Optimizing green sand compaction.

Inside This Story:

* Adequate green sand compaction is essential to produce quality castings and reduce energy and mold cycle times.

* Most high-volume green sand casting facilities utilize automation, but could improve their practices.

* Detailed are several methods to optimize the compaction of green sand molds.

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For green sand molding, having the correct molding parameters is essential, as they will affect the desired casting quality, energy consumption and cycle time. Insufficient compaction, can result in rough casting surfaces and even breakage. Too much compaction requires more energy, can cause casting defects due to low gas permeability and causes more wear on the pattern and equipment.

Today, most green sand molds are produced using automatic molding equipment. Selecting the process parameters for the equipment often is based on the experience of the operator and trial-and-error experiments. In most cases, this leads to the production of quality castings. But material and energy costs for moldmaking and the equipment's productivity may be negatively affected by these trial-and-error approaches because the complexity of multistage compaction is not well known throughout the industry. Also, because moldmaking consumes less energy than melting operations, engineering practices often neglect this source of cost. With many of the compaction patents expiring, there are numerous machines on the market that can make molds in a variety of ways. Many options for molding sequences exist, but the industry lacks a full understanding of the effects and results of compaction.

Therefore, investigations were conducted to observe the compaction process and optimize several green sand moldmaking methods. As more casting firms encounter global competition, small savings per mold, a small improvement in the utilization of the molding equipment or the reduction of mold-related defects could result in significantly improved competitiveness.

Trial Testing

To optimize the mold compaction process, the change in density of the molding material must be recorded during moldmaking. Therefore, investigators developed two special transducer sensors and integrated them into a pattern plate to record the force of moving sand during compaction. The sensors were:

* SP-P, which measured 0.86 in. (22 mm) in diameter and 0.9 in. (23 mm) tall;

* SP-I, which measured 0.86 in. (22 mm) in diameter and 1.06 in. (27 mm) tall.

Trial experiments first were conducted using standard green sand specimens from production sand with a compactability of 38.5%. Here, the compaction pressure was increased from 0.25 to 2 MPa in increments of 0.25 MPa.

When the sensor reading was compared to mold hardness, it was revealed that the new measurement method had better sensitivity than the commonly used mold hardness measurement (Fig. 1). The new method, however, is not as sensitive as the mold strength measurement (Fig. 2) because mold strength is related to the density of the mold and is affected by the clay binder in the molding material.

It was determined that the new measurement method only can be used to conclude the mold density if the compactability remained constant. Changes in the compactability will change the initial density. Due to a different bulk density and more friction, a mold material with higher compactability will start to resist compaction earlier. As shown in Fig. 3, the curves of the other mold properties also will shift but remain parallel. That means an increasing density can be measured above a certain stress level within the molding material.

Airflow and Squeeze Molding

The investigators sought to apply the preliminary data to various production green sand molding practices.

An airflow/squeeze molding machine was examined first. A measurement of the green sand density was taken at a location that is difficult to mold, and then the mold was compacted via airflow with the valve opened for 0.6 sec. Subsequently, the mold was compacted by squeezing at a pressure of 1 MPa, which allowed for quality molds.

It was found that most of the compaction resulted from squeezing and not from airflow. The short period of compaction during airflow meant that the molding material immediately was compacted to a high degree. This resulted in an equal relation between the forces acting on the sand grains and the resistance of the molding material against compaction even before the pressure in the expansion chamber reached its maximum.

It can be seen in Fig. 4 that after a certain point in time prolonged airflow cannot result in increased mold density on the pattern surface, and the pressure difference and the force acting on the molding sand cannot be increased further. Only an increase in squeeze pressure can increase the mold density. This was confirmed with another experiment. Here, the density on the surface of the pattern plate was measured for different periods of airflow and different squeeze pressures (Table 1).

The bulk density was found to be somewhat higher because the sand already was compacted slightly when it was dropped onto the pattern surface. However, airflow did compact the molding material to the desired density of 1,250 kg/[m.sup.3]. Longer periods with airflow did not result in a measurable impact on the density, and mold density only increased with higher squeeze pressures (Fig. 5).

It then was determined that for better optimization:

* squeeze pressure must be considered as the key player during mold compaction;

* the time interval between airflow and squeezing can be decreased. This will decrease the cycle time and increase production output;

* compressed air can be saved if the airflow time is set to 0.3 sec.

Blow and Squeeze Molding

Investigations then examined the relationship between squeeze pressure and density for a flaskless, vertically parted system and blow and squeeze vertically parted molding equipment. This is because molds tend to be compacted as much as possible as lower compaction values require a higher degree of consistency of the molding material. For this experiment, the goal was to reduce energy and wear by using less pressure. Here, the sensors were mounted to the pattern plate on the squeezing side, and measurements were taken at the default squeeze pressure and at 50% of the default squeeze pressure (Fig. 6). It can be seen that reducing squeeze pressure by 50% resulted in only a 19% density reduction.

No mechanical mold defects were observed, but this doesn't mean that the squeeze pressure can be reduced for every molding system. Mold breakage still can occur during transportation or pouring. A lower pressure, however, will reduce energy consumption and tool wear.

Impact Molding

As optimization methods were found during the compaction period, the investigators performed a third experiment to examine impact molding equipment. This molding method often causes scrap due to inadequate pattern removal from the mold, leading to mold breakage. Often a combination of forces results in broken molds.

With the impact process, the forces resulting from the motion of the sand during compaction were recorded at two symmetrically positioned locations on the pattern plate. Sensor 1 was installed at the location where the majority of mold breakage occurs. It was found that the actual compaction finished rapidly, but the compaction at Sensor 1 (problem area) was less than at Sensor 2. Possible causes of this were uneven filling or pre-compaction due to striking off the excess molding sand above the filling frame.

Also, it was found that the valve between the storage chamber and expansion chamber could be closed as soon as the pressure on both sides is equalized. Depressurization started 0.4 sec. after closing the valve, and 0.2 sec. elapsed between the end of depressurization and withdrawing the pattern. This time could be used to increase the output of the molding equipment.

Further, as soon as the hydraulic system used to attach the compaction unit to the molding envelope was turned off, a small gap formed between the pattern plate and mold. Another small movement occurred 0.4 sec. after relieving the pressure (Fig. 7). Immediately at the beginning of mold withdrawal, another force at the sensor was noticed. It was determined then that mold breakage was caused by a combination of the pattern tilt, insufficient mold density and the velocity of mold withdrawal. Thus, every factor must be controlled depending on the effect of the other factors. General remedies to prevent future mold breakage are:

* more even compaction;

* smoother motion during pattern withdrawal from the mold;

* and a slower withdrawal speed.

Therefore, consistent molds can be ensured without unnecessary reproduction of further molds.

This article was adapted from a paper (05-008) presented at CastExpo '05, St. Louis.

For More Information

Visit www.moderncasting.com to view the paper "Optimizing Molding Parameters for Green Sand Compaction" (05-008), in its entirety.

RELATED ARTICLE: Calibrating the sensors.

To accurately measure the different mold properties, the test sensors were calibrated and a database was established to draw conclusions from the sensor readings on the mold properties. Standard test samples of the molding material were produced and molding sand properties were recorded along with the sensor measurements.

Several experiments then were conducted to determine the validity of the sensor data. Measurements were taken at compactability values of 50% and 30% in two different locations of a pattern plate (Fig. A). These locations had different ratios of molding difficulty of 3:1 (very difficult) and 1:1 (moderately difficult).

To double-check the values obtained by the sensors, mold strength also was measured manually at the same locations using a mold sand tester. It was found that the data measured using both techniques showed a strong agreement. The conclusion was drawn that the calibration values obtained via this method could be utilized in a production environment. By keeping track of these values using a database, accurate mold measurements for every mold can be obtained.

Juergen Bast and Andrej Malaschkin are professors in the mechanical engineering division at Freiberg Univ. of Mining and Technology, Saxony, Germany. Thomas Hahn is an Assistant Professor at Pittsburg State Univ., Pittsburg, Kan.

Table 1. Trial Runs to Measure Mold Density with Different
Airflow Times and Squeeze Pressures.

Mold #    Time Period for   Squeeze Pressure
          Airflow (sec)      (N/[cm.sup.2])

1-5            0.3                 70
6-10           0.6                 70
11-15          1.2                 70
16-20          0.3                 85
21-25          0.6                 85
26-30          1.2                 85
31-35          0.3                100
36-40          0.6                100
41-45          1.2                100

Fig. A. The data from
the calibrated sensors
were compared to
conventional mold
testing methods. It was
concluded that the
new testing methods
could be used in high-volume
production.

                                  Compactability

                          50%      30%      50%      30%

                       Difficult Region     Easy Region

Conventional Method        2      3.5      13.1     13.6
New Method               1.9      3.6      13.1     13.5

Note: Table made from bar graph.