Mold Filling Process Control of Molten Aluminum

in Permanent Molds with a Web Gating System

Qingming Chang1,a, David Schwam2,b, Changjun Chen1,c, Jiulang Xiong1,d and John F.Wallace2,b

1 Key Laboratory for Ferrous Metallurgy and Resources Utilization

of Ministry of Education, College of Materials and Metallurgy, Wuhan University of Science and Technology, China

2Case Western Reserve University, U.S.A

achangq@hotmail.com,

bdxs11@case.edu,

cchangjunchen@yahoo.com.cn,

dxiongjiulang@163.com

Keywords: Permanent mold, web gating system, numerical simulation, flow behavior, aluminum castings

Abstract. Web gate system of aluminum castings in permanent molds is investigated in order to

improve the quality of aluminum castings produced in permanent molds. The metal flow in the mold

were observed and conducted using graphite molds and real time X-ray radiography recorded at a rate

of 30 images per second through those molds. The affects of web thickness on flow patterns, gas

entrapment, jetting possibility are studied and discussed. Flow and solidification simulation programs

were employed to predict the flow behavior under the different conditions that can prevail in

permanent mold gating. The study highlights the characteristic features of web gate system used in

permanent mold aluminum foundries and recommends gating procedures designed to avoid common

defects, and provides direct evidence on the filling pattern and heat flow behavior in permanent mold

castings.

Introduction

Web gating system was employed in many cast gating designs because of its favorable filling pattern.

It owns the advantages of both bottom and top gating system, and therefore, can leads the hot molten

metal to fill the cavity in such a sequence that it can forms a temperature gradient suitable for feeding

during solidification.

However, some researches pointed that not all web gating systems have this configure [1]. Only a

specially designed web gating system, f.g., a web gating system with a side riser which has a smaller

cross section at the bottom and bigger one at the top, owns this favorable filling pattern. Lots of

research works [2-5] have been conducted using water flow that is photographed and considerable

information is available from this work. Other studies have shown the behavior of liquid aluminum in

sand molds using real time X-ray photography [6-8]. These previous works have shown several

features in aluminum gating system that have significance, including the maximum velocity of flow

that can be allowed and still maintain a minimum amount of oxide films. However, none of these

X-ray studies is directly applicable to permanent molds. It is not feasible to use metal molds in real

time X-ray studies because the higher density of the normal ferrous mold does not allow appropriates

definition of the flowing aluminum alloys.

In this paper, a graphite mold was utilized which is oriented in a vertical direction and parted the

center of the casting and gating system. The lower density of the graphite allowed the flow of

aluminum through the gating system and into the mold cavity to be monitored. In this way, the actual

flow behavior of molten aluminum alloys in permanent molds could be recorded. With appropriate

adjustments, a computer simulation has been developed which can predict the flow patterns that

actually occur in unfilled molds.

Materials Science Forum Vols. 575-578 (2008) pp 63-68online at http://www.scientific.net© (2008) Trans Tech Publications, SwitzerlandOnline available since 2008/Apr/08

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of thepublisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 221.232.142.124-10/04/08,02:21:30)

Experiment Sets

The equipment employed in this study is shown in Figure 1. The source of X-rays is an electric tube

that can be operated up to 160 kilovolts and can penetrate up to 50 mm of aluminum. The X-rays were

passed through the high strength machined graphite molds. The flow of molten aluminum is picked

up by an image intensifier and then a CCD camera. An insulated crucible is positioned over the

graphite mold. This insulated crucible with the plug closing the hole is set above the vertical sprue and

runner system. Air pressure is employed on the rod to hold it in position until the molten alloy is

poured into the crucible. A pneumatic cylinder is activated by remote control and raises the graphite

rod or plug. This allows the molten metal, which was A356 aluminum cast alloy, to flow through the

20 mm diameter opening, down the sprue and into the mold cavity. During this period, the behavior of

the molten aluminum is recorded by the X-ray arrangement shown in Figure 1.

Figure 1 Schematic of the X-Ray setup

Results and Discussion

The graphite molds were about two inches thick, parted in the middle of the gating systems and the

centerline of the plate casting. The side-feeding method, commonly referred to as web gating, utilizes

a large riser positioned on the side of the casting. This is a gating system where the riser is connected

to the entire height of the casting via a thin ingate section. This design locates the hottest metal in the

top of the riser called a feeder at this location. Forces from the thin ingate provide sufficient resistance

to the metal flow to slow the flow rate. This provides less turbulence of the metal entering the mold

cavity. Several disadvantages, however, occur when using side-feeding. For instance, because of the

location of the ingate, a large amount of machining is often necessary to achieve the desired shape. No

reliable method for calculating the width and thickness of the ingate exists.

The design and some critical dimensions of the gating system and mold are given in Figure 2. The

dimension of the vertical runner cross-section was 20 by 20 mm. The web had a thickness of 3 mm by

a width of 20mm and a 200 mm over the full height of the mold cavity.

The flow pattern of melt metal filling the mold cavity at difference time is shown in Figure 3. As

noted from both the X-Ray and simulation images in Figures 3(a), much of the riser filled prior to

metal entering the mold cavity through the thin ingate. This indicates that a threshold head pressure is

necessary to overcome the frictional forces generated by the very thin ingate. Filling of the riser

reduces the turbulence but as the metal enters into the mold cavity through the ingate, a rather

significant metal velocity was observed. This happens at the base of the mold cavity and the filling of

the mold occurs gradually and uniformly. A votex can be clearly observed in the mold cavity. This

votex not only entraps air in the molten metal, but also enhances the convection and results in a

uniform temperature distribution in the mold cavity. Thus, temperature gradient suitable for feeding

during solidification can be obtained and shrinkage is prone to be observed in this casting.

X-ray

head

Plug

Insulated

Crucible

Image

Intensifier mold

J

64 Physical and Numerical Simulation of Materials Processing

(a) (b)

Figure 2 Mold with a side-riser web gating system

(a) critical dimensions of the mold (b) 3D model of the mold

The mold filling process in this case was also simulated with MAGMAsoft [9]. Relative fine mesh

size is applied in the thin web area so that the more accurate result can be obtained with a cost of

shorter calculation time. The crucible at the top of the sprue was included in the simulation and 90%

of the crucible was filled with molten metal in the beginning of the simulation. The back pressure to

the metal in the crucible can be applied automatically when the sprue is full filled with liquid metal

and a positive pressure is established. The simulation results shown in Figure 3(b) provide a good

prediction of the flow pattern.

(a)

0.3s 0.5s 0.7s 1.0s

(b)

Figure 3. Flow pattern in a web gating system

(a) x-ray experimental results (b) simulation results

To improve the flow pattern and reduce the air entrapment in the mold, a graphite insert was put at

the bottom of the mold cavity so that the bottom of the mold cavity was slightly higher than the

bottom of the web. This insert has the same function with that the mold cavity moves up relatively to

the side feeder. The metal cascades from riser, through the web, to the bottom of the cavity from a

very low height. A smoother filling pattern was obtained when the bottom of the mold cavity was

slightly higher than the bottom of the web and the votex in the mold cavity is alleviated, even not

eliminated.

The flow of molten metal into the cavity is dominated by the thin web. This feature of the web gate

is very advantageous since it prevents entrapment of oxide films into the stream. It is in essence what

Materials Science Forum Vols. 575-578 65

makes this design, originally developed and evaluated by Battelle in the 50’s one of the best gating

designs for vertical molds. As discussed above, the high velocity from the web gate into the mold is

caused by a high pressure head in the side riser. It is easy to image that increasing the web thickness

can reduce the metal pressure head required to overcome the flow resistance in the web. A same web

gating system with that shown in Figure 2 is designed except the web thickness is increased from

3mm to 4.5 mm. The simulation results for this new design are shown in figure 5, in which the flow in

the sprue and runner is not displayed. Compared with the results in figure 3, the metal enters into the

mold earlier before a higher liquid pressure head is established. Correspondingly, the metal flows

more smoothly in the mold, and therefore the votex and air entrapment are reduced. However, if the

web thickness is so increased that it closes to the thickness of the mold, the gating is more like a

bottom gating system, and thus lost the advantages of web gate system.

0.3s 0.5s 0.7s 1.0s

Figure 4 Flow pattern in a web gating system with an insert at the bottom in the mold

0.2s 0.6s 0.9s 1.3s

Figure 5 Flow pattern in a web gating system with 4.5 mm web

Detail investigations on the web thickness were conducted by employing the same gating system

shown in Fig. 2, except that the mold was not included and the web was open to the air. The sensitivity

study yielded an interesting result: in the presence of a riser, the web thickness did not have a major

impact on the velocity of the jet exiting the web. Instead of jetting more or less, the metal tends to rise

higher or lower in the riser. In other words, the thin web acts as a choke. The thinner the web the more

it chokes the flow and the more pressure is required to overcome the resistance to flow. The end result

is that the metal level in the riser needs to be higher before it can flow past the web. Figure 6 illustrates

the flow pattern of molten aluminum A356 with a web thickness of 2.0mm. The velocity of the

exiting jet can be determined from the distance it jets away from the mold. The jetting was videotaped

for different web thicknesses. The jet distance was measured from these videotapes and plotted as a

function of web thickness as shown in Figure 7. While there is a slight drop in the jetting distance as

the web becomes thicker, it is a relatively small drop. The height of the molten metal in the riser is

also plotted as a function of the web thickness in Figure 7. As the web becomes thinner, the level of

molten metal in the riser rises significantly.

The web gating design has a very favorable flow pattern if properly design. Figure 8 shows flow

pattern in a production permanent mold “Box Casting” with a web gate system. The dimensions of the

vertical runner cross-section were 30 by 40 mm at the bottom and 30 by 70 mm at the top. The web

66 Physical and Numerical Simulation of Materials Processing

had a thickness of 6.5 mm by a width of 25mm and a 180 mm over the full height of the mold cavity.

Similar with the case of Figure3, the molten metal fills the riser prior to cascading into the mold

cavity, because of the resistance of the thin web to the metal flow. Of particular interest is the feeding

simulation shown in Figure 9 that predicts shrinkage at the bottom of the casting. Such shrinkage was

occasionally observed in production at Arrow Aluminum in USA, and measures were taken to

eliminate it by adding chill to the bottom.

80

120

160

200

0.8 1.2 1.6 2.0 2.4 2.8

Web thickness, mm

Height or distance, mm

molten metal height

jeting distance

Figure 6 Molten Al jetting Figure 7 Affect of web thickness on maximum jetting

from the web gating system distance and molten metal height in the side riser

0.8s 1.6s 2.5s 8.0s

Figure 8 Flow pattern in a “box casting”

(a) (b)

Figure 9 Solidifications in a “box casting”

(a) feeding (b) cooling curve

The feeder of a gating system is used to provide metal to the casting during solidification. As the

metal begins to freeze in the mold cavity, it begins to shrink. Depending on the geometry of the

casting, shrinkage can form. Therefore, it is vital to provide a constant supply of hot metal to the

system to fill in the voids left by shrinkage. In web gating system, the vertical runner is always

designed with a large cross section. It not only leads the hot metal to the top of the filled part in the

Materials Science Forum Vols. 575-578 67

cavity, but also plays a role of a feeder, and therefore it is named with “side riser” or “side feeder”.

The hottest metal in the system is stored in the feeder and supplied to the casting, via the web gate, as

needed. In most cases, the height of the feeder must be slightly higher than that of the actual casting.

This allows for a small amount of excess pressure to counteract capillary forces. Since this excess

pressure is needed and the feeder size cannot be extreme and reduce yield, the actual dimensions of

feeders in a web gating system require careful calculation. As shown in Fig. 9, the web solidified prior

to the bottom of the casting, and blocks the feeding channel from the feeder to the casting. A chill is

required to eliminate this defect.

Summary

Web gate systems, if properly designed, can provide less turbulent flow into the mold cavity. A big

side vertical riser with a thin web can lead hot metal to the top of the filled part in the mold, results in

a reasonable temperature distribution for solidification and feeding in the casting.

The web thickness plays an important role in mold filling process. A thin web can lead hot metal to

the top of the filled part of the cavity. It is helpful to realize a direct solidification and eliminates

shrinkage. However, too thin web can cause a jet filling pattern, and thus induce a votex and

entrapped gas. On the other hand, a thicker web provides a smooth filling process. But too thick web

loses the advantageous of web gating system, results in a bottom gate filling pattern.

The impact of the web thickness on the exit jetting velocity is relatively small, compared with that

on the molten metal height in the side riser. Thin webs generally block the feeding channel from the

side riser to the casting. Therefore, some measures are usually required to eliminate shrinkage to

obtain sound castings.

Acknowledgment

The authors acknowledge support through CMC program and direction of the work was provided by

the AFS Permanent Mold Committee.

References

[1] Q. Chang, R. Ye, H. Wang and G. An: Journal of Harbin Institute of Technology vol.24 (1992) 10,

p.138

[2] R. E. Swift, J. H. Jackson and L. W. Eastwood: AFS Vol.57 (1969), p.76

[3] G. B. Van Der Graaf, H. E. A. Van Den Akker and L. Katgerman: Metallurgical and Materials

Transactions B Vol.32 (2001) 1, p.69

[4] T. Nguyen and J. Carrig: AFS Vol.94 (1986), p.519.

[5] X. Xue, S. F. Hansen, P. N. Hansen: AFS Vol.101 (1993), p.199.

[6] M. Davandari and J. Campbell: 1st International AFS Conference on the Gating, Filling and

feeding of Aluminum Castings, Oct. 11-13, 1999, p.49.

[7] W. Sun, H. E. Littleton, C. E. Bates: AFS Vol.110 (2002), p1347.

[8] B. Sirrell and J. Campbell: CASTCON '95 (Conference proceeding), Institute of British

Foundrymen: June 15-16, 1995.

[9] Information on http://www.magamasoft.com.

68 Physical and Numerical Simulation of Materials Processing