Does your core sand measure up? This article describes the tests necessary to evaluate sand for coremaking, ensuring foundries realize a cost savings due to reduced resin consumption and improved casting finish.
While cost should play a part in deciding which sand is used in coremaking, performance should be more important, especially if a cost savings can be realized due to reduced resin consumption, less sand needed to produce a core, and improved casting finish.
This article will outline tests used to evaluate various sands for coremaking. These tests are used to find the best performing sand within the foundry's environmental conditions and current resin system. This article focuses on testing for a phenolic urethane coldbox resin system, but the theory applies across the board.
The analysis is grouped into three phases. Phase I tests are performed on raw sand to provide initial insight into the sand system itself. Phase II tests use dog bones or cores to analyze prepared sand characteristics. These tests are used to compare the current sand system to the competing sands. If the new sands can surpass the current sand system or be as good but have some other benefit (such as improved environmental benefits or reclaimability) they will advance to phase III.
Phase III utilizes 2 in. by 2 in. cores and pouring test castings to evaluate the casting quality of the core. If the core produces veins, the surface area is measured along with surface quality of the casting. All sand systems are tested together to ensure they are analyzed under the same conditions.
The following tests are used to screen raw sands before a core is produced.
Sieve Analysis--Sieve analysis is used to test for the grain finess number (GFN). Before analyzing for distribution, the sand must be mixed to be homogenous. Sand tends to become segregated when being transferred into a container. The larger grains tend to roll while the fine grains interlock, causing segregation.
Before the sand is analyzed, a sand splitter is utilized to ensure the sand is thoroughly mixed before a sample is taken. Sand should be split three times to ensure the sand is homogenous. After the sand has been split, a known amount of sand is taken and used for sieve analysis. A sieve shaker is used to sift the sand by rotating and tapping the top of the lid, causing grains of sand smaller than the sieve size to fall through to the next screen.
After the sand is sifted, the screens are cleaned and the retained sand is weighted. The percent of sand and the multiplier for each sieve size gives a quotient. The quotients are added together to derive a GFN.
A three- or four-screen distribution for making cores is preferred because it will make a denser core than a two-screen sand. A three- or four-screen distribution should help in reducing resin due to more sand grain to sand grain contact when making a core.
Sand pH/Adv--This test is used to find the amount of time the sand will be usable. In a chemical binder/resin system, place 25 g of sand into a glass beaker with a magnetic stir bar along with 75 ml of deionized water. Mix the solutions for five minutes using a magnetic stir plate. After five minutes, place the pH probe into the mixture and wait for the pH to stabilize, The pH measures the water-soluble alkaline material in the sand. The sand pH is extremely critical for bench life in a coldbox system. The more basic the sand, the less bench life the prepared sand will exhibit.
To measure Adv, which measures the alkalinity of the sand that is not water-soluble but acid soluble, place 50 g of sand into a glass beaker containing 50 ml of deionized water. While mixing add 50 ml of 0.1N hydrogen chloride (HGI) and continue to mix for five minutes.
After five minutes, place the pH probe into the mixture and begin back titrating with 0.1N sodium hydroxide (NaOH). Back titrate until the pH probe reads 7.0 and stabilizes, Adv is determined by subtracting the ml of NaOH used from the 50 ml of HCI. The lower the Adv the more acidic the sand. Adv is as critical as the pH for the amount of time the sand is workable to make a core in a coldbox system.
The pH/Adv of sands are critical if a problem arises during production. The more basic the sand, the less time the sand will be usable because the high pH causes the resin to react sooner than normal. The sand will set up in the hoppers before the sand can be used to make a core.
Dry Sand Density--To measure density, use a container with a known volume, fill the sand to the top of the container and strike off excess sand. Weigh sand in the container and divide by the volume of the container.
Understanding the density of the sand will help when comparing the incumbent sand to the new sands. If the densities of the sands are different, a direct comparison of resin percentage cannot be done. The resin amount will have to be calculated by using volume. This will be important when it is time to evaluate tensile strengths of various sands.
Density also plays a part in sand cost. More dense sand will cause a foundry to use more sand, which could raise the cost if blow pressure remains the same.
Turbidity--This test compares the level of fines in a sand. This is an unconventional test using deionized water, 10 ml graduated cylinder and a refractometer.
Add sand to the 5-ml level of the cylinder and add deionized water to the 10-ml level. Cap the cylinder and shake vigorously for 20 sec. Allow the sand to settle for 10 sec, then remove the top layer of the solution after this period and measure using the refractometer.
This test is strictly a comparison test; it will help in comparing fines with the different sand systems. The more fines the cloudier the water. The refractometer reading will then be higher. This test will assist in identifying which system has more fines and will help when attaining the correct resin level for the immediate tensile strength in phase II.
The following tests are performed after sands have been screened in phase I tests.
Several tests are used for how well the prepared sands will compare to the current system. When doing these tests, the sand should be prepared at the high side of the sand temperature specifications, or used at the warmest ambient temperatures if sand heaters aren't used. The sand should be heated above the desired temperature since during the mixing, the sand temperature will be lost.
Tensile Evaluation--Immediate tensile strength needs to be established first before any other testing is performed. Immediate tensile is defined as 60 sec after the core has been stripped from the corebox.
The same front-end tensile value should be used for the incumbent sand and as well as the new sand for the evaluation to be valid. Typically, the immediate tensile strengths must be within 6% due to the variation that is seen in tensile strength with coremaking machines. Several attempts may be needed to get the correct level.
To achieve the same immediate tensile strength, the GFN, grain shape and screen distribution must be taken into account. These properties will affect how much resin is required to produce a sufficient immediate core tensile strength.
Dip Redry Tensile Strength--To measure dip redry tensile strength, a set of cores are made with the correct immediate tensile strength. The cores then are coated with a core wash, submersed for a count of three and placed onto the conveyor to be dried in the production oven.
After the cores have dried, they are allowed to cool to the ambient temperature. The cores are then broken to record their tensile strength. The tensile strength of the cores should increase from their respective immediate tensile strength. This is due to the heat causing the solvent
to evolve from the resin.
Core Density--After the resin level has been established to accommodate for the tensile strength, core density must be identified at the current blow pressure (60 psi) used for testing and a lower blow pressure (30 psi) to identify if the pressure can be lowered and still produce a quality core. This will measure the flow ability of the sand with the resin for making a core.
The more rounded the grain of sand is the easier the prepared sand will flow. A rounded grain of sand will allow for less resin to be used as compared to a sub-angular grained sand due to less surface area and more compactibility.
If a new sand system can achieve the same density at a lower blow pressure, this will reduce the cost of the sand since less sand will be used to make the core.
Cure Speed--Cure speed is used to measure how fast a core will be able to cure at the current resin level and core density. To measure cure speed, lower the purge time when making the cores so that the core has not completely cured. Weigh the solidified core and divide by the gas and purge time to calculate the cure speed.
When comparing cure speeds of sands, the density of the sand must be taken into account. A fast cure rate with a high sand density may not be as quick to cure a core as a sand with a low density and a cure rate that is slower. This is important because cure speed will effect the production cycle times in the coreroom.
Bench Life--This test is used to provide information on how long the sand will live or make a core with enough tensile strength to strip from the corebox. A humidity chamber maintaining a temperature around 100F and as much humidity as possible will show conditions seen during summer, which is the season hardest on core production.
Prepared sand is placed in a humidity chamber and sand is removed every 15 mm, up to 1 hr or longer if desired to evaluate tensile strength. The tensile strength of the core is evaluated 60 sec after being stripped from the corebox. Compare the tensile strengths to their respective immediate tensile to evaluate the reduced tensile strength and how long the sand will be useful for core production. Tensile strengths within 6% are not considered a significant difference due to variability of producing dog bones.
Hot Strength--For evaluation of hot strength, a 900F muffle furnace or warmer is needed. Place the cores in the furnace for 10 min, remove the cores from the oven and immediately evaluate the tensile strength. This test will give some insight about the strength of the core when metal is introduced.
Compare tensile strength with the current sand system. If the hot strength is significantly higher, there could be problems during shakeout. The core may not degrade causing cleaning issues or possibly crack defects in the casting. Conversely, if the hot strength of the sand system is considerably lower than the current system, the core may degrade too fast and not produce a dimensionally correct casting.
Permeability--This test is used to help measure the density of the core. High permeability means the core has voids for the air to pass through producing little or no resistance. A core with a high permeability will gain strength faster than a low permeable core because the solvents can evolve more rapidly due to the voids in the core.
Permeability also will affect the cure speed of a core. The denser core will take longer to cure than one with high permeability. This occurs because the catalyst can travel through the core faster if the core is more porous. But the downside to a core with high permeability is that the higher the permeability, the more problems with metal penetration. To compensate for high permeability, a different core wash may be needed, which may raise the cost of the core wash.
Delayed Tensile Strength--Cores are made and evaluated after 1 hr, 4 hr and 24 hr. As the cores are exposed to the environment, they will gain tensile strength due to solvents evolving from the core. At some point, however, their tensile strength will plateau. How fast the core reaches the plateau for tensile strength depends on the permeability and resin level. The more permeable the core the faster the core will plateau because the solvent can escape more rapidly than a core with low permeability.
Humidity Chamber--This test is used to evaluate how the core reacts to high humidity and elevated temperature over a period of time with respect to tensile strength.
A humidity chamber can he made using a steel drum, a metal bucket and heat tape. The heat tape is used to heat the water in the metal bucket to raise the humidity. A thermo-hygrometer is placed in the drum to measure the environment. It is important to maintain a constant 100F temperature and relative humidity of 85% inside the chamber. The warmer the atmosphere the more moisture is absorbed.
The humidity chamber core's tensile strength should always be compared to the tensile strength of cores in the laboratory environment's delayed tensile strength.
This humidity chamber test is useful, since cores may be set in storage as little as a few hours or as long as two weeks before being used. So the environment in which the cores are exposed is very important to the tensile strength and ultimately the quality of the casting being produced.
A test casting is made to measure the veins, surface area and surface finish for each core that is evaluated. The new sands will be compared to the current material. The current material will be used as a standard and placed in every test casting for evaluation. When making a test casting, the current sand should have minimal veining so it will be easier to evaluate.
When the casting has cooled, the sand is carefully removed from the test casting, trying not to disturb any veins during this process. The height and length of each vein is measured in the void where the core was placed. To rate the surface finish, a wire brush is used to clean the outside wall of the cavity. The surface finish is then rated using a casting finish comparator standard.
All of the tests outlined in this article are used for evaluating new core sand systems in a foundry. These tests are relatively easy to perform, but are invaluable in showing the strengths and weaknesses of sand systems. After performing these tests, if a new sand system shows a benefit over the current system, production cores and castings should he made and evaluated.
For More information
"Examining Key Variables of a Coldbox Coremaking Operation, "A FS Cured Sand Committee (4-I), J. Cavanaugh and D. Gilson, MODERN CASTING, June 2000, p.32-34.
"Optimizing Your Coldbox Core Process!" M. Adamovits and K. Horton, MODERN CASTING, March 1998, p.43-47.
About the Author:
Josh Werling is manufacturing engineer at Indianapolis Casting Corp., Indianapolis. His responsibilities include evaluating new raw materials as they relate to the coremaking process and providing technical support.
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|Comment:||Does your core sand measure up? This article describes the tests necessary to evaluate sand for coremaking, ensuring foundries realize a cost savings due to reduced resin consumption and improved casting finish.|
|Date:||Feb 1, 2003|
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