US3844331A - Method of regulating the pressure in low-pressure casting plants - Google Patents

Abstract

Method of regulating the pressure in a low-pressure casting plant, wherein in a first step the furnace is supplied with two gas inputs, one of these two inputs being discontinued when the metal engages a first control means, the pressure in said furnace being memorized when the metal engages a second control means, and in a second step gas is supplied to the furnace until the mould is properly filled, whereafter the previously memorized pressure is utilized for supplying both gas inputs to the furnace; in a third step the memorized pressure is utilized for discontinuing one of the two inputs and regulating the pressure in said furnace during the setting of the metal in the mold.

Description

United States Patent 3,844,331

Py et al. [451 Oct. 29, 1974 54] METHOD OF REGULATING THE 3,768,542 10/1913 Bosworth m1. l64/l55 PRESSURE IN LOW'PRESSURE CASTING FOREIGN PATENTS OR APPLICATIONS PLANTS 4,012,974 6/1965 Japan 164/155 1' [75] Inventors: Alphonse Benjamin Py; Jean Henri Lefebvre; Jean-Claude Pichouron, all of Billancourt, France [73] Assignee: Regie Nationale des Usines Renault,

Billancourt, France [22] Filed: June 8, 1973 [21] App]. No.: 368,341

[30] Foreign Application Priority Data June 22, 1972 France 72.22546 [52] US. Cl. 164/4 [51] Int. Cl B22d 17/32 [58] Field of Search 164/119, 155, 306, 309

[56] References Cited UNITED STATES PATENTS 2,847,739 8/1958 Sylvester 164/119 3,302,254 2/1967 Moriyama 164/155 3,425,483 2/1969 Dearth 164/155 Primary Examiner.l. Spencer Overholser Assistant Examiner-John E. Roethel Attorney, Agent, or FirmStevens, Davis, Miller & Mosher [5 7] ABSTRACT Method of regulating the pressure in a low-pressure casting plant, wherein in a first step the furnace is supplied with two gas inputs, one of these two inputs being discontinued when the metal engages a first control means, the pressure in said furnace being memorized when the metal engages a second control means, and in a second step gas is supplied to the furnace until the mould is properly filled, whereafter the previously memorized pressure is utilized for supplying both gas inputs to the furnace; in a third step the memorized pressure is utilized for discontinuing one of the two inputs and regulating the pressure in said furnace during the setting of the metal in the mold.

6 Claims, 3 Drawing Figures The present invention relates to a method of controlling or regulating the pressure in a low-pressure casting plant comprising a heated furnace containing molten metal and a gaseous space overlying said molten metal, a mould disposed externally and above said furnace, a dipper pipe connecting the bottom of said furnace to the bottom of said mould (bottom casting), first control means inserted between said furnace and said mould bottom, second control means located in the mould bottom, and third control means disposed in the upper portion of said mould, said control means being adapted to regulate the supply of gas to said gaseous space.

In methods of this general type a homogenous quality of the castings cannot be obtained unless the various cycles and operating conditions, and notably the following elements thereof: the gas input in the furnace, which controls the rate whereat the metal rises in the mould; the gas pressure in the furnace and the overpressure exerted on the metal contained in said mould during the setting period, which overpressure is subordinate in turn to the temporary pressure increment in the furnace which follows the mould filling step, are kept as constant as possible.

It is also known that the gas pressure above the molten metal surface in the furnace should be adjusted to keep the mould filling rate at a value consistent with the maintaining of satisfactory casting conditions, irrespective of the momentary metal level in the furnace.

A pressure control method consisting in compensating the increment in the gaseous volume contained in the furnace after each injection of molten metal into the mould is already known. To this end, a reserve of gas is provided for compensating or making up this increment in the gaseous volume at each run of the casting plant, in order to keep the injection time at a substantially constant value.

However, this known method is objectionable in that the elasticity of the gaseous medium and the volumetric differences due to temperature differentials prevent the accurate reproduction, at each run, of the mould filling rate and also of the pressure exerted on the metal contained in this mould.

This is due mainly to the fact that the gas fed under pressure into the furnace is exposed to sudden and important temperature increments such that its initial volume is increased very considerably. Furthermore, the errors occurring in the adjustment of the gas volumes added successively in relation to the initial volumes of gas contained in the furnace increase in the same proportions. Under these conditions, it is clear that this method cannot be applied unless the furnace is perfectly tight, a requirement obviously very seldom attained at the high temperatures usually employed in such plants. In addition, compensating the volume increment in the furnace will not change appreciably the value of the overpressure to be exerted on the liquid metal contained in the mould.

It is the primary object of the present invention to avoid these inconveniences by providing a method of regulating the pressure in a low-pressure casting plant of the type broadly set forth hereinabove, this method being characterised in that, in a first step, the furnace is supplied with two separate gas inputs, one of these two inputs being discontinued when the metal engages the first control means while memorizing the pressure in said furnace when the metal engages the second control means, that in a second step gas is supplied to said furnace until the mould is properly filled, the memorized pressure being then utilized for delivering the two gas inputs to the furnace, and that in a third step the memorized pressure is utilized for discontinuing at least one of said two inputs and regulating the pressure in the furnace during the solidification of the metal in said mould.

A. typical example of a practical application of the method of this invention will now be described in detail with reference to the accompanying drawing, in which:

FIG. 1, divided into two fragmentary FIGS. 1A and 1B are to be read from left to right, respectively, illustrates the wiring and flow diagram of the plant operating according to the method of this invention, the two FIGS. 1A and 18 being readable together by aligning same at the reference letters a to h; and

FIG. 2 is a diagram plotting the time (in abscissa) vs. the gas pressure p (in coordinates) in the gaseous space in the furnace during a run.

In FIG. 1, a furnace or crucible 1 containing molten metal 2 has a gaseous space 3 overlying said metal and containing a gas under pressure. This furnace l is heated in order to keep the metal 2 in the molten or liquid state. A mould 37 disposed at a higher level than said furnace 1 has its bottom connected via a plunger or dipper pipe 38 to the bottom of said furnace. Thus, when pressure gas is fed to the gaseous space 3, one fraction of the molten metal 2 is caused to rise through the pipe 38 into the mould 37, in which this fraction of molten metal 2 solidifies, and then the thus solidified casting is removed after allowing the non-solidified metal in pipe 38 to flow back into the furnace or crucible, and this operation is repeated several times. The pipe 38 encloses an electric switch 52 constituting a first control device, and the lower portion or bottom of the mould 37 encloses another electric switch 53 constituting a second control device; a third electric switch 54 constituting a third control device is enclosed in the uppermost portion of the mould. Each one of these three electric switches is adapted to close a separate electric circuit, as will be explained presently.

Gas under pressure is supplied from a source 4 (upper left-hand corner of FIG. 1A) via a feed line to the system. More particularly, pressure gas is supplied from this line 60 to the gaseous space 3 of furnace 1 (previous filtration in a suitable filter 5 in which the gas is both purified and dried) through a pressureregulator 21 and a pair of valves 18 and 18a. The gas pressure in feed line 60 is in the range of 3 to 6 bars. The gaseous space 3 is thus pressurized through feed line 60, filter 5, regulator 21 and the pair of solenoidoperated valves 18 and 18a, and adapted to be vented to the atmosphere by means of a valve 16 responsive to another solenoid-operated valve 11.

The feed line 60 comprises branch lines 7, 8, 9, l0 supplying gas to three-way solenoid-operated valves l2, l3, l4 and a pressure reducing device 15. Valves l2 and 13 supply gas to a pair of three-way distributors l7 and 17a, respectively, controlling in turn, in a manner to be described presently, a pair of output- control valves 18 and 18a permitting the passage of a relatively large output (valve 18) and a relatively small output q (valve 18a) of gas to point 40 of feed line 60 leading to said space 3.

Another solenoid-operated valve 14 controls a passage valve 33 and the outlet 41 of pressure reducing device 15 is connected to a conduit 32. Branch line of feed line 60-leads to a pressure reducingdevice 21 monitored in a manner to be explained presently.

Another conduit 42 is connected permanently to the gaseous space 3, a first pressure exchange device 25 consists of a vessel containing a liquid overlying which is a gas communicating with said conduit 42, a first liquid column 28 communicates with the liquid in the first exchanger 25, thus displaying at any time the pressure existing in the gaseous space 3; a second pressure exchanger 27 is constituted like the first one 25, and a second liquid column similar to the first column 28 is provided. As will be explained presently, the passage valve 33 shown in its closed condition opens at the beginning of a cycle of operations and then interconnects the second column 30 (of which the height corresponds in this case to the pressure in space 3) and the second exchanger 27. The passage valve 33 is subsequently closed, the second column 30 being thus isolated, and its height will remain constant. This constant height will be utilized in the manner to be set forth hereinafter as a reference or rated pressure in the subsequent operations of the cycle.

A pressure comparator 31 encloses a first chamber 44 communicating with the second column 30 and thus in this first chamber 44 the above-mentioned reference or rated pressure will prevail; said pressure comparator 41 also includes another chamber 46 supplied with gas through a throttled passage 57 from a conduit 32. The pressure comparator 31 equalizes the pressure in its two chambers 44 and 46, so that from the time in a cycle of operations when the second column 30 displays the reference or rated pressure this same pressure also prevails in the second chamber 46 and in the conduit 43 connected thereto. The pressure prevailing in conduit 43 is utilized in a manner to be explained presently for controlling certain operations of the cycle and thus constitutes the monitoring pressure. A third pressure exchanger 26 is similar to the first and second pressure exchangers 25 and 27, and a third liquid column 29 corresponding to the third exchanger 26 is similar to the first and second liquid columns 28 and 30. The gaseous atmosphere in the third exchanger 26 is connected to a conduit 43 whereby the third column 29 will show at any time the monitoring pressure constantly remaining substantially equal to the reference or rated pressure.

Two distributors 19a and 19 responsive to pneumatic differential control means are mounted in the manner illustrated between the pair of conduits 42 and 43; these distributors are adjusted for measuring the discrepancy existing between the monitoring pressure and the furnace pressure, and to deliver a control signal to regulator 21 and valves 18 and 18a through distributors 17a and 17 which can be closed when an electric signal is fed to the solenoid-operated valves 12 and 13. The distributors l9 and 19a are set differently to afford a certain off-set between the closing of the large-output Q and the closing of the small output q.

Three differential pressure- responsive switches 34, and 36 are also connected between the conduits 42 and 43. The first differential switch 34 is adapted to cancel the pressures issuing from distributors 17 and 17a in case the reference or rated pressure were overstepped. The other differential switch 35 is adapted to cut off the large-output flow Q when an electric pulse is supplied to its winding. Likewise the differential switch 36 is adapted to cut off the small-output flow q when an electric pulse is supplied to its winding. These pressure responsive switches 34, 35 and 36 are adapted to preserve the continuity of the injection of molten metal into the mould in case of failure of distributors 19 or 19a.

The pressure reducing device 21 is monitored through conduit 20a by the output of 19 which may be discontinued by actuating the valve 17. This device 21 supplies gas to a line 22 divided into two branch lines 23 and 23a leading to the valves 18 and 18a, respectively, under the control of throttle valves 24 and 24a, respectively.

Electric lines shown diagrammatically at 47, 48 and 49 permit of actuating the solenoid-operated valves 13 and 14 from switches 52, 53, 54 in a manner to be disclosed presently.

The operation of the plant will now be described in detail with reference to FIG. 2 in connection with the successive operations or steps of a cycle.

The gaseous space 3 of furnace l is initially at the atmospheric pressure. This is shown at point A of the diagram of FIG. 2.

The solenoid-operated valves l1, 12, 13 and 14 are actuated simultaneously. Valve 11 cuts off the communication between conduit 6 and the atmosphere through the discharge valve 16. Valve 14 opens the passage valve 33. The pressure in space 3 is transmitted via conduit 42, exchanger 27 and valve 33 delivering at 43 a monitoring pressure equal to the pressure in space 3, thus permittingthe pressure recording phase.

The pair of solenoid-operated valves 13 and 21 are provided for monitoring the pair of distributors 17a and 17, respectively. The other distributors 18a and 18 are monitored by distributors 19a and 19, respectively through valves 17a and 17, respectively, permitting the passage of outputs Q and q, respectively, for supplying gas to the space 3. The pressure increases rather rapidly in this space 3 and drives the molten metal 2 which rises in pipe 38 and thus actuates the first switch 52. This is illustrated by section AB of the curve of HO. 2, whereby the metal rises at a relatively high rate in said pipe 38.

Through the connection 47 switch 52 cuts off the energization of solenoid-operated valve 13, thus discontinuing the monitoring of distributor 17a, and no gaseous flow is permitted through valve 18a, so that the large output 0 is eliminated.

The molten metal continues to rise in pipe 38 but under the control of the small gas output q, until the level of liquid metal is such as to actuate the main switch 53. This is shown in section BC of the curve. Through another connection 49 switch 53 cuts off the energization of solenoid-operated valve 14 and valve 33 is closed, thus isolating the column 30. This column 30 will remain isolated during the subsequent operations of the cycle, and thus creates a fixed pressure which will be utilized for performing these subsequent operations; this fixed pressure is the above-mentioned reference or rated pressure.

The pressure prevailing in the furnace and in the conduit 42 continues to rise slowly and becomes higher than the reference or monitoring pressure produced in column 43, and the distributor 19 controlled by the difference between the two pressures will deliver a pressure signal modulated by this difference and providing an output q. Thus, the molten metal rises regularly and slowly into the mould 37 until it engages the third switch 54. This is shown in section CD of the curve, during which the pressure rises slowly in the furnace.

The third switch 54 actuates the solenoid-operated valve 13 through another connection 48, thus restoring the large output Q through valve 18a and the sequence of steps explained in the foregoing, whereby the pressure rises again at a high rate in the furnace and in conduit 42. This is illustrated by the curve starting at point D. Given a sufficient adjustment difference between the pressure in conduit 42 and the monitoring pressure in conduit 43, the distributor 19a assumes a different position in that it cuts off the flow of gas towards distributor 17a, whereby the latter will cut off the gas flow through valve 18a, and said large output Q is discontinued. This condition corresponds to point E in the curve of FIG. 2, of which section DE has been followed. During this section DE an overpressure is applied to the metal body contained in the mould. Beyond point E there is still a gaseous flow through distributor 19, space 3 is still supplied with gas at said output q, and the pressure continues to increase therein until it exceeds the monitoring pressure by a quantity equal to the preset value of distributor 19. The latter cuts off the small output q, and thus the curve section EF is completed, during which the overpressure in the mould was still increasing. The molten metal then solidifies slowly in the mould, and this correspond to curve section FG.

After a cooling time (point G) the electromagnetic valves 11 and 12 are deenergized.

Valve 12 turns off the small output and valve 11 relieves the furnace pressure by venting same to the atmosphere through valve 16, and this corresponds to section GH of the curve of FIG. 2. The liquid metal contained in pipe 38 then flows back into the furnace, and the final point I of the curve corresponds to the stripping or removal of the casting from the mould.

Although a specific form of embodiment of this invention has been described hereinabove and illustrated in the accompanying drawing, it will readily occur to those skilled in the art that various modifications and changes may be brought thereto without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. In a method of regulating the pressure in a lowpressure casting plant comprising a heated furnace containing molten metal solid at room temperature and a gaseous space overlying the molten metal, a mould disposed externally and above said furnace, a dipper pipe connecting the furnace bottom to the mould bottom, first control means located between said furnace and the mould bottom, second control means disposed in the bottom of said mould, and third control means disposed in the upper portion of said mould, the furnace is supplied in a first step with two separate gas inputs, one of these two inputs being discontinued when the metal engages said first control means while memorizing the pressure in said furnace when the metal engages said second control means; in a secondstep gas is supplied to said furnace until the mould is properly filled, the memorized pressure being then utilized for delivering the two gas inputs to the furnace, and in a third step the memorized pressure is utilized for discontinuing at least one of said two inputs and regulating the pressure in the furnace during the solidification of the metal in said mould.

2. In the method as set forth in claim 1, one of the two gas inputs which is discontinued is greater than the other, non-discontinued input.

3. in the method as set forth in claim 1, said two gas inputs are monitored from a source of gas under pressure.

4. In the method as set forth in claim 2, the monitoring pressure of the larger of the two inputs is eliminated when the metal engages the first control means.

5. In the method as set forth in claim 4, the memorized pressure is converted into a pressure for controlling and monitoring safety distributors.

6. In the method as set forth in claim 5, the gas outputs are responsive to the discrepancy existing between the gaseous pressure in the furnace and the memorized pressure.

Claims (6)

1. In a method of regulating the pressure in a low-pressure casting plant comprising a heated furnace containing molten metal solid at room temperature and a gaseous space overlying the molten metal, a mould disposed externally and above said furnace, a dipper pipe connecting the furnace bottom to the mould bottom, first control means located between said furnace and the mould bottom, second control means disposed in the bottom of said mould, and third control means disposed in the upper portion of said mould, the furnace is supplied in a first step with two separate gas inputs, one of these two inputs being discontinued when the metal engages said first control means while memorizing the pressure in said furnace when the metal engages said second control means; in a second step gas is supplied to said furnace until the mould is properly filled, the memorized pressure being then utilized for delivering the two gas inputs to the furnace, and in a third step the memorized pressure is utilized for discontinuing at least one of said two inputs and regulating the pressure in the furnace during the solidification of the metal in said mould.

2. In the method as set forth in claim 1, one of the two gas inputs which is discontinued is greater than the other, non-discontinued input.

3. In the method as set forth in claim 1, said two gas inputs are monitored from a source of gas under pressure.

4. In the method as set forth in claim 2, the monitoring pressure of the larger of the two inputs is eliminated when the metal engages the first control means.

5. In the method as set forth in claim 4, the memorized pressure is converted into a pressure for controlling and monitoring safety distributors.

6. In the method as set forth in claim 5, the gas outputs are responsive to the discrepancy existing between the gaseous pressure in the furnace and the memorized pressure.

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