Production Technology

J.BABU

AssociateProfessorDepartment of Mechanical Engineering

Sreenidhi Institute of Science and Technology

Gating System Design

Lecture Objectives

Pouring Time

Choke Area

Sprue

Pouring Basin

Sprue Base Well

Gating Ratios

Ingate Design

Slag Trap Systems

- Runner Extension

- Whirl Gate

Pouring time Objective for gating system design

to fill the mould in smallest time Pouring time: Time required to completely fill the mold Too long pouring time higher pouring temperature Too less pouring time turbulent flow in the mold Aim Optimum Pouring Time for any given casting Pouring time depends on

- casting material

- complexity of casting

- section thickness ratio of surface area to volume is imp. rather than total mass of the casting

- size of casting Relations are experimentally obtained rather than theoretical

formulations.

Standards methods to calculate pouring time for different materials:

1) Grey CI (mass < 450kg)

2) Grey CI (mass > 450kg)

kg casting, the of mass W

mm thickness, section averageT 40

inches in iron ofFluidity K where

s W15.59

T1.41Kttime, Pouring

s W15.65

T1.236Kttime, Pouring 3

Pouring time cont..

Casting Mass Pouring time (s)

20 kg 6 to 10

100 kg 15 to 30

100, 000 kg 60 to 180

3) Grey CI (mass < 450kg)

4) Shell moulded ductile iron (vertical pouring)

5) Copper alloy castings

s W Wlog 0.3953 -2.4335Kttime, Pouring

sections heavier for 2.970

thick mm 25 to 10 sections for 2.670

sections thinner for 2.080 K where

s W Kttime, Pouring

1

1

Pouring time cont..

2.80 Bronze Tin

1.90 Brass

1.80 gating Bottom

1.30 gating Top

by given constant a is K

s W Kttime, Pouring

2

32

6) Intricately shaped thin walled castings with Grey CI mass <450kg

where W` = mass of the casting with gates and risers, kg

K3 = a constant as given below

7) For casting above 450 kg and upto 1000 kg

where K4 = a constant as given below

s W Kttime, Pouring 3 '3

Pouring time cont..

T (mm) K3

1.5 to 2.5 1.62

2.5 to 3.5 1.68

3.5 to 8.0 1.85

8.0 to 15.0 2.20

s T W Kttime, Pouring 3 '4

T (mm) K3

Up to 10 1.00

10 to 20 1.35

20 to 40 1.50

Above 40 1.70

Choke Area The main control area which meters the metal flow into the mould cavity

- so that that mould is filled within the calculated pouring time Located at the bottom of the sprue

used system gating the of function a is whichfactor efficiencyC

mm height), (sprue head metal effectiveH

mm/s gravity, to due onacceleratig

kg/mm metal, molten the ofdensity massd

stime, pouringt

kgmass, castingW

mm area, choke Awhere

H g 2C t d

W A Area,Choke

3

2

3

2

Choke Area

Different Gating Systems

Choke Area cont… The effective height ,H of a mould depends on the casting dimensions

and the type of gating used.

cavity mould of height totalc

cope incavity mould of heightp

sprue of heighth where2c

p-hH gate, Parting

2

c-hH gate, Bottom

hH gate, Top

2

Efficiency of the Gating System Efficiency coefficient of the gating system depends on the various

sections that are normally used in the gating system. Elements circular in cross-section

lower surface area to volume ratio reduce heat loss less friction

Streamlining various elements of gating system causes increase in volumetric efficiency of the gating system allow smaller size gates and runners increase in casting yield.

Overall efficiency can be calculated by considering the loss in metal head when a runner changes direction or joins with another runner or gate.

area choke the is A

changes from stream down areas ....are A,A

area or direction in changes at occuring tscoefficien loss ....areK ,K where

AA

KAA

K1

1C factor, Efficiency

21

21

22

2

221

2

1

Sprue

Should be tapered down to take into account the gain in velocity of the metal at it flows down reducing the air aspiration.

The exact tapering can be obtained by

AtVt = AcVc

At = Ac(Vc/Vt)

Since, velocities are proportional to the square of the potential heads,

At = Ac (hc/ht)

Equation indicates that the profile of the sprue should be parabolic as per the above equation.

As it is difficult to make a parabolic a straight taper is made. Straight taper

- reduces air aspiration

- increase the flow rate compared to a parallel sprue.

Sprue cont… Should be tapered down to take into account the gain in velocity of

the metal at it flows down reducing the air aspiration. The exact tapering can be obtained by

AtVt = AcVc

At = Ac(Vc/Vt)

Since, velocities are proportional to the square of the potential heads,

At = Ac (hc/ht)

H= actual sprue heightHt

= h + H

Pouring Basin

The main function of the pouring basin is to reduce the momentum

of the liquid flowing into the mould.

To prevent the turbulence of the molten liquid the pouring basin

should be deep and the entrance into the sprue should be a

smooth radius of atleast 25 mm.

To prevent vortex formation the pouring basin should be kept full.

Pouring Basin cont… Constant flow conditions should be maintained by using a delay

screen or strainer core.

The metal should be poured steadily into the pouring basin keeping the lip of the ladle as close as possible.

Pouring basins are preferable with castings in alloys which form troublesome oxide skins (aluminium, aluminium bronze, etc..)

Sprue Base Well

It is provided at the bottom of the sprue. It helps to reduce the velocity of the incoming metal and mold

erosion. General guideline,

- area of sprue base well = 5 * sprue choke area.

- well depth = runner depth

Refers to the proportion of the cross-sectional area between the

sprue, runner and ingates Denoted as sprue area : runner area : ingate area It is selected depending on the characteristics of molten metal

being cast

Factors that are considered are

- fluidity

- slag or dross forming tendency

- pouring temperature

- mould material characteristics like resistance to erosion,

scabbing tendency, green sand, CO2, dry sand, shell molded, .

Gating Ratios

Some gating ratios used in practice

Metal Gating Ratio

Aluminium

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

Copper2 : 8 : 1

3 : 9 : 1

Ductile iron

1.15 : 1.1 : 1

1.25 : 1.13 : 1

1.33 : 2.67 : 1

Some gating ratios used in practice cont…Metal Gating Ratio

Grey cast iron

1 : 1.3 : 1.1

1 : 4 : 4

1.4 : 1.2 : 1

2 : 1.5 : 1

2 : 1.8 : 1

2 : 3 : 1

4 : 3 : 1

Magnesium1 : 2 : 2

1 : 4 : 4

Malleable iron

1 : 2 : 9.5

1.5 : 1 : 2.5

2 : 1 : 4.9

Steels

1 : 1 : 7

1 : 2 : 1

1 : 2 : 1.5

1 : 2 : 2

1 : 2 : 2

1 : 3 : 3

1.6 : 1.3 : 1

Ingate

It can be considered as a weir with no reduction in cross-section of the stream at the gate.

The rate of flow of molten metal through the gates depend on

- the free height of the metal in the runner and gate area

- the velocity with which metal is flowing in the runner.

where Q = metal flow rate, mm3/s, b = gate width, mm

V = metal velocity in runner, mm/s, g = acceleration due to gravity (mm/s2)

Height of the gate, h1 = h -5 mm

mm 2g

V

gb

Q 1.6hheight, Free

2

2

2

Ingate cont..The points to be remembered while choosing the positioning of the ingates

1. Ingates should not be located near a protruding part of the mould to avoid the striking of vertical mould walls by the molten metal stream.

2. Ingates should be preferably be placed along the longitudinal axis of the mould wall.

3. Ingates should not be places near a core print or a chill.

4. Ingates cross-sectional area should preferably be smaller than the smallest thickness of the casting.

Small castings one ingate Multiple castings multiple ingates In case of multiple ingates,

- they should have uniform area.

- they should be located at constant intervals.

- the runner area should be progressively reduced.

Slag Trap Systems

Runner extension Metal which moves first into the gating system contains slag and

dross. To prevent these from entering the mould cavity the runner is

extended beyond the ingate- the momentum will carry it past the gates into a blind alley

width of runner extension = 2 runner width

Slag Trap Systems cont..

• Whirl gate

Depending on the gating ratio two types of gating systems depending on the choke area:

1. Non-pressurized

2. Pressurized

Sprue

Runner

Ingate

Types of Gating Systems

Non-pressurized gating system Total runner area and ingate areas higher than sprue area No pressure existing in the metal flow system – low turbulence Useful for casting – drossy metals and alloys Gating ratio - 1:4:4

Drawbacks Low casting yield-due to large metal in runners and gates The metal flow should be full in all elements – else - air

aspiration

Pressurized gating system Ingate area is the smallest – more backpressure in the system Turbulent metal – flows full in the system Not used for light alloys, suitable for ferrous castings Provides higher casting yield Gating ratio – 1:2:1

Two basic principles of fluid flow are relevant to gating design:

Bernoulli’s theorem and the Law of mass continuity

Bernoulli’s theorem

Based on the principle of conservation of energy and relates pressure, velocity and elevation of the fluid at any location in the system, and the frictional losses in a system that is full of liquid

2ph+ constant

g 2

v

g h - elevation above a certain

reference lane

p -pressure at that elevation

v - velocity of the liquid

- density of liquid

g -gravitational constant

f - frictional loses

2 21 1 2 2

1 2

p ph h

g 2 g 2

v vf

g g

For two different elevations of a liquid Bernoulli’s equation is :

Law of mass continuity

For incompressible liquids and in a system with impermeable walls, the rate of flow is constant.

Q = AV=constant

For two different locations

Q = A1V1 = A2 V2

Q - Rate of flow m3/sec

A - cross-sectional area of liquid stream

V - average velocity of the liquid

Sprue The sprue should be tapered to take into account the gain in

velocity and thus reduce aspiration

The exact shape can be obtained by applying Bernoulli’s equation and continuity equation

1

2

3

hc

ht

h2

Atmospheric pressure

Open to Atmosphere

mould

sprue

2

3

Actual

Ideal

hc

ht

Ideal and actual shapes of sprue

Casting Yield

• Higher the casting yield higher is the economics of the foundry practice.• Hence the casting yield should be maximized during the design stage itself.

New material Metal melted Scrap metal

Melting losses Metal cast Scrap castings

Fettling losses Actual castingRunners &

Risers

Utilization of the metal in the foundry

mould the into poured metal the of mass - w

mass casting actual - Wwhere

100w

W yieldCasting

Casting Yield cont…

Casting Yields for different metals

Casting description Yield range

Simple shape and massive 0.85 to 0.95

Steels

simple shape 0.75 to 0.85

heavy machinery parts 0.65 to 0.75

Small pieces 0.35 to 0.45

Cast iron

heavy machinery parts 0.65 to 0.75

Small pieces 0.45 to 0.55

Aluminium 0.25 to 0.45

Casting of an Aluminum Piston

Aluminum piston for an internal combustion engine: (a) as-cast and (b) after machining.

Simulation of mold filling and solidification

(a) 3.7 seconds after start of pour. Note that the mushy zone has been established before the mold is filled completely. (b) Using a vent in the mold for removal of entrapped air, 5 seconds after pour.

Types of Internal and External Chills used in Casting

Various types of (a) internal and (b) external chills (dark areas at corners) used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c).

Design Rules for Casting

Suggested design modifications to avoid defects in castings

Elimination of Hot Spots

Examples of designs showing the importance of maintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities.

Examples of Good and Poor Designs

Examples of undesirable (poor) and desirable (good) casting designs. Source: Courtesy of American Die Casting Institute.