Figure 1: Sketch of a compactability sample before and after a force is applied. BUT WHAT EXACTLY IS COMPACTABILITY? Compactability measures the percentage by which a loosely packed sample compresses under applied force, representing the decrease in height of a specific sand volume, as shown in figure 1. Typically, green sand compactibilities range from 35-50%. The test is highly responsive to changes in moisture. The results aid in monitoring moisture levels and guides water additions during the mulling process. As the force is applied to the top of the sample, the sample compacts; the higher the compactability, the more the sample was able to compact in height. The compactability is critical; if it is too low, a result could be friable edges, difficulty drawing pockets, penetration, and crush defects. If the compactability is too high, it can cause poor surface finish, expansion, gas, shrink, pinholes, and blow defects as well as mold-wallmovement which can lead to swell and oversized castings. For many years the 3-ram compactability test was used throughout the foundry. A 3-ram unit could be found at the muller deck and in the sand lab. The test was used to help determine the water addition to the muller. The development of the automatic compactability controller (molding unit) and digital pneumatic sand squeezer (laboratory) has replaced many traditional 3-ram units. The units are shown in figure 2. TRADITIONAL 3-RAM TEST The 3-ram test, dating back to the 1920s, was a significant improvement over the hand-feel method. A sample of green sand is riddled through an AFS standard ¼” screen and funnel assemble into the specimen tube and struck off evenly at the top of the cylinder. The plunger head is raised, and the specimen tube with base is placed underneath the head. It is important to carefully place the specimen tube into the machine, to prevent any pre-compaction. The head is lowered slowly, ensuring additional weight/force is not added to the sample. Next, the cam is slowly turned to raise the weight above the sample and the weight free falls onto the sample, compacting the sand. The dropping of the weight is repeated 2 more times. The operator then reads the value on the vernier scale. History Lesson: Why 3-rams; Why Not 4? Ries and Nevin determined the 3 drops was the ideal amount by dropping a ball bearing on a mold at a steel foundry. The impression was measured. Then back in the lab, it was determined that it took 3 drops of the sand rammer weight to reproduce the same diameter impression on the test specimen. Thus 3 rams of a 14-pound weight falling 2” was developed. More recent studies have proven it is an acceptable procedure. MICHELLE RING Technical Services Manager SIMPSON ARTICLE TAKEAWAYS: • Understanding green sand compactability • Compactibility testing: Best practices Continued on next page Compactability is one of the most essential and common tests in foundry green sand. The ability to properly measure and control compactability, allows the foundry to reduce variation in other parameters, including green strength, moisture, friability, flowability, cone jolt, density, permeability, and wet tensile. Basically, every green sand test except GFN and sand distribution. 15 USING THE COMPACTABILITY TEST TO OPTIMIZE GREEN SAND QUALITY TECHNICAL TOOLBOX ISSUE
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