Simple Solutions That Work! Issue 14

46 per ton cost of the prepared sand. The resulting lower level of gas generation produces fewer casting defects and surface imperfections. If the foundry is reclaiming the sand, the round grain material will provide higher yields and cleaner sand with less energy input. The round grain sand will also compact better, providing around 8-10% higher mold and core densities. The hardness of the sand grain is crucial to withstanding the high temperatures and mechanical forces imposed on the aggregate during the casting process, as well as thermal reclamation. Softer grains will breakdown faster and create fines that drive up binder requirements while decreasing the permeability of the cores and molds. This creates issues in venting the gas away from the mold to metal interface, resulting in casting defects. The silica sand from the Wisconsin St. Peter silica formation is known to be the hardest silica sand grain on the planet and ideal for metal casting applications. The next criterion to review is the chemical composition of the sand. The best silica sands for metal casting applications have at least 96% silica content which provides a fusion temperature of around 3125F. This prevents the sand from melting when exposed to the heat of molten metal and fusing the sand grains together, or to the casting surface. The higher silica content also reduces the amount of foreign materials that can interfere or chemically react with the binders being used in the process. Another important characteristic to be monitored when choosing your sand for an advanced manufacturing solution is the pH control of the material. The chemical binders produced today, as well as the bentonite used for green sand are highly sensitive to wild fluctuations in the pH of the sand. Dramatic changes in the pH will impact the performance of your binder, the quantity of binder demand and the energy necessary to achieve the tensile strength or green strength required for your foundry process. Most sands have some sort of overburden that needs to be stripped away before mining, and over the years some of the overburden material can leach into the sand below. Many silica sands are found under limestone (CaCO2) deposits and lake sands inherently have CaCO2 content from the ancient seashells found in the deposits. Once the material is exposed during the casting process to temperatures between 1500F to 1750F, the CO2 is released leaving CaO in the sand. The CaO is alkaline which neutralizes acids, and if the chemical binder requires an acid catalyst, the quantity needed and the performance (strip time) is negatively impacted. In green sand the pH will impact the amount of energy required to mull the sand as well as the ability to optimize green strength. If thermal reclamation is used, these types of mineralogical transformations during thermal exposure can also be very detrimental to the binders as well. The pH is measured on a scale of 1 to 14 and each numerical step in the pH scale represents a tenfold increase or decrease in acidity. A pH of 5 is 10 times more basic than 6 and 100 times more acidic than a pH of 7, so one can see the importance