21.0 THE EFFECT OF METAL ON DIE CASTING PROCESS
21.1 Introduction:
The effect of metal mainly depends on three variables. 1) metal, 2) Machine and 3) die
1. Metal:
a) Alloy composition
b) Metal temperature
c) Shot weight
2. Machine:
a) Accumulator pressure.
b) Injection line pressure.
c) Intensification.
d) Plunger force.
e) Locking force.
f) Mode of injection.
g) Plunger diameter.
h) Timing of stages in casting cycle.
i) Lubricants.
3. Die:
a) Die temperature.
b) Filling rate.
21.2 Metal:
a. Alloy composition:
To ensure durable castings, the alloy composition must be confirmed to the specifications. If the alloy composition is outside the specification range, immediate difficulties are likely to be experienced but its performance in service is greatly impaired. Satisfactory performance in service can be greatly impaired. Satisfactory performance in casting is therefore no indication that the alloy confirms to the specifications. In some cases two or more of the alloys may be suitable for a particular application and the final decision is then governed by the cost of alloys. Alloys containing secondary aluminium are significantly lower priced.
b. Metal temperature:
Aluminium: Most die-casting, alloys, for e.g. l, 2,4,6 and 24 contain the alloying elements silicon, iron and manganese which form compounds that settle out when a melt is held too low a temperature. These metallic segregates form hard spot inclusions, which can cause considerable difficulties in when machining the casting. The temperature of the metal entering the die cavity is dependent on the holding temperature, the temperature of the shot sleeve, mode of injection and delay between pouring and start of injection.
Zinc: metal temperature has relatively little effect on the casting quality within the normal practicable limits of 395 - 450°c. Little is to be gained by operating outside the normal casting range of 410 - 420°c. If the metal temperature is too high, it shortens the life of the plunger and goose neck and it has been reported that the magnesium loss increases substantially at 450°c. Too low a metal temperature often results in build –up of a metal on the plunger stem unless heating is used in this area. This build-up can reduce the injection speed by mechanical interference.
c. Shot weight:
Variations in shot weight can influence the quality of casting by affecting:
1. The volume of gases in the shot sleeve (% filling of shots sleeve)
2. The average metal temperature entering the die.
3. The size of slug, which in turn influence the transmission of forces from the plunger to the Casting.
4. The position of the metal at the change over from slow to fast injection.
S. The start of intensification on machines when it is initiated from a position switch.
In some cases an excessive shot weight can be dangerous because very long slug can burst and injure the operator as the die opens.
21.3 Machine:
a. Accumulator pressure:
Accumulators are used to store up energy from the hydraulic pump during parts of the cycle when the demand from the pumps is low. This enables the fast injection speed to be achieved with pumps of low out put. Modern machines generally use pistons to separate the hydraulic fluid from nitrogen. For any particular accumulator pre- charge pressure, the range of injection pressures which can be used is small. Therefore, if a substantial change is made in the injection pressure, the accumulator pre - charge pressure must be adjusted. It is very important to have the correct pre - charge. Too high a pre - charge in relation to the injection pressure can result in the separator not being lifted, with the result that the accumulator will not operate. Too iowa
b. Injection line pressure:
This (when no intensification is used) controls the maximum force on the plunger and it also influences the speed of injection. Incases where intensification is used, the line pressure influences the maximum injection force. On some machines the hydraulic pressure control valve for the locking circuit is situated upstream of the valve controlling the injection pressure, with the result that lowering the locking pressure can also lower the injection pressure.
c. Intensification.
The pressure build up at the end of injection, the intensification stage can have a large effect on the porosity of casting. Intensifiers enable large changes to be made in injection force without changing the line pressure and accumulator pre - charge pressure. The rate of pressure build up should not be too fast, otherwise it may cause the die to flash. Alternatively, too long a delay in the pressure build up results in a low pressure being applied during solidification.
d. Plunger speed:
Plunger speed is usually set by a valve, which simply restricts the flow of hydraulic fluid from the accumulator to the injection cylinder or air receiver to the injection cylinder. Small variations in plunger speed can occur due to fluctuations in the injection line pressure and also with large variation in die temperature when producing thin section casting. If a large change is made in the injection pressure, this can produce a significant change in injection speed. Fast injection can increase amount of flash because of the heavy impact and the metal's more fluid condition and the completion of fill.
e. Locking force:
Locking force is dependent upon the locking pressure, tie bar nut setting and temperature of the die and surroundings. As the die heats up, the locking force increases and unless an allowance has being made for this increase, the machine will start when locking. For a given tie bar nut setting, increasing the pressure to the locking ram does not increase the locking force
Locking force, injection pressure and projected area of the casting together with the die distortion are the factors controlling flash. For the best results, the die should, where possible, be designed so that the center of pressure produced in the cavity acts in the center of the area within the tie bars, i.e. The impression should be positioned centrally within the tie bars.
On some machines the hydraulic pressure control valve for the locking circuits is situated upstream of the valve controlling the injection pressure, with the result that lowering the locking pressure can also lower the injection pressure. Little is to be gained by lowering the locking pressure. If one wished to reduce wear in the toggle linkages etc., when producing small casting, the tie bar nuts should be slackened. For this purpose, a plot of locking force against tie bar nut position is useful.
f. Mode of injection:
The use of two stage injections with a slow first stage allows the gases more time to escape from the cavity during the initial fill period of the nozzle and in some cases of the runner.
Two stage injections often also influence the cavity filling speed. Depending on the injection system and where the change over occurs, it can either increase or decrease the filling speed.
g. Plunger diameter:
Large diameter plungers and sleeves are used to increase the shot capacity. Increasing the plunger diameter also reduces the final force on the metal and tends to reduce the plunger speed during injection. If the resistance to metal flow offered by the die is low, the change in injection speed can be very small which results in a significant increase in metal velocity and therefore reduced fill time. If the die offers a large resistance it may be possible for the plunger speed to be lowered so much that the metal velocity is also reduced thus increasing the fill time.
A change in the plunger diameter also alters the injection change over position of the metal. Increasing the plunger diameter makes the actual injection change over position (position of metal) occurs later.
In order to minimize the time spent in changing shot sleeves, and also to reduce the number of spare parts required, it is often convenient to use the same size plunger for the whole range of castings produced on a given machine. Size of plunger is then governed by the largest shot weight from the given range of castings.
h. Timing of stages in the casting cycle:
Some machines have overall cycle time which in effect controls the die open time. Usually the die open time is controlled by the opening and closing speed of the machine and, in the case of semi - automatic machines, by the speed of the operator. During the die open period the cavity surface and sub - surface have a slower cooling rate than at any other time during the cycle (apart from the short time interval between the die closing and injection). The injection timer and the die-cooling timer usually run consecutively, with the result that both influence the solidification time of the casting. The fastest method of operating the die, from the heat extraction point of view is to have the die open time at a minimum.
This increases the temperature of the die face at injection, which is an advantage especially in the case of a thin section casting requiring a good surface finish.
i. lubrication:
Correct lubrication of the die and of shot sleeve and plunger is very important because it affects the soundness and surface finish of the casting and the life of the shot sleeve and plunger.
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21.4 Die:
a). Die temperature:
No one die temperature can be recommended for producing acceptable castings because
Correct temperature varies from die to die and Varies with time within the die both perpendicular to the surface and across the surface
Surface finish is largely controlled by the die face temperature at injection. Casting dimension depends on the temperature at ejection and to a lesser extent by the die temperature. Too high an ejection temperature will cause blisters
b).Filling Rate
Filling rate can be considered in terms of
cavity fill time – unit is millisecond
cavity filling rate of volume flow rate during fill – unit is cm3/s
gate velocity – unit is m/s
Gate velocity is important for the die wear and soldering problems in the gate area.
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