The inventory method; a different paradigm of green sand control.
While the focus of this article is green sand systems, consider for a moment your melt shop and how the melt is analyzed before pouring. A predesigned charge makeup is carefully controlled to ensure that only minor adjustments are needed at tapping. After melting, a metal sample is taken from the furnace to determine its composition. Then, while the metal is still in the furnace, it is adjusted, as necessary, prior to pouring. Testing of the metal composition (not the physical properties translated to composition) is only used to assure that the charge and melting process achieved the desired, predetermined composition.
If that metal sample taken was used only to make adjustments to subsequent heats, the foundry would experience an unacceptable lack of control that would be disastrous. However, this analogy illustrates how most foundries today go about the control of their green sand systems--testing and adjusting the sand and making adjustments to it after (or while) the molds have already been produced.
Green sand systems presently produce about 80% of the ferrous castings poured and sold. These sand mixtures are composed of quartz sand, bentonite, additives (seacoal for iron, cereal for steel) and water. Typical green sand system practice consists of molding, assembling and closing, pouring, cooling, shakeout, metal and refuse removal, cooling, storage, mulling with water, adding bentonite and seacoal or cereal, and returning the sand mixture to molding. This cycle is repeated with a residence time of 10-20 cycles. Obviously, the success of the process depends on the ability to maintain the system sand, consistently and uniformly.
Green sands must be controlled compositionally based on the mass-balance system, as is done in melting. It must be viewed as a system.
This article presents an other-than-traditional way of approaching green sand system control. The idea is that by accurately controlling the addition of the ingredients deactivated by the heat of the solidifying metal at the sand muller, the system sand will be maintained. While a few foundries in the U.S. subscribe to the concepts outlined here, the system has been embraced at a more rapid rate in foundries overseas.
Shortcomings of Current Control Methods
The "Era of Sand Testing" began in the 1920s as the use of the sand lab to simulate foundry variables took on an important role in the foundry. At this time of much research and development, the number of sand tests proliferated and multiplied, giving birth to "sand technology." As a result, the industry came to view the sand muller and melting furnace as the focal and key points for improving casting quality. This sand technology is still seeking the elusive sand test that could predict the quality of the sand mixture that will produce defect-free castings. However, the physical properties of molding sands that produce acceptable or scrapped castings can be identical.
In most foundries today, exact amounts of additions made to the system sand are generally based on experience and sand lab data. One or several sand tests, such as AFS clay, methylene blue clay absorption, compactibility, percent combustibles, green and dry strengths, permeability, etc. are performed on prepared molding sand as it exits the muller and prior to mulling: in effect, testing after the fact. Based on the results of these tests, the subsequent batches of sand mixtures are adjusted for maintaining the system, uniform and in control. From a statistical and practical point of view, this is illogical, risky and dangerous--especially when you consider that molds can be produced at rates exceeding 300 per hr.
This approach of "test and adjust" is almost as if the physical properties of the sand are the ones dictating uniformity and quality. In reality, sand mixture composition determines consistency, and ultimately, casting quality.
If physical properties could be properly and quickly translated to composition, then sand testing would be a feasible tool for foundries in controlling green sand systems, because the sand could still be adjusted while it was in the muller. But this hasn't been shown to be possible.
The "test and adjust" approach depends on the representation of the sample and the accuracy of the test. Considering the poor reproducibility of sand tests measured by gauge R&R (as found by the AFS Molding Methods and Materials Basic Concepts Div. in 1980), "test and adjust" is not effective for good control.
If physical properties do not give foundrymen the answer, what does? The answer is composition, and this article examines how to control green sand systems through composition.
The Inventory (Mass Balance) Method
When metal is poured and solidifies in green sand molds, it heats some of the sand mixture to the pouring temperature of the metal, evaporates some water and deactivates part of the bentonite and additives. The liquid metal contains a known amount of heat and, as it solidifies, that heat is transmitted to the sand. This is the major cause of bentonite deactivation.
Therefore, the weight of the metal poured in the sand is directly proportional to the amount of bentonite deactivated. The deactivated materials must be diluted with the new/reclaimed sand to prevent them from building up within the system, so that casting quality is not adversely affected.
It has been established, through a series of experiments on heat-treating and differential thermal analysis, that the deactivation temperature of western (sodium) and southern (calcium) bentonite is 1180F (638C) and 600F (316C), respectively. Knowing the amount of bentonite deactivated by the heat of the metal provides the information needed to determine the amount of new sand that is required to dilute the deactivated bentonite to maintain a predetermined equilibrium level that has proven to produce acceptable quality castings.
Several research papers on metal solidification showed that the molding sand is heated by the metal, resulting in gradients expressed as lines of constant temperature (or isotherms) that surround the shape of the solidifying casting (Fig. 1). The sand volume between the 1180F (for western bentonite) or 600F (for southern bentonite) isotherm and the metal interface can be determined. All the bentonite contained in this volume will be deactivated. The higher the bentonite content in the sand, the more of it will deteriorate. Studies have indicated that for an iron casting with 3-in. thick sections, roughly 325 lb of the sand mixture will be exposed to temperatures above 1180F. This is regardless of the casting shape, volume of sand in the mold or the size of the mold.
[Figure 1 ILLUSTRATION OMITTED]
The calculation of sand and bentonite volumes and weights was used to prepare Fig. 2. This shows the actual lb of bentonite deactivated by each ton of metal poured in a sand system bonded with 0-20% western and southern bentonite. When casting thickness is below 3 in., the sand volume of the 1180F isotherm is practically the same--325 lb. So for any gray iron foundry producing castings within that 3-in. thickness range, for every ton poured, 325 lb of sand is heated to bentonite deactivation temperatures of 1180F. While figures have been determined for sizes above 5 in., this 3-in. thickness and under size will be used for the purpose of this article, as it fits most commercial castings produced in mechanized, system sand lines.
[Figure 2 ILLUSTRATION OMITTED]
Sand-to-metal ratios do not affect the weight of the bentonite deteriorated by the heat of the metal. The ratio only affects the overall temperature of the system sand and the amount of bentonite added to each batch at the sand muller. Shakeout time also doesn't affect the size of the isotherm when the casting sections are under 3 in. and are shaken out when solidification is complete. When casting section size increases beyond 3 in., the volume and weight of sand in the isotherm also increases.
As an example, see Fig. 2. If 1 ton of metal is poured in sections below 3 in. in a system sand bonded with 10% western bentonite, 32.5 lb of bentonite will be deactivated (no matter what the size of the mold or the sand-to-metal ratio) and must be replaced at the muller to maintain the active bentonite at the 10% level. That is, of the 325 lb of system sand heated to 1180F, 10% of it is bentonite that has become dead clay. Likewise, in a sand system bonded with 5% bentonite, the deactivated bentonite is 16.25 lb.
As the sand recirculates, deactivated bentonite will build up and accumulate within the system sand. This will cause typical casting defects associated with high contents of dead clays. Presently, foundries prevent this accumulation of dead inactive materials by diluting them with new, reclaimed or core sand (core sand entering through the thermal breakdown of the cores is considered a new sand addition). The dilution sand must be bonded to the same level of the system sand; otherwise the other active ingredients also will be diluted. The amount of sand additions to the system is what controls the level of the inactivated materials at equilibrium. Figure 3 shows the level of inactive bentonite at equilibrium, expressed in percent, as a function of the lb of inactivated bentonite per ton of metal poured, as affected by increasing new sand additions.
[Figure 3 ILLUSTRATION OMITTED]
As an example (using Fig. 3), for each ton of metal poured in a 10% western bentonite bonded system, 32.5 lb of bentonite will be deactivated, as has been discussed. If additions of 500 lb of new sand (per ton of metal poured) are made, the deactivated bentonite at equilibrium will be 6.1%. With a 750-lb addition of new sand, the amount of inactive bentonite decreases to 4.15% and to 3.15% with a 1000-lb sand addition. Core sand entering the system is a source of new sand and must be included in the overall calculation.
The clay level at which a foundry must operate is not only dictated by the casting quality expectation, but by other factors. These include the bentonite type and level, the temperature of the sand, the mulling equipment, the molding setup, the conveyor and transport system, the shakeout, the reclamation, the volume and efficiency of the storage facility and the cleaning room practice. The interrelation of these operational steps is significant for the control of system sands.
The addition of bentonite to the system is, therefore, the sum of the bentonite deactivated by the casting process and the bentonite necessary to bond the new and core sand used for dilution plus the exhaust losses. The addition per batch at the muller can be simply computed from the production schedule, which should enumerate the anticipated number of sand batches at the muller, the number of molds and the tonnage and pieces to be remanufactured.
Consider the system bonded with 5% western bentonite and with additions of 500 lb of new sand per ton of metal. The inactive bentonite will be 3.15% (Fig. 2 and 3) at equilibrium. The bentonite consumption per ton of metal will be 16.25 lb (deactivated by the heat of the metal), plus 25 lb to bond the new sand (5% bentonite level), for a total of 41.25 lb of bentonite per ton of metal poured. If the clay content was doubled to a 10% level, the amount of heat-deactivated bentonite per ton poured would double of 32.5 lb, and will require 1000 lb of new sand to maintain the heat-deactivated bentonite to the 3.15% level. The total bentonite consumption will increase to 132.5 lb per ton of metal poured (32.5 lb replacement + 100 lb for new sand bonding).
If it is envisioned that occasionally new or reclaimed sand additions must be made to dilute some types of core sands, it will further increase both the use of consumables (water, clay, cereal and/or seacoal) and the sand volume recirculating. In any event, surges and fluctuations in the system sand are not conducive to uniformity and stability. Clearly, the system sand will grow disproportionately to the needs of reasonable, manageable levels of reclamation and must be discarded. The disposal of system sands, however, is often done in a haphazard way. It should be disciplined by inventorying the size and volume growth of the system to anticipate reclamation and discard.
Worthy of Review
For green sand molding to continue holding the prominent and large share of sand molding it now enjoys, it must improve its performance dramatically. Possibly, this can be achieved by improving the consistency and stability of green sand system manufacturing lines. Obviously, the procedure presently used (testing and adjusting subsequent batches of sand) is not suitable for this task.
The inventory (mass balance) control method, enhanced by the novel developments this industry has seen in the last two decades (sand mulling, core-and moldmaking, shakeout and sand reclamation) is promising and deserves to be studied, evaluated, developed and applied.
However, the sand system itself is only one element of the larger foundry system. Metalcasting must be considered as a process composed of several operational steps. Each should be evaluated and quantified in relation to the others. Equipment, procedures, consumables and processes can be compared and conjointly evaluated, based on their effect on the casting (Fig. 4). for a system to stay in control, the tonnage poured and the consumables used must be closely balanced and controlled.
[Figure 4 ILLUSTRATION OMITTED]
This article was adapted from the 1997 AFS Molding Methods and Materials Div. Silver Anniversary Paper (97-190). The paper reviewed Vingas' 1970 research, "Sand Control of Green System Sands by a Simple Inventory Method." Both papers are available from the AFS Library at 800/537-4237.
Simple Spreadsheet Helps Determine Inventory (Mass Balance) Control
The following spreadsheet helps illustrate the inventory (mass balance) method of green sand system control and is available on the modern casting web site at http://www.moderncasting.com. As an example, Foundry ABC is an iron green sand foundry that uses a 50-ton green sand system. It maintains a clay mixture of 10% western bentonite.
Using western bentonite clay, it is a given that castings with thicknesses 3 in. and under will have 325 lb of sand heated to 1180F or higher per ton poured, see 1. Examining the spreadsheet, at 2, we see that a 10% western bentonite has been deactivated per ton poured. (That 325 lb of sand mixture heated will contain 10% clay.)
As a rule, Foundry ABC adds 500 lb of new sand (which includes 250 lb of core sand) per ton of iron poured. Therefore, examining 3, the system will require a new western bentonite addition of 82.5 lb This number reflects the 32.5 lb to replenish the clay deactivated by the heat + 50 lb to bond the new/reclaimed/core sand to a 10% level (500 lb * 10% = 50).
Now that new sand and material has been added to the green sand system, material must be removed to balance the system. Examining 4, it is noted that 583 lb of sand must be removed (reclaimed). This figure represents the 500 lb of new/reclaimed sands, plus the new clay additions of 82.5 lb. (Note: this level of reclamation assumes 100% efficiency and no losses.)
The system is balanced at the level the foundry has determined is its best composition for producing good quality castings. In order for Foundry ABC to stay in control, must continually control and evaluate its consumable consumption numbers per ton of metal poured.
RELATED ARTICLE: Inventory-Based Green Sand Control at Victaulic
One company using a "modified" version of the inventory (mass balance) concept is Victaulic Co. of America, a mechanical piping products company operating ductile iron foundries in Easton and Alburtis, Pennsylvania. In the early 1980s, Victaulic became one of the first foundries to control is green sand system through the use of a personal computer (PC) network. As noted in a 1987 Transaction Paper by Victaulic's Joe Luckenbaugh and Dave Sharkus, the foundry's control system eliminated the need for direct sand preparation supervision while providing more precise, consistent molding sand. Because additions to the system are individually calculated, it reduced raw material consumption and virtually eliminated batch-to-batch variations in sand at the molding lines. Coupling historical data with statistical control through the PC network has made the operation of the green sand system significantly more understandable and predictable, say company officials.
"If you control the process within narrow constraints, once you arrive at the optimal levels, you'll get good, repeatable results," said Luckenbaugh, manager of foundries.
He agrees that green sand systems are best controlled by composition, and that "testing after the facts is only chasing a problem." At the same time, however, he said that sand tests are necessary to verify that real time controls are maintaining the system at the desired levels.
Victaulic controls its sand to metal ratios to minimize variations in bentonite additions. Because its castings are similar in section size, the sand to metal ratio can relate to the amount of deactivated bentonite in the system.
A key for Victaulic is carefully controlling what enters the system. Its mechanical reclaim system, which was installed in January 1985, has eliminated virgin sand from the system. The only "new" sand entering now is from cores. Luckenbaugh said that Victaulic's purchasing specifications for sand for core use are very stringent (grain size and distribution), since "it eventually becomes the green sand system."
Victaulic purchases higher quality raw materials when it can realize increases in casting quality or reductions in total costs. For instance, he said the foundry buys sand from the Midwest, rather than using locally available subangular sands. "As a result of the higher quality sand, we use less of it because it doesn't break down as easily, it bonds better and we achieve better overall casting quality," he said.
Another key is the control of fines. After reclamation, sands pass through a 20-mesh screen. "Reclamation and dust collector additions are based on the system fines levels. We know that the more debris and deactivated bentonite we take out of the system, the more effective our bond addition is going to be."
Adjustments, he said, are made at the muller based on sand to metal ratios at the mold line, the temperature and clay levels in the return sand and weight of the sand. "Every sand system will change to an extent," he said. "Using data from the 10 previous batches and mold line sand to metal ratios, the system anticipates additive requirements and makes the appropriate adjustments. It keeps us from dealing with sharp peaks. Sand systems are always changing to a degree, but by controlling raw material additions, the system only changes slightly over the course of time.
"We're strong believers in process control. First, you must make the process repeatable. Then, you can work hard to make it the best it can be. But it must be repeatable first."
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|Title Annotation:||includes related material|
|Author:||Vinga, George J.|
|Article Type:||Cover Story|
|Date:||Feb 1, 1998|
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