Green sand system control: from shakeout to mulling.
Over the past few decades, much attention has been given to green sand preparation and control for today's molding processes. As equipment has become more complicated, the foundry industry has come to expect more complex answers to simple problems, primarily because it doesn't look for the obvious.
The industry needs to get "back to the basics" as much today, with sophisticated equipment and processes, as it did decades ago when foundrymen added bond to the muller by hand to prepare sand for floor, bench or squeezer molding. In simpler days, foundries relied on baby scales, pocket watches and the slinger operator's touch to produce "quality" molds and castings. Simple answers to simple problems were learned by trial and error.
But with today's sophisticated and fast equipment, those simple methods are no longer acceptable solutions to today's problems.
Contrary to the beliefs of some foundrymen, the way system sand is handled between shakeout and mulling is important. Success or failure may be determined in the preparation for mixing or mulling as often as in the actual sand preparation itself.
Poor practices and lack of attention to detail in this area can cause serious problems downstream in a sand system. Following these guidelines will provide better success at the muller and help eliminate variations in sand properties.
Keep the System Full
Most green sand systems must have some or all of its sand removed at one time or another, often for maintenance or cleaning of sand storage equipment and rebuilding major sand handling equipment. Normally, green sand is removed on a constant basis because the system tends to fill up from core breakdown sand as well as new dilution sand added on a regular basis. Sand also is removed or added when the sand to metal ratio decreases or increases, which requires less or more sand in a flask.
When sand is removed, it should be stored and added back to the system when a decrease in the sand to metal ratio requires additions. This sand should be metered slowly into the system rather than all at once because of differing physical properties. Metering decreases the effort needed to balance the system again.
The most important thing to remember is the green sand system runs best when it is full. The experience of a large Midwestern gray iron foundry having problems with its flasks illustrates this point. Every weekend, it had to empty its system so all the flasks could be inspected for cracks or weld failures.
When full, their system holds 835 tons of green sand, with a breakdown of the equipment showing:
* two cool return sand hoppers (225 tons each)--450 tons;
* excess sand storage hopper--70 tons;
* delivery belts--150 tons;
* flasks--150 tons;
* molding machine hoppers--15 tons.
Flask inspection and repair takes place on the weekend. To keep overtime costs to a minimum, the system was brought down slowly all day Friday, so that by the end of second shift on Friday the cool return sand hoppers were nearly empty. At the end of the second shift on Friday night, all molds were shaken out and as much sand as possible was returned to the nearly empty cool return sand hoppers. All the sand on the delivery belts, the molding machine hoppers and what was left from the flask was then discarded because there was no room to store it on-site.
When the molding line started up on Monday mornings, it took most of the sand in the coot return sand hoppers just to fill the flasks, machine hoppers and sand belts. This left the cool return sand hoppers down about 315 tons (only about 35% full).
Because of sand dilution rates, core sand dilution, screen distribution and sustained runs for limited parts (as well as a high amount of sand carryout on casting clusters), it took most of the week for the system to become full again, only to be run down again the following weekend. Instead of generating excess sand, most spent green sand additives, shakeout core sand and thermally exposed and abraded sand are returned to the system.
Up to 50 tons of core sand are dumped into the green sand system during each operating shift. At the end of the day (second shift), 100 tons of core sand have been added to the system, which, if full, means a core sand dilution rate of 10.7%. But, on Monday mornings, when the system is at its lowest, the core sand dilution rate is running at 15.7% of the total sand system.
Table 1. Effects of Hot Sand on Control Sand Temperature Sand Characteristics 100F (38C), or no hotter than 15F stable green sand characteristics, such as (8C) above ambient temperature green compression strength, compactibility 120F (49C) relationships between various physical properties deteriorate; extended mull times required 140F (60C) physical properties uncontrollable; friable, weak molds 160F (71C) sand not capable of being mulled--water evaporating faster than clay can be wetted
American and European reports detail how core sand can lower clay rebonding and adversely affect green sand properties. Those reports also refer to how spent green sand additives (clay, seacoal and other additives) can cause problems with green sand physical properties and casting finish.
Recommendation--Keep the green sand system full to reduce the impact spent materials and core dilution sand can make on your ability to control the process. Store green sand to refill the system when sand removal and re-addition is necessary. When adding large amounts of stored green sand back to the system, meter it in to reduce the possible impact of physical property differences.
As seen in Table 1, excessive sand temperature causes more daily control problems than any other control factor in green sand. Allowances are constantly made for sand temperature in tooling, molding and finishing operations. Many foundries shy away from engineering and operating costs for sand cooling because of capital expenditures. But these same foundries may eventually spend more due to allowances made for control problems caused by hot sand. Foundries unable to compete in casting family and weight ranges that should be ideal for them probably haven't resolved problems such as hot sand.
Table 2. Effect of Hot Sand on Molding and Casting Scrap Molding Stickers caused by condensation of water at cold pattern, hot sand interface at strip Friable edges high evaporation rate, especially at mold edges, causes loose sand Sand sticking in hoppers hot sand against cool hopper walls (same phenomenon as stickers) Casting scrap Burn-in up to 6% drop in mold density Blows cool cores set in warm molds cause condensation on core surfaces; dryer molds have less permeability Sand inclusions dry sand washing in mold Erosion scabs lower hot/dry strength results in cuts, washing and erosion of sand surface
Effects of Hot Sand--Upon returning to the mullers, green sand ideal for preparation should be no hotter than 100F (38C), or no hotter than 15F (8C) above the ambient temperature. These temperatures allow all the physical properties of green sand to be developed at normal mulling times. Most foundries don't operate at ambient temperature ranges, therefore foundrymen should understand the risks in doing so.
At 120F (49C), losses in physical properties, such as green and dry compression strength, start to occur. The only way to fully develop these properties is to extend the mulling time. At 120F, however, stickers will start to occur due to warm sand resting against the cooler pattern surface, which causes condensation of water vapor in the molding sand adjacent to the pattern surface.
This "flooding" creates a weak, some times plastic, area in the sand at the mold surface, weakening it and causing it to adhere to the pattern when the mold is stripped. Sand also starts to build up on prepared sand hopper walls at these temperatures, much from the same phenomenon that causes stickers.
At 140F (60C), green sand physical properties are no longer controllable. This temperature is past the maximum permeability of the molding sand. Within one minute, over 20% of the original moisture at the mold surface is lost, and, within two minutes more than 40% of the surface moisture is lost. This leaves a weak, friable mold surface, resulting in poor casting finish and scrap. This scrap may be misleading because it often resembles excess moisture defects.
Above 140F (60C), moisture isn't mulled into the sand because it is lost faster than it can enter the lattice structure of the clay--resulting in moisture loosely attached to the clay and yielding "dead" molding sand. When sand reaches 160F (71C), there is no permanent wetting of the sand/clay mixture because the evaporation rate--relative to the tremendous surface area of the sand mass--is very high.
Reducing Temperatures--Many foundrymen add moisture at the muller to cool green sand to a reasonable or consistent sand temperature. Adding moisture at the muller to reduce temperatures, however, may actually increase control problems.
At 80F (27C), a 3.4% moisture addition is required at the muller to produce a sand with 35% compactibility, while at 140F (60C) it requires 5.1% moisture to obtain 35%. Mull times should be increased to allow for adequate cooling time, adequate distribution of water in the green sand and reduced mulling efficiency.
Because many mulling operations are based on fixed-time muller controls, fluctuating sand temperatures will result in wide variations in the physical properties of mulled sand.
Recommendation--Return green sand to the sand storage hoppers in the ideal temperature range. It is best to cool return green sand before putting it in storage hoppers above the mullers. This reduces the amount of buildup in return sand storage hoppers. If sand isn't cooled to the proper temperature range, extended mull times should be considered.
New Sand Dilution
Often overlooked to control green sand dilution is one of the most important green sand control tools available. By constantly metering the amount of new sand into the system, the effects of sand additive breakdown, sand breakdown and buildup of foreign material in the system are evened out. New sand dilution also should be used to maintain the screen distribution required. Failure to use new sand dilution correctly, or to the extent possible, is almost as bad as not using it at all.
An adequate new sand dilution rate for gray and ductile iron is 300 lb of new sand/ton of iron poured. Table 3 lists recommended starting rates for new sand dilution for other metals. The new sand dilution figure should be tempered by other factors:
* sand to metal ratios;
* core sand dilution rates, including amount of core sand input and core process(es) used;
* screen distribution target for the system sand;
* sand used for distribution, by screen and type;
* fines removal program;
* actual operating experience.
An ignored feature of a good new sand dilution program is how fast corrections can be made using dilution sand after "one time" problems occur. Brief periods of increased new sand dilution rates can help dilute the buildup in the sand system that occurs from sustained runs that are outside the normal operating mode for a green sand system.
Foundry Operation Impact on Screen Distribution--A few years ago, a large Midwestern foundry had a problem that involved recurring burn-in on a motor block. After gathering all the information during the various runs, a distinctive pattern emerged.
A subtle change in green sand screen distribution was apparent during the motor block run that wasn't indicated by green sand testing prior to the motor block run. Going back through the previous runs, there were times when the block exhibited no burn-in problems, while other times the same block had serious burn-in problems. The burn-in only occurred when the screen distribution was on the coarse side.
It was revealed that a large clutch housing occasionally ran just ahead of the motor block. The pattern of clutch housing and motor block being run in tandem was dictated by coreroom production constrictions. Each time the clutch housing ran ahead of the block, sand burned in. When the clutch housing wasn't run ahead of the block, no burn-in problems occurred.
When core weights were factored in, it was evident the clutch housing dumped high amounts of core sand into the system. Once the clutch housing was run in smaller production increments throughout the production day, the core sand dumped into the system at any one time decreased and burn-in on the block stopped.
Core sand influx can have as much effect on green sand screen distribution as new dilution sand. But the adverse effects of high amounts of core sand dilution may sometimes be reduced with proper new sand dilution.
Screen Distribution of Dilution Sand--Another foundry was experiencing sand control problems. As it brought its green sand physical properties into control, officials realized they should be using the new sand dilution equipment to exercise more control until the system started to settle down.
The problem with using the new dilution sand as a control tool was that the coreroom sand was a coarse lake sand. The high influx of core sand from various jobs swung the screen distribution so far to the coarse side that burn-in and penetration occurred. To counter the coarse corn sand influx into the system, a liner silica sand was used for dilution and green sand screen distribution control.
While this was an effective means of distribution control, it precluded the use of new dilution sand for many of the other things it is capable of doing. Every time the new dilution sand was increased to help smooth out the physical property swings in the sand system, the permeability dropped and blows, cold shuts and other problems started occurring.
Although some foundries may save core binder in the coreroom to reduce costs, the effect of coarse sand on the entire operation must be carefully evaluated. It may be that the money saved in the coreroom is wasted in the finishing department in reclaiming castings or in casting scrap.
Table 3. Recommended Starting Point for New Sand Dilution Metal Rate Aluminum 100 lb/ton metal poured Brass/Bronze 250 lb/ton metal poured Iron 300 lb/ton metal poured Steel 500 lb/ton metal poured
Recommendation--Establish a good new sand dilution process. New sand dilution should be recognized as another control tool as well as a means of providing stability to a green sand system. Evaluate the impact of the core sand screen distribution on your green sand system and overall costs.
Retaining green sand in the system is an important part of overall control. Even with shakeout equipment operating correctly and maintained properly, it is still possible to run into problems with excessive sand loss at shakeout. Control problems with moisture, clay content and mulling efficiency can cause high hot and dry strength problems, which cause poor sand breakdown and retention at shakeout.
Sudden changes in sand to metal ratios can result in large amounts of green sand reaching shakeout without ever being exposed to heat. Also, unpoured molds may reach shakeout intact. Both of these instances may result in excessive loss at shakeout.
The most obvious way to decrease sand loss at shakeout is to improve moisture, clay content and mulling control. But aside from the obvious, a good sand dilution program improves shakeout.
A short period of increased new sand dilution can temporarily solve a small problem. Care must be taken to not over-dilute, since mulling efficiency may decrease because clay can't transfer quickly enough to all the new sand during mulling.
Some simple engineering devices may also help improve sand retention at shakeout. A device used by some foundries consists of large multi-point stars cut from heavy steel stock, strung on a chain and hung horizontally over the shakeout area. They are positioned to drag across the top of the mold cake and break up the sand that normally stays on top of the casting clump. The sand is broken up and remains in the system instead of going out on top of the castings.
Another device to aid sand retention at shakeout is to weld angle iron in a fish bone shape on the shakeout deck to slow down sand moving across the deck. If the bars are the correct size, they don't hold castings up long enough to cause damage. But the gates, runners and castings tend to break up the sand clumps on the deck as they move across.
Recommendation--Retain as much sand as possible in the system. Use simple, innovative devices to keep sand in the system.
Screening to eliminate undesirable elements from the sand system is important. Screening equipment must be operated and maintained as recommended.
A good way to determine how serious a foundry is about sand control is to look at the cleanliness of the sand system. A foundry is constantly in and out of trouble if maintenance people drop welding rods, nuts and bolts and other items in the sand system, if workers use the sand system to throw out sandwich wrappers, cigarette butts and other trash, and if the sand system is a good place to dump floorsweeper trash and other debris.
One ductile foundry sampled all the trash it found in the sand system (welding rods, core rack tickets, nuts and bolts, washers, terry cloth glove, chaplets, etc.) and cast them in plates on the jobbing floor by placing them in the sand metal interface. The casting defects were obvious. The samples were mounted on a portable display board in the foundry to educate workers about sand cleanliness.
A good program to educate everyone about sand cleanliness will reduce casting scrap and reclaim costs. But, there is another kind of trash that is as insidious but tends to get overlooked, especially in foundries that have had the problem for some time.
Core butts infestation in green sand is a serious problem. Core butts normally produce excessive amounts of gas, can act as shrink-causing heat sinks and shatter during molding only to wash out into the casting, leaving sand holes or "walnuts and grapefruits" growing on the sides of castings.
A good maintenance program that follows manufacturers recommendations is necessary to keep screen programs operating correctly. Regular inspection and preventive maintenance is important to maintain clean sand.
Recommendation--Keep trash (man-made and process breakdown materials) out of the sand system. Educate everyone on the importance of a clean sand system.
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|Title Annotation:||part 1|
|Date:||Apr 1, 1994|
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