Plastic article or earthenware shaping or treating: apparatus – Female mold and charger to supply fluent stock under... – With product ejector
Reexamination Certificate
2000-11-16
2004-01-27
Mackey, James P. (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Female mold and charger to supply fluent stock under...
With product ejector
C249S059000, C425S443000, C425S444000, C425S577000
Reexamination Certificate
active
06682336
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention relates to the field of plastic gears and plastic gear boxes for gears, and, in particular, to the formation of plastic helical gears and gear boxes utilizing those helical gears for toys and other small motorized devices.
BACKGROUND AND SUMMARY OF THE INVENTION
There is a continuing and long-felt need for inexpensive, plastic gears for use in toys and similar applications. Gears transmit rotational movement and torque forces. Gears may be used to convert the high-speed, low torque output of a rotating electric motor to a low-speed, high torque output of a wheel drive shaft for a toy car. They also may be used to move the mechanical arms of, for example, a toy construction crane. Gears for toys should be safe, inexpensive and wear resistant. Plastic gears are suitable for toys because they are safe as they do not have sharp edges (as do metal gears); may be inexpensively formed by injection molding processes, and are tolerant of the dirt and wear encounter by toys, especially toy cars, trucks and construction vehicles.
There is also a long-felt need for an inexpensive helical gear formed by plastic injection molding. Helical gears can be used in conjunction with a worm gear to transmit rotation and torque from a rotating worm gear to a helical gear. Helical gears have a variety of applications, including engaging a worm gear mounted on the shaft of a small electrical motor to turn the gears of a gear box.
By using helical gears to engage a worm gear on a motor shaft, a motor is not constrained to be mounted perpendicular to the plane of rotation of the gears in the gear box. A motor with a spur gear must be mounted so that its output shaft is perpendicular to the plane of rotation of the gear. This constraint on the mounting of a motor having a spur gear may cause difficulties in arranging the motor and gear box in a small space, such as within a toy vehicle. A motor with a helical gear may be mounted parallel to the axes of rotation of the gears in the gear box. Having the flexibility to orient the motor in relation to the gears is particularly advantageous in a small toy vehicle where the spaces for mounting a motor are limited.
In addition, a helical gear may be used to reduce the rotational speed of the motor shaft to a lower speed of a wheel rotation, with fewer gears than would be practical without helical gears. Reducing the number of gears allows gear boxes to be more compact and have fewer components, than do prior gear boxes with many spur gears. With a standard pair of spur gears their relative speeds of rotation depend on the ratio of the number of gear teeth on each gear. The number of gear teeth on a spur gear depends on the diameter of a gear. A helical gear may be rotated by (or may rotate) a worm gear, which has a small diameter relative to a spur gear. The pitch (or angle of the gear teeth on the helical and worm gears relative to the screw axis) determines the speed of rotation of the helical gear being driven by the worm gear. A relatively small screw and helical gear assembly may be used to dramatically reduce the rotational speed of a motor down to a speed suitable for the wheels of a toy car. By using a relatively small pitch angle, e.g., 6 degrees, on the helical and worm gears, the rotational speed reduction from the rotating speed of the motor gear to that of the driven helical gear, may be much greater than could be practically accomplished with a pair of spur gears.
Helical gears have been difficult to form by plastic injection molding. To form a gear by injection molding, a gear cavity must be formed in the mold. Liquid plastic is rapidly injected into the mold cavity and the plastic is allowed to solidify during a cure period. Once the plastic has hardened, the mold is split apart and the plastic gear removed. This process of injecting liquid plastic curing, opening the mold and ejecting a gear is repeated rapidly in a typical commercial injection mold apparatus. Difficulties arise during the molding process such as: the plastic flows into surface imperfections of the mold cavity; the metal that forms the mold cavity may corrode; the volume of plastic injected in the mold cavity may be excessive; the cooling period needed to hardened the injected plastic may be inadequate; and the ejection process may be too fast for helical gears.
These and other problems with the plastic injection process have in the past made it difficult to form helical gears at sufficiently fast production rates. The production rates must be fast to satisfy the demand for plastic gears and to reduce the cost of manufacturing these gears. If the production speed is too slow, then the cost to manufacturer plastic gears especially helical gears, becomes greater than the cost to use metal gears or other alternatives to plastic gears. If the production of plastic gears is prone to malformed gears or gears that do not properly eject from the mold, then the cost to make the gears becomes excessive. In the past, helical gears have not been made from plastic because the production rate has been inadequate to meet the demand for gears used in toys and the cost has been greater than the cost of metal gears or of other alternatives to plastic gears. Accordingly, there has been a long-felt demand for plastic helical gears.
In the present invention, a helical gear is formed by an injection molding process in which the mold cavity is formed of mirror finished hardened stainless steel. The mirror finish prevents the plastic of the gear from sticking to the mold cavity, and the stainless steel is corrosion resistant. The injection of plastic is carefully metered to dose the proper amount of plastic and to apply the proper pressure to the plastic. By properly metering the plastic injection the invention avoids the problems associated with over-packing the mold cavity with plastic, such as gear warpage and excessive internal stresses in the gears.
Once the plastic is injected, the cooling period allotted to a helical gear is longer than the cooling period for straight gears. Moreover, the pin is balanced and straight such that the gear slides smoothly off the pin as the gear ejects from the mold. A sleeve that forms a collar to the ejection pin slides along the pin to eject the gear from the mold. The ejection of a helical gear is conducted at a slower speed than the ejection of the straight gears. Helical gears have gear teeth that are at an angle with respect to the gear axis. The ejection of gears from a mold is in the direction of the axis of the gear. For a straight gear, the ejection is a straight, non-rotating movement in the direction of the gear axis. To eject a helical gear the gear must rotate slightly as the gear moves out of the mold, to accommodate the angled gear teeth. To allow the helical gear to rotate as it is ejected, the gear must be more slowly ejected from the mold than the ejection speed used for straight gears. If the helical gear is ejected too quickly, the gear teeth may be damaged or stripped off. By slightly reducing the ejection speed of the gears and implementing the other features of the invention, helical gears can formed by plastic injection molding at production rates sufficient to produce low-cost gears for toys and other mass-produced products.
There is also a long-felt need for gear boxes that may be conveniently arranged in or integrated with toy vehicles and other small devices. A gear box transmits rotation and torque through an assembly of intermeshing rotating gears. An input shaft to the gear box transmits a drive rotation to the gears and to an output shaft(s) from the box. As the drive rotation causes the intermeshing gears in the box to rotate, the rotational speed of each of the gears will vary depending on the gear teeth ratios of each pair of gears. The torque and rotational speed of the output shaft will be in proportion to the input shaft speed and torque, where the proportional relationship depends on the arrangement of gears between the input and output shafts.
An embodiment of the present i
Mackey James P.
New Bright Industrial Co. Ltd.
Nixon & Vanderhye P.C.
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