AND FASTENING ELEMENT

The invention relates to a method for injection molding a fastening element from plastic by means of an injection mold that encloses a cavity, wherein the fastening element has a body extending along an axis with at least one wall that encloses a hole extending in the axial direction for accommodating a stud or the like. The invention further concerns a fastening element injection molded from plastic with a hole for accommodating a stud.

Fastening elements of the aforementioned type, especially nuts, are known from DE 298 10 428 U1 , DE 100 48 975 C1 and DE 10 2005 006 592 A1 . The latter document also describes a manufacturing method of the specified type. Thermoplastic materials are used for injection molding such fastening elements, but their thermal stability is limited.

The object of the present invention is to specify a fastening element of the said type and to specify a method for injection molding the same that can be used in an environment where relatively high temperatures, in particular 200° and above, can occur.

The said object is attained by a method with the features specified in claim 1 and by a fastening element with the features specified in claim 10. Advantageous embodiments of the method are specified in claims 2 through 9 and advantageous embodiments of the fastening element are specified in claims 1 1 through 16.

According to the invention, a liquid crystal polymer material (LCP) is used as the plastic in the method for injection molding a fastening element from plastic by means of an injection mold that encloses a cavity and that molds a fastening element having a body extending along an axis with a wall that encloses a hole extending in the axial direction for accommodating a stud or the like. The injection mold here is designed such that the cavity has a uniform thickness and the liquid crystal polymer material (LCP) is injected centrally into the cavity of the injection mold at an axial injection end in such a manner that the flow front of the material advances uniformly axially into the cavity. In this context, the thickness of the cavity is understood to mean the spacing of cavity walls that are directly opposite one another and that together with one another mold a wall of the fastening element. The cavity thickness thus corresponds to the thickness of the wall molded by filling the cavity. Liquid crystal polymers are distinguished by high heat resistance, so that fastening elements molded therefrom can be used in areas with high thermal stress. However, the LCP materials used for injection molding have anisotropic properties with process-induced orientation resulting from alignment of the molecules. In order to achieve the desired molded part properties, it is thus critical to achieve a molecular orientation that is optimal for the strength requirements of the fastening element by means of the direction of flow of the material during injection. The creation of knit lines should be avoided as much as possible here, as LCP material exhibits reduced knit line strengths. It has been shown that a molecular alignment with very advantageous strength properties for a fastening element, preferably usable as a nut, can be achieved by locating the injection point at one axial end and centered with respect to the wall formed by injection molding. The injection process can be well controlled, and the creation of knit lines with reduced strength is largely avoided.

According to another proposal of the invention, central injection of the LCP material can be achieved in an especially advantageous manner by the means that a sprue that closes the hole on one side and into which the polymer material is centrally injected is formed by the injection mold at the injection end of the wall. The sprue results in a very uniform distribution of the material in the annular end of the cavity adjoining the sprue for molding the wall, and thus a uniform flow front as well. The sprue has the further advantage that only one centrally located nozzle is required for the exit of the polymer material. This considerably simplifies the manufacturing and maintenance of the injection molding tools. So that the sprue does not hinder the complete passage through the hole of a stud inserted into the fastening element, according to another proposal of the invention the sprue can be designed to be separable by slots and/or predetermined breaking points. By this means, the stud entering the hole can separate and spread apart the sprue.

Alternatively, according to another proposal of the invention, the injection end of the cavity for molding the wall can be annular in design, with the polymer material being injected into the cavity of the injection mold by means of an annular nozzle.

According to another proposal of the invention, a flange that encloses the hole and has essentially the same thickness as the wall can be molded onto the end of the wall opposite the injection end by means of appropriate design of the injection mold. In addition, the flange can be provided at its circumferential edge with a reinforcing ring extending axially and having the same thickness as the flange.

According to another proposal of the invention, the cavity of the injection mold can be designed such that ribs extending radially outward are molded onto the outside of the wall of the body, wherein the thickness is essentially the same as the thickness of the wall. The ribs serve to reinforce the wall. Some or all ribs can be provided with end sections that are molded onto the flange and that extend radially outward to the circumferential edge of the flange. This achieves reinforcement of the flange and joining of the wall to the flange.

According to another proposal of the invention, tangentially extending reinforcing ribs can be molded onto the outer, axially extending ends of some or all ribs located on the outside of the wall in order to form a tool engagement region on the outside of the body, with the outer surfaces of the reinforcing ribs forming corner regions of a polygonal prism. This design in accordance with the invention makes it possible, by means of walls having uniform and relatively small thickness, to form a tool engagement region on the outside of the body of the fastening element with an outer diameter that is independent of the diameter of the hole and can advantageously be made large.

The invention is explained in detail below with reference to exemplary embodiments that are shown in the drawings. They show:

Figure 1 a perspective view of a first embodiment of a fastening element according to the invention,

Figure 2 a top view of the fastening element from Figure 1 ,

Figure 3 a side view of the fastening element from Figure 1 ,

Figure 4 a cross-section along line IV - IV in Figure 2,

Figure 5 a cross-section along line V - V in Figure 2, and

Figure 6 a perspective view of a second embodiment of a fastening element according to the invention.

Figures 1 through 5 show a fastening element 1 that can be used as a nut for fastening parts to a threaded bolt. The fastening element 1 has a hollow cylindrical body 2 with a wall 3 that, as is shown in Figures 4 and 5, encloses a central cylindrical hole 4. The hole 4 is provided with an internal thread, but can also be designed to be smooth if the fastening element is to be attached to a stud with a self-cutting thread. The top end of the hole 4 in the drawing is closed by a conical sprue 5 whose tip projects away from the body 2. The sprue 5 has a smaller wall thickness than the wall 3, and has three slots 6 arranged at uniform spacing from one another and a predetermined breaking point 7 in the center of the slots 6. The slots 6 and predetermined breaking point 7 make it possible to separate the sprue 5 while the fastening element is being screwed onto a stud when the stud entering the hole 4 presses against the sprue 5 in the axial direction. After separation of the sprue 5, the segments of the sprue that have been separated from one another can be spread apart and pushed aside by the further advance of the stud, so that the stud can emerge unobstructed from the hole 4 at the injection end of the body 2.

Molded onto the end of the body 2 opposite the sprue 5 is a flat, plate-shaped flange 8 that encloses the hole 4 in an annular fashion. The circumferential edge of the flange 8 is reinforced by a reinforcing ring 9 that projects from the flange 8 at its top side facing the body.

Ribs 10, 1 1 extending in the axial direction and radially outward are located at uniform intervals from one another on the outside of the wall 3. Adjacent to the flange 8, the ribs 10 are provided with extensions 12 that extend radially outward to the reinforcing ring 9 and are connected thereto. The ribs 10 and the extensions 12 reinforce the connection between the wall 3 and the flange 8, and increase the stiffness of the flange 8.

The ribs 1 1 are in each case located between the ribs 10, and at their radially outer edges carry reinforcing ribs 13 that extend in the tangential direction on both sides of the ribs 1 1 at a distance from the wall 3. The outer surfaces 14 of the reinforcing ribs 13 form corner regions of a polygonal prism, here a hexagonal prism. This creates a tool engagement region with an advantageously large outer diameter. The ribs 10 have a radial width in the region provided for tool engagement such that their end faces lie in a common plane with the outer surfaces of their respective adjacent reinforcing ribs 13.

Because of its design, the fastening element 1 is especially suitable for manufacture from a liquid crystal polymer material, in particular from a high- molecular-weight, thermotropic LCP material in an injection molding process. All sections of the fastening element 1 , such as the wall 3, flange 5, reinforcing ring 9 and the various ribs, have an essentially uniform, relatively small thickness. Moreover, they are arranged such that with central injection of the material into the tip of the sprue, the flow front advances uniformly into the injection mold, avoiding knit lines where material joins from opposite directions. Central injection also results in a molecular alignment that can be expected to result in favorable strength values for meeting the requirements the fastening element is subject to in use. Consequently, forces that arise in the fastening of parts by screwing the fastening element onto a stud can be accommodated well. The described design of the fastening element and the method for its manufacture by injection molding are also suitable for manufacture of the fastening element from fiber-reinforced plastic, since the fibers likewise take on a prevailing alignment in the axial and radial directions. Figure 6 shows a variant embodiment of a fastening element 21 that differs from the fastening element 1 in that the hole 24 passing through the body 22 is fully open at the injection end facing away from the flange 28. Instead, the body 22 forms an annular surface 30 at the injection end where the material can be injected into the injection mold with the aid of an annular nozzle for primary molding of the fastening element 21 . In the surface of the injection mold that molds the annular surface 30, the annular nozzle has a concentric annular gap with a width that is constant but smaller than the width of the annular surface 30. During the injection molding process, the LCP material exits the annular gap uniformly in the shape of a tube and fills the injection mold with a uniformly advancing flow front. Aside from the difference in the design of the injection means, the injection molding process and the properties of the fastening element 21 that are achievable therewith correspond largely to those described above in connection with the fastening element 1 .