Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2000-11-30
2004-09-07
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S105000, C164S113000, C164S338100
Reexamination Certificate
active
06786272
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to die casting and more particularly to preheating components of the die when casting in order to improve the process and life of the die materials.
BACKGROUND OF THE INVENTION
Die-cast motor rotors are universally produced in aluminum by pressure die-casting. That is a well-established and economical method. While copper possesses more attractive conductivity properties, leading to significantly greater motor efficiency, only small numbers of very large motors utilize copper in the rotors by mechanical fabrication. Tool steel molds that are used for the aluminum die-casting process have proved to be entirely inadequate when casting higher melting point metals including copper because they lack long die-casting life-in-service as they are unable to withstand the associated thermal stresses. Lack of a durable and cost-effective mold material has been the technical barrier preventing manufacture of the die-cast copper rotor, as well as die-casting for other high melting point and high heat capacity metals, such as stainless steel.
Die-casting, when it can be performed, is widely recognized as a low cost manufacturing process. For these reasons, die-casting has become the fabrication method of choice and aluminum the conductor of choice in almost all but the largest frame motors.
Die-cast copper rotors can provide advantages to motor manufacture and/or performance in at least the following ways: (i) improving motor energy efficiency in operation, (ii) reducing overall manufacturing cost, and (iii) reducing motor weight. Motor efficiency is a measure of the amount of power generated versus the amount of power input. Motors losses result from at least primary (i.e. stator winding) I
2
R loss (usually 34% to 39%), secondary (i.e. rotor) I
2
R loss (usually 16% to 29%), iron (core), friction and windage, and stray load. Since the late 1970s, when many aluminum stator windings were replaced by copper, there has been a focus on improving the operating efficiency of motors. Newer motor designs have recently improved efficiencies by increasing the amount of copper in windings, additional core and copper coil size, reduced windage losses, improved core steel, etc. However, the rotor remains die-cast aluminum because aluminum has a relatively low melting point of 660° C. (compared to 1081° C. for copper), and therefore that does not pose a hazard to the integrity of the mold, and because long-lived molds able to withstand repeated high thermal stresses are either not available or not commercialized.
Copper Conductive Rotors (“CCR's”) reduce rotor loss, in addition to achieving overall motor re-optimization of iron, strays, etc. CCR-based designs show overall loss reduction from 15% to 20%. Aside from the higher efficiency (92.5% versus 91% for “premium” efficiency motors), CCRs in today's premium energy efficient (EE) motors can cost approximately 5 to 8% less to manufacture than the current comparable EE motor, and/or produce a motor of comparable EE that weighs 5 to 10% less than the same energy efficient motor with traditional aluminum core rotors.
According to the study “Classification and Evaluation of Electric Motors and Pumps” DOE/CS-0147 published February 1980, sponsored by the US Department of Energy, motors above ⅙ horsepower (“Hp”) used about 60% of the electricity generated in the United States. When extrapolated worldwide, the potential economic and environmental benefits of improving the efficiency of motors by using copper rotors in place of aluminum rotors are substantial. Medium horsepower motors, that is, those in the one to one hundred twenty five Hp range (approximately 0.75 to 100 kW range), use about 60% of the electricity supplied to all motors in the US. Because of the proliferation of electric motors in this horsepower range, the projected energy savings by using the copper rotor motor is a significant national consideration. Efficiency increases (a function of motor size) from improved electrical conductivity are projected to result in total US energy savings in the year 2010 of 20.2 E+12 Btu/yr at only 10% market penetration and 143 E+12 Btu/yr at a market penetration of 50 to 70%.
CCR could be utilized to reduce rotor I
2
R losses in an existing motor design, replacing the existing die-cast aluminum rotor, without re-designing the motor to include more/better quality core, more stator windings, etc., which are the existing methods of improving motor energy efficiency in operation. CCR's can be used in specific motors to achieve a multiplicity of intermediate combinations of these design advantages. For example, where a smaller efficiency increase is required, the CCR could be used to achieve some reduction in manufacturing cost (stator winding, core, etc.) than would otherwise have been the case with traditional aluminum die-cast rotor technology.
The problem encountered in attempting to die-cast copper motor rotors is thermal shock and thermal fatigue of mold materials. Thermal cycling of the mold surface limits the mold life even in aluminum die-casting. Even when the steel mold material is pre-heated, often by circulating hot oil through the die so that it reaches about 250° C., the &Dgr;T with 1081° C. copper still far exceeds the yield strength of steel. That results in cracking (“heat checking”) of the die material. Over repeated casting shots, the tiny cracks grow into larger fractures.
Die-cast motor rotors are typically produced using aluminum because rotor fabrication by pressure die-casting in aluminum has proven to provide cost effective methods and materials for commercially feasible production runs. Copper rotors have typically not been produced because the melting temperature for copper is substantially higher than aluminum, resulting in thermal shock when the copper contacts the steel mold that greatly exceeds the yield strength of the mold material, which leads to commercially unacceptable rates of heat checking, i.e., short steel mold life-in-service, or cracking of the mold due to thermal stress. A low initial temperature of the die results in a large &Dgr;T at the surface of the die, and thus the stress in the die, on each shot. The high melting temperature, high heat of fusion, substantial latent heat and high thermal conductivity of copper combine to maximize the thermal shock.
Lack of durable and cost-effective mold material, and in particular a proven method for die casting with higher melting point electrically efficient materials has been a barrier preventing manufacture of the die-cast copper rotor. It is well known, however, that incorporation of copper for the rotor conductor bars and end rings in the induction motor in place of aluminum would result in attractive improvements in motor energy efficiency due to copper's exceptional electrical conductivity.
It is therefore an object of this invention to provide a commercially feasible method for die-casting high temperature, high performance materials such as copper that will avoid the heat checking and cracking in the mold due to thermal shock of the mold that occurs under traditionally practiced methods of die casting.
It is a further object of this invention to provide processing conditions designed to withstand the copper motor rotor die-casting environment for an commercially and economically acceptable mold material life-in-service.
It is a further object of this invention to provide a die-casting apparatus to facilitate commercial die casting of copper motor rotors.
It is a further object of this invention to employ the die-casting apparatus to die-cast copper on suitable mold materials, such that the thermal shock to the mold material does not result in exceeding its yield strength.
SUMMARY OF THE INVENTION
As a starting point, the solution to the thermal shock problem lies in the use and creation of high temperature mold materials having thermal and thermo-elastic properties conducive to minimizing thermally induced strain. Even the most resilient known mold materials, however, cannot w
Brush Edwin
Cowie John G.
Daugherty Jack
Midson Stephen
Peters Dale T.
Chadbourne & Parke LLP
Copper Development Association, Inc.
Lin Kuang Y.
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