26 be addressed. If there are many consecutive readings in a row at 0.35 it likely means the sand is too hot and you need to find ways to cool the sand. If there are large fluctuations in conductivity readings, or receiving 0.35 readings at nonelevated temperatures, check the conductivity probes to ensure they are in the sand, and are reading correctly. A best practice is to monitor the difference between conductivity probe readings. By checking the difference between probes, it can help identify a probe fault. Again, targets will vary by foundry, but many foundries strive for conductivities between 0.75-1.5, without large variation between data points. WATER ADDITION A good practice is to verify that water requested = water actual. Moisture in molding sand activates the clay bonds, and is classified as either combined or free water. Controlling the moisture content of green sand should be maintained within a narrow range. Moisture in the green sand can affect muller efficiency, working bond, and available bond. It plays a part in nearly every green sand property, except grain fineness. Correlations can be seen between moisture content and compactability. Excess moisture can result in gas related defects, high strengths, lower permeability, and lower flowability. Low moisture produces dry friable sand and can be difficult to mold. BATCH WEIGHT The batch weight is the largest factor in determining water and clay additions. It is important that batch weight stays consistent from batch to batch. The simplest way to improve repeatability is to slow down the sand addition rate. A typical cycle allows over 45 seconds to weigh up the next batch, but many weigh the entire batch in under 5 seconds. Slowing down the speed could potentially reduce variation and improve accuracy. Also, a daily check of the batch hopper weight while empty is also important. The tare weight should be no more than 1% of the normal batch weight. If the weight is more than 1%, it is suggested to inspect the batch hopper for issues. Contact: MICHELLE RING [email protected] GREEN STRENGTH Green compression strength indicates the maximum compression stress the sand mixture is capable of sustaining. It is used to determine the amount of clay addition to a green sand system. Sand to metal ratio, clay content, compactibility, and additives can affect green compression strength. Low green compression strength indicates good flowability but can result in broken molds and poor pattern draw. Low green strength can be caused by low clay content, dry sand, an issue with the muller. High green compression strength can result in broken molds, poor flowability, and difficulties at shakeout. When data is erratic or fluctuations are greater than normal, it is important to check the automatic compactability testing equipment to ensure the wipe-off is aligned and the sample cup is full. If secondary tests confirm the same trends, then start to look at what else has changed in the system. Trending sand data and reporting understandable metrics aids in troubleshooting and determining root causes throughout the foundry process. Repeatable sand properties ensure the sand is strong enough to hold up to the thermal stresses at the mold-metal interface. The sand system is telling us what to do, we just need to ensure we are monitoring and listening.
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