Different Features of Injection Molds
Gates: The gate is a small orifice which connects the runner to the cavity. It has a number of functions. It provides a convenient weak link by which the moulding can be broken off from the runner system. In some moulds the degating may be automatic when the mould opens. The gate also acts like a valve in that it allows molten plastic to fill the mould but being small it usually freezes off first. The cavity is thus sealed off from the runner system which prevents material being sucked out of the cavity during screw-back. Small gates are preferable because no finishing is required if the moulding is separated cleanly from the runner. So for the initial trials on a mould the gates are made as small as possible and are only opened up if there are mould filling problems.
In a multi-cavity mould it is not always possible to arrange for the runner length to each cavity to be the same. This means that cavities close to the sprue would be filled quickly whereas cavities remote from the sprue receive the melt later and at a reduced pressure. To get rid of this problem it is common to use small gates close to the sprue and progressively increase the dimension of the gates further along the runners. This has the effect of balancing the fill of the cavities. If a single cavity mould is multi-gated then here again it may be beneficial to balance the flow by using various gate sizes.
There are three types of gates which are commonly used.
1. Sprue gates: These are used when the sprue bush can feed directly into the mould cavity as in case of symmetrical moulding such as buckets.
2. Pin gates: These are particularly successful because they cause high shear rates which reduces the viscosity of the plastic and so the mould fills more easily.
3. Side gates: It is the most common type of the gate and is simple rectangular section feeding into the side of the cavity. A particular attraction of this type of gate is that mould filling can be improved by increasing the width of the gate but the freeze time is unaffected because the depth is unchanged.
