US20090065170A1

US20090065170A1 - Die cooling apparatus and method thereof

Info

Publication number: US20090065170A1
Application number: US11/853,391
Authority: US
Inventors: Philip Vais; James Paul Lewis
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 2007-09-11
Filing date: 2007-09-11
Publication date: 2009-03-12

Abstract

A die cooling system for cooling a cast wheel for an automotive vehicle is provided that includes a die, multiple thermocouples embedded in the die, which measure the actual cast metal temperature, adjustable cooling valves, and a control system. The control system receives actual casting metal temperature data from each thermocouple to control the operation of the adjustable cooling valves.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an apparatus and method for cooling a die casting product and more specifically to an apparatus and method for measuring the actual temperature of the die cast product and controlling the solidification rate of the die cast product based on the actual temperature of the die cast product.
2. Description of Related Art
Low pressure die casting methods and procedures to produce cast products are well known in the industry. Basically, the low pressure die casting method comprises a metal die mounted above a sealed furnace that contains molten metal. A refractory lined tube, called a stalk tube, extends from the bottom of the die into the molten metal. Air or a gas under low pressure is introduced into the furnace. The air or gas forces the molten metal up the tube and into the die cavity. When the metal inside the die has solidified the pressure in the furnace is released and the molten metal in the tube returns to the furnace. After an additional cooling time the die is opened and the casting is extracted.
Specific methods and procedures during this process can significantly alter the resulting end product. For example, the cooling rate and hence the solidification rate at which the molten metal solidifies can affect the microstructure of the finished product. More specifically, a fast solidification rate produces a fine microstructure, which leads to good elongation properties. If the solidification rate is too fast, however, shrinkage-porosity may develop in the end product. On the other hand, a slow solidification rate results in unacceptable mechanical properties. More specifically, a slow solidification rate creates a coarse microstructure in the material, which in turn creates poor elongation properties.
Conventional methods to control the cooling rate and ultimately the solidification rate of the cast product include using thermocouples to measure the temperature of the die, referred to as an in-die thermocouple system, and using this information to control on/off type cooling valves, which blow cooling air directly onto the die. One disadvantage to the in-die thermocouple system is that the in-die thermocouple system can be affected by variations in atmosphere temperature, die coating thickness, die coating thermal diffusion, die cooling air temperature and humidity, and metal temperature fluctuations. The present invention can adjust to these variations, which results in more uniform properties throughout the entire cast product than in the in-die thermocouple system. This in turn reduces the amount of scrap, which is yet another advantage of the present invention over conventional methods.
In today's the automotive industry, however, the trend is to manufacture larger diameter wheels for aggressive styling. In 2001 the typical wheel diameter on North America automotive vehicles ranged from 14 inches to 17 inches. As of 2005, while the majority of the wheel diameters still ranged between 15 inches to 17 inches, nearly 10% of the wheel diameters were greater than or equal to 18 inches. An increase in the wheel diameter, however, introduced problems in the casting process. More specifically, conventional methods. Like those mentioned above, to control the cooling rate are no longer efficient enough to achieve the proper mechanical properties in all areas of the wheel and more specifically in the spoke portions of the wheel. Thus, what is required is a die cooling apparatus and method to more uniformly cool a cast automotive wheel.

SUMMARY OF THE INVENTION

In accordance with one aspect, the present invention overcomes the above mentioned disadvantages by providing a die cooling system for cooling cast metal comprising a die, multiple thermocouples embedded in the die, adjustable cooling valves, and a feedback control system. The thermocouples are embedded in the die and contact the cast metal and measure the actual casting metal temperature. The control system receives measured temperature signals from each thermocouple to thereby control the operation of each adjustable cooling valve to maintain the proper solidification rate of the casting metal.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which it pertains upon a reading and understanding of the following detailed specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings that form a part of the specification.

FIG. 1 shows a die cooling system in accordance with the present invention.

FIG. 2 is a view of a spoke portion of the die.

FIG. 3 is a view taken along line 3-3 of FIG. 2.

FIG. 4 is a graph showing a die cooling graph.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 shows a die cooling system 10 for cooling a low-pressure die for forming large die cast wheels. The die cooling system 10 includes a die 12, adjustable cooling valves 14, multiple thermocouples (TC1 . . . TCN), and a feedback control system 16. The die cooling system 10 overcomes the above mentioned disadvantages by systematically controlling the operation of the adjustable cooling valves 14 to maintain a solidification rate of the casting metal during the cooling process to produce optimum mechanical properties. It should be noted that although the die cooling system disclosed herein is designed to improve the casting process for wheels with a diameter greater than or equal 18″, the die cooling system 10 can be implemented for casting wheels having a diameter less than 18″.
For simplicity and illustrative purposes only, the present invention will be described with reference to one spoke portion of the die 12, as shown in FIGS. 2 and 3. It should be noted, however, that each spoke portion of the die 21 has the same cooling arrangement as will be subsequently described.
Referring to FIGS. 2 and 3, the die 12 is a conventional die of the type commonly known in the art that includes a top core 18, an inner-side core 20, an outer-side core 22 and a bottom core 24. The die 12 further includes a conventional cooling air delivery structure (not shown) of the type commonly known in the art and will not be described herein. Thus, the adjustable cooling valves 14, shown schematically in FIG. 3, deliver cooling air to the die 12 via the delivery structure. It should be noted that the number of adjustable cooling valves 14 shown in FIG. 3 is for illustrative purposes only and is not intended to limit the scope of the invention. Thus, the number of adjustable cooling valves 14 may vary depending on the size of the die 12 the material of the cast product, etc.
Referring to FIG. 4, the thermocouples TC are embedded into the die 12 such that a tip 26 of each thermocouple TC is either flush with an inside surface 28 of the die 12 or slightly embedded into the casting metal. Thus, each thermocouple TC contacts the casting metal and, therefore, measures the actual casting metal temperature and not the die temperature. Because the thermocouples TC measure the actual casting metal temperature, the die cooling system 10 ensures that casting metal solidifies at the solidification rate, as explained further below. It should be noted that the number of thermocouples TC may vary depending on the diameter of the wheel being cast. For example, a wheel having a diameter of 18″ may require fewer thermocouples than a wheel having a diameter of 20″. In addition, because the casting metal will solidify at the same rate from either side of the die 12, it is contemplated that thermocouples TC may be required on only one side of the die.
A release agent may be applied to the inside surface 28 of the die 12 to facilitate the removal of the cast product when completed. Prior to applying the release agent the thermocouples TC are masked off so the release agent does not hinder the performance of the thermocouples TC.
Referring to FIG. 1, the feedback control system 16 includes a computer 30 and a programmable controller 32. The computer 30 is electronically linked to each thermocouple TC and continuously receives signals indicative of the actual casting metal temperature (temperature data) from each thermocouple TC. The computer 30 transmits the temperature data to the programmable controller 32. In order to achieve and maintain the solidification rate, the programmable controller 32 processes the temperature data to control the operation of the adjustable cooling valves 14. In addition, the programmable controller 32 communicates the status of each adjustable cooling valve 14 back to the computer 30.
FIG. 4 illustrates an example embodiment of a die cooling graph comprising a combination bar graph and line graph. The bar graph represents a cooling zone diagram and illustrates how the die 12 is cooled. The line graph, referred to as metal cooling curves, represents the temperature of the casting metal collected by each thermocouple TC. As an option, the cooling zone diagram and the metal cooling curves can be displayed on a monitor (not shown) connected to the computer 14. It should be noted that the cooling zone diagram and the metal cooling curves shown in FIG. 4 is just one example of a cooling a die and is for illustrative purposes only and is not intended to limit the scope of the invention.
Referring to the cooling zone diagram (bar graph) of FIG. 4, a cooling zone CZ is defined as a location within the die cooling system 10 where cooling air is applied to the die 12. The cooling zones CZ are schematically represented by the adjustable cooling valves 14, as shown in FIG. 3. Thus, the cooling air applied to the die 12 at the cooling zones CZ is supplied by the adjustable cooling valves 14 via cooling air delivery structure and is regulated by the programmable controller 32. As explained below, cooling air is supplied to the cooling zones CZ in a manner such that the die 12 is systematically cooled from the outer most portion of the die 12 toward the inner most portion of the die 12. It should be noted that the operation of the adjustable cooling valves 14, and hence, the cooling zones CZ operate independently of each other. For example, the time at which the cooling zone adjustable cooling valve 14 is activated, the duration of its activation and it subsequent deactivation depends on the casting parameters of the wheel being cast explained further below and the temperature of the casting metal and not necessarily on the status of the previous cooling zone CZ. In addition, activation of adjustable cooling valves 14 for adjacent cooling zones CZ may or may not overlap. Still further, the amount of overlap may vary.
Referring to the line graph of FIG. 4, as mentioned above, the metal cooling curves represent the actual casting metal temperature. For example, the metal cooling curve denoted as TC1 is the actual casting metal temperature as measured by thermocouple TC1, the metal cooling curve denoted as TC2 is the actual casting metal temperature as measured by thermocouple TC2, etc. As illustrated by the metal cooling curves the casting metal is a liquid above a first temperature and becomes a solid below a second temperature. As the casting metal cools from the first temperature down to the second temperature the casting metal is in a semi-solid state and undergoes a transformation from a liquid to a solid. This applies to any metal that has a transformation temperature, such as, for example but not limited to aluminum. This transformation occurs over a time period referred to as a solidification period. The rate at which the casting metal transforms from a liquid to a solid is the solidification rate mentioned above. Maintaining the proper solidification rate, referred to as a critical solidification rate, during the solidification period is critical in order to achieve optimum mechanical properties of the casting metal.
For example, if the casting is not cooled at the critical solidification rate then either macro-shrinkage porosity or micro-shrinkage porosity may occur within the casting. More specifically, if a portion of the casting is cooled too early or too aggressive then the molten metal feed from the gate will be cut off. This in turn causes macro-shrinkage porosity between portions of the casting, such as for example, between the spoke and the flange. Micro-shrinkage porosity can occur in a more localized area if the solidification rate falls below the critical solidification rate. More specifically, if the solidification rate falls below the critical solidification rate then the dendrite arms become too long. This leads to micro-shrinkage porosity between the dendrite arms.
Prior to operation of the die cooling system 10 the feedback control system 16 calculates the critical solidification rate of the casting metal at each thermocouple TC location. The critical solidification rate is calculated by taking into account certain casting parameters for a particular cast wheel, such as the temperature of the casting metal during the casting process, the diameter, width, thickness, design, material, etc. of the wheel, etc. The casting parameters are entered into the feedback control system 16 whereby the feedback control system 16 calculates the critical solidification rate for the cast wheel. It should be noted that the critical solidification rate of the casting metal may vary for each thermocouple TC location due to characteristics of the wheel, such as thickness.
During operation of the die cooling system 10, molten metal is introduced into the die 12 using the low-pressure die casting method described above. The computer 30 continuously processes temperature data from each thermocouple TC and transmits this information to the programmable controller 32. The programmable controller 32 uses the temperature data to control the operation of the adjustable cooling valves 14 for each cooling zone CZ accordingly. In addition, the programmable controller 32 uses the temperature data from a given thermocouple TC to control the operation of a downstream adjustable cooling valve 14. For example, the programmable controller 32 can control the operation of the adjustable cooling valves 14 downstream from thermocouple TC1 based on the temperature data received from thermocouple TC1. Thus, based on the temperature data the programmable controller 32 activates, deactivates and adjusts the flow rate of the adjustable cooling valves 14 such that cooling air is systematically supplied to the cooling zones CZ from the outer most portion of the die 12 toward the inner most portion of the die 12 to thereby maintain the critical solidification rate. As shown in the bar graph of FIG. 4, the presence of a bar indicates that an adjustable cooling valve 14 is activated and is, thus, supplying cooling air to the corresponding cooling zone CZ. It should be noted that when an adjustable cooling valve 14 is activated the programmable controller 32 may also regulate the amount of air flow from each adjustable cooling valve 14. For example, the adjustable cooling valves 14 may be variably opened between 0% and 100%. Thus, based on the temperature data from the thermocouples TC the programmable controller 32 can regulate the on/off operation of the adjustable cooling valves 14 or vary the flow rate of the adjustable cooling valves 14.
As an example, referring to the embodiment shown in FIG. 4, as the cooling process begins the casting metal begins to cool gradually as shown by the metal cooling curve represented by thermocouple TC1. The programmable controller 32 activates the adjustable cooling valve 14 for cooling zone CZ1 and the casting metal represented by metal cooling curve TC1 begins to cool more rapidly. As the casting metal enters and proceeds through the transformation phase described above, the programmable controller 32 may also adjust the flow rate of the adjustable cooling valve 14 to thereby more precisely maintain the critical solidification rate. When the feedback control system 16 determines that the transformation has successfully occurred or will successfully occur at cooling zone CZ1 the programmable controller 32 will either deactivate the adjustable cooling valve 14 for cooling CZ1 or adjust the flow rate of the adjustable cooling valve 14. This process continues through each cooling zone CZ such that the casting metal systematically solidifies from the outer most portion of the die 12 (the wheel flange 34) toward the inner most portion of the die 12 closest to the stalk tube. Once the entire casting metal is transformed from a liquid to a solid the die 12 is quickly cooled to a temperature such that the cast wheel can be extracted from the die 12.
The die cooling system 10 embodiment described above is capable of cooling casting metal at a critical solidification rate calculated from casting parameters to thereby ensure that the casting metal solidifies with optimum mechanical properties. This example embodiment employs a combination time-temperature based method to control the operation of the adjustable cooling valves 14. Thus, the feedback control system 16 controls the operation of the adjustable cooling valves 14 based on the temperature data from the thermocouples TC and on the solidification rate of the cast metal.
It is contemplated, however, that a time based method may be employed to control the operation of the adjustable cooling valves 14. For example, the adjustable cooling valves 14 can be activated, deactivated, or varied based on the based on the critical solidification rate of the cast metal.
It is further contemplated that a temperature based method may be employed to control the operation the adjustable cooling valves 14. For example, the adjustable cooling valves 14 can be activated, deactivated, or varied based on the temperature data received by the feedback control system 16 from the thermocouples TC. Thus, the cooling air flow from the adjustable cooling valves 14 at each cooling zone CZ can be regulated based on the temperature of the casting metal.
It is still further contemplated that the adjustable cooling valves 14 for each cooling zone CZ may be all activated once the cooling process begins and that the programmable controller 32 varies the flow rate of each adjustable cooling valve 14 accordingly to achieve and maintain the critical solidification rate.
While specific embodiments of the invention have been described and illustrated, it is to be understood that these embodiments are provided by way of example only and that the invention is not to be construed as being limited but only by proper scope of the following claims.

Claims

1. A die cooling system for cooling cast metal comprising:

a die;

a plurality of thermocouples embedded in the die, such that each thermocouple contacts the casting metal;

a plurality of cooling zones arranged such that cooling air is supplied to the cooling zones to cool the die; and

a feedback control system,

wherein each thermocouple measures the temperature of the cast metal, and

wherein the control system continuously receives temperature data from each thermocouple and controls the supply of cooling air to the cooling zones based on the temperature data.

2. The die cooling system of claim 1, wherein a tip of each thermocouple is flush with an inside surface of the die.

3. The die cooling system of claim 2 further comprising a plurality of adjustable cooling valves to supply cooling air to the cooling zones, wherein the control system calculates a critical solidification rate of the cast metal and activates and deactivates the adjustable cooling valves to supply cooling air to corresponding cooling zones in such a manner that the die is gradually cooled at the critical solidification rate from an outer most portion of the die toward an inner most portion of the die.

4. The die cooling system of claim 3, wherein during activation of the adjustable cooling valve the control system varies the cooling air flow rate of the adjustable cooling valve to maintain the critical solidification rate of the cast metal.

5. The die cooling system of claim 1, wherein a tip of each thermocouple is embedded in the casting metal.

6. The die cooling system of claim 5 further comprising a plurality of adjustable cooling valves to supply cooling air to the cooling zones, wherein the control system calculates a critical solidification rate of the cast metal and activates and deactivates the adjustable cooling valves to supply cooling air to corresponding cooling zones in such a manner that the die is gradually cooled at the critical solidification rate from an outer most portion of the die toward an inner most portion of the die.

7. The die cooling system of claim 6, wherein during activation of the adjustable cooling valve the control system varies the cooling air flow rate of the adjustable cooling valve to maintain the critical solidification rate of the cast metal.

8. A die cooling system for cooling a cast wheel for an automotive vehicle comprising:

a die;

a plurality of thermocouples embedded in the die such that each thermocouple contacts the cast wheel;

a plurality adjustable cooling valves to supply cooling air to various portions of the die; and

a feedback control system,

wherein each thermocouple measures the actual temperature of the cast wheel, and

wherein the control system continuously receives temperature data from each thermocouple and controls the operation of the adjustable cooling valves based on the temperature data.

9. The die cooling system of claim 8, wherein a tip of each thermocouple is flush with an inside surface of the die.

10. The die cooling system of claim 9, wherein the control system calculates a critical solidification rate of the cast wheel, wherein the control system includes a computer and a programmable controller, wherein the computer receives temperature data from each thermocouple and transmits the temperature data to the programmable controller, and wherein the programmable controller activates and deactivates the adjustable cooling valves to supply cooling air to corresponding cooling zones in such a manner that the die is gradually cooled at the critical solidification rate from an outer most portion of the die toward an inner most portion of the die.

11. The die cooling system of claim 10, wherein during activation of the adjustable cooling valve the control system varies the cooling air flow rate of the adjustable cooling valve to maintain the critical solidification rate of the cast wheel.

12. The die cooling system of claim 8, wherein a tip of each thermocouple is embedded in the cast wheel.

13. The die cooling system of claim 12, wherein the control system calculates a critical solidification rate of the cast wheel, wherein the control system includes a computer and a programmable controller, wherein the computer receives temperature data from each thermocouple and transmits the temperature data to the programmable controller, and wherein the programmable controller activates and deactivates the adjustable cooling valves to supply cooling air to corresponding cooling zones in such a manner that the die is gradually cooled at the critical solidification rate from an outer most portion of the die toward an inner most portion of the die.

14. The die cooling system of claim 13, wherein during activation of the adjustable cooling valve the control system varies the cooling air flow rate of the adjustable cooling valve to maintain the critical solidification rate of the cast wheel.

15. A method of cooling a cast wheel for an automotive vehicle comprising the steps of:

providing a die having a plurality of thermocouples embedded in the die such that each thermocouple contacts the cast wheel, a plurality of adjustable cooling valves to supply cooling air to the die, and a control system;

calculating a critical solidification rate of the cast wheel based on parameters of the wheel;

filling a cavity of the die with molten metal;

measuring the cast wheel temperature continuously at each thermocouple location; and

controlling the supply of cooling air from the adjustable cooling valves to achieve the critical solidification rate during a solidification period.

16. The method of claim 15 further comprising the steps of:

activating and deactivating the adjustable cooling valves to supply cooling air to each cooling zone to gradually cool the die from an outer most portion of the die toward an inner most portion of the die; and

varying the cooling air flow rate of an activated adjustable cooling valve to maintain the critical solidification rate.

17. The method of claim 16 further comprising the steps of:

transforming the molten metal from a liquid to a solid; and

cooling the entire die rapidly to a temperature such that the cast wheel can be extracted from the die.