Controlling green sand compactability.
One key to consistent sand and castings is ensuring that the green sand for molding is in the proper compactability range throughout the molding process. Dry sand will increase the chances of mold breakage or crushing and inclusion defects. Wet sand will result in soft molds, castings with rough surfaces, and sand build-up in flasks and other equipment. Sand compactability can vary for many reasons, causing frustration for foundries trying to control it.
The AFS Molding Methods and Materials Div. Basic Concepts Committee (4-E) undertook a study involving five foundries (iron and aluminum) to understand the change in sand compactability after it is discharged from the muller. A foundry that was experiencing variations in compactability at its molding station, even as the sand was discharged from the muller with consistent compactability, prompted this project.
The study showed that even as the moisture and temperature of the sand discharged from the muller stayed constant, compactability decreased with time while green compression strength remained essentially the same. Following is a discussion of techniques for consistent compactability and a summary of the five-foundry study.
Opportunities for Compactability Variation
Sand compactability levels can vary at several different points in the molding process. Although this article is focusing on sand compactability after it leaves the muller, a brief look at compactability issues before this step in the process is important. Initially, a problem exists in controlling the correct compactability level of the sand in the muller due to changes in the properties of return sand coming back to the muller. Key factors that vary in the return sand are:
* clay content;
* fines (other water absorbing materials);
* the time interval between shakeout and muller.
These variations result from changes in the operation such as sand-to-metal ratio, core sand input, spilled sand, unpoured molds, dust collection and breakdowns. A lower sand-to-metal ratio creates hot and dry sand after pouring, which will require more clay and moisture. An increased amount of core sand reduces the clay and moisture in the return sand mixture, so more must be added.
Both spilled sand and sand from unpoured molds will have more clay and moisture. Breakdowns mean that castings sit in the mold longer, absorbing more heat into the sand and burning out more clay. These variations require varying amounts of moisture and clay to satisfy the compactability set point.
Time also is an important factor. Variations in time elapsed between shakeout and cooling and sand delivery into the muller affects day activation and water absorption. A general rule of thumb is to let the sand sit for 2.5 hr to allow it to relax and absorb moisture (Fig. 1).
Equipment used today is able to control the compactability at the muller by measuring the incoming sand temperature and moisture (conductivity) and calculating the water needed to meet the compactability set point. There may be differences in the calculated water addition and thereby compactability due to the non-homogeneous nature of the sand mixture.
Even after allowing for all these variations, if we are able to control the compactability of the discharged sand within control limits, compactability at the molding machines may still not be within the desired limits. Many foundries make allowances for the drop in compactability by controlling to a higher level at the muller. The actual amount of the drop varies with different sand systems and different seasons of the year.
Monitoring the compactability at the molding machines allows set points to be changed at the muller. This works well if there are no appreciable differences in the operation of the system, such as startups, breakdowns and changes in the job mix. Since anticipating these variations can be difficult, it is necessary to understand what causes them and how to fix them in order to achieve optimum compactability.
Muller vs. Molding Machine: Compactability Variation
Although sand rarely travels far between the muller and molding machine, changes in compactability can occur even in a short time. Variations in compactability between the muller and molding machines primarily are due to:
* variation of the time interval for sand to reach the molding machine--due to line breakdowns or changes in the molding rates;
* temperature fluctuations in the sand--temperature at discharge from the muller should be controlled below 115F;
* ambient temperature and humidity fluctuations--there is a change in the compactability loss from summer to winter;
* variations of sand level in the return sand silo (resident time in the silo)--in a full silo, the sand sits longer, and in an emptier silo, the sand goes in and out too quickly. At least 2.5 hours of resident time in the silos is recommended for consistent operation;
* job changes (sand-to-metal ratio, core sand input)--if the sand system supplies to a single molding line these variations exacerbate control problems;
* variations in the amount of binder added in the muller--increased material additions increase compactability loss. If additions are changed drastically, one can expect increased variations at the molding machine;
* variations in spill sand returned to the system--abrupt variations of spill sand to the shakeout sand result in increased variation of compactability at the molding line.
Variations still can occur between the muller discharge and the molding machines even if the water addition is controlled. Muller cycle times are too short to allow the water added at the muller to be absorbed properly into the clay lattice.
This phenomenon, a function of time and the differences in the moisture levels of the incoming sand, explains the variations in compactability when there is no apparent loss of moisture. Because the water added is on the surface of the particles, the compactability set point would be satisfied with less water than what is really needed if there had been enough time for proper absorption. Tempering bins after the muller provide some time for the dry clay particles to absorb moisture, thus minimizing variations seen at the molding machines.
Compactability Theory in Action
Consider the following example:
It could be assumed that two sands of identical composition having widely differing moisture levels (in this case 0.5% and 1.8%) could be reconditioned to the same degree of compactability by bringing both sands to the same final moisture level. The difference in that amount of water added would seem to be 1.3%, (1.8-0.5). This is not the case in practice because the water added remains on the surface of the clay particles rather than being absorbed in the short time during the mulling cycle.
The sand with 0.5% original moisture reaches the desired compactability level at a lower final moisture than the 1.8% moisture sand.
In other words, the difference in the water-absorbing capacity varies based on initial moisture. Dry sand has greater capacity to absorb water as time goes by compared to moist return sand. This variation shows up as a difference in the compactability at the molding machine even though it had the same compactability when it left the muller. Allowing the sand to sit longer in tempering bins minimizes this problem.
Selecting a Set Point
Temperature--Many foundries adjust the set point for compactability depending on the temperature of incoming sand. For example, the set point maybe moved up by one point for every 10F increase in temperature, compensating for the increased drying of sand or evaporation of moisture that results from the increasing temperature.
The set point scheme may be altered with changing ambient conditions such as hot, humid weather or cool, dry weather. Adjusting the set point for the incoming temperature tends to minimize compactabiity variations measured at the molding machines.
Moisture--Along with temperature, the incoming moisture and the condition of the moisture play a key role in the loss of compactability from the muller to the molding machine. The controller can adjust the water addition based on incoming moisture and temperature to meet the compactability target, but it may not be enough to compensate for the loss of compactability encountered by drier sand. Controllers have the capability to adjust the compactability set point not only based on the incoming temperature but also based on the incoming moisture of the return sand, since both measurements are readily available.
The set point can be moved up with decreasing conductivity for the same temperature (move the set point from 50 to 52, for instance, when the conductivity drops from 0.9 to 0.6).
Sand from five foundries was tested for compactability loss up to 180 rain. after discharge from the muller. The data from the study is shown in Table 1. The data was analyzed using R.W. Heine's clay water strength reference graphs. Except for foundry 2, sodium bentonite with seacoal reference graphs are used to calculate the indices. For foundry 2, sodium and calcium bentonite with a seacoal reference graph is used. Significant indices (equilibrium moisture, equilibrium compactability and clay-water ratio) were calculated using the graphs mentioned above.
From this testing, it was observed that some trends follow the increased loss of compactability (Table 1):
* when equilibrium compactability (EC) is determined at the intersection of test moisture (TM) and methylene blue clay values, EC is much lower than the compactability of sand right after the discharge from the muller. This illustrates that compactability of mulled sand will move towards equilibrium value in time, even when there is no moisture loss. When there is a large difference between the test compactability and equilibrium compactability, loss of compactability with time is also larger.
The EC condition is achieved when the clay has absorbed all the water present and is capable of developing maximum green strength at that moisture content. Other materials existing in a sand system also absorb and hold water, so it is impossible to determine the equilibrium values precisely because water may be partitioned between clay and nonclay materials to different degrees;
* the difference between equilibrium moisture (EM) and test moisture (EM - TM) increases with increasing compactability loss (Fig. 2). EM is determined for a given clay and compactability using the reference graphs;
* a higher clay-to-water ratio indicates a higher compactabiity loss;
* equilibrium values of moisture and compactability depend on the state of mulling of the clay and water in the system. System components and operating details can affect the state of mulling.
As the cycle time for finning sand through the muller is decreased, compactability loss increases. In high production foundries, sandis recirculated without allowing adequate time for the day to relax and absorb water. Water absorption can occur only when the sand is cooled down and is in un-rammed condition.
When sand is in compacted molds, the clay has little freedom to absorb water and swell. Using a large return sand silo ensures that sand rests long enough for the clay to absorb water.
System design and operating parameters determine the nature of sand. EM and EC depend on these characteristics and can be used to monitor the sand system among other indices.
Minimal compactabiity loss does not necessarily indicate that the sand is without problems. Fully mulled sand may not lose much compactability over time, but it may cause other defects (for example, expansion scabs are caused by a lack of material to absorb migrating water as the liquid metal evaporates water in the surface layer of sand).
Adequate size of return sand silos for the amount of new material and heat input are critical for consistent operation of a sand system. Pay close attention to sand storage devices to avoid excessive buildup and see that they are utilized fully. Even if the silos are large enough, if the usable capacity is reduced drastically through build up on sides, quality problems will result. To minimize build up, several design options may be available, including humidity control in the storage silos.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Table 1 Loss of Compactability with Time in Different Sand Systems Properties Foundry 1 Foundry 2 Foundry 3 % Change in Compactability -6.1 -10.9 -22.2 % Methylene Blue Clay (MB) 6 11 8.6 % Test Moisture (TM) 2.6 4.3 3.3 % Test Compactability (TC initial) 40 38 45 Test Green Compression Strength (psi) 30.5 22.8 32 % EC (% equilibrium compactability for the % MB clay and % moisture) 30 22 <20 % EM (% equilibrium moisture for % MB and % TC) 0.2 0.5 0.9 Clay Water Ratio (R)=%MB/%TM 2.31 2.55 2.6 Properties Foundry 4 Foundry 5 % Change in Compactability -4.8 -10.3 % Methylene Blue Clay (MB) 7.8 6.2 % Test Moisture (TM) 3.7 2.7 % Test Compactability (TC initial) 40 54 Test Green Compression Strength (psi) 25 25.6 % EC (% equilibrium compactability for the % MB clay and % moisture) 40 32 % EM (% equilibrium moisture for % MB and % TC) 0 0.6 Clay Water Ratio (R)=%MB/%TM 2.11 2.3
This paper is based on a panel presentation (00-139) at the 2000 AFS Casting Congress.
For More Information
"Compactability, Green Strength, and Moisture as Related to Green Sand Processing Efficiency," R. W. Heine and R. A. Green, 1989 AFS Transaction (89-075), AFS, Des Plaines, IL.
"Evaluation of 8-15% Bentonite Content Green Sand Properties and Behavior: Part IV Summary" T. S. Shih, R. A. Green, and R. W. Heine, 1987 AFS Transactions (87-102), AFS, Des Plaines, IL.
"Compactabitity -- A 2nd Look," 2000 AFS Casting Congress Panel Presentation (00-139), AFS, Des Plaines, IL (2000).
RELATED ARTICLE: Factors that Increase Sand Compactability Loss
The following factors can be detrimental to sand systems, as they contribute to sand compactability loss. If you observe any of these occurrences, institute changes in the system to reach acceptable operating control.
* high clay-to-water ratio;
* increased material addition;
* less storage of system sand;
* low sand-to-metal ratio;
* drier return sand;
* high sand temperature at muller discharge;
* increased number of transfer points;
* aeration of sand.
About the Author
Al Alagarsamy is the corporate director of casting technology at Citation Corp. The AFS Molding & Materials Div. Basic Concepts Committee (4-E) handles specific industry problems related to casting production and the use of mold and core materials.
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|Article Type:||Statistical Data Included|
|Date:||Sep 1, 2002|
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