Method and apparatus for the low-pressure die-casting of metals

• US 4,860,820 A
• Filed: 05/27/1988
• Issued: 08/29/1989
• Est. Priority Date: 07/27/1983
• Status: Expired due to Term

• Current Assignee: A W Plume Limited
• Original Assignee: A W Plume Limited
• Inventors: Pereira, Joseph A. T.
• Primary Examiner(s): Seidel, Richard K.
• Application Number:
  US07/203,589
• Time in Patent Office: 459 Days
• Field of Search: 164/4.1, 164/119, 164/120, 164/150, 164/154, 164/155, 164/306, 164/457
• US Class Current: 164/457
• CPC Class Codes: B22D 18/04 Low pressure casting, i.e. making use of pressures up to a few bars to fill the mould
• CPC Class Codes: B22D 18/08 Controlling, supervising, e.g. for safety reasons controlling or regulating in general G05
• CPC Class Codes: B22D 27/13 making use of gas pressure
• CPC Class Codes: B22D 39/06 having means for controlling the amount of molten metal by controlling the pressure above the molten metal

Abstract

A low-pressure die casting process in which gaseous pressure is applied to molten metal in a bath (C) in three stages from three pressure applying circuits. The first circuit applies pressure to lift molten metal to the charging aperture of the die. When the presence of the metal at that die is detected by a sensor (8), a datum pressure is established and the further application of gaseous pressure to the molten metal is taken over by a second circuit. When the pressure applied to the molten metal differs from the datum pressure by a predetermined amount, an after pressure is applied to the molten metal by the third circuit.

Claims

1. A low pressure die-casting apparatus comprising:

(i) a container for molten metal,

(ii) a riser tube for conveying molten metal from said container to a die having a charging aperture in communication with a cavity,

(iii) a first pressure circuit for applying a first gaseous pressure to the molten metal to force the latter through the riser tube to the charging aperture of the die,

(iv) a second pressure circuit for applying a second gaseous pressure to the molten metal for forcing the latter into the cavity of the die at a controlled rate,

(v) sensor means for establishing for each successive charging operation a datum pressure when the molten metal reaches a predetermined level adjacent said aperture and for causing changeover from said first to said second pressure,

(vi) means for storing gas at a datum pressure,

(vii) a third pressure circuit for applying an after pressure to the molten metal when the die cavity has been filled, the after pressure having a reference value equal to the stored datum pressure plus a constant predetermined difference, said third pressure circuit including pressure control means supplied from a source of pressure gas independently of said first and second pressures and controlled by the datum pressure so as to supply and maintain constant the required after pressure, and

(viii) means in communication with and responsive solely to the second and after pressures to cause changeover from the second pressure to the after pressure at the time the second pressure has risen to that of the reference value and thereafter continuously apply the after pressure on said molten metal until the molten metal in the die has solidified,

(ix) whereby the after pressure is applied during solidification, and despite changes in the molten metal level in the container during successive charging operations, the difference between the after pressure and the datum pressure remains constant.

2. Apparatus as claimed in claim 1 and further comprising means operable by the sensor means for rendering the first circuit inoperative and the second circuit operative when the molten metal reaches the predetermined level.

3. Apparatus as claimed in claim 2 in which means are provided for rendering the second circuit inoperative and the third circuit operative when the gaseous pressure applied to the molten metal to force the latter into the die reaches the value of the reference value.

4. Apparatus as claimed in claim 1 in which means are provided for rendering the second circuit inoperative and the third circuit operative when the gaseous pressure applied to the molten metal to force the latter into the die reaches the value of the reference value.

5. Apparatus as claimed in claim 1 in which the first circuit comprises an inlet connectible to a source of gaseous pressure, a gaseous pressure reservoir, a first gaseous pressure control valve with spring bias interconnecting the inlet and the reservoir, and further flow control means connecting the reservoir to the container for molten metal so as to apply gaseous pressure to molten metal therein.

6. Apparatus as claimed in claim 5 in which the first valve is a pilot-pressure-operated valve and in which the first circuit further includes means for applying a pilot pressure to the first valve.

7. Apparatus as claimed in claim 5 in which the second circuit comprises flow control means for interconnecting the inlet and the container for molten metal so as to apply gaseous pressure to molten metal therein.

8. Apparatus as claimed in claim 1 in which the flow control means of the third circuit includes a second pilot pressure responsive valve with additional adjustable loading, and in which the pilot pressure applied to the second valve is the datum pressure.

9. Apparatus as claimed in claim 1 in which the flow control means of the third circuit also includes a further flow control valve connected between the second pilot pressure responsive valve and the container for molten metal and means for operating the valve to apply the after pressure to molten metal in the container when the predetermined pressure difference is achieved.

10. Apparatus as claimed in claim 1 in which the sensor means comprises a sensor for responding when the molten metal reaches the predetermined level, a flow control valve operable by the sensor and interconnecting the container for molten metal to a second reservoir in such manner that the gaseous pressure acting on the molten metal is communicated to the second reservoir, the pressure therein constituting the datum pressure.

11. Apparatus as claimed in claim 10 in which the second reservoir is connected to the second pilot pressure responsive valve to provide the pilot pressure thereto.

12. Apparatus as claimed in claim 1 in which the predetermined level is that of the top of the riser tube or that of the charging aperture.

13. Apparatus as claimed in claim 1 wherein said third pressure circuit includes pressure control means having an inlet connected to a source of pressure gas so as to be supplied independently of said first and second means, an outlet connected to the container for supplying the after pressure, a pilot inlet connected to the means for storing said datum pressure gas for operating the pressure control means in one sense, an adjustable control determining the pressure difference for operating the pressure control means in the same sense, and means for applying the after pressure to operate the pressure control means in the opposite sense to the effect of the datum pressure and adjustable control so as to maintain the after pressure at the reference value represented by said datum pressure plus said predetermined difference determined by the adjustable control.

14. A method as claimed in claim 13 in which the value of the after pressure is derived from the datum pressure by increasing the latter a predetermined amount.

15. A method of low-pressure die casting comprising the steps of:

(i) applying a first gaseous pressure to molten metal to be cast to convey such metal from a source thereof to a die cavity in a succession of charging operations,

(ii) storing for each successive charging operation the value of a datum pressure necessary to carry the molten metal to a charging aperture of the die,

(iii) thereupon applying a second pressure to carry the molten metal into the die cavity,

(iv) deriving an after pressure supply independent of said first and second pressures having a reference value equal to the stored datum pressure for that charging operation plus a constant predetermined difference,

(v) detecting when the value of the second pressure has risen to that of the reference value,

(vi) removing the second pressure whenever it has risen to that of the reference value and applying the after pressure until the metal in the die has solidified, and

(vii) continuously monitoring and controlling the after pressure being supplied to maintain it constantly at the desired value until the metal in the die has solidified.

16. A method of low-pressure die casting comprising applying to molten metal in a furnace a gaseous pressure sufficient to convey the molten metal to a charging aperture of a die and thereafter to fill the die, storing a datum pressure at the charging aperture to compensate for variance of the furnace pressure prior to each casting operation;

adding to said datum pressure a pressure independent of said gaseous pressure to obtain a reproducible value after pressure; and

continuously monitoring and controlling said after pressure during solidification of the metal in the die to maintain said after pressure at the reproducible value.