US3823760A

US3823760A - Low pressure die casting apparatus

Info

Publication number: US3823760A
Application number: US00296810A
Authority: US
Inventors: F Bloomfield; G Cooper
Original assignee: SOAG MACHINERY Ltd
Current assignee: SOAG MACHINERY Ltd
Priority date: 1971-10-18
Filing date: 1972-10-12
Publication date: 1974-07-16
Anticipated expiration: 1991-07-16
Also published as: FR2156725B1; FR2156725A1; SE389980B; ES407495A1; BE790179A; CH550040A; GB1365938A; DE2250780A1; AU465221B2; IT972205B; AU4761672A; JPS4849625A; DK129506C; CA965576A; DK129506B

Abstract

A method of low pressure die casting in which molten metal is forced upwardly from a furnace into a die through a riser tube extending below the level of the molten metal in the furnace by gas pressure applied to the surface of the metal. To avoid the formation of oxides in the metal, an inert gas, heavier than air, is introduced into the riser tube adjacent the die to fill the riser tube above the level of the metal in the furnace. On application of gas pressure to the molten metal in the furnace, as the metal rises in the tube, the inert gas expels out from the die cavity. After solidification of the metal in the die and removal of the gas pressure to the furnace, excess molten metal falls back by gravity into the furnace, the inert gas again filling the space above the metal in the tube to prevent ingress of air at the interface between the sprue orifice of the die and the riser tube. A low pressure die casting apparatus is disclosed which includes an apertured cap on the riser tube for engagement with the sprue orifice, the cap being provided with an inlet for an inert gas.

Description

United States Patent [191 Bloomfield et a1.

[11] 3,823,760 [451 July16,1974

LOW PRESSURE DIE CASTING APPARATUS Soag Machinery Ltd., Brentford, England Filed: Oct. 12, 1972 Appl. No.: 296,810

Assignee:

Foreign Application Priority Data Oct. 18, 1971 US. Cl 164/113, 164/306 Int. Cl B22d 17/14 Field of Search 164/113, 119, 120, 66,

Galloway... 164/119 X Strom 164/62 Chatourel 164/119 X Great Britain 48387/71 Ho y 164/67 Primary Examiner-.1. Spencer Overholser Assistant ExaminerJohn S. Brown 57] ABSTRACT A method of low pressure die casting in which molten metal is forced upwardly from a furnace into a die through a riser tube extending below the level of the molten metal in the furnace by gas pressure applied to the surface of the metal. To avoid the formation of oxides in the metal, an inert gas, heavier than air, is introduced into the riser tube adjacent the die to fill the riser tube above the level of the metal in the furnace. On application of gas pressure to the molten metal in the furnace, as the metal rises in the tube, the inert gas expels out from the die cavity. After solidification of the metal in the die and removal of the gas pressure to the furnace, excess molten metal falls back by gravity into the furnace, the inert gas again filling the space above the metal in the tube to prevent ingress of air at the interface between the sprue orifice of. the die and the riser tube. A low pressure die casting apparatus is disclosed which includes an apertured cap on the riser tube for engagement with the sprue orifice, the cap being provided with an inlet for an inert gas.

1 Claim, 1 Drawing Figure LOW PRESSURE DIE CASTING APPARATUS BACKGROUND OF THE INVENTION This invention relates to a low pressure die casting method and apparatus therefor. I r

In a low pressure die present operation, molten metal is forced upwardly from a furnace into a die through a riser tube extending below the level of the molten metal in the furnace. The molten metal is fed to the die by applying gas pressure to the surface of the molten metal in the furnace; the molten metal rising up the riser tube into the die where the metal solidifies The gas pressure is then reduced or removed allowing excess molten metal to return via the riser tube to the furnace following which the die is opened and the casting removed from the die.

Problems encountered in low pressure die casting are caused by the generation of oxides in the metal due to air pressurisation, turbulence during the pouring of molten metal into the crucible and the introduction of air when the casting is removed from the die. All these factors cause adulterationof the metal; the structure of the metal in the crucible being modified and the unwanted, detrimental metal oxides being contained in the finished casting.

SUMMARY OF THE INVENTION An object of the present invention is to overcome the disadvantages encountered with present low pressure die casting methods by preventing the generation of metal oxides within the riser tube thereby minimising oxide contamination in the final casting.

A further'object of the present invention is to improve the suitability of low pressure die casting methods for light metals and alloys such as magnesium and magnesium alloys.

With the foregoing and other objects in view, the present invention consists in a method of low pressure die casting, in which molten metal from a closed heated container is conveyed to the die through a riser tube by gas pressure applied to the surface of the metal in said container, one end of which tube extends below the level of the molten metal and the other end of which is in contact relationship with the sprue orifice of the die, wherein an inert gas is introduced into the riser tube adjacent said other end of the riser tube whereby as the molten metal rises up the tube air contained in the die cavity is expelled through vents in the die during charging of the die and as the metal in the riser tube returns to the container on removal of the gas pressure applied to the surface of the metal said inert gas substantially prevents the ingress of air at the interface between said other end of the riser tube and the sprue orifice and fills the space above the metal in the tube.

The invention also consists in a low pressure die casting apparatus, comprising a furnace for containing molten metal, a die mounted over the furnace, a sprue orifice in said die, a riser tube having one end extending below the level of the molten metal in the furnace and its other end aligned with the sprue orifice in liquid tight relationship, means for applying gas pressure to the surface of the molten metal in the furnace to force the metal through said tube into the die via the sprue orifice, and means for introducing an inert gas into said tube adjacent said other end of the tube.

BRIEF DESCRIPTION OF TIIE DRAWING 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, one embodiment in axial section through the upper part of a low pressure die casting apparatus.

DETAILED DESCRIPTION OF THE DRAWING Referring to the drawing, the die is formed in two halves, the top half 2 of the die being attached to a movable top platen 1 and the bottom half 3 being se cured to a stationary bottom platen 4 and defining with the top half 2 a die cavity 5 of the die. The configuration of the die allows vents 6 to be provided in the mating surfaces between the top and

bottom halves

2, 3 of the die.

Molten metal 8 in a furnace 9 having a lid 10 is conveyed into the die cavity 5 through a riser tube 11 by gas pressure, introduced through an inlet 12 controlled by a valve 13, acting on the'surface 14 of the molten metal in the furnace 9. The lower end of the riser tube 11 extends below the surface 14 of the molten metal in the furnace 9 whilst the upper end of the riser tube 11 has an apertured riser cap 15 which in use is aligned in liquid tight relationship coaxially with the sprue orifice 7 A flexible sealing means in the form of a bellows assembly 16 of heat resistant material is located between the lid 10 of the furnace 9 and a collar 17 of the riser cap 15. The bellows assembly 16 is concentric with the riser tube 11 and may be compressed or expanded by wedge means 18 for sealing the interior of the furnace 9 and the exterior of the upper part of the tube 11 from the ambient. The expansion of the bellows assembly 16 exerts a force on the collar 17 to raise the riser tube 11 and press the upper surface of the riser cap 15 against the sprue orifice 7 to form a liquid tight seal between the riser cap 15 and the sprue orifice 7. Other means may be employed for connecting the riser tube 11 to the sprue orifice 7 omitting the bellows assembly and the wedge actuating mechanism completely.

An inlet 1? is provided in the riser cap 15 for introducing an inert gas, heavier than air, into the riser tube 11. Into the aperture 20 the cap 15 there is mounted a nozzle member 21 which extends axially into the riser tube below the level of the inlet 19. The inlet 19 is controlled by a valve 22.

In operation, inert gas, heavier than air, is continuously introduced, at low pressure into the inlet 19 and fills the cavity within the riser tube 11 above the level 14 of the molten metal 8. The amount of gas introduced is sufficient to make up the leakage through the die vents 6 but not sufficient to cause any turbulence in the riser tube 11. When air under pressure is supplied to the furnace 10 via valve 13 and inlet 12, the molten metal 8 is forced up the riser tube 11 pushing the inert gas before it into the die cavity 5 and flushing out air contained in the die cavity, which is vented to the atmosphere through the vents 6. As the metal rises in the tube 11 and passes the nozzle member 21 a back pressure is produced in the cavity 23 formed between the internal wall of the riser cap and the exterior wall of the nozzle member 21 which may be used to shut off or reduce the flow of inert gas whilst maintaining a cushion of gas in the cavity 23.

When solidification of the metal in the die cavity 5 occurs, the pressure of air to the furnace is shut off and excess molten metal returns under gravity down the riser tube 11 into the furnace. As the molten metal clears the lower end of the nozzle member 22 the back pressure exerted on the inert gas is removed and the flow of inert gas is restarted immediately substantially to prevent ingress of air into the riser tube. Thus the formation of oxides on the exposed surface of the metal in the riser tube is prevented since the tube is filled with the gas.

To remove the casting, the upper die 2 is raised thus providing access for air into the sprue orifice 7. However, the ingress of air is prevented from entering the riser tube by the inert gas filling the riser tube above the level of the molten metal therein. When the casting has been removed from the die, the upper die 2 is replaced and the casting operation repeated.

If desired, a control system is incorporated in the system to coordinate the introduction of the inert gas with furnace pressurisation. When pressure is introduced into the furnace 9, a signal is'transmitted from control valve 13 to control valve 22 to initiate the flow of inert gas through inlet 19 to scavenge the die cavity 5. After the die cavity 5 has been-filled with molten metal, a time delay is introduced to allow sufficient time for solidification of the metal casting. During this time valve 13 is closed and the furnace vented to permit the excess molten metal in the riser tube 11 to fall by gravity back into the furnace, the inert gas meanwhile continuing to be introduced to fill the space above the metal in the tube. After a further time delay, the upper platen 1 is raised to allow removal of the upper die 2 and the casting. At this point in the cycle, a signal is sent from the platen l to the control valve 22 to actuate the valve 22 to shut-off the supply of inert gas. Normally, valve 22 can only be reopened in response to further signal from the control valve 13. However, in order to meet the situation which prevails at the beginning of a casting cycle, an override switch 22a is provided to allow valve ll to be opened independently of control valve 13 when the riser tube 12 is not filled with inert gas, but after the initial operation, the valve 22 is controlled solely by valve 13. The control signals may be electrical signals or fluid pressure signals.

We claim:

1. A method of low pressure die casting, in which molten metal from a closed heated container is conveyed to the die by means of gas pressure applied to the surface of the metal in said container, through a riser tube one end of which extends below the level of the molten metal and the other end of which has an inlet nozzle member in contact relationship with the sprue orifice of the die, said inlet nozzle member extending axially into the riser tube below an inlet for an inert gas which is introduced into said other end of the riser tube adjacent said inlet nozzle member to fill the riser. tube with said inert gas above the level of the molten metal when the gas pressure applied to the surface of the molten metal is removed, wherein as the molten metal rises up the tube expelling said inert gas through vents in the die a back pressure is formed between said inlet nozzle member and said inert gas inlet to maintain a cushion of inert gas in said other end of the riser tube adjacent said inert gas inlet to prevent ingress of molten metal into said inlet.

Claims

1. A method of low pressure die casting, in which molten metal from a closed heated container is conveyed to the die by means of gas pressure applied to the surface of the metal in said container, through a riser tube one end of which extends below the level of the molten metal and the other end of which has an inlet nozzle member in contact relationship with the sprue orifice of the die, said inlet nozzle member extending axially into the riser tube below an inlet for an inert gas which is introduced into said other end of the riser tube adjacent said inlet nozzle member to fill the riser tube with said inert gas above the level of the molten metal when the gas pressure applied to the surface of the molten metal is removed, wherein as the molten metal rises up the tube expelling said inert gas through vents in the die a back pressure is formed between said inlet nozzle member and said inert gas inlet to maintain a cushion of inert gas in said other end of the riser tube adjacent said inert gas inlet to prevent ingress of molten metal into said inlet.