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THREAD MOLD Abstract A mold and molded article are disclosed. The article,which may be a container, has a cylindrical portion adjacent one end, including a helical male thread which does not completely encircle the cylindrical portion, with the two ends of the thread obliquely spaced from one another. The parting line of the mold used in producing the article cuts generally perpendicularly to the axis of the cylindrical portion, following the maximum diameter of the helical thread and passing obliquely from one end of the thread to the other.The mold may be used in an injection molding press. 37190
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THREAD MOLD
The invention relates to molding, including injection molding, and more particularly to the molding of a male neck thread on an article such as a container.
In the manufacture of glass or plastic containers requiring a screw thread to accommodate a screw cap it is common practice to use a split mold, called a neck ring, to form the thread. The glass or plastic is forced into the contours of the neck ring while in the molten stage. Due to the pressure applied to the material and the cooler temperature of the neck ring the glass or plastic attains a degree of rigidity and conforms to the contour of the neck ring, thus producing the screw
threads on the container.
In the manufacture of plastic vials by the injection molding process this split neck ring is also used. Since the plastic is injected under pressure of many thousands of pounds per square inch, the split neck ring must be kept closed by the application of a very substantial force. However, the clamp of an injection press develops all of its force in a direction perpendicular to the force required to hold the neck ring closed while the plastic is injected. In order to use the clamping
pressure of the press to hold the neck ring closed, cams or wedges must be designed and built into the injection mold assembly. Furthermore, auxiliary means must be added to cause the neck ring to open, in order to release the finished container, after the clamping pressure has been removed.
This type of injection mold is known as a "cam action" mold. It is much more costly than the normal injection mold and requires substantially more maintenance. Moreover, the number of mold cavities which may be utilized in a particular platen area is generally substantially less if a "cam action" mold is required instead of a conventional mold.
Another type of thread mold parts vertically from the remaining mold portion below the thread, but requires unscrewing of the thread mold from the molded thread. Besides necessitating a great deal of additional equipment, this type mold may encounter problems of sticking to the molded article, particularly in injection nolding. Such unscrewing thread molds are shown in U.S.
Pat. Nos. 2,587,837 and 3,150,222.
U.S. Pat. No. 3,431,967 suggests a solution to a problem similar to that of neck thread molding. Undercut portions of a circular lens holder are limited to slightly less than one half the perimeter of the lens opening and the "male" mold for the lens holder is split obliquely, i.e., at a parting angle between horizontal and vertical,between the undercut portions. The mold is stated to be useable in an injection molding press.
SUMMARY OF THE INVENTION
The present invention makes it possible to manufacture a screw threaded article such as a plastic vial without a split neck ring, "cam action" type mold and without an unscrewing type thread mold. This is accomplished by limiting the male thread contour on the article to slightly less than 3600 and parting the mold cavity generally perpendicularly to the axis of the cylindrical portion on which the thread is located. Accordingly,two cylindrical mold sections are utilized, with the parting line between the molds following the contour of the maximum diameter of the thread projection, then passing obliquely from one end of the thread to the other.In the injection molding of cylindrical threaded articles, this eliminates the necessity for mold closing