22.0 THERMAL BALANCE:
22.1 Introduction:
An important function of the die is to remove heat from the molten metal till it solidifies and cools to a predetermined temperature before it is ejected out of the die. The dies thus act as a heat exchanger.
The heat input into the die per hour depends on:
- The number of shots per hour.
- The mass or volume of the casting including the sprue or biscuit, the runners and overflows.
- The temperature at which the molten metal is injected into the die.
- The casting ejection temperature.
The factors governing the heat out put from the die are:
- Heat loss through radiation - this is a function of the out side surface temperature of the die and the air temperature.
- Heat loss through convection - the vertical die surfaces have the maximum heat loss through convection, followed by the top surface of the die. The bottom of the die exhibits the least heat loss through convection.
- Heat loss through water cooling channels - the cooling channel account for over 80% of heat removal from the die and therefore the design of the cooling system is an important consideration.
Die temperature variations have been found to be one of the foremost causes of defective castings, down time, and reduced production rates. Die temperatures that are too low result in cold - lap defects, while die temperatures that are too high result in sticking of the castings in the die, a decrease in the production rate, die soldering, and die wash problems. Improper balance of temperatures through out the die leads to heat associated problems in one section of the casting, and cold associated in another, and also results in several types of shrinkage defects such as shrinks, lakes and waves. Through improved control of die temperature, these problems can be minimized or eliminated. In the production of thinner - section die castings, temperature control becomes even more critical since the temperature range over which acceptable surface quality will be achieved generally becomes narrower.
22.2 Die temperature at injection
a) The importance of shot rate:
For dies having no internal (or external) heating / cooling system, shot rate is the most important factor controlling cavity surface temperature at injection shot rate equates with heat input rate to the die and for maintaining steady temperatures it must equal the heat extraction rate of the die. When extra heating and! Or cooling is applied, the same balance is required but the influence of shot rate is combined with the extra effect. Thus with extra heating the shot rates may be lower to maintain steady temperature conditions and with extra heat extraction (cooling) the shot rate (heat input) must be higher.
b) Effects of other variables:
1. Cycle time (die- closed time):
the time that the die remains closed after cavity filling ('die – closed Time', 'dwell time') influences the surface temperature at the next injection: by how much depends on the time between shots.
2. Time between shots:
this governs the surface cooling taking place before the next injection.
3. Die surface cooling rate between shots:
This is governed by:
· Heat transfer rate through the die. (material and size factors)
- Heat transfer rate across the die / bolster joint.
- Radiation and convection heat - losses from the die surface
- Internal cooling channels.
- Properties of circulant (coolant).
- Size and location of channels.
- Condition of channel walls.
- Flow rate of circulant (coolant).
- Temperature of coolant.
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