special casting technique foundry tech.pptx

Special casting technique

1. Centrifugal Casting

2. Centrifugal casting products

3. What is Centrifugal Casting? Centrifugal casting, also referred to as Roto Casting, is a specialised casting technique that utilises  centrifugal force  to produce thin-walled cylinders from a variety of materials, including metals, glass, and concrete. Alfred Krupp invented this casting method and has since been widely adopted for its ability to create high-quality products. It is particularly suitable for producing rotationally symmetric components.

4. Working Principle of Centrifugal Casting During the centrifugal casting process, the liquid metal is subjected to centrifugal force, which exerts pressure multiple times that of gravity, effectively dispersing the metal and minimising cracks or flaws on microscopic and macroscopic levels. This is akin to the  die casting  process, where applied  pressure  reduces overall product flaws. As the die begins to fill, the more dense molten metal is forced to the wall of the spinning die. Directional solidification of sound metal progresses from the O.D. towards the bore, while the less dense material, including impurities, “floats” to the I.D. Once the casting has solidified, the part is removed from the die and residual impurities in the I.D. are machined away, resulting in a defect-free structure without cavities or gas pockets.

5. The basic steps of centrifugal casting are as follows: Heating the  metal  beyond its melting point. Rotating the mould at a specific RPM (300 to 3000) to prepare for pouring. Pouring the liquid metal into the rotating mould. Utilising the  rotational motion  to facilitate cooling and ensure pressure within the mould. Removing the mould from the rotation and extracting the solidified material. Refining the final product by removing impurities pushed to the edges during rotation through machining.

6. The various parts of Centrifugal casting include: Ladle Pouring Basin Core Rollers Motor and Metal Mold

7. Ladle The ladle, made of either steel or stainless steel, serves as the vessel used to transfer the molten metal into the pouring basin, as depicted in the diagram. Pouring Basin Once the molten metal is placed in the ladle, it is then used to pour the molten metal into the metal mould. Core In the metal mould, a core, which is a preformed and bonded sand insert, is utilised to shape the interior of the casting, allowing for the creation of hollow sections or cavities within the final product.

8. Rollers In this system, there are four rollers, with two positioned at the bottom and two at the top. The bottom rollers are connected to the motor and rotate together, while the top rollers serve to support the metal mould during rotation. Motor The motor is utilised to impart rotational motion to the rollers. Metal Mould The molten material is introduced using a pouring basin, and with the assistance of a motor and roller, it is rotated continuously at high speed to enable the desired operation.

9. Centrifugal casting can be further categorised into three types: True centrifugal casting Semi-centrifugal casting Centrifuged casting.

10. True Centrifugal casting The revolving speed of the horizontal true centrifugal casting is critical to the effectiveness of the casting process and part quality. It revolves at round 300-3000rpm Molten metal is poured into a rotating horizontal mould to produce tubular parts such as pipes, tubes, rings, and bushes in this casting process. Mould rotation can occur on a horizontal or vertical axis, with the former being more common.

11. True Centrifugal casting set- up Moulds are made of steel, iron, or graphite and can be coated with a refractory lining to extend their life. The surfaces of the moulds are designed to allow for pipe casting with a range of outside styles. The inner surface of the casting remains cylindrical due to centrifugal forces that evenly distribute the molten metal Castings produced by true centrifugal casting have a high density, particularly in the outer portions of the component where centrifugal force is highest. Because centrifugal force constantly reallocates molten metal toward the mould wall during freezing, solidification shrinkage outside the cast tube is not an issue. Any imperfections in the casting tend to be on the inner wall and, if necessary, can be eliminated by machining.

12. How does True centrifugal casting work? Molten metal is poured straight into the mould without any gating mechanism. Once within the hollow, the centrifugal forces of the spinning mould propel the molten material to the cavity’s exterior wall. After the necessary amount of molten metal has been poured, the mould is rotated until the part is hardened. After the casting has been set, the mould is removed, opened, and the part removed for post-processing.

13. Advantages These castings have a high density, high mechanical strength, an excellent outer surface finish, and a fine-grained structure. Impurities and inclusions can be easily removed Gates are risers not required Hollow interiors without cores are formed. Can form very large parts with high accuracy Low equipment and labour cost Generates minimum scrap

14. Disadvantages The casting inner surface diameter will not be accurate. This method is not suitable for all alloys. The true centrifugal casting process is limited to cylindrical parts Secondary machining is often required for inner diameter Long lead time possible

15. Semicentrifugal Casting Semicentrifugal casting uses centrifugal force to produce solid casting rather than tubular casts. The image below shows an example of semi-centrifugal casting where the moulds have their riser at the axis of rotation to feed the molten metal. Expendable moulds are common and used to make parts such as spoked wheels, pulleys, gear blanks, brass bush and nozzles.

16. The exterior regions of items formed by semi-centrifugal casting have a higher density than the centre of the rotating axis. This casting procedure makes things like spoked wheels that have rotational symmetry and can remove the casting centre. Removing the centre section of the cast also removes the cast’s lowest-density component. The quality of the final casting is affected by factors such as rotating speed, component diameter, pouring temperature, pouring speed, mould temperature, and cooling rate

17. How does semi-centrifugal casting work? As shown above, the expendable mould is set up around the central sprue. Molten metal is then poured into the sprue slowly from an external source using a small spout. The molten material may be poured into a rotating mould, or the rotation of the mould may commence after the pouring has taken place. Pouring stops once the correct amount of molten metal for the casting, runner and sprue is poured. The spinning continues after the pouring and distribution and during the solidification process. The equipment will stop rotating after the castings have completely frozen, and the pieces may be removed.

18. Centrifuging Centrifuging , also known as  centrifuge casting , involves the placement of mould cavities of any shape at a certain distance from the axis of spin. The molten metal is poured from the centre, and centrifugal forces push it into the mould cavity through the sprue and the runner. Like true centrifugal casting, the characteristics of the castings might change with distance from the spin axis. This method is used for small parts such as jewellery, small bushes and sleaves .

19. How does Centrifuge casting work? As shown above, the mould is built to have cavities around the central sprue with runners connecting them. Molten metal is then poured into the sprue slowly from an external source using a small spout. The molten material may be poured into a rotating mould, or the rotation of the mould may commence after the pouring has taken place. The pouring stops once the correct amount of molten metal for the number of castings, runner and sprue is poured. The spinning continues after the pouring and distribution and during the solidification process. The mould will be stopped, and the components will be removed after the castings have been set.

20. Advantages Thin-walled and small parts are possible due to the centrifugal forces filling the cavity. Good surface finish due to the amount of forces imparted on the molten metal Since the cast parts are further away from the rotational axis, the part is denser Due to low density, impurities such as trapped gases and inclusions will be near the axis, away from the desired part Disadvantages Not suitable for all the alloys Sprue and runner will have to be machined

21. Types of Centrifugal Casting There are mainly two types of Centrifugal casting process: Vertical Casting and Horizontal casting.

22. Vertical Centrifugal Casting The vertical centrifugal casting process yields components with superior mechanical properties and a more uniform, fine-grain structure. This casting method is specifically employed for cylindrical shapes where the diameter exceeds the height or length of the cylinder. It finds application in various components such as rings,  bearings , balls for ball valves, gear blanks, short bushings, flanges, sprockets (used in automobiles), and even non-cylindrical parts like valves and propellers through specialised castings.

23. Horizontal Centrifugal Casting The horizontal casting process is specifically chosen for manufacturing products with long cylindrical parts, where the length of the casting significantly exceeds the outside diameter. This method is cost-effective and ensures the production of high-quality tubular components. In horizontal casting, a mould or die rotates along a horizontal axis to cast parts with an axis of revolution.

24. Advantages of Centrifugal Casting The centrifugal casting process offers several advantages, including: Lower Casting Defects:  Uniform solidification in centrifugal casting results in reduced casting defects. Energy Efficiency:  Lower pouring  temperature  requirements contribute to energy savings. Easy Removal of Impurities:  Impurities like oxide or slag particles are effectively segregated and easily removable. Thin-Wall Cylinder Production:  Centrifugal casting allows for the efficient manufacturing of thin-walled cylinders. Cost-Effective for Complex Geometries:  Components with intricate geometries can be produced at a lower cost.

25. No Runner and Gating System:  The need for a runner and gating system is eliminated in centrifugal casting. Elimination of Cores:  Cores are not required for casting hollow shapes, such as tubes. Suitable for Mass Production:  Centrifugal casting is well-suited for mass-production applications. High Mechanical Properties:  Components manufactured through centrifugal casting exhibit high mechanical properties and dense metal. Lighter Inclusions and Impurities:  Inclusions and impurities in the casting process tend to be lighter and more easily managed.

26. Disadvantages of Centrifugal Casting The centrifugal casting process has certain disadvantages, including: Difficulty in Temperature Distribution and Solidification Time:  Determining the temperature distribution and solidification time can be challenging in centrifugal casting. Difficulty in Removing Impurities from Small-Diameter Pipes:  If impurities enter the internal diameter of small-diameter pipes during the process, their removal becomes difficult, posing a significant disadvantage. Limitation to Cylindrical Structures:  Centrifugal casting is primarily suitable for manufacturing cylindrical structures, resulting in a loss of structural and purity benefits for non-cylindrical components.

27. Increased Machining Requirements:  Components with non-cylindrical shapes often require additional machining, which can make the process more costly. Size Limitations:  The size of components that can be manufactured through centrifugal casting is limited. Limited Design Options:  Centrifugal casting is only suitable for casting symmetrical shapes, which restricts the design possibilities. Material Limitations:  Not all types of materials are suitable for centrifugal casting, limiting the range of materials that can be utilised. Increased Maintenance and Skilled Operator Requirement:  Centrifugal casting may require more maintenance and skilled operators compared to other casting processes.

28. Applications of Centrifugal Casting Centrifugal casting finds a wide range of applications, including: Hollow Cylindrical Metal Pipes:  Centrifugal casting is commonly employed to cast hollow cylindrical metal pipes. Automotive Industry:  This casting process is extensively utilised in the automotive industry to manufacture pistons and cylinder liners. Aerospace Industry:  Centrifugal casting is widely used in the aircraft industry for casting components like flanges, compressors, and rings. Railway Applications:  It is utilised to manufacture carriage wheels for railways as well as bearings.

29. Electronic Industry:  The electronic industry utilises centrifugal casting for manufacturing switchgear components. Symmetrical Parts:  Centrifugal casting is suitable for creating parts with symmetrical shapes about an axis. Uniform Grain Structure:  Centrifugal casting proves advantageous when a uniform grain structure is required. Clutch Plates and Paper-Making Rollers:  Clutch plates and paper-making rollers are also manufactured using the centrifugal casting process.

30. Continuous Casting As the name suggests, it allows consistent mass production of metal profiles with a constant cross-section. This type of casting is popular in the production of steel bars. Also, the vertical cast creates semi-continuous casting like billets, ingots, bars, etc.

31. Operations In this process, molten metal is poured at a calculated rate in a water-cooled, open-ended mold that allows a surface of solid metal to form on the liquid metal in the center . Metal solidification, thus, happens from the outside in. After this process finishes, strands of metal can be continuously extracted from the mold. Predetermined lengths of products can be cut off by mechanical shears or traveling oxyacetylene torches. Generally, the products created using continuous casting are homogeneous, consistent, and dense. However, it also limits its use to such applications.

32. Advantages Disadvantages Diverse size range of casting products varying from a few millimeters thick strip to larger billets and slabs Requirement of continuous cooling of the molds, otherwise, center-line shrinkage develops Lower costs due to continuous production Casting of only simple shapes with a constant cross-section Lower material wastage Requires large ground space and high initial investment

33. Squeeze Casting Squeeze casting, also known as liquid metal forging, is a combination of casting and forging process.

34. Squeeze casting  & High-pressure die casting Squeeze casting  is a single-step process that combines the casting and forging processes. The molten alloy is poured into the mold and solidified under pressure.  High-pressure die casting  is a single-step process in which the molten alloy is forced, under pressure, into a mold cavity.  Although there are similarities between the two processes, the main difference comes down to velocity

35. Molten metal is poured into the bottom half of a pre-heated die cavity. As the metal starts to solidify, the top half of the die (punch) is pressed into the bottom half and held in position until the casting has solidified. The cast component is then ejected. Pressure is being applied via the upper die, so this is not strictly casting, as it adds forging to create a hybrid technique.

36. By using this technique, the metal will typically come out stronger, with a better grain and less metallic shrinking. This commonly is done with magnesium, aluminum and their alloys, but many other metals can be used. The amount of pressure thus applied is significantly less than used in forging, and parts of great detail can be produced. The porosity is low and the mechanical properties are improved.

37. Stage 1 Die set positioned on hydraulic press, preheated to 200 – 250°C and coated with a releasing agent such as graphite. Accurate metering of liquid metal into die cavity via a "launder".

38. Stage 2 Press actuated to bring two parts of die set together. Metal displaced to fill die cavity and pressure held until solidification is complete.

39. Stage 3 Press ram withdrawn. Die set separated. Component ejected. Components heat-treated and machined when necessary.

40. Casting parameters Casting temperatures depend on the alloy and the part geometry. The starting point is normally 6 to 55°C above the liquidus temperature. Tooling temperatures ranging from 190 to 315°C are normally used Pressure levels of 50 to 140 MPa are normally used Lubrication For aluminum , magnesium, and copper alloys, a good grade of colloidal graphite spray lubricant has proved satisfactory when sprayed on the warm dies prior to casting

41. Advantages Of Squeeze Casting process :  Offers a broader range of shapes and components than other manufacturing methods Little or no machining required post casting process Low levels of porosity Good surface texture Fine micro-structures with higher strength components No waste material, 100% utilization

42. Disadvantages Of Squeeze Casting process :  Costs are very high due to complex tooling No flexibility as tooling is dedicated to specific components Process needs to be accurately controlled which slows the cycle time down and increases process costs. High costs mean high production volumes are necessary to justify equipment investment

43. Direct Squeeze Casting  Direct squeeze casting is similar to the forging process. In this method, molten metal is poured into the bottom half of the mold. The upper half is closed, causing the entire mold to fill with the molten metal. With pressure applied over the whole cavity, the solidification process begins. This process provides better heat transfer to produce stable components. 

44. Indirect Squeeze Casting  Indirect squeeze casting is  similar to high-pressure die casting . In this method, the molten metal is injected into an indirect squeeze-casting machine through a vertical or horizontal shot sleeve. From there, the liquid metal is injected into the die chamber through a thicker gate and a lower velocity than the velocity used in high-pressure die casting. 

45. Vacuum die casting Vacuum die casting (VDC) is  a high-pressure die casting technique that uses a vacuum to remove air and other gasses from the die before molten metal is added . It's also known as vacuum-assisted high-pressure die casting.  Vacuum Die Casting is mainly adopted to reduce some casting defects in parts that occur due to air entrapment. VDC is similar to traditional die casting, but with a vacuum system. The main benefits of VDC include:  Higher quality surface finish Improved mechanical properties Better finished product stability

46. The VDC process is as follows:  Melt the metal Create a vacuum in the die cavity Add pressure to the molten metal to inject it into the cavity Wait for the part to solidify and cool down Eject the part from the die VDC can help avoid porosity and other casting defects.  Vacuum-assisted die casting can be applied for both Hot Chamber and Cold Chamber Die Casting.