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Green sand system controls.

If the green sand equipment needed to screen, cool, convey and mix high quality molding sand can be considered the skeleton and muscles of the system, then the control system is its nerves. A properly designed and maintained control system will ensure safe operation of the equipment, delivery of sand suited to the needs of the molding machine and types of castings to be produced, while attaining high uptimes and low maintenance costs.

Control of a green sand system is usually best accomplished with a programmable logic controller (PLC) and a motor control center. The PLC handles all of the interlock circuits for conveyors and process equipment, inputs from the various sensors located on the system, and outputs to the valves and control devices.

The newer main control panels usually use graphics that depict the equipment arrangement schematically, and with lights showing the operating condition of the system. CRTs are also used frequently as a display mode for the system and its real-time condition.

The critical role of the green sand control system is its ability to control sand properties. Following is a look at four different types of sand property controllers that have been applied successfully to green sand systems.

The first is a moisture additional evaporative cooling system that treats return sand on the conveyor belt. The secret to the success of this type of unit is in the solid-state electronics that allow rapid analysis of changing temperature and volume values. This allows it to almost instantaneously dispense variable quantities of water on the sand stream.

This type of moisture addition system is very beneficial in systems that have wide swings in return sand temperature. Properly utilized, it can help even out peaks and allow the sand muller and its sand property controller to do a better job.

A moisture controller at the muller is one of the critical components in delivering consistent quality sand to the line. Following is a description of how atypical moisture controller may operate.

For a batch muller, a multiple number of thermocouples are set at different locations in the weigh hopper above the muller. The moisture content of the return sand is determined by reading the resistance between the multiple thermocouples. The batch weight, return sand moisture and the multi-temperatures are integrated in the computer section of the controller, which calculates the moisture to within one-tenth of a gallon of the needed water volume to achieve the desired discharge moisture level.

As the mixing cycle starts, water is metered through a pulse transmitting valve, closing at precisely the correct amounts. For the most accurate metering, a weigh hopper is recommended. But if a volumetric hopper is used, level probes are used to calculate the amount of sand in the charge hopper. Changes to the desired discharge results are easily made on the control panel door. Reportedly, moisture control of [+ or -]0.1 % is achievable with this system.

Moisture control systems also are available for continuous mullers. To work properly, these systems require a consistent flow of return sand into the muller. For more accuracy, it is desirable to have a weigh belt feeder or a flow bin that meters the sand going into the muller.

The return sand coming into the muller is monitored for temperature with thermocouples and for moisture by probes. The signals are integrated with set-point valves and the volume of water required is calculated. An electronic water valve and water meter deliver and verify that the proper quantity of moisture is delivered to the muller.

A companion compactibility controller, which works in conjunction with the moisture controller described above, also is available. This is installed after the muller discharge and is on-line with the moisture controller PLC. Deviation from the desired compactibility will cause the PLC to recalculate the values and effect a change in the water settings.

A probe system also has been designed for high-shear type mullers. in this system probes are pushed into the rotating mass of sand in the mixer after a batch of sand is introduced. The probe, which is located inside a vertical cylinder, descends into the sand for a period of ten seconds. During this time both temperature and moisture are measured and the probe is withdrawn. It is then cleaned and cooled to a starting temperature by compressed air. The information from the probe is analyzed by the control unit and water is introduced into the muller.

Another type of continuously operating compactibility controller operates with either batch or continuous mullers and reportedly controls compactibility to a [+ or -]2.5% range. The system consists of a compactability tester and ample feeder mechanism, dual water feed manifold and an electrical control system panel.

Temperature sensors in the batch hopper or incoming sand stream on a continuous mixer automatically regulate water flow to compensate for incoming sand temperature variations and allow the compactibility controller to make smaller, easier to control water additions.

With this system a small, continuous stream of sand is withdrawn by a screw auger. The sand is aerated and moves down a vibratory trough. If the sand is dry, it falls through a slot where a photo cell sensor detects the material and adds water to the muller. As the sand becomes wetter, it bridges the slot and the water addition control is switched to the compactibility controller. The sand then comes off the end of the feeder and fills the slot on the rim of a rotating wheel.

Next, the sand is struck off and passes under a compaction wheel which applies a fixed (but adjustable) load on the sand. The amount of compaction is measured as a change of angle in the arm holding the compaction wheel. A linear variable differential transformer sends a proportional electric signal that controls a mechanism involving two set point relays which controls the low and high limits of a predetermined compactibility range.

In batch muller operations, water is added to the sand mix by trim valves until the upper limit of the compactibility range is reached. The water flow then ceases and the batch discharges. Continuous muller operations are maintained by allowing the controller to add water continuously at a rate that keeps the upper limit set point relay from being de-energized for an extended time period. This type of controller also can control the entire mixing and peripheral equipment operation.

With the use of PLCs for sand handling systems and the sophisticated electronic control systems utilized by sand property controllers, important data can be recorded, analyzed and presented to system operators on a real-time basis for use in assuring quality green sand. The age of electronics has moved sand control from an art to a science.

In summary, the AFS Plant Engineering Committee has attempted to cover the main concept and components needed to operate an efficient green sand system to produce high quality castings. In order to operate a successful green sand molding system, it is essential to have a thorough knowledge of the system components and the relationship that variable amounts of sand, water and additives have on final results. The sand system must be designed for total sand conditioning, ease of maintenance and reliability.

Shakeouts must be effective in not damaging castings, yet must remove excess sand from the casting. Proper sand cooling is essential in achieving high quality sand at the molding station, and subsequently high quality castings. Mullers are the heart of the sand system. Proper selection, horsepower, physical size and cycle times are all key elements in producing quality sand. Moisture and compactibility controllers with state-of-the-art electronic systems provide the final step in the sand reconditioning process.

Just as a winning team must have all players working together and exhibit no obvious weaknesses, a sand system must be a well engineered combination of properly sized and continuously functioning components that will produce quality castings and profits.
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Copyright 1990, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Schulze, W.E.
Publication:Modern Casting
Date:Jun 1, 1990
Previous Article:Selecting a muller for a green sand operation.
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