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System layout and bulk handling.

An efficient green sand system should perform several functions to provide good quality molding sand. First, the sand and castings must be separated, and the larger sand lumps broken down at the shakeout. It is then essential that tramp metal, both magnetic and nonmagnetic, be removed, not only to allow for the reuse of the sand, but to protect other equipment in the sand system.

At the same time, heat f rom the molten metal must be removed and the sand cooled to a controlled, usable temperature for molding. Excess fines should be removed and proper screening and classification performed to achieve proper grain distribution and permeability. Bond, new sand and water can then be added and blended, and the sand mulled to achieve desired molding properties.

In addition, the system should be designed to provide sufficient storage to handle surge situations while maintaining normal operations. A nonferrous or stainless steel foundry requires that screening be performed prior to cooling to protect the cooling equipment.

After molds have been poured and cooled, and the sand and castings separated at the shakeout, the magnetic tramp material must be removed. Surge storage should be provided to allow metering of sand for better control of subsequent operations, The cooling system is next, with water and air being added and the moisture laden air being exhausted. Bond and new sand can be added at this stage as well, depending on the cooler design. If bond is being removed as fines by the exhaust system, it must be returned to the sand and blended in.

Magnetic separation may be performed again, as the cooler temperature will provide for more efficient metal removal. Screening to remove core butts, lumps and other nonmagnetic material is next using either vibratory or rotary screens. Properly sized and designed, either type can be effective. However, rotary screens provide greater screen area for a given footprint and greatly reduce vibration in the system structure. Selection of mesh size is a major variable from system to system and all the operating parameters, such as sand strength, fineness, sand-to-metal ratio and moisture, must be considered. Fines removal by a controlled air stream will provide classification to maintain proper grain fineness, distribution and permeability.

The sand then can be returned to the bulk storage bin. A desirable option for any modern sand plant is a fine screening or laundering system to clean smaller tramp metal that was not removed previously from the sand. This is done by metering a small percentage of the total throughput into a fine mesh screen (generally a gyratory type) and returning this "laundered" sand back to storage.

If properly designed and installed, this system can be run continuously while the sand plant is operating or set to run by itself during downtime if the sand needs extra cleaning. Because of the difficulty of removing fine nonmagnetic material from sand, a laundering system should be a requirement in all modern brass, aluminum or stainless steel foundries.

From the bulk storage, the sand is fed to the muller, where water is added along with bond and new sand, if needed, and the sand mixed and mulled to achieve desired molding properties. Depending on the molding system layout, surge storage and aeration should be provided after mulling.

Aeration is not only important, but is essential on impact molding systems. Impact units depend on quickly accelerating loose sand grains against each other to produce the mold hardness at the pattern face; therefore, the sand must be "fluffy" prior to impact.

Sand Handling

Moving sand between various operations can be accomplished by any of several types of handling equipment. The most common of these is the belt conveyor. Belt conveyors are a very effective method of moving sand, if properly designed and installed. If possible, inclines should be limited to 181 or less from horizontal. Widths should be chosen based on system tonnage and layout factors. Decking between stringers, proper belt scrapers, skirting where needed, troughed idlers where possible and dribble pans over work areas can make for a much cleaner installation.

Vulcanized or taped splices and the use of slider beds also helps to keep the system cleaner. Close spacing of idlers at transfer points or surge hoppers, is needed to prevent spillage. Properly designed transfer points with enough elevation to use wide, steep or vertical-sided chutes will also help to keep the area clean and prevent jam-ups. V-plows should be used whenever possible, with solid plow tables and a continuous transition to the hopper or chute. Backup plows direct the sand remaining at the edge of the belt back toward the center after the plow raises to stop dribbling off the edge.

Bucket elevators have been a much maligned part of many sand systems. Leaks, jam-ups, broken belts and pits full of sand are some of the most common complaints. These problems usually result from improper design, sizing, layout arrangement or installation, but can usually be avoided. Bucket elevators should be fed in line, if possible, and high enough to allow three buckets to fill above the tail pulley. Otherwise, the elevator becomes a digger," increasing bucket wear and loads on the belt and drive.

Discharges are critical and should be correctly designed to allow free flow of sand. Drop-bottom designs improve cleanout and maintenance if enough space is available to use them. The use of plastic or nylon buckets has become common but care must be taken as there are often variations in the capacities from the manufacturers' ratings.

Foundry bucket elevators should be of heavy-duty design with special seals, bearings, shafts, pulleys and ample cleanout access. They are best used for return sand or dry sand, but can be used for prepared sand if care is taken in the design. Properly applied, a bucket elevator can be an efficient, dependable means to save floor space or eliminate the need for several other pieces of equipment.

Another method of moving sand is the vibratory conveyor. For dry sand, these are very effective and can reduce headroom or pit depth requirements because of their low discharge transfer elevation differences compared to belt conveyors. Units also are available for moist sand applications that have a suspended belt over the pan, which reduces sand sticking problems. These are excellent for spill and return systems because of both the lower discharge height requirements and the virtual elimination of spillage. Variable rate vibratory feeders also are used where metering a controlled amount of sand or binder is required.

Screw conveyors also can be used effectively to meter bond into a sand system. They have been used for spill sand, too, because they do not require pits. However, high wear and potential jamming can create maintenance problems.

Pneumatic transporters also have been misused and therefore have had many problems associated with them. The introduction of the dense-phase systems has greatly reduced wear, air consumption, sand degradation and maintenance. These systems are still best used only on dry applications such as new sand, reclaimed sand or bond.

One other handling method should not be overlooked in designing a modern sand system-gravity. When properly used, gravity eliminates expensive equipment, requires no energy to operate and is maintenance free. Stacking components properly and allowing one to feed directly into another is generally the most efficient arrangement. It has the added advantage of decreasing floor space requirements, with the disadvantage of height requirements.

Component Sizing

Critical to the proper use of any of these handling components is sizing for capacity. Sizing for capacity is equally as important as sizing the actual process components, such as mullers and coolers. Many of the problems of bucket elevators and spillage are related to improper initial design sizing or overloads caused by later increases in system operating level. A key design error happens frequently when potential surges are not calculated correctly.

One example of this is a molding line with by-pass capabilities for unusable sand. While the system may operate at a normal throughput of 100 tons per hour, during by-pass the return sand system may see 200 tons per hour or more. If the conveyor, elevator or screen are not sized for this, or a method designed in to even out the surge, then overloading, spillage or even system shut down will occur.

Another part of the sand system that requires proper sizing is sand storage. Many factors affect the required bulk and surge storage capacities. With many molding systems, it is necessary to clean off the line or to shake out all molds when not making molds. This requires that the bulk sand storage be large enough to have empty space during normal operation for the entire contents of the molding, pouring and cooling system.

In a system with proper sand cooling, the total storage of the system should be calculated based on the normal system tons-per-hour rate, and the desired "turnovers" during an operating day. Surge storage must be provided at points in the system, before components such as coolers, and should be fed at a constant rate for best operation. Batch mullers require surge storage to handle the batch characteristics, and then feed evenly to the molding line. Surge may also be required when molding is intermittent or variable to maximize muller utilization.

However, there is no value in having a large quantity of sand stored at any point in the system if you are unable to feed it as required to the system. The shape of the bin is often determined by the space available, except in greenfield installations. One of the most successful bin designs for completely feeding sand has been a round bin utilizing a vibratory bin bottom of a diameter nearly as large as the bin.

Much care must be taken in the design and selection of the vibratory bin bottom. These units are usually manufactured for use in industries other than foundries and will not always feed foundry sand properly. When specifying a foundry bin bottom, the manufacturer should be aware that the size of the unit must be much larger than they would normally recommend. They must also be instructed to enlarge the opening relative to the size of the bin bottom, and the defector cone must be of a special configuration. If all of these design criteria are met, this particular bin design can offer excellent feeding capabilities.

Rectangular sand bins can be made to feed properly if the bin is rounded at the bottom and the same design criteria utilized to specify a vibratory bin bottom for feeding. When a rectangular bin is used that has a belt feed bottom, it is difficult to feed all the sand, particularly from the back side of the bin. One solution to this has been a bin design which features an inverted triangular deflector through the bin.

Generally belt bottom feeders from sand bins create difficulties in feed arrangements and should be avoided, if possible. However, surge bins required at various points in the system may require this design. If so, it should be kept at a low height and with a relatively short length along the belt. When handling prepared foundrys and, hoppers and bins should be coated or lined on the inside with a stick-resistant material, such as a two-coat epoxy.

In a sand system that feeds multiple molding machines, the design of the control system is critical. An automatic control system for keeping sand in all the molding hoppers can make for a much more efficient operation at the muller as well as prevent individual molding stations from running out of sand. In addition, the controls can be designed to allow "topping-up" any hopper that is not full, rather than waiting for a low level signal. This system will enable the muller to run at a steady rate and keep all hoppers full. System Layout

The actual layout of the components in a sand system, in many cases, is determined by factors such as existing building configuration or molding line layout. Actual area available must be taken into account as well as restrictions on height at various points in the system. If possible, the distance for moving prepared sand should be kept to a minimum as opposed to the distances for moving return sand. This helps to prevent changes in moisture level and drying out of the prepared sand.

The most efficient sand system both in terms of initial cost, operation and maintenance will usually be a system with the fewest components. This can be accomplished by a vertically stacked system which utilizes gravity to feed from on component to another as much as possible. This, of course, may require a penthouse, but a properly designed vertically stacked system will often be the most efficient approach.

Whatever the arrangement of the system, a key design element is sufficient access to all areas for both operating and maintaining the equipment. Components that have large drives or gear boxes such as mullers or large coolers require access with heavy equipment to remove these in case of a failure. They also require clearances for removal which must be taken into account.

Conveyors, bucket elevators, screens and other units that have drives must have maintenance access platforms not only to reach these points but to be able to work around them. This is a critical area, which, if overlooked, can be very costly in terms of downtime and maintenance costs. Platforms should be built completely around the top of bucket elevators and at least on one side of all elevated belt conveyors.

Methods should also be provided for removing heavy components such as drive motors, and getting them back to ground level. This may require access doors in platforms for lowering motors from elevated equipment or access doors in the floor for equipment that is located in pits. For elevated equipment where easy access is not available from the floor with a crane, an overhead hoist or at least a monorail should be provided to enable lowering these components to the floor.

Another important design factor for a successful sand system is dust collection. Without proper air flow, sand classification or cooling cannot be performed. In addition, moisture buildup will occur in aerators and bucket elevators, leading to plugging and accelerated corrosion of the equipment.

A well thought out, well arranged green sand system can be a showplace for a foundry rather than an eyesore. Shortcuts in the design phase and layout of a system can be very costly and will continue to cost every day that the foundry is in operation.

(Figures omitted)
COPYRIGHT 1990 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:McMellon, Bruce
Publication:Modern Casting
Date:May 1, 1990
Previous Article:Controlling sand and additives.
Next Article:Failure mode and effect analysis: a step toward total quality assurance.

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