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Testing procedures significant to casting quality.

Bond development during mulling can be controlled to ensure a given level of strength based on amperage drawn by the mixer. Green compressive strength tests are commonly used to test strength development of the prepared sand. While green compressive strength can be used to control uniformity of mix preparation, the relationship between green strength, moisture and mulling are key considerations.

In the past, green and splitting tensile tests were considered redundant or superfluous. Today, especially with high compaction molding, the value of tensile strength testing of molding sand is recognized. Dr. Boenisch, Univ of Aachen, west Germany, points out that tensile testing of molding sands may be a better approach for the control of high compaction molding sands than compressive strength testing.

Compressive strength tests, used successfully to control mix preparation uniformity, do not always correlate with mold or casting defects, Many mold failures, such as pattern stripping, are failures in tensile rather than compressive strength. Figure 10 shows examples of mold defects caused by low green tensile strength.

A decade ago, Dietert and Graham introduced the splitting tensile test (see Fig. 1 1) and an effective clay chart based on compression and splitting strength. This test, in which the specimen is broken along its diameter, provides a simple, reproducible tensile strength measurement of molding sand.

The effects of springback, or rebound cracking, in areas of high compaction become more evident and critical in high pressure molding. Areas of rupture in the plane perpendicular to the direction of the compacting force can occur in a high mold due to springback (Figs. 12a & b).

These areas of rupture are often the cause of reduced tensile strength, and result in severe mold and casting defects. Compressive strength and splitting tensile tests are not sensitive to the effects of springback. Compressive loads cause a sealing of the weak areas of springback rupture.

Splitting tensile loads are perpendicular to the plane of springback rupture (Fig. 12d), and, while this test provides a tensile value on the molding sand, it is not influenced by the effects of springback. As compaction pressure is increased, green compression and splitting tensile strengths increase until the curves f latten out (Fig. 13a).

One test sensitive to the springback effect is that for green tensile strength. Here, the plane of failure is parallel to the planes of springback rupture, and, because the stress is tensile, failure easily occurs through the ruptured areas (Fig. 12c.). As compaction is increased, green tensile strengths increase, peak and then drop off due to the increasing effects of springback (Fig. 13b). The lower the temper and clay level, the lower the compaction level that can be tolerated before the effect of springback is significant (see Fig. 14).

The green and splitting tensile tests provide the tensile strength of the molding sand with and without the effects of springback. The complimentary test information relates importantly to high pressure mold failure more than compressive strength. The green tensile strength test can be used to indicate the maximum compaction that a particular molding sand, at a particular temper and clay level, can tolerate before the effects of springback are significant. (This data relates to compaction and defect occurrence in high compaction mold areas).

Information relating to the compaction of high pressure molds in production can be obtained by testing the sand with increasing degrees of compaction (more rams, raising squeeze pressure). Boenisch notes that, due to increasing compaction, green compressive and green tensile strengths plotted in a "Strength Net Chart," shown in Fig. 15, produce a trend diagram that indicates a character of the sand not evident when based on compressive sand testing only

The curves in the "Strength Net Chart" represent possible examples of sands with different characterstics. Curve A, representing tensile strength increasing linearly with compressive strength as compaction is increased, is the ideal curve, but seldom attainable in production. Curves B through F represent realistic conditions with curve F, the poorest for high pressure molding, showing a strong decrease in tensile strength with increased compaction.

The green tensile/green compressive strength ratio during rising compaction provides important information about the character of the sand that green compressive strength testing alone does not provide. It is evident that green compression tests alone can actually provide misleading information in some cases because they are not sensitive to springback. This increasing compaction/strength ratio should be monitored, and green tensile or splitting tensile tests should be included in routine control. Laboratory vs. On-Line Mold Testing

Higher clay levels are required for handling high speed automated molding processes, but satisfactory molding results (high density/strength at high clay levels) require greater compaction energy due to reduced sand flowability

The compaction gradient created in the mold is primarily a function of the molding method, i.e., jolt-squeeze, high pressure squeeze, impact, etc, and is affected by variables such as pattern changes, changes in the performance of the molding machine and sand quality.

The purpose of laboratory sand tests in the foundry is to provide the foundryman with information to measure uniformity of mix preparation, monitor adjustments and indicate whether changes are occurring in the sand.

Compaction-In a production mold, compaction forces vary vertically and horizontally across the mold due to several influencing factors, i.e., pattern shape, flask wall friction and mechanical compacting force variances, as illustrated in Fig. 16. In any mold a complex compaction gradient exists. Test results vary depending on where in the mold the test is performed.

Time-On prepared production molds, it is difficult to control the time variable and sand properties. Moisture, friability, even strength characteristics can change with time. Even if the tests are taken at the same line point, the time elapsed since mold preparation and testing will vary with other factors, such as line downtime. Variations in the data are then due to the effects of time as well as sand quality and compaction.

Useful information can be gained through mold testing. Mold quality should certainly be monitored, and, if mold quality changes, it is necessary to determine whether it is the fault of changes in sand composition or some other production factor Laboratory data should determine sand changes. Correlation of Sand Test Data

Mold strength, hardness and permeability tests can be performed on the mold. Mold strength tests are more sensitive than mold hardness readings at high compaction, and are recommended for high pressure molds. Readings can be taken at various points in the mold, and the data can be recorded with reference to their location.

Reading variations taken in the same location on subsequent molds is an indicator of mold-to-mold compaction changes. Horizontal variations across a single mold and up and down the vertical walls provide a "picture" of the mold compaction gradient, which varies primarily with the molding process used as well as with pattern shape and complexity, flask wall friction, etc.

Mold test values can then be compared to values obtained on laboratory specimens prepared with varying numbers of rams or squeeze pressures. Properties in different areas of the mold relate to properties of the sand using the number of rams or squeeze pressure that most closely simulates the compaction in that area of the mold.

Effective compaction gages (see Fig. 17) can be used in the laboratory specimen tube, and also can be placed on the pattern to measure the effective compaction pressure. These gages consist of a pressure plate and a steel ball which is imbedded into a piece of lead with a known hardness. The diameter of the impression is measured and related to effective compaction.

Running tests at compaction energies, similar to the actual mold compaction, is a more complicated approach, but it provides data that correlates better with the occurrence of casting defects in certain areas of the mold. The compaction and sand properties in the areas of the highest compaction, like high spots in the pattern, can be very different from the sand properties in places of lowest compaction, such as near the flask wall or in shadow areas.

Areas of high compaction are critical because of their low tensile strength due to the negative effects of mold springback and to the greater expansion tendency of the sand in areas of increased compacted density. Areas of low compaction tend to be soft and have voids that produce penetration defects.

The wet tensile strength test, used to provide bond formulation data, can also be related to sand scabbing. In it, the surface of a compacted specimen is heated to drive back the moisture in the sand, creating a critical over-wet layer in the specimen simulating molten metal contact effects in the mold. Figure 18 illustrates the principle of wet strength testing. To relate the wet tensile test to sand scabbing, the control of bond formation and the compaction of the mold should be constant. The test should be performed using laboratory specimens, the compaction characteristics of which match those of the highest compaction areas of the mold. It is in these areas that the scabbing tendency of the sand is greatest as springback and sand expansion increase. Calibration and Maintenance Certainly, if sand control is to be effective, the sand engineer must have reliable data. Training of technicians is necessary to ensure that tests are performed regularly and that standard procedures for the various tests are followed to prevent differences in operator techniques.

Another important part of sand control involves calibrating and maintaining testing equipment which is operated under harsh foundry conditions. Conclusions Variation in green sand can be attributed to many factors, some of which are in the basic engineering of the sand system. A good sand system should include the following:

* maximum possible sand capacity and sand-to-metal ratio;

* antisegregation equipment installed in the raw sand handling systems that serve the green sand mixers, core room and reclamation output;

* adequate mixing capacity;

* use of computers to anticipate additive requirements with computer file of pattern data;

* use of computers to gather real time data on amperage drawn and sand temperature at various points in the system;

* use of computers to correlate sand test data with molding problems, scrap and defect data as an aid for establishing optimal sand properties;

* mixers equipped with compactability controllers and amp meters;

* ability to weigh sand and additives into mixer;

* ability to remove core sand from castings, reclaim and meter it back into molding sand;

* lumpbreakers installed after shakeout;

* magnetic separators positioned after lumpbreakers;

* ability to dilute shakeout sand with spill and excess prepared sand;

* ability to blend sand in a homogenizer;

sand cooling equipment that does not create balls.

These features, covering some of the most important aspects necessary in the design of an ideal sand system, are lacking in most current sand systems. Future sand system design should incorporate these features to reduce sand variables to more controllable levels, or eliminate them.

In the meantime, identifying and removing the sources of variations in conventional sand systems should be pursued using sand tests. These tests, performed at strategic points along the system, are of fundamental importance to measure the magnitude of these variations to minimize and control them.
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Author:Pedicini, Louis J.
Publication:Modern Casting
Date:Apr 1, 1990
Previous Article:A systematic approach to cast iron defect analysis.
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