Bài tập 18
If its use in trousers was a major factor in establishing the zip as a fashion icon, despite its occasional tendency to trap parts of the wearer's anatomy; another major breakthrough came with the separable zip: the kind that opens at both ends. This type, still widely used in a range of items from jackets to tents, is shown in the diagram.
Let's look first at the right-hand side of the illustration, at the material attached to the item of clothing, the bag, or whatever. This is the tape, which is usually made of fairly tough fabric. At the end of that, there's what is known as the heat-seal patch: the cotton and nylon laminated material used to reinforce the tape.
Now alongside the heat-seal patch is a small piece of metal, used only on a separating zip, whose function is to enable the two halves of the zip to join. This is known as the pin. Opposite that, on the other half of the zip in the diagram, is a device which correctly aligns the pin. The box, as it's called, begins the joining of the zip halves. Running up the inside edge of each half are dozens, possibly hundreds of metal teeth, each of which has a small hook and an equally tiny hollow.
Moving up and down the teeth, to open and close the zip, is a piece of metal called the slider. This is operated by means of a pull tab, so-called because, logically enough, the wearer or user pulls it in one direction or the other. To close the zip, a wedge inside the slider pushes the hook of each tooth on one side into the hollow of each offset tooth on the other; to open it, the wedge forces them apart. To prevent the slider coming off the teeth at the other end, there is a 'top stop' on both sides of the zip.
This basic design has changed little in the many years since it was first introduced, although nowadays, of course, zips - zippers - are available in a whole range of shapes, sizes, and materials.