PRIORITIZING GREEN SAND TESTING
Prioritizing Green Sand Testing
By George DiSylvestro Green Sand Molding Specialist

Introduction
Casting Scrap ConferenceGreen sand molding is one of the oldest and most economical casting methods that has survived time and represents the highest tonnage produced in the metal-casting industry. Those committed to using this process may wish to review and audit their method of controlling their green sand system. Most growth and profit-oriented foundries are interested in improvements in economy, problem solving, decision making and or casting quality.

Our studies of many production metal casting centers, has confirmed that there is very little uniformity or standardization in green sand testing and assigned priorities of tests for quality mold production. Of paramount importance to improve mold production and produce exceptional casting quality, is the consistency of the prepared molding sand.

Many tests are being performed and may not be completely understood. In order to make the information obtained more meaningful, the following is a summary of previous studies that can assist in qualifying and prioritizing the objectives of sand testing.

The Testing of the Sand
Molding sand is tested to control the green sand process in order to produce consistently high quality castings. This includes the ability to reproduce casting dimensions.

As the casting cycle is continuous in high production foundries, using automatic molding systems, an important objective is to monitor and exercise vigilance to the extent of any changes that occur. Success can be rated based on the ability to maintain maximum consistency of the return sand. Different bentonites do not perform the same. There is a synergy effect when they are blended at different ratios. Prediction of sand properties is also made difficult by: pouring temperature inconsistencies, amount of organics added, funneling in storage, hang ups, blend inconsistencies, processing of shakeout sand and changes in casting weights and sizes. Clay activation can vary widely when more than one molding line is fed with the same prepared sand if they run at different speeds and product mix. We can add to these variables the efficiency of fine removal, core sand dilution and new sand additions. We can easily see why the molding sand can be in a constant state of flux. Foundries that can precisely measure the most important properties and interpret them accurately and promptly can achieve molding sand consistency.

The question is, what tests should be run and what are their importance and value. The frequency of testing is determined by the amount of meaningful important changes that occur during the complete cycle of the sand system. It has been determined that with an average sand storage volume, it takes approximately 25 cycles, of average product mix castings, to complete a major change in the sand composition. The previously stated information has to be considered in order to focus on and categorize the tests. They can be separated into what I will call primary and secondary tests.

Primary Tests
A. Sieve Analysis and size frequency of washed molding sand before and after use. Sand is 90% of the mold.

B. Active Clay Content in percent, using the methylene blue method, when using bentonite with water as the major bond. Sometimes called effective clay, it produces the adhesive to bond the sand grains.

C. Moisture Content in percent taken from the prepared sand at the point of entering the mold. (Plasticizes clay)

C-1. Percent Compactibility - to determine the level of resistance to compaction based on the clay / water ratio and other factors to control the sand moldability. (Rel. wetness)

D. Percent of Combustibles - or also known as total loss on ignition. Relates to the total organics in the sand.

D-1. Percent of Volatile Organics @ 1200F - Relates to that portion of the organics that will produce a reducing atmosphere in the mold and prevent wetting by the metal.

The primary tests, listed above in order of importance, virtually affect most other properties and are the primary contributors to mold production, casting finish and integrity. Most metal casters rely on these tests for controlling the molding sand and make decisions based on their results. When the primary tests indicate a value above or below the set limits they have an effect on mold production, shakeout, casting cleaning and ultimately casting quality. These tests should be performed on representative samples and it should be recorded when and where they were taken and if necessary the temperature of the sand sampled. An accurate history of these tests will become necessary when addressing a casting problem.

Some foundries have automatic processing and testing equipment that can manage the variations and they are indeed fortunate. Also important in the controlling of the sand properties is the employment of cooling equipment by most.

To determine, predict and maintain the correct ranges for all the above test results, will demand the coordination of management to integrate the casting product mix with sand processing. It is necessary to operate the molding sand department with the same degree of technical discipline as would be used in the melting and metallurgical departments.

Secondary Tests
E. Compacted Density of a standard AFS test specimen - the actual weight in grams of the sand required to produce the specimen to be used in other tests determines mold density.
 
F. Permeability Number - measures the ability of the steam and gas from burning organics to escape from the mold and reduce back pressure. It affects the venting ability of the mold.
 
G. Dry Compression Strength - in psi it measures the strength developed by the clay/water ratio of a specimen dried to zero moisture.
 
H. Green Compression Strength - in psi it measures the load that the sand can carry and maintain the pattern dimensions when pouring the mold.
 
I. All Other Specialized Tests - to include tensile properties, splitting, tensile, green deformation, friability, mold hardness etc., which are used to supply information needed for a special molding method or casting design.
 
To even begin to obtain control of molding sand systems, it seems that logical steps in the proper sequence should be established. It is recommended that -

  1. Only one person should be responsible and accountable for a foundry’s molding sand system. The size of the foundry will determine whether that person will be able to handle other duties or need help in his primary responsibility for sand control. Provide a job description.
  2. This person, preferably a sand technician, must gain and maintain intimate knowledge of the total molding sand system, from storage through preparation, delivery to the mold station, shakeout and return to storage.
  3. The technician must be conscious of all sources of base sands and additives and their quality.
  4. The technician must question if new sand additions are being made to maintain the level of the system or to purge contaminants. New sand additions will require compensating additive additions. All storage hoppers should be kept full.
  5. The technician must give strict attention to the condition of the sand preparation and sand cooling equipment and all test equipment that is used to determine sand properties. Also, monitor the dust collecting equipment.
  6. Closely monitor sand results from different shifts and if different, determine the cause. He should be responsible for any composition changes.
  7. Graphing of all sand test results is mandatory in order to predict needed changes before major problems occur. Certification is necessary as personnel change and equipment must be standardized. They should be continuously trained in new technology

Control Basics for the Dimensional Reproducibility of Casting

Sand Related Defects That Can Occur

Conclusions
The important conclusions of this abbreviated article which can help get the most out of sand testing for the green sand molding process are as follows:

  1. The four primary tests (A through D) recommended can directly change the consistency and casting performance of the molding sand. They directly affect mold making and can contribute greatly to casting dimensional stability. They affect all other properties separately or in combination.
  2. Any changes that occur in any of the primary tests will affect most all of the secondary test properties. Therefore, exercising a major effort in controlling the primary test properties will contribute to the consistency of the secondary test results.
  3. In the event of equipment breakdowns, material inventory changes, or the introduction of cost reduction programs, these eventualities can be managed by knowing exactly what is happening in the way of physical properties of the prepared sand.
  4. The control of minor or major changes in casting production can be achieved by obtaining meaningful test results in a timely manner and then the changes needed in the sand can be made with confidence.
  5. The combination of the primary and or secondary tests, if the budget allows, are absolutely necessary to control the process of producing castings of consistent quality at the lowest cost with minimum casting losses.

Dialogue
An example would be - Should you want to make a stronger mold, you cannot add green compression strength only. If you want to improve mold rigidity, you must change one or more of the primary tests, such as - increase active clay bond, moisture or change grain size frequency. The same would hold true with permeability. Mold permeability can be the result of all the primary tests to some degree. A review of the time, frequency and technical labor required to perform daily and weekly testing, including the cost of the equipment, should be analyzed.
 
Consistency in the sand composition, sand preparation, maintenance of equipment, physical sand properties and constant personnel education are paramount in the production of high quality castings at the lowest cost.
 
Constant training sessions for sand control technicians will ensure that they are working with the latest technology and this in turn will promote company profitability and growth.
 
"Prioritizing Sand Testing" will ensure the achievement of these objectives.
 
References

  1. Ductile Iron Production Training Seminars - Part 4. "Raw Materials & Molding Sand Control" (Video by George DiSylvestro)
  2. Standard Test Procedures - Compendium of testing raw materials and molding & core sands. 1981 American Colloid Company.
  3. Molding and Core Testing Handbook, AFS
  4. Foundry Sand Practice - C. A. Sanders, American Colloid Company.
  5. Gold Medal Series of Videos by George DiSylvestro -
    "Critical Molding Factors"
    "Experiences in Defect Diagnosis"
    "Penetration"
    "Shrinkage"
    "Inclusions"
    "Gas Related Defects"
    "Pouring Technology"
    "Core Related Defects:
  6. AFS Casting Defect Handbook
  7.   High-Density Handbook 3rd Edition - Chapter 4 "Molding Sand" - AFS Publication
  8. "Does the Squeaking Wheel Control Your Sand System? - AFS Regional Presentation by Dave Boyd of Grede Foundries

DECISION MAKING GUIDELINES TO ASSIST IN IMPROVING CASTING QUALITY

TEST

WHEN COURSE

 WHEN FINE
A. SIEVE ANALYSIS AND SIZE FREQUENCY (Primary) Poor casting finish, lower clay/water required, sticking on patterns, mechanical penetration. Improved casting finish, higher clay/water required, blows, pinholes, boils, expansion defects, increased gas pressure and cope lifts.
 

WHEN LOW

 WHEN HIGH
B. PERCENT ACTIVE CLAY CONTENT BY METHYLENE BLUE TEST (Primary) Broken molds, cuts and washes, erosion, poor draws, lower moisture required, burned on sand and poor casting dimensions.   Narrow moisture range Improved casting dimension, poor shakeout, reduced expansion defects, clay balling and resistance to compaction required for larger heavy castings.
C. PERCENT MOISTURE CONTENT (Primary) Loose sand, cuts and washed, inclusions, drops, broken molds, friable mold edges, improved shakeout, good flowability and increased mold density. Apparent shrinkage, poor casting dimensional control, blows, sticking on pattern, penetration, boils, increased expansion defects, resistance to compaction.
C-1. PERCENT COMPACTIBILITY Friable mold edges, crushes, inclusions, hard to lift pockets, cuts and washes, cope drops, good flowability, increased mold density and improved casting dimensions. Mechanical penetration, apparent shrinkage, oversize castings, rough surfaces, difficult mold compaction, poor flowability and casting knots.
D.  PERCENT COMBUSTIBLES OR LOSS ON IGNITION OF ORGANICS (Primary) Poor casting peel, poor shakeout, lower moisture and clay required and oxidizing mold atmosphere. Higher clay and moisture required, low mold density, high smoke, lower hot strength, blows, erosion, lower expansion defects.
E. COMPACTED DENSITY IN GRAMS PER 2" SPECIMEN (Secondary) Penetration, poor casting dimensional tolerance, run outs, cuts, washed, apparent shrinkage and low mold hardness. Expansion defects, hard molds, penetration, high mold hardness, higher dry and hot strength and poor shakeout.
F. PERMEABILITY NUMBER (Secondary) Blows, pinholes, mold lifts, run outs, metal boils, improved finish, expansion defects, increased venting required for molds and cores. Mechanical penetration, rough surfaces, reduced gas pressure and faster allowable pouring rate.
G. DRY COMPRESSION STRENGTH IN PSI Good shake out, erosion and sand inclusions, indicator of friable sand and poor mold edges, best indicator of western bentonite level in composition. Direct correlation with baked sand lumps at shake out, hot strength can be controlled with composition using additives, good for making heavy castings.  Adds to mold rigidity during casting cycle.
H. GREEN COMPRESSION STRENGTH IN PSI (Secondary)

Poor draws, broken molds, erosion defects based on mold hardness, good sand flowability, lower dry strength, dry mold edges with high mold compaction.

 Good for automatic molding at lower water/clay ratio, higher mold compaction can be tolerated.  Contributes to more uniformity of mold hardness and improved casting dimensions.


 
SPLITTING, TENSILE FRIABILITY, MOLD HARDNESS, pH VALUE, GREEN TENSILE ETC.
Specialized tests can be developed, monitored and compared.   Controlling the primary tests will be the key to adjusting these tests to meet the physical characteristics desired to achieve excellence and maintain profitability.

Mold hardness consistently over 90-92 (B-scale) creates many undesirable results that nullify the primary test results from the laboratory.

Inconsistent hot sand surges of molding sand delivered at the molding station can negate most all control endeavors.
 

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A Gold Medal Series of video training programs are available. They include subjects enhancing the green sand molding process and controls. They include "Defect Diagnosis," covering all major defects and also programs which exhibit "Reduction of Casting Losses." Extensive casting production experience is available.
 
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(847) 825-5620 or fax (847) 825-2512.

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