Jeremy Eastman, Hormel Foods, Austin, MN
The United States government is sponsoring research to identify ways in which foundries can comply with environmental regulations while maintaining their casting quality and global competitiveness. The Casting Emission Reduction Program (CERP) was formed in 1994 to address these issues. The Cooperative Research and Development Agreement, which formed CERP, was drafted between the U.S. Department of Defense, the U.S. Environmental Protection Agency, the California Air Resources Board, and private sector organizations like the American Foundry Society, the Casting Industry Suppliers Association, and the United States Council for Automotive Research.
It is CERP's mission to improve, develop, and/or demonstrate new products, processes, and technologies for the metalcasting industry that reduce negative environmental impact and keep the industry competitive in a global economy. The research performed under CERP enables foundries to meet new environmental requirements and insures an economical supply of domestically produced quality castings.
The CERP foundry is located at McClellan Park, or the former McClellan Air Force Base, Sacramento, CA. Resulting information and research data is shared with industry, helping to secure American jobs and our national industrial infrastructure.
The first prototype of a dual-station, production core-shooting machine developed exclusively for use with the GMBOND Process will be commissioned at the General Motors Saginaw Malleable Iron operations on August 20, 2003. This machine is is designed to accommodate all of the coreboxes required for the core package of the General Motors GEN IV engine block. This aluminum block will be manufactured with the precision sand casting process.
The project has progressed with the active support of General Motors, CERP, Hormel, and the United Auto Workers. It will demonstrate the viability of GMBOND sand binding technology as one that is economical, as well as environmentally friendly.
The Sand Binder and Process
GMBOND Sand Binder consists mostly of animal-derived biopolymers, which are combinations of amino acids linked together to form long chains called proteins. For aluminum or magnesium castings an iron oxide catalyst is incorporated to increase the rate of thermal degradation, improving shakeout of the core. The binder is a dry, fine, slightly tan, water-soluble powder.
Bonding is accomplished by dehydrating the wet core sand mixture. The biopolymers form covalent bonds as water is removed from the core to form a crystalline structure. No chemical reaction ever takes place. Therefore, scrap cores and core butts can be reused without having to add additional binder.
The sand is pre-coated, much like in the shell process, by adding dry binder to hot sand (about 250° F) while mixing. Next, water is added. The sand is then mixed until it is dry and free flowing.
Prior to blowing a core, sand is conditioned (at about 70° F) with water (two parts water to one part binder) and mixed to evenly hydrate each coated particle. Sand should be cool to give the hydrated sand particles optimal flowability for making complex cores like water jackets.
Coremaking is completed by the removal of moisture from the wet sand mixture. During the blowing cycle, conditioned sand is blown into a core box, the same way coldbox sand is blown. The heat from the tooling causes the binder to melt and dry. The purge cycle gives the core strength by removing the water from the corebox.
The machine was designed as two separate components: the sand system and the GMBOND core-shooting machine. This design was necessary due to the overhead space constraints, which were presented to FATA Aluminium by the Saginaw foundry during the design phase. Both the sand system and the core-shooting machine were designed to function independently in order to facilitate the myriad of research and development tests required for process optimization.
The sand system contains different elements required to prepare the GMBOND coated sand and transport the sand to the machine. The process begins with coated sand entering from a storage hopper located directly above the mixer. The sand is introduced into the chiller and then mixed with a metered amount of water. This equipment cools and hydrates the sand to its optimal operating condition.
Because of the limited operating space above the machine, the prepared sand is transported via an enclosed belt conveyor. The conveyor maintains the sand temperature via cool air from the chiller while it is transported into the receiving hopper elevated above the shooting head loading station. This belt is equipped with a cleaning station to ensure that sand remaining on the conveyor belt is removed for reuse.
The core machine consists of a receiving hopper located above the shooting head of the machine, one core shooting station, two purging stations, and one core removal station. The machine is designed to allow for the core unloading and core shooting to take place while the second core is in the air purge, or curing station. This allows for cycle time optimization consistent with customer demand.
The receiving hopper acts as a preload station for the shooting head and also facilitates the removal of any unused sand by discharging it into a sand-receiving flask. The shooting head moves transversely underneath the receiving hopper to receive a metered amount of sand necessary for core production. Immediately before entering the shooting head, the sand is augured from the conditioning hopper.
The shooting head is uniquely designed in that the sand is shot into the mold cavity. This is accomplished by forcing a metered amount of sand into the cavity via air pressure applied behind the sand column. This reduces the total amount of air introduced into the cavity resulting in a more uniformly dense sand core. After the sand is shot into the cavity of the heated metal corebox it is immediately transferred to one of two purging stations.
At this station the sand core is purged of moisture by the convection of warm air. The machine and the corebox are designed to facilitate air purging in different zones, either independently or simultaneously. The cured core is transferred via the cope to a core unloading station where an operator deposits it onto a pallet for unloading. The cope is then transferred back and closed to the drag half of the corebox. Machine and process cycle times are designed to be competitive with current coldbox processes.
This partnership between the private sector and a publicly sponsored research institute is specifically designed to aid in the ongoing competitiveness of the U.S. foundry industry. During the demonstration and optimization of this core and core machine application, a complete lifecycle cost analysis will be performed and eventually published.
General Motors and GMBOND
General Motors Research and Development started working on an environmentally-friendly sand binder system in the early 1990s. Concurrently, GM Powertrain was analyzing the lost foam process for the production of future engine castings. After some initial developmental work, GM's R&D team working on the new binder formally introduced it to the U.S. metalcasting industry in 1994 at the AFS Casting Congress.
Also in 1994, J. Michael Williams of GM Powertrain Manufacturing announced that "lost foam will be the aluminum engine block and head process of choice." Although there were obvious merits to the new sand binder technology, GM Powertrain management had already decided to optimize the lost foam process. Nevertheless, between 1994 and 1996 GM's R&D group worked to develop the process parameters to produce cores with the protein-based sand binder. They realized that despite Powertrain's commitment to lost foam, their suppliers would have a need for this technology because of shakeout and environmental issues related to the phenolic urethane coldbox process.
Between 1996 and 1999, GM conducted several validation trials both internally and with Teksid S.p.A., a large automotive casting supplier. Castings were made in green sand molds, with GMBOND cores for cast iron ventilated brake rotors. Aluminum squeeze cast V6 engine blocks were produced in GMBOND in an effort to determine its dimensional and shakeout tendencies. Many green sand molded aluminum cylinder heads were produced with GMBOND cores. Teksid and GM verified the shakeout of the process with a semi-permanent mold aluminum suspension arm. In every case, the casting quality, surface finish, and shakeout were as good or better than the phenolic urethane coldbox process. Despite this, GM was not in any position to commercialize and optimize this technology for the foundry industry.
As a result of their close relationship from past development work, Hormel Foods entered into an agreement with GM in December 1999. This agreement allowed Hormel to commercialize and develop the new technology. As Hormel has been introducing this technology, they have also been optimizing the coremaking process. Until Hormel's involvement, there was no core machine ever produced specifically for the GMBOND process. Since then, several casting and coremaking trials have been completed, which have verified the technical merits of the new precision sand process.