Simple Solutions That Work! Issue 15

Contact: BRAD HOHENSTEIN [email protected] CASE STUDIES 39 In order to understand the melt quality throughout the entire shift, RPT samples were produced, and specific gravity measured at the start of shift, end of shift, and various times in-between. As Chart #1 shows, the melt would meet the specific gravity target to begin the shift but degrade throughout the shift. From this data it was easily determined that an additional degassing event must occur around mid-shift. RESULTS Mid-shift degassing was implemented as a standard operating procedure for this foundry. Since the process is quick, inexpensive, and effective, it was implemented as part of the company’s quality control system. Chart #2 displays the results of several hundred specific gravity checks before and after the implementation of the mid-shift degassing. The results clearly show that by developing a target specific gravity and using that target to determine degassing time and frequency was an effective method of controlling the hydrogen content of the melt. CASE STUDY 2 Whereas the average part size produced by the automotive supplier in Case Study 1 was about 1lb., the foundry in case study 2 produces parts in the range of 500 to 12,000 lbs. These parts are typically one-off parts for the blow molding industry. Unlike the automotive foundry, porosity is not a structural issue but more of a cosmetic issue for their customers. Using a Porosity Analysis System, the same process steps as in Case 1 were followed: 1) Determine Specific Gravity target 2) Determine degas time 3) Implement procedure and monitor. RESULTS Using the Porosity Analysis System and their newly developed specific gravity target, the foundry was able to update their Standard Operating Procedures to ensure the target specific gravity was met prior to pouring and eliminate hydrogen porosity from their parts. “It was like we were operating blind” stated the President of the foundry. “We now have a defined target that gives us confidence the melt is good.” CASE STUDY 3 The two prior case studies mentioned in this article demonstrate how the use of porosity analysis systems improve melt quality. However, the benefit in case study 3 was dollars to the bottom line. The obvious cost savings is reduced scrap due to elimination of hydrogen porosity in the parts. However, other cost savings such as reduced material costs, labor costs, and increased productivity can have a positive effect on the bottom line. To ensure their castings were porosity free, a medium sized sand foundry (six 1000 lb. melt furnaces) had developed a degassing procedure that was much longer than required. While their degassing procedure insured hydrogen porosity free castings, the cost of this practice was much higher than the foundry manager realized. The foundry was degassing each furnace for a minimum of 30 minutes using nitrogen with rotary inert degassing (RID) units. Each furnace was charged and poured twice a day bringing the total degassing time to 6 hours a day. Once the porosity analysis system was in place and a target density number was set (2.59 for 356 and 2.62 for 319) the degassing time was reduced to 20 minutes without any negative effect on the castings. The reduction in degassing time not only resulted in an additional 2 hours of production time, but also reduced the nitrogen gas consumption by 33%!