20.0 PRESSURE DIE CASTING
20.1 Process description:
Pressure die-casting is a process in which the molten metal is forced under high pressure into a cavity in a metal disc in fraction of a second and allowed to solidify. When the casting has solidified, the die is opened and the die casting is removed. The process is rapid and allows complex shapes to be cast as almost finished parts and many thousands of castings can be produced from a set of dies without significant change in casting dimensions.
20.2 Types of Die casting:
There are two basic die- casting process: the hot chamber and cold chamber processes.
The hot chamber process is used for die-casting metals that melt at lower temperatures, such as zinc and lead. The cold chamber is used for metals that melt at higher temperature such as aluminium, magnesium and brass.
a) Hot chamber process:
The basic components of a hot chamber die-casting machine and die are illustrated in fig. In the hot chamber process, the plunger and cylinder are submerged in the molten metal in the holding furnace. The power to come since in the die cavity is provided by hydraulic accumulator. The oil is supplied to the accumulator by a hydraulic pump at a rate that will bring accumulated pressures up to the desired operating level each time a casting is to be made.
The casting sequence in the hot chamber die-casting process is illustrated in the fig. When a shot is made, the control valve opens causing the shot cylinder to force the plunger down and force molten metal through the nozzle, past the sprue pin, through the runners and gates in the die cavity. The gases that were in the system and some of the molten metal flow through the die cavity and out through the vents and \or into the overflows. After the cavity is filled, the metal is allowed to solidify, the casting is ejected and the cycle repeated. Since the goose neck and plunger are submerged in the molten metal, the system refills automatically when the plunger is withdrawn.
b) Cold chamber process:
The casting sequence for the cold chamber process is illustrated in the fig. In this process, molten material is loaded in to the cold chamber and then the plunger advances to force the metal into the die. Except for the manner in which molten metal is fed in to the shot system and injected into the die, casting sequence for the two processes is similar.
c) Comparison between hot chamber & cold chamber process:
| Hot chamber process | Cold chamber process |
| The plunger and cylinder are submerged in the molten metal in the holding furnace | Molten metal is ladled into the cold chamber |
| After shot, plunger is forced the molten metal through the nozzle | After ladling, the plunger advanced to force the metal in to the die. |
| Since both plunger & goose neck are submerged in the molten metal, the refills automatically when the plunger is withdrawn. | Each time, we have to ladle the molten material in to the chamber. |
| For casting metals that melt at lower temperature such as zinc and lead. | For casting metals that melts at high temperature such as aluminium, magnesium and brass. |
20.3 Clamp mechanism:
Because of the tremendous pressures used to inject the molten metal into the die, large forces are required to hold the two die halves together. This holding force is accomplished with the tie bar –platen toggle type machine constructions shown diagrammatically in the fig. The construction arrangement not only achieves the required clamping force, but it also opens and closes the die rapidly. This speed helps to achieve the high production rates of the die casting process.
20.4 Die casting dies:
Die- casting dies consist of two sections: the cover half which meet at parting line. The basic components of the hot chamber die – casting dies are illustrated in the fig. The cover half of the die is secured to front of the stationary platen of the machine. The sprue for filling the die cavity is in this half and it is aligned with nozzle of the hot chamber machine. The ejector half of the die is attached to the movable platen of the machine. It contains ejection mechanism and in most cases, the runners.
The die cavity, which forms the part being cast , is machined into both halves of the die block or into inserts that are installed in the die blocks. The die is designed so that the casting remains in the ejector when it opens. The casting is then pushed out of the cavity with ejector pins that come through holes in the die and are actuated by the ejector plate, which is powered by the machine. Guide pins extending from the one die half enter hole in the other die half as the die closes to ensure alignment between the two halves.
Dies that produce casting with complex shades may contain stationary and movable cores. The movable cores are moved by cam pins or hydraulic cylinders and are locked in place when the die is closed. Since the die casting m\c operate at high speeds, the heat must be removed from die at high rate. Heat is removed from the die by circulating water or other coolant through channels drilled in the die blocks. Heat sometimes is added to the die by electric or gas heaters for warm up or for making thin sections which transmit insufficient heat to maintain the die at the proper operating temperature.
20.5 The die cavity and machine relationship:
The die-cavity machine may be viewed as a pump that has to fill a die cavity at a given pressure ‘P’ in a given time. The cavity fill time depends on the flow rate “Q”.
A good die must fulfill two basic requirements.
- Cavity fill time should be less than 20 milliseconds for achieving plating grade finish and not more than 40 milliseconds for painting grade finish or mechanical grade finish.
- Gate velocity should ideally be in the range of 40-50 m/s but never out side the range of 30-60 m/s.
The PQ² diagram helps in designing dies to fulfill the above requirements and thereby achieve first shot success.
If a hydraulic pump is considered that at given pressure “P” will be inversely proportional to the square of “Q” (rate of flow). The product P & Q² is a constant. Typical PQ² diagram is shown in the figure (machine line). From the graph it is clear that as the pressure increases the rate of flow decreases. The die-casting machine can be considered as a hydraulic pump.
For water or any liquid flowing through a pipe, P is directly proportional to the square of the flow rate Q. Higher the pressure of the fluid flowing through a given pipe higher would be its flow rate. Molten zinc and aluminium at their respective holding temperature behave and possesses hydraulic constants equal to water.
A graphical representation of the relation between “P” and Q² is shown in the fig. The slope of this line depends on the cross sectional area of the pipe corresponding to the gate/runner area.
20.5.1 The machine / die combination:
If the two lines are super imposed a point of intersection is obtained. This point represents the operation conditions for the given die machine combination.
Let the point of intersection correspond to:
Pressure “P”
Flow rate “Q”
Gate area “A”
And the cavity/casting volume be “Vo”
Then, gate velocity V = Q/A
Cavity fill time “t” = Vo/Q
If the point of intersection gives a gate velocity and a cavity fill time, which uniforms to the requirements, the first shot will be a successful shot. If not, the slopes of either or both of these lines are changed to find an acceptable point of intersection.
The slope of the machine line can be changed by changing one or more of the following machine parameters:
- Plunger speed
- Hydraulic or pneumatic pressure
- Plunger size
The slope of the die line can be altered by a one or more of the following die variables.
1. Runner / gate cross sectional area
2. Resistance in the metal flow system
In practice it is thus possible to have more than a hundred points of intersection available to choose from the choice of the optimum point can be done by plotting a no of lines and using a series of calculation. This exercise takes time and tends to settle for the second best.
Micro computers are used to determine the best parameters.
Besides the locking force the section of the die-casting machine must be based on its PQ characteristics.
The selection of the optimum point of intersection is vital in obtaining the required gate velocity and cavity fill time for the first shot success.
A smaller plunger results in higher metal pressure. It also delivers more metal at low flow rate.
Smaller cross sectional area increases the velocity of the metal and hence higher pressure is required to produce a given flow rate
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