MOULDING & CORE SAND

Transcription

1 MOULDING & CORE SAND MOULDING SAND The principal material used in thefoundry shop formoulding isthe sand. This is because it possesses the properties vital for foundry purposes. Sands are obtained from places whichwerebottomsandbanksofriversandsanddunes. The principal ingredients of the moulding sand are Silica sand grains Clay Moisture Miscellaneous materials Silica in the form of granular quartz is the chief constituent of the moulding sand. Silica sand contains 80 to 90 % silicon dioxide and is characterized by a high softening temperature and thermal stability. The silica sand grains impart refractoriness, chemical resistivity and permeability to the sand and are specified according to their average size and shape. Clay is defined as those particles of sand(< φ 20 μm) that fail to settle at a rate of 25 mm/min, when suspended in water. Clay consists of two ingredients namely Finesilt:itisasortofforeignmatterandhasnobonding power. True clay: it is found to be made up of extremely minute aggregates of crystalline, usually flake shaped, particles called clay minerals. Moulding sands for different gradesofworkcontain5to20%clay. 1

2 Moisture furnishes the bonding action of the clay. When water is added to clay, it penetrates the mixture and forms a microfilm which coats the surface of the clay particles. The bonding quality of clay is dependent upon the maximum thickness of the water film it can maintain. Miscellaneous materials that are fond in addition to silica and clay are oxides of iron, limestone, magnesia, soda and potash. The impurities should be below 2%. The moulding sands may be classified into three types Natural moulding sand: They are also called as green sand. These are taken from river beds or dug from pits. They possess appreciable amount of clay. Due to their ease of availability, low cost and high flexibility of operation, this sand is used for most of the ferrous and non ferrous light castings. Syntheticsand:Theyarealsocalledashighsilicasand.Theycontainlessthan2%or no clay in natural form. They occur as loose or poorly consolidated deposits of sedimentary origin. The desired strength and bonding properties are developed by adding bentonite, water and other materials. Special sand: They are ideal in getting special characteristics. Zircon, olivine, chamotte, chromite, and chrome-magnesite are often used as special sands. Zircon sands are suitable for cores of brass and bronze castings. Chamotte is valuable for heavy steel castings. Chromite sands are useful in increasing chilling tendency to control solidification. PROPERTIES OF MOULDING SAND Proper moulding sand must possess six properties. The properties are determined not only by the chemical composition, but also by the amount of clayey matter in the sand, byitsmoisturecontent andbytheshapeandsizeofthesilicasandgrains. Porosity: Molten metals contain a certain amount of dissolved gases which are evolved when the metal freezes. Further the molten metal produces steam or water vapor when it contacts moist sand. The sand must be sufficiently porous to allow the gases or moisture present or generated to be freely removed when the moulds are poured. This property of sand is called porosity or permeability. Flowability: Flowability of moulding sand refers to the ability to behave like a fluid so thatwhenrammed,itflowstoallportionsofamouldandpackallroundthepatternand take up required shape. Flowability increases as the clay and water content increases. Collapsibility: After the molten metal in the mould gets solidified, the sand must be collapsible so that free contraction of the metal occurs. This would avoid the tearing or cracking of the contracting metal. Adhesiveness: The sand particles must be capable of adhering to another body i.e. they shouldclingtothesidesofthemouldingboxes.itisduethispropertythatthesandmass can be successfully held in the moulding box and does not fall out of box when it is removed. 2

3 Cohesiveness or strength: This is the ability of the sand particles to stick together. The strength of the moulding sand must be sufficient to permit the mould to be formed to thedesired shapeandtoretainthisshapeevenwhenhotmetalispouredinthemould. The property of the sand in its moist state is called green strength. A mould having adequate green strength will retain its shape and will not distort or collapse even after the pattern is removed from the moulding box. The strength of sand that has been dried or baked is called dry strength. It must have then the strength to withstand the erosive forces due to molten metal and retain the shape. Refractoriness: The sand must be capable of withstanding the high temperature of the molten metal without fusing. Moulding sand with poor refractoriness may burn on to the casting. Refractoriness is measured by the sinter point of the sand rather than its melting point. CORE SAND & CORE BINDERS The ingredients of core sands are sand and binders. Core sands are usually silica but zircon, olivine, carbon and chamotte sands are also used. Sandsthatcontainmorethan5% claycannotbeusedforcores.excessiveclayreducesnot only reduces permeability but also collapsibility. The type of sand to be used depends upon the size of the core and the pouring temperature of the metal. The important parameters are size, shape and distribution of the sand grains, the clay content and the mineralogical composition of the clay particles. The smaller grain size of the sand will result in smoother surface of the casting made in contact with these cores. Sand with rounded grains are more satisfactory for cores rather than angular grains because of higher values of permeability. The core sand has no natural bond. Hence some other materials are added to the sand to act as binders which cement the sand particles together before and after the cores are baked. They impart strength and sufficient collapsibility along with good permeability, ample refractoriness. Core binders are generally of three types namely Binders that harden at room temperature Binders that require baking to harden Clays 3

4 Various commercial binders available in the market consist mainly of oils, cereals, dextrine, resins, sulphite-liquor, molasses and proteins. The action of thermoplasting binders such as rosin and pitch depends on the amount of heat which liquefies and disperses the binder in the sand. Rosin is a form of resin and is obtained by distillation and extraction from pinewood. Thermosetting resins are widely used due to high strength, low gas formation, collapsibility and resistance to moisture absorption. The resin binders usually used are of phenol, urea and furan Core oils are more popular as they are very economical and produce better cores. The main components are vegetable oil (linseed oil or corn oil). Specially processed mineral oils are also added to achieve special properties. They are mainly used in making oil sands for the cores. Oil sands can be used for almost any sand casting application. The typical formulation(by weight) is sand (95.8%), cereal flour (1.01%), core oil (1.17%), water (1.86%) and bentonite (0.16%). This mixture is satisfactory for the casting of gray iron and several other metals. Oil sand are very popular because the oil sand core is more collapsible than clay bonded core known as loam core. The baked cores are very hard and not easily damaged. The green and dry strengths of oil sand mixture can be controlled by variation in the proportions of oil and dextrine respectively. TYPES OF CORES The cores used in foundry are typed according to their shape and their position in the mould.thecommontypeofthecoresare Horizontal cores: It is the most common type of core used. The core is usually cylindrical informandislaidhorizontallyatthepartinglineofthemould. Vertical cores: This is placed in a vertical position both in cope and drag halves of the mould. Usually top and bottom of the core are provided with a taper, but the amount of taperonthetopishigher. Balanced cores: When the casting is to have an opening on only one side and only one coreprint isavailableon the pattern, a balanced coreisused. The coreprintinsuchcases should be large enough to give proper bearing to the core. If the core is long, it is supported by chaplets. Hanging and cover core: if the core hangs from the cope and does not have any support atthebottomofthedrag,itisreferredasahangingcore.inthiscase,itmaybenecessary to fasten the core with a wire or rod that may extend through the core. On the other hand,ifithasitssupportonthedrag,itiscalledcovercore. Wingcore:Itisusedwhenaholeorrecessisto beobtainedinthecastingeitheraboveor below the parting line. In this case, the side of the core print is given sufficient amount of tapersothatthecorecanbeplacedinthemould. 4

5 Ram-up core:itissometimesnecessary to setacorewiththe pattern beforethemouldis rammed up. Such a core is called ram-upcore. Itis used whenthe core detail is locatedin an inaccessible position in both interior and exterior portions of a casting Kiss core: When the pattern is not provided with a core print and consequently no seat is available for the core, the core is held in position between the cope and drag simply by thepressure ofthecore.thisisreferred asakisscore. Horizontal cores Vertical cores Balanced cores Cover cores Hanging cores Wing cores Ram-up cores Kiss cores SOLIDIFICATION OF CASTING The casting process is essentially solidification of liquid metal in the mould cavity. Such a phase change from liquid to solid state involves the phenomenon of changes in fluidity, volumetric shrinkage, segregation, evolving of gases absorbed and the size of grains which affect the quality of the final casting obtained. When molten metal enters into the mould cavity, its heat is absorbed by and transferred through the mould wall. In case of pure metals and eutectics, the solidification proceeds layer by layer starting from the mould wall and gradually proceeding inwards. The moving layer between the liquid and solid is called the solidification front. Progressive solidification refers to the solidification in a given cross-section of the casting ideally starting from the mould wall and gradually progressing towards the center of the casting. Directional solidification refers to the sequence of solidification of different regions of the castings ideally starting from the thin regions at one end followed by thicker regions and finally ending at the thickest region. As this front solidifies, it contracts in volume, and draws molten metal from the inner part of the casting(opposite to the mould wall). When the solidification front reaches the inner most region or the thickest part of the casting there is no metal left to compensate the shrinkage and as a result a void space called shrinkage cavity is formed. 5

6 DIRECTIONAL SOLIDIFICATION PROGRESSIVE SOLIDIFICATION This phenomenon can be avoided by putting a suitable feeder so that the feeder solidifies notonly later thanthe hot spot but that shouldcontinuously feed the hot spot with proper location and properly designed riser/feeder neck. Most of the casting alloys do not have distinct melting point. Only pure metals and eutectic alloys solidify at a constant temperature. The interface between the liquid and solid is relatively smooth because the metal is freezing at constant temperature. However casting alloys solidify over a range of temperature. In such cases there are three distinct zones during solidification: completely solid, completely liquid and intermediate mushy zone. The mushy zone is caused by the growth of tree like structures called dendrites and the liquid metal being tapped in their branches. Freezing rate is one of the major factors influencing the microstructure of the casting. Alloys with short freezing range behave like pure metals and eutectics and in this case the solidification proceeds layer by layer. The microstructure comprises columnar grains growing along the direction of heat transfer. In case of long freezing range alloys, the solidification is initiated at a large number of points and the grain grows in size until the neighboring grains hinder them. Thus microstructure comprises eqi-axed grains. 6

7 Freezing rate is greatly influenced by the cooling rate and thermal gradients inside the casting. A long freezing range alloy behaves like a short freezing range alloy in a permanent mould like metal mould. During the time, first skin of solid metal is produced, the latent heat of fusion is released and the remaining liquid rapidly loses most of its under cooling. Growth of nucleation is controlled by the rate of heat transfer from the casting and since this establishes a temperature gradient, it is always directed towards the casting surface and consequently grain growth occurs in a direction opposite to the heat flow. Cooling Rate is defined as the heat loss by the casting/unit time. Naturally the cooling rate mainly depends on the mould material and the air gap formed at the metal mould interface. Metal mould produces higher cooling rate than a green sand mould under same operating conditions. Cooling rate is higher at the beginning of the solidification and lower when solidification progresses. The cooling rate is more at the bottom of the casting where metalplatesareusedasbase. Metals having long freezing range tend to develop casting defects like hot tears and cracks. This happens during the stage of solidification when the solid dendrites while contracting is still not strong enough to overcome the hindrance offered by noncollapsible cores and mould. THANK YOU 7