Special casting Processes

Introduction to Special Casting Process

Now a day’s special Casting process has been developed to effect a saving time and expense to produce a better quality casts. In comparing to Sand Casting the main difference is, in this process do not in all cases require drying or Baking of moulds or cores or rapid hardening action takes place due to chemical reactions in them.

Classification of Special Casting Process:

Need for special casting process

· Sand mould casting process gives satisfactory results at low cost.

· All metals may be cast in sand moulds and there is no limitations as regards the size of the casting which can be made.

· Sand casting enjoys wide applications and a very large quantity of castings even today is produced through sand casting only.

· However, sand moulds are single purpose moulds as they are completely destroyed
after the casting has been removed from the moulding box.

· It becomes therefore obvious that the use of a permanent mould do a considerable saving in labour cost of mould making.

· There are certain other limitations also possessed by conventional sand casting technique which necessitated the developments of Special casting processes.

Advantages of special casting process over sand casting

· Greater dimensional accuracy.

· Higher metallurgical quality.

· Lower production cost (in certain cases).

· Ability to cast extremely thin sections.

· High production rates.

· Better surface finish on the castings; therefore low labour and finishing costs.

· Minimum need for further machining of castings.

· Castings may possess a denser and finer grain structure.

· Castings are slightly stronger and more ductile than solid mould castings.

Centrifugal Casting:

Centrifugal casting is done by pouring molten metal into a rotating mould. The centrifugal force acting on the mould helps in feeding and positioning the metal in the mould. Mould rotation is continued till after the metal is solidified.

Centrifugal casting results in denser and cleaner metal as heavier metal is thrown to parts of the mould away from the centre of rotation and the lighter impurities like slag, oxides and inclusion are squeezed out to the centre.

The castings produced have a close grain structure, good detail, high density and superior mechanical properties. Elaborate gating and risering systems are not required as very simple systems will do the job. There is also a considerable saving of material.

Types of centrifugal casting:

Centrifugal casting can be divided into three categories namely true centrifugal casting, semi centrifugal casting and centrifuging.

True centrifugal casting:

The true centrifugal method of casting is used to produce hollow castings with a round hole. The characteristic feature of this process is that the hole is produced by the centrifugal force alone and no cores are used.

The mould is rotated about the axis of the hole with the axis held horizontal, inclined or vertical. The outside surface of the job may be round, square, hexagonal etc. and should be symmetrical with the whole axis. The central hole should be round to be formed without cores.

Long castings like cast iron soil pipes are cast with the moulds rotated about a horizontal axis. Castings with relatively short lengths are poured with moulds rotated about an inclined or vertical axis. Rotation about the vertical or inclined axis is convenient but the central hole produced will be slightly parabolic with smaller diameter at the bottom because the metal has a tendency to settle down due to gravity. The speed of rotation for true centrifugal casting should be high enough to hold the metal on to the mould wall till it solidifies. A low speed of rotation would result in raining or slipping of the metal inside the mould. Too large a speed of rotation on the other hand may result in internal stresses and possible hot tears. A speed which would provide a centrifugal force of 60 to 75 times the force of gravity on horizontal moulds and 100 times force of gravity for vertical moulds is found to be suitable. The moulds used for the process may be metal moulds or refractory or sand lined moulds. Common products produced by true centrifugal casting include pipes, oil engine cylinders, piston ring stock, gear blank stock, bearing bushes and the like.

Semi-centrifugal casting:

In semi-centrifugal casting process no attempt is made to produce a hole without a core. The centrifugal force resulting from rotation of the mould is used to properly feed the casting to produce a close grained clean casting.

The process is suitable for large axis-symmetrical castings like gear blanks, fly wheels and track wheels. Any hole round or otherwise is made with the use of a core. The mould is clamped to a turn table with casting axis along the axis of rotation.

The metal is poured along or near the axis to feed the points farthest from the axis of rotation under pressure. If made solid the central portion tends to be porous and with inclusion which are removed in subsequent machining.

Centrifuging:

Centrifuging or centrifuge casting is employed to force metal under pressure into moulds of small castings or castings not symmetrical about any axis of rotation. The moulds are made around a central axis of rotation, to balance each other.

The metal is poured along this axis of rotation through a central sprue and made to flow into mould cavities through radial ingates cut on the mould interface. Centrifuging helps in proper feeding of castings resulting in clean, close grained castings.

                                 Permanent Mould Casting:

Permanent mould casting, also sometimes called gravity die casting employs moulds which can be used more than once and hence are permanent. These moulds are usually made in more than one piece to facilitate removal of the finished casting. The mould is assembled; and held together by clamps, screws or toggles during pouring. In the assembled position the parts of the mould make a complete mould with sprues, runners, gates, vents and blind risers. Vent channels may be provided for escape of entrapped air if it is found that the air within the mould cavity cannot escape properly during pouring through the space between parting surfaces.

The moulds are preheated at the start of the run to avoid thermal shock to the moulds. A refractory parting coat is given to the mould once in each cycle by spraying or brushing. French chalk or calcium carbonate suspended in sodium silicate binder is the commonly used refractory coat for aluminium and magnesium castings. It protects the mould and promotes casting ejection.

During operation the mould temperature should be controlled to remain within a close range depending on the metal poured to produce good castings.Permanent moulds are usually made of close grained alloy cast iron which is resistant to heat and repeated changes in temperature. Bronze moulds may be used for casting lead, tin and zinc and wrought alloy steel moulds for some bronzes.

Cores are usually made of alloy steel. Sometimes sand or plaster cores may be used in which case the process is called semi permanent mould casting. Sand and plaster cores are cheaper but the structure, surface finish and dimensional accuracy of cored openings are only as good as that of sand or plaster casting. Metals commonly cast by the permanent mould casting process include lead, tin, zinc, aluminium, magnesium alloys, certain bronzes and cast iron.

Some typical products include refrigerator compressor cylinder block heads, connecting rods, gear blanks, automobile pistons, and kitchen ware and type writer parts.

Weights of castings produced may vary from a few grams to 150 kg but generally range below 25 kg. The life of the moulds varies from 3000 to 10000 castings for cast iron to as many as 100000 castings with softer materials.

Advantages of permanent mould casting process:

· The advantages of permanent mould casting process over sand casting include production of a fine grained structure, smoother surfaces, closer dimensional tolerances, lower floor space requirement and an economical production for large quantities. The fine grain structure produced results from the chilling action of the metal moulds and imparts better mechanical properties to the casting.

· The surface finish obtained in permanent mould castings ranges from 2.5 to 3 microns RMS and dimensional accuracy produced is of the order of ± 0.25 to 1.25 mm / mm across a parting line. Small cored holes up to 6 mm diameter can be produced with metal cores.

Production rates of the order of 15 to 30 castings per hour per mould can be attained. The limitations of the process are higher mould cast, restriction of size and shape of the castings and the lack of flexibility in making any changes in the gating and risering systems.

Die Casting:

The die casting process uses steel dies into which metal is forced under pressure through a runner and gate to fill the dies. The pressure (70 to 5000 kg/cm²) is maintained while the casting solidifies after which the dies are separated, cores are withdrawn and the casting is ejected.

Metals and alloys that are die cast include zinc, aluminium, and magnesium, copper, lead and tin.

Typical applications of die casting process include automobile components, household appliances, railway and aircraft fittings, bath room hardware, business machines, locks, pullers and many other similar parts.

Die Casting Dies:

The dies used for die casting resemble a permanent mould. They are generally made in two parts arranged to open and close with a vertical parting. When mounted on a die casting machine one of the die halves remains stationary during operation and is called a cover die.

The other half moves for opening and closing and is called the ejector die. Die casting dies are made of special die steels which are resistant to heat checking, hammering and mechanical wear and are also dimensionally stable. Die cavities are machined to very close accuracies. Vents and overflow wells are provided in the dies for escape of air. The dies may be water cooled to speed up cooling of the casting.

Die casting machines:

A die casting cycle consists of the following steps:

(i) Closing the die halves

(ii) Clamping the die halves securely together

(iii) Forcing the liquid metal into the dies

(iv) Opening the die halves and

(v) Ejecting the casting.

Die casting machines are designed to perform all these functions. To be effective these machines should be strong and rigidly built to take up die weights and provide holding pressures against the pressure of the molten metal. The machine frame should hold the die halves rigidly in correct alignment. The die holding forces should be in excess of the maximum force developed by the molten metal to ensure leak proof joint in the dies. The die closing and locking arrangements generally used in the die casting machines include hydraulic, hydraulic mechanical or mechanical devices depending on the capacity of the machine.

Modern die casting machines are of two basic types namely

(i) hot chamber or submerged plunger die casting machines ( 7 to 35 MPa ) and               

                      1. Goose Neck type or Air injection type

                      2. Submerged plunger type

(ii) Cold chamber die casting machines (14 to 140 MPa).

Process parameters:

Common metals:

• Alloys of aluminum
• Zinc
• Magnesium
• Lead
• Copper
• Tin

Size limits:

• less than 30 grams upto 7 kg

Thickness limits:

• As thin as 0.75mm to 13 mm

Typical tolerances:

• varies with metal being cast
• typically 0.1 mm for the first 2.5 cm and 0.02 mm for each additional centimeter

Draft allowances:

• 2 degree

Surface finish:

• 1 –25 micrometer Hot Chamber Die Casting Machine:
  
  It is also called a gooseneck machine because of the shape of the metal passage way. In this machine the melting pot, usually made of cast iron, is a part of the machine. The gooseneck containing a cylinder and metal passage way is kept immersed in the metal pot. The plunger in the gooseneck cylinder is actuated either hydraulically or pneumatically. In operation the plunger is withdrawn letting the liquid metal into the gooseneck cylinder through the port provided.
  When the die halves are closed and ready for casting the plunger forces the liquid metal entrapped in the cylinder into the die through the gooseneck passage and a nozzle. After a predetermined time interval the plunger is retracted allowing the liquid metal in the gooseneck channel and nozzle to fall back into cylinder.
  The die halves are opened and the solidified casting is ejected from the die. Hot chamber machines are designed to operate almost automatically and fast. A press button operation will make the machine go through a complete cycle of activities including closing the die halves, forcing the metal into the die, holding the pressure for a predetermined time, withdrawing the plunger, opening the die, ejecting the casting and stop ready for the next cycle. The operator then removes the casting, inspects the dies, gives spray lubrication to the dies and starts the next cycle. Metal injection speeds and pressures can be controlled to suit different metals and castings.
  
  Since the melting pot plunger and cylinder of a hot chamber die casting machine are made of cast iron and cast iron reacts with metals like aluminium at elevated temperatures, only low melting-point metals can be cast by this method. There is also a limit on the maximum pressure which can be applied. Hot chamber machines are mostly operated below 14 kPa. Alloys of lead, tin and zinc are the most common metals cast by this process.
  
  
• Cold chamber die casting machine:
• 
  The metal in this case is melted in a separate furnace and the required quantity of metal is ladled to the machine. A plunger operated hydraulically forces the metal into the die. Injection pressures of 28 kPa to 250 kPa are possible in cold chamber machines. The machine is semiautomatic in that after the metal is ladled into the cold chamber the rest of the operation is automatic. Hot chamber machines are made in capacities varying from 0.25 to 7.5 MN and cold chamber ones from 1 to 10 MN.
  Advantages of Die Casting Process:
  • Advantages of die casting include excellent die life, high production rates, close dimensional tolerances, good details, and excellent surface finish of the castings.
  • Die casting dies retain their accuracies for long production runs
  • Production rates vary- from 5 to 6 castings per minute with hot chamber machines to 2 to 3 castings per minute when cold chamber machine are used.
  • Dimensional tolerances can be held to ± 0.075 mm.
  • Very thin sections can be cast and good surface finish obtained with excellent details.
  Disadvantages of Die Casting Process:
  (1) High cost of dies and machines.
  (2) Restriction on the size of the casting to about 100 kg for zinc alloys and 30 kg for aluminium alloys:
  (3) Only certain non-ferrous metals can be economically die cast.
  (4) Die casting products also are mechanically weaker because of the air entrapped during casting.
  (5) The entrapped air makes die casting unsuitable for heat treatment. When these castings are heated for heat treatment the entrapped air expands causing blisters to be formed on the surface of the castings.

Investment Casting (or) Lost Wax Method

A wax duplicate of the desired casting is created to be invested into a "Ceramic Slurry".

The slurry covered investment can be dipped into alternating coatings of sand and slurry until a suitable thickness of shell is achieved that can hold the molten metal after the investment is burnt out.

The "Burn-Out" process requires that the investment and coating are inverted in an oven that is fired to 1800F so that the investment can flow out and be recovered. The refractory coating is also cured in this procedure.

This process is beneficial for casting metals with high melting temperatures that cannot be moulded in plaster or metal.

Parts that are typically made by investment casting include those with complex geometry such as turbine blades or fire arm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.

Principle

Method also called as precision investment casting. The method involves the use of expendable Pattern with a shell of refractory material surrounded to form a casting mould. Since the pattern made up of wax is melted out and gets destroyed. That is why the name-"Lost wax method".

Process parameters of Investment casting

Process principle

Refractory slurry is formed around a wax or plastic pattern and allowed to harden. The pattern is then melted cut under mould is baked. The molten metal into the mould and solidifies.

Size limits

As small as (1/10) inch but usually less than 10 lb.

Thickness limits

As thickness as 0.025 inch but less than 3 inch.

Typical tolerance

Approximately 0.005 inch.

For the first inch and 0.002 inch for each additional inch.

Draft allowance

Not required.

Surface finish 50 to 125 micron.

Procedure

1. Produce a master pattern

The pattern is a modified replica of the desired product made from metal, wood, plastic, or some other easily worked material.

2. From the master pattern, produce a master die

This can be made from low-melting-point metal, steel, or possibly even wood. If low-melting-point metal is used.

3. Produce wax patterns

Patterns are made by pouring molten wax into the master die, or injecting it under pressure, and allowing it to harden. Plastic and frozen mercury have also been used as pattern material.

4. Assemble the wax patterns onto common wax sprues

The individual wax patterns are attached to a central sprues and runner system by means of heated tools and melted wax. In some cases, several pattern pieces may first be united to form a complex.

5. Coat the cluster with a thin layer of investment material

This step is usually accomplished by dipping the cluster into a watery slurry of finely ground refractory material.

6. Produce the final investment around the coated cluster

After the initial layer is formed, the cluster can be redipped, but this time the wet ceramic is coated with a layer of sand and allowed to dry. This process can be repeated until the investment coating is the desired thickness (typically 5 to 15 mm).

7. Allow the investment to fully harden

8. Melt or dissolve the wax pattern to remove it from the mould

This is generally accomplished by placing the moulds upside down in an oven, where the wax melts and runs out, and any residue subsequently vaporizes.

9. Preheat the mould in preparation for pouring

Heating to 550 to 1100°C (1000 to 2000°F) ensures complete removal of the mould wax, curves the mould to give added strength, and allows the molten metal to retain its heat and flow more readily into all of the thin sections.

10. Pour the molten metal

Various methods, beyond simple pouring, can be used to ensure complete filling of the mould, especially when complex, thin sections are involved.

11. Remove the casting from the mould

This is accomplished by breaking the mould away from the casting. Techniques include mechanical vibration and high-pressure water.

Advantages

i) Smoother surfaces (1500 to 2250 micro-mm root mean-square).Close tolerance (of +0.003 mm/mm)

ii) High dimensional accuracy

iii) Intricate shape can be cast

iv) Castings do not contain any disfiguring parting line

v) Machining operations can by eliminated.

Disadvantages:

i) Process is relatively slow.

ii) Use of cores makes the process more difficult.

iii) The process is relatively expensive than other process.

iv) Pattern is expandable.

v) Size limitation of the component part to be cast. Majority of the castings produce weight less than 0.5 kg.

Applications

The products made by this process are vanes and blades for gas turbines, shuttle eyes for weaving, pawls and claws of movie cameras, wave guides for radars, bolts and triggers for fire arms, stainless steel valve bodies and impellers for turbo chargers.

While investment casting is actually a very old process and has been performed by dentists and jewellers for a number of years, it was not until the end of World War II that it attained any degree of industrial importance.

Developments and demands in the aerospace industry, such as rocket components and jet engine turbine blades, required high-precision complex shapes from high-melting-point metals that are not readily machinable.

Investment casting offers almost unlimited freedom in both the complexity of shapes and types of materials that can be cast.

Ceramic Mould Casting (or) Cope and Drag Investment Casting (or) Plaster Moulding

The ceramic mould casting uses permanent patterns made of plaster, plastic, wood, metal or rubber and utilizes fine grain zircon and calcined, high-alumina mullite slurries for moulding. Ceramic mould casting method uses a ceramic slurry prepared by mixing fine grained refractory powders of Zircon (ZrSiO₄), Alumina (Al₂O₃), Fused Silica (SiO₂) and a liquid chemical binder (Alcohol based Silicon Ester) for making the mould. These slurries are comparable in composition to those used in investment castings. Like investment moulds, ceramic moulds are expendable. However, unlike the monolithic moulds obtained in investment castings, ceramic moulds consist of a cope and a drag setup.

Principle

The Mould is made of Plaster of Paris (Gypsum or CaSO₄ 1/2 H₂O) with the addition of talc and Silica flour to improve strength and to control the time required for the plaster to set. These components are mixed with water and the resulting slurry is poured over the Pattern. After removing the pattern, mould is cured in an oven and it is ready to receive the molten metal.

Procedure

One of the most popular of the ceramic moulding techniques is the Shaw process. A reusable pattern is placed inside a slightly tapered flask, and slurry like mixture of refractory aggregate, hydrolyzed ethyl silicate, alcohol, and a getting agent is poured on top.

This mixture sets to a rubbery state that permits removal of the pattern and the flask, and the mould surface is then ignited with a torch (in an oven for heating to about 100 C).

The patterns used are split gated metal patterns usually mounted on a match plate. The slurry is applied over the pattern surfaces to form a thin coating around it. The slurry fills up all cavities and recesses by itself and no naming or vibration of the mould is required. The pattern is withdrawn after it sets in about 3 to 5 minutes.

During "bum-off, most of the volatiles are consumed, and a three-dimensional network of microscopic cracks (micro crazing) forms in the ceramic.

Advantages:

1. High precision and very good surface finish.

2. The process does not require any risering, venting or chilling because the rate of cooling is very slow.

3. Any patterns made of wood, metal or plastic can be used.

4. The process can be used for all types of metals including highly reactive Titanium or Uranium.

Disadvantages:

1. High cost

2. Difficulty in controlling dimensional tolerances across the parting line.

Applications:

1. The method can be used for producing precision parts like dies for drawing, extrusion, casting, forging etc., pump impellers, components of nuclear reactors and air craft.

Shell Moulding (or) Croning Shell Process

Introduction

Shell moulding is a process for producing simple or complex near net shape castings maintaining tight tolerances and a high degree of dimensional stability. Shell moulding is method for making high quality castings.

Principle

The process is based on the principle of capability of a thermosetting resin and sand mixture to assume the shape of a preheated metal pattern to form a dense, quickly hardened shell mould.

Process parameters of shell moulding Process

Sand coated with a thermosetting plastic resin is dropped onto a heated metal pattern, which cures the resin.

The shell segments are stripped from the pattern and assembled. When the poured metal solidifies, the shell is broken away from the finished casting.

Advantages: Faster production rate than sand moulding high dimensional accuracy with smooth surfaces.

Limitations: Requires expensive metal patterns. Plastic resin adds to cost; part size is limited.

Common metals: Cast irons and casting alloys of aluminium and copper.

Size limits: 30 g minimum usually less than 10kg; mould area usually less than 0.3 m²

Typical tolerances: Approximately 0.005 cm

Draft allowance: 1/4 to 1/2 degree

Surface finish: 1/3 – 4.0 microns

Steps involved

There are different stages in shell mould processing that include:

1. Initially preparing a metal-matched plate

2. Mixing resin and sand

3. Heating pattern.

4. Inverting the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill).

5. Curing the shell and baking it

6. Removing investment

7. Inserting cores

8. Repeating for the other half

9. Assembling the mould

10. Pouring the mould

11. Removing casting

12. Cleaning and Trimming.

The shell mould casting process consists of the following steps.

a) Pattern creation:

A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials.

b) Mould creation:

First, each pattern half is heated to 175-370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.

c) Mould assembly:

The two shell halves are joined together and securely clamped to form the complete shell mould. If any cores are required, they are inserted prior to closing the mould. The shell mould is then placed into a flask and supported by a backing material.

d) Pouring:

The mould is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mould cavity.

e) Cooling:

After the mould has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.

f) Casting removal:

After the molten metal has cooled, the mould can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mould.

Advantages of Shell Moulding Casting

1. Good casting detail and dimensional accuracy are possible.

2. Moulds are lightweight and may be stored for extended periods of time.

3. Has better flexibility in design than die-casting.

4. Is less expensive than investment casting.

5. Capital plant costs are lower than for mechanized green sand moulding.

6. Metal yields are relatively high.

7. Sand: metal ratios are relatively low.

8. Gives superior surface finish and higher dimensional accuracy, and incurs lower fettling costs than conventional sand castings.

Disadvantages:

i) Higher cost of match plate

ii) Size of casting is limited

iii) Serious dust and fume problems

iv) Carbon pickup in case of steels.

Applications

Cylinders and cylinder heads for air cooled IC engines, automobile transmission parts, cast tooth bevel gears, brake beam, hubs, and track rollers for crawler tractors, steel eyes, gear blanks, chain seat brackets, refrigerator valve plate, and small crank shafts.