Zip fasteners are manufactured at a large scale typically by injection moulding from synthetic, thermoplastic materials. In order to reach a high production rate, it is most desirable to manufacture the zip fasteners in as few injection steps as possible, and best in a single injection step, and to obtain different lengths of the zippers within the same tool and without great complications. Another requirement of tools for injection moulding of zip fasteners is that so-called slit-type zip fasteners can be produced. Slit-type zip fasteners are zippers whose teeth have a slit at their heads and a corresponding projection at their base, said projection meshing with a slit in a tooth of the opposite zip fastener tape when the zip fastener is closed. A slit-type zip fastener therefore has an increased strength, and the teeth are secured against lateral pushing-out.
An appropriate tool for this manufacturing mode is disclosed in EP-0,356,600. The mould described therein consists of two tape moulds arranged symmetrically at the left and the right of a slider for two rows of teeth each which are directly moulded on zipper supporting tapes laid into the mould. The slider functions in a first position for providing moulds for the upper and lower stops, also called top and bottom stops, respectively, and in a second position, to be applied to longer zippers, for combining the right and the left mould halves to define a continuous mould for a long zipper tooth chain.
Slit formers having the shape of long strips and a section corresponding to a beveled U are furthermore disposed in the right and the left tape mould. During injection, the slit formers are pushed by a pressing wedge against the tooth moulds thus resulting in the projection of one leg of the U into the moulds for the tooth heads, thus providing the injection mould of the slit. During the ejection of the injection moulded zip fastener tapes, the tooth moulds and simultaneously the slit formers are lifted with respect to the base of the mould. The slit formers loose therefore their contact with the pressing wedge and are pushed by springs out of the tooth slits so that the teeth and the attached zipper tapes can be taken out of the mould.
However, the very pushing out by the action of springs constitutes a particularly problematic aspect of the known construction. It is important for the tape moulds that the slit formers can uniformly slide out of the slits over the whole length of the tape. If this cannot be achieved, for example due to different spring forces over the length, or because the slit former adheres more strongly to the teeth at one end than at the other, or because not the entire length of the right and the left mould halves is used, the slit former may jam, and the mould must be remade fit to function by hand. A further disadvantage is the fact that the known device presents a relatively high limit for the lowest possible length of a zip fastener which can be injection moulded in this mould. This is due, on one hand, to the fact that the right and the left mould halves must be used to the same extent. For each mould half, the shortest possible length results from the position of the moulding nozzle for supplying the injection material into the mould which is closest to the slider. Since, however, the injection material is supplied centrally, about near the slider, non-used, heated material remains in the hot runner at nozzles far from the slider and which are shut down for the manufacture of short zip fasteners, and undergoes a decomposition process.