Simple Solutions That Work! Issue 14

18 UNDERSTANDING THE CHALLENGES WITH ELECTRIC CRUCIBLE FURNACES A lot of companies use electric crucibles for plenty of good reasons (especially space saving). However, sometimes we see foundries using them, because that’s all they are accustomed to operating. We believe that if they did a careful evaluation of all of the high energy and labor costs that they would take a harder look at ways to reduce those costs. Between costly energy losses, maintenance time, crucible bowl, element, and reline costs, and workers compensation/safety costs; these units usually cost more than the dollars for the space they save, metal inventory (3-1 hold to melt ratio) and low up-front costs. If you are melting and casting out of the same crucible, you also need to factor in scrap losses, which are usually quite large. Most manufactures of crucible furnaces still maintain that 3-1 hold to melt ratio. But, unless you charge that crucible no more than ¼ of its hourly melt rate every 15 minutes, you will have a hard time holding metal temperature. The rule of thumb is this – put in whatever you take out every 15 minutes. In fact, if you return your gates, runners, and scrap (while it is still hot), into the furnace, that will help increase its overall efficiency. It’s not unusual for us to see foundries both melting and casting in the same bowl – and not using a baffle in between the casting part and the loading part of the crucible. As a result, inclusions and an increase of hydrogen gas in the metal and into your part are likely outcomes. You generally need to hold temperature at a much higher than normal rate in order to overcome the large temperature swings when loading one of these types of furnaces. There is simply not enough stored BTU’s in that small amount of metal left in the bowl to help with the melting process. It must draw all the energy from the heat source which can cause huge temperature swings which leads to cold shots, increased dross and separation of the alloy constituents. Wet bath reverbs eliminate most of these issues because of their hold to melt ratio. DAVE WHITE Sales Support THE SCHAEFER GROUP ARTICLE TAKEAWAYS: • Understanding each type of furnace • Furnace energy usage • Metal melt loss for each furnace