FOUNDRY MARCH/APRIL, 2012

Gating System for a Piston

G. M. Rajendra PrasadA/14, K. H. B. Colony, PuttenahalliYelahanka, Bangalore - 560 064.

Email : gmrpsbc@gmail.com • Mobile : 094805 04635

1. River Krishna originates in Maharashtra and entersKarnataka, crosses Almatti dam, Narayanpur dam, and thengoes to Andhra Pradesh; crosses Sree Sailam dam andNagarjunsagar dam, and meets the Sea.

Why? River Krishna’s this flow is due to potential energydifference in the flowing water, caused by the ground gradient.

Have you ever seen River Krishna water from sea flowingback to the origin of Krishna River? No, it does not happen,because it is against the positive potential energy gradient.

Similarly, in a solidifying piston casting for heat transfer totake place in the correct direction, the hottest metal shouldbe in the Feeder, and the coldest metal in the Skirt. Thisestablishes a positive temperature gradient from Feeder toSkirt to get a sound Piston casting.

I have seen many foundries, even today, adopting differentgating systems without following this principle of heat transfer,and experiencing continuously high rejection.

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2. First law of Newton states that any moving object continuesto move in the same direction till it is acted upon by an externalforce.

In the mould cavity of a Piston casting, molten metal enteringthe cavity changes its direction of flow after hitting the coretool.

In Fig. 1 upto point ‘a’, it is OK, but above ‘a’ metal is gettingpushed from the Gate portion, and as a result hot spotremains near the Gate. Cold metal goes to the Crown andthe Feeder. So is the case in Fig. 2, despite the fact that youhave your gate right upto the Groove Insert (GI).

Gating system highlighted in Fig. 2 is being followed by oneleading Piston manufacturer in India.

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3. In order to get a positive temperature gradient, Gate couldbe in any of the following patterns:

Fig. 3 Fig. 4

To ensure that you get a sound casting Hot Spot should betaken as close to Groove Insert as possible.

Fig. 5

4. When the solidification time of a casting is 3 minutes andabove, pouring time is only 30 to 40 seconds. Since the dieis in tilted position during pouring, time of flow of metal overGI (Groove Insert) remains the same.

But the fear in “c” is that while fettling, if there is a vibration inthe cutter, it could lead to de-bonding. If you are certain thatthere is no vibration while cutting the gate, you can go in for“c”. It gives sound casting.

“a” and “b” are slightly inferior to “c” as the Hot Spot is justbelow the GI, whereas in “c” Hot Spot is at the Tip (i.e. top) ofthe Crown (an Ideal condition).

This facilitates positive temperature gradient from Crown toSkirt (Please remember the flow of River Krishna!) Hot Spotis dictated by the last metal poured. By the time the cavity isfull, some amount of molten metal would have solidified inthe piston. Please ensure that Hot Spot is as close to Crownas possible to get a sound Piston casting.

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Same as in Fig. 3, but with little bitof turbulence at the bottom.

(a) (b)

(c)

This is IDEAL.

spacefor

author’sphoto

FOUNDRY MARCH/APRIL, 2012

5. So far we are looking at the temperature gradient onlyvertically. We need to look at the temperature gradienthorizontally (i.e. circumferenetially) also.

That can be achieved by making the Piston oval, opposite tothe gate. Casting section is as per the drawing, and at thegate it is slightly thicker.

To combine a positive temperature gradient, both horizontallyand vertically, make the casting oval and give a taper of 1° or2° vertically at the gate (thicker at the crown).

If one doesn’t want to make the piston Oval, he can resort tothis (Fig. 6) :

To explain this point, some time after you put on an electricalwater heating boiler, you will find that the water at the top iswarmer and water below is colder. This is because, warmerwater is lighter in density, hence it comes to the top.

Similarly, in this through & through hole, as the die gets heatedupon pouring molten metal, air gets heated up and rises up,vacuum is created at the bottom, so cold air from atmosphereis sucked in.

This ensures positive temperature gradient circumferentially.

But care is to be taken to see that this hole is cleaned withrotary brush to ensure removal of any scaling formed. (Scaleforms because of moisture in air.)

Days are not far off when the entire die will be water-cooledto reduce the solidification time to get higher production.

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Top of the die

6. Concluding Remarks(5 different cases indicating different gating systems)

Case A

Good for temperature gradient, but results in lot of turbulence,and this leads to inclusions.

Case B

Least turbulence, but the hottest metal is at the bottom (Skirt),and the coldest metal is at the top (Crown). This does notgive a sound casting as temperature gradient is negative.

Case C

Positive temperature gradient present below the gate, butgradient is negative above the gate as the cold metal getspushed up from the gate.

Case D

O.K.

Case E

The Best Result : There is no turbulence. Temperaturegradient is positive right from the Bottom of the Piston to itsTop. Gives the best castings.

(PTO for additional information about Modulus calculation)

Bottom of the die

FOUNDRY MARCH/APRIL, 2012

Steel Aluminium

100 dia x 100 mm heightsolidification time : 5 minutes

100 dia x 100 mm heightsolidification time : 12 minutes

Sand Feeder Sand Feeder

From the above experiment the constants can be arrived at.

Modulus of Feeder (or the casting portion which feeds thelower portion of the casting) is

Mfeeder = 1.2 Mcasting

(for steel casting solidifying in a sand mould).

Ascertain the equivalent value of 1.2, for aluminium alloy inmetallic mould. Assume that it is ‘x’

Solidification Time = ‘x’ (Modulus)2

‘x’ being the constant.

In order to get a sound casting M5 > M4 > M3 > M2 > M1 (inthe Piston casting).

If it is not so, it has to be achieved by

(a) Increasing the section thickness of Piston on OD at thepoint of contact in question, or

(b) Air or Water Cooling to achieve M5 > M4 > M3 > M2 > M1

There are so many other unknowns, such as

– Thermal Conductivity of alloy at 780 °C

– Specific Heat of alloy at 780 °C

– Thermal Conductivity and Specific Heat of the mould (itstemperature is unlikely to go beyond 300 to 350 °C).

A small R & D set up has to be established to take care of allthese factors to get 100 % sound Piston castings.

�

�

10 dia hole

Mild SteelMould

5 mm�

100 H

100 dia

170 dia

200

mm

H

Sectioned Side View

5

4

3

2

1

Additional Information

The Concept of Modulus

This concept is popular in ferrous foundries, but rarely prac-ticed by non-ferrous foundrymen. This concept can be fruit-fully adopted by Aluminium and other non-ferrous foundries.

Modulus = (Volume / Surface Area)

Volume indirectly represents the quantity of heat present.

Surface Area indicates the area through which heat fromthe solidifying casting/riser can be lost (i.e. transferred) tothe mould/core/die.

In case of a solidifying steel casting (in a sand mould)

Solidification Time = 2.1 (Modulus)2

Where solidification time is in ‘minutes’, and modulus in ‘cm’.‘2.1’ is a constant.

Value of the ‘constant’ for Solidification of an aluminiumcasting in a sand mould can be found out.

Solidification time of steel casting in sand mould (for a givendiameter and height (Fig. A), works out to 5.4 minutes(theoretically), but the actual time is only 5 minutes becauseof radiation loss from the top of the feeder.

The following experiment was conducted to ascertain thesolidification time of aluminium in a mild steel mould.

Solidification Time = Constant (Volume/Surface Area)2

“Al-12%Si-1%Ni-1%Cu-1%Mg” alloy was put into threemoulds. Molten alloy temperature was 780 °C.

Three experiments were conducted (Fig. B for die details) :

1. In the first experiment, the 10 mm dia hole was filled withsodium silicate sand (i.e. no external cooling).

2. In the second experiment compressed air was used tocool through 10 mm dia hole.

3. In the third experiment, water cooling was used through10 mm dia hole.

Solidification Time (with different conditions)

Expt. No. No Use of Coating withDie Coating Die Paint

1. 3.5 minutes 5.0 minutes

2. 2.5 minutes 3.25 minutes

3. 2.33 minutes 3.15 minutes

Fig. A

Fig. C

Fig. B