GATING DESIGN CONSIDERATIONS
What is gating system: metal • All passageways through which molten metal enters the mold cavity.
Why Gating System?? Leads to clean molten metal (free of slag and inclusions) pouring ,ensures
• Smooth filling- minimizes bulk turbulence • Uniform filling- all portions of the casting fill in a controlled
manner, usually at the same time
• complete filling- leads molten metal to thin and end sections with minimum resistance. .
COMPONENTS Pouring cup (accepts metal from ladle) Riser
Casting
Sprue (leads molten metal to sprue base)
Sprue Base Runner (changes direction of (takes metal to gate) metal flow)
Gate (leads metal to mold cavity)
Pattern with gating system
Functions: The entry section (basin & sprue) of a gating has two functions: 1) To supply liquid metal free of entrapped gases, slag and eroded sand. 2) To establish a hydraulic pressure head, which will force the metal through the rest of the gating system and into the casting.
The distribution section(sprue base, runner & in-gate) has five functions: 1) To decrease the velocity of the metal stream. 2) To minimize turbulence, both in the gating system as well as in the mold cavity. 3) To avoid mold and core erosion. 4)To establish the best possible thermal gradient in the casting. 5) To regulate the rate of flow of metal into the mold cavity.
Requirements: For defect free casting gating system should have following requirements:
• Should completely fill the mold cavity before freezing • Should ensure smooth metal flow without turbulence A turbulence tends to form dross in the mould
• Metal flow should be such that no gating or mould erosion takes place • The metal entry into the mould cavity should be properly controlled in so that aspiration of the atmospheric air is prevented
• System should incorporate traps for separation of non-metallic inclusions • Should establish proper thermal gradient • Should ensure that enough molten metal reaches the mould cavity. • Should be economical and easy to implement and remove after casting
Following factors are controlled for proper functioning of gating system: 1- Type of pouring equipment; ladles, basins etc. 2- Temperature/fluidity of molten metal. 3- Rate of liquid metal pouring. 4- Type and size of sprue. 5- Type and size of runner. 6- Size, number and location of gates connecting runner and casting. 7- Position of mold during pouring and solidification.
Pouring basin: • Reduces the momentum of the liquid flowing into the mould by settling first into it. It should be deep enough.
• The pouringbasin depth of 2.5 times the sprue entrance diameter for smooth metalflow and to prevent vortex formation.
• To avoid vortex forming, it is necessary that the pouring basin be kept full and constant conditions of flow are established.
• This is achieved by using a delay screen(dam) or a strainer core. A delay screen is a small piece of perforated thin metalsheet placed in the pouring basin at the top of the down sprue.
• Pouring basins are most desirable for alloys, which form troublesome oxide skins (aluminium, aluminium bronze, etc.)
Pouring basin with dam
Sprue: Sprue
is a vertical channel through which the molten metal flows downward in the mould. The
sprues should be tapered down to take into account the gain in velocity of the metal as it flows down reducing the air aspiration. The
exact tapering can be obtained by equation of continuity.
Sprue base: As
the molten metal leaves the sprue, it travels at its highest velocity and develops its maximum energy. At
the sprue base, the direction of flow is abruptly changed, which causes severe turbulence. By increasing
the area of sprue base, both the velocity and the turbulence of metal is reduced. As the sprue base is filled, the molten metal acts as a cushion to absorb the impact of the falling stream. For
the sprue base to function properly, its bottom surface must be flat.
Runners: • connects the sprue to its in-gates, allows metal enter the mould cavity. The runners are made trapezoidal in cross-section.
• It is a general practice for ferrous metals to cut the runners in the cope and the ingates in the drag. The main reason for this is to trapthe slag and dross, which are lighter and thus trapped in the upper portion of the runners.
Gates: • connects the runner to mold cavity, feeds liquid metal to casting at rate consisteny with rate of solidification
• Size of gate depends on rate of solidification Usually a Gate Basin is provide that act as reservoir and trap
Design Of Gating System: turbulence in gating systemFluid flow in duct is related by Reynold number R n : R n= mean velocity flow x dia. of duct x density of liquid = V.d. ρ viscosity of liquid µ Reynold number > 2000, for turbulent flow Ordinary gating system experience Reynold number ranging from 2000 to 20,000, hence metal floe turbulent
Metal flow rate and velocity calculation Based on laws of fluid dynamics: Law of continuity Q=A 1 V 1 =A 2 V 2 Where, Q- vol. rate of flow A- cross-sectional area of flow passage V- linear velocity of flow Bernoulli’s theorem
Total energy of unit weight of fluid remains constant throughout fluid system. Total energy = kinetic energy+ potential energy+ pressure energy = (V 1)² + h1 + P1 = (V 2)2 +h2 + P2 ρg ρg 2g 2g where, V- linear velocity of flow ρ- density
h- height above datum line p- pressure
When
liquid poured in pouring basin at A the velocity can be found at B as: V B = √(2g h A )
Bernoulli’s theorem can be modified considering energy losses as: = h1 + P1 = (V 2)2 +h2 + P2 +hf +h b ρg ρg 2g
h b= energy loss at bend= K (V b)2 2g
K- loss coefficient at bend g- gravity in cm/s2
V b- velocity at bend in cm/sec
hf = energy loss due to friction in kg= fL (V p)2 d 2g Flow velocity at in-gate
f- friction loss coefficient L- length of pipe in cm
d- channel diameter in cm V p – velocity throgh pipe in cm/s
Applicability of laws: Law of continuity holds good only for ducts that run in full Channels are assumed to be of uniform cross-section with single bent Originally sprue is tapered, hence, cross section area changes Gating system may incorporate more than one bend Equations can be employed to find:
Fluid flow efficiency of exsisting gating systems
Effect of changes in gating desing on turbulence & rate of flow
Design of gating can be classified under heads:
Design of pouring basin
Design of sprue
Pouring Time
Design of Runners & Gates
Design criteria for pouring basin: For establishment of proper uniform flow system as under full flow, following measures are taken: Streamlining of pouring basin Use of strainer core Use of dam Use of sprue plug
Design of Sprue: The sprues are tapered down to take into account the gain in velocity of the metal as it flows down reducing the air aspiration. The exact tapering can be obtained by equation of continuity. Denoting the top and the choke sections of the sprue by the subscripts t and c respectively, we get AtVt=AcVc Since the velocities are proportional to the square of the potential heads,
then from Bernoulli’s equation
At Ac
hc
ht
According to eqn. sprue should have parabolic taper but for sake of simplicity its kept straight
In straight sprue there’s a loss of pressure head as metal moves down, hence vortex may form Tapering is done to avoid vortex and aspiration Taper reduces rate of flow in mold cavity prevents erosion Least area at sprue exit controls: Flow of metal in mold cavity Velocity of liquid metal Pouring time
• • •
Choke part of gating system having smallest area Free gating system: Sprue serves as choke Creates non-uniform metal distribution Choked system: gate serves as choke Creates pressurised system
The choke area can be calculated using Bernoulli’s equation as C a
W
c.d .t
2 gH
Ca- choke area c- nozzel coefficient T- pouring time
W- weight of casting d- density of liquid H- liquid metal head
Pouring time: time for complete filling of a mould is called pouring time. High pouring rate leads to mold erosion Low pouring rate may not permit complete filling of mold cavity in time Therefore, its neccesary to know optimum pouring time.
Pouring time for some metals are:-
Design Of Runner & Gate: Gating Ratios: refers to the proportion of the cross sectional areas between the sprue, runner and ingates, and is generally denoted as sprue area: runner area: ingate area Depending on the choke area there can be two types of gating systems:
• Pressurised- cross section area decreases towards mold cavity Back
pressure maintained volume flow rate same from every ingate ratio of order 1:0.75:0.5 Higher casting yield High velocity of metal causes erosion, turbulence, dross formation & aspiration
Non-Pressurised- cross section area increases towards mold cavity have choke at the sprue base has total runner area and ingate area higher than the sprue area no pressure existing in the metal flow system and thus it helps to reduce turbulence The passages remain incompletely filled volume flow different from every ingate ratio of order 1:4:4 Gating ratio and ingates position should be such that: Misruns, laps and cold shuts are avoided Turbulence and erosion in thick sections are reduced
material aluminium
gating ratios 1:2:1, 1:1.2:2, 1:2:4, 1:3:3, 1:4:4, 1:6:6
Aluminium bronze
1:2.88:4.8
brass
1:1:1, 1:1:3, 1.6:1.3:1
copper Ductile irpn
2:8:1, 3:9:1
1.15:1.1:1, 1.25:1.13:1, 1.33:2.67:1
Grey cast iron
1:1.3:1, 1:4:4, 1.4:1.2:1, 2:1.5:1, 2:1.8:1, 2:3:1, 4:3:1
Magnesium
1:2:2, 1:4:4
Malleable iron
1:2:9.5, 1.5:1:2.5, 2:1:4.9
steel
1:1:7, 1:2:1, 1:2:1.5, 1:2:2, 1:3:3, 1.6:1.3:1
Methods adapted for equalising volume of liquid flow at ingates: Bending runner away from mold cavity Enlargements in gating system to nullify momentum effects
Stream lining the gating system: Sharp
corners &junctions are removed Sprue is tapered Providing basin instead of cup
Advantages of streamlining: Low
turbulence, erosion , dross and air entrapment Reduced metal flow rate Sound and clean metal casting
A streamline gating system
