GB1587909A

GB1587909A - Furnace pressurisation of a low pressure die-casting machine

Info

Publication number: GB1587909A
Application number: GB326577A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-01-24
Filing date: 1978-01-24
Publication date: 1981-04-15

Description

(54) IMPROVEMENTS IN THE FURNACE PRESSURIZATION OF A LOW PRESSURE DIE-CASTING MACHINE (71) I JOSEPH AUGUSTINE TERENCE PEREIRA, of No. 2, Ford Close, Shepperton, Middlesex, of British nationality, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an apparatus, for the'low pressure die-casting of metals.

In a low-pressure die-casting apparatus, molten metal is forced upwardly through a riser tube having its lower end extending below the level of molten metal in the bath or crucible of a furnace and having an aperture at its upper end which is connected to the inlet of the die. The molten metal is raised by applying gas pressure to the surface of the molten metal in the furnace; the molten metal rising up the riser tube and into the die where the metal solidifies. The gas pressure is then reduced allowing excess molten metal to fall back down the riser tube to the bath or crucible in the furnace. The casting is then allowed to cool still further after which the die is opened to remove the casting therefrom.

The gas pressure used to raise the molten metal from the crucible in the furnace into the die, performs two main functions. Firstly, it acts as a pump to transfer the molten metal from the crucible up the riser tube and into the die cavity to fill it. The rate of die filling must be closely controlled so as to prevent any undue turbulence which would result in air being trapped within the molten metal resulting in porosity in the casting when the metal solidifies. The second function is to provide an after-pressure in the molten metal as it cools in the die cavity. The cooling metal contracts within the casting and a certain quantity of metal has to be supplied to the liquid core of the casting to compensate for shrinkage during cooling.

In the present state of the art, the normal controls to provide the two desired end-effects are as follows: When furnace pressurisation starts, a simple throttle valve is opened to connect the furnace to a pressure source. The throttle valve regulates the rate of flow of gas into the furnace casing and this gas-flow controls the rate at which the molten metal rises through the riser tube into the die cavity. A pressure regulator valve controlling the pressure level of the external pressure source limits the final pressure attained in the furnace casing, which is reflected as an after-pressure acting on the molten metal cooling in the die cavity.

The gas flow through the throttle valve during the first stage is dependent not only on the throttle valve setting but also on the pressure difference between the external pressure source and the furnace pressure. As the furnace pressure rises, the said pressure difference and the associated gas flow decrease.

The external pressure source, however, is preset to provide the required after-pressure to compact the casting while it is cooling.

Hence the two controls are interdependent resulting in the flexibility of control being impaired.

According to the present invention there is provided a low pressure die-casting apparatus, comprising a furnace for molten metal, a riser tube, one end of which dips into the furnace below the level of molten metal and the other end of which is connected to a die, and a first pressure circuit for applying a gas pressure to the surface of the molten metal to force the molten metal upwardly into the die through the riser tube, and a second pressure circuit for applying an after pressure after the die is filled, said second pressure circuit including a differential pressure switch actuable under the influence of the pressure difference between that in the furnace and a pre-set reference gas pressure whereby on changeover of the differential pressure switch the pressure in- the die on filling of the die is decreased down to or increased up to the reference pressure at a controlled rate, which rate can be adjusted to suit the casting being produced.

The after-pressure is predetermined so as to be that best suited to compact the casting while solidifying. The switching from a first pressure reservoir of the first pressure circuit to a second pressure reservoir of the second pressure circuit is carried out by the differential pressure sensor switch acting under the influence of the pressure difference between the pressure in the furnace and a pre-set reference pressure, whereby when the pressure in the furnace approximately reaches the level of the said reference pressure, the furnace is isolated from the first pressure source and connected to a second pressure source which is maintained at the said reference pressure.

The apparatus in a preferred embodiment is adjustable with respect to the changeover differential whereby the said valve may be optionally biased to swtich over from the first pressure source to the second pressure source at a pressure level higher than the said reference pressure, followed by an adjustable rate of pressure decay down to the reference pressure, or optionally, the second pressure circuit can be arranged so that the switch-over from the first to the second pressure source can occur at a pressure level below the reference pressure, followed by an adjustable rate of pressure rise up to the said reference pressure. In both alternatives, the control of the rate of pressure decay or rise between the switch-over point and the final pressure will be achieved by a throttle valve. Any electrical control system known in the art can be used to incorporate this invention in the die-casting cycle.

In order that the invention may be more readily understood, reference is made to the accompanying drawing which illustrates diagrammatically and by way of example an embodiment thereof and in which the Figure is a circuit diagram of a pressure circuit for a low pressure diecasting apparatus.

Referring to the Figure, a hermetically sealed furnace or crucible contains molten metal which is fed to a die through a riser tube, and lower end of which depends into the molten metal and the upper end of which is connected to the charging aperture of the die.

The molten metal is raised from the furnace to the die by gas pressure applied to the space above the metal level through pipes 14 or 15 as will be described later.

A first pressure circuit effective during a first stage, when the molten metal is raised from the crucible through the riser tube to fill the die, comprises a source of pressure gas 16, a first adjustable pressure control valve 1, a first reservoir 2, a pilot pressure operated pressure control valve with a constant spring bias 3, a first flow control valve 4, a first solenoid actuated 2-way valve 5, a pressure operated 3-way valve 7, a non-return valve 8, and a solenoid actuated 3-way valve 6. The pressure obtaining in the first stage is introduced into the furnace by pipe 14.

A second pressurisation circuit, which is employed to provide an after-pressure while the casting is cooling in the die, comprises the source of pressure gas 16, a second adjustable pressure control valve 9, a second reservoir 10, a second solenoid actuated 2-way valve 11, a second flow control valve 12, a solenoid actuated 4-way valve 21, and a differential pressure operated switch 13 with a spring bias, connected between pipe 18 and pipe 19. The pressure attained in the second stage is introduced into the furnace by pipe 15.

The furnace pressurisation cycle is started by simultaneously energising the solenoid actuated valves 5 and 6. Any suitable control circuit known in the art can be used. Solenoid valve 6 switches on the pilot pressure to the pressure operated 3-way valve 7 through pipe 17, changing the state of valve 7 from a furnace exhaust to furnace pressurisation condition.

Reservoir 2 is now connected to the furnace through valves 3,4, 5 and 7. Valve 4 can be adjusted to give the gas flow rate best suited for the die. The pilot controlled pressure control valve 3 maintains a constant pressure difference across the flow control valve 4.

Valves 3 and 4 used in this manner provide a constant flow of gas, unaffected by downstream pressure conditions. These devices are well known in the art.

Meanwhile, the pressure control valve 9 in the second stage circuit is manually pre-set to provide the desired final pressure best suited for the casting. Gas at this pressure is stored in the second reservoir 10 and a pilot pressure feed is applied to the reference side of the differential pressure switch 13, through the solenoid actuated 4-way valve 21 and pipe 18.

Pipe 19 from the other side of the differential pressure switch 13 is connected to the furnace through the solenoid actuated 4-way valve 21 when the said valve is not actuated as shown drawn on Figure 1. This condition exists when it is required for the swtich-over from the first stage to the second stage to take place at a furnace pressure level which is greater than the said reference pressure. The solenoid actuated 4-way valve 21 is not energised through -out the diecasting cycle.

When the furnace pressure reaches a level equivalent to the said reference pressure plus the spring bias associated with switch 13, the differential pressure switch 13 changes its state and the electrical control circuit is arranged so as to de-energise solenoid actuated valve 5, which isolates the furnace from the first reservoir 2. Simultaneously, solenoid actuated valve 11 is energised. Solenoid actuatec valve 6 and the pressure operated valve 7 are maintained in the energised state to prevent the furnace from exhausting. The second reservoir 10 is now connected to the furnace through pipe 15. The spring bias incorporated in the differential pressure switch 13 allows the furnace pressure to rise above the said reference pressure before the pressure switch changes its state. Since the furnce is at a higher pressure than the pressure in reservoir 10, the gas would flow out of the furnace into reservoir 10 through the second flow control valve 12 and the second solenoid actuated 2-way valve 11, so as to equalise pressures in the furnace and reservoir 10. The rate of pressure decay in the furnace can be controlled by the setting of the second flow control valve 12.

The furnace pressure will eventually stabilize at the reference pressure, which is maintained in the reservoir 10 by pressure control valve 9.

The electrical control circuit is arranged so that the differential pressure switch 13 is isolated electrically for the rest of the die-casting cycle as soon as switch-over to Stage 2 has taken place, so as to prevent a reversal back to Stage 1 conditions when the furnace pressure is allowed to drop to the reference pressure level.

The furnace pressure is maintained for a pre-determined period to allow the metal in the die to solidify. This is effected by a suitable delay circuit incorporated in the control circuit in the usual manner. After the delay has expired, solenoid actuated valves 6 and 11 are de-energised. Valve 11 in the deenergised state isolates the furnace from the second reservoir 10 while the de-energised solenoid valve 6 causes the pressure operated valve 7 to change its state and to exhaust the pressure gas in the furnace through exhaust vent 20, whereby the molten metal in the riser tube is allowed to fall by gravity into the furnace crucible.

Alternatively, the swtich-over from the first to the second pressure circuits and associated first and second pressures can be arranged to take place below the said reference pressure, by maintaining the solenoid actuated 4-way valve 21 and an electrical relay in the energised condition throughout the die-casting cycle. With solenoid actuated 4-way valve 21 in this condition, the gas connections 18 and 19 to the differential pressure switch 13 are now reversed. Pipe 18 is now connected to the furnace while pipe 19 is connected to the reference pressure gas in reservoir 10. With the electrical relay in the energised condition, two electrical connections to the differential pressure swtich 13 are also reversed in readily apparent manner.

Stage 1 of furnace pressurisation will be started in exactly the same manner as described before. When the furnace pressure reaches a level equivalent to the reference pressure minus the spring bias, the differential pressure switch 13 will switch over to start Stage 2 as described before. Since the furnace is at a lower pressure than the pressure in reservoir 10, gas will flow from reservoir 10 into the furnace to raise the furnace pressure. The rate of pressure rise is controlled by the setting of flow control valve 12. The rest of the die-casting cycle will proceed as described before.

While the invention has been described with reference to specific embodiments employing two stages of pressurisation, it can equally be applied to a pressure circuit wherein the first stage when the molten metal is forced up the riser tube and then into the die to fill it, can be carried out in two or more stages.

WHAT I CLAIM IS: 1. A low pressure die-casting apparatus, comprising a furnace for molten metal, a riser tube, one end of which dips into the furnace below the level of molten metal and the other end of which is connected to a die, and a first pressure circuit for applying a gas pressure to the surface of the molten metal to force the molten metal upwardly into the die through the riser tube, and a second pressure circuit for applying an after-pressure after the die is filled, said second pressure circuit including a differential pressure switch actuable under the influence of the pressure difference between that in the furnace and a pre-set reference gas pressure whereby on changeover of the differential pressure switch the pressure in the die on filling of the die is decreased down to or increased up to the reference pressure at a controlled rate, which rate can be adjusted to suit the casting being produced.

2. Apparatus as claimed in Claim 1, in which the said differential pressure switch incorporates a spring, whereby the differential pressure switch is biased to switch over from a first pressure reservoir of said first pressure circuit to a second pressure reservoir of said second pressure circuit at a pressure level in the furnace which is different from the reference pressure by a constant value equivalent to the spring bias.

3. Apparatus as claimed in Claims 1 or 2, in which the differential pressure switch is optionally operable to changeover at a furnace pressure either above or below said reference pressure.

4. Apparatus as claimed in Claim 2 or 3 when dependent on Claim 2 wherein the first pressure circuit connecting the furnace to the first pressure reservoir includes a first adjustable pressure control valve, a first accumulator, a pilot operated pressure control valve with a constant bias, a first flow control valve, a first solenoid actuated two-way valve and a pressure operated three-way valve, actuated by a solenoid actuated three-way valve.

5. Apparatus as claimed in any of Claims 1 to 4 wherein the second pressure circuit connecting the furnace to the second pressure reservoir includes a second adjustable pressure control valve, a second accumulator, a second solenoid actuated two-way valve, a second flow control valve, a solenoid actuated fourway valve and the said differential pressure switch.

6. Apparatus as claimed in Claim 5, whereir the second flow control valve can be adjusted to regulate the rate of decay or rise in furnace pressure to the level of the reference pressure after the differential pressure switch has switched over to the second pressure reservoir.

7. A method of low pressure die-casting in an apparatus having a furnace containing molten metal, a riser tube having one end

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. furnace and reservoir 10. The rate of pressure decay in the furnace can be controlled by the setting of the second flow control valve 12. The furnace pressure will eventually stabilize at the reference pressure, which is maintained in the reservoir 10 by pressure control valve 9. The electrical control circuit is arranged so that the differential pressure switch 13 is isolated electrically for the rest of the die-casting cycle as soon as switch-over to Stage 2 has taken place, so as to prevent a reversal back to Stage 1 conditions when the furnace pressure is allowed to drop to the reference pressure level. The furnace pressure is maintained for a pre-determined period to allow the metal in the die to solidify. This is effected by a suitable delay circuit incorporated in the control circuit in the usual manner. After the delay has expired, solenoid actuated valves 6 and 11 are de-energised. Valve 11 in the deenergised state isolates the furnace from the second reservoir 10 while the de-energised solenoid valve 6 causes the pressure operated valve 7 to change its state and to exhaust the pressure gas in the furnace through exhaust vent 20, whereby the molten metal in the riser tube is allowed to fall by gravity into the furnace crucible. Alternatively, the swtich-over from the first to the second pressure circuits and associated first and second pressures can be arranged to take place below the said reference pressure, by maintaining the solenoid actuated 4-way valve 21 and an electrical relay in the energised condition throughout the die-casting cycle. With solenoid actuated 4-way valve 21 in this condition, the gas connections 18 and 19 to the differential pressure switch 13 are now reversed. Pipe 18 is now connected to the furnace while pipe 19 is connected to the reference pressure gas in reservoir 10. With the electrical relay in the energised condition, two electrical connections to the differential pressure swtich 13 are also reversed in readily apparent manner. Stage 1 of furnace pressurisation will be started in exactly the same manner as described before. When the furnace pressure reaches a level equivalent to the reference pressure minus the spring bias, the differential pressure switch 13 will switch over to start Stage 2 as described before. Since the furnace is at a lower pressure than the pressure in reservoir 10, gas will flow from reservoir 10 into the furnace to raise the furnace pressure. The rate of pressure rise is controlled by the setting of flow control valve 12. The rest of the die-casting cycle will proceed as described before. While the invention has been described with reference to specific embodiments employing two stages of pressurisation, it can equally be applied to a pressure circuit wherein the first stage when the molten metal is forced up the riser tube and then into the die to fill it, can be carried out in two or more stages. WHAT I CLAIM IS:

1. A low pressure die-casting apparatus, comprising a furnace for molten metal, a riser tube, one end of which dips into the furnace below the level of molten metal and the other end of which is connected to a die, and a first pressure circuit for applying a gas pressure to the surface of the molten metal to force the molten metal upwardly into the die through the riser tube, and a second pressure circuit for applying an after-pressure after the die is filled, said second pressure circuit including a differential pressure switch actuable under the influence of the pressure difference between that in the furnace and a pre-set reference gas pressure whereby on changeover of the differential pressure switch the pressure in the die on filling of the die is decreased down to or increased up to the reference pressure at a controlled rate, which rate can be adjusted to suit the casting being produced.

which dips into the furnace below the level of molten metal and the other end which is connected to a die, said method comprising applying a gas pressure via a first pressure circuit to the surface of the molten metal to force the molten metal upwardly into the die through the riser tube to fill the die and then applying an after pressure via a second pressure circuit including a differential pressure switch actuable under the influence of the pressure difference between that in the furnace and a pre-set reference gas pressure whereby the after pres sure is arrived at on changeover at the differential pressure switch from the furnace pressure by decreasing down to or increasing the pressure up to the reference pressure at a controlled rate, which rate is adjustable to suit the casting being produced.

8. A low pressure die casting apparatus constructed and arranged to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.

9. A method of low pressure casting substantially as hereinbefore described with reference to the accompanying drawing.