Cold runner molding system

Details Cold runner molding system Cold runner molding system

2002-08-21

2004-09-14

Utech, Benjamin L. (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Female mold and charger to supply fluent stock under...

Mold having movable core or movable pin

C264S161000, C425S573000

Reexamination Certificate

active

06790029

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved apparatus for injection molding hollow plastic articles and an improved method of molding hollow plastic articles.
2. Description of the Prior Art
The manufacturing process of injection molding has been used for many years to manufacture in bulk a wide variety of different types of plastic articles. Many of these articles are hollow structures. Articles may be made hollow in order to fit onto some other structure. Also, it is often much more economical to manufacture articles in a hollow rather than a solid form, both to reduce the weight of the plastic part and also to reduce the material expense involved in the manufacturing process.
For many years hollow plastic parts have been injection molded utilizing a mold comprised of two or more molding blocks having depressions therein which, when placed together, form a mold cavity. The molding blocks are separable along a parting interface. Prior to the introduction of molten plastic, a core structure is placed in between the molding blocks. Depressions in the molding blocks form one or more seats to hold the core in a proper, predetermined position within the mold cavity. The molding blocks are then closed upon the core to at least partially encapsulate it within the mold cavity. Molten plastic is then forced under pressure through a passageway formed by mating, concave channels in mating surfaces of the molding blocks to enter the mold cavity through a duct called a molding runner gate. The molten plastic is then forced into the mold cavity surrounding the core and fills the entire volume of the mold cavity except that portion of the volume of the mold cavity occupied by the core. The mold is then cooled so that the plastic solidifies to form a molded plastic structure. The core is then removed from within the molded plastic structure.
One problem that frequently arises when fabricating injection molded plastic parts in this manner is that a noticeable mark is formed on the area of the exterior surface of the plastic article that is formed at the runner gate. This mark may take the form of a protrusion, an indentation, or some other surface defect. In any event, the appearance of such a surface blemish is often unacceptable to the customer for whom the part is manufactured.
To remedy this defect the fabricators of plastic articles have sometimes attempted to introduce the molten plastic into the mold cavity through the mold core. To do this the mold core must be hollow in order for the molten plastic to flow through it. In such a system the molten plastic flows outwardly from the hollow center of the core through one or more ejection gates and into the mold cavity from the outer surface of the core, which forms the interior surface of the hollow plastic article. Since the interior surface of the plastic article is normally not visible, the appearance of surface blemishes on the interior surface of the molded article at the location of the ejection gates is normally not a matter of concern.
However, certain difficulties and disadvantages exist with the use of conventional molding systems of this type. Since the molten plastic is conducted through a passageway in the core, a certain amount of plastic is left within the passageway in the core once the molded plastic article has been formed. Plastic left in the core passageway is termed a “runner” and must the removed before the core can be used again. One conventional technique to accomplish this is to maintain the core in a heated condition even after the hollow plastic article has been formed so that the runner remains in a melted condition and can be poured in molten form from the core. Even in molding a plastic article in a single shot an additional amount of thermal energy is required, thus adding to the expense of the manufacturing process. Also, a step of inverting the core to pour the molten plastic from it is often required. The core is unavailable for service in manufacturing a subsequent hollow plastic part during the time that the melted plastic is being poured from it. As a consequence, the throughput of fabricated molded plastic articles is reduced. These problems are quite significant in the competitive field of injection molding.
This conventional technique of injection molding hollow plastic articles is particularly unsatisfactory when the articles to be fabricated are molded in several stages using plastics having different characteristics, for example different colors. In such a situation it is necessary not only to pour the molten runner from the core in order to remove it from the core, but it is also necessary to “purge” the core as well. That is, a certain amount of the next color of plastic to be used in the next stage of fabrication must be introduced into the runner passageway of the core and then poured out of the core while still in molten form in order to collect and carry with it residual amounts of the plastic used in the prior stage of fabrication. The plastic which is employed in the purging operation is discolored and is unsuitable for reuse. It must therefore be discarded as waste. This adds significantly to the cost of manufacturing each multicolored plastic article.
Furthermore, even with purging, not all of the prior plastic in the runner passageway comes out of the core. As a consequence, discoloration in the next subsequent shot of plastic is quite common. Moreover, since the core remains heated with the layer of plastic formed during the earlier stages of manufacture still on it, a loss of definition is likely to occur because the heated core maintains the preliminary portion of the article formed in a soft condition on the exterior surface of the core. This loss of proper definition in the shape of the portion of the article manufactured in the earlier stages of fabrication occurs because it is not possible to maintain the interior of the core in a heated condition without softening the portion of the plastic products already formed on the outer surface of the core. This produces a product of inferior quality. Moreover, because the core never really cools, the several layers of plastic molded onto the core tend to remain at too high a temperature. This causes them to mix at their interfaces, thus creating a further loss of definition at the interfaces between the different colors of plastic.
SUMMARY OF THE INVENTION
The present invention involves a system of injection plastic molding in which the molten plastic is injected into the mold radially outwardly from within a hollow, metal core. The core has opposing ends, at least one of which is an open end. The core has a longitudinal axis that extends between the opposing ends. The core has an outer surface, and defines within its interior, a core runner cavity that has a uniform, longitudinal cross section throughout relative to the longitudinal axis of the core. The core runner cavity extends through the open end of the core. A removable core end closure is located at the open end of the core and defines a core runner extension cavity within its structure. The core runner extension cavity of the end closure is in communication with the core runner cavity and is aligned on the longitudinal axis of the core.
Once the molten plastic has been injected into the mold cavity through the hollow core located therewithin, the mold halves are parted so that the core can be removed. Moreover, and unlike prior systems, the core does not need to be maintained in a heated state so that the plastic from within the core can be drained therefrom. Rather, the mold is allowed to cool. Even though the elongated runner within the hollow core cools, it can still be removed from the core even in a solidified state by merely uncapping the core, gripping the portion of the cold runner that has formed in the core runner extension cavity, and pulling the cold, solidified runner longitudinally out of the hollow core. The plastic runner is resilient enough so that it will pull free of the core. Indeed, the application of l

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