crane also builds charges—the scrap material being loaded into a furnace. Te majority of the scrap bay’s crane time was spent unloading trucks with a magnet and depositing the scrap into its respective inventory bin. To allow the crane to keep up with building a charge every 65 minutes, ME Elecmetal combined multiple scrap classes into single categories and
restructured the bin layout to allow trucks to backup and dump their scrap loads into their respective bin. Due to limited floor space for
truck maneuvering, only the two most commonly used scrap categories were optimized for truck dumping (Fig. 3). Scrap deliveries also were scheduled over all three shifts to avoid crippling a shift with continuous truck deliveries.
Since the scrap bay optimization, delays associated with waiting for charges to be built have not appeared. Ladle responsibility, which had been
handled in melting, was shifted to the pouring department to allow melt staff to focus on continuous processing. Te ladle repair area shares the same bay as the melt furnaces. Te crane associated with the melt
furnaces for transporting ladles, charging, etc., was occasionally required to pick up ladles in ladle repair. Tis took approxi- mately 15-30 minutes per event. To eliminate the need for the crane for this event, a rack was designed to hold the ladle and allow it to be rotated for repair. Te crane is only needed to move the ladle in and out of the rack (Fig. 4). Since the rack has been installed, no delays have been associated with waiting for the crane to return from ladle repair.
Process Optimization With Three Furnaces
After the expansion, delay propaga-
tion became more apparent with three furnaces. Processing, rather than melt- down, became the new rate-limiting step for melting. Any delay associated with processing the current heat also offset the processing times of subse- quent heats by the duration of the delay. For example, if the current heat being
processed had an event that took 30 minutes to correct, processing the next heat would be 30 minutes later. Often with three furnaces, multiple events coincidently occur that are necessary and need to be completed; however, resources are limited and only one event can be performed at a time. Every situation is unique, but in most scenarios prioritizing the task that affects processing mini- mizes delay and propagation. Examples would be back charging
Figure 2. Shown is the process timeline for two and three furnaces. To clarify the event labels: prep furnace refers to gunning the furnace with refractory material and charging the furnace with scrap for the next heat; melt down refers to running the furnace until the heat reaches a specific tempera- ture; and processing refers to all actions taken to process the heat (i.e., oxygen refinement, alloy additions, etc.) and transferring the molten metal to pouring.
24 | MODERN CASTING August 2015
a furnace (adding additional scrap to the furnace that could not originally fit when the furnace was charged) or chill- ing (dropping rail track into the molten bath of the furnace to cool the bath by taking advantage of a thermodynamic phase change). Both tasks require a crane and need to be completed for the heats to progress, but transferring a heat to a ladle is the final step in processing and should be prioritized to begin processing the next heat as soon
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