6.core

core

1. Core

2. CORE Introduction Core is an obstruction-which when positioned in the mold, naturally does not permit the molten metal to fill up the space occupied by the core. In this way a core produces hollow castings. Cores are required to create the recesses, undercuts and interior cavities that are often a part of castings. A core may be defined as a sand shape or form which makes the contour of a casting for which no provision has been made in the pattern for molding. core as a sand shape is generally produced separate from the sand mold and is then baked (hardened) to facilitate handling and setting into the mold. Cores may be made up of sand, metal, plaster or ceramics. 6/30/2015 2

3. Different Functions (Purposes) of Cores For hollow castings, cores provide the means of forming the main internal cavities. Cores may provide external undercut features Cores may be employed to improve the mold surface Cores may be inserted to achieve deep recesses in the castings. Cores may be used to strengthen the molds Cores may be used to form the gating system of large size molds. Essential Characteristics of (dry sand) Cores A Core must possess Sufficient strength to support itself and to get handled without breaking. High permeability to let the mold gases escape through the mold walls. Smooth surface to ensure a smooth casting. High refractoriness to withstand the action of hot molten metal (metal penetration etc.). High collapsibility in order to assist the free contraction of the solidifying metal. Those ingredients which do not generate mold gases. 6/30/2015 3

4. TYPES OF CORES Cores may be classified according to The state or condition of core Green sand core Dry sand core No bake sand core The nature of core materials employed Oil bonded cores Resin bonded cores Shell cores Sodium silicate cores The type of core hardening process employed C0 2 process The hot box process The cold set process Fluid or castable sand process Furan-No-Bake system Oil-No-Bake process D. The shape and position of the core Horizontal core Vertical core Hanging or cover core Balanced core Drop core or stop off core Ram up core Kiss core. 6/30/2015 4

5. A. The state or condition of core 1. Green sand cores Green sand cores are formed by the pattern itself. A green sand core is a part of the mold. A green sand core is made out of the same sand from which the rest of the mold has been made i.e., the molding sand. 2. Dry sand cores Dry sand cores (unlike green sand cores )are not produced as a part of the mold. Dry sand cores are made separately and independent of the mold. A dry sand core is made up of core sand which differs very much from the sand out of which the mold is constructed. A dry sand core is made in a core box and it is baked after ramming. A dry sand core is positioned in the mold on core-seats formed by core-prints on the patterns. A dry sand core is inserted in the mold before closing the same. 3. No-bake sand cores The sand used for preparing no-bake core is similar to that used for making no-bake sand moulds. Synthetic resins like phenol or urea formaldehyde are used as binder for bonding silica sand. Certain chemicals are used as hardeners and catalysts to bring about a chemical reaction with the binder due to which bonding of sand grains takes place. 6/30/2015 5

6. B. The nature of core materials employed 1. Oil bonded cores Conventional sand cores are produced by mixing silica sand with a small percentage of linseed oil. 2. Resin-bonded cores Phenol resin bonded sand is rammed in a core box. The core is removed from the core box and baked in a core oven at 375 to 450°F to harden the core. 3. Sodium Silicate cores These cores use a core material consisting of clean, dry sand mixed with a solution of sodium silicate 6/30/2015 6

7. C. The type of core hardening process employed 1. hot box process It uses heated core boxes for the production of cores. The core box is made up of cast iron, steel or aluminium and possesses vents and ejectors for removing core gases and stripping core from the core box respectively. Core box is heated from 350 to 500°F. Heated core boxes are employed for making shell cores from dry resin bonded mixtures. 2. The cold set process While mixing the core-sand, an accelerator is added to the binder. Curing begins immediately with the addition of accelerator and continues until the core is strong to be removed from the core box. Cold set process is employed for making large cores. 3. Castable sand process A setting or hardening agent such as dicalcium silicate is added to sodium silicate at the time of core sand mixing. The sand mixture possesses high flowability and after being poured in the core box, it chemically hardens after a short interval of time. 6/30/2015 7

8. D. The shape and position of the core 1. Horizontal core Fig. shows horizontal core. A horizontal core is positioned horizontally in the mold. A horizontal core may have any shape, circular or of some other section depending upon the shape of the cavity required in the casting. 2. Vertical core Fig. shows a vertical core. On the cope side, a vertical core needs more taper so as not to tear the sand in the cope while assembling cope and drag. A vertical core is named so because it is positioned in the mold cavity with its axis vertical. A horizontal core is supported in core seats at both ends. Uniform sectioned horizontal cores are generally placed at the parting line. A horizontal core is very commonly used in foundries. The two ends of a vertical core are supported in core seats in cope and drag respectively. A big portion of the vertical core usually remains in the drag A vertical core is very frequently used in foundries. 6/30/2015 8

9. 3. Hanging or cover core Fig. shows a hanging (cover) core It is known as hanging core because it hangs; it is also called cover core if it covers the mold and rests on a seat made in the drag. A simple hanging core is one which is not supported on any seat rather it hangs from the cope with the help of wires, etc. A hanging core is supported from above and it hangs vertically in the mold cavity. A hanging core has no support from bottom. A hanging core is provided with a hole through which molten metal reaches the mold cavity. Hanging cores can be made up of either green or dry sand. 6/30/2015 9

10. 4. Balanced core Fig. shows a balanced core. A balanced core is one which is supported and balanced from its one end only. A balanced core requires a long core seat so that the core does not sag or fall into the mold. A balanced core is used when a casting does not want a through cavity. 6/30/2015 10

11. 5. Drop or stop off core Fig. shows a Drop or stop off core. A stop off core is employed to make a cavity (in the casting) which cannot be made with other types of cores. A stop off core is used when a hole, recess or cavity, required in a casting is not in line with the parting surface, rather it is above or below the parting line of the casting. Depending upon its shape and use, a stop off core may also be known as tail core, saddle core, chair core, etc. 6/30/2015 11

12. 6. Ram-up core A ram-up core is shown in Fig. A ram-up core is one which is placed in the sand along with pattern before ramming the mold. A ram-up core cannot be placed in the mold after the mold has been rammed. A ram-up core is used to make internal or external (surface) details of a casting. 6/30/2015 12

13. 7. Kiss core Kiss core is shown in Fig. A kiss core does not require core seats for getting supported. A kiss core is held in position between drag and cope due to the pressure exerted by cope on the drag. A number of kiss cores can be simultaneously positioned in order to obtain a number of holes in a casting. 6/30/2015 13

14. Method of making the cores Core Making (Preparation) Procedure Steps involved: Core Sand Preparation Making the Cores Baking the Cores. Finishing of Cores. Setting the Cores. 1. Core Sand Preparation The core sand of desired type (dry sand, no-bake etc.,) and composition along with additives is mixed manually or using Muller of suitable type. 2. Making The Cores Cores are prepared manually or using machines depending on the needs. Machines like jolt machine, sand slinger, core blower etc., are used for large scale continuous production, while small sized cores for limited production are manually made in hand filled core boxes. A core box is similar to a pattern that gives a suitable shape to the core. Figure shows a core box used to produce rectangular shaped cores with procedure. 6/30/2015 14

15. Steps Involved in making the core Core box is usually placed on work-bench; it is filled with already mixed and prepared core sand, is rammed by hand and the extra sand is removed from the core box. Weak cores may be reinforced with steel wires to strengthen them. Core box is inverted over the core plate and this transfers the core from the core box to core plate which (i.e., core) is then baked in the oven (over the core plate itself). Larger cores can also be made manually but on the floor (and not on bench). It needs more than one man to work and the cranes may also be used, if necessary 3. Core Baking Cores are baked in ovens in order to drive away the moisture in them and also to harden the binder thereby imparting strength to the core. The temperature and duration for baking may vary from 200 - 450°F and from a few minutes to hours respectively depending on the size of the core and type of binder used. 4. Core finishing The baked cores are finished by rubbing or filing with special tools to remove any fins, bumps, lose sand or other sand projections from its surface. The cores are also checked for dimensions and cleanliness. Finally, if cores are made in parts, they are assembled by using suitable pastes, pressed and dried in air before placing them in the mould cavity. 6/30/2015 15

16. Core binders A core binder, holds sand grains together gives strength to cores makes cores to resist erosion and breaking, imparts adequate collapsibility to cores. core binders are of the following types Organic binders Inorganic binders Other binders. 6/30/2015 16

17. Organic Binders 1. Core oil. They may be Vegetable (i.e., linseed oil) Marine animal (i.e., whale oil), and mineral oil (used for diluting vegetable and marine animal oils) 2. Cereal binders They are Gelatinized starch. It is made by wet milling and contains starch and gluten. Gelatinized corn flour. Cereal binders contribute to green strength. 3. Water soluble binders They are Dextrin, made from starch. Molasses, etc. 4. Wood product binders They are Natural resins (i.e., rosin, thermoplastic). Sulfite binders. They contain Lignin, are water soluble compounds of wood sugars produced in the paper pulp process i.e., as a by-product of paper making. 6/30/2015 17

18. B. Inorganic Binders They are Fire clay Bentonite Silica flour Iron oxide, etc. These binders develop green strength, baked strength, hot strength and impart smooth surface finish. They are finely pulverized materials. They greatly increase the amount of oil necessary in oil sand mixes . Note: Inorganic binders have been discussed under 1 st chapter C. Other Binders They are Portland cement. It hardens at room temperature. Cements (i.e., rubber cements). They harden at room temperature Sodium silicate. The core hardens as carbon-di-oxide gas is passed through it. 6/30/2015 18