Simple Solutions That Work! Issue 14

DEFECT PREVENTION ADVANCED MANUFACTURING SOLU NS 19 In the last two years we converted three foundries from crucible furnace to small gas or electric in cell melters, and virtually eliminated their scrap issues. They are also spending less on energy than with the crucible furnaces, because they were always on 100% output just to try to keep up. Crucibles have their place, don’t get me wrong. If you change alloys a lot, shut furnaces down often for extended periods of time and have extremely limited space, then you really have no choice but to use crucible furnaces. There are some ways to eliminate some of the headaches involved in crucible melting and holding at the machine. Always inspect the design approach when evaluating these crucibles. In electric furnaces a lot of companies are offering quick change elements. The reason is—you have to change them often! Look for elements that typically last about two years and are easily changed when you must also change a bowl. Also, if you do get a burned-out section you can easily splice in a piece of element to keep it going, which also means you no longer need to keep a complete set of elements in stock. CRUCIBLE OPERATION TIPS: • Don’t over charge the crucible. • Use a baffled bowl to keep inclusions out of your parts. • If your crucible bowls are cracking at the top, then you are allowing them to be drawn down more than 4 inches. After that the temperature differential at the top of the bowl and the middle is so great that the top expands and cracks prematurely. • Do not allow ingots to be dropped into the bowl. This can cause cracks to occur in the bottom of the unit. • With a little pre-planning and care you should be getting a year or more of life out of your crucible bowls. Cleaning the crucible bowl and metal tips: 1. Gently scrape the inside of the bowl to clean it once a day. This should remove any oxide buildup that forms. Right after you do this you must introduce your metallurgical flux into the bath of aluminum and work it towards the bottom of the bowl. The deeper you get the flux the more metal it will clean. Don’t forget about the bottom of the bowl to prevent sludge buildup, which will occur if the temperature is allowed to go below the alloy’s sludge point. 2. Make sure your metallurgical flux is both temperature and alloy specific. 3. Degassing to the bottom of the bowl with flux injection will float those heavies (iron, manganese and silicon) that have settled to the top where they can be skimmed off. 4. If you use more than the bowl’s capacity in an 8-hour shift then you should clean your furnace metal every shift. 5. Talk with your flux manufacture regarding their specific instructions on how long to leave the flux in to clean your dross. When buying a crucible furnace, look for four things: 1. Closeness of the burner or elements to the bowl – if it is too close the heat (flame impingement particularly) will oxidize the coating on the bowl and cause premature failure. 2. Make sure the casing temperatures are less than 130°F when looking at 1300°F metal or lower. 3. Ability to hold at least 4–5 times what you are melting per hour. 4. If electric .28kw/# of metal melted connected. If gas 3,000 BTU’s/# connected. In summary, the most important reason to purchase any furnace is to help produce higher quality castings at less cost. Understanding the costs and tradeoffs associated with any furnace will help you to make the best decision. Contact: DAVE WHITE dave.white@theschaefergroup.com