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Gates

Posted by admin on Jun 18th, 2015
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The gate is a channel or orifice connecting the runner with thw impression. It has a small cross-sectional area when compared with the rest of the feed system. This small cross-section area is necessary so that:

(i) The gate freezes soon after the impression is filled so that the injection plunger can be withdrawn without the probability of void being created in the molding by suck-back.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 canbe 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 volume of material to be 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 bade a decision and, therefore, a guide to the dimensions for each gate type is given. The guide applies, except where otherwise stared, to the general case of a molding with a wall section between 0.75mm and 4mm.

Type of gate

To obtain the optimum filling conditions the type of gate must be carefully chosen. On most occasions, however, the choice will be obvious as only one type of gate will meet the particular requirement for the molding on land.

The types of gate commonly used are: sprue gate, edge gate, overlap gate, fan gate,diaphragm gate, ring gate, film gate, pin gate, submarine gate and tab gate.

Posted by Mandy Tang

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