Method For Controlling Die Temperature And For Pacing The Casting Cycle In A Metal Die Casting Operation

Abstract

The method disclosed herein involves placing temperature sensors, such as thermocouples, in a die assembly to sense the operating temperature in both the normally cool zone and the normally hot zone of the die. Temperature distribution in the die is balanced by regulating flow of cooling water into the die to bring temperature of cool zone and hot zone as close together as possible. The temperature sensors are associated with an indicating controller and signal means to provide a visible signal indicative of the desired operating temperature range. Casting cycle pace is adjusted according to signal produced to maintain the die temperature within the optimum operating range which will enable production of castings free of defects.

Description

BACKGROUND OF THE INVENTION

The invention relates broadly to an improved method for metal die casting and, more specifically to a method for automatically controlling temperature distribution in a die assembly to enhance production of salable castings.

In a die casting operation, predetermined amounts of molten metal, called shots, may be dispensed automatically into the plunger-equipped shot well of a die casting machine, by pumping the shots from a melting pot or similar source. Alternatively, the shots may be dispensed into the shot well manually by transferring the heated metal in a ladle or similar device from the melting pot into the shot well. The plunger, in coordination with the intermittent action of the pump, injects the metal shot into the die cavity of a close, multipieced die assembly (usually two major die pieces). The metal usually solidifies in the die cavity within a few milliseconds and the casting is ejected from the open die. The procedure is repeated with the die closed to cast additional pieces.

The optimum casting cycle required to produce salable castings depends on various factors such as the geometry of the die cavity, the size of the casting, the type of metal being cast and, most significantly, by the heat input into the die assembly. Heat input into the die is directly related to the amount of metal and the cycling rate, so that the die temperature during casting is chiefly determined by such heat input. To regulate the heat input, the common practice is to pass a cooling liquid, usually water, through channels positioned behind the die cavity. Water flow into the die is usually controlled by manually opening and closing valves in the water inlet lines to keep the die cool. The manual operation is somewhat unsatisfactory, however, in that it frequently results in the die temperature being too hot or too cold, causing casting defects visible to the operator. The substantial amount of scrap resulting from the known die casting methods, therefore, emphasizes a need in the art for a method which overcomes the problem.

SUMMARY OF THE INVENTION

One broad object of the present invention is a method for controlling the temperature distribution in a die assembly to achieve an optimum temperature range which will produce a salable metal die casting.

Another broad object is a method for pacing the casting cycle to achieve optimum production of metal die castings.

A more specific object of the invention is a method for die casting magnesium base alloys in which flow of cooling liquid to the die assembly is manually or automatically regulated to minimize the temperature difference between the hottest and coolest portions of the die cavity.

Broadly stated, a preferred die casting method of the present invention comprises positioning a temperature sensor, such as a thermocouple, in the lowest operating (i.e. coolest) temperature zone of the die cavity on one side of a die assembly and another temperature sensor in the highest operating (i.e. hottest) temperature zone of the die cavity in the opposite side of the die assembly. More than one temperature sensor may be used on each side of the die assembly if the size of geometry of the die cavity requires it. The temperature sensors are associated with an indicating controller and operatively connected to a signal means on the controller which gives a visible signal in response to the temperature indicated by the temperature sensors. The die assembly is heated to a predetermined temperature range operable for casting a metal suitable for die casting, such as a magnesium base alloy. To commence the die casting cycle, a cooling liquid, preferably water, is passed through the die assembly. Flow of cooling liquid into the die assembly is adjusted until the temperature differential between the temperature sensors is within about 10.degree. F. of the median temperature point of the predetermined operable temperature range. Once the temperature differential between the sensors is determined, the low limit and high limit casting temperatures are set on the indicating controller. The die casting cycle may then be maintained at an optimum pace to hold the temperature of the die casting assembly between the low limit and high limit casting temperature, as a result of visually observing the signal produced by the signal means of the indicating controller.

BRIEF DESCRIPTION OF THE DRAWING

The single FIG. of the drawing is a front elevation view of one side of a die assembly and, in schematic, an indicating controller and cooling liquid piping associated with the die assembly, illustrating typical die casting apparatus employed in the practice of the invention.

The apparatus illustrated herein represents only one embodiment of apparatus which may be used in the practice of the invention, the form shown being selected for convenient illustration and clear demonstration of the principles involved.

DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to the die casting apparatus shown in the drawing, there is illustrated the stationary half or side of a two-piece die assembly, as designated generally by the numeral 10. The die piece 10 includes a die cavity or face 12, which for purposes of this illustration represents the female portion of a die cavity for a chain saw housing. At the bottom of the cavity 12 is a gate 14 communicating with a shot well 16, the gate providing means for filling the cavity with the molten casting metal injected from the shot well.

Extending through the die piece 10 immediately behind the gate 14 is a passageway 18. Passageway 18 communicates with a conduit 20, which, in turn, connects with a main supply conduit 21, to provide means for conducting a cooling liquid, usually water, through the die piece adjacent the gate stream filling area, in which the operating temperature of the die tends to be highest. A valve 22 in conduit 20 provides means for regulating flow of the cooling water into passageway 18. At the end of the die cavity 12 opposite the gate stream filling area, where the die operating temperature tends to be lowest, is a second passageway 24 which extends through the die piece immediately behind the die cavity. Cooling water is circulated into passageway 24 through a conduit 26 which branches into conduit 20 and is intersected by main conduit 21. A solenoid valve 30 in conduit 21 may be operated automatically (as explained hereinafter) to regulate the main water supply into conduits 20 and 26. Where it is desired to by-pass solenoid valve 30 and direct the water supply only through conduit 20 (as explained hereinafter), the water flow may be regulated by a manually operated valve 32 in conduit 20. It will be understood that the water outlet lines for each die assembly are elevated to assure uniform flow of the cooling water through the die.

Means for sensing the temperature of the die are provided by temperature sensors, such as thermocouples or thermistors, with one sensor preferably being positioned in each piece or half of the die assembly. Alternatively, both temperature sensors may be positioned in the same piece or half of the die assembly. For example, one thermocouple 34 is positioned in a well below the die cavity 12 close to the gate 14 on one side of the die assembly (female die piece 10), to sense the operating temperature in the highest temperature zone of the die. A second thermocouple 36 is similarly positioned in the opposite side of the die assembly, that is, at the end of the die cavity away from or opposite the gate area, to sense the operating temperature in the lowest temperature zone of the die. With reference to the drawing, the thermocouple 36 is actually positioned in the male half of the die assembly (the side opposite die piece 10), which is not shown in the drawing to simplify the description of the invention. More than one temperature sensor may be used in each side of the die assembly if, for example, the size or geometry of the die cavity requires additional sensors to more accurately measure the operating temperature of the die.

Each thermocouple is positioned in its well housing in the die assembly such that the heat-sensitive tip of the device is within about 0.090 in. to 0.125 in. of the die face. If the thermocouple tip is placed closer than 0.090 in. from the die surface, the amplitude response to the die temperature, as registered by the indicating controller 38, will exceed the low limit and high limit temperature set point on the controller, so that it is difficult to establish an operable casting temperature range on the controller. Conversely, if the tip of the thermocouple is placed farther than about 0.125 in. from the die surface, the thermal wave from the casting is dissipated or damped out in the die piece such that the temperature response by the thermocouple is not sensitive enough to be picked up by the controller. Typical of temperature sensors which may be used are nickel-chromium alloy ("Chromel" or "Chromel-P") thermocouples or iron-constantin alloy thermocouples. For the indicating controller 38, a preferred instrument is a conventional indicating electronic potentiometer controller (Versatronic two-relay, Honeywell, Inc.).

Thermocouple 36 is connected to controller 38 through a lead 40 , which connects into one side of a selector switch 42. Switch 42 is connected into a temperature indicating needle 44 on the controller 38 by a common lead 46. The thermocouple 34 is connected to controller 38 and indicator needle 44 through a lead 48 which connects into the opposite side of selector switch 42. The controller 38 includes a temperature-indicating dial 50, which is graduated in Fahrenheit degrees, with a range of from 0.degree. F. to 800.degree. F. A knob 52 on controller 38 provides a manual control for indicator needle 54, the needle being set on dial 50 at the low limit casting temperature of the die assembly. Similarly, the knob 56 on controller 38 is a manual control for indicator needle 58, which is set on dial 50 at the high limit casting temperature.

Indicating controller 38 further includes a battery of indicator lights 60, 62, 64, which provide a visible signal in response to the die temperature, as sensed by either the thermocouple 34 or 36. Indicator light 60, which is red, is connected into controller 38 through leads 66, 68 and a relay 70. Indicator light 62, which is green, connects into controller 38 through leads 72, 74. A blue indicator light 64 connects into controller 38 through leads 76, 78 and a relay 80. Power for operating the controller 38 is provided by a line 82 (120 V. A. C.) which connects the controller with an appropriate source of power.

The method of this invention has been found to be particularly useful in die casting of magnesium base alloys, but it is contemplated to be adapted to casting of any suitable die casting metal, such as aluminum, zinc, brass, or alloys of these metals. It is further contemplated that the present method would be applicable to injection molding of plastic materials.

In a typical magnesium base alloy die casting operation, utilizing the present method, the die assembly is heated to a temperature suitable for casting a magnesium base alloy. This temperature should be between about 515.degree. F. and 535.degree. F., the median temperature point, therefore, being about 525.degree. F. The molten metal is injected into the shot well 16 to commence the casting cycle. At the same time cooling water is circulated freely and continuously through the passageways 18 and 24 of the die assembly, as received through main conduit 21 and the secondary conduits 20 and 26. The cooling water flow is adjusted by leaving valve 22 in conduit 20 fully open and closing valve 28 in conduit 26 only far enough to restrict the water flow sufficiently to "balance" the temperature distribution in the die assembly. The temperature distribution is "balanced" by adjusting valves 28 and 22 until the die temperature in both the cool zone and hot zone, as sensed by the thermocouples 34 and 36 and registered by the needle 44 on dial 50, read as close to the median temperature point (525.degree. F.) as it is possible to attain. Indicator needle 54 is then set on dial 50 at the low limit casting temperature set point, i.e. at about 515.degree. F. Similarly, indicator needle 58 is set on dial 50 at the high limit casting temperature set point, which is about 535.degree. F.

Once the operable casting temperature of the die assembly is determined, the casting cycle can be maintained at the optimum pace for each casting job by merely observing the signals emitted by the indicator lights 60, 62 and 64 on controller 38. The operable casting temperature for each job will vary according to the type of material being cast, size of the casting, and the like. In a typical magnesium die casting cycle, for example, if the die temperature drops below the low limit set point, as sensed by either thermocouple 34 or 36 (as selected by switch 42) relay 70 is actuated to light up the red indicator light 60. The casting cycle pace can then be increased to bring the die temperature above the low limit set point, i.e. somewhere between 515.degree. F. and 535.degree. F. When the die temperature comes up to the desired operating range the green indicator light 62 will light up and remain on so long as the temperature of the die stays in this range. The green light thus gives the operator a visible signal which assures him that the die is operating at the temperature required to produce castings free of defects. Conversely, if the die temperature exceeds the high limit set point (535.degree. F.), as sensed by either thermocouple, relay 80 will be energized to light up the blue indicator light 64. The casting cycle pace can then be slowed down until the die temperature falls back into the desired operating range, at which point the green light 62 will again come on to indicate that the die is again operating at the proper temperature.

With regard to regulating the cooling water flow into the die assembly, it is contemplated that solenoid valve 30 could be electrically actuated to close the valve and shut off the main water supply through conduit 21, in the event the casting cycle pace slows down too much or stops completely. The water is thus diverted through conduit 20 (acting as a bypass), in which the flow can be manually regulated with valve 32. By this arrangement, the flow of cooling water to the die can be reduced to a safe minimum to prevent cracking of the die upon restarting the casting cycle.

An additional embodiment contemplated within the practice of this invention is to regulate the casting cycle time with an automatic controller operatively associated with the temperature sensors in the die, to thereby maintain the die temperature within the desired operating range.

Claims

I claim:

1. In a metal die casting operation, a method for controlling the die temperature and for pacing the casting cycle of the die, which comprises the steps of:

positioning a first temperature sensor in the highest operating temperature zone of one side of a die assembly and a second temperature sensor in the lowest operating temperature zone of an opposing side of said die assembly, said temperature sensors being associated with an indicating controller, the controller including signal means operatively connected to said first and second temperature sensors, the signal means providing a visible signal in response to the temperature indicated by at least one of said temperature sensors,

heating the die assembly to a predetermined temperature range operable for casting a metal desired to be die cast,

injecting the metal to be die cast into said die assembly to commence the casting cycle, while passing a cooling liquid through said die assembly,

adjusting the flow of said cooling liquid into said die assembly until the temperature differential between the first and second temperature sensors is within 10.degree. F. of a median temperature point within said predetermined operable temperature range,

setting on the indicating controller a low limit casting temperature and a high limit casting temperature, as determined by the temperature differential between said first and second temperature sensors,

maintaining the die casting cycle at an optimum pace to hold the temperature of the die casting assembly between the low limit and high limit casting temperature, as determined by visually observing the signal produced by the signal means of said indicating controller.

2. The method of claim 1 in which the metal to be die cast is a magnesium base alloy.

3. The method of claim 1 in which said first and second temperature sensors comprise a set of thermocouples.

4. The method of claim 1 in which said first and second temperature sensors comprise a set of thermistors.

5. The method of claim 1 in which the temperature sensor in each side of the die assembly is positioned within from about 0.090 inch to 0.125 inch of the die face.

6. The method of claim 1 in which more than one temperature sensor is positioned in each of the opposing sides of the die assembly.

7. The method of claim 1 wherein the first temperature sensor and second temperature sensor are each positioned in the same side of the die assembly.

8. The method of claim 1 in which the cooling liquid comprises water.

9. The method of claim 1 wherein the signal means comprises a battery of indicator lights mounted on the indicating controller and operatively connected to the first and second temperature sensors.