A critical review on sand casting technology

Transcription

1 A critical review on sand casting technology DR. T.R. VIJAYARAM Prof., SMBS VIT University S and casting technology also known as sand molded casting process. It is a metal casting process characterized by using sand as the mold material. Sand casting is a basic manufacturing method involving pouring molten metal into sand mold. Different types of sand are used in sand casting process. The factors determined by the sand are casting surface finish, gas permeability of the mold, and mold strength. Green sand moulding is still the major method of moulding in foundry industry due to its environmental and nancial advantages. Sand compaction plays an important role for the quality of the mould and therefore for the quality of the casting. Molding Sand is relatively cheap and sufficiently refractory even for steel foundry use. A suitable bonding agent, usually clay, is mixed or occurs with the sand. The mixture is moistened with water to develop strength and plasticity of the clay and to make the aggregate suitable for molding. The term "sand casting" can also refer to a casting produced via the sand casting process. Sand castings are produced in specialized factories called foundries. Over 70% of all metal castings are produced via a sand casting process. Designing and fabrication of pattern The pattern is a full size model of the part that makes an impression in the sand mold, with dimensional allocation for shrinkage and finishing. If the casting is hollow, additional patterns called cores are used to create these cavities in the finished product. Patterns are usually made of wood, plastic, metal, or plaster; however, other materials or combinations of materials are used if there are additional specific properties required of the pattern. Every Pattern must have a draft angle of approximately 2-3 to all walls parallel to the parting direction to facilitate removing the part from the mold. Paint the surface of the Pattern with Varnish or Shellac to make it water tight and to ensure that the sand does not stick. Some of the patterns used in green sand molding process are displayed below. SOLID PATTERN SPLIT PATTERN MATCH-PLATE PATTERN COPE AND DRAG PATTERN 42 February 2015

2 Patterns are commonly made larger than the casting because of the shrinkage effect. Shrinkage allowances are usually 1-2%. The pattern surfaces are never made perpendicular to the mold parting surface. The taper of the pattern surface, which provides narrowing the mold cavity towards the mold parting surface, is called draft. Draft allows easy removal of the pattern and the casting from the sand mold. The draft angle is commonly 1-3%. Pattern material Patterns may be constructed from variety of materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins. To be suitable for use, the pattern material should be: 1. Easily worked, shaped and joined 2. Light in weight 3. Strong, hard and durable 4. Resistant to wear and abrasion 5. Resistant to corrosion, and to chemical reactions 6. Dimensionally stable and unaffected by variations in temperature and humidity 7. Available at low cost The usual pattern materials are wood, metal, and plastics. The most commonly used pattern material is wood, since it is readily available and of low weight. Also, it can be easily shaped and is relatively cheap. The main disadvantage of wood is its absorption of moisture, which can cause distortion and dimensional changes. Araldite is the new material for pattern making, which is referring to a range of engineering and structural epoxy, acrylic, and polyurethane adhesives. A parting release agent is applied on the pattern surface in order to provide easy removal of the pattern from the mold. Pattern allowances Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting. Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with the particular specification can be made. The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns and core boxes are as follows: 1. Shrinkage or contraction allowance 2. Draft or taper allowance 3. Machining or finish allowance 4. Distortion or camber allowance 5. Rapping allowance Sand molding Molding is the multi-step process in which molds are created. In horizontal casting, the mold is contained in a two piece frame, called a Flask. The upper portion of the flask is called a Cope and the lower portion is a Drag. First, molding sand is packed into a Flask around the pattern. After the pattern is removed, Gating and Runner arrangements are positioned in the drag half of the mold cavity and the Sprue is placed in the cope portion. Gating systems are necessary for the molten metal to flow into the mold cavity. Cores are also placed in the drag portion of the mold if they are needed. To finish the mold, the Cope section is placed on the Drag section, and the mold is closed and clamped together. Molding sand Two main routes are used for bonding the sand moulds: The "green sand" consists of mixtures of sand, clay: Bentonite and moisture. If the sand can be squeezed together and hold its shape, it is suitable for use. The "dry sand" consists of sand and synthetic binders cured thermally or chemically. The sand cores used for forming the inside shape of hollow parts of the casting are made using dry sand components. The sand is rejuvenated by adding water and mulling: mixing and smashing. Fine sand will give good detail; coarse sand will give a pebbly or rough texture. The following picture is the actual green sand used in foundries. GREEN SAND Green sand is not green in color, but "green" in the sense that it is used in a wet state (akin to green wood). Unlike the name suggests, "green sand" is not a type of sand on its own, but is rather a mixture of: silica sand (SiO2), or chromite sand (FeCr2O), or zircon sand (ZrSiO4), 75 to 85%, or olivine, or staurolite, or graphite, bentonite (clay), 5 to 11%, water, 2 to 4%, inert sludge 3 to 5%, anthracite (0 to 1%) Generally, the following tests are performed to judge the molding and casting characteristics of foundry sands 43 February Moisture content Test Clay content Test Chemical composition of sand Grain shape and surface texture of sand. 5. Grain size distribution of sand 6. Refractoriness of sand 7. Strength Test 8. Permeability Test 9. Flowability Test 10. Shatter index Test 11. Mould hardness Test.

3 Molding sand requirements Ability to retain mold shape during packing and pouring. High temperature stability. Permeability for the gases liberated from the mold and solidifying metal. Collapsibility - ability of the sand to be shake out. Silica sand with additives is used for sand casting. Types of sand Green sand - a mixture of silica sand (quartz) with 4-15% of a clay (bonding agent), about 5% of water and some other additives (iron silicates, zircon, chromites). The green sand is prepared from silica sand, water and a certain quantity of clay (bentonite, kaolin). Green sand is the most popular sand type. Resin bonded sand - a mixture of silica sand with a polymeric resin as the bonding agent. If the resin hardens at room temperature, the mixture is called no bake. Sodium silicate bonded sand - a mixture of silica sand with 3-4% of sodium silicate (water glass, NaO*nSiO2*mH2O). The bonding forms when sodium silicate reacts with CO2, which is applied to the mixture: NaO*nSiO2 + CO2 = Na2CO3 + SiO2. The mixture is called no bake since the binding process does not require heat treatment (baking). Sodium silicate bonded sand is widely used for the preparation of cores. The main disadvantage of sodium silicate bonded sands is their bad collapsibility. Steps involved in Green sand molding process 1. Prepare a Pattern. This must be made of a smooth solid substance to withstand ramming, for example sealed wood or plaster. 2. To prepare the Mold: divide the flask (wooden box, made of 2 parts- (top) Cope and (bottom) Drag). Turn over the Drag Face down on a board. 3. Place the pattern in the upturned Drag. Ensure there is enough space around the pattern for Gating (room for runner and Sprue- pouring hole for the metal) 4. Dust with Parting Agent to prevent it Sand control chart SAND CONTROL CHART USED IN FOUNDRIES sticking. (Parting agent is a hydrophobic material which repels moisture e.g. crushed limestone. 5. Use a fine riddle (large sieve) to just cover the pattern. 6. Use a Paddle to ram the sand, lightly at first to protect the pattern, and then harder to compress the sand. 7. Fill with sand, and ram it until full. Level off and smooth with a trowel. 8. Turn over the compressed Drag and the pattern will be immersed in sand. 9. Put the Cope on the Drag and bolt 44 February 2015 together. 10. Repeat steps 4-7. Take care not to ram the sand too hard as to affect the Drag below. 11. Take Cope off and set it aside. You should be able to see an imprint of your Pattern. Depending on its location, use your hole-cutter and cut a Sprue hole by placing your hand on the back of the Cope and push hole cutter through the other side, twisting slightly. Green sand molding process pictures are shown below.

4 Sand casting process flow chart is shown below. Green sand mold section is shown below. SAND CASTING PROCESS FLOW CHART Ė A typical pattern attached with gating and risering system 45 February 2015

5 SAND CASTING PROCESS DETAILS Sand casting mold A typical mold for a sand casting is shown below. The set of channels through which a molten metal flows to the mold cavity is called gating system. Typical gating system consists of a pouring cup and a sprue receiving the poured melt, runner a channel through which the melt is supplied to the gates through which the molten metal enters the mold cavity. A gating system may include a riser (feed head) a cavity connected to the gating system feeding the casting when it is shrinking. Air within the mold cavity and gases formed when a molten metal contacts the mold surface are removed through the vents. The interior cavities of a casting are formed by a separate inserts called cores. Cores are usually made of sand and backed. A mold frame (flask) consists of two parts: cope (the upper part) and drag (the lower part). A mold cavity is formed in the process of pattern molding, when the pattern (commonly wooden) is embedded in sand in the flask forming an impression of the casting. After the sand packing the pattern is removed from the flask and the cores and the gating system are arranged. Cores, runner and gates are arranged in the drag; pouring cap and sprue are placed in the cope. Then the two parts of the mold are assembled and poured. After the metal has solidified and cooled to a desired temperature, the casting is removed from the mold by the process called shakeout. 46 February 2015

6 SAND MOLD-OPENED SAND MOLD-CLOSED Sand casting is able to make use of almost any alloy. An advantage of sand casting is the ability to cast materials with high melting temperatures, including steel, nickel, and titanium. The four most common materials that are used in sand casting are shown below, along with their melting temperatures. Sand Casting Defects Production of castings involves a large number of steps including casting design, pattern making, moulding, melting, pouring, shake out, fettling, inspection and finishing. It is not uncommon for one or more of these steps to be performed unsatisfactorily due to use of defective material or equipment, carelessness of the operator or lack of skill. Such unsatisfactory operations result in a defective casting which may be rejected at the final stage. Since reclamation of defective castings is often costly and sometimes outright impossible, care should be taken to avoid the occurrence of the defects in the first instance. It is therefore necessary to understand the various defects that occur in sand castings and the main factors that are responsible for their occurrence. Some of the common sand casting defects are mentioned below 1. Open Blows and Blow Holes 2. Pin Hole Porosity 3. Entrapped Air and other gases 4. Cracked Casting 5. Bent or Twisted Casting Sample castings produced by sand casting process 47 February 2015

7 Sand casting is able to make use of almost any alloy. An advantage of sand casting is the ability to cast materials with high melting temperatures, including steel, nickel, and titanium. The four most common materials that are used in sand casting are shown below, along with their melting temperatures. Sand Casting Defects Production of castings involves a large number of steps including casting design, pattern making, moulding, melting, pouring, shake out, fettling, inspection and finishing. It is not uncommon for one or more of these steps to be performed unsatisfactorily due to use of defective material or equipment, carelessness of the operator or lack of skill. Such unsatisfactory operations result in a defective casting which may be rejected at the final stage. Since reclamation of defective castings is often costly and sometimes outright impossible, care should be taken to avoid the occurrence of the defects in the first instance. It is therefore necessary to understand the various defects that occur in sand castings and the main factors that are responsible for their occurrence. Some of the common sand casting defects are mentioned below 1. Open Blows and Blow Holes 2. Pin Hole Porosity 3. Entrapped Air and other gases 4. Cracked Casting 5. Bent or Twisted Casting 6. Dropped Mould 7. Fusion 8. Swell 9. Run out 10. Mismatch 11. Mis-run and Cold Shut 12. Shrinkage-Faults 13. Rat Tail and Buckles 14. Core Shift 15. Inclusions 16. Cuts and Washes 17. Metal penetration 18. Hard Spots 19. Scabs 20. Hot tears Pictures of sand casting defects MISRUN INCLUSIONS SHRINKAGE 48 February 2015

8 The advantages and disadvantages of green sand casting process Advantages 1. Simple production process The production process of green sand casting is very simple comparatively, so easily handled. The materials are simple, and easily available. 2. Lower production costs Since the materials are simple and its prices are lower comparatively, the production costs of green sand casting are lower. Therefore, the green sand castings are cheaper than resin sand castings and shell molding castings. 3. Higher production rate The green sand casting process can achieve high production rate. The floor molding with green sand can produce castings of 100 to 200 sand boxes each day, e.g. about 100 pieces to 2000 pieces each day. The automatic molding process with green sand can produce castings of 20 tons each day. Moreover, green sand casting process can produce the castings with unit weights from hundreds of grams to several tons. Disadvantages The disadvantages of green sand casting process are also inevitable. 1. More casting defects Green sand mold is a kind of soft mold, so it is not hard enough as the resin sand molds and shell molding. So, there are more casting defects such as sand residuals, sand holes, air holes and shrinkages. 2. Rough surface quality The casting surfaces by green sand casting process are very rough and coarse. If the iron foundries use the very fine green sand, the rough surfaces will be better. 3. Bad casting dimensions Since the green sand will have larger shrinkage, the casting dimensions will have larger changes during molding and cooing periods. Therefore, the green sand casting process will cause larger dimensional tolerances. Normally, it could reach casting dimensional tolerances of CT10 to Ct Unstable casting quality The temperature has some influence to the casting quality made by green sand casting process. Therefore, the iron foundries should keep the good temperature in cold winter. Moreover, as the manual floor molding, the worker's skill will have key affects to the casting quality. Miscellaneous sand molding processes like DISA are shown below, which are self explanatory. Conclusions Casting is an age old production technique wherein cavities are formed by a pattern into a porous and refractive material, usually sand, and then liquid metal is poured into the cavity so that it takes up the shape of the cavity, thus forming the required metal product. Green sand casting process involves many process parameters which affect the quality of the casting produced. Although there are many new advanced technologies for metal casting, green sand casting remains one of the most widely used casting processes today due 49 February 2015 to the low cost of raw materials, a wide variety of castings with respect to size and composition, and the possibility of recycling the molding sand. The Green sand casting process is one of the most versatile processes in manufacturing because it is used for most metals and alloys with high melting temperatures such as iron, copper, and nickel. Acknowledgement The author expresses his thanks to the School of Mechanical and Building Sciences, SMBS, VIT University Chennai India for granting permission to publish this review paper.