Metal Casting
Introduction:
Virtually nothing moves, turns, rolls, or flies without the benefit of cast metal products. The metal casting industry plays a key role in all the major sectors of our economy. There are castings in locomotives, cars trucks, aircraft, office buildings, factories, schools, and homes. Metal Casting is one of the oldest materials shaping methods known.
Casting means pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify. When solidified, the desired metal object is taken out from the mold either by breaking the mold or taking the mold apart. The solidified object is called the casting. By this process, intricate parts can be given strength and rigidity frequently not obtainable by any other manufacturing process. The mold, into which the metal is poured, is made of some heat resisting material. Sand is most often used as it resists the high temperature of the molten metal. Permanent molds of metal can also be used to cast products.
Advantages
The metal casting process is extensively used in manufacturing because of its many advantages.
- Molten material can flow into very small sections so that intricate shapes can be made by this process. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.
- It is possible to cast practically any material that is ferrous or non-ferrous.
- As the metal can be placed exactly where it is required, large saving in weight can be achieved.
- The necessary tools required for casting molds are very simple and inexpensive. As a result, for production of a small lot, it is the ideal process.
- There are certain parts made from metals and alloys that can only be processed this way.
- Size and weight of the product is not a limitation for the casting process.
Limitations
- Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique. Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish. Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process.
- The metal casting process is a labor intensive process.
History
Casting technology, according to biblical records, reaches back almost 5,000 years BC. Gold, pure in nature, most likely caught Prehistoric man's fancy as he probably hammered gold ornaments out of the gold nuggets he found. Silver would have been treated similarly. Mankind next found copper, because it appeared in the ash of his camp fires from copper-bearing ore that he lined his fire pits with. Man soon found that copper was harder than gold or silver. Copper did not bend up when used. So copper, found a nitch in man's early tools, and then marched it's way into Weaponry. But, long before all this man found clay. So he made pottery something to eat from. Then he thought, "now what else can I do with this mud" . Early man thought about it, "they used this pottery stuff, (the first patterns), to shape metal into bowls ".
3200 B.C. A copper frog, the oldest known casting in existence, is cast in Mesopotamia.
233 B.C. Cast iron plowshares are poured in China.
233 B.C. Cast iron plowshares are poured in China.
500 A.D. Cast crucible steel is first produced in India, but the process is lost until 1750, when Benjamin Huntsman reinvents it in England.
1455 Dillenburg Castle in Germany is the first to use cast iron pipe to transport water.
1480 Birth of Vannoccio Biringuccio (1480-1539), the "father of the foundry industry," in Italy. He is the first man to document the foundry process in writing.
1480 Birth of Vannoccio Biringuccio (1480-1539), the "father of the foundry industry," in Italy. He is the first man to document the foundry process in writing.
1709 Englishman Abraham Darby creates the first true foundry flask for sand and loam molding.
1750 Benjamin Huntsman reinvents the process of cast crucible steel in England. This process is the first in which the steel is completely melted, producing a uniform composition within the melt. Since the metal is completely molten, it also allows for alloy steel production, as the additional elements in the alloy can be added to the crucible during melting. Prior steel production was accomplished by a combination of forging and tempering, and the metal never reached a molten state.
1809 Centrifugal casting is developed by A. G. Eckhardt of Soho, England.
1896 American Foundrymen's Association (renamed American Foundrymen's Society in 1948 and now called the American Foundry Society) is formed.
1897 Investment casting is rediscovered by B.F. Philbrook of Iowa. He uses it to cast dental inlays.
1947 The Shell process, invented by J. Croning of Germany during WWII, is discovered by U.S. officials and made public.
1953 The Hotbox system of making and curing cores in one operation is developed, eliminating the need for dielectric drying ovens.
1958 H.F. Shroyer is granted a patent for the full mold process, the forerunner of the expendable pattern (lost foam) casting process.
1968 The Coldbox process is introduced by L. Toriello and J. Robins for high production core making.
1971 The Japanese develop V-Process molding. This method uses unbonded sand and a vacuum.
1971 Rheocasting is developed at Massachusetts Institute of Technology.
1996 Cast metal matrix composites are first used in a production model automobile in the brake rotors for the Lotus Elise.
1971 Rheocasting is developed at Massachusetts Institute of Technology.
1996 Cast metal matrix composites are first used in a production model automobile in the brake rotors for the Lotus Elise.
Metal Casting History (India)
3000 BC Earliest castings include the 11 cm high bronze dancing girl found at Mohen-jo-daro.
2000 BC Iron pillars, arrows, hooks, nails, bowls and daggers or earlier have been found in Delhi, Roopar, Nashik and other places.
500 BC Large scale state-owned mints and jewelry units, and processes of metal extraction and alloying have been mentioned in Kautilya's Arthashastra
500 A.D. Cast crucible steel is first produced in India, but the process is lost until 1750, when Benjamin Huntsman reinvents it in England
Casting Terms
- Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of the flask in the molding structure, it is referred to by various names such as drag lower molding flask, cope upper molding flask, cheek intermediate molding flask used in three piece molding.
- Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.
- Parting line: This is the dividing line between the two molding flasks that makes up the mold.
- Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions.
- Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to give a better surface finish to the castings.
- Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.
- Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured.
- Spruce: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold.
- Runner: The channel through which the molten metal is carried from the sprue to the gate.
- Gate: A channel through which the molten metal enters the mold cavity.
- Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.
- Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as feed head.
- Vent: Small opening in the mold to facilitate escape of air and gases.
Steps in Making Sand Castings
There are six basic steps in making sand castings:
- Patternmaking
- Core making
- Molding
- Melting and pouring
- Cleaning
Pattern making
The pattern is a physical model of the casting used to make the mold. The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting. If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities.
Core making
Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings. Thus the void space between the core and mold-cavity surface is what eventually becomes the casting.
Molding
Molding consists of all operations necessary to prepare a mold for receiving molten metal. Molding usually involves placing a molding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity and finishing and closing the mold.
Melting and Pouring
The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled.
Cleaning
Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects and general quality is performed.
Pattern
The pattern is the principal tool during the casting process. It is the replica of the object to be made by the casting process, with some modifications. The main modifications are the addition of pattern allowances, and the provision of core prints. If the casting is to be hollow, additional patterns called cores are used to create these cavities in the finished product. The quality of the casting produced depends upon the material of the pattern, its design, and construction. The costs of the pattern and the related equipment are reflected in the cost of the casting. The use of an expensive pattern is justified when the quantity of castings required is substantial.
Functions of the Pattern
- A pattern prepares a mold cavity for the purpose of making a casting.
- A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.
- Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern.
- Patterns properly made and having finished and smooth surfaces reduce casting defects.
- A properly constructed pattern minimizes the overall cost of the castings.
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:
- Easily worked, shaped and joined
- Light in weight
- Strong, hard and durable
- Resistant to wear and abrasion
- Resistant to corrosion, and to chemical reactions
- Dimensionally stable and unaffected by variations in temperature and humidity
- Available at low cost
The usual pattern materials are wood, metal, plastics, rubber and waxes.
Wood: It is the most commonly used pattern material because it is,
· Easy to work and readily available
· Can be cut and fabricated into numerous forms by glueing, bending, and curving
· It is easily sanded to a smooth surface and may be preserved with shellac
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.
The wood products gaining more popularity for pattern work in recent times includes compressed wood laminates and laminated wood impregnates. The laminates are available as plywoods and as laminated boards, plain or veneered. Laminated wood impregnates are simply impregnated with resins so as to fill up the cell cavities or are impregnated and compressed to increase density and hardness.
Metal: When a large number of casting are desired from a pattern or when conditions are too severe for wooden pattern. Commonly a metal pattern is itself cast from a wooden pattern called master pattern. When metal patterns are to be cast from master patterns, double shrinkage must be allowed. For example, if the metal pattern is to be made of brass and the castings are to be of cast iron, the shrinkage allowed on the wood master patter will have to be 14mm per metre for brass, plus 10mm per metre for cast iron, making a total of 24 mm per metre.
Commonly used metals are: Cast Iron, Steel brass, Aluminium, and White metal.
Plastics: Do not absorb moisture, are strong and dimensionally stable, resistance to wear, have a very smooth and glossy surface, and are light in weight. Because of glossy surface it can be withdrawn from the mould very easily without injuring the mould and no dry or liquid parting compound is necessary. Plastic has a very low solid shrinkage.
Rubbers: Certain types of rubbers, such as silicon rybber, are favoured for forming a very intricate type of die for investment casting.
Waxes: Excellent for investment casting process. Commonly chosen waxes are paraffin wax, shellac wax, bees- wax, cerasin wax, and micro-crystalline wax.
Types of Patterns
The type of pattern selected for a particular casting will depend upon:
· Ease or difficulty of the moulding operation to come.
· Number of castings- small or large
· Type of moulding process
Most commonly used patterns are:
1. Single-piece pattern
2. Split pattern
3. Match plate pattern
4. Cope and drag pattern
5. Gated pattern
6. Loose piece pattern
7. Sweep pattern
8. Skeleton pattern
9. Segmental pattern
10. Shell pattern
11. Built-up pattern
12. Boxed-up pattern
13. Lagged-up pattern
14. Left- and right-hand pattern
Single piece or solid pattern
A pattern that is made without joints, partings, or any loose pieces in its construction is called a single-piece or solid pattern.
· Not attached to a frame or plate and is, therefore, sometimes known as a loose pattern.
· Cheaper patterns.
· Moulder has to cut his own runners and feeding gates and risers.
· Operation takes more time & thus they are not recommended except for limited production.
· Usually used for large castings of simple shapes.
The simplest type of pattern classified under this heading is the flat-back. It may have few or no irregularities, may not have a core print, but very seldom does it have lose pieces. When completed, the mould cavity will be either entirely in the drag or entirely in the cope. The flat-back jas the largest horizontal cross-sectional area and it serves as the parting surface in the mould. Soil temper, stuffing box and gland of a steam engine are few examples of casting which are made by making solid patterns.
Pattern Allowances
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:
- Shrinkage or contraction allowance
- Draft or taper allowance
- Machining or finish allowance
- Distortion or camber allowance
- Rapping allowance
Shrinkage or Contraction Allowance
All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types:
- 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.
- Solid Shrinkage: it refers to the reduction in volume caused when metal loses temperature in solid state. To account for this, 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.
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
