core and core making for foundry tech .pptx

Core and core making

1. CORE AND CORE MAKING CONTENTS : Core Function of core Properties of core Core making procedure Types of core

2. Core

3. INTRODUCTION Core is an obstruction which when positioned in the mold , naturally does not permit the molten poured metal to fill up the space occupied by the core. In this way a core produces hollow casting. A core is essentially a body of material which forms a component of the mould It possesses sufficient strength to be handled as an independent unit. Cores are required to create the recesses, undercuts and interior cavities that are often a part of castings Cores may be made up of sand, metal, plaster or ceramics

4. DIFFERENT FUNCTIONS (PURPOSES) OF CORES (i) For hollow castings, cores provide the means of forming the main internal cavities. (iii) Cores may provide external undercut features. (vi) Cores may be employed to improve the cavity surface finish . (v) Cores may be inserted to achieve deep recesses in the castings. (vi) Cores may be used to strengthen the molds. (vii) Sometimes the mold may be completed simply by assembling the core pieces or cores. (viii) Cores may be used to form the gating system of large size molds

5. ESSENTIAL CHARACTERISTICS OF CORES Cores must possess: (i) Sufficient strength to support itself and to get handled without breaking. (ii) High permeability to let the mold gases escape through the mold walls. (iii) Smooth surface to ensure a smooth casting. (iv) High refractoriness to withstand the action of hot molten metal penetration. (v) High collapsibility in order to assist the free contraction of the solidifying metal. (vi) Those ingredients which do not generate mold gases .

6. CORE MAKING (PREPARATION) PROCEDURE Steps Involved : Core Sand Preparation Making the Cores Baking the Cores Finishing of Cores Setting the Cores

7. CORE-SAND PREPARATION Core sand ingredients Basic Ingredients of core sand Refractory sand grains Binders Waters binders are of the following types (A) Organic binders (B) Inorganic binders (C) Other binders

8. MAKING THE CORES Small cores can be made manually in hand rammed core boxes. Cores on mass scale are rapidly produced on a variety of core-making machines; to name a few Jolt machine. Core roll over machine (c) Sand Slinger. (d) Core extrusion machine (e) Core blower (f) Shell core machine

9. Hand Making of Cores Small sized cores for limited production are made manually in hand-filled core boxes. Steps Involved: 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

10. Core Boxes A core box is basically a pattern for making cores. Core-boxes are employed for ramming cores in them. Core-boxes impart the desired shape to the core-sand. Core-boxes range from simple wooden structures to precision metal assemblies which possess long life under exacting conditions.

11. Core prints Core print are extra projection provided on the pattern Core print form core seats in the mold Core seats supports the core against the buoyancy of molten metal Venting Cores surrounded by molten metals are vented to help escape mold gases Venting done by piercing metal wires and pipes.

12. Baking the Cores Once the cores have been prepared, manually or on a machine they are placed on core plates and sent for baking. After the green sand cores are adequately supported on the core plate or core drier, they are sent to ovens for BAKING. Core-baking develops the properties of the organic binders. Core-baking drives off moisture from the cores, oxidizes the oil and polymerizes the binder. Cores are baked at up to 650°F. At 212°F, the moisture from the cores is driven off. At 400 to 500°F or so, core oil and other organic binders change chemically and molecularly from liquid to solid by oxygen absorption and polymerization.

13. Equipment used for the baking of cores Core-baking equipment may be categorized as follows: (A) Core Ovens 1. Batch type 2. Continuous type (B) Dielectric bakers (C) Radiant bakers (A) CORE OVENS Core ovens are available for baking cores of various types and sizes. Core-ovens are heated by gas or oil and they operate within a specified temperature range

14. Continuous type In continuous type ovens, cores move slowly on a convey The baking time is controlled by the oven temperature and the speed of the conveyor The cores are being loaded at one end, then they enter the oven and are unloaded at the other end. 1. Batch Type They bake cores in batches Batch type core ovens can be further classified into two types (a) Drawer type - Cores are placed in the sliding drawers . It can bake small size cores (b) Rack type - Cores are placed on the racks and transferred to the oven . Big size cores can also be accommodated in these ovens

15. FINISHING OF CORES Core finishing consists of (a) Cleaning (b) sizing (c) core assembly. Cleaning : Cleaning involves 1. Trimming: Trimming means removing fins and other sand projections from the cores by rubbing them with file or an abrasive tool. 2. Brushing: Brushing removes loose sand from the cores. 3. Coating: (i) Coatings produce smooth core surfaces and make cores resistant to molten metal penetration. (ii) A fine refractory coating may be applied by dipping, swabbing or spraying. (iii) Core coating materials are finely ground graphite, silica and zircon flour. (iv) Coating materials are held in suspension in water by a suitable emulsifier. (v) A torch or burner may be used to dry the coated core surfaces

16. (b) Sizing (i) Sizing means making a core dimensionally accurate. (ii) Cores can be corrected to size by grinding, filing or scraping. (iii) Initially while making, cores can be given a sizing allowance , cores may be made a little oversized. This little excess material will be consequently removed during sizing. (c) Core-assembly Core assembling means joining together by pasting , leading or bolting two or more component parts of the core before the core can be set in the mold. A Core paste may be formed out of talc, dextrin, molasses flour , water etc . Leading is the process of joining core parts with molten lead . Lead on freezing forms a strong joint. Leading is preferred for making small core assemblies Large core parts are assembled by nuts and bolts. Nuts and bolts are of course covered with mud after joining the core parts

17. Setting the Cores Core-setting means placing cores in the mold. In order to obtain correct cavities in the castings, the core should be accurately positioned in the molds Cores in the molds should be firmly secured so that they can withstand the buoyancy effect of the being poured molten metal. Small cores are set in the molds by hand whereas big cores may require a crane for the purpose . Location of Cores : Surfaces in the mold cavity can be used for locating and positioning of the cores. Core-ends are supported or balanced in the core seats formed by Core prints provided in the patterns.

18. Core Shifting and Chaplets : (i)If the core gets shifted from its position in the mold, it will result In a displaced cavity and hence a defective casting. (ii)A core must be firmly supported in the core seat especially to Overcome vertical movement of the core due to the buoyant forces exerted On core by the poured molten metal (iii)Chaplets serve to support the cores. (iv)Chaplets are metal shapes which are positioned between mold and core surfaces. As soon as the molten metal is poured, chaplet melt and becomes a part of the casting. (v) Chaplets should be of the same material which is being cast.

19. TYPES OF CORES Cores may be classified according to: (A) The state of core 1. Green sand core 2. Dry sand core (B) The nature of core materials employed 3. Oil bonded cores. 4. Resin bonded cores. 5. Shell cores. 6. Sodium silicate cores.

20. (C) The type of core-hardening process employed 7. CO-process. 8. The hot box process. 9. The cold set process. 10. Fluid or castable sand process. 11. Nishiyama process. 12 Furan No-Bake system .Oil-No-Bake Process. (D) The shape and position of the core 14. Horizontal core 15. Vertical core 16. Hanging or cover core 17. Balanced core 18. Kiss core.

21. Horizontal core: A horizontal core is positioned horizontally in the mold. A horizontal core is supported in core seats at both ends. Vertical Core: A vertical core is named so because it is positioned in the mold cavity with its axis vertical The two ends of a vertical core are supported in core scats in cope and drag respectively. hanging or cover core : A hanging core is supported from above and it hangs vertically in the mold cavity. It is known as hanging core because it hangs A hanging core has no support from bottom

22. Balanced core : A balanced core is one which is supported and balanced from its one end only A balanced core require a long core seats so that the core does not sag or fall into the mold Kiss core: 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 and drag Ram up core : A ram up core is placed in the sand along with the pattern before ramming the mold A ram up core cannot be placed in the mold after the mold has been rammed