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- 1. Moulding
- 2. Moulding Moulding is the process of making a mould cavity by packing prepared moulding sand around the pattern and removing the pattern from the mould to form the mould cavity. Moulding Sand Silica Grains Clay Water 90%
- 3. 1. Green sand 2. Dry sand 3. Loam sand 4. Facing sand 5. Backing sand 6. Parting sand Moulding Sand Also called wet state; 8% water, 16 to 30% clay, soft, light and porous ,High damping capacity. Dry in nature, High strength and rigidity Fine silica, fine refractories, clay, graphite, fiber and water, becomes very hard when it is dried Mixture of silica and dry sand ,used to cover the surface of the pattern (fine finish) eg. Coal dust mixed with sand Old sand, can be used repeatedly Mixture Silica and brick powder, used to avoid the sticking of patterns while removing (split pattern)
- 4. Refers to the sand ability to withstand the temperature of the liquid metal being cast without breaking down. Refractoriness Flowability is the property of the sand to respond to the moulding process so that when rammed it will flow all around the pattern and take the desired mould shape. Flowability Ability of the sand to be easily stripped off the casting after it has solidified. Collapsibility Ability of the moulding sand to stick with walls of the moulding boxes Adhesiveness Refers to the sand ability to exhaust gases Permeability Properties of moulding sand
- 5. Defects In Casting 1. Gas defects 2. Shrinkage cavities 3. Molding Material Defects 4. Pouring Metal Defects 5. Metallurgical defects
- 6. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects 1. 2. 3. Blow holes and open blows Air Inclusion Pin Hole Porosity
- 7. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects These are the spherical, flattened or elongated cavities present inside the casting or on the surface. On the surface they are called open blows and inside, they are called blowholes. ― Inadequate core venting ― Excessive release of gas from core ― Excessive moisture absorption by the cores ― Low gas permeability of the core sand ― Moisture content of sand too high, or water released too quickly ― Gas permeability of the sand too low ― Sand temperature too high ― Improve core venting, provide venting channels ― Reduce moisture content of sand ― Improve gas permeability ― Reduce sand temperature. Install a sand cooler if necessary ― Dry out cores , thus reducing absorption of water and reducing gas pressure Remedies Possible causes Blow holes and open blows 1.
- 8. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects 2. Air Inclusion The atmospheric and other gases absorbed by the molten metal in the furnace, in the ladle, and during the flow in the mould, when not allowed to escape, would be trapped inside the casting and weaken it. ― Faulty gating ― Faulty pouring ― Inferior molding or core sand ― Soft ramming of mold ― Rough handling of mold and core ― Modify gating system ― Improve pouring to minimize turbulence. ― Use of superior sand of good strength ― Provide hard, ramming Remedies Possible causes
- 9. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects 3. Pin Hole Porosity Agglomeration of small, rounded cavities up to around 5 mm in size. Surface of the cavities mostly smooth and polished. ― High pouring temperature ― Gas dissolved in metal charge ― Molten metal not properly degassed ― Slow solidification of casting ― High moisture and low permeability in mold ― Regulate pouring temperature ― Control metal composition ― Ensure effective degassing ― Modify gating and risering ― Reduce moisture and increase permeability of mold Remedies Possible causes
- 10. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities. ― Faulty gating and risering system ― Improper chilling ― Ensure proper directional solidification by modifying gating risering and chilling Remedies Possible causes
- 11. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects 1. 2. 3. Run out Cuts and washes Metal penetration 4. Fusion 5. Swell
- 12. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects A run out is when the liquid metal leaks out of the mold because of a faulty mold or flask. Scabs are a thin layer of metal that sits proud of the casting. ― Faulty molding ― Defective molding boxes ― Improving molding technique ― Change the defective molding boxes Remedies Possible causes Run out 1.
- 13. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sand not having enough strength and the molten metal flowing at high velocity. ― Low strength of mold and core ― Lack of binders in facing and core sand ― Faulty gating ― Improve mold and core strength ― Add more binders to facing and sand ― Improve gating Remedies Possible causes Cuts and washes 2.
- 14. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects Metal penetration 3. When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration. ― Large grain size ― Soft ramming of mold ― Molding sand or core has low strength ― Molding sand or core has high permeability ― Pouring temperature of metal too high ― Use sand having finer grain size ― Provide hard ramming ― Suitably adjust pouring temperature Remedies Possible causes
- 15. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects Fusion 4. This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high. ― Low refractoriness in molding sand ― Faulty gating ― Too high pouring temperature of metal ― Poor facing sand ― Improve refractoriness of sand ― Modify gating system ― Use lower pouring temperature ― Improve quality of facing sand Remedies Possible causes
- 16. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects Swell 5. Enlargement of the mould cavity by metal pressures, results – localized or overall enlargement of castings. A proper ramming of the mold will correct this defect. ― Too soft ramming of mold ― Low strength of mold and core ― Mold not properly supported ― Provide hard ramming ― Increase strength of both mold and core Remedies Possible causes
- 17. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects 1. 2. Slag Inclusion Mis -runs 3. Cold Shuts
- 18. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects Slag Inclusion 1. As, in most cases, this defect is related to furnace or melt treatment slag it can occur in all casting materials, independent of the molding or casting process. This defect is often observed on casting surfaces on top in the mold, cores, and protruding mold sections. These non- metallic inclusions can be accompanied by gas bubbles. ― Addition of flux/flow in molten metal ― Adoption of some of the slag trapping methods like pouring basin screens and runner extensions. Remedies Possible causes
- 19. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects Mis-runs 2. A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design. ― Lack of fluidity in the molten metal ― Metal freezes before mold is filled ― Faulty design ― Fluidity can be improved by changing the composition of t the metal ― Adjust proper pouring temperature ― Modify design Remedies Possible causes
- 20. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects Cold Shuts 3. A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the molten metal is poured into the mold cavity through more- than-one gate, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the poured metal and thick enough walls in the casting design. ― Lack of fluidity in the molten metal ― Faulty design ― Fluidity can be improved by changing the composition of the metal ― Adjust proper pouring temperature ― Modify design Remedies Possible causes
- 21. Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Metallurgical defects Hot tears 1. If a section begins to shrink while still hot and there is not a sufficient supply of liquid metal, the resulting internal stress will tear the metal. This is because while hot the metal has relatively low strength. ― Lack of collapsibility of core ― Lack of collapsibility of mold ― Faulty design ― Improve core collapsibility. ― Improve mold collapsibility ― Modify casting design. Remedies Possible causes
- 22. Die Casting
- 23. Die casting is a permanent-mold casting process in which the molten metal is injected into the mold cavity under high pressure. Typical pressures are 7 to 350 MPa. The pressure is maintained during solidification, after which the mold is opened and the part is removed. Molds in this casting operation are called dies; hence the name die casting. Die Casting 1) Hot Chamber Die Casting 2) Cold Chamber Die Casting Classification
- 25. Disadvantage Advantage ― Hollow shapes are not readily casted because of the high metal pressure. ― Limited sizes of the products can be produced based on the availability of the equipment. ― High melting temperature alloys are practically not die casted. ─ High production rate ─ High accuracy in part dimensions ─ Smooth surface finish for minimum mechanical finishing ─ Ability to make many intricate parts such as hole opening slot trademark number etc. ─ Much thinner wall sections can be produced which can’t be produced by other casting methods. Die casting process is preferred for nonferrous metal parts of intricate shapes. Examples of products are automobiles appliances, hand tools, computer peripherals, toys, optical and photographic equipment etc. Application
- 26. In hot-chamber die casting, the metal is melted in a container attached to the machine, and a piston is used to inject the liquid metal under high pressure into the die. Typical injection pressures are 7 to 35 MPa. Production rates up to 500 parts per hour are not uncommon. Hot-chamber die casting imposes a special hardship on the injection system because much of it is submerged in the molten metal. The process is therefore limited in its applications to low melting-point metals that do not chemically attack the plunger and other mechanical components. The metals include zinc, tin, lead, and sometimes magnesium. Hot Chamber Die Casting
- 27. Cycle in hot chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber; (2) plunger forces metal in chamber to flow into die, maintaining pressure during Cooling and solidification; (3) plunger is withdrawn, die is opened, and solidified part is ejected. Finished part is shown in (4). Process Cycle
- 28. In cold-chamber die casting machines, molten metal is poured into an unheated chamber from an external melting container, and a piston is used to inject the metal under high pressure into the die cavity. Injection pressures used in these machines are typically 14 to 140 MPa. Compared to hot-chamber machines, cycle rates are not usually as fast because of the need to ladle the liquid metal into the chamber from an external source. Nevertheless, this casting process is a high production operation. Cold-chamber machines are typically used for casting aluminum, brass, and magnesium alloys. Low-melting-point alloys (zinc, tin, lead) can also be cast on cold-chamber machines, but the advantages of the hot- chamber process usually favour its use on these metals. Cold Chamber Die Casting
- 29. Process Cycle Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber; (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification (3) ram is withdrawn, die is opened, and part is ejected.
- 31. In this process we make use of the centrifugal force which is directed away from the centre and exists at high speeds. In this method the mould is rotated about its central axis and there exists a continuous pressure as metal is solidifying, slag being lighter gets segregated towards the centre. Centrifugal Casting
- 32. True Centrifugal Casting Semi-centrifugal casting Centrifugal casting In true centrifugal casting process, ― The axis of rotation of mold can be horizontal, vertical or inclined. Usually it is horizontal. ― Basically, symmetrical shaped components (hollow cylindrical parts) like hollow pipes, hollow bushes, gun barrels and tubes. ― No core is required. This casting technique is employed when axi-symmetrical objects with uniform diameter are to be produced. Core is not employed in these castings
- 33. Advantages of true centrifugal casting 1) Relatively very light impurities move inwards towards centre. So they can be removed easily. 2) These castings have a directional solidification starting from outside to inside. 3) Central core is not required for making a hole or pipe 4) Employment of gates and risers is not required 5) This technique is best suited for the mass production of symmetrical objects. Disadvantages of true centrifugal casting 1) Centrifugal castings require very high investments. 2) Skilled labours are to be employed for this process. 3) Only some shapes can be generated by this casting process. Applications 1) Bearings for machines 2) Pipes 3) Rings and other annular components
- 34. True Centrifugal Casting Semi-centrifugal casting Centrifugal casting ― It is similar to true centrifugal casting but only with a difference that a central core is used to form the inner surface. This casting process is generally used for articles which are more complicated than those possible in true centrifugal casting, but are axi-symmetric in nature. ― Semi centrifugal casting is employed for casting gear blanks, sheaves, and wheels etc. ― low spinning speeds can be used speeds ranging between 180 to 200 RPM is recommended. These castings are normally prepared in vertical machines. The mold cavity is arranged with in the mold so that its central axis is vertical and concentric with the axis of rotation. Article produced by semicentrifugal casting process
- 35. 1) Number of molds can be stacked together, one over the other can be fed by a common central sprue in order to produce more than one casting at a time. 2) Similar to true centrifugal casting it ensures purity and density of the product poorer structure forms the center of the casting so this can be machined off. Advantages Semi Centrifugal Casting
- 36. True Centrifugal Casting Semi-centrifugal casting Centrifugal casting ― This type of casting method is used for casting unsymmetrical castings in groups. This is done in groups for obtaining balance of whole casting. ― In these castings the axis of mold doesn’t coincide with each other to induce the pressure on molds the casting rotated at the center. ― Feeding to the mold cavities is done by central sprue by the action of centrifugal forces.
- 37. 1) Ensures better quality. 2) Castings when generated on large scale will be economical. 3) Cleaning and repair cost of the castings can be reduced. 4) Rejection percentage is very low Advantages Centrifugal Casting
- 39. 1) The investment casting process initiates with the production of wax replicas or patterns of the required shape of castings. Each and every casting requires a pattern to be produced. Wax or polystyrene is made used as the injecting material. The assembly of large number of patterns are made and attached to a wax sprue centrally. 2) Metallic dies are used to prepare the patterns. 3) The pattern is immersed in refractory slurry which completely surrounds it and gets set at room temperature forming the mold. The mold is further heated, so that the pattern melts and flows out, leaving the required cavity behind. After heating, the mold gets further hardened and molten metal is poured while it is still hot. After the casting gets solidified, the mold is broken and it is taken out. Investment Casting
- 41. Advantages Investment Casting 1) Closed dimensional tolerances and intricate geometries are easily obtained. 2) A single process gives good surface finish 3) Alloys with higher melting points can also be easily cast Limitation 1) Expensive process since it uses the wax, which increases the cost. 2) The cross section of molds being thin, the process is limited by size and mass of casting. 3) Removal of the wax from the mold again adds up time and cost.
- 42. Comparison of Casting Process Process Merits Limitation Sand Almost any metal is cast; no limit to size, or weight; low tooling cost Some finishing required, wide tolerance Die Excellent dimensional accuracy and surface finish; high production rate Die cost is high; part size limited; usually limited to nonferrous metals Centrifugal Large cylindrical parts with good quality; high production rate Equipment is expensive, part shape limited Investment Intricate shapes; excellent surface finish and accuracy; almost any metal cast Part size limited; expensive patterns, molds and labour.