Sand moulding process


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This Presentation gives the information of Manufacturing process-1 of Mechanical Engineering course as per VTU Syllabus. Please write to me at: HAREESHANG@GMAIL.COM for suggestions and criticisms or visit : for more info.

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Sand moulding process

  1. 1. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 1
  2. 2. Syllabus • Sand Moulding : – Types of base sand, requirement of base sand, Types of sand moulds. • Sand moulds: – Moulding sand mixture, ingredients (base sand, binder & additives) for different sand mixtures, Method used for sand moulding. • Cores: – – – – – Definition, Need, Types. Method of making cores, Binders used. Concept of Gating & Risering, Principle involved and types. Fettling and cleaning of castings, Basic steps involved. Casting defects, causes, features and remedies. • Moulding machines : – Jolt type, squeeze type, Jolt & Squeeze type and Sand Slinger. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 2
  3. 3. MOLD MATERIALS • A mold material is one, out of which the mold is made. • A mold material should be such that the mold cavity retains its shape till the molten metal has solidified. • Castings can be made in: – Permanent molds—made of ferrous metals and alloys (steel, Grey C.I. etc.). – Temporary refractory molds — made up of refractory sands and resins. • Permanent molds are normally employed for casting low melting point materials. Permanent molds are too costly. • For the above mentioned reasons, most of the foundry industry has its castings produced using refractory mold materials like Refractory Sands. • As compared to permanent molds, the refractory sand molds can cast high melting point materials and bigger objects, whereas permanent molds produce small castings of better quality and dimensional accuracy. 12/18/13 3 Hareesha N G, Asst. Prof, DSCE, BLore
  4. 4. TYPES OF BASE SAND • The primary and basic material used for preparing moulds is sand, due to its high refractoriness. • Sand usually referred to as 'base sand‘ • Nearly 90 - 95 % of the total moulding sand is occupied by sand and the remaining is binder and additives. • Basic types of base sand are given below – Silica Sand – Chromite sand – Zircon – Olivine sand 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 4
  5. 5. I. Silica Sand • Silica sand is essentially silicon dioxide (Si02) found in nature on the bottoms and banks of rivers, lakes and seashore. • Silica deposits tend to have varying degree of organic and contaminants like limestone, magnesia, soda and potash that must be removed prior to its use, otherwise which affects castings in numerous ways. • Silica sand is available in plenty, less expensive and possess favorable properties. • Thermal expansion leads to certain casting defects; the reason for which not being used in steel foundries. • However, silica sand when mixed with certain additives like wood flour, (corn flour), saw dust etc., defects can be eliminated. • These additives burn by the heat of the molten metal thereby creating voids that can accommodate the sand expansion. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 5
  6. 6. II. Olivine sand • Olivine sand is typically used in non-ferrous foundries. • With its thermal expansion about half of that of silica sand, makes it suitable for production steel castings . • But the high cost restricts its wide use III. Chromite sand • This is African sand with cost being much higher compared to other sands. • Due to its superior thermal characteristics, it is generally used in steel foundries for both mould and core making. IV. Zircon or Zirconium silicate • This sand possesses most stable thermal properties of all the above sands. • The choice for this type of sand arises when very high temperatures are encountered and refractoriness becomes a consideration. • But the major disadvantage is that, zircon has trace elements of Uranium and Thorium which is G, Asst. Prof, DSCE, BLorenature thereby restricting hazardous in 12/18/13 6 Hareesha N its use in foundries.
  7. 7. PROPERTIES OF MOLDING SANDS • The very important characteristic of a molding sand is that it should produce sound castings. • For doing so, the molding sand should possess certain desirable properties and they are: – Flowability – Green Strength – Dry Strength – Hot Strength – Permeability or Porousness – Refractoriness – Adhesiveness – Collapsibility – Fineness – Bench Life – Coefficient of expansion 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore – Durability 7
  8. 8. 1. Flowability • • • Flowability is the ability of the molding sand to get compacted to a uniform density. Flowability assists molding sand to flow and pack all-around the pattern and take up the required shape. Flowability increases as clay and water contents increase. 2. Green Strength • • • It is the strength of the sand in the green or moist state A mold having adequate green strength will retain its shape, Will not distort, Will not collapse, even after the pattern has been removed from the molding box. Green strength helps in making and handling the molds. 3. Dry Strength • • • – – – • It is the strength of the molding sand in the dry condition. A mold may either intentionally be dried or a green sand mold may lose its moisture and get dried while waiting for getting poured or when it comes in contact with molten metal being poured. The sand (of molding cavity) thus dried must have (dry) strength to withstand erosive forces due to molten metal, withstand pressure of molten metal, and retain its shape. There should be an optimum balance between dry strength, and collapsibility of the 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore molding sand. 8
  9. 9. 4. Hot Strength • • It is the strength of the sand (of mold cavity) above 212°F. In the absence of adequate hot strength, the mold may Enlarge, break, erode or, get cracked. 5. Permeability or Porousness • • • • • • The moisture, binders (organic compounds) and additives present in mould sand core produce steam and other gases. Though much of these gases escape through vents and open feeder heads, yet a good amount of the same tends to pass off through the pore spaces of the molding sand. Thus to provide a path for free escape of the gases, the molding sand should be permeable or porous. Sands which are coarse (Bigger in size) or have rounded grains exhibit more permeability. Soft ramming and clay addition in lesser amounts also improves permeability. In the absence of adequate permeability, defects like surface blows, gas holes, mold blast etc. may be experienced. 6. Refractoriness • It is the ability of molding sand to withstand high temperatures (experienced during pouring) without – – – – • fusion, Cracking Buckling experiencing any major physical change. As compared to castings of low melting point alloys, refractoriness is much more 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore essential in the production of high melting point alloy castings (e.g. steel etc.). 9
  10. 10. 7. Collapsibility • Collapsibility is that property of the molding sand which determines the readiness with which the molding sand or mold, – automatically gets collapsed after the casting solidifies, and – breaks down in knock out and cleaning operations. • If the mold or core does not collapse, it may restrict free contraction of the solidifying metal and cause the same to tear or crack. 8. Fineness • • • Finer sand molds resist metal penetration and produce smooth casting surfaces. Fineness and permeability are in conflict with each other and hence they must be balanced for optimum results. Fineness and permeability, both the properties of the molding sand can be maintained by using mold coating on highly permeable mold cavity walls. 9. Bench Life • It is the ability of the molding sand to retain its properties during storage or while standing (i.e., in case of any delay). 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 10
  11. 11. 10. Coefficient of expansion • Molding sands should possess low coefficient of expansion. 11. Durability • • • The molding sand should possess the capacity to withstand repeated cycles of heating and cooling during casting operations. Molding sand should be chemically immune to molten metals. Molding sand should be reusable. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 11
  12. 12. TYPES OF SAND MOULDS • Moulds prepared with sand are called 'sand moulds' or 'temporary moulds', as they are broken for removing the casting. • The different types of sand moulds are: – Green sand mould – Dry sand mould and – No-bake sand mould 1. Green sand mould • The word 'green‘ signifies that the moulding sand is in the moist state at the time of metal pouring. • The main ingredients of green sand are silica sand, clay and moisture (water). • Additives may be added in small amounts to obtain desired properties of mould/casting. • Nearly 60 % of the total castings are prepared from green sand moulds. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 12
  13. 13. Advantages of green sand moulds • • • • Preferred for simple, small and medium size castings. Suitable for mass production Least expensive Sand can be reused many times after reconditioning with clay and moisture Disadvantages • • • • • • • Moulds/cores prepared by this process lack in permeability, strength and stability. They give rise to many defects like porosity, blow holes etc., because of low permeability and lot of steam formation due to their moisture content. Moulds/cores cannot be stored for appreciable length of time. Not suitable for very large size castings. Surface finish and dimensional accuracy of castings produced are not satisfactory. Difficult to cast thin and intricate shapes. Mould erosion which is common in green sand moulds is another disadvantage. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 13
  14. 14. 2. Dry sand mould • The word 'dry' signifies that the mould is dry or free from moisture at the time of metal pouring. • The absence of moisture makes dry sand moulds to overcome most of the disadvantages of green sand moulds. • A dry sand mould is prepared in the same manner as that of green sand mould, i.e., by mixing silica sand, clay and water. • The entire mould/core is dried (baked) in ovens to remove the moisture present in them. • Baking hardens the binder thereby increasing the strength of moulds/cores. • The temperature and duration of baking ranges from 200 - 450°F and from a few minutes to hours respectively depending on the type of metal being poured and size of the casting. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 14
  15. 15. 2. Dry sand mould Advantages • Strength and stability of moulds is high when compared to green sand moulds. • Baking removes moisture and hence, defects related to moisture are eliminated. • Better surface finish and dimensional tolerance of castings. Disadvantages • Consumes more time, labor and cost due to baking process. Hence, not suitable for mass production. • Not suitable for large and heavy size castings, as they are difficult to bake. • Capital cost of bake ovens. • Under baked or over baked moulds/cores is another disadvantage. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 15
  16. 16. 3. No-Bake sand moulds • A no-bake or self-setting sand mould is one that does not require baking. • The main ingredients of no-bake sand are silica sand, binder (resin type), hardener and a catalyst or accelerator (if necessary). • The bonding strength developed in moulds/cores is by means of a self-setting chemical reaction between the binder and the hardener. • In some cases, a catalyst or an accelerator is added to speed up the chemical reaction. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 16
  17. 17. Advantages • • • • • • • 3. No-Bake sand moulds Higher strength - about 50 to 100 times that of green sand moulds. Patterns can be stripped within a few minutes after ramming which is not possible in both green and dry sand moulds. Moulds/cores can be stored for longer periods. Highly simplified moulding. Hence, reduced need for skilled labour. Better dimensional accuracy and stability. Improved casting quality with increased freedom from defects. Surface finish is excellent. In many cases, castings can be used in as-cast condition without machining. Disadvantages • Use of resins and catalysts causes lot of environmental problems both within (i.e., during mixing and pouring) and outside (dumped sand) the foundries. • Resins and catalysts are expensive. • Unsafe to human operators. • Due to high strength and hardness of moulds/cores, sand reuse is a slightly 12/18/13 17 Hareesha N G, Asst. Prof, DSCE, BLore difficult process.
  18. 18. Skin-dried molds • • • • • Sands used for making skin dried molds contain certain binders like linseed oil which harden when heated. The mold is made with the molding sand in the green condition and then the skin of the mold cavity is dried with the help of gas torches or radiant heating lamps. Unlike dry mold, a skin dried mold is dried only up to a depth varying from 6 mm to 25 mm. A skin-dried mold possesses strength and other characteristics in between green and dry sand molds. If a skin-dried mold is not poured immediately after drying, moisture from green backing sand may penetrate the dried skin and make the same ineffective. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 18
  19. 19. MOULDING SAND MIXTURE- INGREDIENS FOR DIFFERENT SAND MIXTURES • A moulding sand is a mixture of base sand, binder and additives. • ingredients of – green sand – no-bake sand mixture – dry sand mixture 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 19
  20. 20. • Ingredients for Green Sand Mixture Green sand mixture is composed of base sand, binder, moisture and additives. • Base sand – Silica sand is used as the base sand. – It possesses favorable properties, inexpensive and can be reused many number of times. – The amount of silica sand added may vary from 85 - 92 % depending on the requirements. • Binder – Bentonite (clay binder) is the widely used binder for bonding sand particles. – It is activated in the presence of water. – A best bond between the sand particles can be obtained with Bentonite varying from 6 - 12 % and water 3 - 5 %. • Additives – Additives are added in small quantities to develop certain new properties, or to enhance the existing properties of moulding sand. 12/18/13 20 Hareesha N G, Asst. Prof, DSCE, BLore – Sea coal, silica flour, wood flour and iron oxide are a few commonly used
  21. 21. • Ingredients for No-bake most widelymixture in sand used binder system Ingredients of 'alkyd binder system' which is one of the Indian foundries is discussed below. • Base sand – Silica sand is used as the base sand. • Binder – The alkyd binder system consists of three parts: Part A (binder), Part B (hardener) and Part C (catalyst). • Part A (Binder): – The binder is an alkyd resin which is obtained by reacting linseed oil with a polybasic acid like isopthalic and solvents like turpentine, kerosene or mineral spirit to improve flowability. – Its addition ranges from 2 - 5 % based on weight of sand. • Part B (Hardener): – The hardener is a reacted product between cobalt/lead salts and napthanic acid. – Its addition ranges from 5 - 10 % based on weight of binder. • Part C (Catalyst): – Methylene-diphenyl-Di-isocyanate commonly known as MDI is used as catalyst to speed up the chemical reaction. 12/18/13 21 Hareesha N G, Asst. Prof, DSCE, BLore – Its addition ranges from 20- 25 % based on weight of binder.
  22. 22. Ingredients for Dry Sand Mixture • Ingredients for dry sand mixture is similar to that of green sand. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 22
  23. 23. Loam sand ingredients • • • • • Loam sand contains much more clay as compared to ordinary molding sand. The clay content is of the order of 50% or so. The ingredients of loam sand may be fine sands, finely ground refractories, clays, graphite and fibrous reinforcement. A typical loam sand mixture contains silica sand 20 volumes, clay 5 vols, and moisture 20%. Molds for casting large bells etc., are made up of brick framework and lined with loam sand and dried. Sweep or skeleton patterns may be used for loam molding. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 23
  24. 24. • MOLDING METHODS Various molding methods are: – – – – Bench molding Floor molding Pit molding Machine molding a) Bench molding • Molding is carried out on a bench of convenient height. • Small and light molds are prepared on benches. • The molder makes the mold while standing. • Both green and dry sand molds can be made by bench molding, • Molds, both for ferrous and (especially) non-ferrous castings are made on bench molds. • Both cope and drag are rammed on the bench. b) Floor molding • Molding work is carried out on foundry floor when mold size is large and molding cannot be carried out on a bench. • Medium and large-sized castings are made by floor molding. • The mold has its drag portion in the floor and cope portion may be rammed in a flask and inverted on the drag. 24 Hareesha N G, Asst. Prof, DSCE, BLore • 12/18/13 Both green and dry sand moulds can be made by floor molding
  25. 25. c) Pit molding • • • • • • • • • • • • • Very big castings which cannot be made in flasks are molded in pits dug on the floor. Very large jobs can be handled and cast easily through pit molding. The mold has its drag part in the pit and a separate cope is rammed and used above the (pit) drag. The depth of the drag in pit molding is much more than that in floor molding. In pit molding, the molder may enter the drag and prepare it. A pit is of square or rectangular shape. The sides of the (pit) drag are lined with brick and the bottom is covered with molding sand . The cope (a separate flask) is rammed over the pit (drag) with pattern in position. Gates, runner, pouring basin, sprue etc. are made in the cope. The mold is dried by means of a stove(heater) placed in the pit. Cope and drag are then assembled. A crane may be used for lifting and positioning the cope over drag. Cope can be clamped in position. 12/18/13 25 Hareesha N G, Asst. Prof, DSCE, BLore Mold is ready for being poured.
  26. 26. d) Machine molding • • • • • • • In bench, floor and pit molding, the different molding operations are carried out manually by the hands of the molder, whereas in machine molding, various molding operations like sand ramming, rolling the mold over, withdrawing the pattern etc. are done by machines. Machines perform these operations much faster, more efficiently and in a much better way. Molding machines produce identical and consistent castings. Molding machines produce castings of better quality and at lower costs. Molding machines are preferred for mass production of the castings whereas hand molding (bench, pit and floor) is used for limited production. Machine molding is not a fully automatic process; many operations can though be performed by machines, yet some others have to be carried out by hands. A few different types of molding machines are listed below: – – – – Jolt machine Squeeze machine Jolt-squeeze machine Sand Slinger 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 26
  27. 27. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 27
  28. 28. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 28
  29. 29. TYPES OF CORES Cores may be classified according to A. The state or condition of core 1. Green sand core 2. Dry sand core 3. No bake sand core B. The nature of core materials employed 1. Oil bonded cores 2. Resin bonded cores 3. Shell cores 4. Sodium silicate cores C. The type of core hardening process employed 1. C02 process 2. 3. 4. 5. 6. The hot box process The cold set process Fluid or castable sand process Furan-No-Bake system Oil-No-Bake process 12/18/13 D. The shape and position of the core 1. Horizontal core 2. Vertical core 3. Hanging or cover core 4. Balanced core 5. Drop core or stop off core 6. Ram up core 7. Kiss core. Hareesha N G, Asst. Prof, DSCE, BLore 29
  30. 30. 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 G, Asst. Prof, DSCE, to bring about a chemical reaction 30 and catalysts BLore 12/18/13 Hareesha N with the binder due to which bonding of sand grains takes place.
  31. 31. 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 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 31
  32. 32. 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 12/18/13 32 Hareesha box, it chemically hardens after a N G, Asst. intervalBLore time. short Prof, DSCE, of
  33. 33. 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. 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. 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. • 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 33
  34. 34. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 34
  35. 35. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 35
  36. 36. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 36
  37. 37. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 37
  38. 38. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 38
  39. 39. Method of making the cores Core Making (Preparation) Procedure Steps involved: 1. Core Sand Preparation 2. Making the Cores 3. Baking the Cores. 4. Finishing of Cores. 5. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 39
  40. 40. 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 40
  41. 41. 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 A. Organic binders B. Inorganic binders C. Other binders. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 41
  42. 42. A. 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 N G, Asst. Prof, DSCE, BLore as a by-product of paper 42 pulp process i.e., 12/18/13 Hareesha making.
  43. 43. 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 1st 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. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 43
  44. 44. PRINCIPLES OF GATING SYSTEM • The term gating system refers to all passageways through which the molten metal passes to enter the mold cavity. • The gating system is composed of – – – – – Pouring cups and basins Sprue Runner Gates Risers. • Fig. shows the various components of the gating system. • Since the way in which liquid metal enters the mold has a decided influence upon the quality and soundness of a casting, the different passages for the molten metal are carefully designed and produced. • A gating system should avoid sudden or right angle changes in direction. • Sudden change in direction causes mold erosion, turbulence and gas pick-up. • If possible the gating system should form a part of the pattern 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 44
  45. 45. REQUIREMENTS, PURPOSES OR FUNCTIONS OF THE GATING SYSTEM • A Gating system should, – fill the mold cavity completely before freezing; – introduce the liquid metal into the mold cavity with low velocity and little turbulence, so that mold erosion, metal oxidation and gas pickup is prevented; – incorporate traps for the separation of non metallic inclusions which are either introduced with the molten metal or are dislodged in the gating system; – regulate the rate at which liquid metal enters into the mold; – be practicable and economical to make and; – consume least metal. In other words, the metal solidified in sprue, runner, gates and risers should be minimum because gates, risers etc., are removed from the final casting; the gating system should provide for the maximum yield. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 45
  46. 46. Characteristics GATES • A gate is a channel which connects runner with the mold cavity and through which molten metal flows to fill the mold cavity. • A gate should feed liquid metal to the casting at a rate consistent with the rate of solidification. • The size of the gate depends upon the rate of solidification. • A small gate is used for a casting which solidifies slowly and vice-versa. • More than one gates may be used to feed a fast freezing casting. • A gate should not have sharp edges as they (i.e., edges) may break during pouring and (sand pieces) thus be carried with the molten metal into the mold cavity.. • Moreover, sharp edges may cause localized delay in freezing, thus resulting in the formation of voids and inclusions in the cast objects. • A gate may be built as a part of the pattern or it may be cut in the mold with the help of a gate cutter. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 46
  47. 47. Types of Gates • The major types of gates are, 1. Top Gate 2. Bottom gate 3. Parting line side gate 1. Top gate • Fig. shows a few types of top gates. • A top gate is sometimes also called as Drop gate because the molten metal just drops on the sand in the bottom of the mold • In top gate, a stream of liquid metal impinges against the bottom of mold cavity until a pool is formed and this is kept in a state of agitation until the mold is filled. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 47
  48. 48. Advantages of Top Gating • Simplicity for moulding. • Low consumption of additional metal. • Generation of favourable temperature gradients to enable directional solidification from the casting towards the gate which serves as riser too. Disadvantages of Top Gating • The dropping liquid metal stream erodes the mold surface. • Dropping metal does cutting action, lifts portions of the surface and causes scab (Skin). • Splashing of molten metal associated with the liquid metal stream increase chances of oxidation. • There is lot of turbulence and pick-up of air and other gases. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 48
  49. 49. 2. Bottom Gate • Fig. shows a few types of bottom gates. • A bottom gate is made in the drag portion of the mold. • In a bottom gate, liquid metal fills rapidly the bottom portion of the mold cavity and rises steadily and gently up the mold walls. • Types of bottom gate are – Simple bottom gate – A horn gate Simple bottom gate 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore A horn gate 49
  50. 50. Advantages of Bottom Gating • • • There is no scouring(Rubbing) and splashing in the bottom gate. As compared to top gate, a bottom gate involves little turbulence and metal erosion. Bottom gate produces good casting surfaces. Disadvantages of Bottom Gating • • • • In bottom gates, liquid metal enters the mold cavity at the bottom. If freezing takes place at the bottom, it could choke off the metal flow before the mold is full. A bottom gate creates an unfavorable temperature gradient and makes it difficult to achieve directional solidification especially when the bottom gate has a riser at the top of the casting. A bottom gate involves greater complexity of molding. The liquid metal cools as it rises the mold walls and results in cold metal and cold mold near the (top) riser and hot metal and hot mold near the gate. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 50
  51. 51. 3. Parting line side gate • • • • • • In parting line gates, the liquid metal enters the mold cavity from the side of the mold (cavity) at the parting line separating cope and drag at or the level of mold joint. Examples of parting line gates are shown in Fig. Parting line gate can be made by the pattern itself or it can be cut afterwards. As regards fluid flow, parting line gates stand in between top and bottom gates. A parting line gate has sprue formed in the (sand of the) cope. A recess is generally provided at the base of the sprue to avoid the cutting of sand at this place. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 51
  52. 52. Advantages of Parting Line Gating • Parting line gates are simple to construct. • Parting line gates are very fast to make. • Parting line gates produce very satisfactory results when drag is not very deep. • Parting line gating makes best compromise between molding convenience and the ideal gating arrangement. Disadvantages of Parting Line Gating • In case the parting line is not near the bottom of the mold cavity or the drag portion is deep, some turbulence will occur as the liquid metal falls into the mold cavity. • Cascading (Spilling) of molten metal from a height in the mold cavity will cause erosion or washing of the mold. • Cascading in non-ferrous metals will promote air pickup by the liquid metal and thus result in an inferior casting. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 52
  53. 53. PRINCIPLES OF RISERING Definition • • • • A riser or a feeder head is a passage of sand made in the cope (mold) during ramming the cope. The molten metal rises in the feeder head after the mold cavity is filled up. This metal in the feeder head (or riser) compensates the shrinkage as the casting solidifies. Fig. shows a riser. Functions of Riser • • • • • • Metals and their alloys shrink as they cool and solidify. It creates a partial vacuum within the casting. Partial vacuum leads to a shrinkage void. This shrinkage void will grow and form shrinkage cavity if extra liquid metal from outside the mold (cavity) is not supplied. The primary function of the riser (attached with the mold) is to feed metal to the solidifying casting so that shrinkage cavities are get rid of. A riser permits the escape of air and mold gases as the mold cavity is being filled with the molten metal. A riser full of molten metal indicates that the mold cavity has already been completely filled up with the same. A casting solidifying under the liquid metal pressure of the riser is comparatively sound. Risers promote directional solidification. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 53
  54. 54. TYPES OF RISERS 1. 2. Open Riser Blind Riser 1. Open Riser • • It is shown in Fig (top riser). Open Riser The top of the open riser is open i.e., exposed to atmosphere. • The liquid metal in the riser is fed to the solidifying casting under force of gravity and atmospheric pressure till the top surface of the riser solidifies and thereafter gravity is only the feeding force. • An open riser is connected either at the top of cope, or on the side at the parting line. • An open riser is generally cylindrical. Advantages • An open riser is easy to mold as compared to a blind riser. • An open riser is open to atmosphere, thus it ensures that unlike a blind riser it will not draw metal from the casting as a result of partial vacuum in the riser. Limitations • Open riser is not placed in the drag. • Open riser is generally larger than a comparable blind riser. • An open riser is more difficult to remove from the casting as compared to blind riser. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 54
  55. 55. 2. Blind Riser • A blind riser is closed at its top; however, a vent or permeable core at the top of the riser may be provided to have some exposure to the atmosphere otherwise the vacuum created between the top of the riser and the liquid metal level in the riser may not permit proper feeding of liquid metal from riser to the casting. Gravity is the only Blind Riser feeding force. • A blind riser is connected, either at the top of cope, or on the side of the casting at the parting line or in the drag. • A blind riser is a rounded cavity and it represents the minimum practical ratio of surface area to volume and thus associates a slow cooling rate and is more efficient. Advantages • • • • Blind risers can be placed at any position in the mold. A blind riser is smaller than a comparable open riser. A blind riser can be removed more easily from a casting. Blind risering promotes directional solidification better than the open risering. Limitations • • It is difficult to mold a blind riser. A blind riser may draw liquid metal from the solidifying casting. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 55
  56. 56. FETTLING & CLEANING OF CASTINGS • • Fettling is the name given to cover all those operations which help giving the casting a good appearance after the same has been shaken out of the sand mold. Fettling includes 1. Removal of cores from the casting. 2. Removal of adhering sand and oxide scale from the casting surface (surface cleaning). 3. Removal of gates, risers, runners etc. from the casting. 4. Removal of fins, and other unwanted projections from the castings. 1. Removal of Cores • It may be difficult to remove dry sand and hardened cores in the absence of suitable equipment. • Hammering or vibrations imparted to cores does loosen and break them up. • Sand portions sticking inside the castings are removed by poking action using a metal rod. • Cores from larger castings may be removed effectively by pneumatic rapping and hydro blasting. 12/18/13 56 Hareesha N G, Asst. Prof, DSCE, BLore .
  57. 57. 2. Cleaning of Casting Surfaces • • • • • • The outside and inside surfaces of castings are cleaned of adhering refractory (sand) particles and oxide scale and they (i.e., surfaces) look smooth and pleasing. The extent of surface cleaning required depends upon the metal/ alloy of the casting and size of the casting. Steel castings (because of their high melting and pouring temperatures and consequent burning of the sand in contact with the molten metal) require considerable more cleaning than those of iron and brass. Aluminium castings are virtually free from burned-on sand. Since heavy castings suffer more than light castings from the burning-on of sand, their cleaning is more difficult. Sand may be removed from the surfaces of castings using hand methods or mechanical equipment 3. Removal of gates and risers • • • Numerous methods are available for removing feeding and gating systems. The choice of a particular method depends upon the type of metal/alloy, — size of the casting, — size of runners, gates and risers. A few commonly used methods are given below: 1. Chipping hammers 2. Flogging (knocking off). 3. Shearing. 4. Sawing 5. Abrasive wheel slitting 6. Machining. 12/18/13 7. Flame cutting. Hareesha N G, Asst. Prof, DSCE, BLore 57
  58. 58. 4. REMOVAL OF FINS AND OTHER UNWANTED PROJECTIONS FROM CASTINGS • Castings are trimmed to remove fins, chaplets, wires, parting line and the stumps of feeder heads and ingates. All these unwanted projections are dressed flush with the surface. • The methods employed to remove unwanted projections from the castings are – – – – – – – – Chipping Sawing Flame cutting Flame gouging and flame scarfing. Grinding Abrasive belt machining Rotary tools cutting Trimming and sizing. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 58
  59. 59. CASTING DEFECTS • Casting process involves a number of variables and a loss of control in any of these variables can cause defects under certain circumstances. • Some of the common casting defects, their features and remedies to prevent such defects are discussed below. 1. Shrinkage defect • Shrinkage is a void on the surface of the castings resulting from concentrated contraction or shrinkage of metal during solidification.Refer figure. • Although a riser is used to over come the shrinkage effect, in some cases it fails to feed the molten metal efficiently to the casting as it solidifies. Remedies •Use large sprue and riser to promote directional solidification. •Locate risers and gating systems in correct positions. •Gates to be cut as wide as possible. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 59
  60. 60. 2. Porosity defect (Blow hole and Pin hole) • Molten metal absorb gases from various sources such as fluxes, moisture in sand, binders, additives and normal atmospheric gases like oxygen and nitrogen. • If these gases are not allowed to escape, they get entrapped in the mould cavity forming small balloon shaped voids or cavities leading to porosity defect in castings. • Two types of gas related defects occur in castings. They are: – blow hole – pin hole defect. • Blow holes occur below the surface of the castings and are not visible from the outside surface. • Pin holes are small gas cavities, many in number at or slightly below the surface of the casting. Remedies •Avoid excess ramming of mould. •Provide proper vent holes. •Avoid use of excess carbonaceous or other organic material in the sand/core binders, because these materials react with the molten metal producing large amount Hareesha N G, Asst. Prof, DSCE, BLore of gases. 12/18/13 60
  61. 61. 3. Misrun • Misrun occur when the mould cavity is not completely filled with molten metal. • it is a defect wherein a casting solidifies before the molten metal completely fills the cavity. Remedies • Fluidity of metal should be high. • Pouring rate and time should be controlled. • Thin sections should be suitable designed. 4. Penetration When fluidity of liquid metal is high, it may penetrate into the sand mould/core (into the voids between the sand particles) causing a fused aggregate of metal and sand on the surface of the casting leading to defect. Remedies • Sand should be properly rammed. .. . • Moulding sand/core sand should not be too coarse to promote metal penetration. 12/18/13 Hareesha N • Control proper metal temperature. G, Asst. Prof, DSCE, BLore 61
  62. 62. 5. Mould shift • It is a step in the cast product at the parting line caused by sidewise relative displacement of cope and drag box. Remedies • Proper alignment of cope and drag box. • Proper handling of assembled cope and drag box during operations. 6. Cold shut • Two portions of metal flow together, but lack of fusion due to premature freezing results in a defect known as cold shut. Remedies • Place gates and risers at proper locations. • Metal fluidity should be high. 12/18/13 Mould shift Hareesha N G, Asst. Prof, DSCE, BLore Cold shut 62
  63. 63. 7. Hot tears • A hot tear is an internal or external ragged discontinuity formed in the casting due to the pulling action of the metal just after it has solidified . Remedies • Provide adequate fillets at sharp corners. • Proper metallurgical and pouring temperature. • Place gates and risers at proper locations. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 63
  64. 64. MOULDING MACHINES • • • When large number of castings is to be produced, hand moulding consumes more time, labour and also accuracy and uniformity in moulding varies. To overcome this difficulty, machines are used for moulding. Based on the methods of ramming, moulding machines are classified as follows: 1. 2. 3. 4. Jolt machine Squeeze machine Jolt-squeeze machine Sand slinger 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 64
  65. 65. 1. Jolt Machine • A jolt machine consists of a flat table mounted on a piston-cylinder arrangement and can be raised or lowered by means of compressed air. • In operation, the mould box with the pattern and sand is placed on the table. The table is raised to a short distance and then dropped down under the influence of gravity against a solid bed plate. The action of raising and dropping (lowering) is called 'Jolting'. • Jolting causes the sand particles to get packed tightly above and around the pattern. The number of 'jolts' may vary depending on the size and hardness of the mould required. Usually, less than 20 jolts are sufficient for a good moulding. • The disadvantage of this type is that, the density and hardness of the rammed sand at the top of the mould box is less when compared to its bottom portions. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 65
  66. 66. 2. Squeeze Machine • • • • • • In squeeze machine, the mould box with pattern and sand in it is placed on a fixed table as shown in figure A flat plate or a rubber diaphragm is brought in contact with the upper surface of the loose sand and pressure is applied by a pneumatically operated piston. The squeezing action of the plate causes the sand particles to get packed tightly above and around the pattern. Squeezing is continued until the mould attains the desired density. In some machines, the squeeze plate may be stationary with the mould box moving upward. The disadvantage of squeeze machine is that, the density and hardness of the rammed sand at the bottom of the mould box is less when compared to its top portions. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 66
  67. 67. 3. Jolt Squeeze Machine • Jolt squeeze machine combines the operating principles of 'jolt' and 'squeeze' machines resulting in uniform ramming of the sand in all portions of the moulds • The machine makes use of a match plate pattern placed between the cope and the drag box. • The whole assembly is placed on the table with the drag box on it. • The table is actuated by two pistons in air cylinders, one inside the other. One piston called 'Jolt piston' raises and drops the table repeatedly for a predetermined number of times, while the other piston called 'squeeze piston' pushes the table upward to squeeze the sand in the flask against the squeeze plate. In operation, sand is filled in the drag box and jolted repeatedly by operating the jolt piston. • After jolting, the complete mould assembly is rolled over by hand. • The cope is now filled with sand and by operating the squeeze piston, the mould assembly is raised against the squeeze plate. By the end of this operation, the sand in the mould box is uniformly packed. • The match plate is now vibrated and removed. The mould is finished and made ready for 12/18/13 67 Hareesha N G, Asst. Prof, DSCE, BLore pouring.
  68. 68. 4. Sand slinger • A sand slinger is an automatic machine equipped with a unit that throws sand rapidly and with great force into the mould box. Figure shows a sand slinger. Sand slinger consists of a rigid base, sand bin, bucket elevator, belt conveyor, ramming head (sand impeller) and a swinging arm. • In operation, the pre-mixed sand mixture from the sand bin is picked by the bucket elevator and is dropped on to the belt conveyor. • The conveyor carries the sand to the ramming head, inside which there is a rotating impeller having cup shaped blades rotating at high speeds (around 1800 rpm). 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 68
  69. 69. • • • The force of the rotor blades imparts velocity to the sand particles and as a result the sand is thrown with very high velocity into the mould box thereby filling and ramming the sand at the same time. The density of the ramming sand can be controlled by varying the speed of the impeller. Rest of the operations, viz., removal of pattern, cutting gates etc., are done manually. In the initial stages of ramming, the blades are rotated at slow speeds; around 1000 - 1200 rpm to avoid damage to the pattern due to the abrasive action of the high velocity sand particles. 12/18/13 Hareesha N G, Asst. Prof, DSCE, BLore 69