Automatic iron pouring: necessity or luxury?
* This article details the benefits of automatic pouring for an iron casting operation.
* An economic analysis of a foundry considering automatic pouring is presented, providing details on easily forgotten wastes that hurt the bottom line.
* Metal delivery and floor space requirements are discussed for automatic pouring systems to help determine if this option is right for your plant.
With the competitive pressures of today's casting market and increased competition from offshore foundries, any reductions in manufacturing costs can quickly become significant. One area of the foundry that often isn't examined during cost-reduction sessions is the melt deck. But in reality, inefficiently or incorrectly poured molten metal results in scrap and excess costs. For example:
* Short pours create partial casting formation and shrink defects.
* Cold metal results in poor and incomplete casting formation and chill-off in gating.
* Interrupted pours cause cold laps.
* Fast pours create erosion defects such as sand inclusions.
* Overfilling the sprue (pouring) cup wastes metal and the cost to melt it.
* Spills during the starting and stopping of pours damages mold flasks and wastes melt.
While each of these items is small by itself, they all can add up to hundreds of thousands of dollars per year.
The correct procedure for pouring iron into a mold is four steps.
1. A rapid "pre-pour" is performed to raise the molten metal to the level of the sprue cup, allowing control of the rest of the pour. This assures proper filling of the entire gating system from the outset, and provides a good visual check on metal flow rate.
2. For the rest of the pour, the metal is poured at the maximum rate that the pattern and gating design will allow the mold to absorb the metal.
3. During the pour, the level of metal in the pour cup must be steadily maintained, creating constant pressure to properly feed the gating, casting cavities and risers in a controlled manner. Maintaining a constant level in the cup assures consistent metal flow, avoiding mold erosion from turbulence in the mold and metal chilling in the gates.
4. Finally, after the mold is filled, the pour is stopped at the proper level to maintain the required head pressure during solidification, while not overfilling or spilling.
To ensure that these steps are properly followed, many of today's iron foundries (large and small) have installed automatic pouring systems. The use of automatic pouring can virtually assure the correct filling of every mold. In addition, with the proper controls, these systems can record all required process control and ISO data.
The true benefit to automatic pouring is its ability to control variables that are sometimes "up in the air" with manual pouring. These variables include:
* rate of pour during pre-pour, mold fill and end-of-pour. Every pattern has an optimal pouring profile;
* total amount of metal to be poured, which is controlled by casting weight;
* compensation for pour times as the viscosity of the molten metal changes due to temperature fluctuation;
* short pours;
* interrupted pours;
* cold iron;
* pour rate variation from the changing level of iron in the pouring vessel.
Regardless of the size of the foundry, all of these variables affect efficiency and profitability in the same manner, only at different magnitudes. With the pouring controllers available today, hundreds of different pattern "pouring profiles" can be stored and then activated when any pattern change occurs on the molding line. This means that true jobbing foundries with casting runs in the hundreds can reap some of the efficiency benefits of automatic pouring that high-production foundries see.
Automatic Pouring Options
Essentially three types of automatic pouring are available--robotic, stopper rod and automatic ladle.
Robotic-In this process, an industrial robot manipulates a ladle for pouring. This typically is not used in iron applications, but rather aluminum and small brass castings. As a result, it isn't discussed any further in this article.
Stopper rod--Stopper rod pouring vessels are either unheated or heated and pressurized, depending on the type of iron being poured. This method of pouring typically is used in larger, higher production foundries with molding line rates of 275 molds/hr and above.
Automatic ladle--These systems are used in conjunction with molding rates up to 300 molds/hr. In addition, these systems provide benefits to a foundry pouring multiple grades or alloys of iron.
Stopper Rod Pouring Systems
Of the two automatic pouring approaches for iron, stopper rod pouring has dominated high-volume foundries due to its large capacities and the ability to heat the metal. This method uses large holding and pouring vessels (typically up to 20 tons, but can be larger) that are heated with inductors. These systems can deliver in excess of 4 tons of metal every 10 min. if necessary.
The pouring is accomplished through a nozzle in the bottom of the vessel that is opened and closed by a stopper rod. This stopper rod can be controlled by either an operator (semiautomatic) or by an automated controller (fully-automatic). Typically, the controller is a feedback unit in a closed loop system in which the pour cup is monitored by a sensor, which utilizes a vision or laser system.
As the sensor system monitors the pour cup, it can control the stopper rod to adjust for changes in molten metal viscosity due to temperature or reduced metal flow due to material build-up in the nozzle. Additional controls can sense the pour cup position and adjust the pouring in the x and y axes.
Automatic Ladle Pouring
Automatic ladle pouring systems have been in use in foundries since the early 1990s, but haven't been fully accepted because they haven't been considered flexible enough and have a high relative cost. More recently, automatic ladle pouring systems have generated interest as their costs have been reduced and they can generate high performance across a range of molding rates.
Automatic ladle systems have ladle capacities up to 5500 lb and can pour synchronously with continuously moving or indexing mold lines. These systems can automatically accommodate pattern specific pour cup location changes. In addition, these systems can integrate in-stream inoculation and are able to accommodate changes in grades of iron to be poured.
The biggest drawback of ladle systems is the lack of heated vessels. As a result, these systems must pour the iron they hold within 5-6 minutes to ensure proper pouring temperature. As a result, ladles are sized to match a specific foundry's requirements.
Examining Pouring Economics
To understand the cost savings obtainable with an automatic pouring system, take a look at the following analysis performed by a foundry examining the addition of an automatic ladle pouring system to its operation.
Foundry Requirements--A green sand foundry running a tight flask, indexing mold line produces 850 molds/day. With one shift, this amounts to 1700 castings/day with an average casting weight of 150 lb.
Potential Scrap Reduction--By utilizing automatic ladle pouring, the foundry can optimize pouring performance. This will reduce scrap by a conservative 2%. With an estimated scrap cost of $0.30/lb, this would amount to a scrap savings from automatic pouring of:
1700 castings/day x 250 days x 150 lb/casting x 2% x $0.30/lb scrap cost = $382,500/yr
Potential Spilled Metal Reduction--The foundry estimated that the total spillage (over-pour in the sprue cup and iron that missed the sprue cup at the start of pour and end of pour) for a given day was at or more than 2500 lb. This was based on the metal removed from the tops of the molds after cooling. Assuming the cost to melt iron is $0.08/lb and that at least 90% of this waste can be eliminated by automatic pouring, then potential spilled metal savings is:
2500 lb/day x 250 days x $ 0.08/lb x 90% = $45 ,000/yr
Potential Increased Yield (decreased iron in sprue cup)--In this foundry, most of the castings are flat and do not require head pressure from a large sprue cup at the end of the pour. Although the sprue cup does require a specific volume during the pour, the foundry estimated that in 67% of the molds, the cup can he empty after the pour is complete.
The sprue cup weighs 20 lb. The automatic "end of pour" by weight can reduce this by 67% (13 1b). As a result, the potential savings are:
1700 castings/day x 250 days x 13 lb/casting x $ 0.08/lb melt cost = $442,000
Potential Labor Savings--In this foundry, the automatic pouring unit would eliminate two workers. Based on this, the potential savings is:
1 man x 1 shift/yr x $45,000/man x 2 men= $90,000
Potential F/ask Repair Reduction-- This foundry's own maintenance records showed that more than $140,000yr was spent repairing flasks from hot metal damage. The automatic pouring unit eliminates this cost, yielding costs savings of: $140,000.00
Total Cast Savings $1,099,500/yr
From seemingly small items and every day occurrences, this foundry can save more than $1 million/yr by installing an automatic pouring unit. In addition, this estimate does not include more difficult figures to quantify such as quality control and assurance cost savings accumulated through the automatic storage of critical data in the automatic pouring unit as each mold is poured.
Metal Delivery & Floor Space
The last two things to consider with automatic pouring systems are delivery of metal and floor space.
To fully reap the cost saving benefits of automatic pouring, the system must not be kept waiting for molten metal.
For stopper rod pouring, the only viable system is overhead monorail delivery, which will carry a large ladle to the pouring vessel. Therefore, when looking at installing a stopper rod system, proper planning is necessary for an unrestricted path for the monorail (with the required overhead elevations) from the melt deck to the pouring deck.
For metal delivery, ladle systems are more forgiving. In Europe and Asia, forklift trucks are used to bring full ladles to the pouring units (many foundries in North America would not accept this practice). In North America, monorail hoist systems have been used to bring metal, and the ladles are refilled directly on the pouring unit. In another option, the automatic pouring ladles can be filled in this same manner, while they are on an automatic ladle changer (saving valuable time).
A fourth option is that covered ladles from the pouring unit can be transported by monorail hoist systems between the pouring unit and the holding furnaces. In this case, a ladle changer is required that automatically swaps a full ladle for an empty one.
In terms of space for automatic pouring, stopper rod systems must have the required elevation to be above the mold line. Beyond that, these systems do not require additional floor space.
For the automatic ladle pouring systems, floor space must be allocated for:
* filling ladles;
* changing ladles during production (if this is the method of metal delivery used);
* a length of track for the unit to run on during the pouring process. This ranges from 12-30 ft, depending on the molding line and the required production rates.
* safety enclosures.
Is It for Your Foundry?
As with all capital improvements, justification based on payback is the key analysis to determining if an automatic pouring system for iron is right for your operation. Perform a detailed and true cost analysis of your melt operation as was performed in this article and see the costs related to scrap and waste that are hurting your bottom line each year.
Even though you may think your foundry has tightened its belt in every way possible to compete in today's global economy, ways still exist to improve your "competitive edge." Your foundry will never know if automatic pouring is a necessity or luxury if you don't check the numbers.
For More Information
"Successful Automated Pouring of Ductile Iron: Design and Operation," William Plug, Proceedings from the AFS Induction, Melting, Holding & Pouring of Iron Conference, AFS, Des Plaines, IL (1999).
"Controls for Automatic Molten Metal Pouring Systems," Emad Tabatabaei, Proceedings from the AFS Induction, Melting, Holding & Pouring of Iron Conference, AFS, Des Plaines, IL (1999).
About the Author
Dan Spinner has served as manger of equipment sales for the foundry design, build and engineer firm The Mount Co. for five years. Spinner's expertise in foundry engineering for the last thirty years has focused on pouring and casting cleaning and finishing.
|Printer friendly Cite/link Email Feedback|
|Date:||Oct 1, 2002|
|Previous Article:||Development, evalation and application of solid fluxes.|
|Next Article:||Hone your strategic planning skills! With planning never more critical, these exercises can help you make informed, forward-looking decisions on your...|