Metal casting

Gating system

A GATING SYSTEM is the conduit

network through which liquid metal

enters a mold and flows to fill theenters a mold and flows to fill the

mold cavity, where the metal can

then solidify to form the desired

casting shape

.A pouring cup or a pouring basin provides an opening for the introduction ofg g g

metal from a pouring device.

A sprue carries the liquid metal down to join one or more runners, which

distribute the metal throughout the mold until it can enter the casting cavity

through ingates

Pouring Basin: An enlarged portion at the top of the sprue.

S A ti l th h hi h th lt t l t t thSprue: A vertical passageway through which the molten metal gets to the runner.

Choke: A restriction in the gating system that limits the flow rate of the molten

metal.

Runner: The passageway metal flows through to get to the gates from the sprue.

Runner Extension: Short extension of runner which goes beyond the last gate.

In-gate: a Small passage from the runner to the mold entry .

Riser: A vertical passageway which provides a source of hot metal to prevent

shrinkage in the castingshrinkage in the casting.

Desirable design considerations for gating system

• Rapid mold filling

The design considerations are many times conflicting

p g

• Minimizing Turbulence

• Avoiding Mold and Core Erosion

• Removing Slag, Dross, and Inclusions

• Promoting Favorable Thermal Gradients

• Maximizing Yield

• Economical Gating Removal

• Avoiding casting distortion• Avoiding casting distortion

• Compatibility With Existing Molding/Pouring Methods

• Controlled Flow Conditions• Controlled Flow Conditions.

Pouring time

The pouring time is calculated based on the integrated energy balance equation on the basis of unit mass flow (Bernoulli’s equation)

Case –a:  

• Assumptions:

• Pressure at point 1 and point 3 is equal  (i.e.  P3  =  P1)p p q

• Molten metal level at point 1 is maintained constant  .  i.e. v1 = 0 

• Frictional losses along the mold surfaces are neglected.

Applying Bernoulli’s equation between point 1 and point 3 givesApplying Bernoulli s equation between point 1 and point 3 gives

Time taken to fill the mold cavity, tf : f

V= Volume of the Casting

Ag = Cross sectional area of the  gate.g g

• Care should be taken to ensure that pressure anywhere in the liquid metal does not fall below atmospheric pressure.

• Applying Bernoulli’s equation between (3) and (2) for case (a)• Applying Bernoulli s equation between (3) and (2)  for case (a),

If pressure at point (3) is atmospheric,  P3 = 0

Then         . (since  v2 = v3)

Thi i t t bl i t t ( i tiThis is not acceptable since gas entrapment  (aspiration effect)from the porous mold in to the molten metal will take place due to the negative pressure at point 2.

The sprue should be tapered to prevent this.  

Let in the limiting case, p2 = 0.      From the previous equation,

The ideal case can be obtained as follows:

From continuity equation, A2v2 = A3 v3

Also P1 = p3 = 0   and v1 = 0

The limiting condition for preventing aspiration effect is 

i.e.

During fluid flow the non-uniform velocity distributions can be

accounted by replacing the (velocity)2 by Where is the average velocity and

= 0.5 for laminar flow and = 1 for turbulent flow

Energy loss due to friction in a circular conduit based on per unit mass, Ef

= average velocity, D,L = diameter and length of the conduit,f =friction factor.

This should be added to the energy at station (2) in the integrated energy equation, when the flow is from station (1) to station (2).For a smooth conduit

h

for turbulent flow, which can be modified to 

where

The frictional loss (per unit mass ) associated with sudden enlargement or contraction in flow area

= the average velocity in the small conduit and= friction loss factor

The integrated energy equation between (1) and (3) thus becomes

Where = average fluid velocity in the sprue.

With P1 = p3 and using above equations, we get

d = diameter of the sprue, l = length of the sprue (=h2)p , g p ( 2)

where discharge coefficient,

Pouring time (t)Pouring time (t)

The time for complete filling of the mold

Pouring Time of casting

(i) Grey cast iron. Mass less than 450 Kg

(ii) Grey cast iron, mass > 450 Kg

K = fluidity factorK fluidity factorT = average section thickness of castingW = mass of Cast iron

(iii) Steel casting,

(i ) Sh ll ld d d til i

K1 = 2.08 for thinner section= 2.67 for 10 mm < thickness < 25 mm

(iv) Shell molded ductile iron

= 2.97 for heavier casting

For further details regarding other alloys and casting shapes, Refer to ,

P.N. Rao, Manufacturing technology, Foundry, forming and welding, Vol 1 eds 3 MGH 20111, eds. 3, MGH, 2011.

Choke area

Choke is the main control are which decides the metal flow in to the mould cavity so that mould is completely filled within the pouring time. This is the minimum area in the gating system g g g yand can be either at the bottom of the sprue or at the in-gate.

A = Area of the choke , mm2W = Casting Mass , kgW Casting Mass , kgt = Pouring time, sD = Mass density of molten metal , kg/mm2

H = Effective metal head mmH = Effective metal head, mmC = Efficiency factor

Effective Metal head, H

Efficiency coefficient of gatingsystem depends on the various sections in the gating system i e C/s change in directions of flow surfacein the gating system. i.e. C/s, change in directions of flow, surface roughness, etc.

Efficiency coefficients for various types of gating systems

Type of system Taperedchoked sprue

Straight spruerunner choke

Straight runner 0 90 0 73Straight runnerentering  runner

0.90 0.73

Two runner with multiple ingate no bends in runners

0.90 0.73ingate, no bends in runners

Two runner with multiple ingate, 

0.85 0.70

90° bends in runners

Gate elements Sharp Round

Loss coefficients for various gate elements

p

Sprue entry from pouring cup 0.75 0.20

Bend of sprue in to runner 2.00 1.00

Right angle bend in runnerRight angle bend in runnerSquare Cross sectionRound  cross section

2.001.50

1.501.00

Junction at right angles to runner 4 0 ‐ 6 0Junction at right angles to runner 4.0   6.0

Junction with 25% or more area reduction from runner in to in‐gates

2.00 0.50

Runner choke when choke area is approximately oneRunner choke when choke  area is approximately one third runner area, plus bend of sprue in to runner 13.00 ‐‐‐‐

Loss from wall frictionRound channel loss = 0.02 (L/D)Round channel loss   0.02 (L/D)

Square channel  loss = 0.06 (L/D)

Rectangular channel loss = 0.07(A+B)/(2 A B)where A= one side of rectange and B= other side of rectange

Gating ratio

Gating ratio is the ratio of ;Gating ratio is the ratio of ;

Sprue area : runner area : in‐gate area

Two types of gating:

a. Non pressurized gating system

b. Pressurized gating system

i d iNon‐pressurized gating system: Choke at the bottom of the sprue

No pressure existing in the gating system No pressure existing  in the gating system

Reduced turbulance

For alloys forming dross , Al., Mg., etc

T d f d Tapered sprue preferred

Typical ratio  1: 4: 4

Disadvantages of Non‐pressurised gating system

• To be designed carefully to see that all parts flow full• To be designed carefully to see that all parts flow full

• Runners are kept in the drag and ingates in the cope

• Low yield due to large volume of metal.y g

Pressurized gating systemPressurized gating system Normally ingate is the choke region

Maintains a positive pressure in the entire gating system Maintains a positive pressure in the entire gating system

Metal is more turbulent

Higher casting yield due to less volume of metal in runner Higher casting yield due to less volume of metal in runner

and in-gates

Used for ferrous alloys

Typical ratio : 1:2:1

Types of gates

Design of gating system   must consider  Ease of moulding

Avoid turbulent flow

Prevent washing of sand from mold walls

Prevent aspiration of air

Prevent dross or slag entering the mold along with molten metal

Various designs of gates are being practiced

Parting Gates

Skin bob and relief sprues collect dross or slag

Principles of Metal casting, R.W. Heine, C.R. Loper, P.C. Rosenthal, Tata McGraw Hill

Shrink bob serving dual purpose ofShrink bob serving dual purpose of dross collector and metal reservoir

Core inserts to filter the metal or prevent mold erosion

Principles of Metal casting, R.W. Heine, C.R. Loper, P.C. Rosenthal, Tata McGraw Hill

Bottom Gate

Branch gate used to feed a singleBranch gate used to feed a singlecasting at several points or anumber of individual casting

Top gates

Riser

M t i l V l t i Sh i k (%)Material Volumetric Shrinkage (%)

Medium carbon steel 2.5‐3.00

1% carbon steel 4.00

Pure Aluminum 6.60

Pure copper 4.92

i lGrey cat Iron 1.90 to negative values depending on carbon %

White cast iron  4.0 ‐5.0

Functions of a riser is to feed metal to the casting as it solidifies.Directional solidification promoted

Requirements of a good riser

Th h ld b ffi i t lt t l t• There should be sufficient molten metal to feed the casting.

• Facilitates directional solidification

• The riser should solidify after the casting .

Modulus method

• Modulus (M)  is the inverse of the cooling characteristics (i.e. surface/volume ratio).

Th l l i• The general rule is, 

MR ≥ 1.2 McR c

Divide the whole section in to simple geometrical shapes Divide the whole section in to simple geometrical shapes.

Determine the modulus of each section considering by not

considering the surface area in contact between adjacent sections

Determine the section with the most significant modulus.

Cooling characteristics of a casting can be represented by the ratio ofby the ratio of

If this ratio of the casting is higher than riser thenIf this ratio of the casting is higher than riser, then casting will solidify faster.

Chvorinov’s ruleChvorinov s rule 

t = solidification time s k = mold constantts = solidification time, s,    k = mold constant

Freezing ratio XFreezing ratio, X

Caine’s method

Riser volume determination by Caine’s method

Y = riser volume /casting volumea, b, c are constants and depends on the materials

Naval Research Laboratory

Here a shape factor is considered where the shape factor is defined asshape factor is defined as

The length, width and thickness are computed from the maximum dimensions of the casting section.

The shape factor replaces the freezing ratio of the caine’s method.

Riser volume determination by NRL methodRiser volume determination by NRL method.

ChillsEff t f hill f di di tEffect of chills on feeding distance

Chills