Controlled nozzle cooling (CNC) of permanent mold casting

Abstract

A process for permanent mold casting of metals and their alloys includes the steps of providing at least a mold equipped with a plurality of cooling nozzles, making a layer of coolant permeable materials covering the nozzles and maintaining the materials at desired temperatures, delivering a molten metal into the mold, supplying predetermined amount of coolant to each nozzles to contact the external surface of the casting at desired rate, time, and duration to achieve an acceptable level of progressive solidification from the distal end of the casting towards the riser until the casting has reached desired temperatures.

Claims

1. A process for casting of metals to maintain an acceptable level of progressive solidification from a distal end of a casting to its riser or its downsprue, comprising the steps of: preparing permanent or semi-permanent molds with a plurality of nozzle holes spaced at predetermined intervals; lining the molds with a coolant permeable liner composing of at least an aggregate to form a cavity for castings and a plurality of nozzle holes spaced at predetermined intervals, each hole for hosting a cooling nozzle with a tip of the nozzle separated from a surface of the casting by a thin layer of coolant permeable materials; preparing a plurality of venting systems in the molds and the liner to release used fluids, moisture and mold debris; holding at least part of the mold or the liner at predetermined temperatures; introducing a molten metal into the mold cavity to form castings; embedding nozzles into the nozzle holes in the at least a permanent or semi-permanent mold with the tip of each nozzle separated from the surface of the casting by a thin layer of coolant permeable materials; delivering a predetermined amount of a selected coolant including water to the tip of each nozzle at predetermined rates, times, and durations to break the thin layer of coolant permeable materials in order for the selected coolant to directly contact external surface of a solidifying casting to cool the solidifying casting sequentially; and cooling the casting under controlled conditions to predetermined temperatures before removing the casting out of the mold.

2. A process according to claim 1, wherein the permanent or semi-permanent molds are made of materials including metal, graphite, or ceramic materials.

3. A process of claim 1, wherein the coolant permeable liner is a sand liner made using a molding method including sand blowing and 3D printing, a sand liner with a coating, or simply a layer of coating including a thermal barrier coating, a thick coating used in permanent mold casting process, or a thin die lube used in a die casting process.

4. A process according to claim 1, wherein the thin layer of coolant permeable materials composed of at least an aggregate and a binder.

5. A process according to claim 1, wherein the thin layer of coolant permeable materials is a coating.

6. A process according to claim 1, wherein the nozzle holes are either directly molded in a molding machine or machined on the molds.

7. A process according to claim 1, wherein the venting system comprises fluid soluble cores.

8. A process according to claim 1, wherein the venting system comprises a solid plug that can be removed out of the molds at predetermined times.

9. A process of claim 1, wherein the venting system comprises a metal chill that can be removed out of the mold at predetermined times.

10. A process of claim 1, wherein the venting system comprises a cooling nozzle that can be removed out of the mold at predetermined times.

11. A process according to claim 1, wherein the molten metal is introduced into the mold cavity by gravity or by pressure.

12. A process according to claim 1, wherein heating sources are embedded in the mold or the liner for holding part of the mold or liner to predetermined temperatures.

13. A process according to claim 1, wherein part of the mold or liner are maintained at predetermined temperatures by using embedded heating sources, external heating source including infrared heating, or by a combined use of embedded heating sources or external heating source.

14. A process according to claim 1, wherein the nozzles are mounted on a rigid fixture with each nozzle being controlled for translational motion to form an apparatus that can be retrofitted into existing production lines to make the casting.

15. A process according to claim 1, wherein the thin layer of coolant permeable materials is less than 15 millimeters thick.

16. A process according to claim 1, wherein the thin layer of coolant permeable materials is less than 3 millimeters thick.

17. A process according to claim 1, wherein a controller is used to regulate the delivery of a predetermined amount of coolant to each embedded nozzle in the molds at predetermined rates, times, and durations.

18. A process according to claim 1, wherein the coolant includes a liquid, a gas, a mixture of gases, or a mixture of liquids and gases.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B are schematic views of a layout of one embodiment of the present invention.

(2) FIGS. 2A and 2B are schematic views of a layout of one embodiment of the present invention.

(3) FIG. 3 is a schematic view of a layout of one embodiment of the present invention using a sand liner.

(4) FIG. 4 is a schematic view of a layout of one embodiment of the present invention using heaters to heat a sand liner.

(5) FIG. 5 is a schematic side view of a layout of one embodiment of the present invention.

(6) FIG. 6A is a schematic side view of a layout of a prior art of making a cast steel railway wheel.

(7) FIG. 6B is a schematic side view of a layout of one embodiment of the present invention on making a cast steel railway wheel.

(8) FIG. 7 shows a photograph of an aluminum wheel, a schematic side view of a layout of a prior art of making the cast wheel, simulated solidification times in the cast wheel made using a prior art, and a cross sectional view of the cast wheel.

DETAILED DESCRIPTION OF THE INVENTION

(9) Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

(10) The invention deals with a controlled cooling sand casting process using an array of nozzles embedded in molds that delivers a desired amount of selected coolant at desired times to contact the surfaces of a casting to ensure progressive solidification from the distal end of a casting to the riser.

(11) FIGS. 1A and 1B illustrate a method and an apparatus of one embodiment of the present invention of CNC sand casting. Sand mold 20 is made with or without a flask 40. The mold 20, which is permeable to liquid or gases, is composed of at least an aggregate and a binder that are conventionally used to make sand molds. The mold 20 can be made using a mold making machine, sand blower, 3D printing, and etc. The mold is kept at predetermined temperatures to ensure that a molten metal can fill the casting cavity and to ensure a progressive solidification from the distal end to the riser or downsprue of the casting.

(12) The cavity 24 in the mold 20 is used to make a casting. A plurality of nozzle holes 18 are made for hosting the nozzles, 10, 12, 14, and 16 that are to be embedded in the sand mold 20. There is a thin layer 15 of coolant permeable materials such as sand or coating materials that are used for separating the nozzle, 10, 12, 14, or 16, from directly contacting the molten metal during mold filling. Nozzle holes 18 are molded using a pattern or are machined. The nozzle hole 18 can also be a through hole so that the nozzle can be placed in the mold flush with the surface of the cavity 24. Conventional coatings can be applied on the surface of the mold cavity 24, especially if the tip surface of the nozzle 10, 12, 14, or 16 is flush with the surface of the mold cavity 24 if the nozzle hole 18 is a through hole. As a result, there is a thin layer of coolant permeable materials 15 for separating the tip of a nozzle from direct contacting the molten metal during mold filling.

(13) Having made the mold 20, nozzles 10, 12, 14, and 16 mounted on a rigid fixture 48 are placed manually or using a robot in the nozzle holes 18 before or after the molten metal is poured into the mold cavity 24. Each nozzle 10, 12, 14, or 16 can be control individually for translational motions so that it can be placed into or removed out of the nozzle hole at predetermined times. The fixture 48 can also be used to lock the molds in place to prevent the molds from opening and the resultant metal leakage from the molds due to the static pressure that the molten metal in cavity 24 applies on the mold 20. A gap 11 is designed to allow the used coolant and the resultant gases to escape from the tip of the nozzle 10, 12, 14, or 16. A number of venting system 22 (only one is shown in FIGS. 1 and 2) is also recommended to release the gases in the mold cavity 24 and molding materials of the mold 20. The nozzles 10, 12, 14, and 16 are connected to a controller 44 using conduit 42. Coolant is provided from coolant supply 46, regulated by the controller 44, and delivered to each nozzle. The controller 44 controls the delivery of a desired amount of coolant to each nozzle at predetermined times and rates. The coolant delivery system consists of nozzles 10, 12, 14, 16 and their conduit 42, the fixture 48, the controller 44, and the coolant supply 46. The mold 20 and its flask 40 can be prepared in a conventional casting production line. The coolant delivery system can be associated to the molds before or after pouring molten metal into the mold cavity 24 and removed out of the mold 20 after the casting has been cooled to desired temperatures. Thus, only one or a limited number of coolant delivery systems are needed for an entire conventional casting production line for the mass production of castings.

(14) The sequence of coolant delivery to each array of nozzles is shown in FIGS. 2A and 2B. After molten metal fills the mold cavity, a predetermined amount of coolant is delivered to the first array of nozzles 10. As the coolant is released from the nozzle 10 and breaks through the layer of coolant permeable materials 15 to the molten metal of the casting 26 in a flow pattern shown as 13 in FIG. 2A, solid dendrites 28 start forming from the molten metal near the distal end of the casting 26 due to the combined cooling of the mold 20 and the coolant. A freezing front 29 thus forms near the distal end of the casting 26 and moves gradually towards the riser 30. When the freezing front 29 moves close to the second array of nozzles 12, coolant starts to be delivered to nozzles 12 shown in FIG. 2B. As the freezing front 29 moves further towards the riser 30, nozzles 14 and 16 will be actuated and nozzles 10 and 12 will be gradually switched off as needed. Any air gap 27 at the casting/mold interface that forms due to the contraction of the casting 26 is filled with the coolant. The coolant discussed in the present invention can be a liquid such as water, mineral oil, and liquid nitrogen, or gases such as CO.sub.2, compressed air, moisture, N.sub.2 or helium. In fact, liquids and gases that are conventionally used in the casting industry as coolant can be used as coolants in the present invention.

(15) The delivery of coolant is such that the angle, ?, is greater than the value that is required for an adequate feeding of liquid metal to the solidification shrinkage in the mushy zone at the left side of the freezing front 29. An important feature of coolant delivery is to break the thin layer of coolant permeable materials that separate the molten metal 26 and the tip of the nozzle 10, 12, 14 or 16. As a result, the coolant delivery system is designed to deliver a predetermined amount of a selected coolant including water to the tip of each nozzle, 10, 12, 14 or 16, at predetermined rates, times, and durations to break the thin layer of the coolant permeable materials so that the coolant contacts the external surface of the solidifying casting 26 in order to maintain an acceptable level of progressive solidification from the distal end of the casting to the risers or the downsprue of the casting.

(16) The thickness of the thin layer of coolant permeable materials separating the nozzle, 10, 12, 14 or 16, from the casting 26, has to be thick enough to withhold the static pressure of the molten metal but thin enough so that the nozzle could deliver a coolant to break up this thin layer of coolant permeable materials to directly contact the external surface of the casting 26. It is recommended that the thickness be less than 15 millimeters, preferably less than 3 millimeters.

(17) The spacing or interval between neighboring array of nozzles, for example between nozzle 10 and nozzle 12, is between 4 to 10 times of the local wall-thickness of a steel plate-shaped casting to ensure a progressive solidification from the distal end to the riser or downsprue of the casting. The minimum spacing between the neighboring array of nozzles is dependent on the casting materials and casting methods. Ideally, the spacing should be such that the feeding angle, ?, is greater than the value that is required for an adequate feeding of liquid metal to the solidification shrinkage in the mushy zone of the casting at given rates of coolant delivery from the nozzles.

(18) Venting has to be used in order to release used coolant, mold debris, and resultant moisture from the molds. FIGS. 1-2 illustrates only one venting system 22. The venting system can be made of porous materials, coolant-dissolvable cores, or fluid-soluble cores, such as water-soluble cores conventionally used in the metal casting industry. When a venting system 22 is located at the distal end of the casting 26 as shown in FIGS. 2A and 2B, the venting system can be simply a solid plug, a metal chill or a cooling nozzle which is initially used as a chill to solidify the distal end of the casting first and then, after a solid shell is formed at the distal end of the casting, is removed out of the mold, leaving behind a hole allowing used coolant from nozzle 10, mold debris, and moisture to escape from the mold. After the thin layer 15 neighboring nozzle 10 is broken, a pathway is formed between nozzle 10 and the venting system 22 via the air gap 17. The air gap 17 becomes thicker when a coolant flows through the gap and erodes the coolant permeable materials. The next array of nozzles, i.e., nozzle 12 is then operating, delivering a coolant to breaking the thin layer 15 in front of the nozzle to contact and cool the external surface of the solidifying casting. Used coolant from nozzle 12 and mold debris formed adjacent to nozzle 12 can travel through the just formed pathway via nozzle 10 and be discharged out of the mold. By turning on the nozzles sequentially from the distal end to the riser of the casting to directly contact the external surface of a solidifying casting and cool the solidifying casting, the casting is solidified progressively towards the riser/downsprue. Used coolant, mold debris, and moisture arising from array of nozzles that are under cooling operation can be removed through venting systems and nozzle holes where nozzles were removed after the nozzles have completed their cooling operations.

(19) One of the nozzles 10 can also be placed at the distal end of the casting as shown in FIG. 3 and be used as a chill discussed above. After delivering a coolant to cool the distal end of the casting and to form a solid shell there, this nozzle 10 can be removed out of the mold, leaving behind a hole to release used coolant, mold debris, and moisture generated by the next array of nozzles, i.e., nozzle 12. Used coolant from nozzle 12 is designed to break through the thin layer 15 in front of the nozzle and flow through the just formed pathway via nozzle 10 and be discharged out of the mold. Nozzle 10 can also be removed so that the nozzle hole can be used for discharging used coolant and mold debris produced adjacent to nozzle 12. By turning on the nozzles sequentially from the distal end to the riser of the casting to directly contact the external surface of a solidifying casting and cool the solidifying casting, the casting is solidified progressively towards the riser/downsprue. Used coolant, mold debris, and moisture arising from array of nozzles that are under cooling operation can be removed through venting systems and nozzle holes where nozzles were removed after the nozzles have completed their cooling operations. Used coolant can also be discharged via gap 11 shown in FIG. 3.

(20) FIG. 3 illustrates a method and an apparatus of another embodiment of the present invention of CNC casting using molds with a sand liner. Since sand is used only for the purpose of a smooth mold filling and the cooling of the casting is controlled by using a coolant from nozzles, the thickness of the sand layer can be significantly reduced so that less sand and binder are needed to make molds. FIG. 3 shows an expendable sand liner 21 in metal molds 20. The sand liner 12 can be made using sand blower. Nozzles 10, 12, 14, 16 are fixed on a rigid fixture 48 and are embedded in the mold 20 and the liner 21 during or after the liner 21 is made. The sand liner 21 contacts the molten metal 26 and 30 during mold filling. The nozzles 10, 12, 14, and 16 deliver the predetermined amount of coolant at predetermined times to the locations where rapid cooling is required. The coolant delivered from the nozzles 10, 12, 14, or 16, penetrates and breaks the permeable sand layer or coating 15 and directly contacts the external surfaces of the casting 26 to maintain a progressive solidification from the distal end of the casting 26 to the riser 30 as shown in FIG. 2. The gap 11 allows the used coolant and resultant gases to escape from the cooling zone. The venting system (not shown in FIG. 3) and the nozzles can be used to release used coolant, mold debris, and moisture from the mold as described above. Fluid-soluble cores can be used in the venting system.

(21) The sand liner 21 is expendable and used for only once. The metal mold 20 supporting the sand liner 21 can be used for many times. Since a new sand liner 21 needs to be made for each casting 26, the dimensional accuracy of the casting 26 is ensured regardless of deformation/distortion that may occur in the metal molds 20.

(22) FIG. 4 illustrates a method and an apparatus of yet another embodiment of the present invention of CNC casting using a liner in metal molds. The liner 21 can be made of sand, insulation materials, or thermal barrier materials. By using a thin liner 21 and a heating source 52 shown in FIG. 4, the liner 21 can be quickly heated up to elevated temperatures before or after the molds 20 are closed and in waiting to receive molten metal. To speed up the heating of the liner 21, external heating sources such as infrared heating (not shown in FIG. 4) can be used to assist the heat source 52 in heating up the cavity side of the liner 21 after the cope (top mold) or drag (the bottom mold) are made but before they are closed. The liner or mold can also be heated up and maintained at predetermined temperatures by using heating source 52 embedded in the mold 20 or sand liner 21, external heating source such as infrared heating, or a combined used of embedded heating source 52 and external heating source. Moisture and gases in the liner 21 can be better removed at high temperatures for producing castings 26 with improved internal integrity. Molten metal 26 and 30 can be poured slowly to avoid oxide formation and entrapment during mold filling. More importantly, an extremely large casting 26 can be made using a metal mold 20 supported thin liner 21 which is heated to close to the liquidus temperature of the casting metal 26. When the sand liner 21 is at the liquidus temperature of the casting metal 26, heat loss from the molten metal 26 during mold filling is minimized so that the molten metal 26 can be poured into the mold cavity very slowly and flow easily to fill a thin mold cavity with a great length. Theoretically, there is no limit to the size of a thin-wall casting 26 to be made using the present invention if the liner 21, supported by rigid molds 20, is held at the same temperature as the cast molten metal 26. A controlled delivery of coolant through nozzles 10, 12, 14, and 16 penetrates and breaks the sand layer or coating 15 to directly cool the surfaces of casting 26 which ensures progressive solidification from the distal end of the casting 26 to the riser 30. The benefits using the present invention shown in FIGS. 3 and 4 include 1) reduced use of sand and binder, 2) smooth filling of the mold cavity, and 3) the capability of making large and thin-wall castings with high internal integrity and mechanical properties.

(23) FIG. 5 illustrates a method and an apparatus of yet another embodiment of the present invention of CNC casting using semi permanent molds or permanent molds. Nozzles 10, 12, 14, and 16 are embedded in the molds 20. A layer of coating 50, which is permeable to the coolant to be used, is applied on the internal surfaces of the molds 20. Controlled delivery of the coolant from nozzles 10, 12, 14, and 16 penetrates and breaks the coolant permeable coating 50 to directly cool the surfaces of the casting 26 and to ensure progressive solidification from the distal end of the casting 26 to the riser 30. Gaps 11 and venting plugs 22 allow the used coolant and resultant gases to be released. The benefits of using the present invention shown in FIG. 5 include 1) reduced solidification time of the casting, or reduced production cycle for each casting, and 2) improved internal integrity and mechanical properties of the casting.

(24) The invention further provides examples of the present invention of CNC casting. The examples provided below are meant merely to exemplify several embodiments, and should not be interpreted as limiting the scope of the claims, which are delimited only by the specification.

Example 1

(25) Mold filling during sand mold casting can be relatively well controlled compared to that during high pressure die casting [12-13]. However, the freezing rate is much lower in sand molds than that in metal molds because sand has a lower thermal diffusivity than metal. As a result, sand castings usually have coarse solidification microstructures and poor mechanical properties. Grassi et al tested making automotive steering knuckles of aluminum A356 alloy using a typical sand casting process and an ablation casting process [1-2]. They found that that the tensile strength, yield strength, and elongation in samples taken from the conventional sand casting were 228 MPa, 179 MPa, and 3.5 respectively. Using water as solvent in the ablation casting process, the tensile strength, yield strength, and elongation in samples taken from the casting were 325 MPa, 261 MPa, and 12.5 respectively, much higher than that in the sand casting. It is expected that castings made using the present invention of CNC casting as shown in FIGS. 1A and 1B using water as the coolant should have identical mechanical properties as those made using the ablation casting process. However, compared to the ablation casting process which has to use a soluble binder to make the sand molds, the present invention of CNC casting shown in FIGS. 1A and 1B can be used for aggregate mold using any conventional binder including clay/water, sodium silicate/water, resin, and oil. Thus, the present invention shown in FIG. 1 can retrofit in the existing sand casting production lines for mass production of castings.

Example 2

(26) Steel railway wheels were initially made using a sand mold with a metal ring to chill the tread of the wheel to encourage progressive solidification starting from the tread surface to the wheel hub [14]. Later, a graphite mold technology was developed [15]. Steel wheels produced using graphite molds are more consistent in quality than those made using sand molds. U.S. Pat. No. 3,302,919 to Beetle et al. describes a method of using a sand liner in graphite molds to make a cast steel railway wheel. As shown in FIG. 6A, part of the graphite mold is used for cooling the tread and rim of the wheel during solidification. The sand liner is used to slow down the cooling rate of the wheel plate, allowing a progressive solidification from the tread to the hub of the wheel [15]. However, the cost of using the graphite technology is much higher than using the sand mold technology to make cast steel railway wheels. It is expected that the present invention shown in FIG. 3 is capable of producing railway steel wheels without using expensive graphite molds. FIG. 6B is a schematic presentation for producing steel wheels using present invention. Metal molds are lined with a sand liner. Nozzles are placed near the tread and rim of the wheel. The coolant is administrated to the surfaces of the tread and rim of the wheel to achieve identical cooling rates equivalent to or higher than that of the graphite to ensure progressive solidification from the tread to the hub of the wheel. Since the expendable sand liner is molded for every wheel casting using a sand blower, the dimensional accuracy of wheel made using the present invention should be better than that made using graphite molds which suffer a gradual damage at the surfaces in contact with the high temperature molten steel.

Example 3

(27) In the automotive industry, thin-walled large aluminum castings are usually made using the high pressure die casting (HPDC) process because the sand casting process is not capable of producing such castings. HPDC is also termed as die casting. During die casting, high pressures have to be used to inject molten aluminum at high speeds into the cavity in molds made of steel in order to be able to fill the entire die cavity [13,16]. Still, there is a limit on the size of a casting that the die casting is capable of making. U.S. patent application Ser. No. 15/874,348 by Kallas of Tesla, Inc. discloses a giant die casting machine for the production of the entire body frame of a car in a single press. The body frame part may be the largest thin-walled aluminum casting to be made in the casting industry. The present invention shown in FIG. 4 provides a method and an apparatus to make castings such as the entire body frame of a car. The expendable liner provides the dimensional accuracy of the casting; the heated liner maintained at high temperatures allows a slow and smooth mold filling of the entire die cavity; and the embedded cooling nozzles allow a progressive solidification from the distal ends of the casting to the riser. It is expected that the body frame of a car made using the present invention should have much better internal integrity and mechanical properties than that made using the HPDC process. This is because 1) progressive solidification is not achievable for thin-walled castings solidifying in metal molds, and 2) entrapment of oxides and gases are unavoidable during die casting which is associated with a turbulent mold filling process. Furthermore, it is extremely expensive to build a giant die casting machine for the production of extremely large thin-walled castings.

Example 4

(28) Because progressive solidification is not achievable in HPDC process, the industry has been using various means to achieve local progressive solidification using cooling lines in the metal mold or cooling pins. Cooling lines are drilled into a block of a metal die so the cooling lines are usually straight. The coolant, usually water or oil, is not in direct contact with the casting. Instead, it is only circulating in the cooling lines to take away heat from the die. To prevent damage to the expensive metal die, the cooling lines are usually drilled at least 10 mm away from the cavity surfaces. Heat extraction of these cooling lines from the solidifying casting is limited by the thermal diffusivity of the at least 10 mm thick steel. It is widely believed that the cooling lines are effective only in maintaining the dies at certain temperatures and are ineffective in reducing local solidification time in the casting. Cooling pins are more effective in achieving local progressive solidification in a casting. The cooling pins are made of metal and have coolant circulating within them as well. Still the chill effect of the cooling pins is limited by the thermal diffusivity of the metal separating the coolant from the casting, although the thickness of this metal layer becomes thinner using 3D printing technologies. By delivering a desired amount of a selected coolant through nozzles to contact the surfaces of the casting, more effective progressive solidification can be achieved at least locally using the present invention shown in FIG. 5 than that using cooling lines or cooling pins. The cooling nozzles can also be used to drive bubbles away from the surfaces to be machined as that the machined casting will be more pressure-tight and leak-tight [17].

Example 5

(29) Aluminum automotive wheels are made using permanent mold process. The molds are made of steel. A relative thick coating is applied on the mold surface to protect the mold steel from erosion during mold filling under low pressure or under gravity casting conditions. The use of a thick coating also slows down the flow speed of the metal during mold filling. A photograph of a wheel is shown at top left image in FIG. 7. The wheel casting is made using three molds: a top die, a bottom die, and a side die, and is fed from a riser tube shown at the top right image in FIG. 7. Numerical modeling indicates that there are hot spots at the junctions between the spoke and the rim. These hot spots cannot be properly fed by the liquid metal through the big spoke from the riser tube shown in the bottom images in FIG. 7, leading to porosity formation in the junctions. By using an array of nozzles embedded in the side die which delivers a desired amount of a selected coolant to contact the surfaces of the casting near the junctions at desired times, the hot spots and the resultant porosity in the junctions can be minimized using the present invention shown in FIG. 3. In addition, the solidification time in the spoke area is a little too long. Long solidification times in the spoke regions limit the production rates of the wheels. A reduction of the solidification times in the spoke area would lead to significant cost savings. By using an array of nozzles embedded in the top die which delivers a desired amount of a selected coolant to contact the surfaces of the casting near the big spokes at desired times, the solidification times of the big spokes can be reduced using the present invention shown in FIG. 3.

(30) While the invention has been described in connection with specific embodiments thereof, it will be understood that the inventive methodology is capable of further modifications. This patent application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth and as follows in scope of the appended claims.

REFERENCES

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