2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-074 (11 pages) Qualification Case Study for Chemically Bonded Sand System P. Patel, L. Wells, S. Ramrattan Western Michigan University, Kalamazoo, MI Copyright 2020 American Foundry Society ABSTRACT INTRODUCTION Chemically bonded sand systems have been used to BACKGROUND manufacture complex near-net-shape castings for more The United States is the third largest supplier of castings than sixty years. Over this period, many foundries have in the world and the metal casting industry is the sixth substantially invested in advanced technologies for largest industry in the United States, according to melting and filling processes, such as automated pouring American Foundry Society.1 Metal castings are essential systems, spectrometers, etc. Despite these investments, to a majority of industries such as defense, automotive, the casting industry still suffers from high variability and construction, agriculture, aerospace, oil and gas, mining, scrap/rejection rates. One cause of these quality issues is railroad, transportation, and health care. Casting quality is the inability to effectively monitor the quality of incoming a major contributor to the success of these industries. sand-binder systems. Most foundries are dependent on their sand-binder suppliers to detect differences in Casting quality losses and rejection rates are directly incoming sand-binder systems as foundries expect to related to foundry operational costs, such as; labor, receive consistent sand-binder systems. This dependency energy, raw material, and delayed customer delivery. In stems from the fact that foundries use traditional addition, sand consumption and cost continue to increase. chemically bonded sand tests, such as the hot tensile Metal casting industries use about 9% of the industrial strength test, which suffer from excessive levels of sand for foundry purposes.2 Figure 1 shows the dramatic variability that makes them insensitive sand-binder increase in unit price of sand and gravel3 over the last system shifts. century. In addition, since sand is the second most widely consumed natural resource4, there are some concerns Recent research has shown that disc-shaped specimen about sand shortages in the future.5 It can be considered a tests can detect wide range of differences in sand-binder responsibility of all industries in the world to minimize systems. In this paper, a qualification methodology for waste of sand. Foundries can minimize sand waste in chemically bonded sand systems is proposed, that focuses several different ways, such as the widely implemented on combining casting quality to statistical process control recycling or reusing of sand. However, it is impossible to for chemically bonded sand-binder systems. An achieve 100% sand reclamation, as losses are inevitable in implementation of the qualification methodology is casting and recycling processes. A second approach presented through a case study. Principal component towards minimizing sand waste is to reduce rejection analysis (PCA) was applied on thermal distortion test rates. If castings are acceptable every time a foundry (TDT) data to monitor an in-control resin coated sand- pours molten metal into molds, then indirectly, sand waste binder system with a control chart. In this study, two is reduced. different resin coated sand2 -binder systems with the same binder type and binder level as the in-control system but Sand is majorly used either as a green sand or as a different sand origins were considered as out-of-control chemically bonded sand in foundries. Green sand is a systems. These sands are round grain silica sands with mixture of sand, clay, and moisture. Several practices similar silica content, grain fineness number, and sand have been identified to control green sand mold making grain shape. In this case-study, if a sand-binder supplier process. These practices include sand testing, analysis of uses these sands interchangeably, the control chart variance, linear regression, Taguchi method, simulation, detects the out-of-control system while 2the tensile and particle physics. In the United States, green sand strength test cannot. Casting trials suggests that casting testing is used for sand control and these tests can be surface quality from these three sand-binder systems are prioritized according to a foundries need.6 Jacobson significantly different for gray iron, but it is similar for introduced a method of mass balance which calculates aluminum. Therefore, with respect to this case study, the how much new clay should be added for a specific job.7 ability to detect this shift (for a gray iron system) would In addition, a data-driven modelling approach has been help prevent surface-quality related losses. developed that provides rejections predictions, to assist in sand control, based on historic data.8 Despite having Keywords: Qualification, resin coated sand system, many different approaches, foundries still struggle to thermal distortion test, quality control, principal maintain green sand casting quality, primarily due to of component analysis. dynamic nature of a green sand systems. Page 1 of 11 2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-074 (11 pages) 80 70 60 50 40 Unit Value ($/t) 30 20 10 0 1910 1914 1918 1922 1926 1930 1934 1938 1942 1946 1950 1954 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002 2006 2010 2014 1902 1906 Years Figure 1. Unit price of sand and gravel. In contrast to green sand, chemically bonded sand obvious uncommon sand systems changes, which restricts systems use a chemical binder(s) and catalyst to cure and its effectiveness for use in statistical process control harden a mold/core. There are different chemically (SPC) to detect small sand-binder system changes10, 11. bonded sand processes, such as hotbox, cold-box, no bake, injection transfer molding, and 3D printing. Each of In contrast, recently adopted AFS standardized, disc- these processes have a specific binder(s) used to make shaped specimen tests, such as; thermal distortion test, molds/cores. Chemically bonded sand is widely used for abrasion test, impact test, and hot permeability, developed manufacturing molds/cores because of its high at Western Michigan University, collect data regarding productivity and dimensional accuracy compared to green independent as well as coupled thermal, mechanical, and sand. physical san-binder system properties. In addition, these tests have shown reduced specimen-to-specimen and test- The quality of chemically bonded sand casting is highly to-test variability, compared to traditional tests. Previous sensitive to the sand-binder system being used to make research has also shown that these disc-shaped specimen molds/cores. When a chemically bonded sand mold/core tests are able to differentiate various sand-binder is exposed to molten metal, unnecessary casting defects systems12. Finally, research has shown that an SPC can occur if appropriate sand system was not used. method, based upon principal component analysis (PCA), Therefore, it is important to detect shift (identify can be very effective in detecting small shifts in assignable causes for changes) in a sand-binder system. chemically bonded sand systems13. The premise of this work is that testing procedures that determine chemically bonded sand system properties can A new concept of qualification for chemically bonded also be used to detect shifts in a sand-binder system. sand systems was proposed in 201714. In the following section, the quality control framework component of the In the United States, the American Foundry Society’s qualification process is presented. The use of this (AFS) standardized sand tests, such as tensile strength framework will be demonstrated through a case study. test, loss on ignition, disc-transverse test, etc., are widely used to monitor for changes in a chemically bonded sand QUALITY CONTROL FRAMEWORK system. These tests are primarily based on independent The proposed quality control framework is shown in Fig. physical, mechanical, chemical, and thermal sand-binder 2. The objective of this framework is to combine SPC properties. However, sand-casting processes are with casting trials to ensure sand-binder system shifts do inherently thermo-mechanical, thermo-chemical and not affect quality. After detecting a sand-binder system thermo-physical. Foundry engineers have recognized that shift, a foundry needs to verify that this “new” sand- certain AFS standardized sand tests provide limited binder has no effect on casting quality, as not all process information regarding sand-binder system behavior, shifts negatively quality. In this paper, the quality control which restricts engineers from successfully controlling framework adopts a surface defect casting trial that uses sand-binder related quality losses.9 In addition, tests such disc-shaped specimens to validate casting quality and as hot tensile strength has high variability in their “qualify” a chemically bonded sand system. A sand- measured data,10 which makes them insensitive to small binder system is deemed qualified if casting trials are free or medium-sized process shifts. Previous research has of a specific defect under given process parameters. The shown that this test (hot tensile strength) can only detect proposed quality control framework can help foundries, Page 2 of 11 2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-074 (11 pages) through a systematic process, efficiently administer a on SPC to identify any shifts that cause the sand-binder mold/core making process. systems to become unqualified. The quality control framework shown in Fig 2 is, for the most part, self-explanatory. The important aspect of this framework is to perform diagnostic casting trials when a possible sand-binder system shift is detected. Foundries can use disc-shaped specimen tests to perform the SPC component of the framework. For this case study, an SPC method based upon PCA which only relies upon results for the TDT13, is considered. If a potential process shift is detected, a casting trial(s) is performed to observe the effects, if any, of this change on casting quality.