COREA core is a body of material, usually sand, used to produce a cavity in or on a casting for which no provision has been made in the pattern. Cores are required to create the recesses, interior and undercuts in castings.Cores are generally made separately from the sand mould and are then baked to facilitate handling.An example of a core in a casting is one forming the water jacket in a water cooled engine block: an example of a core on a casting is forming the air space between the cooling fins of an air cooled cylinder.

FUNCTIONS (PURPOSES) OF CORESCores provide the means for producing hollow castings and internal cavities.Cores may provide external undercut features.Cores are employed to improve the mould surface.Cores are used to achieve deep recesses in the castings.Cores are used to form the gating system of large size moulds.Cores are also used to strengthen the mould.Sometimes, a mould is completed by the assembly of a large number of cores and core pieces.

TYPES OF CORESCores can be classified as follows:According to the Condition of the Core(!) Green sand core ii) Dry sand core.According to Shape and Position of the CoreHorizontal coreVertical coreBalanced coreDrop core Hanging core

GREEN SAND CORESGreen sand cores are made by green sand containing moist condition about 5% water and 15-30 % clay. It imparts very good permeability to core and thus avoids defects like shrinkage or voids in the casting. Green sand cores are not dried. They are poured in green condition and are generally preferred for simple, small and medium castings. The process of making green sand core consumes less time. Such cores possess less strength in comparison to dry sand cores and hence cannot be stored for longer period.

DRY SAND CORESDry sand cores are produced by drying the green sand cores to about 110C. These cores possess high strength rigidity and also good thermal stability. These cores can be stored for long period and are more stable than green sand core. They are used for large castings. They also produce good surface finish in comparison to green sand cores. They can be handled more easily. They resist metal erosion. These types of cores require more floor space, more core material, high labor cost and extra operational equipment.

Core sand properties Core sand must possess the following properties: -Adequate green strength to retain its shape after the removal from the core box and transportation before baking.High dry strength and hardness after baking.Good refractoriness to withstand the effects of high temperature of molten metal.Adequate permeability to allow easy escape of gases generated during and after pouring of molten metal.Core sand should be able to impart good smoothness and finish to the core surface.Core sand should possess good falliability so that the core crumbles and falls apart when it is to be removed from the casting.

Core should generate minimum amount of gases during the pouring of molten metal.It should possess good collapsibility so that the core gives away easily as the casting cools and shrinks. It avoids cracks and hot tears in castings.Core sand must be capable of retaining its properties during storage.It must be capable of resisting the effects of molten metal like erosion and thermal shock. The above properties are imparted to cores by addition of the proper type and amount of binders, additives and water, followed by baking of the core.

CORE SANDThe main constituents of the core sand are pure silica sand and a binder. Silica sand is preferred because of its high refractoriness. For higher values of permeability sands with coarse grain size distribution are used. The main purpose of the core binder is to hold the grains together, impart strength and sufficient degree collapsibility.Beside these properties needed in the core sand, the binder should be such that it produces minimum amount of gases when the molt metal is poured in the mould.

CORE SAND INGREDIENTSThe various ingredients most commonly used for core making are: Refractory materialsCore bindersWaterSpecial additives

(I) Refractory Materials: Refractory material may be defined as that material which can withstand high temperatures. Silica, olivine and carbon are the commonly used refractory materials, but silica is mostly used. Coarse white silica sand is used in steel foundries and lake sand for cast iron foundries.

(ii) Core Binders: The functions of a core binder are to give strength to a core and hold the sand grains together. Binders also resist breaking of cores and impart collapsibility to cores after use. The commonly used core binders are:Inorganic binders: Inorganic binders are finely pulverized materials used to provide green strength, hot strength and baked strength to a core. Fireclay, bentonite. limonite. silica flour and iron oxide, etc. are inorganic binders. They are generally very fine powders and act as binding materials after baking of the cores.(b) Organic binders: They are classified as:Cereal bindersMolasses ,dextrin. Core oils(Linseed oil ,Whale oil, mineral oils)Protein binders.

CORE MAKINGCore making basically is carried out in four stages namely core sand preparation, core making, core baking and core finishing. Each stage is explained as under.Core Sand Preparationcore sands are generally mixed with the help of any of the following mechanical means namely roller mills and core sand mixer.for getting better and uniform core sand properties using proper sand constituents and additives.Core Making Process Using Core Making MachinesThe process of core making is basically mechanized using core blowing, core ramming and core drawing machines.

Core blowing machinesThe basic principle of core blowing machine comprises of filling the core sand into the core box by using compressed air. The velocity of the compressed air is kept high to obtain a high velocity of core sand particles, thus ensuring their deposit in the remote corners the core box. On entering the core sand with high kinetic energy, the shaping and ramming of core is carried out simultaneously in the core box. The core blowing machines can be further classified into two groups namely small bench blowers and large floor blowers.

Core ramming machinesCores can also be prepared by ramming core sands in the core boxes by machines based on the principles of squeezing, jolting and slinging. Out of these three machines, jolting and slinging are more common for core making.

Core drawing machinesAfter ramming sand in it, the core box is placed on a core plate supported on the machine bed. A rapping action on the core box is produced by a vibrating vertical plate. This rapping action helps in drawing off the core from the core box. After rapping, the core box, the core is pulled up thus leaving the core on the core plate.

Core bakingCores cannot be used directly after removal from core boxes as they are too soft and practically have no strength. The main purpose of baking is to drive away the moisture and harden the binder, thereby giving strength to the core. They are baked in a baking ovens or furnaces. Core baking is done between 350 to 430CC for several hours, depending upon the shape and size of the core. Care should be taken that the core is properly baked. An under baked core will generate a huge amount of gases, producing blowholes in the die casting, whereas an over baked core has little strength and is just like loose sand.

Types of ovensContinuous type ovensContinuous type ovens are preferred basically for mass production. In these types, core carrying conveyors or chain move continuously through the oven. The baking time is controlled by the speed of the conveyor. The continuous type ovens are generally used for baking of small cores.Batch type ovensBatch type ovens are mainly utilized for baking variety of cores in batches. The cores are commonly placed either in drawers or in racks which are finally placed in the ovens. The core ovens and dielectric bakers are usually fired with gas, oil or coal.

CORE FINISHINGThe cores are finally finished after baking and before they are finally set in the mould. The fins, bumps or other sand projections are removed from the surface of the cores by rubbing or filing. The dimensional inspection of the cores is very necessary to achieve sound casting. Cores are also coated with refractory or protective materials using brushing dipping and spraying means to improve their refractoriness and surface finish. The coating on core prevents the molten metal from entering in to the core.Bars, wires and arbors are generally used to reinforce core from inside as per size of core using core sand.

core boxA core box is a type of pattern being used for making cores.it made of wood, brass, aluminum or any suitable material. In core boxes, sand is rammed or packed to form the cores and thus impart the desired shape to them. A core box should be very accurate with all the allowances as needed for construction of a pattern and must conform to the size of core print given on the pattern.

CORE PRINTSCore Prints are extra projections provided on the pattern that form a seat in the mould when a pattern is embedded in the sand for making a mould. Core prints are of various designs and are used to support all types of cores in the mould,. Though the core prints are a part of the pattern, yet they do not appear on the cast pan.