The gate is a channel or orifice connecting the runner with the impression. It has a small cross-sectional area when compared with the rest of the feed system. This small cross-sec
tional area is necessary so that: (i)The gate freezes soon after the impression is filled so that the injection plinger can be withdrawn without the probability of void being created in the molding by suck-back . (ii)It allows for simple degating and in some mould this degating can be automatic. (iii)After degating only a small witness mark remains. (iv)Better control of the filling of multi-impressions can be achieved. (v)Packing the impression with material in excess of that required to compensate for shrinkage is minimized . The size of the gate can be considered in terms of the gate cross-sectional area and the gate length, the latter being known as gate land. The optimum size for a gate will depend on a number of factors including: (i) the flow characteristics of the material to be molded; (ii) the wall section of the molding; (iii) the volume of material to injected into the impression; (iv) the temperature of the melt; (v) the temperature of the mould.
No theoretical size exists for the ideal gate. The gate size chosen in practice for a particular component is normally based on past experience. However, the reader may not have this experience upon which to base a decision and, therefore, a guide to the dimensions for each gate type is given. The guide applies, expect where otherwise stared, to the general case of a molding with a wall section between 0.75mm and 4mm.