Tips for auditing your green sand system.
Many variables exist in a green sand system, which by its nature is in a constant state of change. Problems can occur when one or more key control properties fall outside of the control range. When an effective system is in place to monitor these key control properties and correct the variances, green sand systems can be more effectively controlled.
Following is the AFS Green Sand Molding Committee's (4-M) procedure for auditing green sand systems, which was developed out of numerous plant visits. This article identifies the main areas of the sand system that foundry personnel should self-audit for process improvement and scrap reduction.
Because operations and equipment are so different, absolute rules for control do not exist. However, foundries are advised to examine and question their methods of green sand control - and recognize that yesterday's process may not necessarily still be the best one for today.
Controlling your green sand system requires breaking the process down into key areas and then working to reduce the variability of those key areas. To properly focus the audit, it is recommended that foundrymen examine three key areas - return sand, prepared sand and sand properties.
Proper control of the percentage of clay, sand, moisture and temperature in the return sand will greatly enhance the ability to hit target properties coming out of the muller. The following areas are critical and should be given priority during the return sand system audit.
* Contamination. The audit should verify that core butts, trash, metal and other contaminants are being purged from the system sand.
* Size Distribution. Much about the state of the return sand can be learned from the prepared sand screen analysis and AFS clay or 25 micron test. While some may think this is a prepared sand issue, the fact is that with the exception of the new sand additions at the muller, most changes to the size distribution of the sand occur outside the muller.
The washed 140 screen in the prepared sand should be maintained between 1015% to reduce penetration and increase the number of grain-to-grain contacts for reduced friability and mold erosion. This has a positive effect on all strength readings (green, dry and wet tensile).
The 200+270+PAN should be kept as low as possible because these materials contribute only minimally to penetration resistance, while causing higher moisture levels and lower permeability. A level of 4% or less is recommended.
* Moisture Sponges. The moisture sponge percentage (defined by the committee as AFS Clay minus methylene blue clay) should be generally maintained between 2-5% (varies with foundry and metal poured). This dead, light, moisture-absorbing material competes with the clay for water, thereby increasing the moisture needed to hit the compactibility target.
Excess moisture sponges can lead to pinholes, rough surfaces, scabs and erosion. Insufficient moisture sponges, meanwhile, make compactibility more difficult to control because the sponges "even out" errors in moisture addition. They also reduce compactibility loss on the belt as the sand travels from the muller to the molding line. The moisture sponge percentage is affected by:
* dust collector location and flow rates. These light, powdery materials are easily pulled off, especially in areas where the sand is churned or free-falls;
* dryness of the return sand, which allows higher amounts of light material to be drawn off;
* changes in the rate at which fines or blackwater are added back in to the system. Addition rates and percentages must be consistent;
* the rate that new sand and core sand is added to the system. Adding sand dilutes the percentage of moisture sponges in the return sand. If the system is not purged with enough sand, sponges will build up. On the other hand, too much new sand can create green sand that is too "clean" (moisture-sensitive) and friable, since new sand requires more energy to coat it than system sand.
* Return Sand Moisture & Temperature. Because clay won't adhere to sand over 130F (54C), temperature is one of the most critical elements of return sand. The lower the sand-to-metal ratio is, the quicker the temperature of the sand rises (see chart at right). Long cooling lines and high pouring temperatures also drive sand temperature up. Many foundries straggle to control the temperature of their sand and look for ways to cool it. Adding water before the sand hits the muller cools the sand and starts the rehydration process. Because time in the muller is generally limited, water often is applied to the sand before it enters the storage bin so that rehydration can begin.
Most foundries check the moisture of their prepared sand, but the moisture levels of the return sand should not be overlooked. If the sand is too wet, it can plug equipment. If it's too dry, the complete rehydration of the sand will be difficult. The tighter the rerum sand's moisture, temperature and moisture sponges are held, the easier it is to consistently maintain compactibility in prepared sand.
GREEN SAND PREPARATION
The goal of green sand preparation is to homogenize, moisturize and effectively smear clay onto the sand grains to create a sand that has the optimum properties for the molding machines.
* Muller Maintenance & Adjustment. A written record of preventive maintenance on the muller is a must. Nothing should be assumed - records should be verified. Checks should include:
* plow and wheel spacing - this affects the smearing of the clay;
* lining condition - poor conditions cause a drag effect, thus increasing the muller's amperage needed to turn the sand mix. If the doors are of the type that open when high amperage is reached, the door may open before sufficient mulling is achieved;
* dust collection - air flow rates, especially over the muller, can cool the sand. Vigorous dust collection can also draw off bond before it has had a chance to be mulled in;
* compactibility controller - the compactibility controller must be maintained and calibrated periodically. Keep in mind that the controller performs much better when the moisture sponges and other characteristics of the return sand are controlled well;
* aerator - this is an important piece of auxiliary equipment for the muller. It fluffs the prepared sand prior to molding, and the longer the transport distance from the muller to the machine (more precompaction), the more important fluffing becomes.
* equipment for bond and water addition - this can offer the biggest challenge when it comes to maintaining reliability and consistency. Verify that both bond and water are being fed consistently. Is there an alarm that provides an alert if either stops feeding?
* Materials Addition. The first thing to examine is how, when and where each of the additions are made. For new sand additions, a common rule of thumb is that 300 lb of sand should be added per ton of iron poured. In reality, however, there is no "one size fits all" formula for new sand additions. Each foundry must take into account its equipment, core sand addition rate and level of moisture sponges. The core sand addition rate becomes important when considering adverse effects of partially burned core sand that is coated with carbon. Clay does not adhere well to carbon-coated sand grains.
The rate of clay addition required to maintain target methylene blue clay levels in the prepared sand depends on the amount of five clay in the return sand and the amount of new sand being added. Return sand clay levels are dictated mainly by the burnout rate of clay due to the molten metal, the amount of clay removed from the system by the dust collectors and the amount of clay removed from the system in dump sand. Clay burnout rates are dictated by the sand-to-metal ratio-heavier parts expose the sand to higher temperatures for a longer period of time, burning out more clay.
For batch mullers, are additions made in the most favorable sequence? If dry materials are added first, foundries must consider that no tempering can begin until the water is added. Many foundries find the best results by adding water first. This cleans off the plows and begins the hydration of the sand as soon as possible.
Where possible, it can be beneficial to add clay to return sand to create mixing and tempering (prehydration) of the clay before it reaches the muller. Many foundries hydrate the return sand and clay as much as possible without causing silo plugging so that water added in the muller is only trim.
In the batch muller, any changes that increase the amount of time the water is in contact with the return sand and bond will increase muller efficiency (less water to achieve desired properties). The total time the water is in contact with the premix and sand is called the "true mulling time." How long does it take to add premix and water? As the time to make the addition increases, the time available to mull the water and premix into the sand is reduced.
Continuous mullers use "retention time" instead of "true mulling time" when discussing how long the premix and water are being mixed. Retention time equals average mass of sand in the muller divided by the feed rate into the muller (assuming feed rate in equals feed rate out). The retention time must be long enough to achieve consistently good properties. Some foundries speed up the input rates in continuous mullers because of higher sand demands. These foundries must judge the cost/benefit of scrap vs. production rates.
What are the components of the premix and why is each being used? Examine whether each additive is still necessary and actually achieving the characteristics originally intended. Should the percentage be changed or should an additive be eliminated? Over the years, premix requirements often change as equipment changes or casting requirements change.
GREEN SAND PROPERTIES
The three critical areas for examination of green sand properties are material percentages, sand properties and test equipment/methods.
* Material Percentages
Material percentages in green sand (methylene blue clay, moisture, grain size distribution, volatiles and combustibles) are all independent of mulling. Each foundry establishes target levels based on the molding equipment in their plant. Once the targets have been set, a system must be in place to keep them there.
For example, the methylene blue clay must be set at an appropriate level for your molding equipment and the defects seen. (Higher pressure molding machines generally run better with more clay, as do foundries that run parts prone to scabbing defects.) Does the methylene blue clay show sharp swings?
Moisture Levels - Tight moisture control is critical for controlling compactibility. High moisture levels can cause can metal penetration, pinholes and scabbing. Low moisture allows friability, erosion and sand inclusion problems.
Free Water - For this discussion, free water is defined as the water held by the moisture sponges in the green sand. This means dead clay, coked seacoal, wood flour and any other nonclay, moisture-absorbing materials in the sand. Some free water is good because it helps "even out" the moisture in the sand and helps combat compactibility drop, offs due to moisture losses. Too much of this water, however, can interfere with flowability and cause sticking. Pay attention to the clay/water ratio. Is it consistent? Many things can throw the ratio off. If mulling is not adequate, the system may be using excess water to reach the target compactibility. If dust collector fines are staying in the system, more water is needed due to moisture sponges.
Carbon Levels - The volatiles test is an index of low-temperature carbons and the combustibles are an index of the high-temperature carbons. Are the volatiles and combustibles levels appropriate for the molding equipment and castings being produced? Seacoal swells to fill the gaps between sand grains when heated, and prevents scabbing by relieving the stress of expanding sand grains. Generally, as the machine's ability to produce harder molds increases, the level of combustibles that are needed is reduced.
Sudden increases in core sand input can cause volatiles to surge. Fines levels usually index combustibles since the size distribution of seacoal is usually highly peaked on the below-270 screen. The level of fines can change when dust collector flow rates or return sand moisture changes. Also look at blackwater - has the rate of re-addition or removal changed?
Besides scabbing, low carbon levels can cause pinholes and poor peel. When carbon levels surge, there may be increased erosion, blows, or sand inclusions.
* Sand Properties Reported
The most common sand tests performed are compactibility, green compression strength, specimen weight, methylene blue clay, permeability and moisture.
Methylene blue clay is affected by the ratio of western (sodium) to southern (calcium) bentonite, moisture, level of moisture sponges, mulling quality and time.
Compactibility is a function of the % methylene blue clay, moisture, mulling quality and time, moisture sponge level, and the western/southern bentonite ratio. After your foundry has decided on its target compactibility, a plan should be developed to keep it there. If you're having problems with stickers, not getting flowability into mold pockets or losing mold details, the sand may be too wet from attempts to achieve a high compactibility.
Green compression strength is a function of the same variables as compactibility plus grain size distribution, core sand/new sand ratio and quantities being added to the system and other special strength-altering materials in the premix (like wood flour).
Specimen weight is mainly a function of grain size distribution and the moisture sponge level. Specimen weight is a good indication of moisture sponge problems. For instance, if an iron foundry's specimen weight falls below 148, it will expect to find a moisture sponge value exceeding 4% (the system choked with fine, dead material) and a clay/water ratio below 2. This can culminate in moisture-related defects previously mentioned. If the specimen weight for an iron foundry is above 162, the foundry may well find moisture sponge levels below 1.5% (too clean) and difficult to control compactibility levels also previously mentioned. Know where your foundry typically runs well and how much it varies from day to day.
* Test Equipment and Methods
Before you make any decisions based on the results of sand tests, you must consider how confident you are with the testing equipment and methods. Are you sure the data is reliable? Are samples representative of the sand in the system?
The more variation that exists in the sand, the more frequently it should be tested. Also take into account the size of the system and the turns of sand per shift. Although automatic controllers can reduce the frequency of testing, most foundries test their sand at least once per turn.
Audit the test lab on sample preparation and test procedures. Check all shifts and operators. Are they consistent?
Equipment and procedures affect reliability. For example, compactibility testing using 3 rams is affected by the rigidity of the rammer base, how freely the weight falls along the shaft and the balance of the arm.
Methylene blue solution should be calibrated against a clay standard stored in airtight container. Due to the accuracy required to prepare the solution, purchasing the solution may be worth the extra cost. Do all technicians have the same judgment as far as the methylene blue clay halo end point? Note that because methylene blue clay solution calibration standards vary, methylene blue clay test results between labs do not always compare well.
Different crucible sizes are thought to affect volatile readings. Larger crucibles have more oxygen to react with the carbon to produce C[O.sub.2].
Keep in mind that the choice of oven temperatures affect the volatiles test. The higher the temperature, the more carbonaceous material is burned off. Temperature control should be verified and the thermostat should be scheduled for replacement at a set interval of 6-12 months. Besides ensuring accuracy, replacing the thermostat is less expensive than a burnt-out heating element.
Is test equipment calibrated on a regular basis? Verify, through records, that calibration is performed regularly.
Periodically send samples to an outside lab for testing. Many foundries consider this practice valuable, as it can alert managers if equipment is going awry and help evaluate lab performance. Results from two labs may not match, but they should track each other. Foundries do this anywhere from once a week to once a month. The outside lab also may perform additional tests that your lab doesn't have equipment for.
As long as every variable in the system is held constant, the system will be in equilibrium. However, one of the many absolutes for foundries is that every variable always is changing to some extent.
There are, however, some key equilibrium concepts worth understanding. The clay level in the system is a balance between what is added at the muller, burned out at pouring, removed through dump sand or sucked out through the air ducts. When dust collector or blackwater fines are added back in, the equilibrium shifts toward higher clay so that the clay addition must be reduced. When the maintenance department "fixes" the dust collectors, the clay addition rate must again be reviewed. Core sand plus new sand additions dictate the dump rate of sand from the system and therefore also affect equilibrium. Western/southern bentonite ratios also affect the equilibrium, since the southern clay bums out at a lower temperature than western.
Similarly, moisture sponges have their own equilibrium factors. "Generated" during pouring, they are affected by the same factors noted in the previous paragraph, but also the target methylene blue clay level since the higher the clay percentage in the mold, the more will be burned out.
The auditor should be aware of these key equilibriums so not only can the "out of bounds" property be identified, but also the reason it is out of bounds - and what the best route is for getting it back in control.
A systematic audit of the green sand system through return sand, sand preparation and property verification, followed by corrective action to maintain target properties of the green sand, will lead to a reduction of scrap and a more consistent quality performance.
This article was adapted from a panel presentation at the 1998 AFS Casting Congress.
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|Title Annotation:||guidelines for metalcasters|
|Author:||Wrobel, Mark D.|
|Article Type:||Cover Story|
|Date:||Mar 1, 1999|
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