Case Study, Minnesota Technical Assistance Program, University of Minnesota
Progress Casting Group, Inc., located in Plymouth, Minnesota, is an aluminum foundry that casts parts for use in a variety of applications, including: food processing equipment, motorcycles and airplanes.
In 1993, Progress Casting used approximately 35,000 pounds of 111-trichloroethane (TCA) as a carrier in the wash used to coat sand cores. After sand cores are dipped into the wash, the TCA quickly evaporated leaving a protective coating on the cores. The coating prevented molten metal from penetrating the cores during the parts-casting process and left a smooth finish on the casting surface. TCA was used as a wash carrier because it: held other wash components in suspension, dried quickly to give a consistent coating thickness, and ensured adequate tensile strength to prevent the coating from chipping.
The following factors contributed to Progress Casting’s interest in finding alternatives to TCA:
Environmental commitment. Progress Casting made a commitment to prevent pollution by participating in the U.S. Environmental Protection Agency’s (EPA) 33/50* toxic chemical use reduction program. Its goal was to reduce TCA use by 30 percent in 1992, and eliminate TCA use by January 1994.
Economics. Economics was an important factor in eliminating TCA use. TCA costs were increasing due to excise taxes and phasing-down of TCA production, as mandated by the 1990 Clean Air Act Amendments (CAAA). In 1992, Progress Casting spent $44,250 on TCA purchases. In 1993, the same volume of TCA cost $59,000.
Customer demand. Customers demanded that Progress Casting stop using TCA because of new product-labeling rules mandated by the CAAA. These rules require that products made with ozone-depleting chemicals (ODC) be labeled stating that ODCs were used in the production of the product.
Progress Casting evaluated a variety of water- and IPA-based coatings as alternatives to TCA. Each coating was tested for its ability to provide a uniform coating thickness and quality without damaging the sand cores.
Progress Casting uses a low-pressure permanent molding technique to cast some of its parts. The pressure can cause molten metal to burn into the sand core if the coating is inadequate; therefore, the coating selected had to withstand the pressure and protect the core.
Other problems with using water-based coatings included:
Keeping solid components suspended in the mixing tank.
Developing dipping techniques that provided even coating thickness.
Finding drying methods that dried fast enough to leave a strong coating on the sand cores.
The primary problem with using an IPA-based coating was the fire hazard caused by IPA’s high flashpoint.
Drying Requirements
The water-based coatings required significantly more time to dry than TCA; therefore, Progress Casting had to determine the appropriate drying time and temperature requirements. In addition, vendors could not supply "turn-key" equipment that would work with the washes for all the core sizes used at Progress Casting. Consequently, Progress Casting evaluated four different drying techniques to determine which one would work best. These included: high intensity lights, microwave, drying tunnels, and infrared ovens.
The high intensity lights and drying tunnels did not dry fast enough to prevent coatings from dripping and losing thickness uniformity. Microwave drying used nonselective heat that penetrated the sand cores and caused them to disintegrate. Infrared ovens, however, quickly dried the coatings without damaging the sand cores.
Customized Equipment
All the water-based coatings tested required numerous dips of the sand cores into the corewash slurry (TCA coated adequately after only one dip). To accommodate this requirement, Progress Casting worked with vendors to design custom equipment to automate dipping and drying the smaller sand cores. Automation is a key factor in producing consistent coating thickness and quality.
The equipment layout was designed for efficiency and consists of: a dip tank containing a continuously recirculating water-based slurry; two separate infrared ovens; and four rotating racks used to hold the cores and lower them into the dip tank and ovens.
First, a row of sand cores is placed on one of the racks, and then lowered into the dip tank containing continuously recirculating corewash and held in the coating solution for approximately one minute. With the corewash forced upward in the tank, the countercurrent effect facilitates wetting of the core surface and helps ensure uniform coating. The aeration action also minimizes bacterial growth in the corewash.
After dipping, cores are then lifted out of the tank and rotated into place over one of the infrared ovens as the operator adds another row of cores to the second rack. The racks then simultaneously lower the cores into either thedip tank or the infrared oven. By having four racks, two sections of cores are simultaneously dipped into either the slurry tank or the infrared ovens as the operator removes one row of cores and adds another row. Each core is dipped and dried twice.
When the automated dipping/drying equipment was first tested, Progress Casting evaluated core coating quality to determine the right ratio of dipping and drying times required to get a uniform, quality coating. Other problems with using this equipment included: fungal growth in the slurry mix, which was resolved by the core wash vendor; and curdling or separating of the corewash, which was resolved by adjusting the mixer speed in the dip tank.
Safety concerns. IPA has a flashpoint of 53°F; therefore, using IPA in a foundry situation can be extremely dangerous. To reduce the fire hazard, Progress Casting installed an exhaust hood in the work area to remove the IPA from the plant floor as it is sprayed onto parts. In addition, a sprinkler system was also installed in the work area.
IPA was almost a "drop in" substitute for TCA, except for the fire hazard and a slightly longer drying time. Through testing, Progress Casting found that the IPA can be sprayed onto a part and then hand rubbed into the surface. No heating of the part is required to set the IPA surface coating.
As of October 1994, Progress Casting has completely eliminated TCA use by substituting water- and IPA-based coatings for the TCA. As a result, Progress Casting has eliminated $59,000 in TCA purchase costs. Approximately 35,000 pounds of water- and IPA-based coatings are purchased at an annual cost of $14,500. The net annual cost savings resulting from eliminating TCA is $44,500.
In addition to the economic benefits, Progress Casting has satisfied customer demands to eliminate TCA use; thereby, eliminating the ODC use labeling requirement mandated by the CAAA.
Progress Casting has also achieved its 33/50 Program goals by first reducing and them completely eliminating its use of TCA.
MnTAP has a variety of technical assistance services available to help Minnesota companies manage and reduce their industrial waste. If you would like assistance or more information about MnTAP, call 612/624-1300 or 800/247-0015 if calling from greater Minnesota.
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