METAL CASTING

Hemendra Grover08MT35

Introduction

• Casting may be defined as a ‘metal object obtained by allowing molten metal to solidify in a mold’ i.e. forming metal objects by melting metal and pouring it into molds.

Alternatives to casting :• Where resources are limited casting is unlikely to be the first choice

when manufacturing metal objects. For this reason it is sensible to look at the alternatives before describing those open to the caster.

• Forging • Pressing• Fabricating• Metal joining

Advantages of Casting :• Most intricate of shapes(both internal and external) can be

cast.• Construction may be simplified.• Mass production• Casting of extremely large, heavy metal objects is possible.• EconomicMetallurgical advantages :• Machinability and vibration damping capacity in cast irons.• More uniform properties from directional point of view.• Strength and lightness• Good bearing quality

Foundry

• Commercial establishment for producing castings.• Castings are used in transportation, communication,

construction, agriculture, power generators etc., due to this widespread use, casting are produced almost everywhere that manufacturing occurs.

Types of Foundries :• Based on type of metal poured, Foundry may be classified as- Ferrous or non-ferrous, gray iron, steel, malleable, brass and

bronze, or light metal( Aluminium, Magnesium )

• According to the nature of their work and their organizational framework –

Jobbing : physical plant such that it usually contracts to produce a casting or a small no. of castings of a given kind.

Production : highly mechanized shop which requires that large no. of a given kind of casting be made economically.

Semiproduction : a portion of the work is of a jobbing nature while the balance is production casting.

Captive : integral part of some manufacturing company and whose castings are consumed mainly in the products of the parent organization.

Independent : It is usually a separate company that produces casting for any no of customers.

Steps in making Sand Castings :(1) Patternmaking (including core boxes)(2) Coremaking(3) Molding(4) Melting and Pouring(5) Cleaning

PATTERNS

• Foundryman’s forming tool.• The mold cavity i.e. a casting is made up of pattern.• Many casting can be made from a single pattern.

Patternmaking :• A vast majority of Patterns are made by pattern shops

independent of the foundry .• Pattern dept. in foundries are concerned with modifying

existing pattern equipment and preparing it for molding(Rigging) .

Types of Patterns :• Depending upon the casting requirements, the pattern may

conform to one of the following types :1. Single or loose patterns : • Single copies of the casting but incorporating the allowances

and core prints necessary to producing the casting.• Generally made up of wood but metal, plaster, plastics, wax,

or any other suitable material can be used.• Since hand molding is practiced which makes it slow and

costly.2. Gated Patterns :• Improvement of ungated loose patterns.• Gating system is a part of pattern and eliminates hand

cutting of gates.

• More rapid molding of small quantities of castings.3. Match-plate Patterns :• Mass production of small castings.• The cope and drag portions of the pattern are mounted on

opposite sides of a wood or metal plate conforming the parting line.

• Gating systems are almost always attached to the plate .• Generally used with some type of molding machine in order

to get maximum speed of molding.• A limitation arises in the weight which can be handled by the

molder.• Offset parting surface makes it more accurate dimensionally.

4. Cope and Drag Pattern plates :• Consist of the cope and drag parts mounted on separate

plates.• Molding of medium and large castings is greatly facilitated.• Expensive as plates are separate for both cope and drag but

increased production or making of large molds is a advantage.5. Special Patterns and Devices :• For extremely large castings, Skeleton patterns can be

employed. Large work of symmetrical shape sometimes involves the use of sweeps for forming a mold surface.

• Loose patterns having an irregular parting line are difficult to mold without a Follow board or ‘match’. It serves to support the loose pattern during molding of the drag half of the mold and also establishes the parting surface when the match is removed.

• For casting of metal patterns, Master pattern is used as an original which is made up of wood.

• Several patterns may be cast from the master and mounted on a pattern plate after they have been finished to the proper dimensions.

Pattern Allowances :• For metallurgical and mechanical reasons , a number of

allowances must be made on the pattern if the casting is to be dimensionally correct.

Shrinkage allowance :• Correction for solidification shrinkage of the metal and its

contraction during cooling to room temperature.• It is the amount the pattern must be made larger than the

casting to provide for total contraction.

Machining Allowance :• Amount dimension on a casting are made oversize to provide

stock for machining.• Influenced by the metal, the casting design, and the method

of casting and cleaning.• It may be minimum if the surfaces to be machined are entirely

in the drag half of the mold, since dimensional variation and other defects are usually least prevalent there.

Pattern Draft :• Draft is the taper allowed on vertical faces of a pattern to

permit its removal from the sand or other molding medium without tearing the mold-cavity surfaces.

• In the case of pockets or deep cavities, considerably more draft is necessary to avoid tearing the mold during withdrawal of the pattern.

: Taper or draught allowance

Size tolerance :• The variation which may be permitted on a given casting is called its tolerance and is equal to the difference b/w the min and the max limits for any specified dimension

Distortion Allowance :• Certain objects such as large flat plates and dome or U-

shaped castings sometimes distort when reproduced from a straight or perfect pattern.

• Distorted pattern then produces a casting of the proper shape and size.

Function of Patterns :• Main purpose is its use in molding.1) Molding the gating system.2) Establishing the parting line.3) Making core prints.4) Establish local points.5) Minimize costing defects attributable to the pattern.6) Provide for ram-up cores.7) Economy in molding.

Colour code for patterns :• Pattern makers use a colour code so that it is clear to the

metal caster which surfaces are which. This is as follows:• 'As cast' surfaces which are to be left unmachined - Red or

Orange• Surfaces which are to be machined - Yellow• Core prints (see below) for unmachined openings and end

prints - Black• Core prints for machined openings - Yellow stripes on Black• Seats for loose pieces and loose core prints - Green.

Core prints and core boxes• The core print is the extra piece on the pattern which leaves a

cavity (or print) into which the end of the core fits, in order to hold the core in place.

• The core box is the negative shape, usually made of wood in which the core is formed.

Molding Processes

• Good castings can’t be made without good molds.

Classification :1. Sand Casting a) Green-sand molds, green sand molding b) Dry-sand molds c) Core-sand molds d) Cement-bonded-sand molds e) Loam molds f) Shell molding g) Pit and floor molding

2. Permanent-mold casting3. Die casting4. Centrifugal casting5. Plaster-mold casting6. Investment casting7. Special processes; graphite molds, ceramic molds

• Each of the process listed above has a field of most appropriate application, certain advantages, and limitations.

SAND CASTINGS

• Molding processes where sand is used to make the mold produce by far the largest quantity of castings.

Green-sand Molding :• Green molding sand is defined as a plastic mixture of sand

grains, clay, water, and other materials which can be used for molding and casting processes.

• The sand is called ‘green’ because of the moisture present and is thus distinguished from dry sand.

The basic steps in green-sand molding are the following :1) Preparation of the pattern ( mostly done with match-plate or

cope and drag patterns)2) Making the mold ( by ramming of sand to develop strength

and rigidity using machine or hand )

3) Core setting ( cores are set into the mold cavity to form the internal parts of the casting )

4) Closing and weighting ( to prevent cope from floating over drag during pouring )

Advantages :• Great flexibility as a production process as green sand can be

further reused many times.• The most direct route from pattern to mold ready for pouring.• Economical Limitations :• Thin, long projections of green sand in a mold cavity are

washed away by the molten metal or may not even be moldable.

• Certain metals and some castings develop defects if poured into molds containing moisture.

• More intricate castings can be made by some other castings.• Dimensional accuracy and surface finish of green-sand

castings may not be adequate.• Large castings require greater mold strength and resistance to

erosion than is available in green sands.Dry-sand Molds :• The sand mixture is modified somewhat to favor good

strength and other properties after the mold is dried.• Mold-cavity surface is coated or sprayed with a mixture which

upon drying imparts greater hardness or refractoriness to the mold followed by drying in an oven at 300 to 650 F or by circulating heated air through the mold.

• Because the mold is dry, the volume of gas formed when the casting is poured is much less than with green molds, and casting defects attributable to moisture should be absent.

Floor and Pit Molding :• The production of large intricate castings weighing from 1 to

over 100 tons is one of the special advantages of the casting processes.

• When molds are medium to large in size, considerably heavy equipment, floor space, and time must be allocated.

• Floor molding is done on the floor of bays of the foundry set aside for these heavy molding jobs.

• When the pattern being molded is too large to be handled in flasks, the molding is done in pits. These are concrete-lined box-shaped holes in the molding floor. The pattern is lowered into the pit and molding sand is tucked and rammed under the pattern and up the side walls to the parting surface.

Loam Molds :• When a large mold for a gray-iron casting can be multiple-

piece flasks or by bricking up a large portion of the mold, loam is used as the molding material.

• Loam is a molding sand containing about 50% sand grains and 50 % clay.

• It is troweled onto a brickwork surface and brought to the pattern dimensions by using skeleton patterns, sweeps, or templates as the molding progresses.

• Loam molds must also be thoroughly dried.

Reference :• Principles of metal casting _ Heine and Rosenthal• Metal Casting _ Steve Hurst

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