Mold for Making an Internally Threaded Housing

The present invention relates to molds for making internal threads or other mechanically engaging detail for a threaded length-adjusting device, such as a micrometer device used with agricultural implements, like the manually propelled wheeled spreader disclosed in our copending application, entitled Collapsible Drop Spreader, Ser. No. 08/504,473, filed Jul. 20, 1995, and assigned to the same assignee as the present invention.

Heretofore the making of such devices having an internal thread or threads by injection molding techniques has required a mold form requiring multiple parts for disassembly after the molding operation, or in other cases requiring the mold core to be rotated or collapsed, an expensive maintenance-prone and time-consuming operation.

It is the principle object and primary purpose of the present invention to address the aforementioned needs and thus provide a new and efficient mold for forming the internally threaded member of a linear or length-adjusting device, such as a micrometer device used with a spreader.

It is a further object of the present invention to provide a simple two-part mold for forming both a thread member on the inside surface of a hollow housing and the hollow housing itself.

According to one embodiment of the invention first and second mold members are fitted one within another to define a first annular space for forming a thin-walled housing for the micrometer device, as well as a second annular space coaxial and contiguous with the first annular space, the second annular space defining segments or portions of a single helical thread for the interior of the thin-walled housing.

The invention will be better understood as well as further features, objects and advantages thereof become more apparent from the ensuing detailed description taken in conjunction with the drawings.

FIG. 1 is a schematic view partly in cross-section showing the two-part linear adjustment micrometer device used with the spreader, according to the invention; and

FIG. 2 is schematic cross-sectional view of the mold for making the housing part of the micrometer device shown in FIG. 1.

FIG. 1 shows a linear adjustment device, for example, a micrometer device 10 with its threaded extension member 12 that is connected by conventional means to one end of a flexible cable, as described in our aforementioned patent application, which is incorporated herein by reference. Extension member 12 is screw-threaded into the micrometer housing 14 of the micrometer device 10 in a known manner and the housing 14 by means of a bracket member 15 is connected to an operating lever on the handle of the spreader (not shown). The degree of rotation of the extension member 12 with respect to the housing 14 will vary the length of the micrometer device as a whole and hence the flexible cable and the size of the opening the shutoff bar or slide makes with the opening in the bottom of the hopper. Towards this end, the exterior threads 16 of the extension member are intended to cooperate with an annular array of radially extending teeth 18 disposed on the interior of the housing 14 for the necessary threaded engagement between the two members. The teeth 18 are seen to be discrete elements, each spaced from the other and together forming a single annular row on the internal surface of the housing 14. While this arrangement is preferable, the teeth need not be discrete but could be defined by a single and continuous rim or thread instead. However, the discrete teeth members shown facilitate a working fit without binding between the teeth and the threaded portion 16 of the member 12.

In accordance with the invention, the teeth 18 are fabricated by the mold shown in FIG. 2, which is seen to comprise two parts 20, 22 that fit together, as shown, to form an annular cylindrical space for the plastic material to be injected therein to thereby form the micrometer housing 14. The lower section of the housing 14 is seen to be formed by an annular space in the lower part 22 of the mold. The top part 20 of the mold is seen to have annular extension members 24 that become more narrow in their diametrical or transaxial dimension as they extend downwardly, thus defining a core which caps or telescopes over an upstanding solid core section 26 of the lower part 22 which gradually narrows in its transaxial direction as it extends upwardly along its axis. Together the two cores define a cylindrical core of uniform diameter. Thus, the inner wall of the mold 22 and the outer walls of the extension members 24 of the mold 20 complete the annular space for forming the thin-walled housing 14. It should be understood that the mold section 22 may comprise separate parts (not shown) to allow for the separation of the lower part of the housing 14 from that part of the mold. Preferably, however, the lower part 22 is a unitary piece, as shown.

According to the invention, the faces 28 of the extension members 24 of the upper mold 20 terminate just above sloped or beveled ledge portions 30 at the lower end of the upstanding core section 26 of the lower mold 22 and thus define an auxiliary or second space contiguous with the above-mentioned annular space that forms the housing. This second space defines the triangular shaped teeth 18, as shown in FIG. 1. Each of the teeth 18, then, are formed by the aligned juxtaposition by each of the top mold extension members 24 with each of the underlying ledges 30 of the core 26, thus forming the spaces for the plastic material to be injected therein in a well know manner. The triangular cross-section of the teeth provides a thin edge on the teeth that facilitates entry of the teeth into the threads 16 without binding. However, it is within the scope of the invention to provide other cross-sectional shapes, such as an acme thread, a square thread, or a buttress thread, all well known in the art. If it is desired to form a single continuous thread, that is, one full thread, rather than the discontinuous thread shown, then a continuous annular core is used rather than the discontinuous core composed of separate extension members 24, and this core member then terminates in the same manner just above a continuous ledge portion which, of course, replaces the ledge portions 30.

Thus, according to the invention, an inexpensive two-part mold is provided that obviates the need for an elaborate mold form defining an extended thread structure for matching the external threads 16 of the extension member 12.

The foregoing relates to preferred embodiments of the invention, it being understood that other embodiments and variants thereof are possible within the scope of the invention, the latter being defined by the appended claims.