SCREW AND MOULDING APPARATUS THEREFOR

Abstract: A plastic headed metal screw or bolt (23) has a screwdriver slot (26) in the plastic head (25) which is aligned with and intercommunicates with a complementary screwdriver slot (24) in the metal screw or bolt (23). Injection moulding apparatus for the manufacture of plastic headed metal screws, includes means defining an array of mould cavities (6) with connecting conduit means (7) for injected plastic, each cavity (6) formed so as to accommodate the head of a metal screw (23); a complementary array of projecting members (8), each member being shaped to engage the slot (24) of a said screw (23), and being located so as to project it into a mould cavity (6); and means for rotating each projecting member (8) or each screw (23) about its longitudinal axis and for biasing each projecting member (8) or screw (23) towards a cavity so as to cause each projecting member (8) to engage each slot (24).

Description: Screw and Moulding Apparatus therefor This invention relates to plastic headed screws and to apparatus for producing plastic headed screws, bolts and the like by an injection moulding process. Metal fixings, such as pins and nails, can be provided with a plastic head by a simple injection moulding process in which the head of the pin or nail is encased in moulded plastic. Such pins are highly decorative and are in much demand for fixing laminated plastic sheeting, for example in the interior finishing of caravans and boats. The injection moulding process is relatively simple. A mould block is provided, containing a linked series of mould cavities into which molten plastic can be injected in the usual way. The mould cavities are formed, so that the head of a metal pin is accommodated in the cavity while the shank of the pin rests in a cylindrical housing leading from the mould cavity in the block. There is clearly a need for fixing screws having a similar moulded plastic head. However, screws with a solid plastic head are not very strong and the sides of the screwdriver slot break or burr very easily. If a plastic head is to be applied to a screw or bolt having a slot, as opposed to a nail which has a plain head, it is essential that the slot in the metal screw or bolt is aligned with the slot in the plastic head when the screwdriver is inserted. In GB 916161 screws and bolts are disclosed in which the metal head is covered by a relatively thin plastic cap, in order to provide electrical insulation. The plastic caps are designed so as to rotate about the axis of the axis of the screw or bolt and cannot thus be firmly bonded to the metal as in the case of plastic headed pins. We now provide a metal screw or bolt havng a slotted or otherwise recessed head having integrally moulded thereto a solid plastic head having a slot or recess permanently aligned with that of the metal screw or bolt. The manufacture of such a screw is complicated by the fact that the inner metal screw head must have a slot which communicates with a .corresponding slot in the moulded plastic head so that a screwdriver can be inserted to engage with the inner metal head. While it would be a simple matter to provide an appropriately shaped mould cavity for a plastic head having a slot, it is difficult to ensure that the slot in the metal screw is accurately aligned with the slot-forming component of the mould. This invention provides apparatus which conveniently solves this problem, enabling the rapid manufacture of plastic headed screws having a slot in the plastic head which communicates with the slot in the metal head. By the term "slot" used in this specification, we mean not only a straight cross-slot of the conventional type which is open at each end, but also the various types of closed-end slot, including straight close-end slots, and cross-shaped slots of various patterns, e.g. Phillips and 'Torx' - type screws, as well as polygonal cavities of the Allen type. The apparatus is based on the provision of screwdriver-like projections which are biased into the mould cavities and can be turned until they engage the slots. According to the invention, therefore, there is provided injection moulding apparatus for the manufacture of plastic headed metal screws, including means defining an array of mould cavities with connecting conduit means for injected plastic, each cavity formed so as to accommodate the head of a metal screw; means for holding an array of metal screws with their longitudinal axes parallel; a complementary array of projecting members, each member being shaped to engage the slot of a said screw, and being located so as to project it into a mould cavity; and means for rotating each projecting member or each screw about its longitudinal axis and for biasing each projecting member or screw towards a cavity so as to cause each projecting member to engage each slot. A small degree of bias towards the mould cavities is essential for each screw or projecting member, so that as it is turned it can move axially so that the projecting member engages the slot. While the biassing can be achieved by suitable springing, we find that in one preferred embodiment of the invention, the members or screw array are arranged so as to be able to drop under the influence of gravity. Conveniently, in one embodiment, the moulding apparatus is formed with two cooperating components: a first component comprising a first mould plate including an array of housings in communication with apertures in a face of said first plate to accommodate the shank portions of the screws; and a second component comprising a second mould plate provided with a complementary array of housings in communication with apertures in one face of the second mould plate, each housing accommodating a projecting member; means being provided for bringing the two said faces into abutment and cavities being formed in one or both of said faces and arranged so as to form, when the two faces are in contact and screws are located in the first mould plate, an array of mould cavities surrounding the heads of the screws. As indicated above, the cavities may be in either of said faces, but it is convenient to have the cavities communicating with the housings carrying the projecting members. In a preferred embodiment of the apparatus according to the invention, a base plate carries the array of housings for the screw shanks and an upper plate carries the housings for the projecting members, each of the latter housings opening into a mould cavity, the two plates being hinged for close abutment. In this embodiment, screws can easily be inserted into _ the housings in the base plate, the upper plate can be lowered into contact, thus forming a series of moulds surrounding the screw heads. Then, as the projecting members are turned, they can drop into alignment in the slots. Such an embodiment is particularly suited to plastic heads which are domed, frustroconical etc. Naturally, if it is desired to produce a plastic head which is undercut, i.e. is of narrower diameter nearer the shank, then the cavities must be provided in the plate which houses the screw shanks, in order that the moulded heads can be removed from the moulds at the end of the injection process. In an alternative embodiment, one mould plate includes an array of cavities, each including a projecting member which is fixed, while the other mould plate includes an array of housings for screws, each housing including gripping means for holding a screw firmly so that it projects into the mould cavity formed between the abutted mould plates. In this embodiment the means for rotation is arranged to provide rotation to the screws rather than to the projecting members. It will be seen that this embodiment is a simple corollary of the first embodiment. The means provided for rotating the projecting members or screws can comprise any convenient form of powered drive whereby linear movement is converted into rotary movement, or rotary movement is transmitted. In a preferred embodiment of the apparatus according to the invention, each projecting member or screw gripping means is provided with a pinion in the form of a collar and a rack is provided into which the pinions can mesh. Linear movement of the rack then rotates each of the projecting members or screws. Alternatively, but less preferably, the members or screw gripping means can be provided with pinions which interlink in a gear train. Other analogous forms of drive train can be envisaged. In the general embodiment in which rotation is provided to the projecting members or screw gripping means, it is convenient, as stated above, to provide a pinion collar directly to the member or gripping means. However, problems can sometimes arise if the slots in the screws are of variable depth in any one batch. In the case where a slot is particularly shallow, for example, the projecting member (if above) does not drop as far as others so that its collar can become axially disconnected from the rack or neighbouring pinion in the gear chain. Similarly, if the screw is above, it does not drop as far to engage the projecting member and drive is again disconnected. To overcome this drawback, it is preferable that the rotatable item is located within a rotatable housing in such a way that rotation of the housing causes rotation of the item (either projecting member or screw gripping means) regardless of its relative axial disposition. Thus, for example, a projecting means can be located inside a generally tubular driven housing so as to be rotatably engaged therewith. In one preferred embodiment the tubular housing is provided with a terminal collar having an axial aperture having the profile of the slot-engaging portion of the projecting member, e.g. a cross-shaped aperture for a Phillips-type screwdriver head. The projecting member then passes through the aperture and is rotationally driven by the housing and collar at any degree of axial movement of the projecting member. A housing for a screw-gripping means can be similarly constructed. Naturally, the housings for the screw shanks and the housings for the projecting members must offer a close fit. in order to prevent leakage of injected plastic. Two preferred embodiments of the apparatus will now be described with reference to the accompanying drawings in which:- Figure 1 represents a schematic sectional view of the apparatus at the point of moulding; Figure 2 is a plan view in the direction A-A; and Figure 3 is a plan view in the direction B-B. Figure 4 represents a plan view of an alternative embodiment in the direction A-A of Fig. 5, similar to that in Figure 2, but with details of the train of pinions shown in cutaway Figure 5 represents a schematic part-sectional view of the alternative embodiment to that shown in Figure 1; and Figure 6 is a sectional view of a . screw head. The apparatus, in one embodiment, comprises a lower mould plate 1 carrying an array of 24 cylindrical housings 2 to accommodate screw shanks. The diameter of the housings 2 is such that an inserted screw sits with its head proud of the top surface. An upper mould plate 3 is mounted for pivotal movement on a support 4. The upper mould plate 3 carries an array of 24 housings 5 communicating with mould cavities 6. The mould cavities 6 are all connected to a branched conduit 7 serving to direct injected plastic to each of the mould cavities 6. Housed within each housing 5 is a projecting member 8 having a screwdriver-like end 9 for engagement with a screw slot. Immediately above the mould plate 3, each projecting member 8 is provided with a pinion collar 13 arranged in mesh with a linear toothed rack 14. In operation, the upper mould plate 3 is hinged upwards to expose the housings 2 in the lower mould plate 1. Screws are dropped into plate, their slots being randomly oriented. The upper mould plate 3 is then lowered into position in close abutment with the lower mould plate 1. At this point, the upstanding heads of the screws are housed within the mould cavities 6 and most, if not all of the projecting members are not yet located in the screw slots. The rack 14 is then caused to move, thus rotating the pinions 13, causing the projecting members 5 to rotate. As each projecting member 5 aligns with its screw slot, it drops under gravitational bias to engage the slot. Further rotation of the projecting member 5 then merely rotates the screw in the housing 2. In an optional modification of this embodiment, the upper mould plate 3 can be provided with spring biasing means 15a to complement (or replace) the effect of gravity. Once all the projecting members 5 are dropped and located in their slots, the upper mounting plate 10 is brought into contact with the upper mould plate 3. The pistons 12 which are provided with spring biasing means 15 are aligned with, and come into contact with, the tops of periphery members 5. The mould is then clamped together. Any projecting members 5 which may not have dropped and located in their respective slots (because of faults in screws etc) are thus protected from damage by the clamping force. Molten plastic, e.g. polypropylene or acetal, is injected via conduit 7 into- - each of the mould cavities 6. The finished screws are then removed from the apparatus by again hinging upward the upper mould plate 3. In a second embodiment, shown in Figures 4 and 5, the housings 5 in the upper plate 3a, 3b are replaced by generally tubular housings 16 having an inner bore 17 in which the projecting member 8 is located. At the bottom end of each tubular housing 16 the inner bore 17 is reduced to a smaller aperture 18 having a cross-section matching that of the driving end 19 of the projecting member 8. In contrast to the arrangement shown in Fig.l, the driving end 19 has a cross-section matching the profile of the screw slot (e.g. a Phillips 'cross' pattern) throughout its length. A driven pinion collar 13 is mounted on the tubular housing 16. Rotation of the tubular housing 16 thus causes rotation of the projecting member 8 (by direct drive on the driving end 19) regardless of the relative axial position of the projecting member 8 in the tubular housing 16. In a second change from the format of Figure 2, the rack 14 is replaced by pinions 13 which intermesh as in a gear train, (see Fig.5). A simple lever 20 is mounted on a drive shaft 21 which carries a driving pinion 22. Rotation of the lever 20 about the axis of the shaft 21 causes the drive pinion 22 to rotate the driven pinions 13. As before, as soon as the driving end of the projecting member 8 engages the slot of a screw, the screw turns. The whole train of pinions 22, 13 is housed in a recess in the bottom surface of an upper section 3a of the mould plate. For reference. Figure 6 shows a sectional view of the head of a screw manufactured according to the present invention. A conventional countersunk head 23 Phillips-type screw having a conventional 'cross'- shaped slot 24 is encapsulated by an integral solid plastic head 25 having a complementary slot 26 which forms an extension of the screw slot 24.

Claims: CLAIMS : 1. A metal screw or bolt having a slotted or otherwise recessed head having integrally moulded thereto a solid plastic head having a slot or recess permanently aligned with that of the metal screw or bolt. 2. Injection moulding apparatus for the manufacture of plastic headed metal screws, including means defining an array of mould cavities with connecting conduit means for injected plastic, each cavity fo med so as to accommodate the head of a metal screw; means for holding an array of metal screws with their longitudinal axes parallel; a complementary array of projecting members, each member being shaped to engage the slot of a said screw, and being located so as to project it into a mould cavity; and means for rotating each projecting member or each screw about its longitudinal axis and for biasing each projecting member or screw towards a cavity so s to cause each projecting member to engage each slot. 3. Injection moulding apparatus according to claim 2 comprising two cooperating components: a first component comprising a first mould plate including an array of housings in communication with apertures in a face of said first plate to accommodate the shank portions of the screws; and a second component comprising a second mould plate provided with a complementary array of housings in communication with apertures in one face of the second mould plate, each housing accommodating a projecting member; means being provided for bringing the two said faces into abutment and cavities being formed in one or both of said faces and arranged so as to form, when the two faces are in contact and screws are located in the first mould plate, an array of mould cavities surrounding the heads of the screws. 4. Apparatus according to claim 2, in which one mould plate includes an array of cavities, each including a projecting member which is fixed, while the other mould plate -includes an array of housings for screws, each housing including gripping means for holding a screw firmly so that it projects into the mould cavity formed between the abutted mould plates. 5. Apparatus according to claim 3 or claim 4 in which the means for rotating the projecting members or screws comprises a toothed rack arranged to mesh with pinion collars on the projecting members or screw gripping means. 6. Apparatus according to claim 3 or claim 4 in which the means for rotating the projecting members or screws comprise intermeshing pinion collars on the projecting members or screw gripping means and an intermeshing drive pinion. 7. Injection moulding apparatus substantially as herein described with reference to the accompanying drawings.

Inventors: WHETSTONE STEPHEN, WHETSTONE, STEPHEN
Applicants: SEAC LIMITED
Application year: 1988
Main classification: F16B23
IPC codes: B29C45, F16B37