Lec_Casting_Gating_20 _21 Jan (1 . . . ) .pptx

manufacturing

1. Manufacturing Processes (ME F219) Prof Amrita Priyadarshini Associate Professor Department of Mechanical Engineering

2. Metal Casting Processes 2

3. Shrinkage Allowance Machining Allowance Draft or Taper Allowance Distortion Allowance Rapping or Shake Allowance Types of Allowances

4. A job shown in the Figure is to be made of steel by casting process. The mould for this job is made from a wooden pattern. Determine the dimensions of the wooden pattern. Assume machining allowance of 2mm on each side, shrinkage allowance of 2% and a taper allowance of 1 o. Example Machining Allowance Taper allowance Shrinkage allowance Shake allowance

5. Solution Machining allowance Taper allowance Shrinkage allowance Final Dimensions

6. The casting shown is to be made in cast iron using a wooden pattern. Assuming only shrinkage allowance, calculate the dimension of the pattern. All Dimensions are in Inches The shrinkage allowance for cast iron for size up to 2 feet is 0.125 inch per feet ( as per  Table 1) Problem#3

7. The shrinkage allowance for cast iron for size up to 2 feet is 0.125 inch per feet ( as per  Table 1 ) For dimension 18 inch, allowance = 18 X 0.125 / 12 = 0.1875 inch »  0.2 inch For dimension 14 inch, allowance = 14 X 0.125 / 12 = 0.146 inch »  0.15 inch For dimension 8 inch, allowance   =  8 X 0.125 / 12 = 0.0833 inch »  0. 09 inch For dimension 6 inch, allowance   =   6 X 0.125 / 12 = 0.0625 inch »  0. 07 inch

8. Moulding materials A large variety of molding materials is used in foundries for manufacturing molds and cores. molding sand, system sand or backing sand facing sand parting sand core sand. The properties that are generally required in molding materials are: Refractoriness It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest. Permeability Gas flow rate through specimen During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity.

9. Green Strength The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape. Dry Strength When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic forces of the liquid material. Hot Strength As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength. Flowability Ability of the sand to flow around and over the pattern when the mould is rammed.

10. Melting (Ghosh Mallik, 2.3) Gases in melting Furnaces 1/21/2025 10

11. Pouring (Gating Design)

12. If liquid metal poured very slowly Time is taken to fill mould cavity longer Solidification may start before the mould cavity gets filled completely If the liquid metal impinges with high velocity Erosion of mould surface A compromise has to be made in arriving at optimum velocity A good gating design ensures the distribution of the metal in the mould cavity at a proper rate without excessive temperature loss, turbulence and entrapping gases and slags Gating Design

13. Gating system Sprue: It is a circular cross-section minimizing turbulence and heat loss its area is quantified from choke area and gating ratio. Ideally it should be large at top and small at bottom Sprue well : It is designed to restrict the free fall of molten metal by directing it in a right angle towards the runner It aids in reducing turbulence and air aspiration Runner: Mainly slows down the molten metal that speeds during the free fall from sprue to the ingate . The cross section of a runner should be greater than the sprue exit. It should also be able to fill completely before allowing the metal to enter the ingates . In systems where more than one ingate is present, it is recommended that the runner cross section area must be lowered after each ingate connection to ensure smooth flow. Ingate : It directs the molten metal from the gating system to the mold cavity. It is recommended that ingate should be designed to reduce the metal velocity Riser: Molten metal rises in it after filling the mould cavity completely. The molten metal in the riser compensates the shrinkage during solidification of the casting thus avoiding the shrinkage defect in the casting.

14. Characteristics A gating system should avoid sudden or right angle changes in direction. A gating system should fill the mould cavity before freezing. The metal should flow smoothly into the mould without any turbulence. A turbulent metal flow tends to form dross in the mould. Unwanted materials such as slag, dross and other mould materials should not be allowed to enter the mould cavity. The metal entry into the mould cavity should be properly controlled in such a way that aspiration of the atmospheric air is prevented. Metal flow should be maintained in such a way that no mould erosion takes place. The gating system should ensure that enough molten metal reaches the mould cavity. It should be economical and easy to implement and remove after casting solidification.

15. Types of Gating System Vertical gating: liquid metal is poured vertically to fill the mould with atmospheric pressure at the base Bottom gating: liquid metal is filled in the mould from the bottom to top, thus avoiding splashing and oxidation

16. Comparison Top Gate Generation of favorable temperature gradients to enable directional solidification from the casting towards the gate which serves as a riser too. The dropping liquid metal stream erodes the mould surface. There is a lot of turbulence. Bottom Gate Involves little turbulence and sand erosion. If freezing takes place at the bottom, it could choke off the metal flow before the mould is full. Creates an unfavorable temperature gradient and makes difficult to achieve directional solidification.

17. Vertical gating analysis Time taken to fill up the mould ( t f ): Ag is cross-sectional area of gate V is the volume of the mould   Assuming entire mold is at atmospheric pressure and velocity of melt at point 1 ~ 0

18. Bottom Gating Applying Bernoulli’s equation between point 1 and 3: If a riser is used then t f should include the time required to fill the riser. Calculated by replacing A m by A r h m by h t

19. Example Two gating designs for a mold of 50 cm x 25 cm x 15 cm are shown in Fig. The cross sectional area of the gate is 5 cm 2 . Determine the filling time for both the design.   Top/Vertical gating Bottom gating (b) Bottom gating

20. Aspiration Effect As the liquid metal passes down the sprue it loses its pressure head but gains velocity. Pressure anywhere in the liquid column should not fall below atmospheric pressure Fluid may loose contact with the sprue walls, if sprue has constant cross section

21. Sprue Design

22. Aspiration Effect

23. Practice Problems

24. Problem 1: A strip of metal is originally 1.5 m long. It is stretched in three steps: first to a length of 1.75 m, then to 2.0 m, and finally to 3.0 m. Show that the total true strain is the sum of the true strains in each step, that is, that the strains are additive. Show that, using engineering strains, the strain for each step cannot be added to obtain the total strain. Test # 1