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Good pouring practice contributes to quality castings.

Good Pouring Practice Contributes to Quality Castings

Because the soundness of a casting, in large part, depends upon how the metal enters the mold and solidifies, the pouring and handling of molten metal is critical to the success of the metalcasting process. This installment of The Metalcasting Process focuses on the importance of proper pouring practices and molten metal handling. Development of mechanical and automatic metal transfer and pouring systems are described as well.

Metal Pouring

The metal pouring operation in the foundry can be described simply as the transfer of molten metal in some fashion from the melting or holding furnace to the molding line and then placing the metal into a mold in a way that will result in a sound, saleable casting. While this description may seem overly simplistic, good molten metal handling and mold filling techniques are crucial in producing sound castings and should not be downplayed.

Poor pouring and handling practices lead to a wide variety of casting defects and, almost always, render the part unusable. For example, badly designed or poorly implemented transfer methods can lead to excessive temperature loss, which can result in cold shuts and a variety of other casting defects. If the metal is superheated to overcome a poor transfer system, the waste comes in the areas of energy use and refractory wear, as well as damage to the alloy.

In the case of nonferrous metals that are susceptible to gas pickup, conveying, handling and pouring techniques that create excessive turbulence of the molten metal can result in gas porosity problems in the final casting.

Other examples of poor pouring practice include not keeping the mold sprue full during mold filling. This can lead to entrapped air in the metal, causing a myriad of gas and air problems, along with slag and dross inclusions, in the casting. Another is not providing enough metal to the mold, or breaking the molten metal stream and then restarting the pour, which can lead to pour shorts or cold shuts.

On the other hand, overpouring the mold also creates significant problems. These may include wasted metal, safety hazards and additional work in subsequent operations like shakeout and sand re-use operations.

These are just a few examples of the price a foundry pays for poor molten metal handling and pouring practices. In short, lack of good technique and control of the pouring operation can undo all of the good work performed in the foundry that leads up to mold filling, as well as creating problems further down the line.

The actual pouring operation can be done manually, mechanically or automatically. In a manual operation, the pourer (or pourers) carries the ladle of molten metal to and directs the stream of molten metal into the mold. Since ladles have to be physically carried, the amount of metal in each ladle is dependent upon the strength of the pourers. This limits the number of molds and the size of casting that can be poured from each mold. Hand pouring, shown in Fig. 1, is usually reserved for lower volume runs and smaller castings.

Mechanical pouring of molten metal is accomplished by carrying the ladle to the pouring station by means of an overhead monorail system, as shown in Fig. 2. By turning a handwheel (Fig. 3), a series of gears tilt the ladle and allow the metal to be directed into the gating system. Another type of mechanical pouring ladle is tilted by using a lever. The lever replaces the handwheel and gear box.

In many modern high production foundries, molten metal is poured into the molds automatically by pouring machines which use a variety of measuring and/or weighing apparatus and other controls. In most automatic pouring operations, the molds are transported to the pouring machines, rather than the metal being moved to the molding line, which is the usual case in manual and mechanical operations.

Generally, the ideal molten metal handling and pouring system would be one that moves the correct amount of metal the shortest possible distance. Upon arrival at the mold line, the metal would be the correct temperature and slag or dross free. It would be poured from the proper height to achieve the required ferrostatic pressure necessary for proper mold filling. Ideally, this would result in sound, gas-, slag- and dross-free castings, accomplished in a safe and productive manner. While this ideal operation is not always possible, the metalcaster should nonetheless aim to provide as efficient a metal transfer and pouring operation as possible.


Ladles are the vehicles most often used in all types of foundries for conveying and pouring molten metal. Generally, a ladle is a steel shelled vessel with refractory lining in which molten metal can be conveyed, or from which molten metal can be poured into other ladles, molds or ingots.

A distributing ladle receives its metal either directly from the cupola spout, melting furnace, forehearth or duplexing furnace. From there, the metal is conveyed to and deposited in smaller ladles for pouring. In other cases, the ladle that receives the tapped metal from the primary melter or holding furnace is also used as the pouring vessel.

While a wide variety of ladles are used in foundries, they are most often characterized by their external shape and spout design. The most common external shapes include straight-sided, tapered and cylindrical. Common spout designs are top lip, teapot spout and bottom nozzle. These spout designs are illustrated in Fig. 4.

The teapot ladle receives its name from the fact that it has a deep, internal well adjacent to the lip. Since the opening to this well is near the bottom of the ladle, as in a teapot, metal enters the pouring spout from beneath the metal surface, thereby retaining in the ladle any particles of floating slag or dross.

As is shown throughout this article, there are ladles available to meet the needs of nearly any casting operation. For example, many small nonferrous foundries use crucible pouring, in which metal is poured from the same crucible in which it is melted. Melting is done in crucible furnaces of different types. The crucible is removed from the furnace by means of tongs. For pouring, the crucible is transferred to a pouring shank, a device which holds the crucible and permits manual tilting.

Many steel foundries, on the other hand, may use very large bottom-pour ladles. In any case, the ladle or ladles should be designed and sized according to the individual foundry's needs.

And, the important factor in ladle selection is heat loss. For example, a 300 lb ladle of iron tapped at 2800F (1537C) and held at room temperature will normally lose heat more rapidly than a 2000 lb ladle of iron tapped and held at the same temperature.

Also, a tall, small diameter ladle will lose less heat in a given amount of time than a shorter, larger diameter ladle lined with the same refractory and holding the same amount of molten metal. This is because there is less surface area of metal exposed to the air in the tall, smaller diameter ladle.

All ladles should be preheated, thoroughly dried and kept close to service temperature by good scheduling of metal delivery. Lining life will be extended when ladles are kept at a high temperature to reduce damage caused by the thermal shock of cooling and reheating. Gas and oil torches and electric ladle heating covers are commonly used for ladle preheating.

However, ladles should be allowed to cool as often as practical for cleaning. Dross and slag should be chipped away periodically to avoid recontamination of the melt. When necessary, refractory patches can be rammed- or gunned-in to fill areas where the refractory has eroded. Also, lips and spouts should be kept free of accumulated slag or dross.

All ladle refractory patching should be dried completely before the ladle is returned to use. An uncured lining can create steam during filling and cause a serious explosion. Another safety measure is to provide vent holes in the steel ladle shell. These should be kept open to permit steam to escape.

Uncovered hot ladles cool rapidly if there is a delay between fills. Insulated covers help reduce heat loss from an empty ladle as well as from a full ladle. Ladle covers reduce temperature loss from the metal surface and also retain heat during a delay in pouring schedules. They also serve to protect pourers from the intense heat.

Economics also dictates that heat losses be minimized and that the proper size ladle be used for each production run. It is uneconomical to continually pig that portion of the metal which is too cold to pour. Holding the cooled portion, or heel, until a new hot batch is received is not a good alternative, because it will chill the next ladle of hot metal. Figure 5 demonstrates one method for calculating ladle capacities.

Pouring Nonferrous Alloys

Handling and pouring some nonferrous alloys present some unique problems, so it is important to identify some characteristics of nonferrous alloys which will affect their pouring. Nonferrous alloys may be distinguished on the basis of their oxidation behavior during melting and pouring. Several families of alloys, including aluminum alloys and some bronze, brass and zinc alloys form insoluble films of oxide or dross on the surface of the melt.

These alloys are very susceptible to defects caused by the entrapment of these films as inclusions in the casting. To avoid these conditions, stirring of the molten metal should be avoided in order to prevent oxide films from being pushed under the surface. After the ladle is filled, or the crucible has been removed from the furnace, the metal should be carefully skimmed just before pouring.

If the metal must be transferred to a pouring ladle, an effort should be made to minimize the height of fall of the metal during transfer. Likewise, pouring should be done smoothly and evenly to avoid splashing and interrupting the metal stream. With careful pouring of these skin-forming alloys, it is possible to form and maintain and oxide "sleeve" around the metal stream, thus protecting the molten metal from undue exposure to the surrounding air.

Pumps have been used to transfer nonferrous metals for over 30 years, but improved design, closer component tolerances and new materials have resulted in improved reliability and increased acceptance. Pumps are used in a number of nonferrous applications including: transport between melting and holding furnaces; direct filling of molds, tundishes or casting units; and supplying refining vessels.

The two most common types of pumps are electromagnetic induction and centrifugal. Most are made either of graphite or stainless steel. In a centrifugal pump, either air or electricity is used to turn an impeller. Electrically driven centrifugal pumps are capable of a discharge velocity as high as 1100 ft/min.

Operation of electromagnetic pumps may best be described using the analogy of an electric motor in that the metal itself is electrically charged and acts like the rotor, while the pump acts as the stator. Metal is transferred by continuously reversing the flow of the metal many times each second. The flow rate is controlled by changing the level and frequency of the current in the stator.

Automatic Pouring

Due to the very nature of the molten metal pouring operation, it can be one of the most uncomfortable jobs in the foundry. And because it is so critical in producing quality castings, many foundries have automated the process where practical.

Automatic pouring devices can offer the advantages of increased productivity and metal yield, improved quality and process control, labor savings and reduced scrap. Productivity improvement is due to the consistency of pours from one mold to then next. Increase in yield owes to increased accuracy, resulting in the elimination of overpours, spills and pigging of ladle heels. Labor savings, of course, come from reducing the number of people needed in the pouring operation.

The elimination of interrupted pours and irregular pour rates bring about major reductions in scrapped and defective castings. Some automatic pouring units also improve melt temperature control and permit a reduction in pouring temperatures, resulting in better process control and savings in energy and refractory consumption. Also, isolation of the operator from the effects of molten metal spatter, gases, heat and smoke provide health and safety benefits.

There are two basic methods of automatic pouring: mechanized ladle pouring and automatic direct pouring.

Mechanized Ladles--These are used to transfer measured amounts of metal from a storage vessel or controlled pouring unit to the mold. Automated dip and pour mechanized ladles also are used, primarily in nonferrous operations. Units are programmed to duplicate pouring sequences for a given mold requirement using load cell weighing systems and controlled tilt rates.

Carrousel System--One automated mechanized ladle system, which has been in use for over a decade, uses a carrousel to synchronize pouring with a continuously moving molding line. The ladles are filled under a pour box attached to a holding furnace. The pouring ladles rotate from pour to fill and back on a circular carrousel track. A timed, air-activated stopper rod controls the amount (said to be accurate to within 2%) of metal dispensed to the ladle.

Direct Pouring--Direct bottom-pour ladles use either slide gates or, more commonly in foundry operations, stopper rods and nozzles to control the flow of metal to the mold. Metal is metered directly into the mold from the pouring vessel. An automatic iron pouring system with ladles and stopper rod control is shown in Fig. 6.

With the addition of resistance or induction heating equipment to the pouring vessel, the metal temperature can be controlled. Also, the vessel will hold the molten metal and replace heat losses during pouring interruptions.

Electric Pouring Furnaces--Metal transfer and pouring vessels are eliminated entirely when the molding line is brought to a stationary melting/pouring vessel. Resistance furnaces typically are bottom-pour units with stopper rods and nozzles mounted in the furnace bottom or in an attached pour box.

Automatic pouring from a channel furnace, illustrated by Fig. 7, can be accomplished either through a pour spout or a nozzle, with or without a stopper rod. This is because the vessels can be sealed and pressurized with an inert gas. The pressure can be varied to control the rate of pouring or to establish a constant head pressure above the nozzle. Consistent pouring is the result in either case.

Pouring Controllers

Some pouring controllers control only the volume of metal poured per mold. More sophisticated units control both volume and rate of molten flow, while others control the positioning of the mold and nozzle as well.

Some systems use simple timing devices which may be reset depending upon the size of the mold. Most mechanized ladling systems use an automatic scaling device to measure the weight of metal dispensed based on the exact weight of metal required for the mold. Once programmed with this information, programmable controllers permit repeatable, controlled pouring duration.

Control of the rate of molten metal flow (pour rate) is necessary to maintain the proper height of metal in the sprue to achieve the required ferrostatic pressure necessary for proper mold filling. To achieve such control in automatic pouring, more sophisticated stopper rod controls are used.

If the metal flow rate is consistent through the nozzle, programmed repetition of calculated stopper rod positions provides a controlled, consistent pouring rate. However, slag buildup on the nozzle and/or variations in metal head pressure can cause flow variations. In such cases, a real-time feedback loop can compensate for these variations to maintain a consistent pour rate.

Sensing systems that measure the rate of mold fill or level of metal in the pour cup have been developed for this purpose. The level of the molten metal in the mold may be measured either with a video camera or laser measuring probe. These measurements are used to control the position of the stopper rod and thus, the pour rate.

Video System--The video system displays an edited and digitized picture that separates the molten metal stream from the rest of the pouring basin, based on the brightness level of the stream. The area of metal fill can be compared to the actual area of the pouring basin, providing a comparative value used to control the position of the stopper rod.

Laser System--A laser measurement system makes use of optical triangulation between the laser probe source, the top of the mold and a photodetector. The laser probe focuses on the top of the mold when positioning the next mold for pouring. Also, it focuses on the level of metal in the sprue cup when controlling the pour rate. The photodetector measures the distance of light reflected back from the mold top or sprue cup.

During the positioning phase, the probe monitors the top surface of the mold and detects the leading edge of the pour cup as it moves into place under the pouring nozzle. Because the indexing stroke has been programmed into the controller's memory, the controller can calculate the final pouring position of the incoming sprue cup. The pouring until will be automatically adjusted to the proper position above the sprue cup during mold indexing.

During the pour, the system continuously measures the level of the molten metal in the pour cup. This level is continuously compared to a predetermined, ideal level. A servo actuator throttles the stopper rod, allowing the desired level of metal to be reached quickly. The system then maintains the proper level.

Mold Filling

Automatic mold filling methods are ideal methods for production of precision castings, meeting requirements for greater casting integrity, as well as thinner wall thicknesses enhanced mechanical properties. They utilize such techniques as pressurized pouring vessels or flasks with vacuum capability to effect metal transfer, or both.

CLA/CLAS--Developed in the U.S., these patented counter gravity low pressure processes (CL) for filling of investment ceramic molds with air melt alloys (CLA) and vacuum melt alloys (CLV); and for filling of chemically bonded molds with air melt alloys (CLAS), permit highly consistent mold filling with less turbulence.

The basic concept of the CLA process, typically used to cast aluminum alloys, is to place the mold in a special casting chamber that has vacuum capability. The mold, open at the bottom, is submerged into the molten metal vessel. The porosity of the mold permits it to be filled when a vacuum is created in the chamber.

Similarly, in the CLAS process, which is most applicable for a variety of iron and steel alloys, castings are gated from the drag half of the mold. The cope is attached to a vacuum chamber and the drag is submerged in the molten bath. Metal is pulled up into the mold cavities by vacuum and decomposition gases are drawn up into the vacuum system.

One important feature of both these processes is a reduction in nonmetallic inclusions. Floating slag is held at the meltlining area by the meniscus established by the holding force on the melt and is not carried into the mold. Instead, molten metal is drawn into the mold cavity from the clean portion of the melt, below the slag layer.

Cosworth Process--A low pressure sand casting process to produce high integrity aluminum alloy castings was developed in England in 1978. The mold is filled from below in such a manner that the metal rises with an unbroken surface.

Filling head and feeding pressure is provided by an electromagnetic pump which is permanently submerged in the holding furnace. Flow rate can be controlled with extreme precision. Efficient degassing is provided in the furnace. An inert gas blanket minimizes subsequent gas pickup and oxidation.

The mold is allowed to solidify under pressure and any further requirement of the mold for additional feed-metal is supplied under pressure by the pump. Zircon sand is used as the molding medium because of its low expansion and high heat extraction rate. The high heat capacity, said to approach that of permanent molds, along with pressure solidification, creates very rapid, directional solidification of the casting back to the ingate.

FM--The font mince (thin iron) or FM process was developed by a French foundry. It is a casting process using a controlled differential pressure to produce complex, very thin walled irons, steels and superalloys.

Controlled, very rapid filling of molds is accomplished by combining:

* a low pressure exerted on the liquid metal;

* a negative pressure on the mold;

* evacuation of gases from the mold;

* casting in open molds with air gating.

The effects of reduced pressure in the mold reduce the quantity of gas to be evacuated. The air riser at the top of the mold improves evacuation of remaining gases in the mold cavity. After risers have solidified, the pressure of the liquid metal is increased to allow for shrinkage. Following this controlled solidification, the pressure is released and the excess liquid metal in the gating is recovered in the furnace.

Vacuum Lift Foam Filled Casting Process--Perhaps the newest counter-gravity casting process is one especially designed to improve the cleanliness of high quality steels produced in the evaporative pattern casting (EPC) process.

A vacuum pump attached to the top of an EPC flask creates a negative pressure throughout the flask due to the permeability of the unbonded sand. The vacuum draws the metal from the pouring vessel up into the flask through a porous media tube. The process reduces turbulence and atmospheric contamination of the molten steel.

Castyral--Another new EPC process adds isostatic pressure to the sand flask in order to improve soundness and mechanical properties in EPC-cast aluminum alloys. The method consists of placing the flask in a pressure vessel. Immediately after pouring, the pressure is increased gradually to a value which is held until solidification is complete.

PHOTO : Fig. 1. Hand pouring is usually reserved for smaller nonferrous castings.

PHOTO : Fig. 2. Mechanical pouring of molten metal by means of an overhead monorail system is

PHOTO : shown.

PHOTO : Fig. 3. By turning a handwheel, a series of gears tilt this ladle.

PHOTO : Fig. 4. Shown are typical foundry ladle types--(a) teapot; (b) bottom pour; (c) teapot

PHOTO : shank; (d) lip-pour shank. Reprinted from "Metals Handbook, Ninth Edition, vol 15",

PHOTO : courtesy of ASM International.

PHOTO : Fig. 5. A practical method for calculating ladle capacities is shown. Reprinted from

PHOTO : "Useful Information for Foundrymen," courtesy of the Whiting Corp.

PHOTO : Fig. 6. An automatic iron pouring system utilizing intermediate ladles and stopper rod

PHOTO : control is shown, courtesy of Asea Brown Boveri Corp.

PHOTO : Fig. 7. This automatic pouring furnace utilizes an induction channel loop to maintain

PHOTO : temperature, a pressurized vessel to maintain the proper metal head above the pouring

PHOTO : nozzle and a laser controlled stopper rod to control the pouring rate. Photo courtesy of

PHOTO : Inductotherm Corp.
COPYRIGHT 1989 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Bralower, Paul M.
Publication:Modern Casting
Date:Sep 1, 1989
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