Temperature control by process timer method
What is process-timer method?
A process timer facilitate pouring-temperature control by showing a green light when pouring workman may start the pouring cycle and red light when he must stop in accordance with temperature of molten metal.
Use of this method is based on knowledge of both the cooling rate of metal in the ladle and the optimum pouring temperature range for particular castings.
Why use this method?
When critical castings-such as cylinder blocks and heads are being produced it is essential to maintain the pouring temperature within a closely controlled range. Temperature measurements by thermocouple tip and manpower cost is an expensive part of process control- particularly in high production foundries. By use of the process-timer method of control, temperature can be measured less frequently during the pouring cycle, yet control of optimum casting temperature range is maintained.
What is the process timer?
It is an electronic timing device that will operate for predetermined period, which is a set value, entered using a keypad. An audiovisual signal comes on automatically at end of each preset time to indicate when an operation can be started and must be ended.
What is needed for the process-timer method?
An immersion pyrometer to determine the temperature of each freshly filled ladle of metal at filling station. Two process timers units are wired to operate in sequence. One is connected to a green light, which indicates when the metal is at the correct temperature for poring to begin, and the other to a red light, which indicates when pouring, must cease. They are both mounted on a panel convenient to the ladle filling station.
Duplicate lights should be mounted in the casting bay so that the casting operator can easily see them. The timers and lights should check daily before pouring commences.
How is the system established?
The rate of metal temperature loss is influenced by initial metal temperature, the capacity and shape of ladle, the degree of ladle insulation and preheat temperature and whether the ladle is fitted with a cover.
The average rate of fall in temperature per minute should be established by a series of measurements using an immersion pyrometer and expressed in degrees centigrade per minute.
Once the rate of metal temperature loss has been determined it is then possible to predict the time it will take for a ladle to fall to the temperature at which pouring may begin and also the temperature at which pouring must stop.
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