Simple Solutions That Work! Issue 12

785 Btu per pound and holding in our low energy holders that utilize electric immersion elements can be accomplished in the 18-20 Btu per pound range if you convert the electric usage to Btu’s. We happen to think that this will be the foundry of the future using electric melter holders at the machine to give high quality inclusion free aluminum to the machines. CRUCIBLE MELTER/HOLDERS Let me start by saying that if you are comfortable using crucible furnaces, you really don’t know how much they cost you a year. With worker comfort issues, maintenance, crucible bowl costs, downtime, reline costs when they leak (and they will leak eventually) and workers comp or safety costs associated with these units, they can cost more than the dollars you save in space, metal inventory (3-1 hold to melt ratio) and lower upfront costs. 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 go to another crucible furnace. Also remember crucible furnaces do not recover very quickly so they must be very evenly charged. Whatever you take out in 15 minutes you put back in as scrap or ingots. There are some ways to eliminate some of the headaches involved in crucible melting and holding at the machine. Play particular attention to the design approach in making crucibles. There are a lot of companies out there offering quick change electric elements – because you have to change them often. Elements should last at least two years. Some companies use only one burner on large units and they should be using two. BASIC ADVICE ON CRUCIBLE OPERATIONS • Never allow a bowl 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 the 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. We have always taken basic melt rates very conservatively in order to ensure good crucible life and metal quality. Through years of experience of foundry men pushing melt-to- hold rates on crucibles at 2 to 1 range have resulted in drastically reduced life. Also, many times chilling or sludging results, causing alloy desegregation, inclusions and metal chemistry problems. Due to these facts, we have gauged our melt rate on the conservative side. We have a minimum 3-1 hold to melt ratio and a lot of times go to 4-1 to ensure a more even temperature. If a customer wants to melt beyond rated capacity then make sure the furnace has the input power to perform the task. Evenly charging the furnace with ingot or small scrap (while it is still hot) will increase the efficiency of a crucible furnace. The rule of thumb is put in whatever you take out every 15 minutes. Unless you are casting very large sand castings or permanent mold casting that require most of the liquid metal in the bowl do not batch charge a crucible! It will greatly reduce the life of the silicon carbide bowls. Follow these few guidelines to minimize down time usually associated with crucible furnaces. 20 Gass fired Crucible Electric Crucible Elements