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Types of core Core in different shape and size are used in mould
as per design. If core is prepared from the mould sand and is a part of main-pattern, then it is called 'Green Sand Core', but if core is prepared separately with help of core box and heated at required temperature and fitted in mould known as ‘Dry sand core'. Classification of core is based on types of core and its position in mould, which given below : Horizontal
core Vertical core Balanced core Hanging core Drop core or stop off core Ram up core
This core is positioned vertically inside the
mould. It is a usual practice to have greater part of the core in the drug position of the mould. To make assembly simple and easy the core seat is prepared with large taper, as shown
Horizon tal core
This is the most common and simplest type. This core is arranged horizontally in mould. Generally round cross-section core are used. It is
placed on mould seats as shown in fig. The ends of the core rest in the seats provided by the core print of the pattern.
Balanced core When blind hole is required in component, balanced core is used.
A balanced core is one which is supported and
balanced from its one end only. Fig. shows that seating of core having length which makes cantilever end and balance other parts of mould. Chaplets are used to support core. This is used when a casting does not required a thorough cavity, for supporting the core in
Seat is not required for this core It is support with help of cope and drag box. It is used when number of holes are required
Ram up Core
This core is fitted with the pattern in the send
after that ramming is done
Hanging Or Cover Core
If the core over hang from the cope and does
not have any support at the bottom in the drag it is called hanging core
Drop or Stop off Core
When cavity in the component is not on
parting line but above or below the line this core is used
Chaplets are often required to be placed between the mould
wall or base and core, in order to avoid deflection of the core and achieve the exact section thickness of the casting. As far as possible, chaplets should be of the same composition as the cast metal so that a homogeneous structure is obtained and no internal flaws are developed For ferrous castings, steel chaplets are available as hardware items in different shapes
Core making or core preparation : It is necessary to study the core making after
knowing its types and characteristics core making is divided in four parts as shown below : (A) (B) (C) (D)
Preparation of sand Core moulding Core baking Core finishing
(A) Preparation of sand :
Core sand is a mixture of sand and binder. Sand
is basically silica, with less than 5% clay. Grain size of sand is very small. Type of sand is depend on core dimension and metal pouring temperature. Round sand gives better result. Core binders : Pure sand does not have natural bond so binders are added in sand so it makes tight bond with atoms. Binders play following role : (i) Makes bond with sand particles. (ii) Improves strength of core (iii) Resists abrasion (iv) Gives durability to core
Binders are divided in two parts : organic and
inorganic binders. Linsead oil, stratch, wheat powder, dextrin, resin and peach are organic binder. Thermosetting plastics like urea, phenol are also core binders. It is also available in the form of liquid and powder. At higher temperature organic binders are burn with molten metal so it is having limited use. Bentonite, silica floore, ferres oxide and fireclay are inorganic binder. Bentonite and silica floor are maximum used as a binder. They are available in fine powder and used in silica. At higher temperature inorganic binder are not burn so it gives strength to core. Inorganic binders gives better surface finish to core
is now a days maximum used for core binder. Leensead oil, resin, mineral oil are used in core oil. The following advantages gives core oil : Minimum time in preparation of mixture, so preparation core is easy, It is easily removed after from casting. Strength of core is easily controlled at wet core and baked core. Baked core are tough so it is easy to handle.
Making core from send is known as core
making. Core making with manually or by machine. Different core machine such as squeezing m/c , sand slinzer , jolt m/c, core extrusion
(C) Core Backing Generally baking is carried in ovens The temp. of baking is depedes on 1.Type of binder used 2.Dimensions and size of core 3.Time of baking Following ovens are used 1.Batch Type oven 2.Continuous type oven 3.Die-electric baker
Batch Type oven When requirement of core is in specified quantity, this type of oven is used. Prepared batch of core is placed on portable rake or on dryer and backed. Continuous type oven When similar sized small and in mass quantity core is required, continuous type oven is used. Here for loading of core inside the oven, conveyer or rail is used and passed slowly inside the oven and the other end unloading of core is taking place. Baking time for core is generally maintained by controlling conveyor motion. Die-electric baker For high quality and fast baking, this die-
Before placing core inside the mould, finishing is
required. Core finishing is done with following steps : (i) Cleaning : Baked core hailing unwanted fin, sand particle, and projection which are removed by brush, file or abrasive tool called cleaning. Coating is also done for protection against corrosion and moisture, it also improve surface finish, fine sand or graphite or zircon used for coating, it is applied by spray or measuring components or with brush. it is also known as core dressing. (ii) Sizing : To give accurate dimension as per design on core, different operation for sizing are carried out such as file work, grinding. Template or gauge is used for accurate measurement. (iii) Core assembly : If one or more one core are required to join, is called core assembly. Different parts of core is joined by talk, dextrin, powder
Carbon Dioxide Core
Moulding processes Moulding processes may be classified as hand moulding or machine moulding according to whether the mould is prepared by hand loots or with the aid of some moulding machine. Hand moulding is generally found to be economical when the castings are required in a small number. On the other hand when the castings are required in large quantities, hand moulding is more time-consuming and laborious and becomes expensive. Considerable skill is also needed to make good moulds by hand. In such cases machine moulding is generally employed. The main advantages of machine moulding are as follows: (i)It affords great saving in time, especially when a large number of similar castings in small sizes are required. (ii)When the number of castings is substantial, the additional cost of metallic patterns and other equipment is compensated by the high rate of production and the overall cost per piece works out lower than in the case of hand moulding. (iii)The castings obtained arc more uniform in size and shape and more accurate than those obtained by hand moulding due to steadier lift of the pattern. (iv) A semi-skilled worker can do the machine job whereas hand moulding requires skilled craftsmanship.
Types of Sand Natural Sand
Refractoriness : The capability of the moulding sand to withstand the high temperatures of the molten metal without fusing is known as refractoriness. Refractoriness is depends on silica in sand, increasing in silica increases refractoriness. Porosity / permeability : Molten metal always contains a certain amount of dissolved gases which are evolved when the metal solidifies, also when the molten metal comes in contact with the moisture sand, generates steam and water vapor. If these gases and water vapor do not find passage to escape completely through the mould they with form gas holes and pores he casting. The ability of the sand to allow the gas to pass through it is called permeability. It depends on size and shape of grains, moisture content and degree of ramming. Plasticity or Flowability : This refers to the ability of the moulding sand to acquire a predetermined shape under pressure and retain the same when the pressure is removed this will increase with increase in clay moisture content.
Cohesivness or strength: It is ability of sand particles to stick to each other
Adhesiveness : The property of dashes with other material Collapsibility : it is the property of the moulding sand that permits it to collapse (break) easily Co-efficient of expansion : Moulding sand should possess law coefficient expansion. Chemical resistivity : The moulding sand should not react chemically with the molten metal, otherwise shape of casting will be distorted and smooth surface will not be obtained.