GATING SYSTEM Gating System It refers to all the sections through which the molten metal passes while entering into the mold cavity. ELEMENTS OF GATING SYSTEM : 1. Pouring cup 2. Sprue 3. Sprue well 4. Runner 5. Ingates (Gates) 6. Riser FUNCTIOS OF GATING SYSTEM Fill the mould cavity completely before freezing. Minimizing turbulence. Avoiding erosion. Removing inclusions. Regulate flow of molten metal. Consume least metal-less scrap. Trap contaminants. Establish directional solidifications. Design of Pouring Cup Pouring Cup Ceramic Pouring Cup Use of Anti-Swirl bars to reduce turbulence Anti-Swirl bars • Anti-Swirl bars avoid the swirling of the liquid metal and allow it to flow straight, hence reduce turbulence. Typical dimensions of pouring cup Round inlet and round outlet Inlet Diameter (inches) 2 5 8 10 Outlet Diameter (Inches) 1 2.5 3 4 Height (Inches) 1.5 5.25 5.5 8 Typical dimensions of pouring cup Square inlet and round outlet Inlet Dimensions (inches) 3.06*3.56 4.13*5.25 5.50*6.25 Outlet Diameter (Inches) 1.28 1.50 2.00 Height (Inches) 4.59 5.00 6.00 Design of sprue Law of continuity of mass The rate of flow of mass of the fluid is constant at any cross-section. m=dA1V1=dA2V2=dA3V3 where m = rate of flow of mass d = density of liquid metal A1 = area of cross-section at (1) A2 = area of cross-section at (2) A3 = area of cross-section at (3) V1 = velocity of liquid metal at (1) V2 = velocity of liquid metal at (2) V3 = velocity of liquid metal at (3) Q = A1V1= A2V2= A3V3 Q = volume rate of flow A2V2 = A3V3 𝑉2 = 2𝑔ℎ1 𝑉3 = 2𝑔ℎ2 𝐴2 𝐴3 = ℎ2 ℎ1 As the liquid flows down, the cross-section of the fluid decreases. So the taper is provided in the sprue. Liquid loses contact if the sprue is straight which could cause ‘ aspiration’. Design of choke Choke area The smallest area that occurs at the bottom of the sprue is known as ‘choke area’. (1) (2) choke area Choke area is designed based on Bernoulli’s theorem. Bernoulli's theorem It is based on principle of conservation of energy and relates pressure, velocity and elevation. Bernoulli’s Equation : 𝒑 𝒅𝒈 𝑝 𝑑𝑔 + 𝑽𝟐 𝟐𝒈 + 𝒁 − 𝜟𝑭 = 𝑯 is the FLOW ENERGY per unit weight (p – pressure, d = density) 𝑉2 2𝑔 is the KINETIC ENERGY of the fluid per unit weight 𝒑 𝒅𝒈 + 𝑽𝟐 𝟐𝒈 + 𝒁 − 𝜟𝑭 = 𝑯 Z is the POTENTIAL ENERGY of the fluid per unit weight Δ𝐹 is the FRICTIONAL LOSS H is the TOTAL ENERGY of the fluid per unit weight which is always CONSTANT along the same streamline Applying Bernoulli’s theorem between 1 and 3, 𝑊 𝐴𝑐 = 𝑑𝑡𝑐 2𝑔𝐻 Where, 𝐴𝑐 = choke area at section (3) W = casting weight d = density of liquid metal 𝐴𝑐 = 𝑊 𝑑𝑡𝑐 2𝑔𝐻 t = Pouring time (seconds) c = Efficiency factor g = Acceleration due to gravity H = Effective height of metal head Calculation of pouring times 1. Gray-Iron Castings < 1000 pounds Pouring time, 𝑇 𝑡 = 𝐾(0.95 + ) 𝑊 0.853 Where, K is the FLUIDITY FACTOR ,depends on temperature and composition of the molten metal (1.0 to 1.6). W is the weight of the casting in pound T is the average thickness of the casting in inches. t is the pouring time in seconds. 2. Gray-Iron Castings > 1000 pounds Pouring time, 𝑇 3 𝑡 = 𝐾(0.95 + ) 𝑊 0.853 2. Gray-Iron Castings > 1000 pounds Pouring time, 𝑇 3 𝑡 = 𝐾(0.95 + ) 𝑊 0.853 3. Shell moulded ductile iron pouring time, 𝑡=𝐾 𝑊 Where K is about 1.4 for castings of thinner section K is about 1.8 for castings of medium section K is about 2.0 for castings of heavier section 4. Steel Castings Pouring time, 𝑡=𝐾 𝑊 Where K is about 1.2 for castings of weight 100 pounds. K is about 1.8 for castings of weight 100,000 pounds. Sample calculation of pouring time Casting weight : 400 lb. Average thickness : 1 inch. 1. Gray cast iron (Fluidity factor : 1) 𝑡 = 1(0.95 + 1 ) 0.853 400 = 42 seconds. 2. Shell moulded ductile iron (medium section) 𝑡 = 1.8 400 = 36 seconds 3. Steel 𝑡 = 1.0 400 = 20 seconds Design of Runner and Ingates Types of gating system 1. Pressurized Gating System The total cross-sectional area gradually DECREASES from choke to ingates. 2. Un-pressurized Gating System The total cross-sectional area gradually INCREASES from choke to ingates. Typical Gating Ratios Pressurized gating system : AC : AR : AG = 1 : 1.3 : 1.1 ( for grey cast iron) AC : AR : AG = 1 : 2 : 1 ( for aluminum) AC : AR : AG = 1 : 2 : 1.5 ( for steel) Un-pressurized gating system : AC : AR : AG = 1 : 4 : 4 ( for grey cast iron) AC : AR : AG = 1 : 3 : 3 ( for aluminum) AC : AR : AG = 1 : 3 : 3 ( for steel) C- Choke , R – Runner , G- Gate Comparison of gating system Pressurized Gating Un-pressurized gating Total cross-sectional area decreases Total cross-sectional area toward the mold cavity. increases toward the mold cavity. More turbulence and chances of mold erosion. Flow of liquid metal is almost equal from all ingates. Casting yield is more. Less turbulence. Complex and thin sections can be successfully cast. Complex and thin sections may not be successfully cast. Flow of liquid metal is different from each ingate. Casting yield is less. Design and location of ingates Multiple ingates are often preferable for large castings. A fillet should be used where an ingates meets a casting (Produce less turbulence). The minimum ingates length should be three to five times the ingate’s width, depending on the metal being cast. Curved ingates should be avoided, as far as possible Design of Riser Primary function of a Riser It should act as a reservoir of molten metal in the mold to compensate for shrinkage during solidification. Guidelines for Riser Design and Location The riser must not solidify before the casting. The volume of riser(s) must be large enough to feed the entire shrinkage of the casting. The pressure head from the riser should enable complete cavity filling. Risers must be placed so that then liquid metal can be delivered to locations where it is most needed. Important method of Riser Design 1. Caines Method 2. Modulus Method Riser Design : Caine’s Method Relative riser and casting geometry to obtain sound castings. 𝑅𝑖𝑠𝑒𝑟 𝑣𝑜𝑙𝑢𝑚𝑒 𝐶𝑎𝑠𝑡𝑖𝑛𝑔 𝑣𝑜𝑙𝑢𝑚𝑒 𝐶𝑎𝑠𝑡𝑖𝑛𝑔 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎/𝑣𝑜𝑙 𝑅𝑖𝑠𝑒𝑟 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎/𝑣𝑜𝑙 Riser Design : Modulus Method Modulus od solidification : Modulus of solidification of casting (or riser) is defined as the ratio of its volume and surface area. 𝑉𝑜𝑙𝑢𝑚𝑒 𝑀𝑜𝑑𝑢𝑙𝑢𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 = 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 Modulus method is based on Chvorinov’s Rule. Chvorinov’s Rule 𝑻𝑺𝑻 = 𝑽 𝒏 𝑪𝒎 ( ) 𝑨 Where, TST = total solidification time. V = Volume of the casting. A = Surface area of casting. n = exponent usually taken as 2. 𝐶𝑚 = Constant depends on mold material What Chvorinov’s Rule Tells Us? 𝑻𝑺𝑻 = 𝑽 𝒏 𝑪𝒎 ( ) 𝑨 A casting with a higher modulus (volume to surface area ratio) cools and solidifies more slower than the one with a lower modulus. To feed molten metal to the casting, TST of the riser must be greater than TST of the casting. Since mold constants of riser and casting will be equal, design riser to have a larger modulus so that the main casting solidifies first. Requirement of the riser to feed the casting. 𝑴𝑹 = 𝟏. 𝟐 × 𝑴𝑪 where, 𝑀𝑅 = Modulus of casting. 𝑀𝐶 = Modulus of riser. Feeding Distance Casting without a riser - influence of END EFFECT Casting with riser - influence of END and RISER EFFECT Feeding distance for steel castings (plates/bars) : • End effect - promotes a distance of 2.5 *t • Riser effect – promotes a distance of 2*t Total feeding distance = 4.5*t Feeding distance with two Riser Steel plate casting with two riser Influence of END and RISER EFFECT Total feeding distance is about 13*t Feeding Distance (Large steel casting) : Shape Factor, SF = (L +W)/T where, L denotes the length W denotes the width T denotes the thickness (Note : L>W>T) Riser on large steel castings Fluidity of Molten Metal Fluidity of molten metal It is the capability of the molten metal to flow and fill into the mould cavities. pure metals act with good fluidity. Factors affecting fluidity a) Related to molten metal : 1.Viscosity 4.Surface tension 2.Inclusion 5.Freezing range of alloy 3.Latent heat of fusion 6.Thermal diffusivity of alloy. b) Related to casting parameters : Mould material – thermal conductivity, surface characteristics, etc. Method for determining the fludity 1) Fluidity spiral tube method 2) Suction tube method Measuring Fluidity • The FLUIDITY INDEX of the material is the length of the solidified metal in the spiral passage. • The greater the length of the solidified metal greater is the fluidity. Solidification of castings Cooling curve for a pure metal during casting A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point) `a) Phase diagram for a copper-nickel alloy system b) Cooling curve for a 50 % Cu alloy during casting Most alloy freeze over a temperature range rather than at a single temperature Solidification of casting Solidification involves two steps that are NUCLEATION and GROWTH. NUCLEATION : It refers to the process in which tiny solid particles called, ‘Nuclei’, are formed when liquid metal cools below its liquidous temperature. Two types of Nucleation : a) HOMOGENEOUS NUCLEATION : It occurs without the help of foreign particles. b) HETEROGENEOUS NUCLEATION : It occurs with the help of foreign particles (such as the mould material, impurities, and added nucleation materials.) Homogeneous nucleation Critical nucleus : The nucleus grows to give rise to the solid structure only when its size is greater than certain size which is known as ‘Critical Nucleus’. This requires supercooling. The critical nucleus radius r is given by, r = -2σ/ΔFV σ= specific surface energy ΔFV= change in free energy Heterogeneous Nucleation Solid phase crystallizes on a foreign particle – larger nucleus. Degree of super cooling needed for solidification is smaller. Heterogeneous nucleation is the dominating mechanism in the early stage of solidification. Scarcity of foreign particles in the central part od the casting – results in homogeneous nucleation. Grain Structure At the surface, heterogeneous nucleation takes place for few layers. These grains are known as ‘Equiaxed Grains’. Inside absence of sand particles leads to homogeneously nuckeated grains. Their orientation will be from the surface to the center. These grains are known as the ‘Columnar Grains’. Equiaxed grains Columnar grains When columns forms side arms, it is known as a ‘Dendrite’ and the grain structure is known as the ‘Dendritic grain structure’. New nuclei resist the growth of neighboring nuclei. Hence an equiaxed grain structure is produced at the center. Columnar and dendritic grain structure are coarse and directional-undesirable in most situation. This can be changed in practice by adding ‘Nucleating Agents’, which produce an equiaxed grain structure in the entire casting. Nucleating agents for different alloys : Metals Al alloys Plain carbon steel Stainless steel Mg alloy Cast iron Nucleating Agents Ti compounds(TiAl3, TiB2, TiC) Al compounds(AlN, Al2O3) Ca and Mg cynides ZrC, ZrN, Zr oxides Sulphur compounds Three basic types of cast structure: A) Columnar dendritic B) Equiaxed dendritic C) Equiaxed nondendritic Cooling Shrinkages When liquid metal cools from its pouring temperature in a casting , it undergoes the following types of shrinkages a) Liquid shrinkage : It occurs during cooling up to the liquidous temperature. It is about 0.5 % of the room temperature volume. b) Solidification shrinkage : It occurs during cooling from liquidous to solidus temperatures. c) Solid shrinkage : It occurs during cooling from solidus temperature to room temperature. It is a function of the coefficient of thermal expansion of material. The liquid and the solidification shrinkage are taken care of by a riser Solidification casting for important cast metals/alloys Sr No. 1 2 3 4 5 6 7 Metal or Alloy Aluminum Zinc Gold Copper Magnesium Lead Al-4.5 % Cu Solidification Shrinkage (% vol) 7.1 6.5 5.5 4.9 4.2 3.2 6.3 Sr.No. Metal or Alloy 8 9 10 11 12 Al-12 % Si Brass (70-30) 90 % Cu- 10 % Al Carbon Steel White Iron Solidification shrinkage (% volume) 3.8 4.5 4 2.5 to 4 4.5 to 5 Liquid shrinkage is about 0.5% by volume with normal superheat. Thank You