Simple Solutions That Work! Issue 13

DEFECT PREVENTION 53 but in a range that depends on their composition. After solidification is over, solid cooling (ref. SCC in the Figure 1 ) occurs at different rates. Figure 2 shows the cooling curves obtained on aluminum castings for reference. The curves were obtained with a data pack (data logger) wired to an aluminum casting with a set of thermocouples. The data logger stores the temperature trend from the casting pouring until the end of the cooling stage. The missing area is due to the disconnection of the data pack before manipulator loads the casting on the steel belt conveyor. Figure 2 shows again two cooling curves: a liquid cooling curve (LCC) and a solid cooling curve (SCC). The point “A” represents the beginning of the forced air cooling when the castings enter the castings cooler conveyor. The main purpose of the present article is to discuss the solid cooling curve (SCC). CASTING COOLING TECHNOLOGIES The most common technologies to perform the casting cooling process downstream the molding line are: • Cooling drums • Vibrating coolers • Steel belt coolers The steel belt coolers have multiple advantages compared to the other conventional technologies: • Smooth castings handling with no vibrations, dust or noise. • No relative motion between material and belt, thus no wear. • Flexible layout arrangement, including greater inclined ramps for material lifting. • No heavy foundations are required. • Castings indexing according to foundry needs. Moreover, an automated system performs a dynamic control on the process parameters, such as: • The temperature of the castings, through a set of optical pyrometers at different points along the transportation. • The casting ID in order to adjust the air flow rate and the steel belt speed, according to the casting type. HEAT TRANSFER METHODS In order to reduce the cooling time, the thermal energy has to be efficiently removed from the castings. One of the most critical factors in the casting cooling process is the heat exchange method adopted to cool the castings down. In a parallel-flow pattern, also referred to as “co-current” flow, both the airflow and the castings Figure 3: Typical combination of heat exchange methods Continued on next page enter the cooling tunnel at the same point and then moves together in the same direction. This method is not as effective because there is a large temperature gradient at the inlet of the cooling tunnel and the cooling medium cannot reach a given temperature to maximize the overall efficiency of the cooling process. Counter-flow pattern, also referred to as “counter-current” flow, is by far the most common arrangement for heat exchange. It occurs when