 1 bar). The application of pressure allows the mold to remain filled and reduces shrinkage during cooling. Also, finer details and thinner walls can be molded.

• Vacuum Permanent Mold Casting - Similar to low pressure casting, but vacuum pressure is used to fill the mold. As a result, finer details and thin walls can be molded and the mechanical properties of the castings are improved.

Permanent mould casting Advantages Can form complex shapes

Good mechanical propertiesMany material optionsLow porosityLow labor costScrap can be recycled

Disadvantages: High tooling cost

Long lead time

PossibleApplications:Gears, wheels, housings, engine components

Die Casting A permanent mold casting process in which

molten metal is injected into mold cavity under high pressure Pressure is maintained during solidification,

then mold is opened and part is removed Molds in this casting operation are called

dies; hence the name die casting Use of high pressure to force metal into die

cavity is what distinguishes this from other permanent mold processes

Die Casting

Die Casting Process -Hot chamber

Metal is melted in a container, and a piston injects liquid metal under high pressure into the die

High production rates - 500 parts per hour not uncommon

Applications limited to low melting-point metals that do not chemically attack plunger and other mechanical components

Casting metals: zinc, tin, lead and magnesium

Die Casting Process -Hot chamber

Cycle in hot-chamber

casting:(1) with die

closed and (2) plunger

withdrawn, molten metal flows into the chamber

Die Casting Process –Cold chamber• Molten metal is poured into unheated chamberfrom external melting container, and a pistoninjects metal under high pressure into die cavity

• High production but not usually as fast ashot-chamber machines because of pouring step

• Casting metals: aluminum, brass, andmagnesium alloys

• Advantages of hot-chamber process favor itsuse on low melting-point alloys (zinc, tin, lead)

Die Casting Process –Cold chamber

Cycle in cold-chamber casting:(1) with die closed and ram withdrawn, molten metalis poured into the chamber

Die Casting Process –Advantages $ Limitations

• Advantages:– Economical for large production quantities– Good dimensional accuracy and surface finish– Thin sections are possible– Rapid cooling provides small grain size and good strength to casting

• Disadvantages:– Generally limited to metals with low melting points– Part geometry must allow removal from die cavity

Die Casting Machines

Hot-Chamber Die Casting Machine

Cold-Chamber Die Casting Machine

Die Casting

Centrifugal castingCentrifugal casting, sometimes called

rotocasting, is a metal casting process that uses centrifugal force to form cylindrical parts. This differs from most metal casting processes, which use gravity or pressure to fill the mold. In centrifugal casting, a permanent mold made from steel, cast iron, or graphite is typically used. However, the use of expendable sand molds is also possible. The casting process is usually performed on a horizontal centrifugal casting machine (vertical machines are also available) and includes the following steps:

Mold preparation - The walls of a cylindrical mold are first coated with a refractory ceramic coating, which involves a few steps (application, rotation, drying, and baking). Once prepared and secured, the mold is rotated about its axis at high speeds (300-3000 RPM), typically around 1000 RPM.

Pouring - Molten metal is poured directly into the rotating mold, without the use of runners or a gating system. The centrifugal force drives the material towards the mold walls as the mold fills.

Cooling - With all of the molten metal in the mold, the mold remains spinning as the metal cools. Cooling begins quickly at the mold walls and proceeds inwards.

Casting removal - After the casting has cooled and solidified, the rotation is stopped and the casting can be removed.

Finishing - While the centrifugal force drives the dense metal to the mold walls, any less dense impurities or bubbles flow to the inner surface of the casting. As a result, secondary processes such as machining, grinding, or sand-blasting, are required to clean and smooth the inner diameter of the part.

Centrifugal casting steps

Centrifugal casting

Centrifugal casting is used to produce axi-symmetric parts, such

as cylinders or disks, which are typically hollow. Due to the high centrifugal forces, these parts have a very fine grain on the outer surface and possess mechanical properties approximately 30% greater than parts formed with static casting methods. Centrifugal casting is performed in wide variety of industries, including aerospace, industrial, marine, and power transmission. Typical parts include bearings, bushings, coils, cylinder liners, nozzles, pipes/tubes, pressure vessels, pulleys, rings, and wheels.

Centrifugal casting use, advantage, limitation

Advantages Can form very large parts , Good mechanical properties

Good surface finish and accuracy , Low equipment costLow labor cost , Little scrap generated

Disadvantages: Limited to cylindrical parts , Secondary machining is often required for inner diameter , Long lead time possible Applications:Pipes, wheels, pulleys, nozzles

Investment casting Investment casting is one of the oldest manufacturing

processes, dating back thousands of years, in which molten metal is poured into an expendable ceramic mold. The mold is formed by using a wax pattern - a disposable piece in the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens into the mold. Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mold after it has been formed. Lox-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mold is destroyed during the process, parts with complex geometries and intricate details can be created.

Investment casting

Investment casting can make use of most metals, most commonly using aluminum alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that can not be molded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.

Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding. The process steps include the following:

Investment casting steps Pattern creation - The wax patterns are typically injection molded into a metal die

and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity.

Mold creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting.

Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.

Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used.

Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. The ceramic mold is typically broken using water jets, but several other methods exist. Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen).

Finishing - Often times, finishing operations such as grinding or sandblasting are used to smooth the part at the gates. Heat treatment is also sometimes used to harden the final part.

Investment casting

Investment casting use, advantage, limitation

Advantages: Can form complex shapes and fine details

Many material optionsHigh strength partsVery good surface finish and accuracyLittle need for secondarymachining

Disadvantages: Time-consuming process

High labor costHigh tooling costLong lead time

possibleApplications: Turbine blades, armament parts, pipe fittings, lock

parts, handtools, jewelry