Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Patent
1994-07-20
1997-01-21
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
2643281, 264334, 249 59, 425330, 425542, 425DIG58, B29C 4533
Patent
active
055957021
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of molding screws of a synthetic resin and a molding apparatus for carrying out the method.
BACKGROUND ART
Screws made of a synthetic resin are not only remarkably light in weight in comparison with screws made of a metal and can be mass-produced, but also their production cost is relatively low because their material is relatively inexpensive. In addition, they can be easily produced with different pitches and/or different thread shapes in section. Therefore, they have been increasingly widely utilized as being incorporated in various feeding devices.
For example, paper sorters operatively associated with copying machines are required to incorporate such a mechanism that can move a number of vertically disposed sorter trays successively to their prescribed positions so as to bring them to be in spaced arrangement or to be collected at a bottom position, or can broaden the spacing between adjacent sorter trays on the side of the copying machine at which copied papers are discharged than on the other side, and synthetic resin-made screws are utilized in this mechanism. Synthetic resin-made screws are also used in various devices, such as the zoom mechanisms of cameras or the head transporting mechanisms of printers.
If upper and lower molding blocks are used to mold such a synthetic resin screw by means of a cavity the center axis of which lies in a parting plane formed between the upper and lower molding blocks, it will be difficult to withdraw a molded screw longer than one pitch from a mold assembly. Specifically, if the molded screw is observed from one side thereof along its horizontal axis, by noting either one of the side faces (flights) of a particular thread, it can be seen that a top half which corresponds to an angular extent of 90.degree. (a 1/4 pitch of the thread) above the center line is visibly exposed, i.e., the top half is in the "non-undercut state" in which the molded product can be withdrawn from the mold assembly in the direction of the eyes of the viewer. Further, it will be seen that a bottom half which follows the top half and corresponded to an angular extent of 90.degree. below the center line is not visibly exposed, i.e., the bottom half is in the "undercut state" in which no molded product can be withdrawn from the mold assembly.
Further, another top-half side face which is opposite to the non-undercut side face of the thread above the center line is in the undercut state. By noting the opposite side faces of two adjacent threads formed on the opposite sides of a particular root, it will be seen that one of the side faces is in the non-undercut state, whereas the other side face is in the undercut state.
As will be understood from the above description, in each one-pitch portion of a screw thread, a so-called undercut portion which hinders a molded product from being withdrawn from a mold assembly is formed every 90.degree. along either side face of the screw thread. In addition, if either side face is in the non-undercut state, the opposite side face is in the undercut state.
The shape of the screw thread is formed as a twisted plane in the above-described manner. A molded screw having thin threads made of a soft molding material, such as polyethylene resin, can forcedly be released from a mold assembly, but a molded product made of a resin suitable for use as a structure material, such as nylon, cannot be withdrawn from the mold assembly because an undercut portion of the molded product will be caught in the cavity of the mold assembly. Accordingly, it is substantially difficult to carry out molding with a mold assembly having a simple structure made up of a small number of movable members.
Another problem is involved in the cross-sectional shape of a screw thread, such as a square thread or a wedge thread. For example, if a square-threaded screw is molded, the thread is formed in such a manner that each side face of the thread erects at right angles to the adjacent root, so that a major part of the screw thread is
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Hiroki Toyohisa
Suzuki Tadashi
Heitbrink Jill L.
Optec Co. Ltd.
Toska Co., Ltd.
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