Injection molding apparatus

Details Injection molding apparatus Injection molding apparatus

2000-11-08

2002-04-30

Davis, Robert (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Distinct means to feed, support or manipulate preform stock...

Opposed registering coacting female molds

C425S125000, C425S468000, C425S577000

Reexamination Certificate

active

06379138

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a cam ring for actuating retractable pins for a golf ball injection mold.
BACKGROUND OF THE INVENTION
It is standard practice in the fabrication of an intermediate layer or cover layer of a golf ball to utilize an injection mold having two mold plates with hemispherical cavities that mate to form a spherical shape when the mold halves are joined. At the initial stage of the injection molding process, a golf ball core placed inside the mold is supported centrally within the mold by a plurality of retractable pins located near the upper and lower poles of the mold cavity so as to leave a space for forming an intermediate or cover layer about the core. The retractable pins are activated by a plate that controls movement of the pins in a vertical direction to engage with the core to hold it securely in place. After the pins have engaged with the core of the ball, thermoplastic or thermnosetting material then is injected into the mold cavity in a horizontal plane from a primary supply through a plurality of edge gates that usually are evenly distributed near or around the parting line of the mold halves and the equator of the inner hemispherical surface of the golf ball. The retractable pins hold the core in place while the injected material fills the void between the core and the inside wall of the mold. Trapped air and gasses escape through vents located at the upper and lower poles of the ball as flow from injected material from each of the plurality of gates eventually joins to fill the void between the golf ball core and the mold. Once the void is nearly filled but before the injected material has completely hardened, the pins are retracted from the mold cavity in a vertical direction until the faces of the pins form a portion of the mold cavity wall. If the pins are retracted only after the injected covering material has contacted the pins, any voids formed from retraction of the pins are filled by the injected material. Once the injected material has substantially hardened, the mold is opened and the ball is removed.
Use of a plurality of retractable pins to securely position the golf ball core during the injection process is known to cause wear at the interface between the surfaces of the pins and the surfaces of the pin holes in the mold plate through which the pins are inserted. Typically, the face of the pins that contact the core of the ball are not normal, i.e. perpendicular, to the direction of the axial force applied to the pins to cause them to engage with the core. In a conventional retractable pin golf ball injection mold, illustrated in FIGS.
1
and
2
, the pins are arranged in a circle and are engaged with the core by being inserted into the mold in a vertical direction. The faces of the pins, however, are angled so that they contact the core essentially along a tangent to the contacted surface of the ball in order to have a good grip on the golf ball core. For forming intermediate layers, the faces of the pins that contact the ball core typically are curved to conform with the mold cavity wall (e.g. having a radius cut) or have an angle cut matching the tangent to the point of ball contact, whereas the tips of pins in an injection mold to form a cover layer are shaped with a dimple radius formed on the end thereof. In both instances, the faces of the pins contact the ball at an angle not normal to the direction of the vertical axial force applied to the pins. As a result, the ball core applies a counterbalancing force on the pins that has an axial load component and a cantilever load component. As the pins move under this cantilever load when engaging or disengaging from the ball core, the pin holes are worn out of round and the pins may spread, flex and/or experience extensive wear. In some instances, galling of the pin and pin hole may result. Wear between the pins and pin holes eventually becomes excessive and allows injected material to flow into the worn area, causing undesirable flash on the surface of the molded layer of the ball. The result of this undesired wear is that the manufactured balls require additional process steps to remove the flash and the mold must be shut down periodically for inspection, repair and/or replacement of worn tooling. Thus, it would be desirable to engage and disengage the retractable pins in a direction that is essentially normal to the tangent of the contacted surface of the ball.
A second disadvantage of conventional retractable pin injection molding that results from having the retractable pins not operate in a direction normal to the tangent of the point of contact with the ball core is the time, expense and precision required for forming the face of the pin. Because the faces of conventional retractable pins are angled to better grip the ball, forming a dimple radius on the end of each pin requires expensive, high-precision processing, such as using an EDM process to “burn” the shape of the dimple on the end of the pin or using a compound angular set up to produce an oblique conical radius on the pin. The dimple formed on the face of the pin is elliptical because the face of the pin is elliptical due to its angled cut, pins being generally circular in cross-section. Noticeable cosmetic defects can result if the elliptical dimples formed on the face of the pin are even slightly out of place. Moreover, because each pin is custom-made to match the geometry of the mold, the pins can not be used in a mold having a different geometry. Thus, the retractable pins can not be reused or repositioned should the mold geometry or dimple pattern change.
Yet another disadvantage associated with conventional injection molding is that the use of multiple gates dispersed around the equator of the mold cavity is known to cause “knit lines” on the newly formed ball layer when injected layer material from neighboring gates intersects as the material fills the mold cavity. “Knit lines” are seams along the newly formed intermediate layer or cover layer that are formed where the injected material intermixes from different gates during the formation of the layer.
FIG. 2
illustrates the formation of knit lines
10
as flow from any one gate
12
intersects with flow from a neighboring gate. When a golf ball cover is formed by a conventional retractable pin injection process with multiple edge gates to inject a layer material into a mold, the injected material from each gate has a flow front that eventually intersects with layer material entering the mold from other edge gates. Knit lines are formed at the intersection of each of these converging flow fronts. The multiple knit lines of the newly formed layer ultimately intersect at the flow terminus of the layer material near the upper and lower poles of the mold cavity. As such, there are a number of knit lines or flow fronts throughout a layer where layer material from various gates flows together as it fills the void between the golf ball core and the mold. Depending on the composition of the injected material, the material tensile strength can be reduced by as much as 10% to 60% along the knit lines. Thus, because the intermediate or cover layer is inherently weaker along the knit lines, it is desirable to minimize the occurrence of knit lines when forming a golf ball layer. Therefore, there exists a need for a method of making golf ball layers by an injection molding process that does not result in the occurrence of knit lines, thereby increasing the durability of the layer and extending the useful life of the golf ball.
In addition to resulting in knit lines that may weaken the golf ball cover, conventional multiple edge gate injection molding also may not maintain balanced flow or uniform filling of thermoplastic resin blend cover material between the core and the inside wall of the mold, which may further weaken the golf ball cover. For example, non-uniform filling can cause the flow terminus of the cover to not meet at the poles of the ball where trapped air and gasses typically are released through a vent. When the flow ter

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